FUNDAMENTALS OF PHOSPHORS
Edited by
Willian1. M. Yen Shigeo Shionoya (Deceased) Hajime Yamamoto
o ~,~~F;'~:~~O"P
Boca Raton l ondon New York
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This mat erial was previously published in Phosphor Handbook, Second Edition © 2007 by Taylor and Fran cis Gro up, LLC.
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Dedication
Dr. Shigeo Shionoya 1923-2001
This volume is a testament to the many contributions Dr. Shionoua made to phosphor art and is dedicated to his menlOry.
In Memoriam Shigeo Shionoya Formerly of the University of Tokyo The Institute for Solid State Phy sics Tokyo, Japan
Shosaku Tanaka Tottori University Department of Electrical & Electronic Engineering Tottori , Japan
The Editors William M. Yen obtained his BS. degree from the University of Redlands, Redlands, California in 1956 and his Ph .D. (physics) from Washington University in St. Louis in 1962. He served from 1962-65 as a Research Associate at Stanford University under the tutelage of Professor A.L. Schawlow, following which he accepted an assistant professorship at the University of Wisconsin -Madison. He was promoted to full professorship in 1972 and retired from this position in 1990 to assume the Graham Perdue Chair in Physics at the University of Georgia-Athens. Dr. Yen has been the recipient of a J.5. Guggenheim Fellowship (1979-80), of an A. von Humboldt Senior US. Scientist Award (1985, 1990), and of a Senior Fulbright to Australia (1995). He was recently awarded the Lamar Dodd Creative Research Award by the University of Georgia Research Foundation. He is the recipient of the ICL Prize for Luminescence Research awarded in Beijing in August 2005. He has been appointed to visiting professorships at numerous institutions including the University of Tokyo, the University of Paris (Orsay), and the Australian National University. He was named the first Edwin T. Jaynes Visiting Professor by Washington University in 2004 and has been appointed to an affiliated research professorship at the Uni versity of Hawaii (Manoa). He is also an honorary professor at the University San Antonio de Abad in Cusco, Peru and of the Northern Jiatong University, Beijing, China. He has been on the technical staff of Bell Labs (1966) and of the Livermore Laser Fusion Effort (1974-76). Dr. Yen has been elected to fellowship in the American Physical Society, the Optical Society of America, the American Association for the Advancement of Science and by the U.S. Electrochemical Society.
Professor Shionoya was born on April 30, 1923, in the Hongo area of Tokyo, Japan and passed away in October 2001. He received his baccalaureate in applied chemistry from the faculty of engineering, University of Tokyo, in 1945. He served as a research associate at the University of Tokyo until he moved to the department of electrochemistry, Yokohama National University as an associate professor in 1951. From 1957 to 1959, he was appointed to a visiting position in Professor H.P. Kallman's group in the physics department of New York University. While there, he was awarded a doctorate in engineering from the University of Tokyo in 1958 for work related to the industrial development of solid-state inorganic phosphor materials. In 1959, he joined the Institute for Solid State Physics (lSSP, Busseiken) of the University of Tokyo as an associate professor; he was promoted to full professorship in the Optical Properties Division of the ISSP in 1967. Following a reorganization of ISSP in 1980, he was named head of the High Power Laser Group of the Division of Solid State under Extreme Conditions. He retired from the post in 1984 with the title of emeritus professor. He helped in the establishment of the Tokyo Engineering University in 1986 and served in the administration and as a professor of Physics. On his retirement from the Tokyo Engineering University in 1994, he was also named emeritus professor in that institution.
During his career, he published more than two hundred scientific papers and authored or edited a number of books-the Handbook on Optical Properties of Solids (in Japanese, 1984) and the Phosphor Handbook (1998). Professor Shionoya has been recognized for his many contributions to phosphor art. In 1977, he won the Nishina Award for his research on high-density excitation effects in semicond uctors using picosecond spectroscopy. He was recognized by the Electrochemical Society in 1979 for his contributions to advances in phosphor research. Finally, in 1984 he was the first recipient of the ICL Prize for Luminescence Research. Hajime Yamamoto received his B.s. and PhD. degrees in applied chemistry from the University of Tokyo in 1962 and 1967. His Ph.D . work was p erformed at the Institute for Solid State Physics under late Professors Shohji Makishima and Shigeo Shionoya on spectroscopy of rare earth ions in solids. Soon after graduation he joined Central Research Laboratory, Hitachi Ltd., where he worked mainly on phosphors and p-type ZnSe thin films. From 1971 to 1972, he was a visiting fellow at Professor Donald S. McClure 's laboratory, Department of Chemistry, Princeton University. In 1991, he retired from Hitachi Ltd . and moved to Tokyo University of Technology as a professor of the faculty of engineering. Since 2003, he has been a professor at the School of Bionics of the same university. Dr. Yamamoto serves as a chairperson of the Phosphor Research Society and is an organizing committee member of the Workshop on EL Displa ys, LEOs and Phosphors, International Display Workshops. He was one of the recipients of Tanahashi Memorial Award of the Japanese Electrochemical Society in 1988, and the Phosphor Award of the Phosphor Research Society in 2000 and 2005.
Preface This volume originated from the Phosphor Handbook which has enjoyed a moderate amount of sale success as part of the CRC Laser and Optical Science and Technology Series and which recently went into its second edition. The original Handbook was published in Japanese in 1987 through an effort of the Phosphor Research Society of Japan. 111e late professor Shionoya was largely instrumental in getting us involved in the translation and publication of the English version. Since the English publication in 1998, the Handbook has gained wide acceptance by the technical community as a central reference on the basic properties as well as the applied and practical aspects of phosphor materials. As we had expected, advances in the display and information technologies continue to consume and demand phosphor materials which are more efficient and more targeted to specific uses. These continuing changes in the demand necessitated an update and revision of the Handbook and resulted in the publication of the second edition which incorporates almost all additional topics, especially those of current interest such as quantum cutting and LED white lighting phosphor materials. At the same time, it has also become apparent to some of us that the evolution of recent technologies will continue to place demands on the phosphor art and that research activity in the understanding and development of new phosphor materials will continue to experience increases. For this reason, it has been decided by CRC Press that a series of titles dedicated to Phosphor Properties be inaugurated through the publication of correlated sections of the Phosphor Handbook into three separate volumes. Volume I deals with the fundamental properties of luminescence as applied to solid state phosphor materials; the second volume includes the description of the synthesis and optical properties of phosphors used in different applications while the third addresses experimental methods for phosphor evaluation. The division of the Handbook into these sections, will allow us as editors to maintain the currency and timeliness of the volumes by updating only the section(s) which necessitate it. We hope that this new organization of a technical series continues to serve the purpose of serving as a general reference to all aspects of phosphor properties and applications and as a starting point for further advances and developments in the phosphor art. William M. Yen Athens, GA, USA October, 2006 Hajime Yamamoto Tokyo, Japan October, 2006
Contributors Chihaya Adachi Kyushu University Fukuoka, Japan
Hiroyuki Matsunami Kyoto University Kyoto, Japan
Pieter Dorenbos Delft University of Technology Delft, The Netherlands
Mamoru Mitomo National Institute of Materials Science Tsukuba, Japan
Gen-ichi Hatakoshi Toshiba Research Consulting Corp. Kawasaki, Japan
Noboru Miura Meiji University Kawasaki, Japan
Naoto Hirosaki National Institute of Materials Science Tsukuba, Japan Sumiaki Ibuki Formerly of Mitsubishi Electric Corp. Amagasaki, Japan Kenichi Iga Formerly of Tokyo Institute of Technology Yokohama, Japan Tsuyoshi Kano Formerly of Hitachi, Ltd ., Tokyo, Japan Hiroshi Kobayashi Tokushima Bunri University Kagawa, Japan
Makoto Morita Formerly of Seikei University Tokyo, Japan Shuji Nakamura University of California Santa Barbara, California Eiichiro Nakazawa Formerly of Kogakuin University Tokyo, Japan Shigetoshi Nara Hiroshima University Hiroshima, Japan Hiroshi Sasakura Formerly of Tottori University Tottori, Japan
Hiroshi Kukimoto Toppan Printing Co., Ltd . Tokyo, Japan
Masaaki Tamatani Toshiba Research Consulting Corporation Kawasaki, Japan
Yasuaki Masumoto University of Tsukuba Ibaraki, Japan
Shinkichi Tanimizu Formerly of Hitachi, Ltd . Tokyo, Japan
Tetsuo Tsutsui Kyushu University Fukuoka, Japan
Hajime Yamamoto Tokyo University of Technology Tokyo, Japan
Rong-jun Xie Advanced Materials Laboratory, National Institute of Materials Science Tsukuba, Japan
Toshiya Yokogawa Matsushita Electric Ind. Co., Ltd. Kyoto, Japan
Contents Chapter 1
Chapter 2
Index
Fundamentals of luminescence 1.1 Ab sorption and em iss ion of light... 1.2 Electronic sta tes an d optical transition of solid crystals 1.3 Luminescence of a localized center 1.4 Impurities and luminescence in semiconductors 1.5 Luminescence of organic comp ound s 1.6 Luminescence of low-dimensional systems 1.7 Transient characteristics of luminescence 1.8 Excitation energy transfer and cooperative optical phenomena 1.9 Excitation mechanism of luminescence by cathode-ray and ionizing radiation 1.10 Inorganic electroluminescence 1.11 Lanthanide level locations and its impact on phosphor performance Principal phosphor materials and their optical properties 2.1 Luminescence centers of ns --type ions 2.2 Luminescence centers of transition metal ions 2.3 Luminescence centers of rare-earth ions 2.4 Luminescence cen ters of complex ions 2.5 Ia-VlIb compounds 2.6 IIa-VIb compounds 2.7 IIb-VIb compounds 2.8 ZnSe and related luminescent materials 2.9 IIIb-Vb comp oun ds 2.10 (Al,Ga ,In)(P,A s) alloys emitting visible luminescence 2.11 (AI,Ga,In)(P,As) alloys emitting infrared luminescence 2.12 GaN and related luminescence materials 2.13 Silicon carbide (SiC) as a luminescence material... 2.14 Oxynitride phosphors
1 1 11 25 39 51 61 73 89 101 111 129 145 145 157 181 205 217 221 237 265 273 283 291 299 313 321 329
chapter one - section one
Fundamentals of luminescence Eiichiro Naka za wa Contents 1.1 Absorp tion an d emission of ligh t... 1.1.1 Abso rp tion and reflection of light in cry stals 1.1.1.1 Optical consta n t and com p lex d ielectric cons tan t.. 1.1.1.2 Absorp tion coefficient 1.1.1.3 Reflec tivity and tran sm issi vity 1.1.2 Absorp tion and emission of light by impu rity a toms 1.1.2.1 Cla ssical harmonic oscilla tor model of op tica l cen ters 1.1.2.2 Elect ro ni c tran sition in an a tom 1.1.2.3 Electric dipole tran sition p robabili ty 1.1.2.4 Intensit y of light emission an d absorption 1.1.2.5 Os cilla tor strength 1.1.2.6 Impurity atom s in cry stals 1.1.2.7 For bidden transition 1.1.2.8 Selection ru le Referen ce
1.1
1 2 2 .3 3 4 .4
5 6 7 8 9 9 9 10
A bsorption and emission of light
Most phosphors are comp osed of a transparent microcrystallin e host (or a matrix) and an activa tor, i.e., a small amo un t of in ten tion ally added im p ur ity atom s dis tri bu ted in the host crys tal. Th eref ore, th e lu min escen ce processes ofa phosp h or can be divided into two parts: the processes main ly related to th e h ost, and th ose that occu r around an d within the activa tor. Processes rela ted to optical absorption, reflec tion, and tra ns m ission by the host crys tal are d iscussed, from a macroscopic p oint of view, in 1.1.1. O ther h ost processes (e.g., excitation by electron bombardment an d th e migration and transfer of the exc ita tion en ergy in the host) are di scu ssed in a later sect ion. 1.1.2 d eals wi th ph enomena rela ted to the activa tor atom ba sed on th e theory of atomi c spectra. The interaction betw een the h ost and th e activa tor is not explicitl y discussed in thi s section; in th is sense, the ho st is treat ed onl y as a m edi um for the acti va tor. The interaction processes such as the transfer of the h ost exci ta tion energy to the activat or w ill be discussed in detail for eac h ph osphor elsewhere.'
1
2
1.1.1
Fundamentals of Phosphors
Absorption and reflection of light in crystals
Since a la rge number of phosphor host mater ials are tran sparent and nonmagnetic, their optical p rope rtie s can be represented by the optical con st ants or by a complex dielectric constant.
1.1.1.1 Optical constant and complex dielectric constant The electric and magnet ic field s of a light wa ve, propagating in a uniform matrix with an ang ular frequency to (= 2n:v, v:frequency) and velo city v = wl k are:
E = Eo expH k.r - wt)]
(1)
(2)
-
where r is the position vector and k is the complex w av e vector. E and H in a nonmagn etic di electric material, w ith a ma gn et ic permeability that is nearly equal to th at in a vacuum (u = ~o) and with uniform dielectr ic cons tan t £ and electric conductivit y 0 , sati sfy the next two equations derived from Maxwell 's equations.
(3)
aH + £~o -a H 22
2
V' H
= 0~o -
at
(4)
at
-
-
In order that Eqs. 1 an d 2 satisfy Eqs. 3 and 4, the k -vector and its length k , whi ch is a complex number, should sa tisfy th e following relation:
(5) where £ is the complex dielectric const ant d efin ed by: _
£
,
•
= £ + 1£
1/
cr == E + 1 •
(6)
W
Therefore, the refractive ind ex, which is a real number defined as n == clv = ck no in a transparent media, is also a com p lex number :
11
.
-
l
£
= 11 + IK == ck/w = ~
Jl/2
(7)
where c is the velocity of light in vac u um and is equal to (£ Oflot1/ 2 from Eq. 5. The last term in Eq. 7 is also derived fro m Eq. 5. The real and im aginary parts of the comp lex refr acti ve index, i.e.. the real refractive index n and the extinction index K, are call ed optical constants, and are the rep resentat ive
Chapter one: Fundamentals of luminescence
3
constants of the macroscop ic optical properties of the m at erial. The op tical constan ts in a nonmagneti c material are related to each other using Eqs. 6 and 7,
(8)
(9)
Both of the optical cons tan ts, n and K, are functions of ang ular frequency wand, hen ce, are referr ed to as dispersion relations. The di spersion rel ati on s for a mat erial are obtained by mea suring and analyzing the refle ction or transmission spectrum of the material ov er a wid e spectra l region.
1.1.1.2 Absorption coefficient The inten sity of the light pr opagating in a med ia a d istance x from the incident surface havin g been decreased by the optical absorp tion is given by Lambert's law. 1= 10 exp( -ax)
(10)
wh ere 10 is the incid ent light intensity minus reflection losses at the su rface, and « (cm') is the absorp tion coefficient of the media. Using Eqs. 5 and 7, Eq, 1 may be rew ritt en as:
E = Eo exp( -soxx] c) exp[ - iw(t + nx]c)]
(11)
and, since the intensity of light is p roportional to the square o f its elec tric field s treng th E, the absorp tion coeffic ien t may be ide n tified as :
a = 2WK/ C
(12)
Therefore, K is a factor that represents the extinction of light due to the ab sorpti on b y the medi a. There are sev eral wa ys to rep resent the absorption of light by a medium, as d escribed below. 1. Absorption coefficien t, a(cm - 1) : l/ In = rOO< 2. Absorption cros s-section, a l N (cm-). Here, N is the number o f ab sorption center s pe r unit volume. 3. Optical d ensit y, abs orbance, 0 = -loglQ(1lIo) 4. Absorptivi ty, (10 - 1)110 x 100, (%) 5. Mola r extin ction coefficient, t = a loglQclC. Here, C(mol/ I) is th e molar con centration of absorption centers in a so lu tion or gas .
1.1 .1.3 Reflectivity and transmissivity When a light beam is incident normall y on an optically smooth crys tal surface, the ratio of the intensities of the reflected light to the incident light, i.e.. normal surface reflectivity Ro' can be written in terms of the optical cons tan ts, n and K, by
Fundamentals of Phosphors
4
(13)
Th en , for a sa mp le w ith an absorp tion coefficie n t a and th ickness d th at is lar ge enough to negl ect interference effects, th e overall normal reflec tivity an d transmissivit y, i.e., th e ra tio of th e transmitted light to the inciden t, a re; resp ectively:
R = Ro(1+ f
exp( -ad) )
(14)
(15)
If ab so rption is ze ro (a = 0), then,
(16)
1.1.2
Absorption and emission of light by impurity atoms
Th e emission of lig ht fro m a ma terial orig ina tes from two typ es of m echan isms: thermal emission and luminescence. Whi le all th e a toms composing th e solid participa te in the light em iss ion in th e thermal process, in the luminescen ce process a very small n um ber of a toms (impuri ties in m ost cases or crystal defects) are exci ted and take p art in the emission of light. The impuri ty ato m or defect and its surro u nding a toms form a lumin escent or an emi tting cen ter. In m ost phosphors, the lumin escence center is forme d by intentionall y incorpora ted impurity a tom s called activators. This secti on treat s the absorp tio n and emission of light by these impuri ty a toms o r local defects.
1.1.2.1
Classical harmonic oscillator model of optical centers
The absorption and emission of light by an a tom can be described in the mos t si mplified sch em e by a linea r h arm onic osci lla tor, as shown in Figure 1, composed of a posi tive charg e (+e) fixed a t z = 0 and an electron bo u nd and osc illa ting around it a long the z-axis. Th e elec tr ic dipole moment of th e osci lla tor w ith a cha racteristic angular freq uency W o is given by: M
= ez = M o exp(iwJ)
(17)
a nd its energy, th e sum of th e kinetic an d potential en er gies, is (mew;/2e2 )M;, where me is the m ass of th e electron. Such a vibra ting electric dipole transfer s energy to electromagneti c radiati on a t an average ra te of (w;/121rt oc3 )M5per second, and therefore has a tot al ene rgy decay rat e given by:
(18)
Chapter one:
Fundamentals of luminescence
5
B= 0
Z
Figure 1 Electrom agn etic rad iation from an electric dipole osc illato r. The len gth of the arrow gives the intensity of the rad iation to the direction .
When the cha nge of the energy of this oscillator is expresse d as an exponential function e'! », its time constant T" is equal to A o-l, which is th e radiat ive lifetime of the oscillator, i.e., the time it tak es for the oscillator to lose its energy to r 1 of the initi al en ergy. From Eq. 8, the radi ative lifetime of an oscillator with a 600-nm (CDo = 3 X 1015 S- I) w avelen gth is To = 10-8 s. The int ensity of the emission from an electric dipole oscilla tor dep ends on the direction of the propagation, as shown in Figure 1. A more detailed an alysi s of absorption and emission processes of light by an atom will be d iscussed usin g quantum mechanics in the following subsection.
1.1.2.2 Electronic transition in an atom In quantum mech an ics, the energy of the electrons localized in an at om or a molecul e have discrete valu es as sho wn in Figure 2. The absorption and emission of light by an
- .....----m
(a)
(b)
(c)
-...I-----n Figure 2 Absorp tion (a), spontaneo us emission (b), an d induced emission (c) of a photon by a two level system .
Fundamentals of Phosphors
6
at om, th erefore, is not a gradual and continuous process as discussed in the abo ve sectio n usin g a classical dipole oscillator, but is an instantaneous transition betw een two discrete ene rgy levels (stat es), m an d n in Figure 2, and should be treated statistically. The ene rgy of the photon absorbed or emitted at the tran siti on m H n is: (19)
w here E" and £"/ are th e ene rgies of the initial and final sta tes of the transiti on , resp ectively, and CO/l1I1 ( =2 1tV m ,,) is th e ang ular freque ncy of light. Th ere are tw o possibl e emission processes, as shown in Figure 2; one is called spo ntaneous emission (b), and th e othe r is stim ula ted emis sion (c). The stimulated emission is ind uced by an inciden t photon, as is the case with the absorption process (a). Laser action is based on this typ e of emission p rocess. The in tensi ty of th e absorp tion and em ission of photons can be enumerated by a transition p rob abil ity per a tom per second . Th e probability for an atom in a radiation field of ene rgy d en sity p(com,,) to absorb a photon , m aking the transition from n to m, is given by
Wmil -- B
fl -;l N
p(co "1/1 )
(20)
w he re BII_ is the transiti on probability or Eins tein's B-coefficient of optical absorption, and p(co) is eq ua l to l(co)/ c in which 1(co) is th e light intensity, i.e., the energy per second per unit area perp en d icular to the direction of light. On the othe r hand, th e p rob ab ility of th e em ission of light is the sum of the spontaneous emi ssion p robability A m->" (Einst ein 's A-coefficient) and the stimulated emission probability BII1 ->IIP(col/I,J Th e stimula ted emission probability coefficien t Bm _ is equal to B'Hm' The equilib rium of op tica l absorp tion an d emission between the atoms in the states m and n is expressed by th e followin g equa tion. , Hr
H /
(21)
where N mand N" ar e the number of at om s in th e sta tes m and n, resp ectively. Takin g into account the Boltzmann d istribution of the sys tem and Plank's equa tion of radi at ion in thermodynamic equilibrium, th e follow ing eq ua tion is obtaine d from Eq. 21 for the spontaneous mission probability. (22) Therefore, the probabilities of optical absorp tion, and the spo n taneo us and ind uced emissions between m and n are related to one another.
1.1.2.3
Electric dipole transition probability
In a quantum mechanical treatment, op tical tran siti on s of an atom are ind uced by per-turbing the energy of th e system by L.,(-erJ E, in wh ich Yj is the pos ition vec tor of the electron from the atom cen ter and, th erefore, L.,(-erj ) is the electric d ip ole moment of the atom (see Eq. 17). In thi s electric d ip ole ap proxima tion, the tran sit ion probability of optical absorption is gi ven by:
W
-
_
1t_ 2
"''' - 3£ octl
2
)IM
l(co "'"
1
"'"
(23)
Chapter one:
Fundamentals of luminescence
7
Here, the dipole moment, M",n is defined by:
(24)
where \jim and \jill are the wavefunctions of the states m and n, respectively. The direction of this dipole moment determines the polarization of the light absorbed or emitted. In Eq.23, however, it is assumed that the optical center is isotropic and then (M l1m ) z 2 = 2 I M mn 1 / 3 for light polarized in the z-direction. Equating the right-hand side of Eq. 23 to that of Eq. 20, the absorption transition probability coefficient BII--;m and then, from Eq. 22, the spontaneous emission probability coefficient A m can be obtained as follows: 1
1
_ ) 11
(25)
1.1.2.4
Intensity of light emission and absorption
The intensity of light is generally defined as the energy transmitted per second through a unit area perpendicular to the direction of light. The spontaneous emission intensity of an atom is proportional to the energy of the emitted photon, multiplied by the transition probability per second given by Eq. 25.
(26)
Likewise, the amount of light with intensity I o(w l1rll ) to be absorbed by an atom per second is equal to the photon energy wmll multiplied by the absorption probability coefficient and the energy density la/C. It is more convenient, however, to use a radiative lifetime and absorption cross-section to express the ability of an atom to make an optical transition than to use the amount of light energy absorbed or emitted by the transition. The radiative lifetime 'nm is defined as the inverse of the spontaneous emission probability A I1H
rr
•
-1 "t11/11
-
A
(27)
m-:,n
If there are several terminal states of the transition and the relaxation is controlled only by spontaneous emission processes, the decay rate of the emitting level is determined by the sum of the transition probabilities to all final states: Am
= L... ~ A m-"m
(28)
rr
and the number of the excited atoms decreases exponentially, exp(-t/'t), with time a constant, = A",-I, called the natural lifetime. In general, however, the real lifetime of the DC
Fundamentals of Phosphors
8
excited state m is controlled not only by radiative processes , but also by nonradiative ones (see 1.7). The absorption cross-section G represents the probabili ty of an atom to absorb a ph oton incident on a unit area. (If there are N absorptive atoms per unit volume, the absorption coefficient a in Eq. 10 is equal to GN. Therefore, since the intensity of the light with a photon per second per unit area is 10 = O)mn in Eq. 23, th e absorption cross-section is given by:
G l/J1l
1.1.2.5
[2
ItO) mil 1
3E
Cn M
a
(29)
11m
Oscillator strength
The oscillator streng th of an optical center is often used in order to represent the streng th of light absorption and emission of the center. It is defined by the following equation as a dimensionless quantity.
-
J,,,,, -
2m e0) nm
ne2
IM (
2 - 2me0) nm M
1
mn). -
3ne2
I m"1,
(30)
The third term of this equation is giv en by assuming that the tr an sition is isot ropic, as it . is the case w ith Eq. 24. The radiative lifetime and absorption cross-section are expressed by usin g the oscillator strength as:
(31)
(32)
Now on e can estimate the oscill ato r strength of a harmonic oscillator with the electric dipole moment M = - er in a quantum mechanical manner. The result is that onl y one electric dipole transition between the ground state (n = 0) and the first excited state (m = 1) is allowed, and the oscillat or s treng th of this transition is flO = 1. Therefore, the su m mation of all the oscillator strengths of the tran sition from the state n = 0 is also 'In!;110= 1 (m =/=- 0). This relation is true for anyone electron system; for N-electron systems, the following fsum rul e should be sati sfied ; that is,
(33) m-.r. n
At the beginning of this section, th e em ission rat e of a line ar harmonic oscillator wa s classically obtained as A o in Eq. 18. Th en , the total tran sition pr obability given by Eq. 32 with f = 1 in a quantum mechanical schem e coincid es with the em ission rat e of the classical linear oscillator A o, multiplied by a factor of 3, corresponding to the three degrees of freedom of th e motion of the electron in the present system.
Chapter one:
Fundamentals of luminescence
9
1.1.2.6 Impurity atoms in crystals Since the electric field actin g on an impurity atom or optical cen ter in a cryst al is different from th at in vacuum d ue to the effect of the p ola riza tio n of the su rro und ing a toms, and the light ve locity is reduced to c/ 11 (see Eq. 7), the radiat ive lifet im e an d th e ab sorption cross-section ar e changed from those in vac uu m . In a cubic cryst al, for exampl e, Eqs, 31 and 32 are chan ged, by the intern al local field, to:
(34)
(35)
1.1.2.7 Forb idden transition In the case that the electric d ip ole moment of a transition M ll m of Eq. 25 becom es zero, the probability of the electric d ip ole (E1) tran sition in Eq, 25 and 26 is als o zero. Sinc e th e electric d ipole tran sition generally ha s the largest tra nsi tion proba bility, this situation is usually expressed by the ter m forbidd en transition . Since th e electric di p ole m oment operat or in the in tegral of Eg. 24 is an odd func tion (od d p arity), th e electr ic d ipole mom ent is zero if the initial and final s tates of the tran sition h av e the sa me parity; th at is, both of the w av efunction s of these sta tes are either an even or od d function , and the transit ion is said to b e parity forbid d en . Likewise, sinc e the electric dipole moment op erator in the integral of Eg. 24 has no spin op era tor, transition s be tw een ini tia l and final states w ith different sp in m u ltip licities ar e spi n forb idden . In Eq . 24 for the dipole moment, the effects of the high e r-ord er pertur ba tio ns are neglected. If th e neglected ter ms are in cluded, the transition mome n t is w ritten as follows: 2
I ., -I( ., M
- er
2
2
2
c 37tffi 2 + -- r x + __11/_ " er· r ( 2mc P}"" 40c2 I( )11/, 1
I
(36)
1
where the firs t term on the rig h t-hand side is the con trib ution of the electric dipole (E1) term previously given in Eg. 24; the second term, in w h ich p d enotes th e momen tum of an electron, is that of magnetic d ip ole (M1); and th e th ird term is that of an elec tric quadrupole transition (E2). Provided th at (r)"," is abou t th e ra d ius of a hydrogen a tom (0.5 A) and ffi mll is 1015 rad / s for visible light, rad iati ve lifetimes es tima ted from Eq, 26 an d 36 are - 10-s s for El, - 10-3 s for M1, an d _ 10-1 S for E2. El-tr'a nsitions ar e forbidden (par ity forbidden ) for f -f and d-d tran sit ion s of free rar eearth ions and transition- metal ions because the ele ctron con figur ations, and hence the parities of the initial and final sta tes, are the same. In crystals, however, the E1 tran sition is pa rtially allowed by the od d com pone n t of the crystal field, an d thi s pa rtially allow ed or forced E1 tran sition h as the rad iat ive lifeti me of - 10-3 s. (See 2.2).
1.1.2.8 Selection rule The se lection ru le governi ng w h ethe r a d ipole tra ns ition is allo w ed be tw een th e s ta tes m and n is determin ed by the tran sition matrix elemen ts (er)1I/1l and (r x P)'WI in Eg. 36. How ever, a group theoretic al inspection of the sy m me tries of the wavefun ctions o f these states and the opera tors er an d r x P enables th e d et ermination of the selec tion rules w ithout calculating the ma trix eleme n ts.
Fundamentals of Phosphors
10
When an a tom is free or in a sph erica l symmetry field , its elec troni c states are denot ed by a se t of th e quantum numbers S, L, an d J in the LS-coupling scheme. Here, S, L, and J denote the quantum number of the spin, orbital, and total angu la r momentum, resp ectivel y, a nd L'iS, for exa m p le, denotes the difference in S between th e states m and n. Then the sele ction rules for E1 an d M1 transit ions in the LS-coupling scheme are giv en by: .15=0,
L'iL = O or
±1
L'if = 0 or ± 1 (J = 0 ~ J = 0, not allowed)
(37) (38)
If th e sp in -orbi t in teraction is too large to use the L5-coupling scheme, the JJ-coupling scheme might be used, in which many (5, L)-terms a re mix ed int o a J-state. In the JJcoupling schem e, therefore, the L'iS and L'iL selection rules in Eqs. 37 ad 38 are less strict, and only the L'iJ selection rule applies. While th e E1 transitions between the sta tes with th e same parity are forbid den, as in th e case of th e f-f transitions of free rare-earth ions, th ey become part ially allow ed for ions in cryst als due to the effe cts of crys tal fields of odd pari ty. The se lection rule for th e p artially a llow ed E1f-f tr ansi tion is 1.1 JI :::; 6 U= 0 - 0,1 , 3,5 ar e forbidden). M1 transitions are always parity allowed because of the even parity of the magnetic dipole operator r x p in Eq. 36 .
Reference 1. Practical Applications of Phosphors, Yen, WM. , Shionoya, 5., and Ya mamoto, H., Eds., CRC Press, Boca Raton, 2006.
chapter one - section two
Fundamentals of luminescence Shigetoshi Nara and Sumiaki Ibuki
Cont ents 1.2 Electronic states and optical transiti on of so lid cr ystals 1.2.1 Outline of band theory 1.2.2 Fundamental absorption, d ire ct transition, and indirect transition 1.2.3 Exciton References
1.2 1.2.1
11 11 18 22 24
Electronic states and optical transition of solid crystals Outline of band theory
First, a brief d escription of crystal properties is given. As is w ell known, a crystal consists of a periodic con figura tion of a tom s, which is called a crystal lattice. There are man y di fferent kinds of crystal lattices and th ey are classified, in general, according to their symmetries, which specify in variant properties for translational and rotational operation s. Figure 3 shows a few, typical examples of crystal structu re s, i.e., a rock-salt (belonging to on e of the cubic gro u ps ) s tructu re, a zinc-blende (also a cubic group) structure, and a wurtzeite (a he xagonal group) structure, respectivel y. Second, con side r the electronic st at es in these crystals. In an isolated state, ea ch atom has electrons that exist in d is crete electronic energy levels, and the states of these bound electrons are characterized b y a to m ic wavefunctions . Their di screte en erg y levels, however, will have finite spectral width in th e condensed state because of the o ve rlaps between electron ic w avefunctions belonging to different atoms . This is because electro ns can be come itinerant between atoms, until finally they fall into delocalized ele ctron ic states call ed electronic energy bands, w hi ch also obey th e sy m metries of cr ystals. In these energy bands, the states with lower en ergies a re occupied by ele ctro n s origin a ting from bound electrons of a toms and a re called valence bands. The energy bands having higher energies are not occupied by e lectro ns and a re called conduction bands. Usuall y, in materials having crystal sym m etrie s such as rock-salt, zinc-blende, or wurtzeite structures, there is no electronic s ta te b etween the top of the val ence b and (the highest sta te of occupi ed bands) and the bottom of th e conduction band (the low est state of un occupied bands); thi s region is called the bandgap. The reason why unoccupied s ta tes a re called
11
12
Fundamentals of Phosphors
.-- -or..:-...-,- - -- : : _-; ....... ... ...
-
/"'T
.,/
./
I
... ~ ::
~
,
./
,..~
-'..; ' .:::=....---IH:;ec
Q
Q
-
./ .,/
I
:;>
./ I
./
V
- - ~ - ....... ::.~
.,/ ,/
-
_ _----::~F-
./
./'
.......
---
- ---;- ' ........
V
./
• : Na
O :CI
O :s
. : Zn
. : Zn
O:s
Figure 3 The confi g uration of th e a to ms in thre e impor tan t kin ds of crysta l s tru ctures. (a) rock -sa lt type, (b) zinc-blen de type, and (c) wur tze ite typ e, respectively.
con duction bands is due to th e fac t th at an elec tron in a cond uc tion ban d is almost freely m ob ile if it is excited from a val ence band by so me me thod: for exa mp le, by abso rp tion of light quanta. In con tras t, electrons in va len ce bands cannot be mob ile becau se of a fundamen ta l p rop erty o f elec tro ns ; as [ermion s, only two elec trons (sp in up and do w n ) ca n occupy an electronic sta te. Th us, it is necessary for electrons in the va lence ba nd to have emp ty states in order for them to m ove freely w hen an elec tric field is ap plied . After an elec tro n is exci ted to th e cond uc tion ba nd, a hole that remains in the va lence band behaves as if it w ere a m obile pa rticle wi th a p ositiv e ch arge. Th is hyp othetical particle is ca lled a positive hole. The schema tic d esc ripti on of the se exci tatio ns are show n in Figure 4. As noted above, ba nd gaps are strongly rela ted to the op tical p roper ties and the electric cond u ctivity of crystals. A m ethod to evaluate th ese electronic band struc tu res in a qu antitative way using q uan tu m m ech ani cs is briefly d escribed. The m otion of elect-rons under th e in fluence of electric field s ge ner a ted b y a toms th at ta ke so me d efin ite space config ura tion spe cified by th e sy mmetry of the crys tal lattice, can be descri be d by the foll owing Schrodinger equa tion .
(39)
whe re VCr) is an effective potential applied to each electron and ha s the p ro pert y of: (40)
due to th e translati on al sy mme try of a give n crysta l lattice. R" is a latt ice vec tor indica ting the nth positi on of atoms in the lattice. In th e Fourier rep resentation, the potential VC r) can be writt en as :
V(r) =
I 1I
V" eiG"r
(41)
Chapter one: Fundamentals of luminescence
13
E
E
Conduction Band
...
Forbidden Band
----- - --- - --- ----
~~
Valence Band
---il--K Figure 4 The typic al band dispe rsion near the mini mum band gap in a se micond uc tor or an insulator wi th a direct bandgap in the Brillouin zon e.
where Gil is a reciprocal lattice vector. (See any element ary book of solid -state physics for the defin ition o f G,J It is d ifficult to solve Eq. 39 in ge ne ra l, but w ith the help of the translation al and rotational symme tries inh erent in the eq uation, it is possible to p red ict a general fun ctional form of solutions. The solu tion wa s first found b y Bloch and is ca lled Bloch's theorem. The solution \jI(r) should be of the form : (42) and is called a Bloch f unction. k is the wave vector and uk(r) is th e period ic fun ction of lattice translations, such as : (43)
As one can see in Eq. 40, uk(r) can also be expanded in a Fourier se ries as: (44)
where C,,(k) is a Fourier coefficient. Th e form of the solution represented by Eq . 42 shows that the wave vectors k are well-defined quantu m n umbers of the electronic sta tes in a given crys tal. Putting Eq. 44 into Eq. 42 an d usin g Eq. 41, on e can rew rite Eq . 39 in the following form :
Fundamentals of Phosphors
14
(45)
w here E eigenvalues d etermined b y :
I {~(k + G,)2- E}8 2rn
C G I
"
+ Vc - G I
II
1=0
(46)
Henceforth, th e k-dep endence of the Fourier compon ents CII(k) are negle cted . These formulas are in the form of in fini te dimensional determinant equa tions . For finite dimensions by considering amplitudes of v c,_c" in a given crystal, one can solve Eq. 46 approximately. Then the energy eige nv alues E(k) (energy band) m ay be obtaine d as a function of wave vector k and th e Fourier coefficients CII" In order to ob tain qu alit ative interpretation of ene rg y band an d properties of a wa vefuncti on , one can s ta rt wi th th e O th order approximati on of Eq . 46 by taking (47)
in Eq. 44 or 45; thi s is eq uiva len t to taking VII = 0 for all n (a va nis hing or constant crystal potential m od el). Then, Eq. 46 give s:
(48)
This corresp onds to the free electron model. As th e ne xt ap proxim ation, consider the case th at the nonvanishing compon ents of VII are only for n = 0, 1. Eq . 46 becomes:
Vc I 2
~(k+G )2_ E 2m
=0
(49)
1
Thi s me ans th at, in k- space, th e tw o free electrons ha ving E(k) and E(k + G) ar e in indep endent sta tes in the absence of the crystal potential even when Ilk I = Ilk + Gil; this energy degener acy is lifted under the existence of nonvanishing VG ' In the above case, the eigenvalue equation can be solved ea sily an d th e solution gives
E= ~ {E(k) + E(k + G)}± .J { E(k) - ~(k + G) r +V~
(50)
Fig ure 5 shows the glob al profile of E as a fun ction of k in one dimension. One can see the exis tence of ene rgy ga p at the wave vec tor th at sa tisfies: (51)
Chapter one:
Fundamentals of luminescence
15
E (k) Eo (k-G , )
_
_
- l -<e::::...
Eo (k)
----L
---""
k
a Figllre 5 The emergence of a band gap resulting from the int erferen ce between two plan e waves satisfying the Bragg condition, in a one-dimens ional model. This is called the Bragg condition. In the three-dimensional case, the wa ve vectors that satisfy Eq. 51 form clos ed polyhedrons in k space and are called the I st, 2nd, or 3rd, ..., nth Brillo uin zone . As sta ted so far, the electronic energy band structure is d et ermined by the sy m metry and Fourier amp litud es of the crystal potential VCr). Thus, on e need s to tak e a m ore realist ic model of them to get a more accurate description of the ele ctronic pr op erties. There are now many procedures that allow for the calculation of th e ene rgy band and to get the wavefunction of electrons in crystals. Two representative methods, the Pseudopotential method and the LCAO method (Linar Combination of Atomic Orb ital Method ), which are frequently app lied to outer-shell valence electrons in sem icond uc tors, are br iefly in troduced here. Firs t, consider the pseudopotential method. Eq. 46 is the fu n d am ental eq ua tion to get band struc tu res of electrons in crystals, but the size of the d et erminant eq ua tion will become very large if one wishes to solve the equation with suffic ien t acc uracy, because, in general, the Fourier components Vc do not decrease slo wly d ue to the Coul omb potenti al of each atom. This correspond's to the fact th at th e w av e fu nc tions of valen ce electrons are free-electron like (plane-wave like) in the intermed iat e reg io n between atoms and give rapid oscillations (atomic like) near the ion cor es. Therefore, to avoid this difficulty, one can take an effective potential in which the Coulomb potential is canceled by the rapid oscillations of wavefunctio ns . Th e rapid osciUation of w avefunctions originates from the orthogonalizati on between atom ic-like propert ies of wavefu nctions near ion core s. It means th at one in tro d uces new wavefu nclions and a wea k effective potential instead of pl an e wa ves and a Co u lombic po ten tia l to represent the elec tro nic states . Thi s effective p otenti al gives a sm all n umber of reciprocal wave vectors (G) that can reproduce band s truc tures wi th a corres po ndi ng sma ll number of Four ier compo nen ts. Thi s potential is called th e pscudopotential. TIle pseudopo ten tial method necessaril y results in some ar bitra riness wi th respect to th e choice of th ese effective potenti als, depe nd ing on th e- selec tion of effective wav efunction s . It is even p ossible to parametri ze a sma ll number of co mp one n ts in Vc " and to de ter mine th em em pirically.
Fundamentals of Phosphors
16
For example, taking severa l Vc va lues in high sy m me try p oints in the Brillouin zone and, aft er adjus ting them so as to reprod uce the bandgaps ob taine d w ith expe rimen tal measur emen ts, on e calculates the band dispersion E(k ) over th e en tire region. In contrast, the LCAO m ethod ap p rox ima tes the Bloch sta tes of val ence electrons by using a lin ear combination of bound a tomic w ave fun ction s. For examp le, (52)
sa tisfies th e Bloch condition s tated in Eq, 42, w here <\J (r) is one of the bound atomic wa ve functions. In order to show a simple exa m p le, assu me a on e-dimension al crys tal consisting of atoms having one electron per at om bo und in the s-or bital. The H amiltonian of th is cry stal can be written as:
H
tz2
= __
2m
V 2 + V(r)
= H o + 8V( r)
(53)
where H o is the Hamiltonian of each free atom, and 8V(r) is the term that rep resents the effect of p eriodic potential in the crystal. Usin g Eg. 53 and the wavefun ctio ns exp resse d in Eg. 52, th e expectation value obtained by multiplying w ith <\J'( r) yie lds :
E(k)=E + o
E +L 1
e'kR" 5 (R )
,,~o .
1
"
1+ L n;tOe'kR"S0 (R" )
(54)
where Eo is the energy level of s-orbi tal satisfying Ho<\J(r) = Eo<\J(r), and E] is the energy shi ft of Eod ue to 8V given by H ' (r)8V(r)<\J(r)dr. So(R,,) is called the ove rlap in tegra l and is d efined b y: (55) Simi larly, Sl(R
Il
)
is defi ne d as: (56)
Typically speaking, the se qu antities are regarded as parameters, and they are fitted so as to best rep roduce experimen tally observed results. As a matter of fact, other orbitals such as P> d-orbi tals etc. can also be used in LCAO. It is even possible to combine th is method with th at of p seudopotentials. As an example, Figure 6 rev eal s tw o band structure calculation s due to Ch ad i-: one is for Si and the other is for GaA s. In Figure 6, energy = a in the ordinate corresponds to the top of the valence ban d . In both Si and GaAs, it is locat ed at the I point (k = (000) point). The bottom of the con d uction band is also locat ed at th e I point in GaAs, while in Si it is locat ed near the X poin t (k = (100) poin t). It is diffi cult an d ra re th at the en ergy bands can be calculat ed acc ura tely all through the Brill ouin zone wi th use of a small number of paramet ers d etermined at high symmetry p oint s. In th at sense, it is gu ite conven ien t if on e ha s a simple perturbat ion al method to
Chapter one: Fundamentals of luminescence
17
6 4 2 0
:>
>Q)
>.
>.
Ol
OJ
~ -2 ....
-2
OJ
"Q)
-4
LU
llJ
-6
c
c
GaAs
-8 -10 L 1 -12
-
L
XU, K
k
--
XU, K
k
Figure 6 Calculated band structures of (a) Si and (b) GaAs using a combined pseudopotential and LCAO method. (From Chadi, D.J., Phys. Rev., B16, 3572, 1977. With permission.)
calculate band structures approximately at or near specific points in the Brillouin zone (e.g., the top of the valence band or a conduction band minimum). In particular, such procedures are quite useful when the bands are degenerate at some point in the Brillouin zone of interest. Now, assume that the Bloch function is known at k = k o and is expressed as 'Vll k o (r) . Define a new wavefunction as: (57)
and expand the Bloch function in terms of 1lIlk(r) as: (58) II'
Introducing these wavefunctions into Eq. 39 obtains the energy dispersion E(ko + k) in the vicinity of k., In particular, near the high symmetry points of the Brillouin zone, the energy dispersion takes the following form:
(59)
where (l/m\ is called the effective mass tensor. From Eq. 59, the effective mass tensor is given as:
Fundamentals of Phosphors
18
(i, } = x, y, z)
(60)
For th e iso tro pi c case, Eq . 60 give s the scalar effective m ass m' as:
1 m
(61)
Eq. 61 indicat es th at m' is proportional to the inverse of curva ture near th e extr emal poin ts of the di sp ersion relation, E vs. k. Furthermore, Figure 5 illus tra tes the tw o typica l cases that occu r near th e bandgap, th at is, a positive effectiv e mass at the bottom of the con duction band and a ne gative effectiv e m ass at the top of the va len ce band, d ep ending on the sign o f d 2E / dk 2 a t each ex trema l p oint. Hence, und er an applied electric field E, the specific cha rge el m' of an elec tro n becomes negative, w hile it becomes p ositi ve for a hole . This is the reason why a hole look s like a p articl e w ith a p ositi ve charge. In th e ac tua l calcu la tion of physical p roperties, the followi ng quantity is also im portant: (62) Th is is ca lled the density of states a nd represents th e number of states bet w een E an d E + dE. We ass u me in Eq. 62 th at space is isotropic and m' can be used . Th e band stru ctu res of se m ico nd uctors h a ve been in tensively investi gat ed expe rimentally using optical ab sorption and/ or refl ection spec tra. As shown in Figure 7, in many co mpou n d semiconductor s (most of III-V an d II-VI combinati on in th e periodic table), cond uction bands consist main ly of s-orbital s of the cation, and vale nce bands consist p r in cip all y of p-orbi ta ls of th e anion. Man y comp oun d semicon d uctors have a direct bandgap, w h ich means th at the con d uc tion ban d m inim u m an d th e valenc e band m a ximum a re both a t th e I p oint (k = 0). It sho uld be n ot ed that the sta tes jus t n ear th e m a ximum o f the vale nce band in zi nc-blen de typ e se micon d uc tors consis t of tw o orbitals, n amely 1 8 which is tw ofold d egenerate an d 1 7 w ithout d egener acy; th ese originate from th e spin-orbit interaction. It is known th a t the twofold d egen er acy of I S is lifted in th e k =f. 0 region correspond ing to a light and a h eavy hole, resp ectively. On the other h an d , in wurzite-type cryst als, the valence b and top is split b y b oth th e spin-orb it in terac tio n an d th e crys ta lline fie ld effect; th e b and maximum then consists of three orbi tals: 1 9, 1 9, and 1 7 w ith o ut d egen era cy. In Ga P, th e con d u ction band m inimum is at th e X p oint (k = [100]), a n d thi s com po und h as an in direct band gap , as described in th e n ext sec tio n .
1.2.2 Fundamental absorption, direct transition, and indirect transition When so lid crystals are irrad iat ed by light, various op tica l phenomen a occur : for example, tr an smi ssion , reflection, an d absorp tion . In p articul ar, absorp tion is the annihila tion of light (photon) res u lting fro m th e crea tion of an elec tro nic excitation or lattice excitation in crys ta ls. O nce electrons ob tai n energy from light, th e electrons are exci ted to higher sta tes . In s uch quantum m ech an ical phenomena, one can on ly calculate the probability of exc ita tion . The probability d ep ends on the d istribution of mi croscop ic ene rgy level s of
Chapter aile: Fundamentals of luminescence
19
\
\
E
r g(Al
r , (B)
r , (C)
r , (B)
10001
[0001
[000]
(a)
(b)
(C)
[100]
Figure 7 The typical band dispersion near r-point (k = 0) for II-VI or III-V semiconductor compounds. (a) a direct type in zinc-blende s tructure; (b) a direct type in wurzeite structure; and (c) an indirect type in zin c-bJende structure (GaP) .
electrons in that system. The excited electrons will come back to their initial states after they release the excitation energy in the form of light emission or through lattice vibrations. Absorption of light by electrons from valence bands to conduction bands results in the fundamental absorption of the crystal. Crystals are transparent when the energy of the incident light is below the energy gaps of crystals; excitation of electrons to the conduction band becomes possible at a light energy equal to, or larger than the bandgap. The intensity of absorption can be calculated using the absorption coefficient a(hv) given by the following formula : (63) where n, and nr are the number density of electronic s ta tes in an initial state (occupied byelectron) and in a final state (unoccupied by electron), respectively, and Pi[ is the transition probability between them. In the calculation of Eq. 63, quantum mechanics requires that two conditions are satisfied. The first is energy conservation and the second is momentum conservation. Theformer means that the energy difference between the i.nitial state and the final state should be equal to the energy of the incident photon, and the latter means that the momentum difference between the two states should be equal to the momentum of the incident light. It is quite important to note that the momentum of light is three or four orders of magnitude smaller than that of the electrons. These conditions can be written as (t? 12m' )k; = (tl 212m' )k;2 + hv (energy conservation); Jik.r = n( k, + q) (momentum
20
Fundamentals of Phosphors
- -- - Ef
hv
L----------
k
Figure 8 The op tica l absorption due to a direct transiti on from a valence band s tate to a conduction band s ta te.
conser vat ion); and v = cq if one assumes a free-ele ctron -like dispersion for band structure E(k), w here kfa nd k, are the fina l and initial wave vec tors, respectively, c is the light velocity, and q is th e ph oton momentu m . One can neglect th e momentum of abso rbed ph otons compared to those of electrons or la ttice vibration s. It res ults in op tical tran sitions occurring alm os t ver tically on the energy dispersion curve in the Brillouin zo ne . Thi s ru le is called the momentum selection rule or k-selection rule. As shown in Fig ure 8, consi der first the case th at th e minimum ban d gap occurs at the top of valen ce band and at the bo tto m of conduc tio n band; in such a case , the electrons of the va lence band are exci ted to the cond uc tion band wi th th e same momentum. This case is called a direct transition, and the m at er ials having th is type of band struc ture are called direct gap materials. The absor p tion coefficient, Eq. 63, is written as: ' a(hv)=A , (hV- Eg )1/2
(64)
wi th the use of Eqs . 63 and 64. A ' is a co ns tan t rela ted to the effective m asses of elect rons and holes. Th us, one can experi men tally measure the ban d ga p Eg, beca use th e absorp tion coe fficient increases steeply from the edge of the bandgap . In actua l measurem ents, the abs orption inc reases exp onenti ally because of th e existence of impuri ties near Eg . In some materi als, it can occur th at the transition at k = a is forbidden by some selection ru le; the transiti on probability is then propo r tion al to (hv - EJ in the k "'" a region and the absorption coefficien t becomes: '
a(hv) = A' ( hv - Eg )
3/ 2
(65)
Chapter one: Fundamentals of luminescence
21
E
k
Figure 9 The op tical abso rp tion du e to an ind irect tran sition from a va lence band state to a conduction band sta te. The mom entum of electron changes due to a sim ultaneous absorp tion or emission of a ph on on .
In contrast to the direct transition , in the case sh own in Fig ur e 9, both the energy and the momentum of elec trons are cha nged in the process; exci tation of thi s typ e is called an indirect transition. This tran siti on corre sp onds to cases in which the minimum bandgap occurs bet ween two sta tes with d iffere n t k-valu es in the Brillouin zo ne . In this case, conservat ion of m om entum cann o t be provided by th e photon, an d th e tran sition necessarily mu st be associated with the exci ta tion or ab sorption o f phonon s (la ttice vib ra tions) . This leads to a decrease in transiti on probability due to a hi gher-order stoc has tic p rocess. The materials having such band str uc tur e are called indirect gap materials. An expression for the abso rption coefficien t accom panied by phonon absorption is:
(66)
while the coefficien t accompanied by phonon emission is:
(67)
where, in both formulas, Ep is the phonon energy. In closin g this section, the light emission process is briefly di scussed . The in tens ity of light em ission R can be writt en as: (68)
Fundamentals of Phosphors
22
where n il is the number density of electro ns exis ting in upper energy states and n, is the number density of empty states with lower energy. The large difference from absorption is in the fact that, usually speaking, at a given temper ature electrons are found only in the vicinity of conduction band minimum and light emission is observed only from these electrons. Then, Eq. 68 can be written as: Conduction Band
/11/IIJ/////II/ /IJ/I/ lI!It/
n=,
'""-- n =; >.
OJ .... CD
C
W
Valence Band Figur e 10
Energ y levels of a free exciton.
(hVk-TE J
L = B' ( hv - Eg + Ep ) 1/2 exp -
(69)
g
B
confirming that em ission is only observed in the vicinity of Eg . In the case of ind irect transiti on s, light emission occurs from electronic transitions accompanied by ph onon emission (cold band); light em ission at higher energy corresponding to phonon absorption (hot band ) ha s a relati vely small probability since it requires th e presence of thermal phonon s, Hot-band emission vanishes completely at low temperatures.
1.2.3 Exciton Although all ele ctrons in crystals are specified by the energy band states they occupy, a character istic excited s ta te ca lled the exciton, which is not derived from th e band theo ry, exist s in almost all se m icond uc tors or ionic crystals. Consid er the case where one electron is excited in th e cond uction band an d a hole is left in the valence band. An att rac tive Coulomb pot ential exists between them and can result in a bound state analogous to a h ydrogen atom . This config ura tion is called an exciton. The binding energy of an exciton is calculated , by ana logy, to a hydrogen atom as:
(70)
where n (= 1, 2, 3, ... ) is a quantum number specifying the states, E is the dielectr ic cons tan t of cr yst als, and m'. is the reduced m ass of an exciton. An exci ton can mov e freely th rough the crystal. The energy levels of the free exciton are shown in Figur e 10. Th e s ta te corresponding to the limit of n ---7 is the minimum of 00
Chapter one:
23
Fundamentals of luminescence
the conducti on band, as shown in the figure. The energy of the lowest exciton s tate obtained by putting n = 1 is: (71)
Two or thr ee kinds of excitons can be gene rated, depending on th e splittin g of the valence band, as was shown in Figure 7. Th ey are named, from th e top of the valence band, as A- and B-excitons in zinc-blende typ e crys tals; and A, B, and C-excitons in wurzite-type crys tals. The re are two kinds of A-excitons in zinc-blende m aterials originating from the existence of a light and heavy hole, as ha s already been n oted . Wavelength [A] 4600
4800
4900
B1
,-, I , , ,
12
J
/
I
I
I
A1
I
I I I
10
I
I
I
Jell
I I I
I
,-
8
"'-, <:»I
I I
I I I I I
I
E U
-e0
~
C Q)
'u
I
\
6
I I
:E Q)
,
\
0
0
\ \
C
0
'li 0
,,
Aoo
, "
4
t\
(/)
.D
«
Boo
I I
I
\ \
I
2
I
I
I I I I I I
I"v, I
I
0 2.65
2.60
\
2.5 5
Photon Energy reV]
Figure 11 The exciton absorp tion spectru m of CdS (at 77K). The so lid line and the broken line corresp ond to the cases that th e po larization vector of in cident ligh t are parallel an d perpend icu lar to the c-axis of the crys tal, resp ectively. (From Mitsuhash i, H . an d Fujishiro, Y, p ersonal com m unication. With p ermission .)
Fundamentals of Phosphors
24
Excitons create several sharp absorption lines in the energy region just below Eg . Figure 11 shows the absorption spectra of excitons in CdS. 2 One can easily recognize the absorption peaks due to A-, B-, and C-excitons with n = 1, and the beginning of the interband absorption transition corresponding to n ~ (Eg) . The order of magnitude of absorption coefficient reaches 105 crrr' beyond Eg, as seen from the figure. As noted previously, the absorption coefficient in the neighborhood of Eg in a material with indirect transition, like GaP, is three to four orders of magnitude smaller than the case of direct transition. An exciton in the n = 1 state of a direct-gap material can be annihilated by the recombination of the electron-hole pair; this produces a sharp emission line. The emission from the states corresponding to the larger n states is usually very weak because such states relax rapidly to the n = 1 state and emission generally occurs from there. With intense excitation, excitons of very high concentrations can be produced; excitonic molecules (also called biexcitons) analogous to hydrogen molecules are formed from two single excitons by means of covalent binding. The exciton concentration necessary for the formation of excitonic molecules is usually of the order of magnitude of about 1016 crrr'. The energy of the excitonic molecule is given by: 00
(72)
where G," is the binding energy of the molecule. The ratio of Gm to Gex depends on the ratio of electron effective mass to hole effective mass, and lies in the range of 0.03 to 0.3. An excitonic molecule emits a photon of energy Eex - G"" leaving a single exciton behind. If the exciton concentration is further increased by more intense excitation, the exciton system undergoes the insulator-metal transition, the so-called Mott transition, because the Coulomb force between the electron and hole in an exciton is screened by other electrons and holes. This results in the appearance of the high-density electron-hole plasma state. This state emits light with broad-band spectra.
References 1. Chadi, D.}., Phys. Rev., B16, 3572, 1977. 2. Mitsuhashi, H. and Fujishiro, Y, unpublished data.
chapter one - section three
Fundamentals of luminescence Hajime Yamamoto
Contents 1.3 Luminescence of a localized cen ter.. 1.3.1 Classification of localiz ed centers 1.3.2 Con figura tional coordinate model 1.3.2.1 Description by a classical model 1.3.2.2 Quantum me chanical description 1.3.3 Spe ctral sha pes : 1.3.3.1 Line broadening by tim e-dependent perturbation 1.3.3.2 Line broadening b y time-independent p erturbation 1.3.4 Nonradiative transitions References
25 25 26 26 28 30 34 36 36 37
1.3 Luminescence of a localized center 1.3.1
Classification of localized centers
When con sidering op tical ab sorption or emission within a single io n or a g ro u p of ions in a so lid, it is ap p ro p riate to tr eat an optical transition with a localized model rather than the band model d escribed in Section 1.2. Actuall y, m ost p h osp hors have localized luminescent center s and contain a far large r va riety of io ns th an d el ocali zed centers. The principal locali zed centers ca n be cla ssified b y their el ectronic tr ansitions as follow s (belo w, an arr ow to the ri ght indi cat es op tical abs or p tio n an d to the left, emission ): 1. Is # 2p; an exa m ple is an F center. 2. ns? ~ nsnp. Thi s g ro up includes Tl+-typ e ion s; i.e., C a", In', 'n-. Ge 2+, Sn 2+ , Pb 2+, Sb3+, Bi3+, Cu-, Ag-, Au-, etc. 3. 3d10 # 3d94 s. Examples are Ag +, Cu ' , and A u" , Acceptors in Ilb -Vlb compounds are not included in thi s gro up . 4. 3dn# 3dn, 4dn # 4dn. The first and sec ond row transition-metal ions form thi s group. 5. 4fn # 4l, 5l ~ 5fn; rare-earth and actinide ions.
25
26
Fundamentals of Phosphors 6. 4fn ~ 4l- 15d . Examples are Ce 3+, Pr3+, Sm 2+, Eu 2+, Tm 2+, and Yb2+. Only absorption transitions are observed for Tb3+. 7. A charge-transfer transition or a transition between an anion p electron and an empty cation orbital. Examples are intramolecular transitions in complexes such as VOl-, WO/-, and MoO/-. More specifically, typical examples are a transition from the 2p orbital of 0 2- to the 3d orbital of V5+ in V0 43- , and transitions from 02-(2p) or S2-(3p) to Yb 3+(4f). Transitions from anion p orbitals to Eu 3+ or transition metal ions are observed only as absorption processes. 8. J[ ~ J[' and n ~ J[' . Organic molecules having J[ electrons make up this group. The notation n indicates a nonbonding electron of a heteroatom in an organic molecule.
1.3.2 Configurational coordinate modeir' 1.3.2.1 Description by a classical model The configurational coordinate model is often used to explain optical properties, particularly the effect of lattice vibrations, of a localized center. In this model, a luminescent ion and the ions at its nearest neighbor sites are selected for simplicity. In most cases, one can regard these ions as an isolated molecule by neglecting the effects of other distant ions. In this way, the huge number of actual vibrational modes of the lattice can be approximated by a small number or a combination of specific normal coordinates. These normal coordinates are called the configurational coordinates. The configurational coordinate model explains optical properties of a localized center on the basis of potential curves, each of which represents the total energy of the molecule in its growld or excited state as a function of the configurational coordinate (Figure 12). Here, the total energy means the sum of the electron energy and ion energy. To understand how the configurational coordinate model is built, one is first reminded of the adiabatic potential of a diatomic molecule, in which the variable on the abscissa is simply the interatomic distance. In contrast, the adiabatic potential of a polyatomic molecule requires a multidimensional space, but it is approximated by a single configurational coordinate in the one-dimensional configurational coordinate model. In this model, the totally symmetric vibrational mode or the "breathing mode" is usually employed . Such a simple model can explain a number of facts qualitatively, such as:
1. Stokes' law; i.e., the fact that the energy of absorption is higher than that of emission in most cases. The energy difference between the two is called the Stokes' shift. 2. The widths of absorption or emission bands and their temperature dependence. 3. Thermal quenching of luminescence. It must be remarked, however, that the onedimensional model gives only a qualitative explanation of thermal quenching . A quantitatively valid explanation can be obtained only by a multidimensional model." Following the path of the optical transition illustrated in Figure 12, presume that the bonding force between the luminescent ion and a nearest-neighbor ion is expressed by Hooke's law. The deviation from the equilibrium position of the ions is taken as the configurational coordinate denoted as Q. The total energy of the ground state, Ug, and that of the excited state, U e, are given by the following relations. U =K x
fL
s 2
(73a)
Chapter one:
27
Fundamentals of luminescence
Excited State
>-
e> OJ
c
W
~ I-
Ground State
o
00
Configurational Coordinate
Figure 12 A schematic illustration of a configurational coordinate model. The two curves are modified by repulsion near the intersection (broken lines) . The vertical broken lines A ~ Band C ~ D indicate the absorption and emission of light, respectively.
(73b)
where Kg and K, are the force constants of the chemical bond, Qo is the interatomic distance at the equilibrium of the ground state, and Uo is the total energy at Q = Qo' The spatial distribution of an electron orbital is different between the ground and excited states, giving rise to a difference in the electron wavefunction overlap with neighboring ions. This difference further induces a change in the equilibrium position and the force constant of the ground and excited states, and is the origin of the Stokes' shift. In the excited state, the orbital is more spread out, so that the energy of such an electron orbital depends less on the configuration coordinate; in other words, the potential curve has less curvature. In Figure 12, optical absorption and emission processes are indicated by vertical broken arrows. As this illustration shows, the nucleus of an emitting ion stays approximately at the same position throughout the optical processes. This is called the FranckCondon principle. This approximation is quite reasonable since an atomic nucleus is heavier than an electron by 103 to 105 times. At OK, the optical absorption proceeds from the equilibrium position of the ground state, as indicated by the arrow A ---7 B. The probability for an excited electron to lose energy by generating lattice vibration is 10 12 to 1013 s', while the probability for light emission is at most 109 S-l. Consequently, state B relaxes to the equilibrium position C before it emits luminescence. This is followed by the emission process C ---7 0 and the relaxation process 0 ---7 A, completing the cycle. At finite temperature, the electron state oscillates around the equilibrium position along the
28
Fundamentals of Phosphors
configu ra tion al coord ina te curve up to the thermal energy of kT. The amplitude of this oscillation causes the spectral width of the abs orp tio n tran sition. Wh en two con figurational coo rd ina te curves inte rsect with each othe r as sho w n in Fig ur e 12, an electron in the exci ted sta te can cross the intersection E ass isted by thermal en ergy an d can reac h the gro un d state n onradiatively. In other word s, on e can ass ume a nonr adiative relaxation process with th e activ ation energy t:..U, and wi th the tran sition p rob ability per un it time N given by:
-su-
N = s exp -
(74)
kT
w here s is a prod uc t of th e tran si tion p rob ab ility between the ground an d excit ed sta tes and a fre qu ency, with which th e excited sta te reach es the intersection E. This quantity s can be treated as a cons tan t, since it is only w eakly depen d ent on temperat ure. It is called the frequency factor and is typicall y of the orde r of 1013 S-I . By em p loy ing Eq . 74 a nd letting W be the lum inescence proba bi lity, the lumi nescence efficiency T] can be expressed as :
T] =
W [
1
-t:..UJ-
s - = l + -exp - W+N W kT
(75)
If the equili b riu m position of th e exc ited stat e C is lo cat ed ou tsid e th e configura tiona l coordin at e curve of the groun d stat e, the excited sta te int ersects the ground state in relaxi ng from B to C, lead in g to a nonrad ia tive p rocess. It can be shown by quantum mec hanics th at the configurational coo rdi n ate curves can actuall y intersect each other on ly when th e tw o states belon g to differen t irreducib le representati ons. Otherw ise, the tw o curves beh ave in a repulsive way to each other, givi ng rise to an energy gap at the expected int er secti on of the p ot entials. It is, however, possible for eith e r sta te to cross over w ith h igh p robabil it y, because the wa vefunctions of the tw o sta tes are admix ed n ear th e inter section. In contrast to the above case, the in tersection of two configu rational coord in ate curve s is generall y allow ed in a multidimension al model.
1.3.2.2
Q uan tum mechanical description
The classical d escription di scu ssed above cannot satisfactorily explain observ ed phenomena, e.g., spectra l sh apes an d nonradiat ive transition probabilities. It is thus necessary to discuss the conf igura tiona l coordinat e model based on quantum mechanics . Suppose that the ene rgy state of a localized center invo lved in lu minescen ce p rocesses is d esc rib ed by a wavefunct ion \fl. It is a fun ction of b oth electronic coo rdi na tes rand n uclear coordi na tes R, bu t can be separa ted in to th e electron ic part and the nu clea r part b y the adiabatic approximation:
(76)
where nan d k are th e qua n tum nu mb ers indica tin g the energy sta tes of the electron and th e nucle us, resp ectiv ely. For th e nuclear wave function Xnk(R), the tim e-independent Schr odinger eq ua tion can be wri tte n as follows:
Cllapter one:
Fundamentals of luminescence
29
u n»
I.
'-- ---- ~ 4
ue
'IIo
e
~-----....
....
-
>~
hv o
(j)
c:
hVnm
W
Cii
a
U9
f-
1\
\
I.
\ \
......
r-....
/
\. \.
a
9
'IIm 9
~
\
nm
I I
,
"" .'L __ _"
"t ---- - ""-
/
/ /
/'
a
Q
Configurational Coordinate Figure 13 Discrete energy levels due to latti ce vi bration, each with the energy of tu» and the wavefun ction s IJIU and 'lfi;, of ha rmonic oscillators represen ting the two s tates. The notation V o means the frequ ency at the emi ssion peak. A luminescent transition can occur at v",,, .
(77)
with a being the nuclear number, M a th e ma ss of the a th nucleus, .1Ra the Laplacian of Ru' and Enk the total energy of the localized center. The energy ter m Uk(R) is composed of two pa rts: the energy of the electrons and th e energy of th e elec tro static interaction between the nuclei arou nd the loca lized cen ter. Co nsidering Eq. 77, one finds th at Uk(R) plays the role of the po ten tial energy of the nuclear w avefunction Xnk' (Reca ll th at the electron energy also d epends on R.) Th us, U k(R ) is an ad iabatic p otential and it for m s the configurational coordina te curve w hen one takes the coord ina te Q as R . When U k(R) is expanded in a Taylor ser ies up to second order aro u nd the eq uil ibrium positio n of the ground sta te, the po ten tia ls are exp ressed by Eq. 73. For a harm onic os cilla tion, the seco nd term is the first non vanishin g term, w hi le the firs t term is no n-zero onl y w hen th e eq uilibrium position is di splaced from the origi nal posi tion . In th e latter case, the firs t term is related to th e Iahn-Teller effect. Som etim es, the four th term in th e expansion ma y also be present , sig na ling anha rmonic effects. In the following, consi de r for s im plicity only a sing le coordinate or a tw o-dimensional model. Cons ider a harm oni c oscillat or in a po tentia l shown by Eq . 73. Thi s osc illator giv es discrete energy levels insi de the confi gura tiona l coordi na te curves, as illust rat ed in Figure 13. Ell/ = (m+ lj2)nw
where
to
is the pr oper ang ular frequency of th e ha rm onic oscill at or.
(78)
30
and
Fundamentals of Phosphors The electric dipole transition probability, W"w between the two vibrational states n In is given by:
(79) Here, (80)
When the transition is allowed, M eg can be placed outside the integral, because it depends weakly on Q. This is called the Condon ap p ro xim ation and it makes Eq. 79 easier to understand as:
(81)
The wavefunction of a harmonic oscill ator has the shape illustrated in Figure 13. For = 0, it has a Gaussian shape; while for In (or n) i= 0, it has maximum amplitude at both ends and oscill ates In times with a smaller am plitud e between the ma xima. As a m (or n)
If
consequence, the integral X~,XgmdQI takes the largest value along a vertical di rection on the confi gurational coordinate model. This explains the Franck-Condon principle in terms of the shapes of wavefunctions. One can also state that this is the condition for which
If
2
if
2
W"m oc X;/Ix g/ll dQI holds. The square of th e overlap integral X;nXg/lldQI is an important quantity that d etermines the streng th of the optical transition and is often called the Franck-Condon factor.
1.3.3 Spectral shapes As described above, the shape of an op tical absorption or em iss ion spectrum is decided by the Franck-Condon factor and also by the electronic population in the vibrational levels at thermal equilibrium. For the special case where both ground and excited states have the same angular frequency W, the absorption probability can be calculated with harmonic oscillator wavefunctions in a relatively simple form:
W = e-s[m! ]5"-/11 [I.;,-m(5)]2 11m
n.,
m
(82)
Here Lp(z) are Laguerre's polynomial functions. The quantity 5 can be expressed as show n below, with K being the force constant of a harmonic oscillator and Qo the coordinate of the equilibrium position of the excited state.
5 = 12 11 ~(Q _ Q 0 )2 hw
(83)
Chapter one: Fundamentals of luminescence
31
Ascan be see n in Figure 14, 5 is the number of em itted phonons acco m p anying the optical transition . It is commonly used as a measure of electro n -p h on on in teraction and is called the Huang-Rhys-Pekar factor. At OK or m = 0, the tran sition probabilit y is give n b y the simple relati on: -s
W - 5" !:...-. n!
(84)
110
A plot of W IIO agains t 11 giv es an absorption spectrum consisting of man y sh arp lines. This result is for a ve ry special case, but it is a conveni ent tool to demonst rate h ow a spectrum var ies as a fu nction of the inten sity of electron-p hono n in terac tion or the d isplacement of the equilibrium position in th e excited s ta te. Th e results ca lcula ted for 5 = 20 and 2.0 are sh ow n in Figures 14 (a) and (b),? resp ectively. Th e pe a k is locat ed at n == 5. For 5 == 0 or wea k electron-p hono n interact ion, th e spectrum consists only of a sin gle lin e at 11 = O. Th is line (a zero-p ho non line) becomes p rominent w he n 5 is relatively sma ll. For luminescen ce, transit ions accompanied by phonon emission show up on the low-energy side.of the ze ro-p ho no n line in con trast to abs orp tion sh ow n in Figure 14(b). If the energy of the phon on , noo, is equ al both for the ground a nd excited s ta tes, th e abs or p tion an d emission spectra form a mi rr or im age abo u t the zero-p h onon line. Typ ical exam ples of this case are the spectra of YPO:4Ce3+ shown in Figu re 15,Rand that of ZnTe:O show n in Figure 16.9 Examp les of o ther 5 va lues a re described. For th e A emission of KCI:TI-i' having a very broad band width, 5 for the grou nd s ta te is found to be 67, w hi le for th e correspond ing A absorption band , S of th e excited state is about 41.10 Mean while, in A I20 3 :C r' + (ru by ), 5 = 3 for the n arrow 4A 2 ~ 4T 2 absor p tion band , and 5 = 10- 1 for the sharp R lin es (4A2 H 4T2 ) were reported ." A ve ry sma ll va lue sim ilar to th at of R lin es is expected for sharp lines due to 41" intraconfigurational trans ition s. Th e spectra of YP0 4 :Ce 3 + in Figure 15, which is due to 4f H Sd transition, sh ow 5 = L8 The above di scu ssion has treated th e ideal case of a tran sition between a p air of vibrationa l levels (gm) and (en) res u lting in a si ng le line. Th e fac t is, h owever, th at each line has a finit e w id th even a t OK as a res u lt of ze ro-poin t vibra tion. Next, conside r a spectral shap e at finite temper ature T. In thi s case, ma ny vib ra tion al levels at thermal equilibrium can act as the initial state, eac h level con tributing to the transition w ith a probability proport ion al to its popu la tion d ensity. Th e total tran sition probability is the su m of suc h weigh ted probabil ities fro m these vi bra tion al levels. At sufficiently high temperature, one can treat the final st at e classicall y and ass u me the wavefun ction of the final sta te is a b-function and the populati on density of the vibra tional levels obeys a Boltzmann d istrib u tion . By thi s approxima tion, the abso rp tion spec tru m has a Gaussian shape g iven by:
W(noo ) =
1 . r;;--
'1/ 2 1t0
n
) ( l
exp
- 1'100 - U
.
2
I
2]
(85)
20 n
Here,
(86)
32
Fundamentals of Phosphors
c-,
Sl1ro
e' OJ
c
W
ro
o
I-
o
o
00
Configuration al Coordinate (a)
I
o
10
20
I
o
10
I 20
III
I 30
I 30
Phonon Number (b)
Figure 14
(b) sho w s the spec tra l sha pe ca lcu la ted for the config u rational coordinate mod el, in which the vibra tiona l frequency is identical in the ground and excited states shown in (a). The upper figure in (b) shows a res ult for 5 = 20, while the lower figure is for S = 2.0. The ordinate shows the number of p ho no ns 11 accompanying the optical transition. The tran sition for n = 0 is the zero phonon lin e.
Chapter one: Fundamentals of luminescence
33
.0 ~i
: /C
330
340 Wavel ength (nm)
Figure 15 Opti cal sp ectra of 5d ~ 4j(2F3/2 ) transit ion of Ce3 + doped in a YP0 4 sing le cry stal. The upper figure is an excitation sp ectrum, with the lower luminescence spectrum at 4.2K. Th e tw o spectra are positioned symmetrically on both sid es of the zero-ph onon line at 325.0 nm . Vibr onic lines are observed for bo th spec tra . The notation s rt and 0 ind icat e that the pola rization of lu minescence is par allel or perpendicular to the crys tal c-axis, res pective ly.
{(J a (Tn
2
( tu» )
== S CO
3
nw"-s
"" 2S ·kT · e
tu»
coth '~ 2kT
(nw,f (nws t
(87)
(88)
where IJw is the energy of an abs orbed phonon, an d S" de n otes S of the exci ted sta te. Th e coefficient on the righ t-hand side of Ego 85 is a norm ali zation factor d efined to give fW(1iw)dw = 1 . By d ef in in g w as the spec tra l w id th, w h ich sa tis fies the cond it ion W(Uj + w) = W(Uj) / e, on e find s:
34
Fundamentals of Phosphors
40 I
w
.
30
~
f-
30 z w 0
u::
1.98130 eV
a:
o
3
'E
;0 .026: eV
o z
~ (f) w
40
I
1.L
w 20 0
20
1.L 1.L
0
Z
o
0 10 i= 0...
>- 10
f(iJ
a:
z
W fZ
ABSORPTION 0
0 1.82
1.86
1.90
1.94
1.98
2.02
2.06
2.10
2.14
0 m
(fJ
«
2.16
PHOTON ENERGY (eV)
Figure 16 Absorpti on and lu min escence spectra of ZnTe:O at 20K. (From Merz, J.L., Phys. Rev., 176, 961, 1968. With permission.) w=
)20a
(89)
At sufficien tly hi gh temperature, the spectral width w is propo rtional to -If and the peak height is in versely proportion al to ·i f . The relation s for the luminescence process are found simply by exchanging th e suffixes e and g of the above eq ua tions . In experiments, a Gaussian sha pe is most comm on ly ob served . It appears, however, onl y w hen certain condi tions are sa tisfied, as is ev iden t from the above discussion . In fact, mo re com plica ted sp ectral shap es are also obser ved. A well-known example is the structu red band shape of a transition obs erved for Tit-ty pe ions in alk al i halides." It has been sho wn that th is shape is induced by the [ahn-T eller effect and can be described by a configurational coordinate model based on six vib rati onal modes aro un d a Tlv-typ e ion . Another example is the asy m metric luminescen ce band of Zn 2Si04 :Mn 2 +. To expl ain this sha pe , a config ura tional coord in ate model w ith a sma ll diff erence between the excited and gro und-s tate potential minima (S = 1.2) ha s been proposed. " In summarizing the discussion of the spectral shape based on the configurati onal coord in ate model, on e can review th e experim en tal results on luminescence bandw idths. In Figure 17,13 the halfwidth of the luminescence band of typical activators in phosphors is pl otted against th e peak wa velen gth .!' The act ivat or s are clas sified by the type of op tical transition describ ed in the Section 1.3.1. When the d ~ d (Mn 2+),j ~ d (Eu 2+), and 52 ~ sp tran sitions (Sn 2+, Pb 2+, and Sb 3+) are seque n tially compared, one finds that the halfwidth incre ases in the sa me order. This is apparently because the overlap of the electron wa ve functions between the excited and gr ound states increa ses in the ab ove ord er. The difference in the wavefunction overlap increases the shi ft of the equilibrium position of the excited state, Qo, and consequen tly the Stokes' shift and the halfwidth increase as we ll. Weak ele ctron-phonon interactions gi ve line spe ctra. The line width in this case results from factors other than those inv olv ed in the con figurational coordinate model. Such factors are briefly rev iewed below.
1.3.3.1
Line broadening by time-dependent perturbation
The mo st fundamental origin of the lin e width is the ene rgy flu ctuation of the initial and final states of an op tica l transition cau sed by the uncertainty principl e. With 1: being the
Chapter one: Fundamentals of luminescence
35
tungstats and titanates
,·---L-x ----'l , ~--j--;----~ x 'I
I
I
'I
6 000
I
V x I xl
I
0
:
0
, I
:
0
I
x
l.-
x
•
2
,
x .... --------l<-J
I.
S
......;.;.,._. _ _ .
.
J
,.....
'I
.§.
-~
4 000
.!:
deep donor-acceptor pairs
r-----~
"0
.....
I
I
,
2 000
I , L
X (1)
t__ ~ ~ _'1::__
r - - - )- - - - - - - - - - - - - - - - - -
+~ - - - + +
I
L _+\_±-+_---.J' Eu2 +
t. __
~
(2)
t.
in ZnS
j - - ., ~
(3)
r t.----------,
I
t.
I t. !.-_t._"\
f" -~61
,
t.1
..J
\
u_ \_.J
I t.
t.
I
Mn 2+-orange
Mn 2 +-green
400
500
600
700
Peak Wavelength (nm) Figure 17 A p lot of peak wavelength and half-width of various phosphors. The p oints (1)-(3) indicate the followin g ma ter ials. The luminescence of (2) and (3) origina tes fro m Mn 2+ principa lly. (1)(Sr,Mgh(P 0 4h :Sn2+ ; (2) Srs(P0 4h F:SbJ+,Mn 2+(3) CaSi0 3:Pb 2+, Mn 2+. (From Narita, K., Tech. Dig est Phosphor Res. Soc. 196th Meeting, 1983 (in Japanese) . With perm ission.)
harmonic mean of the lifetim es of the initial and fina l s tates, the sp ectra l line wid th is given by 11/1 and the sp ectra l sha pe takes a Lorentzian form:
I(v) -..!.. -
TC
1/vL
l + ( V-V S /v~
(90)
where V L == (ri l + ri )/4Jrc, v is the freq ue ncy of light, Yo th e frequency at the line cen ter, and t, ana 'tf ar e m e nrenm es of th e ini tial and fina l sta tes, res pectively.
Fundamentals of Phosphors
36
In ad d ition to the spectra l width given by Eq. 89, th ere are other kinds of timed ependent perturbation con tribu ting to th e w idth. They are absorption and emission of a phot on , which makes " the natural width," and absorption and emission of pho nons. The flu orescent lifetime of a tr ansition-met al ion or a rare-earth ion is of the ord er of 1 ~ s at th e sho r test, which correspon d s to 10-6 em: ' in spectral w id th. This is much sha rpe r than the actually observed w id ths of about 10 crrr' : the latter arise fro m other so urces, as discussed below. A t high temperatures, a sign ifican t contribution to th e wi d th is the Raman sca tterin g of phonons . Thi s process d oes not ha ve any effect on the lifet ime, but do es make a Lor entzian con trib u tion to th e width. The spe ctra l width due to the Raman scattering of phonons, .1.E, depends strongly on temperature, as can be seen below :
_ (T J71xO xOe
.1.E -a -
TO
0
X
2
(ex - 1)
dx,
x
=
o
tU»a
(91)
KT
where T Dis Debye temperatu re and a is a con stant that includes the scattering probability of phonons.
1.3.3.2 Line broadening by time-independent perturbation When th e crys tal field around a fluor escent ion ha s s tatistical dist ribution, it produces a Ga uss ian spec tra l sha pe.
i
1 exp - (v - v2 o) ) I(v) =-/'-', 2n:0"
(92)
20"
w ith 0" being the s ta nda rd d eviat ion . Lin e broad en ing by an inhomogene ously dist ributed crystal field is called inhomogeneous broadening, while the p rocesses described in Section 1.3.3.1 result in homogeneous broadening.
1.3.4
Non radiaiiue transitions
Th e classi cal theory d escrib es a nonradiatiue transition as a process in which an excited sta te re laxes to the gro u n d sta te b y crossing ov er the intersection of the configura tional coord inate curve through thermal excita tion or other means (refer to Sectio n 1.3.2). It is often observed , however, that th e expe rim en tally d eter mi ned activation ene rgy of a nonra diative process d ep ends upon temperature. This problem has a quantum mechanical expl an at ion : that is, a n op tical transiti on acco m p an ied b y ab sorpt ion or emission of m - n phonon s can tak e p lace when an n th v ib ra tional level of the excited s tate and an mth vibra tiona l level of the ground state are located at the sa me energy. The proba bility of su ch a transition is also proportional to a product of th e Fra nck-Con d on coeff icien t and th erm al distribution of po pulation in the grou nd state, giv ing the required temperature-dep endent probability. Wh en the ph on on ene rgy is th e same both at the gro und and exci ted sta tes, as sh own in Figure 14, the nonradiative relaxati on probab ility is given by:
N =N p
cg
ex P
{-S(2(n)+I)}~ (S(n)y {S(I +(n)W+ L..J ]=D
l'', (P+ J')1.
i
(93)
Chapter one: Fundamentals of luminescence Here, let p ""
111 -
37
n, and (n) denotes the mean number of th e vibrati onal qu anta n at
r.
temperature T expressed by (n) = {exp (tlw/kT) - 1
The notati on Neg implies th e ov erlap
integral of the electron w avefunctions . The temperature dependence of N; is im pli citly in clu de d in (n) . Obviously, Eq. 93 does not hav e a form characterized by a sing le activat ion ene rgy. If writt en in a form such as Nil oc exp (- Ep / kT ) , one obtains: (94)
where (n) ptv» is the m ean ene rgy of the excited st ate subject to the nonr adiati ve process. The energy Ep increases with temperatu re and on e obtains Ep < t:,U at sufficie n tly low temperature. If 5 < 1/ 4 or if electro n-pho no n inter action is sma ll en ough, Eq. 93 can be sim plified by leaving only the term for j = O. N I,
=
r/p!
N eg ' exp {-S(l + 2(n))}{ - S(l + (n))
(95)
Ina material th at shows lin e sp ectra, such as rare-earth ions, the dominating no nradiative relaxation process is due to multiph on on emission . If Egop is the energy sep aration between two levels, the nonrad iati ve relaxa tion probability be tween th ese levels is given by an equation deri ved by Kiel:" (96)
(97)
where A K is a rate cons tant and E is a coup ling cons tant. Eq. 95 can be transforme d to the same form as Eq. 96 using th e conditions S = 0, exp{- S(l+ 2(n))} '" 1, Sp/ p! = EP and AK = Neg' altho ug h Eq. 95 was d eri ved ind ep endently of the configurational coord ina te mod el. If tw o configura tional coo rd in ate curves have the same curva ture and th e sam e equilibrium p osition, the curves will never cros s and there is no relaxation process by thermal ac tiva tion between th e tw o in th e framework of the classical theory. However, thermal quenching of luminescen ce can be explai ned for such a case by taking phono n-e mission relax ation into accoun t, as predicted by Kiel's equation.
References c.c. and Sch ulman, }.H., Solid State Physics, Vol. 5, Sei tz, F. and Tu rnb ull, D., Eds ., Acad emic Pre ss, 1957, pp . 97-116. Curie, D., Luminescence in Crystals, Methuen & Co., 1963, pp . 31-6 8. Maed a, K., Luminescence, Maki Sho ten, 1963, pp. 6-10 and 37- 48 (in Japa nese) . Diba rtolo, B., Optical Interactions in Solids, John Wile y & Sons, 1968, p p. 420 -427 . Kam imura, A., Sugano, S., and Tanab e, Y, Ligand Field Theory and Its Applications, Firs t Edi tion , Shokabo, 1969, pp. 269-321 (in Jap an ese). Fukuda, A ., Bussei, 4, 13, 1969 (in Jap an ese ). Keil, 1., Phys. Rev., 140, A601, 1965.
1. Klick,
2.
3. 4.
5. 6. 7.
Fundamentals of Phosphors
38 8. 9. 10. 11. 12. 13. 14. 15.
Nakazawa, E. and Shionoya, S., f. Phys. Soc. [pn., 36, 504, 1974. Merz, ].L., Phys. Rev., 176,961 , 1968. Williams, F.E., f. Chern . Phys., 19,457,1 951. Fonger, WH. and Struck, C.W, Phys. Reo., Bll1, 3251, 1975. Klick, c.c. and Schulman, ].H., J. Opt. Soc. Arn., 42, 910, 1952. Narita, K., Tech. Digest Phosphor Res. Soc. 196tl1 Meeting, 1983 (in Japanese). Struck, C.W. and Fonger, WH., f. Luminesc., sm. 3251, 1975. Kiel, A., Third Int. Con! Quantum Electronics, Paris , Grivet, P. and Bloembergen, N., Eds., Columbia University Pres s, p. 765, 1964.
chapter one - section four
Fundamentals of luminescence Sumiaki Ibuki
Contents 1.4 Impurities and luminescence in semicon d u ctors 1.4.1 Impuriti es in semiconductor s 1.4.2 Lu minescence of excitons bound to impurities 1.4.3 Lum inescen ce of isoel ectronic traps 1.4.4 Luminescence of donor-acceptor pa irs 1.4.5 Deep levels References
39 39 .40 .43
43 46 49
1.4 Impurities and luminescence in semiconductors 1.4.1
Impurities in semiconductors
As is well known, when semi con d uc tors are doped with impurities, the lattices of the semiconductors are di storted a nd the ene rgy level str uc tures of the semiconductor s are also affected . For example, whe n in Si an As atom (Group V) is substi tu ted for a Si a tom (Group IV), one electron in the ou term ost electronic orbit in the N sh ell of the As atom is easily released and moves freely in the Si lattice, becau se the number of electro ns in the N she ll of As (5) is one more th an th at in the M shell of Si (4). Thus, impurities th at sup ply electrons to be freed easily ar e call ed donors. On the contra ry, when a Ga.atom (Group III) is su bs titu ted for a Si atom, one electron is attr acted from a Si a tom, form in g a hole that moves freely in the Si lattice; thi s is because the number of electrons in the N shell of Ga (3) is one less than that in the M shell of Si (4). Thus, imp ur ities that su pp ly free holes easily are called acceptors. In compound semiconductors, it is easily understood in a similar w ay what kinds of impurities pl ay the role of d onors and acceptors. Usu ally, in compound semiconductors such as ZnS an d GaAs, the stoichiometry does nothold stri ctly. Therefore, when more positive ions exit, ne gative ion vacancies ar e crea ted and work as donors. Similarly, when more negative ions exit, positive ion vacan cies work as accep tors. In a donor, one excess electro n orbits around th e p ositively charged nucleus, as in a hydrogen atom. This electron m ov es around in a semiconductor crystal (which usually has a la rge dielectric con stant) so that the Coulomb in teraction between the nucleus and 39
40
Fundamentals of Phosphors
Conduction Band
Donor
Acceptor
Valence Band Figure 18
I1111IIIIIIIIIII
Sha llow impur ity levels in a sem icon ductor.
the elec tro n is wea kened . Th e radius of the elec tro n orbi t becomes large under these co nd itio ns and the electron is greatly affected b y the pe riodic po ten tial of the crystal. For exa mple, whe n the effective m ass of the electron is 0.5 and th e d ielectric constan t of the crystal is 20, th e Bohr ra d ius of the electron becomes 40 tim es larger than that in the h yd rogen atom . Therefore, the excess electron of the donor can be released from its binding to th e nucleu s by an excita tion of small energy. This means th at the d on or level is located very close to the bottom of the conduction band, as shown in Figure 18. Similarly, the acceptor level is locat ed very close to the top of the valence bond . Impurity levels with small ionizat ion energies ar e called shallow impurity levels. Ot her imp urity levels can also be located at deep posit ions in th e forbidden land. Light absorp tio n takes p lace between the valence band and imp u rity levels, or between impurity levels and the cond uc tion band. When a large qu antity of impurities exists, th e b and sha pe can be observed in absor p tion. Luminescen ce takes pl ace through these impurity levels w ith waveleng ths lon ger than the bandgap waveleng th . When th e d opant imp uri ty is cha nged, th e luminescent wa velength and efficiency also cha nge. It is usu all y found th at in n-typ e sem icond uctors, lumine scence bet ween th e conducti on band an d accep tor level s is stro ng; w hereas in ptyp e sem icond uctors, luminescen ce between donor levels and the va lence band is strong.
1.4.2 Luminescence of excitons bound to impurities Th e number of impu rities in cluded in semiconductors is of the order of magnitud e of 1014 to 1016 crrr' , even in so-ca lled pure semi con d uctors. Therefore, excitons movi.ng in a crys tal are generall y cap ture d b y these impur ities and bound exciton states are created . Luminescence from such bound exciton s is, in ordinary crystals, stronger than that fro m freeexcitons. Excitons bound to d on or s or accep tors create H 2 molecule-type com pl exes. Those bound to ionized donors or accep tors crea te H 2 + molecular ion-type complexes . Bind ing energies of excitons in th ese comp lexes depend on the effective ma ss ratio of electro n to hole, and
Chapter one:
Fundamentals of luminescence
41
11 Lin e(4888)
11-LO
Ionized Donor-LO
I
~
12
Lin e(486 7)
~ Ion ized Don or
16 - Exciton (48 57)
r s - Excit on
It
(4853) Excit on (B-band)
5260
51 80
5100
502 0
4940
4860
I,+LO
t
4780
W av elength (A)
Figure 19 Luminescence s pectru m near th e b and gap of CdS (1.2K). (From Litton, C.W ., Reynolds, D.C., Co llins, r.c. and Par k, vs, Pl1ys. Rev. Lett., 25, 1619, 1970. With permission .)
are abou t 0.1 to 0.3 of ioniza tion energies of the d on or or acce p tor impuri ties . The radiati ve recombination of bound excitons takes pla ce efficiently w ith en ergy less than that of free excitons. The hal fwi dths of luminescence lines ar e very narrow. As an examp le, a luminescenc e spec tru m of CdS, a II-VI compound of the direct transition type, n ear the band edge is shown in Fig ure 19.' In the figu re, the I" 12, and 13 lines correspond to the luminescen ce of excitons bo u nd to n eutral accepto rs, neutral donors, and ionized do nors, resp ectively. They were identified by m easurements of th eir Zeeman effect. Th e bi nd in g energies of these b ound excitons a re 19, 8, and 5 meV, resp ectively. The halfw idths of the luminescence line are very na rr ow, about 2-3 cm' . and are much less than those of the free exciton lines shown as T, and [ 6 exci tons. In II-VI compoun ds like CdS, excitons couple stro ngly with the longitud ina l op tica l (LO) phonons that genera te a polarized elec tric field . As a result, exc iton luminescence lin es accompan ied by simultaneous emiss ion of one , two, or mor e LO p honons are observed strongly. as shown in the fig ure . The oscillat or streng th of the 12 bound exciton was obtaine d from the a rea of the absorption spec tr um and fou nd to be very large, abo u t 9.2 Th e osc illa tor s tre ngth of the free exciton is 3 X 10-3, so that of the bound exciton is enhan ced b y ~ 103 . Th is enhan cement effect is called the gian t oscilla tor streng th effect. From a theoreti cal point of view, th e ratio of the osc illa tor stren g th of the b ound exciton to th e free exci ton is given by th e ra tio of the volu me in which the bound exci ton moves around, to that of the unit cell . In CdS, this ratio is - 103, so that the ve ry large va lue observed for the 12 bound exc iton is reason ab le. This value gives a calculated lifet ime of 0.4 ns for 12, The life times of excitons are deter mined from lu minescence de cay measu remen t.' For the 12 bound exc iton, a va lue of 0.5 ± 0.1 ns w as obta ine d, which agrees well with the calcu la ted va lue . This also indica tes that the luminescen ce quantum efficie ncy of th e 12 bo u n d exciton is close to 1. In the case of indirect transition-type se m icond uc to rs, on th e ot her h and, the lu m in escence efficiency of bound exc itons is very low. A typical exam ple is the cas e of S donors in GaP. The luminescence qu antum efficiency h as been estimat ed to be 1/ (700 ± 200).4The reason for the lo w efficiency is ascr ibe d to the Auger effect. The sta te in w hic h an exci ton
Fundamentals of Phosphors
42
Wave Length (A)
4950
4900
48 50
4800
1.0
0.5
'Mi'--- NO=4.4Xl0 18
z-
o
'iii c
(1)
E (1)
o
c
No=2.1Xl 0 IB
(1) ()
'"c
(1)
E :J
-'
IK-- - - No=1 .8Xl 0
1B
K - - - - - No=9.2 Xl 0 ' 7
I~
No=6.5X l 0 17
ND=1 .9Xl 0 17 2.50
2.55
2.60
Photon Energy (eV)
Figure 20 Ch anges of luminescen ce spectra of exci tons bound to Cl d onors in CdS (1.8K ) wit h the Cl con cen tration . N D :C1 d onor concentration (crrr-') . (Fro m Kukimoto, H ., Shion oya, S., Toyoto rni, S., an d Mori gaki, K., J. Phys. Soc. Jap an , 28, no, 1970. With permi ssion .)
is bound to a neutral donor includes two electrons an d on e h ole, so when on e electron and on e hole recombine, the recombinati on en ergy does not result in light emission, but is instead transfe rred to the rem aining electron to raise it into the conduction band. N ext, th e effec t of hi gh con centrati ons of impurities on the bound exciton lum inescence is discussed. As an example, cons id er the case of an 12 bound exciton in Cd S:Cl as sho wn in Figure 20.5 With increasing Cl d ono r concen tration N o, the spectral width broadens . Beyond N o - 2 X 1018 crrr' , the emission peak shifts toward the high-energy side with further inc reases of N[l. Simultan eously, the spectra l wid th b road ens more and the shape becomes asym me tric, havin g lon g tails toward the low-energy sid e. Th ese facts can be interpreted theoretically. " At higher No, an exciton bound to a donor collid es w ith other donors. Don or ele ctrons can thus be virtua lly exci ted and can exert the scree ning effect on the bound excitons through changes of th e dielectric con stant. This br ings abo u t the high-en ergy shi ft of th e emission peak . The asy mme try of the spectra l
Chapter one:
Fundamentals of luminescence
43
shape wi th lon g tails is interpreted as bein g cau sed by th e Stark effect due to ionized impuri ties, i.e., com pensated d onors and accep tors.
1.4.3 Luminescence of isoelectronic traps In sem icond uctor crys tals, if an isoelectroni c elem en t, (i.e., an element belonging to the same column in the per iod ic ta ble as a con stituent elemen t) is s ubstitu ted for a con stituent element, eith er a free electron or a hole in the se micond uctor is a ttracted to th e iso electron ic elemen t. This is because of the differences in electronega tivi ty bet w een the isoe lectronic elemen t and the mother element. Such isoelectronic elements are ca lled isoelectronic traps. When an elec tro n is trapped in an isoelectonic tr ap , a hole is attra cted to the top trap by the Coulomb force , and an exciton bound to an isoelec tro nic tr ap is crea ted. Th is state produces lu minescence that is quite different fro m that due to an exci ton bound to a d on or or accep tor. In such cases, an electron or hole is attracted to the do nor or acceptor by a long-range Co ulomb for ce. On the other hand, the isoelect ric trap attrac ts an electro n or a hole by the sho rt-range typ e forc e that com es from th e difference in th e electronegat ivity. Therefore, the wavefu nct ions of the electron or hole trapped at th e isoelectro nic trap is very mu ch localized in real space and, ins tea d, is grea tly ex ten ded in k-space, Thi s pl ays an important role in the case of indirect tran sition-typ e se mi con d uctors. Figure 21 sho w s the wavefu nction of the electron bound to an N isoel ectron ic trap in GaP.? Th e bottom of th e con d uction band of GaP is loca ted at th e X point in k- space, and the electro n ha s a rela tive ly large amplitu de, even at th e r po in t. Th erefore , the electron can recombine w ith a hol e a t the r p oint wi th a high p rob ability for con d itions applicable to direct transitions. Th e emis sion spectrum is shown in Figure 22.8 The recombina tion probability is 100 times larger than th at of an exci ton bound to a neutral Sdonor, for w hich onl y th e ind irect transition is possibl e. Moreover, in the GaP :N sy stem , there is no th ird particle (elec tro n or hole), so the Auger n onrad iat ive recombinati on does not occur, and the recombinati on p rob ab ilit y is actua lly close to 1. Wh en the conce n tra tion of N traps is high, luminescence of an exciton stro ng ly bound to a pair o f N traps closel y located to eac h other is also observed at a slightly longer w avelen gth. Other isoelectr ic traps in GaP, Zn-O, an d Cd -O centers, in which tw o elem en ts are located in th e nearest neighbor sites, are kn own. Th ese cen ters also prod uce efficient luminescen ce, as do isoelectric trap s in d irect tran sition-type se micond uc tors, of w hic h CdS :Te9 an d ZnTe :O lO have been iden tified.
1.4.4 Luminescence of donor-acceptor pairs When the wavefu nction of an elec tron trapped at a d onor overlaps to some exten t with the wavefunction of a hole located at an accep tor, both particles can recombine ra d ia tive ly. Theluminescence thus p roduced has som e interest ing characteristics because the elect ron and the hole in this pair are located in la ttice sites ap ar t from each other. As exp lained below, the luminescen ce waveleng th and probab ility will d ep en d on th e ele ctron-hole distance in a p air. As shown in Figure 23, at the s tar t of luminescen ce, th e el ect ron is locat ed a t th e donor D and the hol e at the acceptor A. Th e energy of thi s ini tial stage is exp ressed, taking th e origin of the ene rgy axis to be the accep tor lev el A, as Ei = Eg - (ED + EA ) , where Eg, ED, an d EA are th e band gap ene rgy, ion ization en er gy of a neutr al donor, and that of a neutral acce p tor, respect ivel y. Aft er the rec ombina tio n, a p ositi ve effec tive charge is left in the do nor and a negat ive effect ive cha rge in th e accep tor. Th e fina l s ta te is determi ned by th e Co ulomb interaction bet ween them , giving th e fina l s ta te energy to be E, = - £2/4m:r, where E is th e sta tic dielectri c constant of the cryst al, and r is th e
44
Fundamentals of Phosphors
GaP: N 3
>Ql E
0
~
I
> ~
uJ
2
>-
e'
Q'
S
Ql
c w
I"-
1:::-
uT
(')
N II
S
Ol
lU
o
x k= ~ (100)
(000)
k
Figure 21 Energy level and wavefun ction of N isoelectron ic trap in GaP in the k-space. (From Holon yak. N ., Campbell, J.e., Lee, M.H, et al.. ]. Appl. Phys" 44,5517, 1973. With permi ssion .)
distance between the d ono r and acce p to r in th e pair. Th erefore , the recombinati on energy E, is given by :
t; == E cf j -
(98) == 1':: -
(ED + E'I) + e2
4rrcr
In this formul a, r takes d iscrete valu es. For sma ller r va lue s, ea ch D-A pair emission line sho u ld be se pa ra ted , so that a se ries of sha rp em ission lin es should be obser ved . For larg er r values, on the other hand, interval s among eac h em issio n line are small, so that they will not be resolved an d a broad em ission band will be obser ve d . The transition probabilit y should be p roportional to the squa re of th e overl ap of th e electro n and h ole wavefu nctions . Usually, the wa vefunction of a donor electron is more w ide ly sp rea d than that of an acceptor hole. Th e elect ron wavefunc tion of a h ydrogenlike donor is assumed to d ecrease expo ne ntially with r. Therefore, the tran sition p robability W(r ) is exp ressed as:
Figure 22 Luminescence spectrum of GaP:N (4.2K). (From Thomas, D.G. and Hopfield, J.J., Phys. Rev., 150, 680, 1966. With permission.)
o ----..--
~
E(r)
Figure 23 Energy levels of a donor-acceptor pair. (99)
where r s is the Bah!' radius of the donor electron and Wo is a con stant related to the D-A pairs. As a typical exa mp le of D-A pair luminescence, a spectrum of S donor and Si acceptor pairs in GaP is sh own in Figure 24.11 Both Sand Si substitute for P. The P site, in other words the site of on e of the tw o elements cons titu ting GaP, composes a face-centered cubic lattice. In this lattice, r is given by l(l /2)mp /2a, where m is the shell number and a is th e lattice constant. For the shell number s m = 1, 2, ' " 12, 13, 15, 16, ..., there exis ts atoms;
Fundamentals of Phosphors
46
100
c 80
C 60
r
'iii
C')
c
Q)
C 40
B A
Rb
20 0 2.18
2.2 0
2.2 2
2. 24
2.26
2 .28
2.30
2.32
Photon Energy (eV)
Figure 24 Lwnine scence spectr um of D-A pai rs in GaP:Si,S (1.6K). (From Tho mas, D.G., Gershenzan, M., an d Trum bore, F.A., Phys. Rev., 133, A269, 1964. With permission .)
but for m = 14,30,46, ... , ato ms d o not exist. Assuming th at th e p osition of each em ission lin e is given by Eq . 98 with r given in thi s way and ED + EA = 0.14 eV (E g = 2.35 eV), it is poss ible to determ ine th e sh ell nu mber for each lin e, as shown in Figure 24. As expecte d, lines for m = 14,30, ... d o no t appear as seen in th e figure. Agreeme n t be tw een ex perime n t and th eor y is surprisingly good . As underst ood from Eqs . 98 an d 99, th e sm alle r the r value is, the sh or ter the lu minesc ence w avelength em itte d and the hi g her the transition p rob ability beco mes; in o ther words, the shorte r the d ecay time. The refore, if one observes a time-resol ved e mission spectrum fo r a bro ad ba nd co m posed of many unresolved pair lin es, th e em ission p ea k of th e broad b and shou ld sh ift to longer w av elen gths with the lapse of time. The broad band peaking a t 2.21 eV in Figure 24 is the en semble of m any unresolv ed p air lin es. Figure 25 12 shows tim e-resolved luminescence sp ec tra of thi s ba nd . It is clearl y seen th at th e p eak sh ifts to longer wavelengths wi th time, as expec ted . Simi lar tim e shifts in D-A p a ir lumin escence h ave been observ ed in II-VI comp ounds su ch as Zn Se and CdS. (See 2.7.)
1.4.5
Deep levels
As th e final s tage of thi s sec tion, lu m in escence and related phen omena cau sed by de ep lev els in semiconductors are discussed . Certain d efects and impurities crea te de ep localized levels with la rge ioni za tion ener gi es. In th ese d eep levels, elec tron- la ttice in teractio ns ar e ge ne ra lly stro ng, so th a t th e n onradiative recom bination takes pl ace via the se levels, thus low ering the lumin escen ce efficien cies of emittin g cen ters . Fu rther, th ese deep centers sometim es m ove an d multip ly by th ems elves in crystals, and ca us e th e d eteriora tio n of luminescence d evices because of th e local he ating by multiphon on emission . Cha nges of th e states of d eep levels ca used by photoexcit ati on are stu died from measurements of conductivity, capacitance, and magn etic prop er ties. In this w ay, the struc ture, den sity, position of energy levels, and ca p tu re and release proba bilities for carrier s have been determine for various deep lev els. Calculat ion s of binding ene rgies of d eep levels usin g w avefunc tions of th e con d uc tion and va lence ban ds h ave also been p erformed . In th is way, bin d in g energies of in GaP and GaAs an d th ose of Ca and As vacancies in GaAs are obtained. Calculat ions are fu r th er made for complex defects in cludin g 0, for exampl e, a compl ex of and Si o r Ge vaca ncy, and a toms occupying antisites.P
°
°
47
Chapter one: Fundamentals of Luminescence 5
C Crystal M78200K
5
o 5
10'
5 ~
'00
c
100 usee
(j)
:f
10
5
10- 1
5
10- 2 5
10- 3 5 2.12
2.16
2.20
2.24
2 .28
2 .32
Photon Energy (eV)
Figure 25 Time-resolved luminescence spectra of D-A pa irs in GaP:Si,S (20K). (From Th omas, D.G., Hopfield, J.J., and Au gu stniak, W.M., Phys. Reo., 140, A202, 1965. With perm ission.)
Transition metal s incorporat ed in semicon d uc tors usu all y creat e d eep levels and exhibit luminescence. Since elec tron- la ttice in teractions are s tro ng, broad-band spectra with rela tively weak zero-ph onon lin es are usu all y observed. Figure 26 shows luminescence spectra of Cr 3+ in GaA s14 as an exa m ple. Co u pling with ph on ons results in the phonon side ba nds shown in Figure 26. As for the nonradi ative recombina tio n th rough defects, no t only the Auger reco mbination p rocess but also m an y phonon emission process are ob served. The tran sition probabilities of the latter increase wh en related levels are d eep and crys tal te mp eratures are hig h. In cer tain cases, the en ergy level of a locali zed trap is sha llow before trapping
48
Fundamentals of Phosphors Wavelength (nm)
1700
1650
1600
1550 LO
LA
I I
1500 TA IAr
1450
I
LPE M 228
OJ
U
C
OJ
-11-
U CfJ
OJ
c
Resolution
'E ::l
--l
.840
.860
.880
5900 6000 6100 6200 6300 6400 6500 6600 6700 6800 6900 7000 7100 Wavenumber (cm' ) Figure 26 Lum inescen ce spectrum of Cr3+ in GaAs (4.2K ). (From Stocker, H.J. an d Schmidt, M., App!. Phys., 47, 2450, 1976. With permi ssion .)
J.
E
L-
_ _.....I_
.........;::..-"--~
Q
Figure 27
Config ur ationa l coordinate model of d eep defect level. (C: cond uction band, V: val ence band, D: d eep d efect.)
an electron; but after trapping, lattice relaxation and th e rea rrangement of surrounding a toms tak e place and the energy level is m ade d eep, as shown by the config ur ationa l coord inate model (1.3.2) in Figure 27. In th is state, the diffe ren ce betwe en the op tical
Chapter one:
Fundamentals of luminescence
49
activation en ergy an d th ermal activation energy is large, and nonrad iat ive recombin at ion through the emiss ion of ma ny phonons occurs w ith high probab ility."
References 1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11.
12. 13. 14. 15.
Litton , C.W, Reynold s, D.C., Co llins, r.c. and Par k, YS., Phys. Rev. u u.. 25, 1619, 1970. . Thom as, D.G. and Hopfield, ].J., Phys. Reu., 175, 1021, 1968. Henry, c.H. and Nassau, K., Phys. Reo., 81, 1628, 1970. Nels on, D.F., Cu th bert, ].D., Dean , PJ ., and Thomas, D.G., Phys. Rev. Leti., 17, 1262, 1966. Kukimoto, H ., Shionoya. S., Toyotomi, S., an d Morigaki. K., f. Phys. Soc. [pn., 28, no, 1970. Han amur a, E., J. Phys. Soc. [p n., 28, 120, 1970. Holonyak, [r., N., Campbell, I.C; Lee, M.H ., Verdeyen. J.T., Johnson, WL., Cr aford, M.G., and Finn, D., I. App!. Phus., 5517, 1973. Thomas, D.G. and Hopfi eld , J.J., Phys. Reo., 150, 680, 1966. Aften, A.c. and Haaus tra, J.H., Phys. u «. 11, 97, 1964. Merz, J.L., Phys. Rev., 176, 961, 1968. Thomas, D.G., Ce rshenzon. M., and Tru mb ore, F.A., Phys. Rev., 133, A269, 1964. Thom as, D.G., Ho pfield . J.J., and Augustyn iak, WM., Phys. Rev., 140, A202, 1965. AIt, H.Ch ., Materials Science Forum , 143-147, 283, 1994. Stocker, B .]. and Schm id t, M., f. Appl. Phys., 47, 2450, 1976. Kuki mo to, H ., Solid State Phys., 17, 79, 1982 (in Jap an ese).
chapter one - section five
Fundamentals of luminescence Chihaya A dachi and Tetsuo Tsutsui
Contents 1.5 Luminesc ence of orga ni c compounds 1.5.1 Origin of luminescence in org anic compounds 1.5.2 Electronically excited st ates of or gan ic molecules and their p hotolu m inesce nce 1.5.3 Fluo rescence of organic molecu les in a solid state 1.5.4 Q uan tum yield of fluo rescence 1.5.5 Organic fluorescen t and phosphorescence compou nds with hi gh gua n tu m yield s References
51 51 52 54
56 56 59
1.5 Luminescence of organic compounds 1.5.1 Origin of luminescence in organic compounds The luminescen ce of orga nic compoun d s is essentially based on localized rt-electron systems wit hin ind ivid ual organic m olecules' . Th is is in clear con trast to inorgan ic phosphors where luminescen ce is de termined by the ir latti ce structures, and thu s their luminescence isaltered or d isappears altogether when the crys ta ls me lt or d ecom p ose. In organic lumi nescent compo unds, in con tras t, it is the rr-electron systems of individ ua l m olecu les tha t are respo nsible for luminescence. Therefore, even when organ ic crystals melt in to amorphous agg regates, lu minescence still persists. Further, when molecules are in vapor p hase or in solu tion, they basically demonstrate sim ilar lu minescence sp ectru m as in soli d films . Lum ines cence from organic compoun ds can be class ified into two ca tegories: luminescence fro m electronica lly exci ted sing let (51) or triplet (T )) s ta tes . Em ission from sing le t exci ted sta tes, called " fluorescence," is commonly observed in conventiona l organic compound s. Em ission from triplet excited sta tes, called "p hos phorescence," is rarely observed in conventional organic com pounds at ambien t tem peratures due to the small radiative decay ra te of phosphorescence. Electronically exci ted s ta tes of organic compou nd s are ea sily prod uc ed not only via photoexci ta tion but also by other exci ta tion methods (such as chemical rea ction s, electrochemical reactions, mec ha ni ca l forces , hea t, and electric charge recomb ina tio n) capable of prod ucing electronically excited s ta tes in organic mo lec ules, as d epicted in Fig ure 28.
51
52
Electroluminescence Triboluminescence
Fundamentals of Phosphors
Chemilumine scence
Ground state
Figure 28 Th e various excitation methods, i.e., light absorption, thermal, chemical and chargedparticle, and d ecay process es, i.e., photoluminescence, thermal deactivation, and energy transfer and migration, in organic molecules. It should be emphasized that any kind of luminescence in organic compounds is due to
well-defined electronically neutral singlet or triplet excited states in the organic molecules, ev en though luminescence can be produced by a variety of excitation methods having different names Like photoluminescence, chemiluminescence, electrochemiluminescence, triboluminescence, th ermoluminescence, and electroluminescence. In add ition to radiative deca y, the excited molecules also decay nonradiatively through thermal deactivation and energy transfer and migra tion.
1.5.2 Electronically excited states of organic molecules and their photoluminescence Electronic transiti ons in organic molecules ar e described by the molecular orbitals of 0 ele ctrons and It-electrons. Each molecular orbital can accept two electrons with antiparal1el spins accord ing to Pauli's exclusion principle, and both a and It-electrons participate in chemical b onding. He re, It-electrons demonstrate a variety of photo- and electronic activities compared with a-electrons, sin ce a- electrons become located a t deeper energy levels compared with those of It-electrons (Figure 29). The ground state is characterized by the nelectrons in the highest occupied mol ecul ar orbital (HOMO) . In ord er to produce an electronically excited sta te, a mol ecul e must absorb energy equal to or greater than the en ergy difference between th e HOMO and the lowest unoccupied molecular orbital (lUMO) levels (100)
With ab sorption of en ergy, an electron is promoted from HOMO to lUMO, and this constitutes an electronic transition from the ground state (So) to an electronically excited state (S1)' Here, the energy level diagrams (Jablonski dia gram) for molecular orbitals for the ground
Chapter one: Fundamentals of luminescence
53
LUMO
1 ~o" I tJE
n-electrons {
:,J,.,
I/J n- l
-H-- "
a-electrons
--r-r-
¢2
---t-t-I/Jl Ground state
~O
1
. .,
¢M1
tJE
l'
l'
HOMO
~ ¢n
-t--T-H-- ¢i ¢n-l
--r-r---t-t-
1J2
¢,
Lowest excited state
Figure 29 Energy leve l diagrams of molec u lar orbitals and electro n con fig ura tio ns for gro und and singlet excited states.
and excited sta tes are commonly used for the description of electronic transitions in organic molecules (Figu re 30). In Figure 30, the transition of an electron from HOMO to LUMO is expressed in terms of its spin s tates. Th e electronic tra nsitions are expressed in terms of the difference in energy bet w een the gro lmd and excited s tates in the electron ic-sta te diagram. The spin multipl icity of the states is implicitly ind icated by the notations of 5 (sing let) or T (triplet). The gro un d state, 50' and lowest singlet and triplet sta tes, 51and T" are com posed of multiple vibra tional states, d ue to vibron ic and rota tion ene rgy levels of the molecules. When an energ y larger th an the HOMO-LUMO ene rgy differen ce is abs orbed by a molecu le, either higher vibronic sta tes within th e 51 sta tes or higher singlet excited states 52 and 53 are pro duce d . Th e highe r vibro nic states of 5, rela x to the low est vibron ic state of 51 w ithin a timescale of - 10- 12 s. The high er energy sing le t s tates s uch as 52 and 53 rela x to the 5, sta te via nonradi at ive, in ternal conversion (IC) p ro cesses. Trip le t exci ted states are usually prod uced via an in tersys tem crossi ng (I5C) p rocess from 5, ~Tl' since th e transiti on probability of di rect exci ta tion from 50 int o T] is ve ry sma ll. Also, the higher energy triplet states such as T2 and T3 relax to the T 1 sta te via nonr ad iat ive processes. Thus, radi ativ e tran sitions take pl ace as all. electronic tran sition from the lowest excited states of 51 or T, to the gr ound sta te 50' The radiative tran sition from 51 ~50 is classified as a spin-a llowed tra nsition and the refore the tim escale of the transition is of the order of - 10-9 S . On the other hand, the tim escale of the T 1~ So tran sition is m uch lon ger, ranging from micro- to milliseco n ds because the pro cess is intri ns ically spin- forbidden . The emission sp ectr a of org anic m olecul es o ften exhibi t a vibron ic structure beca use the ground state also con tains vibronic and rota tiona l fine structures. Figur e 31 sho ws schematically the relation between a bsorp tion an d emission spectra . An emission spec trum looks like the mirror image of the electronic absorpti on sp ectru m of a molecule d ue to the pre sence of vibra tional levels in each energy leve l. The emission wa velength for the rad iative transition from the lowest 5, state to the lowest 50 state, the 0-0
Fundamentals of Phosphors
54
(2)
(6) (3) Q)
ro
(2)
81
(4)
'0
(2)
>-
T1
0>
Ci>
C
W
(1) (1)
(1)
(5) "( ( (6)
2
( )
)
Figure 30
Th e Jablonski di agram, which explains photophysical processes in molecular systems: (1) light absorption, (2) vib rational rela xation, (3) internal con version (IC), (4) intersystem crossing (ISC), (5) radiative transition, and (6) nonradiative tran sition.
emiss ion, corresponds to the wavelength for the electronic transition from the lowe st So state to the lowest 51 state, the (}-{) absorption. However, in the actual absorption and emission spectra, the peaks of the (}-{) transitions do not coincide with each other because of Stokes ' shift.
1.5.3 Fluorescence of organic molecules in a solid state Fluorescence in organic solids is essentially the same as that from the individual molecules of the solid. This is be cause molecular orbitals assumed for isolated molecules are only weakly perturbed in th e solid state by the presence of weak van der Waals interactions among the m olecules. H owever, on e has to note that drasti c changes of fluore scence can appear in sol id s because of the formation of intermolecular com plexes and due to energy migration and transfer among the m olecules. Intermolecular complexes that are formed in their excited states, called excim ers and exciplexes, giv e ch aracteristic emissions at wavelengths diff erent from those of the component molecules. An excimer is an excited -state complex formed between two sa me m olecules. Ar omatic h ydrocarbons such as anthracen e an d perylene, for example, sho w a cha racteris tic broad, featureless excimer fluorescence. On the other hand, exciplexes are excited -s ta te complexes made of two different molecules. Further, charge-transfer interactions between donor and acceptor molecules sometimes ca use the form ation of CT-complexes, which have broad and redshifted weak fluorescence spectra. Such intermolecular complexes formed in th eir ground states also show changes in their abs orption sp ectra . The ab sorption spectra of molecular aggregates with parallel arrangements are shifted significantly to shorter wavelen gths com pared with tho se of isolated com ponent mol ecules, and their flu orescence spec tra are a lso different. Molecul ar ag grega tes called
55
Chapter one: Fundamentals of luminescence
(1)
o
Ql
o c co
c
(1)
o
.0
(J) (1)
o
o
(J)
.0
~
u:::
+----;--»7
Wavelength
/ 1\
1\
/\ / 1\ I
(0,0)
\1/ \1
\ 1; \11
1\1 Figure 31
Explanation for the re lationshi p between absorption an d emission sp ect ra base d on the energy s tate diagram.
J-aggregates in w hich m olecul es a re arr anged in a head-to-tail fashion, in p articular, show very sharp, reds hifted ab sor pti on an d fluorescen ce. Very ra pid and efficient migration or tran sfer of excitation energy among mol ecul es occurs in a solid state, and the se mechanisms induce drasti c changes in fluorescence spectra. Anthracene crys tals doped with a trace amount of tetracene, for exa mple, never give characteristi c an th racene em ission, but yield the tetracene emission spec tru m instead , although the abso rp tion sp ectrum still looks the sa me as that of anthracene. The excit ed states produ ced in an thracene molecul es by photoexcitati on are efficien tly tran sferred from anthracene to tet racen e mo lecu les. Therefore, the effects of trace am ounts of impurities incorpor ated in organic soli ds s ho uld be carefu lly consid ered. In so me cases, impurities act as effective exciton-quenching cen ters, reducin g the intrinsicall y high fluorescen ce yields of orga nic solids. He re, we make a distinction between en ergy m igrat ion and en ergy transfer. Energy migration impl ies coheren t en ergy tran sfer between like m olecules, w here energy is transferred as an exciton . Therefore, it is the dominant m ech anism in sin gle crystals w here iden tical molecules are regularly aligned . On the other hand, energy transfer
Fundamentals of Phosphors
56
involving dipole-dipole interaction via the near-field electromagnetic interaction, i.e., the Forster mechanism, dominantly occurs in donor-acceptor molecular combinations where the emission spectrum of the donor and the absorption spectrum of the acceptor overlap. Therefore, the Forster mechanism occurs between singlet-singlet energy transfers where spin conservation is maintained. Further, the Dexter mechanism involving direct electron exchange between adjacent molecules dominates in the case of triplet-triplet energy transfer.
1.5.4
Quantum yield offluorescence
The fluorescence "quantum yield" is given as the ratio of the emitted and absorbed photons and is normally determined experimentally through careful photoluminescence measurements using an integrated sphere. A more general definition of quantum yield of fluorescence is the ratio of the radiative transition rate, k, and total (radiative and nonradiative) transition rate, k, + k nr , from a singlet excited state to the ground state. (101)
Here, nonradiative processes include the direct radiation-less transition from 51 to 50 and the I5C from 51 to T 1• In a solid state, energy migration and energy transfer processes also need to be included. The experimentally determined quantum efficiency values of isolated molecules are not always useful for the evaluation of the efficiency of organic solids, because molecular aggregation occurs. For example, coumarins are laser dyes with high quantum yields of -90% in dilute solutions, but they yield weak fluorescence in bulk solid states. Deactivation processes intrinsic to solid states, called concentration-quenching processes, occur in many compounds, although some aromatic hydrocarbons such as anthracene and pyrene show high quantum yields both as single molecules and crystals. The quantum yields of fluorescence are strongly dependent on temperature because thermal nonradiative decay processes are highly dependent on temperature. At temperatures lower than the liquid nitrogen temperature, almost all rotational and vibrational motions of pendant groups are frozen and quantum yields tend to be high.
1.5.5
Organic fluorescent and phosphorescence compounds with high quantum yields
Organic molecules with well-developed n-conjugated systems usually show intense electronic absorption in the ultraviolet to visible wavelength regions due to a transition from the bonding rt' orbital into the antibonding n' orbital (i.e., a transition from 50 to 51 states). The absorption maxima shift to longer wavelengths as the length of the n-conjugated systems increases. Incorporation of heteroatoms such as nitrogen, oxygen, and sulfur within the nconjugated systems usually causes a redshift in the absorption peaks. Attachment of electron-donating groups such as -NH z, -CH3, and -oCH3 or electron-accepting groups such as -CN and -NOz also causes redshifts in the absorption spectra. Organic compounds with strong absorptions in the near ultraviolet to visible regions usually exhibit visible fluorescence. The highest bonding n orbital and the lowest antibonding n' orbital mainly govern the quantum yields of fluorescence, as well as do the emission peak wavelengths. However, it should be noted that nonbonding rt orbitals playa role in determining quantum yields in some cases.? Organic fluorescent molecules can be classified into the following categories.' 1. Aromatic hydrocarbons and their derivatives. This category includes polyphenyl hydrocarbons, hydrocarbons with fused aromatic nuclei, and hydrocarbons with arylethylene and arylacetylene groups
57
Chapter one: Fundamentals of luminescence
Naphthal ene
Pyrene
Perylene
Anthracene
p-Terph enyl
Phenanthlene p-Quarterphenyl
Q9 C=CH-CH=C
frans-Stilbene
db Tetraphenylbutadi ene
Distylylb enzene
Figure 32 Examples of aroma tic hyd rocarb on s with intense fluorescen ce.
2. Compounds with five- and six-me m bered heterocycles 3. Compounds with carbony l gro ups 4. Complexes of metals with or gani c ligands Arom atic hydrocarb ons form one of the mo st importan t groups am ong fluorescent compounds, which emit light in the violet to blue reg ions, and their quantum yields are fairly high even in crys talline sta tes as we ll as in solutions", Introduction of substituents can shift the fluorescence toward longer wavelengths. N aphthalene, an thracene, ph en anthlene, pyrene , perylen e, p-terphenyl, and p-quarterp henyl are typical examples of this class of compou nds (Figure 32). Some polyphen yl hydrocarbons and hyd rocarbons with fused
58
Fundamentals of Phosphors
aromat ic nuclei show poor solubility in conven tional organic solvents, which m ay limit their practica l ap p lica tions. Polyphen yl h ydro carbon s with arylethylene an d ar ylacetylene g roups, such as trans-stilbenes, tetraphenylbutadiene, and d ist ylylb enzen es, sho w ver y similar optical characteristics with polyphenyl hydrocarbons. Syn theses of these m olecule s are relati vel y easy, an d th erefore, these compounds are useful in vario us ap p licatio ns . One of th e m ost p opular classes of flu or escent compou nds comprises derivati ves of fiv e- and six-mem bere d heterocycles. Intense flu orescen ce is observed, simply w he n heterocycles are inc o rpora ted in d ev eloped n-conjugated systems . Almost all the com p ounds of this gro u p sho w this p roperty, both in solu tio n and in th e crys talline sta te, wi th sp ectral ranges sp anning th e vi olet to the red. 2,5-Diphynyloxa zole, 2-phen y-S-(4-biphen yl)-1,3,4oxa d iazole, an d 1,3,5-triphenyl-2-pyrazoline, sh ow n in Figure 32, are typical fluorescent comp o un ds with fiv e-membered heterocycles.
2.5-Diphynyloxazole
4-Methyl -7diethylaminocoumar in
2-Pheny-5-(4-biphenyl)1,3,4-oxadiazole
N H
o
3-Phenylcarbostyryl
1,3,5-Triph enyl-2-pyrazoline
1,8-Naphtholylene-1',2'benzimdazole
4-Amino -N-p henylnaphthalim ide
Figure 33
Exam p les of fluo rescence mo lecules con taini ng het eroatom s.
Chapter one:
59
Fundamentals of luminescence
Fluorescence
o:f
cfj\::a ~ ~
-
,' .
N b \
\
\
Alq3 AI, Ga, Mg complexes 1 _ * 1t n
/,
/,
Phosphorescence
cSfb ~
~~f) CF 3
'" I
~-;, - "!)r- ---N/_"
3
Eu(tta)3phen
PtOEP
Ir(pPYb
Rare-earth complexes
Transit ion metal
Heavy metal ' MLCT
I-f
3n:._n *
Figure 34 Examples of metal complexes demonstrati ng intense photoluminescence.
It is more d ifficult to d escr ibe the flu orescen ce b eh a vior of arom a tic com pound s containing carbonyl groups, becau se a delica te balance exis ts be tw een the IT-IT' tran sit ion of n:-con jugated systems , and th e IT-IT' tran sitions rela ted to the carbon yl grou ps govern the trans ition from singlet excited s ta tes to th e groun d s ta tes . One finds a va riet y of useful compounds with intense fluorescence a mon g carbony l-con taining molecul es, for example, cournar ins. carb ostyryls, naphthalimides, and n aphtholylene benzimd azoles. Typical carbonyl-containing com pounds a re d epicted in Fig u re 33. Metal com pl exes wi th or ganic ligands ar e ano the r im p orta nt class of fluorescent compounds (Figure 34).5 Some of these compou nd s exhibit rather broad fluorescen ce spectra si mi lar to those of orga nic ligands. One su ch com p oun d is (8-h ydroxyquinolino)aluminum, w h ich has been used for organic thin-film electrolum inescen t d evi ces. Metal com p lexes, such as some eu rop ium and terbium com p lexes, that exhib it na rrow band luminescen ce specific to incorp ora ted m et al ions are also kn ow n . Further, he avy metal comp lexes such as Ir, Pt, Ru. and A u com p lexes were recently revealed to h ave intense phosphorescen ce based on th e transition of th e metal-to-ligand charge tran sfer (MLCT) comp lex triplet sta te. Sin ce th e MLCT h as a natu re of mixin g sin glet and tr ip let excited sta tes, radia tive decays a re a llowed , lead in g to in tense phosp horescen ce . In particular, Ir com plexes h ave been w id ely d eveloped in recen t years, and iri di u m tris(phenypyridine). in p articular, shows al most 100% phosphorescence efficiency."
References 1. Pope, M. and Swenberg. C.E., Electronic Processes in Organic Crystals, Oxford University Press, New York, 1982, chap. 1. 2. Krasovitskii, B.M. and Bolotin, B.M., Organic Luminescen t Materials, VCH Publishers, New York, 1988, chap. 7. 3. Krasovitskii, B.M. and Bolotin, B.M., Organic Luminescent Materials, VCH Publishers, New York, 1988, chap. 18. 4. Becker, R.5., Theory and Interpretation of Fluorescence and Phos phorescence, John Wiley & Sons, New York, 1969, chap. 13.
5. Yersin. H., Transition Metal and Rare Earth Compounds: Excited States, Transitions, Interactions I (Topics in Current Chemistry), Springer Verlag. 6. C. Adachi, Baldo, M.A., and Forrest, S.R., Nearly 100% internal quantum efficiency in an organic light-emitting device, f. Appl. Phus., 90, 5048, 2001.
chapter one - section six
Fundamentals of luminescence Yasuaki Masumoto 1.6 Luminescence of low-dimension al systems References
_
_
61 72
1.6 Luminescence of low-dimensional systems Low-d ime nsional syst ems discu ssed in this section are tw o-dimensional (20) sys tems, one-dimensiona l (10) systems, and zer o-d imen sion al (00) systems. 20 systems in clude layered materials and quan tum we lls; 10 systems incl ude linea r chain-like m at erials and quantum wires; and 00 sy stems include sma ll m icrocryst all ites an d quan tu m do ts. Op tical properties of low-dim ensional sys tems are subst antially di fferen t fro m th ose of th reedimension al (3D) systems . The m ost remarkable modification com es from d ifferent d istribution s of energy levels and densities of sta tes origina ting from the spa tial confine ment of electrons and ho les. Different d istribu tions of energy levels in low -dimensional systems arise from th e quantum confine me n t of elec tro ns an d holes. Th e sim ples t mode l for 20 systems is th at of a particle in a bo x with an infinitely deep well po ten tial, as is shown in Figure 35. The wavefunctions and energy levels in th e we ll are known from basic qu an tu m me cha nics and are desc ribe d by : 'V" (z) == (2/ LJ 1/ 2 cos( nns] LJ
E == II
!!.~_ ( rrn )2 2m
Lz
(102)
n == 1, 2, 3, .. .
'
(103)
In type-I semi conductor quantu m wells, bo th electrons an d holes are confined in th e sa me wells. The ene rgy levels for electrons and holes are described by:
E,==E , s
2.' ( rrne )2+ ~ 2 (k2+ e) +-.!2Zm L 2m x
t'
Z
y
(104)
C
61
62
Figure 35
Fundamentals of Phosphors
n
3
n
2
n
1
A p article in a box m ad e of in finitely tall poten tial bar riers.
2 ( n;n" )2+ 2 E +~ "2m L 2m "
Z
.!!"-'(k + e)v 2
x
(105)
h
where me' and m.,' are the effective m asse s of electron and hole, respec tively. If elect ric dipole transit ion s are allo wed from th e va lence band to the conduction band, the optical transition occurs fro m the sta te d escribed by n h, k., and ky to the state described by no, k., and k},. Th erefore, the op tica l tr an siti on tak es place at an energy:
(106) w here 11 is th e red uce d mass given by W1 = m,>' + m h ' - I It is w ell kn own th at th e join t density of states P.3D for the 3D fo r an allowed and di rect transit ion in se micond uc tors is represent ed by:
(107) H ere, E, is the bandgap energy and 11 is the reduced mass as above. The joint d ensities of s tates for 20 , 10, an d OD sys tems are given, respectiv ely, b y the exp ressi ons
P2 0 (E ) = ~ n;1i ~ " L..J a(E- E - Eg ) II
(108)
63
Chapter one: Fundamentals of luminescence (1)
(c)
(a)
(d)
(b)
(2)
E
E
(a)
Er--------
E
(b)
(c)
(d)
Figure 36 (1) Schematic illustrations of (a) 3D, (b) 2D, (c) 1D, and (d) OD systems. (2) Densities of states for (a) 3D, (b) 20 , (c) 10, and (d) 00 systems are shown . 2
(21l) 1/ ' " 1 p\D(E) = - h- LJ ( )1/ 2 IT E -Em -E n -Es
(109)
11/,/1
I
Poo(E) = 2
O(E - E, - Em- E" - Eg )
(110)
l,m ,!"1
where e is a step function and 0 is a delta function. The sum of quantum confinement energies of electrons and holes are represented by E1, Ern' and E", where E J , Em' and En refer to the three directions of spatial confinement. Figure 36 shows schematically the joint densities of sta tes for 3D, 2D, 1D, and OD systems given by Eqs. 107-110. The optical absorption spectrum a(E) is proportional to the joint density of states, if the energy dependence of the optical matrix elem en t and the other slowly varying energy dependence are neglected. As a result, the absorption spectral shapes of 3D, 2D, 1D, and 00 systems are well de scribed by the joint density of states as shown in Figure 36. Although the exciton effect has been ne glected thus far, it dominates the ab sorption spectru m around the bandgap. The exciton is a composite of an electron and a hole due to the Coulomb attraction. As in the hydrogen atom, the Coulomb attraction forms bound levels of the exciton. The lowest-energy bound state is characterized by the effective
Fundamentals of Phosphors
64
Rydberg energ y Ry' and th e effective Bohr radius as' . The lowest exciton sta te is lower than th e unbound continuum sta te by Ry', and its radius is given by as'. In the 3D case, the effe ct ive Rydberg ene rgy and the effective Bohr rad iu s are given by:
(111)
(112)
w h ere £ is th e d ielectric consta n t, m o is th e elec tro n mass, an d Ry = 13.6 eV and as = 52.9 pm are th e Rydberg energy and Bohr radius of the hyd rogen a tom, resp ecti vely. Th e ex citon energy levels are d es cribed by: E - E _ Ry' ~I
-
n;
0 ' )
c
(n = I, 2, 3, . ..)
,
(113)
an d the absorption spectrum is m odified as shown in Figure 37.J In the 20 cas e, th e binding energy of the lowest -en erg y exci ton is enhanced to be 4Ry', because th e exc iton energy levels are describ ed by:
E 11,n!
=E +E _ g
"
Ry' (
1)2'
(m=O, 1, 2, .. .)
(114)
m+ 2
w here 11 is th e qu antum number for ele ctrons an d holes, and m is the quantum number for th eir rel ati ve m otion. Th e w av efunction of a 20 exciton shrin ks in the 20 pl an e and its ra d ius becomes (,.,/3/ 4 ) as'. This means that th e overlap between the electron w avefu nc tion and hole w avefunction is en ha n ced compared with the 3D case. As a res u lt, the osci lla to r strength of a 20 exciton is la rger than that of a 3D exc iton . Th e oscillat or strength of a 20 n th excit on per unit laye r fo2D is written as: a'/, 30 B "
(115)
where fn 3D is th e oscilla tor strength of th e n th 3D exciton. The enhan cement of th e exci ton binding energy and th e oscillator stre ngth lead to th e stability of th e 20 exciton a t roo m temperature. Figure 38 shows an exam p le of th e observ a tion of a 20 exc iton in th e op tical absorp tion spectrum of GaAs quantum wells a t room temperature.' The binding energy and th e oscilla to r streng th of a n exciton increase w ith a d ecrease in size an d dimen sion. ' Figure 39 s hows th e in crease of the binding energy of 20,10, and 00 exci to ns with th e d ecrease in size and dimensional ity. Here, th e 10 exciton is con fin ed in a sq u a re parall elepiped and the 00 exciton is confin ed in a cu be, w he re the side-leng th of the squ a re or the cu be is L. Sin ce th e radiative lifetime is in ver sel y proportion al to the oscillator strength, it decreases with a d ecre ase in size and dimension al ity. Sho r teni ng of th e radia tive lifet im e of th e exci ton wi th decreasing size is observed in GaAs qua n tu m wells."
Chapter one:
65
Fundamentals of luminescence
(a)
IZ
3D EXCITON
W
o
u:: W o
n == 1
LL
n == 2
o z
o
WITH EXCITON EFFECT
n==3
i=
"'"
0-
cr:
o (f)
--- - - - -
/'
.: "- WITHOUT EXCITON EFFECT
[()
<(
/
/
J "
J
E
E -Ry* n
== 1
9
(b)
20 EXCITON
rz w G
20 n ==2
u:LL W
WITH EXCITON EFFECT
n"~
o o z
o
i=
1------
0..
a:
oen
-,
co
WITHOUT EXCITON EFFECT
J
\.
)
E Figure 37 Absorp tion s pectra for (a) 3D or (b) 20 excitons. Ab ov e the energy gap Eg, the abs orption coefficient is enha nced from its va lue by the Sommerfeld factor, as a resu lt of the Coulomb interaction between electrons and hold s. (From Weisbuch, C. and Vin ter, B., Qua ntum Sem iconductor Structure s, Academic Press, Boston , 1991. With permi ssion.)
Discussion thus far has focused on the optical properties of low-dimensional sy stems, with special emphasis on se mico n d uctor quan tum wells. However, in phosphor s, th e m ore important low-dimensional systems are sm all semiconducto r micro crystallites and quantum dots. Man y kind s of nan om eter-size microcry stallites made b y various means beh ave as quantum dots. For example, mi crocrystallites of II-VI and I-VII comp ounds can be chemically grow n in polymers, solutions, and zeo lites . Poro us Si. m ade by means of electroche mical etching, is regarded as an ens emble of quantum do ts and quan tu m wires. III-V semico nd uc tors GaAs and GaIn As mic rocrystallites can be ep itaxially grow n on the oriented GaA s crys tal surface. The above-men tion ed, n an ome ter-size se m icon d uc tor m icrocr yst allites ca n be regarded as quantum dots in the sen se that th ey show the qu antum size effect. Tha t is with a d ecrease in size, the absorption bands show a blue-shift due to thi s effect, because
66
Fundamentals of Phosphors
X 103
300 K
20 GaAs BULK CRYSTAL
n=2
',I \
\ \ \ \ \ \
10
\
....1 I I
GaAs-A1As QUANTUM WELLS L ; = 8.3nm L b = 9.3nm
o"--
.l......-
--::>~
I I
I I -'---
800
700
600
I
.....--
900
WAVELENGTH J (nm)
Figu re 38 Absorption spectra of GaAs-AJAs quantum well struc tures and a bulk GaAs crys tal at 300K . (Fro m Ishib ashi , T , Tarucha, S., and O kam oto, H., lnsi. Phys. Conf. Ser. No . 63, 1982, chap . 12, 587 . With perrniss ion .)
th e spa tial con finem en t of elec tro ns, holes, and excitons increases the kin etic energy of th ese particles. Simultaneously, the same spatia l confi nement increases the Co ulom b in teractio n betw een electrons and holes. The quantum confine me n t effect can be classified into thr ee ca tegories'': th e weak co nfineme n t, the inter med ia te confinemen t, an d the strong confinemen t reg imes, d ep end ing on the relat ive size of the ra d ius of m icrocrystallites R compared to an electron a,'. a h ole all" an d an exci ton Bohr radius a B' , resp ectively. Here, the m icrocrystallites are ass umed to be sp heres, and a,' an d ah' are defined, respectively, by: .
ae
I/ E = -
' -2 '
me ,.
and
(116)
Chapter one:
Fundamentals of luminescence
67
--->- 18.00 -l<
0: .........-
>-
o
0: W
Z
W
o
12.00
z 0
z CO
z
0 f-
6.00
o X
W U) r-
2D 0.00 0.00
1.00
2.00
L/a B* Figure 39 Binding energies of Is excitons in a plane L thick, a square parallelepiped, and a cube of a side L. (From Matsuura, M. and Kamizato, T., S urf. Sci., 174, 183, 1986. With permission.) The exciton Boh r radius a B' is given by:
(117)
and an inequality a;'. a h ' < a B' holds.
• Strong confinement (R « a;, a.,' < aB*). Th e in d iv id ua l m oti on of electrons an d holes is quantized and the Coulomb in teraction en ergy is much smaller th an the qu antized kinetic en er gy. Nanometer-size C aA s. CdS, CdSe, and CdTe microcrystallites ar e good exam p les of the s trong con fine m en t regim e. Th e groun d -sta te energy is: 2n 2
2
E(R) = E + h _ 1.786e g 21-lR 2 eR
_
0.248R '
Y
(118)
where the secon d term is the kinetic energy of electro ns and h oles, th e thi rd term is th e Coulomb energy, and th e last term is the correlat ion en ergy.
• Intermediate confinement (a h ' < R < a e' ) . In th is case, the elec tron m oti on is quantized, while the hole is bound to the electron by the ir Coulombic attraction. Many Il-VI microcrystallites belong to the in termedia te confinemen t regim e.
68
Fundamentals of Phosphors • Weak conf inement (R »
as' > a,', a,'). In this cas e, the center-of-mass motion of excitons is quantized. Nanometer-size CuCl mi crocrystallites are typical examples of th e w eak confinement regime; th e gro un d -state en ergy is written as: (119)
where M = m ,' + m hois th e translational m ass of the exciton. Typical experimental d at a for three categories are shown in Figure 40.6 CdS, CuBr, and CuCl mi crocrystallites belong to strong, intermediate, and weak confinement regimes, res pectively. With a decrease in mi crocrystallite size, the continu ou s band ch anges into a se ries of di screte levels in CdS, alth ough th e levels ar e broadened becau se of the size di stribution. In the case of Cu Cl, the excit on absor p tion b ands show blue-shifts with a decrease in siz e. The lumines cence of semiconductor microcr ystallites not only depend s on the mi crocrys ta llites themselve s, but a lso on their s urfaces a nd th eir surroundings since the sur face:v olume ratio in these syste ms is large. Th e luminescen ce sp ectrum then depends on the preparation co nd itions of mi cro crystallites . Thus it is th at some samples sh ow donorac cep tor pair re combination, but other samples do not; in others, the edge luminescence a t low temperature consists of exciton and bound exciton lum inescence. The exciton luminescence spec trum of m any sa m p les sh ows Stokes shift fro m the ab sorption spectru m, indicating th e pres ence of localized e xcitons. Typical examples of the luminescence spectra of CdSe mi crocr ystall ite s an d CuCI m icrocryst all ites are show n in Figures 41 and 42.7,8 Impurities or d efects in insulating crystals oft en dominat e their luminescen ce spectra; thi s is also th e case w ith semiconductor microcrystallites, but ad dition al effect s occur in the latter. Nanometer-size semiconductor microcrystall ites can be composed of as few as lOL 106 a toms; if the con centration of cen ters is less than ppm, considerabl e amoun ts of th e microcrysta llites are free from impurities or defects. Thi s conjecture is ve ri fied in AgBr microcrystallites, w h ich are indirect tr an sition m at erials." Figure 43 sho w s luminescen ce spectra of AgBr microcryst allites with averag e radii of 11.9, 9.4, 6.8, and 4.2 nm. The h igh er-energy band observ ed at 2.7 eV is the in direct exciton luminescence, and the lower-en ergy band observ ed at 2.5 eV is the bound exciton luminescen ce of iodine im p ur ities . In contrast to AgBr bulk crys ta ls, the indire ct exci ton luminescen ce is strong comp ared w ith the bound exciton luminescence a t iodine impuritie s . The rati o of the ind irect exciton lumin escence to th e bound exciton lu minescence a t iodine impurit ies increases w ith the decrease in size of AgBr microcrystallites. This increase in ratio shows that the number of impurity-free mi crocryst all ites increases with the decrease in size. Simultaneously, th e dec ay of the indirect exc iton luminescence ap p roaches single exp onential d ecay approxim ating the rad iative lifetime of the free in d irec t exciton. Th e blue-shift of th e indirect excito n luminescen ce shown in Figure 43 is due to th e exciton quantum confinement effect, as di scussed previou sly. Nano m eter-size se mi con d ucto r microcrystallites can be us ed as a laser medium.'? Figure 44 shows th e lasing spectrum of CuCI microcr ystallites. When the micro cryst allites embedded in a N aCl cry sta l are pl aced in a cavi ty and excited by a nitrogen laser, lasing occurs a t a certain threshold . Th e em iss ion sp ec tru m bel ow th e thresh old , sho w n in Figure 44, arises from ex citon ic molecule (biexciton ) luminescen ce. Above th e th reshold, th e broad excitonic m olecu le em ission band is con ver ted to a sharp emission spe ctru m ha v ing a maximum p eak at 391.4 nm. In this cas e, the lasing spectrum is composed of a few longitudinal m odes of the laser cav ity consi sting of mi rro rs se pa ra ted by 0.07 run . The optical gain of th e CuCl m icrocryst allites com pa red with th at in a bulk Cu CI sa mple is found to be much large r. The h igh optical ga in of CuCI m icrocrystallite s comes from th e s pa tia l confinement of exc itons, resu ltin g in th e enhanced formation efficiency of excit onic molecules.
Chapter one:
69
Fundamentals of luminescence
~
Z3
" ,,
;-
Z W
" f 1 t:
0
J
if)
"" ""
I
-J
"", ,
I I
< U
CuCl
f f
i= CL
a
2 3.4
~ z
:
.....
Z3 ....
W
I
<
/
U
i=
CL
a
1
.
f
/2
/3
I
I
/
3.1
0 -J
o<
i=
CL
a
,/
CuBr
I
-J
Z W
- .... .... .' ..;. ........ /
I
I
0
~
.
<,
"3
I
if)
(/)
,
~
J ,/
I
3.2
3.3
1d 1p 15
/'
/
CdS I
,, I
:
/3
.-
2.5
I
3.0
/
3.5
4,0
PHOTON ENERGY (eV) Figu re 40 Absorp tion spectra at 4 .2K for CuO, CuBr, and CdS m icrocrys tallites . For Cu C!, the average radius R = 31 nm (1), 2.9 nm (2), and 2.0 nm (3); for CuBr, R = 24 n m (1),3.6 nm (2), and 2.3nm (3); for CdS, R = 33 nm (1),2.3 nm (2), 1.5 nm (3), and 1.2 nm (4). (From Ekimov, A.L, Phyica Scripta T, 39, 217, 1991. With per mission.)
After the initial report of visible p hotoluminescence from po rous Si," m uch effort has been devoted to clarify the mec h anism of the photolum in escence. Figure 45 sh ows a typ ical example of a luminescence sp ectrum from porous Si. As the first appro xim ation , porous Simade by electrochemical etch ing of Si wafers can be treated as an ensemble of q ua ntum dots and quantu m wires. However, rea l porous Si is a m uch mor e complica ted system, consisting of amorpho us Si, SiO z, Si-oxygen-hy drogen compounds, Si microcrys tallites, and Si wires. This complexi ty obscures the qua ntu m size effect with other effects, and the physical orig in of the visible luminescence from po rous Si re mains a p uzzle. Depend ing on the sample preparation me thod, porous Si shows red, green, or blue luminescence.
70
Fundamentals of Phosphors
15 K
8 ill
o z
A
f@ .5
o
Luminescence
Absorption
(f)
co
«
o 450
4D0
550
500
600
WAVELENGTH (nm) Figure 41 Absorption and luminescence spectra of wurtzite CdSe microcrystallites (R = 1.6 nm). (From Bawendi, M.G., Wilson, wt.. Rothberg, L., et a!., Phys. Rev. Leii., 65, 1623, 1990. With permission.)
~ Cf) z
w
fZ
w
0
z
w
0
Cf)
w
z
~
::>
---l
CuCI in NaCI
Z3 bulk
JA
1
JJS=3 ~
3.2
4
3.3
PHOTON ENERGY
(eV)
Figure 42 Exciton luminescence spectra of CuO microcrystalJites at 77K The average radius R is 5.7 nm (1),4.9 nrn (2), 3.4 nm (3), 2.7 nrn (4), and 2.2 nm (5). The energy of Z3 exciton for bulk CuCl crystals at 77K is indicated by a vertical bar. (From Itoh, T., Iwabuchi, Y, and Kataoka, M., Phys. Stat. Solidi (b), 145,567, 1988. With permission.)
Chapter one: Fundamentals of lumin escence
71
.......
til .-
2K
c:: :::l
..c >C1:I
R=4.2nm
'-'
>..... 0 U)
Z LU
..... Z
0
LU
U
Z LU
9.4
o
U) LU
0
-z
~
...J
11.9 0
2.0
2.5 ENERGY
PHOTON
3.0 (eV)
Figure 43 Photolum inescen ce spectra of AgBr microcryst allites at 2K. Th e aver age rad ius R of microcrystalli tes is 11.9, 9.4, 6.8, and 4.2 run. The lum in escence sp ectra are n ormalized by their respective peak intensities. The 2.7-eV band is indirect exciton luminescence , and the 2.5-eV ba nd is boun d exciton luminescence at iodine impur ities. (From Ma surnoto, Y, Kaw amura, T, Ohze ki. T, and Urab e, S., Phys. Reo., B446, 1827, 1992. With pe rmission.)
~
!:: l/l z ILl
4
17K r-r-->
-
....
Z
Z2
o
l/l l/l
lmm
~ ILl
388
390
394
WAVELENGTH Figure 44 Emission spe ctra of the laser device made of CuCI mi crocryst allites at 77K bel ow and above the lasing th resh old . The thre sh old I'll is about 2.1 MW crrr". Th e solid line sho ws the spectrum under the excitati on of 1.08 [ th' The dash ed line shows th e spectrum under the exci tation of 0.86 [ th o (From Matsumoto, Y, Kawamura, T , an d Era, K., Appl. Phys. Lett., 62, 225, 1993. With p ermi ssion .)
72
Fundamentals of Phosphors PHOTON 1.4
ENERGY (eV) 1.6
1.B
20
~
U5 Z
ill
IZ W
o Z
ill
o
(f)
ill
Z
2
::> -.J
a a
II
0...
1.0
0.9
08
0.7
WAVELENGTH (jJm) Figure 45 Room-temperature photoluminescence from the p orous Si. Anodization time s are indicated. (From Canham , L.T., Appl. Phys. Leii., 57, 1046, 1990. With permission.)
The photoluminescence quantum efficiency of porous Si exhibiting red luminescence is as high as 35%, but its electro lum in escen ce quantum efficien cy is 0.2% Light-emitting diodes made of porou s Si have also been demonstrated but th e quantum efficiency is too low for practical application. If the electroluminescence quantum efficiency is improved substantially, porous Si will be used in light-emitting de vices because Si is the dominant material in the semiconductor industry. Note: An updated discussion on the size affect on radiative properties alluded in Reference 10 below appears elsew here."
References 1. Weisbuch, C. an d Vin ter, B., Quantum Semiconductor Structures, Acad emic Press, Boston, 1991. 2. Ishibashi, T , Taru cha, S., and Okamoto, H., Int. Symp. GaAs and Related Compounds, Oiso, 1981, Inst. Ph ys. Conf . Ser. No. 63, 1982, chap. 12,587. 3. Matsuura, M. and Kamiza to, T, Surf Sci., 174, 183, 1986; Masumoto , Y and Matsuura, M., Solid State Phys. (Kotai Butsuri), 21, 493, 1986 (in Japanese) . 4. Feldmann, L Peter, G., Gobel, E.O., Dawson, P., Moore, K., Foxon . C, and Elliott, R,J., Phys. Rev. Lett., 59, 2337, 1987. 5. Yoffe, A.D ., Adv. Phys., 42, 173, 1993. 6. Ekimov, A.I., Phyica Scripta T, 39, 217, 1991. 7. Bawendi, M.e. , Wilson, W.L., Rothberg, L., Carroll, P.J., Jedju, TM ., Steiger wald, M.L., and Brus, L.E., Phys. Rev. Lett., 65, 1623, 1990. 8. Itoh, T, Iwabuch i, Y, and Kat aok a, M., Phys. Stat. Solidi (b), 145, 567, 1988. 9. Matsumoto, Y, Kawamura, T , Oh zeki , T , and Urabe, S., Phys. [<.ev., B446, 1827, 1992. 10. Masumoto, Y, Kaw amura, T , and Era , K., App/. Phys. Lett., 62, 225, 1993. 11. Canham, L.T , App/. Phys. Lett., 57, 1046, 1990. 12. Properties of Porous Silicon , Can ha m, L., ed., The Institute of Electrical Engineers, 2005. 13. Practical Applications of Phosphors, Yen, W.M., Shionoya, S., and Yamamoto, H., Eds ., CRC Press, Boca Rat on , 2006.
chapter one - section seven
Fundamentals of luminescence Eiichiro Naka zauia
Contents 1.7 Transien t characteri stics of lu m inescence 1.7.1 Decay of lu minescen ce 1.7.1.1 Decay of flu orescen ce 1.7.1.2 Quasistab le s ta te a nd phosphorescen ce 1.7.1.3 Traps and phosp horescence 1.7.2 Thermoluminescence 1.7.3 Pho tos timu lation an d photoquenching References '"
73 73 74 76 77 80 85 87
1.7 Transient characteristics of luminescence This sec tion focuses on tran sien t lu mi nescent p henomena, tha t is, tim e-depen den t emission proc esses such as luminescence after-glow (phosphorescence) , th erm all y s timu la ted emission (thermal glow ), phot o (in frar ed)-stimulated emission, an d photoqu ench in g . All of these p henome na are rela ted to a guasistable state in a luminescent center or an elec tro n or hole tra p.
1.7.1
Decay of luminescence
Light emission that persists after th e cessation of excita tion is called after-glow. Following the terminology born in the old d ays, luminescence is divided in to fluo rescence and phosphorescence according to th e dura tion time of the af ter-g low. Th e length of the du ration time requi red to dis tinguish the tw o is n ot clea rly defi ned. In lu min escen ce phen omen a in inorgani c materials, the after-glow that can be perceived by the h uman eye, na me ly tha t persis ting for longer than 0.1 s after cessa tion of excita tion, is us u ally called phosphorescence. Fluoresce nce implies light emission du rin g excita tion. Therefore, fluorescence is the p rocess in which the em ission dec a y is rul ed by th e life time «10 ms) of the em itting s ta te of a lumin escen ce cen ter, while th e phosp horescenc e pro cess is ruled by a quasis table sta te of a center or a trap . In organ ic m olecul es, flu orescence and phosphorescen ce ar e dis tinguished by a q uite differen t definition. Th e tw o are disti n guish ed by wheth er th e transition to em it light is
73
Fundamentals of Phosphors
74
allow ed or forb idden by sp in selection rules . Light em ission due to an allowed transition is called fluorescence, wh ile that due to a forb idden transition, usually showing a long afterglow, is called phosphorescence (see 1.5).
1.7.1.1 Decay of fluorescence The d eca y process of th e luminescence intensity I(t) after th e termination of excitation at t = 0 is gen erally represented b y an exponential function of the elapsed tim e after the excita tion. (120) where 1" is the d ecay tim e constan t of the emission. It should be noted that the emiss ion d ecay curve of nonlocal ized centers, donor-acceptor pairs for exa mple, is not alw ays represented in the exp onen tial form of Eq. 120. (See 1.4.) If on e denotes the number of excited luminescenc e centers in a unit volume by n' , and th e radiative and nonradiative transition probabilities by WR and W NR, resp ectivel y, then the rate equa tion for n' is: dn'
-
dt
= -
(
) ,
WR + WNR n
(121)
and the so lu tion of th is equation is: (122) H ere, n~ is the value at t = 0, that is, a t the end-poin t of excit ati on or, in oth er words, at the star t point of th e after-glow. Therefore, the lifetime of the center, which corresp onds to the elapsed tim e for n' to be d ecreased by the factor of e-1 of n~ is (W R + W,W:)-l. Since the em ission int ensity is proportional to n' , the de cay time of the aft er-glow in Eq. 120 is equa l to th e lifetime of the cen ter: I
(123) and the luminescence efficiency of the cen ter is given by:
(124)
The radiative transition p robability WR of an emitting s ta te is the s um ma tion of the spo n ta neo us emiss ion p roba bility A "I""" from the state m to all th e final states n, (see Eq. 29)
W [{ --
I
n
A m~ 11 --
I
-'t
I /111 1
(125)
75
Chapter one: Fundamentals of luminescence
I
·--f t1E
~;:7"".- - - - - - _1. (b)
(a)
Figure 46 Con figura tional coord ina te mod els of nonrad iati ve relaxa tion p rocesses: therma l activation typ e (a), and multi p hon on typ e (b).
wh ere "t"", is given b y Eqs. 32 or 35. Th e ratio of th e transition probab ili ty to a p articular final s ta te n to W R, Atn....,,/WR, is call ed th e branching ratio. Wh ile the nonrad iat ive tr ansition probab ili ty W N1\ is ge n era lly ruled b y thermal rel axation p roc esses (i.e., the emission o f energy into lattice vi bra tions), it is also increased b y the effect of resonan t ene rg y transfer bet w een optical ce n ters. (See 1.8 .1.) The thermal rela xati on in a lu mi nescence center ca n be divided in to tw o types of mech an isms as shown b y the two configurational coo rd in a te diagrams (a) and (b) in Figure 46. In the fir s t ty pe (a), the center is thermally ac tiv a ted from poi n t A, the p oint of the low est energy on th e exite d s ta te 1I, to th e crossin g p o int C where th e electro nic s ta tes of the excited and gro u nd s ta tes a re intermi xed, and th en thermall y released from C to B on th e grou nd s tate 1. The energ y I:: n ece ssa ry to exci te th e center fro m A to C is ca lle d the thermal activation energy. The p rob ab ility that the cen te r will m a ke th e transition from state II to state I b y th ermal activation via point C is ge n erally gi ven b y:
a = sex p (;; )
(126)
Therefore , the n onrad iative transition probability by th ermal ac tivatio n is gi ven by:
(127)
wher e k is the Bolt zmann constant an d s is the frequen cy factor. Th is typ e of nonrad iat ive transition is strongly depen d en t on tem perature, resu lt in g in therm al q uen ch ing, that is, the decrease of emission efficie ncy an d sh orten ing of the emission decay time a t hi gh temperature (see Eqs . 123 and 124). An exam p le of th e th ermal quenc h ing effect is shown in Figure 47 for Y20ZS:Yb3+.1 The emission from the ch arge-transfer s ta te (CTS) of Yb3+ ions a t 530 nm is s tro ngly red uced by thermal quench in g a t high temperature. The 4f emission under CT S exci ta tio n (313 nrn), how ever, is in creased at high temperature due to th e increased amou n t of exci ta tion tran sfer from th e CTS. Th e Figu re also sh ows that, as exp ecte d, th e em ission fro m the
76
Funda mentals of Phosphors
30 00
:;;
-:3
10
>-
t::
(/)
z
w
3
f~
z
0
(/) (/)
::E
W
o : 4f excite (918 n01) 0.3
-
: theo ry
100
200
300
TEM P ERATURE [ K] Figu re 47
Tempera ture dependence of two types of the emission in YZ02S :Yb3+, from the CTS (charge-transfer s ta te) and from a 4f-emitting level of Yb3+ ions . (From Nakazawa . E., f. Luminesc., 18/19,272,1979. With pe rmission.)
4flevel at 930 nm is n ot so th ermall y quenched un d er direct 4f excit ati on in to the emitting level (918 nm) . The secon d type of nonradia tive transiti on is a m ul tip honon process shown in Fig ur e 46b. Th is type is often observed in th e relaxa tion between the 4f exci ted levels of rare-earth ions, w h ere no cross-poin t exis ts between curves I an d II in the config ura tion coordina te diagram because of the si mi lar ity of the electronic sta tes. The transition betw een states I and II occ urs a t point A, where an energy gap ~E exists betw een the sta tes: namely, the tran sition fro m the p ure electronic s ta te of II to th e electron -p h onon-coupled s tate of I wi th 11 phonons tak es place at A, which is follow ed by the ins tan tane ous trans fer to point C and relaxation to B. Th e nonradiative transition probability is, therefore, dependen t on ~ E or 11, th e n u mber of phonons necessary to fill th e energy ga p, since ~E = n w/" where wI' is th e largest phonon energy. The nonradi ati ve multi phonon tran sition probabili ty is then given by.'
WNR (~E) = WNR (0) (, -l l.c\ [
(128)
w here a. depends on th e character of th e phonon (la ttice vibration). Sin ce the process is m ain ly due to the sp ontaneous emission of phonons, th e tempera tu re dependence of the probab ilit y is small. An experim ental resul t- showi n g th e applicabili ty of Eq. 128 is shown in Figure 48.
1.7.1.2 Quasistable state and phosphorescence If one of the exci ted s ta tes of a lum inescent cen ter is a quasis tab le state (i.e., an excited sta te wi th very long lifetim e), a percen tage of th e cen ters wi ll be st abilized in th at state d uring exci tation. After exci ta tion has ceas ed, af ter-g low is caus ed by the th ermal ac tivation of the state. This situation is illust rated using th e configura tion al coordinate diagra m in Figure 49, where state III is a quasistable stat e and s ta te II is an emi tting state wi th a radiative transition probabilit y WR. The cen ter, once stabilized a t A', transfers from sta te HI to sta te II by thermal acti vati on via point C. The probabili ty of this activation, all/_d /, is given by Eq, 126. Then, if WR ~ a lt/ -+ /I' the decay time constant of the emi ssion beco mes
Chapter one: Fundamentals of luminescence
77
107 0 0 r--I
.....
106
Vl
2
~
.....(l)
co
2
105
P 3/2
t:: 0
Vl Vl
E
6
H9/2
~
.-
•
d
I
A
Neodymium Europium Holmium Erbium Thulium
(3 P 1 ,3 D3) 4 F 9/ 2
0
104
5 D1
(l)
c
0 t:: 0
103
...c:
.-.....
0..
........
='
:E
102
~
:i';
~
YAI0 3
10
77K
1 1 000 LlE
2000
3 000
5 000
4 000
Energy gap to next - Io w e r level
[em-
1]
Figure 48 Energy gap law of nonrad iative relaxa tion d ue to multip honon processes. (From Weber, M.J., Phys . Rev., 88,54, 1993. With permission .)
almost equ al to l/a iIJ-. J1 , that is, the lifetime of the quasist able s ta te. The decay curve of the after-g low is rep resented by an exponen tial func tion th at is simila r to Eq. 120, and is strong ly tem perature d ep end ent. The decay tim e cons tan t of an emittin g cen ter w ith quasis table sta tes is not usually longer than a seco nd.
1.7.1 .3 Traps and phosphorescence Excited electrons and holes in the conduction and va lence band s of a phosp hor can often be cap tured by impu rity centers or crys tal d efects befo re they are cap tured b y an emitting center. When the probabili ty for the elec tro n (ho le) cap tured by an impuri ty or d efect cen ter to recombine wi th a hole (electro n) or to be reac tiva ted in to th e cond uction ba nd (valence band ) is ne gligibly small, the cen ter or defect is called a trap. The electrons (ho les) cap ture d by tra ps m ay caus e phosphorescence (i.e., long af terglow) w hen they are therm all y reactivated into the cond uc tio n ba nd (va lence band) and then rad iatively recom bined at an emi tting cen ter. Th e d ecay tim e of phosph orescence
78
Fundamentals of Phosphors
Figur e 49 Configurat ion al coord ina te mod el of the lu minescence after-glow (phosp horescence) via a qu asistabl e state.
•
I
0
CONDUCTION BAND
I
TRAP
RECOMBINATION
CENTER
////////////lll/lllill/i/ll// VALENCE BAND Figure 50
Luminescence after-g low p rocess via a trap in an en ergy ban d scheme.
due to traps can be as long as several hours and is oft en accompa nied by p hotoconductive phen omena. Th e dec ay curv e of the after-glow due to traps is not generaJly rep resen ted by a simp le expone ntial fu nction. The form of th e curve is d ependent on the concen tra tion of the traps an d on th e electron cap ture cros s-sec tions of the trap an d the emitting center. Furthermore, it also d epends on th e excitation inten sit y level. Whil e severa l kinds of traps usuaJly exist in p ractical phosp ho rs, on ly one kind of electron trap is presumed to exis t in the simple model shown in Figure 50. Let N be the trap concen tra tion, and I1c and n l th e n um ber of electrons per unit vo lume in the cond uc tion band an d trap sta tes, respectively. Th e num ber of h oles d enoted by p is eq ual to lie + 11/. The rate equ ation rep res en ting th e d ecaying processes of the conce n tra tion of elec trons and holes after the ter m in at ion of excita tion is:
79
Chapter one: Fundamentals of luminescence
-dn, =
dt
- an + b( N -) n n ' I e (129)
dp
dt = - rpne where a is th e p rob ability per second for a trapped electron to be th ermally excited into the cond uc tion ba nd and is given by the sa me form as Eq. 126 with the density of st at es in the cond uction band included in s. Th e prob abilities that a free electron in the conduction band w ill be cap tur ed by a trap or to rec om bine with a h ole are given by band r, respectively. It is sup posed that the number of the electrons li e in th e cond uction band in the after -glow process is so small that p = n, and dp/ dt = dn .Ldt . Th en , the abo ve tw o equa tions give:
dn, _ dt -
-an;
(130)
n, + (b/r)(N - n,)
This eq ua tion can be sol ved ana ly tically for two case s: b ~ rand b = 1'. First, the case of b ~ 1', w hich presumes that th e electro ns once released from traps are not retrap ped in the af ter-glow p rocess. Eq. 130 th en simplifies to: dn ,
-
= - an '
dt
Since the em ission intensity is given by 1(t) above, then
ex;
(131)
dp/dt, an d dp/ dt
=
dnJ d! as me n tioned
J(t) = 1 exp (- at) 0
(132)
This simple exponential d ecay of af ter-glow is the sa me as the one due to th e qu asist abl e state mentioned previou sly and is called a first-ord er or mono molecu lar rea ction typ e in the field of chemical reaction kin etics. In the case b = 1', which mean s that the traps and emitting centers have nearl y eq ua l capturing cross-sections, Eq. 130 can be simplified to: dn ,
-
di
a
2
N
I
=- - n
(133)
and then the number of tra pped elec tro ns per un it vo lu me is g iven by:
(134)
Approxim ating J(t) oc dn.]dt Cl;s before, the d ecay curve of the after-glow is ob taine d as: _
J(t)-
Jo
(1 + yt)
2 '
(135)
80
Fundamentals of Phosphors
ZnS
-200
Cu
-100
0
200
TEMPERATURE Figure 51 Glo w curve s of ZnS :Cu p hosp hors co-activat ed with AI, Sc, Ga, and In. (From Hoogenstraa ten, w., Philips Res. Rept., 13, 515, 1958. With p ermi ssion.)
Thi s form is called the second -order or bimolecular rea ction typ e, where the d ecay curve is cha ng ed by excita tion intensity as well as b y temperature. Wh ile the treati se mentioned above is a simple model presuming a single kind of trap , the real phosphors may have se ver al kinds of trap s of different d epths. In many real cases, therefore, the af te r-glow decay curve is not represented simply by a monomolecul ar or bimolecular rea ction cur ve. It often fits into the following eq ua tion.
I (t)
-
I 0
(l+yt)"
(136)
where 11 is around 0.5-2 . If t ~ 1/y, this d ecay curve can be approxim ated by I (t ) cc t:". Th e d ecay time cons tan t of an aft er-glow is therefor e denoted either by the 1/ e decay time or the 10% de cay time.
1.7.2
Thermoluminescence
When a phosphor w ith deep trap s is excited for a w hi le at rather low temperatures and then heated, it shows an increased after-glow called th ermally s tim ula ted luminescence due to the recombinat ion of electrons thermally reactivated from the d eep traps. Th is emission is also called thermoluminescence, and th e temperature dependen ce of the emissio n intensity is ca lled the glow curve, which is a good me ans to measure the depth (i.e., the activation energy of traps). Figure 51 shows the glow curves of ZnS:C u phosphors with va rious co-activat or s.'
Chapter one:
Fundamentals of luminescence
81
The measuremen t of a glow curve of a ph osphor sampl e proceed s as follows. 1. The sam p le is coo led to a low tem per ature (liq u id n itrogen is often used as coo lan t). 2. The sa m p le is excited by UV light until the trap s are filled w ith electro ns or h oles. 3. The excitation is terminated, and the temperature of the sa mp le is raised a t a cons tan t rate, dT/ dt "" ~, while the intensity is recor d ed. 4. The temperature de pe n dence of flu orescence is th en measured und er a constan t UV exci ta tion, which is used to calibra te the effect of temperature quench in g on the therm oluminescen ce int en sity. Th e glow curve thus obtained is ana lyzed w ith th e following theory. Ass um e th at (1) a sing le kind of trap exists; (2) th e deca y of after-glow is of the first -ord e r type g ive n by Eq. 132, an d (3) the p rob ability for the tr apped ele ctrons to be thermall y release d into the con d uc tion ban d is give n by Eq . 126. Sinc e the ret rappin g of the released electro ns is neglected in the firs t-order kin eti cs, the change in th e number of trapp ed elec trons is:
dn dt
- ' = - ntsexp(-£/ kT )
(137)
Integrated from a tem perature To to T wi th the rel ati on dT / dt = number of residual elec tro ns in th e traps at T as:
~,
thi s eq ua tion giv es the
(138)
where n10 is the n umber of th e trap p ed electrons at th e initial temper ature To. Therefore, the emission inten sity a t T, ap proxi ma ted by 1 ex; dn.] dt as menti on ed in the previou s section, is gi ven by :
1(T) ex;
n /05
exp(_~F~) exp( -STs exp(~) U) kT 70
d:)
(139)
I-'
Based on this equa tion, the follow ing techniqu es h a ve been proposed for ob ta ining the trap depth (activa tion en ergy e) from a glow curve. (a) In the initi al rising p art of th e glow p eak on the low -tem p erature side w here the number of trapped electrons is n early constant, Eq. 139 is appro xim at ed by -£
1(T) ex; s exp -
(140)
kT
Then the slope of the Arrhenius p lo t (l/ T vs. In l) of the curve in this reg ion gives the trap depth s. In fac t, however, it is not easy to d et ermine the d epth wi th thi s meth od becau se of the un certainty in fixin g the initial rising p ortion. (b) Let th e pea k posi tion of a glow cu rv e be T,II' Then, the foll ow ing equ a tion d erived from dI /dT = 0 should be va lid .
l<£ - s ex ( - £ - I-'-
kT1112
-
P kTIII
J
(141)
82
Fundamentals of Phosphors Table 1 ~/s
K [K ieV]
[K]
10-1 10- 5 10-6 10- 7 10-8 10-9 10- 10 io- » 10- 12 10-13 10- 14 10- 15
833
35 28 22
725 642 577
17 13
524 480 441 408
10 7 6 6 5
379
353 331 312
5 4
From Curie, D., Lum inescence in Crystal s, John Wiley & Sons, 1963, cha p. VI. With permission.
If the frequency factor s is obtained in some manner, E can be estimated by this relation from the observed val ue of TII/ ' Note tha t th e temperature rise rate ~ should be kept constant throughout the measurement for this analysis. Randall and Wilkins' performed a numerical calcu lation based on this theory and obtained the following equation, which approxima tes the trap depth £ with 1% error.
e = ~II
-
To(~/s)
(142)
K(~/s)
Here, To(~/s) and K(~ /s) are the parameters determined by ~ / s as listed in Table l. For ZnS :Cu, s = 109 S-I is assumed and the following estimations have been proposed for various values of ~ .5
EleVI = T,,, 1500 EleV EleVI
(~
~1I /400
= 1K/s)
(~= O.OlK :;)
= (~" -7)/433
(~ = O.06Kl s)
(c) If the glow cu rve is me asured with two d ifferent ri se rates, ~l and ~2' it is apparent that one can obtain the value of E without assuming the value of s in Eqs. 141 or 142, using the following equation.
~ (_ l k
T II/2
__ 1) In(~ . T,~2 J T,"I =
~2 T,:'I
(143)
Hoogen straaten- extend ed this method for m any rai sing rates ~i' and, by plotting the curves (1 /T I11 • vs. In(Ti~ i/~;)) ' obtained the trap depth from the slope £/k of the straight line connecting the plotted points as shown in Figure 52. 6 A numerical an alysi s? has shown that Hoogenstraaten's method gives the best result among several methods for obtaining trap depths from glow curves.
Chapter one: Fundamentals of luminescence
83
5.-----------------,
4 3
2
lL--L--__L--------'_------'_------'
2.76
2.80
' - -__
2.88
2.84
100D /T m [K- 1] Figure 52 H oogens traaten p lot showing the d ependence of the peak tempera ture (T m ) of a glow curve on th e temperature-raising ra te (~) . The slop e of this line gives the dep th (activation energy) of the trap . (From Avouri s, P. and Morgan, T.N., f. Chern. Phys., 74, 4347, 1981. With perm ission.)
I 1
I
I I I
I I
- - - - 1- - I I
I I
I I
I
I
I
1
'
I
I
. I
I I
4- A~ O"-71
~ (J) ~I
I
Tz
------i~_
Figure 53
Predicted shape of a glow cur ve.
T
84
Fundamentals of Phosphors
------. ~
5
Z
~ ~
4
if)
Z 0
U
3
~ ~
U
2
~
0
<:»:
......C
1L--L.-..l--...J.-...J.-....l....---I....-......L----'----'-----'-----'------L--'---'------'----'
2.20
2.30
2.40
(DECAY TEMPERATURE) -
2.50 1
X
10 3
Figure 54 Temperature d ep end ence of the de cay time cons tant of ZnS:Cu . (From Bube, R.H. , Phys. Rev., 80, 655, 1950. With permission .) (d) Many me thods for obtaining the d e pth f from the width of th e peak of a glow curve h ave been p roposed." Th e res u lts are listed be low, w here th e wi d th for a peak is d efin ed in va rious ways, as shown in Fig ure 53.
1.
e=
2.
E
= 2kT,"(1.25T,./ W - 1)
3.
E
= 1.52
kT,~
(J
kT,~
I A- 3.16 kT",
4. £ = (l + W IA)kT,~ I (J
N ote th at these methods are usable un d er certain res tricted conditions ." There is a m ethod to ob ta in tr ap d epth other than the glow-c urve m ethod d escrib ed above . It is to use th e te mp era ture depen dence of th e d ecay time of aft er-glow, th at is, p h osphorescen ce. As mention ed in rel a tion to Eq . 132, the decay tim e consta n t of the expone ntial af ter -glow due to the firs t-order rea ction kinetics is eq ual to the inverse of the th e rm al detrapping probability, a- J , and its tem p erature d ep en d ence is given by :
£ ) ' Ve = s - I exp ( kT
(144)
Th erefore, the trap d epth £ ca n be ob tained from th e me asurements of the p h osp hore scen ce d ecay tim e 'lie at several di fferent temper at ures (TJ An example is shown in Figu re 54, in which th e d epth is ob tained fro m the slope of the stra igh t lin e connecting th e Arrhe n ius pl o ts of the obse rv ed va lues" fo r L j and T; A usable me th od wi th w hich trap d epths and rel ative trap d ensiti es are obtained m ore easily and acc urately was recen tly p rop osed. " In th is me thod, the sample is exci ted periodically un d er a slowly va rying temp erature and th e after- glow (ph osphorescen ce) in tensity is m eas ured at several delay times (til) af ter th e termination of excitatio n in eac h cycle.
Chapter one:
Fundamentals of luminescence
85
x 35
(a)
2 3 4
x 1.4
5
130ms 400 ms 1 300ms 4000ms 3
2
( b)
x 1.4
100
200
300
400
500
600
700
TEMPERATURE [K] Figure 55 Tem pera ture depen d encies of the after-gl ow in tensity of (a) Zn 2 SiO, :Mn 2 + a nd (b) Zn 2Si04:Mn L+ , As. The d elay time (t.t) is 0.13, 0.4, 1.3, and 4.0 s, respectively, for the cur ves numbered from 2 th rough 5 in the figures. (Fro m Nakazawa. E., JJ.!lI . f. Appl. Phys., 23(9), L755, 1984. With perrnission .)
The temperature dependence of th e after-glow in tensity at ea ch d elay time m ak es a p eak at a cert ain tempe ratu re Til,. Fro m the eq uation dI / dT = 0 and us in g either Eq. 132 or Eqs. 135 and 136, the following relation is ob tained bet ween the pe ak temper ature T and the del ay time td . I11
(145) Since Eq. 145 is similar to Eq. 144 (i.e., the d ecay tim e metho d ), th e me th od use d there for obt aining th e trap depth E can be applied hereby, substituting td for 1 , and Til, for T; An example of the measured after-glow in tensity cur ves is sh own in Figure 55.
1.7. 3 Photostimuiation and phoioquenching When a phos p hor wi th d eep traps is once excited and th en irrad iat ed b y infrare d (IR) or red light d uri ng the decay of its ph osp horescence, it sometim es sh ows phot ostimulat ion
86
Fundamentals of Phosphors
UY-lI I
: -IRI
(a)
I I I
I I
>f-
-z C /)
UJ
~
Z 0
C /) C/)
UV
UV+IR-
(b)
~ UJ
Tl M E Figure 56 Photostimulation and photoquenching simulation for the case (b) under a constant excitation and (a) in the after-glow process after the termination of excitation.
or photoquenching of luminescence; that is, an increase or decrease of the emission intensity as schematically shown in Figure 56(a). Under a stationary excitation shown in Figure 56(b), the stimulation enhances and the quenching reduces the emission intensity temporarily. These phenomena are utilized for IR detection and radiographic imaging, in which the intensity of the stimulated emission is used to measure the intensity of IR light or Xvravs.!' Photostimulation is caused by the radiative recombination of the electrons (holes) released by photoactivation from deep trap levels, as shown in Figure 57(a). On the other hand, photoquenching is caused by the nonradiative recombination of holes (electrons) photoactivated from luminescent centers as shown in Figure 57(b). Figure 58 depicts the configuration coordinate model of photostimulation. The activation energy Eo of photostimulation is not generally equal to the thermal activation energy e, of trapped electrons discussed before with reference to Figure 49. Since the optical absorption process takes place in a very short time period without changing the configuration of the atoms in the center at that moment, the process is represented by the straight vertical transition in Figure 58 from state III (a trap or the quasi-stable state of emitting centers) to state II (the conduction band or emitting centers). On the other hand, the thermal activation needs energy t l to overcome at least the lowest barrier between states II and III in Figure 58; hence, the activation energy 101 is generally smaller than £0. Photostimulation spectra (i.e., excitation spectra for IR stimulation) can be used to measure the optical activation energy to.
Chapter one:
Fundamentals of luminescence
87
...
C.B .
RADIATIVE RECOMBINATION
(a)
(b)
NONRADIATIVE RECOMBINATION
V.B. Figure 57
Photostim u lation process (a), an d photoq uenc hing process (b) in an en e rgy ba n d sch eme. C.B. and VB. indica te the co nduction ba n d and the val ence ban d of th e hos t crysta l, respective ly.
II
Figure 58
Pho tostim ula tion in configurational coordina te m odels.
References 1. 2. 3. 4.
5. 6. 7. 8. 9. 10.
11.
Nakazawa, E., J. Lum inesc., 18/19, 272, 1979. Weber, M.L Phys. Reo., B8, 54, 1973. Hoogenstraaten, v«, Philips Res. Rept., 13,515, 1958. Ran dall, J.T. an d Wilkins, M.H. F., Proc. Roy. Soc., Al84, 366, 1945. Cur ie, D., Luminescence in Crystals, John Wiley & Son s, 1963, ch a p. VI. Avouris. P. an d Morgan , T.N., J. Chern. Phys., 74, 4347, 1981. Kivits, P. and Hagebe u k, H.J.L., J. LII11Iinesc., 15, 1, 1977. Bube, R.H., Phys. Rev., 80, 655, 1950. Nakazaw a, E., [pn. J. App!. Phys., 23(9), L755, 1984. Na kazawa , E., Oyo Buturi, 55(2), 145, 1986 (in Jap a nese). Practical Applications of Phosphors, Yen, W.M., Sh ion oya, S., an d Yam a moto, H , Ed s.. CRC Press , Boca Raton, 2006.
chapter one - section eight
Fundamentals of luminescence Eiichiro Nakazawa
1.8 Excitat ion en ergy tran sfer and coop er ati ve op tica l phenomena 1.8.1 Excitatio n energy transfer 1.8.1.1 Theory of resonan t en ergy transfer 1.8.1.2 Diffusion of excitation 1.8.1.3 Sens itiza tio n of luminescence 1.8.1.4 Concentra tion quenching of luminescen ce 1.8.2 Cooperat ive optical phenomena Referen ces
1.8
89 89 90 94 95 96 97 100
Excitation energy transfer and cooperative optical phenomena
1.8.1
Excitation energy transfer
The process of excitation en ergy transfer from an excited point in a cryst al to a luminescent center can be classified into the followin g two types. 1. Migration of free electrons (holes), electron -hole pairs, or quasi-p articles such as excitons and plasmons conveys the excitati on energy to luminescent cent ers. Thi s typ e of transfer seems to be active especially in s uch semicond uctor-like hosts as ZnS and CdS, which are w id ely used as cathode-ray tube (CRT) phosphors. In th e ini tial excitation process of CRT phosphors, the local excitation b y a high-energy electro n pr oduces several hund reds of these particles, and th ey are di sp ersed in th e crystal by this typ e of transfer. (See 1.9.) 2. Excitation energy is tran sferred from an excited cen ter (ene rgy donor) to an unexcited center (energy accep tor) by means of quantum mechan ical resonan ce.l? Thi s typ e of tran sfer is practi cally utilized for the sensitiza tion of lu minescence in lamp phosphors, w hich are mostl y oxid es or oxoacid salts w ith less- mobile electrons and holes than in CRT phosphor mat erials. In this section, the res on ant energy transfer process and related phenomena, such as the se nsitization and quenching of luminescen ce, w ill be discus sed . More recent experimental studies on this topi c are referred to in the referen ce list. '
89
Fundamentals of Phosphors
90
e
~------R -----~~
D Figure 59
A
Coulomb in teraction in a resonant en erg y transfer process.
~--R ------:~
D
A
Figure 60 Resonant energ y trans fer by the exchange interaction, in wh ich the ove rla pping of the wavefu nctions of D and A (sha d ed region) i.s necessa ry.
1.8.1.1
Theory of resonant energy transfer
De xter 's theory of resona nt energy transfer- has elucidated that two optical centers w ithin a certain di stance may be in resonance and tr an sfer th e excit ation en ergy from one (donor) to the other (acceptor). The close proximity of th e center s en ables th em to be connected by the electrostatic interaction shown in Figure 59 or by the quantum mechanical exchange interacti on shown in Figure 60. Th e energy donor, which is called a sensitizer in p ractical usa ge, is d enoted hereafter by D and the acceptor by A. For resonant energy transfer to take place, it is necessary that the tran sition energies of D and A be equal. (a) Multipolar Interaction. The me chanisms of resonant en er gy transfer can be classified into sev eral typ es based on the character of th e transitions in 0 and A. Wh en both transiti ons in 0 and A are of electric dipole character (dipole-d ipole interaction), the probability pe r second of en er gy transfer from D to A is given by :
(146)
Chapter one: Fundamentals of lumin escence
91
Here, R is th e sep ara tion b etween 0 and A, n is the re fractive index of the crystal, 0 A is the absorption cross-sec tion of A, an d is the rad iati ve lifetime of D . Likewi se, the tran sfer probability du e to th e dipole-quadrupole interact ion is:
'0
(a = 1.266)
(147)
In these equa tions , f o(E) and FII (E) re p rese n t th e sh ape of the 0 emission an d A absorption spec tra, res pectively, which ar e normalized (i.e., ff d E)dE = 1 an d fFA E)dE = 1. The integrals in Eqs. 146 and 147 are, therefore, th e ov erl apping ra tios of these two spectra, which is a measure o f the resonance con dition. The tr ansfer p robabilities du e to all multipolar interact ion s-i .e., dipole-d ipole (d-d) in Eq. 146, d ip ole-qu adrupole (d- q) in Eq. 147, an d quadrupole-quadrupole (q-q), ar e sum ma rized in Eq. 148 with thei r R d epe n den ce being noticed .
(148)
Here, s in the third term is 6, 8, an d 10 for (d d) , (dq), and (qq), respect ively. If the d ip ole tran sition is allo wed for b oth D an d A, the m agnitudes of a , are a dd > a dq > a qq, an d the di pole-dipole interacti on has the h ighest transfer prob ability. How ever, if the dipole tran sition is not complet ely all owed fo r 0 andl or A , as is the case with the f-f transition of ra re-ear th ions, it is p robable that the higher-ord er interaction, d-q or q-q, may h ave the larger transfer probab ilit y for sma ll d ist ance p airs due to the higher-order expone n t of R in Eq . 148.4•5 Since th e emission in tens ity an d the radiat ive lifetime of 0 are d ecreased by en ergy transfer, the me ch anism of th e transfer can be an alyzed using the d ependence of the transfer p robability on th e pair d istance give n by Eq . 148, an d hence the domina n t mechanism among (dd ), (dq ), an d (qg) can be determined . When the acceptors ar e randomly distributed w ith various di st ances from a donor 0 in a crys tal, the em iss ion decay curv e of D is not an expone n tial on e. It is giv en by th e following equa tion for the multipolar in teractio ns."
~(t) = exp [ - _t _ r(l- ~) ~ [_t J Co
3/
1:0
5
' ],
(5 = 6, 8, 10)
(149)
1:0
ro
is the ga m ma func tion, a nd C and Co ar e, respectively, th e con centration of A Here, and its crit ical concen tra tion a t which the tran sfer probability is eq ua l to the ra d iat ive probability (1/1:0 ) of D. Thus, the emission efficiency 11 and th e em issio n d ecay time cons tan t 1:111 can be estim ated using Eq. 149 an d th e foll ow ing eq ua tions :
-2l = 11 1J
L~(t)dt 1:0
(150)
Funda mentals of Phosphors
':JL
1.00
>f:-< 0.50
>-<
(/)
Tb - Nd 5 =8
z
~
f:-<
z z a .....
- : calc
>-<
(/) (/)
0.10
•
: exptl
6
7
.....
z:
~
0.05
o
3
2
4
5
TIM E ems] Figure 61
Ca(P0 3) 2'
Decay curves of Tb3+ em ission (504 - 7Fj ) affecte d by the energy tra nsfer to Nd 3+ ions in (Fro m Naka zawa , E. and Shio noya, S., t. Chem. Phys., 47, 3211, 1967. With p ermission.)
l~
(t)dt
t
= .;::("1-.-_
}II
_
(151)
So (t)dt
Figure 61 sh ows the decay curve s of Tb3+ emi ssio n (5 0 4 - 7Fj ) in Ca(P03h, which are affected, due to en ergy tr ansfer," by th e concen tration of the co-activated Nd3+ ions. The dependence of the emi ssion intensity and decay time of the donor Tb 3+ ion on the concen tratio n of the acce p tor Nd 3 + ion in th e same system are sho w n in Figure 62. Theoret ical curve s in the se fig ur es are calculat ed using Egs. 150 and 151 w ith s =8 (qua d ru po le-di po le in terac tion). (b) Exchange Interaction. When an en ergy donor 0 and an accepto r A are locat ed so close th at their electronic wavefunctions overlap ea ch othe r as show n in Fig ure 60, the excitation energy o f 0 could be tran sferr ed to A due to a quantum m echan ical excha nge interaction betw een the two. If th e ove rlap of w av efunctions vari es as exp(- R/L) with R, th e transfer p rob ability due to thi s interaction becomes:" (152) w here 1(2 is a cons tan t with d ime nsion of energy squared and L is an effective Bohr radius; that is, an ave rage of the radi i of 0 in an excited state and A in the ground s tate. Th e emi ssion d ecay curve of 0 , taking into account a randomly distributed A interacting through the excha nge m echanism, is given (simi lar to Eq. 149) by:"
(153)
93
Chapter one: Fundamentals of lumin escence
Tb-Nd s =8 o
0.8
®
exptl
calc
0
~
<, ~
z
0.6
I I [0
0
!--t
C/) C/)
0.4
!--t
::E r..i1
0.2
0.01
1 !
I l! "
I
10
I II "
0.01 (Nd)
0.1
Figure 62 Intensity and d ecay tim e of Tb3+ em ission in the sa me system as that in figu re 61. Solid curves are theoretical on es for qu adrupole-di pol e in teraction (s = 8). (From Nakazaw a, E. and Shionoya, S., J. Chem. Phys., 47, 3211, 1967. With perrn ission .)
The emi ssion efficienc y and the av erage d em iss ion de cay time t ., can b e estima ted using Eqs. 150, 151, and 153. Since the exchange interaction requires the overlapping of the electron clouds of 0 and A (see Figure 60), the ion can be no further away than the second nearest site in the host crystal. Note that while Eq . 152 requires the spectra l overla p for the resonanc e cond ition, it is irrelevant to th e spectral intensities. Therefore, if A is located ne xt to 0, an d if th e transition s are not comp letely electric dipole allowed, the tran sfer p robability by exchange interaction can be larger than for multipolar in teractions. As de scribe d later in reference to Figures 64 and 65, th e emissio n of Mn2+ in the halophosphate phosphor, the m ost general lamp phosphor, is exc ited by the en ergy tr ansfer from Sb3+ due to the exchange interaction since the corresponding transiti on in th e Mn 2 + ion is a forbidden d-d transition." Perrin's mod el" tre ats the emi ssion decay of D under general en ergy transfer in a simple manner. In this model, it is as sumed th at the transfer probability is a cons tan t if A exist s w ithin some crit ical distance and is zero ou tside the range. Th is model, therefore, is thou ght to be most applicabl e to the short-ran ge exchange interaction. (c) Phonon-Assisted Energy Transfer. Phon on-assisted en ergy transfer occurs when the resonance condition is not w ell sati sfied between 0 and A, resulting in th e spectral ove rlap in Egs. 148 and 149 being sma ll. In thi s case, the difference oE between the transition energies of 0 and A is compensated by phonon emission or absor p tio n . The transfer p robability? is given by: (154)
Fundamentals of Phosphors
94
lOS. - - - - - - - - - - r - - - - - - , - - - - - - - - - - - - ,
~
I
~
104 f --
-
-
-T--
-+-
Er
-
-
-
4 5 3/ 2
-
-----t-
-
-
-
-
-----j
-- Yb 2 F S / 2 Eu
I
D2 - - Yb 7 F 712 o
5
10 3 r------+---~-___j_----_____j Eu SD o -4~m6F I 1 / 2 0
Eu SD o -- Yb 7 F S 12 0
1a2 L--...L.----'---'-----L----'-----L-----'_L-...L----'---'---l..--"---J.-.J 2 000 3 000 1 000 a En e rgy ga p [e m- I] Figure 63
Relati on between the nonresonan t phonon- assisted energy transfer rate and the energy m ismatch I) in Y20 3 :R3+ (R = rare ear th ions). (From Yamada , N., Shion oya, S., and Kushi d a, T., J. Phys. Soc. Japa n, 32, 1577, 1972. With perrnission.)
where Pa,(O) is equal to th e resonant transfer p roba bility g ive n by Eqs. 148 or 149, and ~ is a p aramet er th at d e pends on the energy an d occupa tion n u mbe r of p articip ating phon ons. Th e ene rgy gap 6 £ is equa l to n wp with 11 an d wI' being the number and energy, resp ectively, of the lar gest energy phon on in the h ost. Figure 63 shows the energy transfer rates for va riou s D-A systems of rare-earth ion s in Y203 ho st ' ", these are in excellen t agreem en t wi th Eq . 154.
1.8.1.2
Diffu sion of excitation
Energy transfer to th e sam e type of ion is called excitati on migration or energy mi gr ati on . While th e effec t of en ergy mi gration among donors (D ~ D) prior to 0 ~ A tran sfer is neglected in the above di scu ssion, it m ust be taken in to accoun t in the emissio n de cay of sensitiz ed ph osp hors. The effect of 0 ~ 0 mi grat ion on 0 ~ A systems is theoretically . exp ressed by the following eq ua tio n.U'?
<)l(t) = exp -
[J...- + -.lJt 1:0
(155)
"t M
w here m igra tio n rat e is defined as "t",,-I = 0.51·4n:Nl\cx Il 4D 3/ 4, in w hich 0 is a d iffusion co nstant, typically 5 x 10-9 crrr's' for th e Pr J+-P r 3 + p air in LauKPro2ClJ:Nd and 6 x 10-10 cm 2s-1 for the Eu 3+-Eu 3+pair in Eu( PO J) glass.12,13
Chap ter one: Fundamentals of luminescence
>C)
95
,-, ,,, ,,,
A
I..
\
.I \ "
0::
n: .. II""\-" ~\ ::
W
Z
:,1'
W
A
W
, ,,,
>
.....
:5w
,,
,.
I
\.
"
E
II'"
,
"
.\ ' " "1.:\'
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ J J~
25000
\
, \ \' " , ..\\,
I
-»
a:::
, ,,
-, \ \
-- -,
23000 21000 19000 17000 1 WAVE NUMBER [cm- ]
,,
',\
,
\.,
.\.'
-
",,~~,
" ......
""
15000
Figure 64 Emis sion spe ctra of a lamp ph osp hor, Ca s(P0 4 MF,Cl):Sb3 +, Mn 2+, in which the Sb 3 + ions sensit ize the emission of Mn 2+ ions by an energy transfer process. The Sb3+ concentration is fixed to be 0.01 mollmol Ca. The Mn 2+ concentra tion is changed, AO , B:0.005, C O.OlD, D:0.020 and E:0.080 moll mol Ca . (From Baller, K.H. and Jerome, c.w.,;. Electrochem. Soc., 97, 265, 1950. With perm ission.)
1.8.1 .3 Sensitization of luminescence Energ y transfer p ro cess es ar e often us ed in practical phosphors in order to enha nce the emission efficiency. The process is called sensitization of lum inescence, and the en ergy donor is called a se ns itizer. The emission intensi ty of Mn -'
96
Fundamentals of Phosphors
I
I I I I
>r(f)
z
/"'\
\
, \~Sb3+
, ,
I I I I I I
I I
\
I I
w
r-
z
140 160 180 200 220 240 260 280 WAVELENGTH [nm] Figu re 65 Excitat ion sp ec tra for the blu e Sb 3+ emi ssion and the red Mn 2+ em ission in the sa me sys tem as that in Figure 64. (Fro m Johnson, P.D., f. Electrochem. Soc., 108, 159,1961. With permission .)
1.8.1.4
Concen tration quenching of luminescence
If the con cen trat ion of an acti v ator is higher th an an appropria te va lue (usually se veral w t %), the emission of the phosp h or is usually lowered, as show n in Figure 66. This effec t is called concentration quenching . The origin of th is effect is though t to be one of the follow ing:
1. Exci tation ene rgy is lost from the emitting state due to cro ss-relaxation (describ ed later) between the ac tiva tors . 2. Excit a tion migration due to the resonance betw een the activa tors is increa sed wi th the concentration (see 1.1.2), so that the energy reaches remo te killers or the crys ta l surface acting as quenching cen ters.20•21 3. The act ivator ions are paired or coag u la ted, and a re ch anged to a quenching cen ter. In some rare-earth activa ted phosphor s, the effec t of concentration qu en chin g is so small tha t even stoichiometric p hosphors, in which all (100%) of the ho st con sti tu en t cations are substitu ted by th e activa tor ion s, have been develop ed. Figure 67 sh ows the conce n tra tion depen d ence of the emission intensity of the Tb3+activ at or in TbxLal_,.P,Ow a s toichiome tric phosph or, in which the emission intensity from the 504 emitting leve l (see the energy level diagram of Tb " in 2.3) at tains the maximum a t x = 1.22 Thi s phosphor ha s the sa me crystal
97
Chapter one: Fundamentals of luminescence
1
10- 1
°
(Y1-x Pr x)202S
c: : ZnS
10- 3 '--10- 5
(hexag.) : Cu [x], Al [2 xl
----'---
10- 4
'---
10- 3
-'-
10- 2
"'----_
10- 1
ACTrvATOR CONCENTRATION [g . atom/mol] Figure 66 Acti va tor con centration de pe nd en ce of th e ca th od e-lu m inescen ce inten sities of Kuboniwa, S., Kawai, H ., and Hoshina. T., [pn . f. Appl. Phys., 19, 1647, 1980. With permission .)
Yp ZS:Eu 3+ and ZnS:Cu . (From
stru ctu re as NdP50W a typical stoichiometric phosphor, in which ea ch Nd 3+ ion is isolated by th e sur ro un ding P0 4 groupS. 23 Wh en the concentration quenchin g due to cross-re la xa tion (rela xation due to re son an t energy transfer between the sa me element atoms or ion s [see th e in sertion in Figure 68]) occurs on a particul ar lev el among several em itt ing le vel s, the em iss ion color of the ph osphor changes with the activat or concen tra tio n . For exa m p le, while the emissi on color of Tb-r-activated phosphors is blue-white due to mixing of blue emission from the 5 0 3 emi tting level and green emission from the 50 4 level at concent rations below 0.1%, the color cha ng es to g ree n at th e h igh er concentrations. The cha nge is caused by crossrelaxat ion, as shown in Figure 68,24 between th e 50 3 and 5 0 4 emitting lev els, thereby dimini shing the p opulat ion of Tb3+ ions in 50 3 sta te an d in crea sing th e one in th e 5 0 4 state.
1.8.2 Cooperative optical phenomena In emission and abso rp tion spectra of crystals hi ghl y doped w ith two types of ra re-earth ions, lab eled A and B, som etim es show weak ad d itional lines other than th e inherent spe ct ral lines specific to the A an d B ions. Th ese ad d ition al lin es ar e due to th e coo p era tive optical processes ind uced in an AB ion-p air co upled by ele ctro st ati c o r exchange interac tion s. The coop erative optical p rocess can be divided into th ree types as shown in Figure 69. They are: (a) cooperative absorp tion (A B + CO,H B -7 A ' B'); (b) Ram an luminescence (K B -7 AB' + COil-B); and (c) coop era tive luminescence (A ' B' -7 AB + COil+lI)' The obse rved int ensit ies of all th ese coopera tiv e sp ec tra are ve ry w eak (10-5 of the n ormal f-f
98
Fundamentals of Phosphors
100
(fJ
c
.....C1>
.-C 10
1
0.1
0.01
x : Tb 3 +
1
Ion concentration
Figure 67 Activa tor concentra tion dependence of photoluminescen ce in tensity of a "s toichio me tric" phosp hor, Th,Lal_xPSOI4' whose emission reaches the maximum int ensity at the complete s ubs titution of host La" ions by the ac tiva tor Tb- ions (x = 1). (From Tanaka, S., Na kamura, A., Kobayashi, H " and Sasa kura, H.,Tech. Digest Phosphor Res. Soc., 166th Meeting, 1977 (in Japa nese). With perm ission .)
tr an sit ion ). Coopera tive absorption has been observed for Pr 3+-PrJ+, Pr3 +-Ce3 +, and Pr3+Ho3+ p airs,25,26Ram an luminescence for Gd 3+- Yb3+ and Tm 3+-Tm 2+pairs 27.28, an d coope rative luminescence for Yb3+-Yb3+ and Pr 3+-Pr3+ pairs. 2Q,3o The coop erative abs orp tion transition AB + nO) ~ A'B' p roceed s via an intermediate sta te Ai or B' in a m anner A B ~ AiB' ~ A'B' ,31 Th ese th ree sta tes are combined by the multipolar o r excha nge interaction operator H A B, which is also opera tive in ene rgy transfer p rocesses described in 1.8.1, and the perturbation P by the ra d ia tio n field given by -er-E for electric d ipole transitions as d escribed in 1.1. Th en , the mom ent o f the transition (see 1.1) is given by:
(156)
The coo pe ra tive absorption intensity of the P r1+-p rJ+ ion pai r in Pr03 crys ta ls, estimat ed b y thi s equa tion, agrees well with th at of observed coo pe rative spe ctra.F-" The es tima tion ind icat es that the dq or higher-order multipolar in ter ac tion is effective in this p air.33
99
Chapter one: Fundamentals of luminescence
\ \ \ \
\ \
\
06-
I
c:
o rf) rf)
o
2
4
6
8
10
Tb cone. [mol %J Figure 68 Activator concen tration d ep end en ce of the emi ssion intensities of the tw o emitting levels of Tb3+,IJ (50 3) , and 1. (5 0 4 ) , The relative in tens ity between the tw o and therefore the emission colo r is change d from blu e-white to gree n wi th the inc rease of the activa tor con centrati on d ue to cross relaxa tion betw een 50 3- 5 0 4 and 7F6JFo. (Fro m Na kazaw a, E. and Sh ionoya, S., J. Phys. Soc. Japan , 28, 1260, 1970. With permission.)
A*-~-
hv
-----
A
--~I (a)
A .. -..,.....-
A*-~-
B'IhY ---;-
B'±hP --.
--~
A
(b)
---~--
A
(c)
Figure 69 Transition in coop erative op tical processes: (a) coop era tive absorp tion, (b) Ram an luminescence and (c) cooperative luminescence.
100
Fundamentals of Phosphors
References 1. Foers ter, Th ., An n. Phys., 2, 55, 1948. 2. Dex ter, D.L., J. Chern . Phys., 21, 836, 1953. 3. Yen, W.M., Modern Problems in Condensed Matter Science, Vol. 21, Elsev ier, Am sterdam, 1989, pp. 185-249. 4. Na kazawa, E. and Shionoya, 5., J. G em. Pltys., 47, 3211, 1967. 5. Kushida, T , J. Phys. Soc. Japan , 34, 1313; 1327; 1334; 1983. 6. Inokuti, M. and H irayam a, F., f. Chem. Phys., 43, 1978, 1965. 7. Soules, TH., Bateman, RL., Hews, RA ., and Kreidl er, E.H ., Phys. Rev., B7, 1657, 1973. 8. Perrin, F., Compt . Rend., 178, 1978, 1924. 9. Miyakawa, T and Dexter, D.L., Phys. Rev., B1, 2961, 1970. 10. Yamada, N., Shionoya, 5., and Kushida, T, f. Phys. Soc. Japa n, 32, 1577, 1972. 11. Yokota, M. and Tanim oto, 0 ., f. Phys. Soc. Ja pan, 22, 779, 1967. 12. Weber, M.J., Phys. Rev., B4, 2934, 1971. 13. Kras utky, N . and Moose, H.W., Phys. Rev., BS, 1010, 1973. 14. Batler, K.H . and Jerome, c. w., J. Electrochem. Soc., 97, 265, 1950. 15. John son , P.O., f. Elecirocnem . Soc., 108, 159, 1961. 16. Shio noya, S. an d Na kazawa, E., A ppl. Phys. Leit, 6, 118, 1965 . 17. Blasse, G. an d Bri!, A., J. G em. Phys., 51, 3252, 1969. 18. Tabei, M. and Shio noya, 5., Jpn. J. App . Phys., 14, 240, 1975. 19. Suzuki, A., Yamada, H ., Uch ida, Y., Kohno, H ., an d Yoshida, M., Tech. Digest Phosphor Res. Soc. 197th Meeting, 1983 (in Japanese) . 20. Ozaw a, L. and Hers h, H .N., Tech . Digest Phosphor Res. Soc. 155th Meeting, 1974 (in Japanese) . 21. Kub oni wa, S., Kawai, H ., and H osh ina, T , [pn. J. Appl. Phys., 19, 1647, 1980. 22. Tan ak a, S., Nakamura, A ., Kobayas hi, H ., an d Sasak ura, H ., Tech. Digest Phosphor Res. Soc. 1661h Meeting, 1977 (in Jap anese) . 23. Dani elm eyer, H. G., f. Luminesc. , 12/13, 179, 1976. 24. Nakaza wa, E. and Shionoya, S., f. Phys. Soc. Ja pan, 28, 1260, 1970. 25. Varsani, F. and Dieke, G.B, Phys. Rev. Lett., 7, 442, 1961. 26. Dor m an, E., f. G em. Phys., 44, 2910, 1966. 27. Feof iJov, P.P. and Trifim ov, AX, Opt. Speci., 27, 538, 1969. 28. Nakazaw a, E., f. Luminesc., 12, 675, 1976. 29. Nakazawa, E. and Shionoya, S., Phys. Rev. u u.. 25, 1710, 1982. 30. Ran d , S.c., Lee, L. S., and Schaw low, A.L., Opt. Commu n., 42, 179, 1982. 31. Dexter, D.L., Phys. Rev., 126, 1962, 1962. 32. Shinagawa, K., f. Phys. Soc. Japan , 23, 1057, 1967. 33. Kus hida, T , J. Phys. Soc. Japan , 34, 1318, 1973; 34, 1327, 1973; 34, 1334, 1973.
chapter one - section nine
Fundamentals of luminescence Hajime Yamamoto
Contents 1.9 Excitation mechanism of luminescence by cathode-ray and ionizing radiation 1.9.1 Introduction 1.9.2 Collision of primary electrons with solid surfaces 1.9.3 Penetration of primary electrons into a solid 1.9.4 Ionization processes 1.9.5 Energy transfer to luminescence centers 1.9.6 Luminescence efficiency References
1.9 1.9.1
101 101 101 103 105 107 107 108
Excitation mechanism of luminescence by cathode-ray and ionizing radiation Introduction
Luminescence excited by an electron beam is called cathodoluminescence and luminescence excited by energetic particles, i.e., a-ray, ~-ray or a neutron beam, or by y-ray, is called either radioluminescence or scintillation." The excitation mechanism of cathodoluminescence and of radioluminescence can be discussed jointly because these two kinds of luminescence have a similar origin. In solids, both the electron beam and the high-energy radiation induce ionization processes, which in turn generate highly energetic electrons. These energetic electrons can be further multiplied in number through collisions, creating "secondary" electrons, which can then migrate in the solid with high kinetic energy, exciting the light-emitting centers. The excitation mechanism primarily relevant to cathodoluminescence is discussed here.
1.9.2
Collision of primary electrons with solid surfaces
Energetic electrons incident on a solid surface in vacuum are called primary electrons and are distinct from the secondary electrons mentioned above. A small fraction of the electrons is scattered and reflected back to the vacuum, while most of the electrons penetrate into • The word originally means flash, as is observed under particle beam excitation.
101
102
Fundamentals of Phosphors
(a)
( c)
t (/)
c:
0 ....... l-<
U
..........
4-<
0
l-<
.D
(b)
6
;::l
Z
0
50
100
150
EI ectron energy (eV) Figure 70 The energy dis tribution of elec tron s emitted from the Ag surface ex posed by th e pr ima ry electro ns of 153 eV: (a)ele ctron s emi tted by elastic scatterin g, (b) elec trons by inelastic sca ttering and (c) seco nda ry electrons. (From Dekker, A.J., Solid Stale Physics, Pren tice-H all, Tokyo, 1960,418-420. With permission .)
the so lid . Th e reflected electro ns can be classified into th ree types: (a) elastically sca ttere d p rim ar y electrons, (b) inelas tically sca ttered primary electro ns, and (c) secondary electrons .' The second ary electron s mention ed here are those electro ns generated by th e p rim ar y electrons in the so lid and are ene rgetic enough to overcome the wo rk fun ction of the solid surface. This phenomen on, i.e., the escape of seco nd ary electrons from the solid, is similar to the photoelectric effect. The relative numbers of the th ree types of sca ttered electrons observed for the Ag surface are shown in Figure 70.1.2 As sh own in this figure, the inelastically scattered primary electrons are much smaller in number than the other two types . The ratio of th e number of the emitted electr ons to th e number of th e incident electr ons is ca lled the secondary yield an d usually denoted as o. With this terminology, 0 sho uld be d efin ed only in terms of th e sec ond ary electron s (c), exclu ding (a) and (b). How ever, in most cases, 8 is s ta ted for all th e sca ttered electrons- (a), (b), an d (c)-for practical reasons. For an insulat or, 8 d epends on th e surface p otential re lative to the cathode as is schematically shown in Fig ure 71. For 0 < 1, the ins u lator surface is negativel y charged; as a consequence, the po ten tial of a phosphor s ur face is not raised abo ve Va sho wn in Figure 71, even for an accelera ting voltage high er than VJ(' In other words, the surface potential stays at VII and is called the sticking potential. To p reve n t electr ical charging of surface, an aluminizing technique is employed in ca tho de-ray tub es (CRTs) . Negati ve cha rgin g of a phosph or is also a problem for vac uum flu orescent tub es and some of field emission displays, which use low- energy elect rons at a vol tage below VI' The alu min izing technique cannot be used in th ese cases, however, because the low-en ergy p rimary
Chapter one:
Fundamentals of luminescence
103
OL....-_ _-----l. VI o
----l
_
Surface potential - Figu re 71 A schematic illust rati on of seco n da ry yie ld as a func tion of the surface po tential of an insul ator. The seco ndary yie ld 0 is un stab le a t p oint A, while it is s ta ble at B and C. At these p oint s, the state shi fts toward the di rection of the give n arrows w ith a cha ng e in the p otential. N ear poi n t C, whe re the po tential is in a reg ion of a few to several ten s of volts, the yield ap proaches 1 becau se the incident pri mary electrons are reflected . (From Kazan , B. and Knoll, M., Electron Image Storage, Academic, New York, 1968, 22. With perrnission .)
electrons, for exa mp le a few ten or hundred eV, cannot go through an alum inum film, even if it is as thin as 100 nrn, which is practically the m inimum thickness required to prov ide sufficien t electrical conductivity and optical reflectivity. It is, therefo re, required to mak e the phosp hor surface electri cally conductive. To.evalua te a ca thod olum in escen ce efficiency, one must exclude the sca ttered primar y electrons (a) and (b) in Figure 70. The ratio of the electrons (a) and (b) to the number of the inciden t electrons is called back-scattering factor, denoted by 110' Actuall y, th e electrons (b) are negligibl e com pa red with the electrons (a) as shown in Figure 70. Th e va lue of 110 depend s we akly on th e primary electron energy but increases w ith the a to mic number of a solid . 11 0 obeys an em pirical formula, with the atomic number or th e number of electron s per mo lecu le bein g 2",3; that is: 110 = (1/ 6)ln Z III
-
(1/4)
(157)
Th e v alue calcu la ted by this formula agrees well with experimental results ob tai ne d for sing le-c rys tal sa mp les. For example, the calculated values for ZnS with 2 ", of 23 is 0.25 and for YV0 4 wi th 2 m of 15.7 is 0.21, while the observed values for single cr ystals of these comp ounds a re 0.25 and 0.20/ respectively." In contrast, a smaller va lu e of 110 is found for a powder layer becau se some of th e reflected electrons a re abs orbed by th e pow der th rou gh multiple-scattering. The obs erved values of 110 are 0.14,4 both for Z nS and YV0 4 in pow d er form . It has also been reported that 110 va ries by several p er cent d ep ending on th e packing density of a po wder layer."
1.9.3 Penetration of primary electrons into a solid The pen et rati on p ath of an electron in a solid has been directly observed w ith an op tica l microscope by using a fine electron beam of 0.75 urn d iamet er (Figure 72). Thi s experime n t shows a narrow cha nnel leading to a nearly sph erica l region for electron en ergy hi gher than 40 keV, while it shows a semisphericalluminescent regi on for lower electro n ene rgies.
104
Fundamentals of Phosphors
Primary electron
Figure 72 A sche ma tic illus tration of a region excited by an electron beam. Th is reg ion can be vis ua lize d as the luminescent profile of th e so lid phosphor part icle as seen through a mi croscope. The energy of the prim ar y electrons incre ases in the order of A, B, and C. For A, the energy is several keY and for C, 40 keY or higher. (From Ehren berg, W. and Franks, ]., Proc. Phys. Soc., B66, 1057, 1953; Garlick, G.F.]., Br. J. App!. Phys., 13, 541, 1962. With permission .)
The former feature is found also for high-energy particle excitation, i.e., th e excitation v olume is confined to a narrow ch annel until the energy is dissipat ed by ionization processes. This result indicates th at the sca tterin g cro ss-sec tion of an ele ct ron or a particle in a solid is larger for lower electron ene rgy. The en er gy lost by a charged p a rticle passing through a so lid is ex p res sed by Bethe 's formula": d Ef d x = (2nNZ",e 4 /E) ln( E/E, )
(158)
where E d enot es the energy of a primary electron at d istance x from the solid surface, N the elec tron d ensity (crrr-' ) of th e so lid, 2 m th e m ean a tom ic number of the solid, and E; the m ean ionization potential av e raged over all th e elec tro ns of th e con stituent atoms. Variou s fo rm u las have been proposed to giv e the relation bet w een E and x. Among them, the m ost frequently used is Thom son -Whi dding ton's formula, to w h ich we ca n derive from Eq. 158 sim p ly by putting In(EI E) = const ant.
E = Eo( l- xjR )
1/2
(159)
Here, Eu is th e primary elec tron en ergy at th e surface an d R is a constant called as th e range, i.e., the penetration d epth a t E = 0*. It is to be noted th at an incremental ene rgy loss, -dEl dx, increases with x acc or d in g to Eg. 159. In a range of 1 0 = 1-10 keV, the dependence of R on Eo is given by!': (160) where n = 1.2 /(1-0.29Iog 211/)' p is the bulk d ensity, A the atomic or m olecular w eight, 2 ", the atomic number per m olecul e, and Eo an d R are expressed in units of keV and A, respectively. When Eu = 10 keV, Eq. 160 gi ves R == 1.5 11m for ZnS and R = 0.97 urn for CaW0 4 . The ex perim en tal values agree well w ith the calculated va lues . • Other form ulas define the ran ge as the pe netration d e p th a t E = Eo/e.
Chapter one:
Fundamentals of luminescence
105
5
....
~
(:;
;:l
....
~
4
(:;
;:l
>-. ;....
(\
c::l
t
,D
;....
c::l
u: (:;
....u0 2 ;....
~
(!)
E ;:l
z
6
(/)
(:;
0;....
....u
5
4
(!)
0
,D
c::l
'-"
(!)
(!)
4;.... (!)
8
c::l
t 7 :0 ;....
3
'-"
>-. ;....
9
1
# 0 5 LJE [eV]
4-
0
;.... (!)
,D
E
;:l
z
3
2 1 0
0
10
30 40 LJE [eV ]
20
50
60
Figure 73 Electron energy loss spectra of YV 0 4 : (a) peaks A to D origina te in the electronic transi-
tions of the V04- 3 comp lex; (b) peak E can be assigned to plasmon excitation. Peak G is du e to a transition from Y 4p orbital to the cond uction band and, peak H from V 3p to the conduction band. The origin of peak F is not identified. The strong peak at 0 eV indicat es the incident electrons with no energy loss. (From Tonomura, A., Endoh, L Yamamoto, H., and Usarni, K., f. Phys. Soc. Japan , 45, 1654, 1978. With permission.) Wh en Eois d ecrea sed at a fixed electron beam current, luminescence vanish es at a cer tain positive vo ltage, called the dead voltage. One of the explanations of the dead volt age is that, at sha llow R, th e primar y electron energy is dissipated within a dead lay er nea r th e surface, where nonradi ative processes dominate as a result of a high concentration of lattice d efects.'? It is also known, h owev er, that the dead voltage decreases with an in crea se in electrical con d uctivity, indicating that th e dead voltage is affected by electrical charging as we ll.
1.9.4 Ionization processes A charged p article, s uch as an elec tro n, loses its kinetic energy through various m odes of electrostatic interaction with constituent atoms when it passes through a so lid . Elemen tary processes leadin g to en ergy dissipation can be ob served exp erimen tally by the elec tron energy loss spec troscopy, which measures the energy lost by a p rim ar y elec tro n d ue to inelasti c sca ttering (corresp on di ng to the electrons (b) in Figure 70). Main loss p rocesses observed by this me tho d a re core-elec tron excita tion s and crea tion of pl asmons, w hic h are a collective excita tion mo de of th e va lenc e e lectro ns . Core- elect ron excita tion is observed in the range of 10 to 50 eV for m at er ials having elem en ts o f a la rge a to mic number, i.e., rare-earth compounds or hea vy me tal oxid es suc h as va na da tes or tungstates .P Th e p lasmo n energy is found in th e reg ion of 15 to 30 eV. Co m p ared wi th th ese excita tion m od es, the co ntrib ut ion of th e band-to-ba nd tran siti on is sma ll. As an example exhi b iting vario us mo de s of exci tation, th e electron ene rgy loss spec tr u m of YV04 is shown in Fig ure 73.14
Fundamentals of Phosphors
106
Primary electron
Plama creation and other processes
Emission of optical phonon Ionizing process
c-,
bO .....
V
s:::
u..J
Threshold for ionization -
Ei
Secondary electrons
Carriers in thermal equilibrium Figure 74 A schem at ic illus tra tion of exci ta tion processes by a high -en ergy electron, whi ch p enetrat es in to a sol id . (From Robb ins, D.]., f. Electrochem. Soc., 127, 2694, 1980. With permission.)
Pla smons are converted to sing le-ele ctron excitation s in an extre mely short period of time, _10-15 s. As a con sequence, ele ctrons with energies of 10 to 50 eV are created eve ry tim e an energetic primary elect ron is scattered in a so lid as a result of core-electron exci ta tion or plasmon crea tion . This results in a series of ionization processes in a solid . Most of the electrons gen erat ed by the scattering events, or the seco ndary electrons, are still energetic enough to create other hot carr iers by Auger p rocesses. Seconda ry electron multiplication can last until th e ene rgy of the ele ctron falls below the threshold to crea te free carriers . All through thi s electro n energy loss process, sca tte ring is accompanied by phonon crea tion, as schematicall y shown in Figure 74 .' 0 Seco nd ary electro n multiplicati on is essen tially the same as the ph oto excitati on process in the va cu u m ultraviolet region. The ave rage en ergy required to crea te an electron-hole pair near the band edges, Ea" is gi ven by th e follo wing em pirical formula. " Em' = 2.67Eg + 0.87 reV]
(161)
wher e Eg is the bandgap energy ei the r for the direct or th e indi rec t ga p . This formula wa s origin ally obtained fo r eleme n ts or binary compound s with tetrahed ral bonding, but it is applied often to phosphors w ith more complex chem ica l co mpositions and cryst al stru ctur es. It is not , h owever, s tra igh tforward to define the bandgap energy for a materi al ha ving lo w-l yin g en ergy levels chara cte ris tic of a m olecular group, e.g.,
Chapter one:
Funda mentals of luminescence
107
vanada tes or tungst at es. The re fore, one m us t be care fu l in applyi ng th e above formul a to so me p hosphor s. As d escribed above, the average crea tion energy of an electron-hole pair is closely related to the catho d olu m inescence efficiency (see also Section 1.9.6). There is, h ow ever, another way to cons ider the luminescen ce efficiency; it foc uses on phon on emission," which compe tes with the electron-hole pa ir creation in th e ioniza tion processes. The phonon emission probability, denoted as R; here, is proportion al to th e in teraction of an electron wi th an op tical phonon, and is expressed as:
(162)
wh ere W w is the ene rgy of a lo ngi tudina l op tica l phon on in terac ting w ith an elec tron, an d an d £0 are high-frequency and sta tic d ielectric consta n ts, respectively. When multipl ied with the p ho non energy, the proba bility R; contributes to the pair creation energy EI1V as a term ind ependent of Eg, e.g., the second term 0.87 eV in Eq. 161. The lu minescence excited b y energetic particles is radi olum inescence.'? Th e exci ta tion mechan ism of ra d iolu m inescence h as its ow n characteri st ic processes, though it involves ion ization processes sim ilar to the ca tho dolu minescence processes. For exa mple, the ene rgy of y-rays can be d issipated b y three p rocesses: (1) th e Com pton effect, (2) the photo electric effect d irectly followed by X-ray emission and A uger effect, and (3) the creation of electro n-posi tron pair s. Su bse q ue n t to these processes, highl y energe tic seco nd ary electron s are created, follow ed by the exci ta tion of lum inescence centers, as is the case with ca tho do lumi nescence. A characteristic energy loss process of neutron s, which has no electr ic cha rges but much larger mass than an elec tron, is due to the recoil o f h ydrogen atoms. If the neutron energy is large enough, a recoiled hy d ro gen is ioni zed an d crea tes secondary electrons. It mu st be added, how ever, th at hydrogen atoms are n ot con ta ine d intentionally in inorganic ph osphors. £~
1.9.5
Energy transfer to luminescence centers
The fina l products of the seco ndary-electron m ultip lica tion are free electron s and free holes nea r the band edge, i.e., so-called ihermaliz ed electro ns and holes. Th ey reco mbine with each othe r, and a part of the recombination energy may be converted to luminescen ce light emission. The process in w hich eith er a thermalized elec tron-h ole pair or the en ergy released by their recom bin ati on is tran sferred to a luminescen ce cen ter is call ed host sensitization becau se the luminescen ce is se nsi tized by th e op tica l absorp tion of the h ost lattice. This p rocess is analogo us to the optica l excitation near the band edge. Det ailed studies were made on the optical exci tatio n of luminescen ce in IIb-VIb and lIIb-Vb com p ounds, as described in 2.7 and 2.8. Luminescence of rare-earth ions and Mn 2+ ion s arises becau se these ions capture electrons and holes b y actin g as isoe lec tro n ic traps.P-'" In inorganic compound s having com plex ions an d organic compounds, the excitation ene rgy is tran sferred to the luminescence cen ters through the m olecul ar energy levels.
1.9.6 Luminescence efficiency The catho dolu minescence energy efficiency 'Il, for all the p ro cesses d escribed above can be expressed b y 20 :
108
Fundamentals of Phosphors Table 2
Examp les of Ca tho do lu min escen ce Efficiency
Ch emical co m pos ition
WTDS designation
Zn 2SiO; :Mn2+ CaW04:Pb ZnS:Ag,Cl ZnS:Cu,A1 Y202S:Eu o, Y203:Eu 3 + Gd 20 2S:Tb3+ CsI:TI+ CaS:Ceo+ LaOBr:Tb3+
GJ BJ X X X RF GY
En ergy efficiency
Peak wavelength
(%)
(nm)
8 3.4 21 23, 17 13 8.7 15
525 425 450 530 626 611 544
11
22 20
544
Lu minescence colo r G reen BIlle Blue Gre en Red Red Yellowis h green Green Yellowis h green Yellow ish g reen
Note: The ph osphor screen designat ion by WTDS (Worldwide Phosphor Type Designa tion Sys tem) is presented. Man y da ta are collected in Alig, R.C. and Bloom,S., f. Electrochem. Soc., 124, 1136,1977.
(163) where 110 is the back-scattering factor given by Eq. 157,l1x th e me an ene rgy efficiency to create thermalized electro ns an d hole s by the primary electrons or Ex! Em" q the quantum efficiency of the luminescence excited by thermalized ele ctron-hole pairs, a nd E"III the mean energy of the emitted photon s. Thus, (164)
and also 11, < 1/3 acco rd ing to Eq. 161. TIle ene rg y efficienc y, luminescence peak wavelength and color are shown in Table 2 for some efficien t phosphors. For the commercial phosphors, ZnS:Ag,CI; ZnS:Cu,Al; Y20 25:Eu3+; an d Y zO,:Eu3+, w e find 11x = 1/3 from Eq. 163 by ass um ing that 110 = 0.1 and q = 0.9-1.0. Thi s va lue of 11, sugges ts that the energy efficiency is close to the limit predicted by Eq. 163 fo r these phosphors. It is to be emphasized , h ow ever, that this estimate does not exclu de a possibilit y for further improv ement in the efficiency of these phosphors, for example by 10 or 20%, si nce the calcu la ted values ar e based on a nu mber of approximations and simplifying ass u m p tions . It sh ou ld also be not ed that the band gap energy is not known acc ura tely for the phosphors gi ven in Table 2, except for Zn 5, CsI, and CaS. For the other phosph or s, the optical ab sorption edge must be us ed instead of the bandgap energy, leavin g the estimation of 11 approxim ate. For CaS, the indirect bandgap, 4.4 eV, gives 11, = 0.21, while the direct bandgap, 5.3 eV, g ives the value exceeding the lim it predicted by Eq . 163.
References 1. 2. 3. 4. 5. 6. 7.
Dekker, A.}., Solid State Physics, Prentice-Hall, Mar uzen, Tokyo , 1960, 418-420. Rud berg. E., Proc. Roy. Soc. (London), A127, 111 , 1930. Toml in, S.G., Proc. Roy. Soc. (London), 82, 465, 1963. Meyer, V C ., J. App l. Phys , 41, 4059, 1970. Kazan , B. and Kn oll, M ., Electron Image Storage, Academi c Press, New York, 1968, 22. Kan a, T. a nd Uchida, Y, [pn. J. Appl. Phys., 22, 1842, 1983. Ehrenberg, W. and Franks, L Proc. Phys. Soc., B66, 1057, 1953.
Chapter one: Fundamentals of luminescence 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
109
Ga rlick, G.F.J., Br. ]. Appl. Phys., 13, 541, 1962. Beth e, H.A., An ll. Physik, 13,541, 1930. Whidding ton, R., Proc. Roy. Soc. (London), A89, 554, 1914. Feldma n, C , Phys. Rev., 117, 455, 1960. Gerg ley, Gy., J. Phys. Chern. Solids, 17, 112, 1960. Yam am oto, H. an d Tonornura, A ., J. Luminesc., 12/13, 947, 1976. Tonomura. A., Endoh, J., Yamam ot o, H ., and Usarni, K. , J. Phys. Soc. Japa n, 45, 1654, 1978. Robb ins, D.J., J. Electrochem. Soc., 127, 2694, 1980. Klein , C A., J. A ppl. Phys., 39, 2029, 1968. For example, Brixner, L.H., Materials Chemistry and Physics, 14, 253, 1987; Derenzo, S.E., Moses, w.w., Ca hoon, J.L., Perera, R.L.C , and Litton, I.E., IEEE Trans. Nuc/. Sci., 37, 203, 1990. Robb ins, D.J. a nd Dean, P ]., Adv. Phys., 27, 499, 1978. Yama mo to, H . and Kano, T., J. Electrochem. Soc., 126, 305, 1979. Garlick, G .F.J., Cathode- and Radioluminescence in Luminescence of Inorganic Solids, Go ldberg, P., Ed., Academic Pre ss, New York, 1966, 385-417. Alig, R.C and Bloom, S., J. Electrochem. Soc., 124, 1136, 1977.
chapter one - section ten
Fundamentals of luminescence Shosaku Tanaka, Hiroshi Kobayashi, Hiroshi Sasakura, and Noboru Miura
Contents 1.10 Ino rgan ic elec trolumi nesc ence 1.10.1 Introdu ction 1.10.2 Injection EL 1.10.3 High-field EL 1.10.3.1 Injection of carriers 1.10.3.2 Electron energy distribution in high elec tr ic field 1.10.3.3 Excitation mechanism of luminescen ce centers Referenc es
111 111 112 113 114 118 l22 127
1.10 Inorganic electroluminescence 1.10.1
Introduction
Electro lum inescence (EL) is the generation of light by the application of an electric field to crys talline mat erials, or resulting from a current flow through semiconductors. Th e EL of ino rg an ic materi als is classified into the two groups: injection EL and high electric field EL. The high- field EL is further divided into two types: powder phosphor EL and th infilm EL. The classification of EL with regard to typical device applications is su m mar ize d as follows: EL
-r Injection EL L
Hi gh-fi eld EL
• Light-emitting diodes (LED), Laser di od es (LO)
-r Powder phosphor EL L
Thin-film EL
EL illumination p an els • EL d ispla y panels
Hi storically, the EL phen om enon was first observed by Destriau'-' in 1936, who observ ed luminescen ce produced from Zn S powder phosphors sus pend ed in cas tor oil wh en a strong electric field was a pp lied . Thi s type of EL is, today, classified as pow de r phosp hor EL. Lat er on, in the early 1960s, pol ycr ystalline ZnS thin film s were prep ared and use d as EL materials. Th is typ e of EL is typ ical of thin-film EL.
111
Fundamentals of Phosphors
112
On the other hand, in 1952 Haynes and Briggs" reported infrared EL from forwardbiased p-n junctions in Ge and Si diodes. This type of EL is classified as injection EL. Visible EL is observed in diodes made of wide bandgap semiconductors, such as GaP. These diodes are called light-emitting diodes (LEOs) and have been widely used since the late 1960s. Semiconductor lasers, first demonstrated in 1962 using GaAs diodes, operate by stimulating injection EL light in an appropriate optical cavity. As will be described below, the mechanisms of light generation in injection EL and high-field EL are quite different from each other. In addition, the applications of these EL phenomena to electronic devices are different. Usually, the term EL is used, in a narrow sense, to mean high-field EL. In this section, therefore, the description will focus on the basic processes of the high-field EL, in particular on the excitation mechanisms in thin-film EL. The mechanisms of injection EL are described only briefly.
1.10.2 Injection EL The term "injection EL" is used to explain the phenomenon of luminescence produced by the injection of minority carriers. Energy band diagrams for p-n junction at thermal equilibrium and under forward biased conditions (p-type side.positive) are shown in Figures 75(a) and (b), respectively. At thermal equilibrium, a depletion layer is formed and a diffusion potential Vd across the junction is produced. When the p-n junction is forward-biased, the diffusion potential Vd decreases to (Vd - V), and electrons are injected from the n-region into the p-region while holes are injected from the p-region into the nregion; that is, minority carrier injection takes place. Subsequently, the minority carriers diffuse and recombine with majority carriers directly or through trapping at various kinds of recombination centers, producing injection EL. The total diffusion current on p-n junction is given by:
(165)
where Op and On are diffusion coefficients for holes and electrons, Pno and o"n are the concentrations of holes and electrons as minority carriers at thermal equilibrium, and L p and L, are diffusion lengths given by where '"C is the lifetime of the minority carriers. The LEOs that became commercially available in the late 1960s were the greenemitting GaPN and the red-emitting GaP:Zn,O diodes. GaP is a semiconductor having an indirect bandgap; the Nand (Zn,O) centers in GaP are isoelectronic traps that provide efficient recombination routes for electrons and holes to produce luminescence in this material (See 2.8.2). Very bright LEOs used for outdoor displays were developed using III-V coxipound alloys in the late 1980s to early 1990s; these alloys all have a direct bandgap. Green-, yellow-, orange-, and red-emitting LEOs with high brightness are fabricated using InGaAlP, GaAsP, or GaAlAs (See 2.8.3). In 1993 to 1994, GaJnN (another alloy with a direct bandgap) was developed, leading to very bright blue and green LEOs (See 2.8.5). Thus, LEOs covering the entire visible range with high brightness are now commercially available.
,ro:t ,
Chapter one: Fundamentals of luminescence
113
n-rype
p-type
Ec ----:O·········.·.···l·····
-qV d
••
Ef<
.
Ey
(a)
• EL~
EFp Ev
..... 1:
OOOO~
~--=--O-----",----o
valence band
(b) Figure 75 Energy band di agrams for the p-n junction under (a) thermal equilibrium and un d er (b) forwar d-biased conditions.
1.10.3 High-field EL In the case of thin-film EL used for d isplay panels, a high electric field of th e order of 106 V em? is applied to the EL materi als. Electrons, which are the m ajority carriers in thi s case, are injected into th e EL emitting layers. The se electrons are accelera ted by the elect ric field until some of th em reach en ergies sufficien t to ca use impact exci tation of luminescent centers generating EL light. Th e m ost com mon luminescent cen ters in ZnS and other EL hosts are Mn 2 + and the rare- earth ions; these ac tiva tors offer a wi de variety of em iss ion
Fundamentals of Phosphors
114
colors. As noted above, the mechanism of high-field thin-film EL is quite different from that of injection EL. Here, the basic processes of high-field EL-that is, (1) the injection of carriers, (2) the carrier energy distribution in the high electric field region, and (3) the excitation mechanism of the luminescence centers-are discussed. In the case of high-field, powder phosphor EL, electrons and holes are injected by tunnel emission (field emission) induced by high electric field (106 V crrr') applied to a conductor-phosphor interface. The excitation mechanism is similar to that of thin-film EL, and is also discussed in this section.
1.10.3.1
Injection of carriers
Injection of majority carriers through a Schottky barrier:' When a semiconductor is in contact with metal, a potential barrier, called the Schottky barrier, is formed in the contact region. Before interpreting the Schottky effect in the metal-semiconductor system, one can consider this effect in a metal-vacuum system, which will then be extended to the metalsemiconductor barrier. The minimum energy necessary for an electron to escape into vacuum from its position within the Fermi distribution is defined as the work function q
vacuum
metal X=Q
r-----r::::::-:~==:::::=====~~~X
16rr
cOX
-.. -----.
--- ..
-,
"
.
..........
Figure 76 Energy band diag ram representing the Schottky effect between metal surface and vacuum. The barrier lowering under reverse bias is q(¢m - ¢B)'
Chapter one:
Fundamentals of luminescence
115
by U = q2/161If ox. When an ex tern al field E is applied, th e total p otential en ergy is given by: UT = q2 /1 61Ifox + qEx, as shown in the figure. Thus, at high field s, the potential barrier is lowered considerably, and th e effective metal work function for thermoionic em ission q$B is reduced. Th is lowering of th e potential barrier induced b y the ima ge charge is known as the Schottky effec t. The energy band diagrams for an n-typ e se micon d uc tor in con tact with a metal ar e show n for the case of thermal equilibriu m and under reverse-biased con di tions (se mic onductor side: positive) in Figures 77(a) and (b), resp ective ly. When an electric field is applied to the met al-semiconductor con tac t regi on , the potential energy is lowered in the semiconductor by the ima ge for ce or Schottky effect. The barrier height qcI>B is lowered as discu ssed and electr ons can be thermally injected into the se m icond uc tor. Th e current densit y for this p rocess is expressed as:
J = AT 2 exp
_ <1> - ( Ej 41If [
q
~T
Il
)V2 ]
s
(166) 1f2
aV -Texp( -y - k; 2
q<1»
where the permitt ivity in th e semiconductor e, is used instead of that in vacuum, Co. The injected electron s are then accelera ted by the electric field and exc ite the luminescent centers by imp act.
metal
semiconductor (n-type)
(a) \
...
············_ ·_···---~,;l
-~
(b)
conduction band
valence band
Figure 77 Energy band di agrams for the n-type semiconductor in conta ct with metal (a) un d er thermal equilibriu m and (b) under reve rse-b iased cond itions.
Fundamentals of Phosphors
116
In the case of ZnS:Cu EL p ow d er ph osphors, a Schottk y barrier is thou ght to be formed between th e n-Zn S semicon d u ctor and th e C u metal or th e conduct in g CU,S microparticles found in th e p h osphors . In th e latter, ele ctron injec tio n occ u rs from the conductive phase throu gh th e Scho ttky barrier and causes the elec tr oluminescence.
Injection of carriers due to Peele-Frenkel emission.' Semiconductors with fairly wide gaps of 3.5 to 4.5 eV (such as ZnS , CaS, and SrS) are used as EL materials. In these comp ounds, a large number of electro ns are usually trapped in traps caused by latti ce defects. Wh en an elec tric field is applied, trapp ed electrons are released into th e cond uction band, as sho w n in Fig ure 78 . This process is known as the Poole-Frenkel emi ssio n process and is due to field -enha nced th erm al excita tion of trapped electrons into the con d uction band. For an electron trapped by a Co u lom b-like potential U 1/ r", the exp ression for this process is id ent ical to th at for Scho ttky emiss ion. With the barrier height qlj>u red uced by the elec tric field as sh ow n in the fig ure, the current density due to Poole-Frenkel emissio n is expressed as : 0<:
J r = Eexp [ - q a - (qE/1tEJI/2 . kT
(167)
In th e case of th in-f ilm EL d evices, a fractio n of th e initial electro ns is injected by this p ro cess.
.... .... .... ........
ti····l··-. t
.
":-',"4.
....~
......... - qE x "
..... ... ..... "
......
-,
electron trap (q
~ S)
• x Figure 78 Energy ban d dia gram for deep electron traps un der high electric field. Electro n injection. due to Pool e-Frenke l effect is illustrated.
Chapter one: Fundamentals of luminescence
- qE
117
- qE
X
d:S: 100 A --------3.~
X
X
d:S: 100 A - - - - - - - - :.. ~ X
(a)
(b)
Figure 79 Energ y ban d diagram for (a) Scho ttky barrier and for (b) deep electr on traps under high electric field . Electron injection du e to the tu nneling effect is illustrated .
Injection of carriers due to tunnel emission (field emission)." When an extremely high electric field of over 106 V crrr" is applied to a Scho ttky barrier or to electro n traps, th e barrier width d becomes ver y thin, with a thickness in the ne ighborhood of 100 A. In this case, electrons tunnel directly into th e conduction band, as illustrated in Figure 79. Th e cur ren t den sit y due to this p rocess d ep ends onl y on the electric field and does not depend on temperature, and is d escr ibed by:
J
2
[-4(2r1l·t 2(QepB)Jl2]
= E exp ----''-----'---'------'3Qli E
(168)
where m' is the effective electron m ass. Since the ave rag e electri c field within thin p h osp h or films us ed in EL p an els is ne arly 106 V cm- I , it is possible to conclude that electron injection due to tunnel ing takes pl ace in add ition to Schottky and Poole-Frenkel emiss ion, w ith tunneling emiss ion becoming predominant at high electric field conditions. For powder-typ e EL d evices, it is kn own th at thin embedded Cu.S cond uc ting needles are formed in the ZnS mi crocryst als. Although the aver age applied electric field in the devic es is about 104-105 V em:' . the electric field is conce n trated a t the tips of these microcryst als and the locaI electric field can be 106 V crrr-' or more. Electrons are inje cted by tunnelin g from one end of the ne edl e and holes from the other end. This me ch an ism is kn own as th e bipolar field-emission model. The injected electrons recombine with holes, which were injected by the sam e process an d were trapped at cen ters pr ev iously, thu s produ cing EL.
Injection of carriers from interfacial state? When semicond uc tors are in con tact w ith insulators (diel ect ric materials), states are form ed at the interface having energy levels
Fundamentals of Phosphors
118
dielectric semiconductor (ZnS)
~-----'~. thennoelectronic emission
conduction band
valence band Figure 80 Energy ban d d iagr am of a dielectric materi al and a se m icond uctor in con tact. Electron injection from interfacial s ta tes under hi gh elec tric field is illu stra ted .
di stributed in the forbi d den bandgap of the se micond u ctors, as illu strated in Figure 80. The d ensity of the in ter facial sta tes is of the order of 1012_1013 crrr". When an electric field is applied, elec tro ns trapped in these states are injected into the cond uc tion band due to tunneling and/ or Poole-Frenk el emission. A typical ac-thin -film EL p anel has a doubly insulating s tructu re con sisting of glass substrate /ITO (indium-tin oxid e) transparent electrodes / insul atin g lay er /EL phosphor layer/insulating layer /metal elec trod es. For' this type of EL devi ce, the d ominant electron injection mechanism into th e EL phosphor layer is field emission from the insul ator /EL phosphor interfacial sta tes .
1.10.3.2 Electron energy distribution in high electric field At thermal equilibriu m , electrons in semiconductors em it and abso rb phon on s, but the net rate of energy excha nge between the electrons and the lat tice is zero. The en ergy distribution of electrons at thermal equilibrium is expressed by the Maxwell-Boltzmann distribution fun ction as :
f (e) = exp ( -
k~
l
m'v 2 2
E= -
(169)
Thi s di st ribution fun ction is spheri cal in m omentum sp ace, as illu str at ed in Figure 81(a). In the p resence of an electri c field, the electrons acquire ene rgy from the field and lose it to the lattice by emi tting m ore phonons. Simultaneously, the electro ns m ove with the drift veloci ty vd , proportion al to and in the direction of th e electric field . In this case, the energy di st ribution of th e electrons changes to a displaced Maxw ell-Bolt zmann distribution function (see Fig ure 81(b)) given by:
f(e ) = ex p ( -
k~
J
m'(v - vJ
E= -
-'--
2
"';';";'-
(170)
At moderately hi gh elect ric field ( ~10" V crrr' ), th e mo st frequent sca tter ing even t is the emission of optical pho no ns . Th e ele ctrons acquire on the average mo re energy than
Chapter one:
Fundamentals of luminescence
119
Vz electric field ~
Vy
Figure 81 Electron energy distributions is in the momentu m space as a func tion of electron velocity: (a) Maxwell-Boltzman n, (b) displaced Maxwell-Boltz mann, and (c) Baraff' s dis trib ution function.
they h ave a t thermal eq uilibrium describ abl e by an effective temperature T, h igher th an the lattice temperat ure Tv These electrons, therefo re, a re called hot electrons. Howev er, the en erg y of ho t electrons is still too low to excite lu minescent cen ters or to ion ize the lattice; the therm al en ergy of hot electrons is on ly 0.05 eV, even a t T, = 600K, while an energy of at least 2 to 3 eV is required for the im pact exci ta tion of lu minescent cen ters. When the electric fiel d in semicond u ctors is in creased above 10 5 V crrr' , elec tro ns ga in eno ug h ener gy to ex cit e lumines cent center s by imp act excitation and also to crea te elec tro n-hole pairs by im pac t io n iza tion of th e latti ce . Th e en ergy dis trib ution of th ese h ot ele ct ron s can be ex p ressed by Ba ra ff' s d ist ribution func tio n" (see Fig ure 81(c» , given by:
f( E) = e
' I OS
exp( -bE)
E ll - qE'A 2Eo +qE'A
a = - -· -
(171)
120
Fundamentals of Phosphors
,••••••.••.•......... _-._._--_.-_ •..........
1
··, ·
0) ..... ro
~
/ ....-
c
Shockley's theory
0 ......
ro
.-c N
·· · ··
0.5
0
~
..... () ro
l+- Wolffs
c,
E
theory
~
0
0
1
234
5
Average Energy qE A (eV) Figure 82 Dependence of impact ionization rate on the average electron energy calculated from Shockley's and Wolff's theories.
where Eo is the optical phonon energy and Ie is the mean free path of electrons.
In the case of moderately high electric field (qEA < E). When the electric field is moderately high, and the average electron energy, gEA, is smaller than the optical phonon energy El> ' Eq . 171 can be reduced to the following form:
(172)
This function agrees with Shockley's distribution function." and implies that some electrons with very high energy can exist, even in the case of relatively low electric fields. This model is, therefore, called the lucky electron model. The impact ionization rate increases when the average electron energy is increased, as illustrated by the solid curve in Figure 82. The threshold energy-in other words, the threshold field-for impact ionization is relatively low in this model.
In the C!1SC of extremely high electric field (qEA J> Eo). When the electric field is extremely high, and the average electron energy, gEA, is larger than the optical phonon energy Eo, Eg. 171 can be rearranged into the following form:
f( e) ex: exp - -3£E -() ] [
(qEAl
(173)
121
Chapter one: Fundamentals of lumin escence
This function agrees with Wolff's distribution function derived using th e diffusion appro ximation "; Eq. 173 gives a threshold energy for the impa ct ionization th at is higher th an that for the lucky electron m odel, as shown in Figure 82. Recently, Bring u ier v'? investigated electron tran sport in ZnS-type, thin-film EL. Two basic tr ansport modes in th e lu cky-drift theory are considered. Firs t, the ballistic regime, which is defined in terms of the optical-phonon m ean fre e path A and th e electron-phonon colli sion rate 1h m • This regime implies a co llis ion -free (balli stic) mode. Second is the drift regime, which is characterized by th e length Ae and the rate 1h c of th e energy rela xati on. Thi s m ode predominates aft er the electro n has suffered one co llision sin ce, on ce it has collided , it is d efle cted and the probability of other colli sions is g rea tly increa sed. In the ballistic mod e, an electron tr avels with a group ve locity Vg(f), so th at A = V g'l:m; while in the drift mod e, th e motion is go verned by a field-d ependent drift velocity v d(f) an d A= v d'l:e' The lucky-drift m odel may be applied to the ca se w h ere r, s- 'l:m and A" ;» A, which should hold true for wide-gap semiconductors in the hi gh-field regime. When these two ineq ua lities are fulfilled , ea ch collisi on results in an appreciable momentum loss for the electro n, with little ene rgy loss. Over th e energy relaxa tio n len gth, an electro n drifting in the field los es its m omentum and direction , but con serves much of its en ergy. The en er gy exch ange between electron s and phonons is d escribed by th e electronphonon interaction Hamiltonian, where elec trons ca n em it or abs or b o ne phonon at a time. Because a phonon is a boson , th e p robability of th e phonon occupation number ch anging from n to (n+ 1) is proportional to (n+ 1), while a ch ang e from n to (n-1) is proporti on al to n . Therefore, the ratio of the phonon em ission re(n --7 n+ 1) to th e phonon ab so rption ra(n --7 n-1) rates is given by (n +1)/n . Because r, > r a, a n elect ro n experiences a net ene rg y loss to the lattice, tending to stabilize th e electron drift. Hot e lec tro ns in hi gh electric field lose energy mostl y to optical phonons an d also to zone-edge acous tic ph onons, though somewhat less effect ivel y. A t temperature T, th e phonon occu pa tion n um be r n(O)) is g iven as nuo) = l / (exp( O)/kT)- l) . For ZnS, the optical phone ener gy (I) is 44 m eV Thus, one obtains an occu p a tion number, n(O)) = 0.223 a t 300K. The analytica l expression fo r the saturated drift vel ocity V s in th e lucky-d rift th eory is given by : I/2
nO)
V
s
.
= ((2n +1)m' J
(174)
whi ch yields 1.38 x 107 em S-1 a t 300K for electrons in th e energy minimum r p oint a t k = (000) of the cond uc tio n band. In order to assess th e elect ro n- phonon coupling, the electron-phonon sca tterin g rate 1h (= r,_ + ra, re/ r. = (n + l) /n) need s to be d etermined. From th ese rates , th e a verage en er gy loss per un it time of an elec tro n ca n be deri ved; in the s teady sta te, thi s loss offsets the energy gained by drifting in the field, yielding: nO)(r - r ) = c
{/
flO)
(2n + 1)'I:
= qEv
.;
= 10 13 eV
S- I
(175)
By su bstituting n = 0.223 an d 0) = 44 meV into Eq . 175, one obtains 1h = 3.2 x 10 14 s-', or an elec tron m ean free tim e of 'I: = 3 fs. The co mpetition between heating by the fiel d and cooling by a lattice sca ttering deter mines not only the av erage energ y f a\' but a lso th e nonequilib rium ene rgy d istribution function. The en ergy b al ance cond ition is obtained by setting the following eq uation to zer o.
Fundamentals of Phosphors
122
3 ,,-.,
>
(l)
'--'
;>
ro W
:>-.
2
•
on ~
(l)
I::
r..Ll (l)
on ro I-t
>
0.3
0.5
1
6
Solid line sh ows average electro n energy
lOa"
3
00 V/cm)
Electric Field E Figure 83
2
as a function of elect ric field E in ZnS at
300K obta ined from the ene rgy balance cond ition. Solid circles ar e the Monte-Carl o calculated
Ea "
(From Bringuier, E., f. A ppl. Phys., 75, 4291, 1994; Bhattach aryya, K., Goodn ick, S.M., and Wager, I.F. , f. A ppl. Phys., 73, 3390, 1993. With permission .) dc
-
dt
= qEv d
tiro
(2n + 1)1(E)
(176)
where l/"r(E) is the energy-dependent scattering rate. The average electron ene rgies Eav obtained from this equation are plotted in Figure 83 as a fun ction of the electric field E. It can be seen th at the average electron energy Ea ,. in creases sharply when th e electric field exceeds 2 x 106 V crrr'. An average electron energy Eov exceed ing 2 eV is suffici ent for the impact excitat ion of luminescent centers, as described in the next section . Recently, an ensemble Monte-Carlo sim ulation of electron tran sport in Zn S bulk at hi gh electric fields was performed." Scattering me chanism s associated with polar optical phonons, acousti c phonons, in ter-valley scattering in the conduction band, and im purities were included into a nonparabolic multi-valley model. The av er age electron en ergy Ea \, calculated in this way is also shown in Figure 83. Close agreem en t w as obtained between the Ea y values calculated by the Monte-Carlo method and those obtained b y the luck ydrift theory. Simulated results of the electron energy di stribution are show n in Figure 84, tog ether with the impact excitation cro ss-section for the Mn2+ center discussed in the next section . The results show th at energetic electrons are available at field strengths exceed ing 106 V crrr' to cause impact excitation, and that transient effects such as ballistic transport can be disregarded in explaining the excitation mechanism of th in-film EL.
1.10.3 .3
Excitation mechanism of luminescence centers
In EL phosphors presently used, there are two types of luminescent centers. One is the d onor-acceptor pair type, and the other is the localized center type. For the latter, Mn 2+ ion s producing luminescence due to 3ds intra-shell tran sitions are th e mo st efficient centers used in ZnS thin -film EL d evices. Some di valent and trivalent rare-earth ions emitting luminescence due to 4d n- ISd ~ 4f or 4fnintra-shell tran sit ions are also efficien t luminescent
Q
SI-
sII
f( e: )
~
'--'
.....
'"""a
;::
11(e:)
~
.."
411-
-->
ES
s~
\
"'::!
4
6
2 X 10 V/cm
1.5
X
;::
4
(ZnS)
;:: !:>. ;:::,
/' 4E(4D)
'" :::. ;:::,
...e:
6
10 V/cm
2+
Mn
6
1 X 10 V/cm
3
3
3
»
00
_ 4T2 (40) -
(4A 1.4E) (40)
\
4T2 (40) 4T 1 (40)
1-0 ~
... ~
1j)
~ ~ ~
S·
'" Vl
C
UJ
g
2
o
«
I
o
!
100
I
I
200
«
I
300
[
I
400
Electron Energy Distribution
(a)
)Ir
2
2
1
1
0,
o
I
,
20
40
I
60
I
80
I
)t
o
"'";:: "'"
6A1(6S)
100
Excitation Cross Section
(b)
(c)
(a) Electron energy distribu tion f(£), (b) Mn 2+ impact excitation cross-secti on a(£) as a fun ction of ene rgy, an d (e) energy levels of Mn 2 +. (From Bhattacharyya, K., Coodnic k, S.M., and Wager, J.E, f. App l. Phys., 73, 3390, 1993. With permission.)
Figure 84
>-l
N
W
Fundamentals of Phosphors
124
cen ters ; these ions are poten tial cand id ates for color EL. The excitation processes of these luminescent cen ters are described in this sec tion."
Electron-hole pair generation by hot electron impact ionization. In the ZnS host lattice, a high electric field of 2 x 106 V cm' is eno ug h to produce hot electrons. Consequently, th ese h ot electro ns ion ize the ZnS latti ce by collis ion, an d b y creating elec tron-hole pairs. This process is call ed impact ionizat ion of th e lattice. If imp ur ities, d on or and / or accep tor exist, they w ill als o be io nized . The electron-hole p airs are recaptured by the se ionized do nors and accep tors, an d luminescence is prod uced as a result of th e reco mbina tion of elec trons and holes. These p ro cesses are illustra ted in Figure 85(a). Th e ioniza tion rat e Pion of the la ttice is calc ulat ed us ing the follo wing eq uation:
~Vt1 l~( e)f( £ )d£ DC
(177)
g
where 0( £) is the ioni zat ion cross-section of the lat tice, 109 is the bandgap energy, and f(£) is the electron ene rgy distrib u tio n func tion. 0 (£) is p roportiona l to the produc t of the density of sta tes of th e va lence and cond uctio n bands. In cat hode-ray tubes, luminescence due to donor-accep tor pa ir reco m bination is very efficien t, an d ZnS:Ag,Cl and ZnS:Cu,Al(Cl) phosp hor s are widely and commonly used as blue and gr een phosp ho rs, resp ective ly. Zn S:Cu,Al(Cl ) p hosphors are also use d for powd er-typ e EL. However, these p hosphors are not efficien t w he n used in th in-film EL devices. Thi s is unders tood in terms of the reionization of the captured electrons and holes by the applied electric field p rior to their recombination .
Direct impact excitation of luminescent centers by hot electrons. If hot electrons in the host latti ce co llide di rec tly wi th localized lumin escent cen ters, the gro und -state elect rons of the cen ters are excited to higher levels, so th at lu m inescence is p rod uced, as illu strated in Fig ure 85(b). EL of ZnS:Mn 2+ is d ue to the impact excitation of the 3ds intra-sh ell configuration o f Mn 2+ cen ters . Sim ilarly, EL of trivalent rare-earth (RE)-do ped Zn S is ba sed on the im pact excita tion of the 4fn intra-she ll configura tio ns. This excitation mec hanis m is tho ugh t to be do mi nant in thirr-fil m EL d evi ce operation. Ass uming direct impact excitation, the excitation ra te P of cen ters can be exp ressed by:
(178)
w here 0(£,y) is the im pact excita tion cross-sec tion to the exci ted sta te y of the cen ters, f(£) is the energy d ist rib uti on of ho t elec tro ns discussed a bove, and Eo is the th reshold ene rgy for the excita tion . Alth oug h calculations of imp ac t exci ta tion and ion ization cross-sectio ns in free atoms or ions are very sop histicated and accu ra te, they are s till crude in solids. Allen 13 has pointed ou t tha t the proble ms lie in the form of the wavefu nc tions of the lu m in escen t cen ters to be used , especially w he n cova len t bonding wi th the host crystal is includ ed . The re is also a prob lem of die lectric screening. This screening sh ould be p roperly tak en as dependent on the energy and wave vector of carriers, or be taken appro ximatel y as a functio n of d ist ance r using the screened Coulomb po ten tia l exp resse d by <\l(r) = (-A /r)exp(-r/A D) , where AD is the po ten tia l decay coefficient. In add ition, the carrier ve loci ty is not a simple function of its ene rgy. Allen'> calc ulated a cross-section 0 for impac t excitati on using a
9 l':l
""':::l
c; ...,
(b)
(a)
(c)
<:>
::s ~
-.~
conduction band
~
;Il ~
if} ::l ::s ~
[
V>
impact ionization
~ .011I(
Y
impact excitation
12"
-s... .
/ /'
valence band
"" "::s""
V>
8
energy transfer
Mn2+ generation of e-h pairs and recombination
Mn2+, RE 3+
Figure 85 Three excitation pr ocesses of lumines cent cen ter s in thin -film EL de vices: (a) impact ion ization and recombina tion , (b) direct impact excitation, and (c) energy transfer. N
N
CJl
126
Fundamentals of Phosphors
simple Born-B ethe treatment of th e direct C oulomb term and obtained the following expression.
(179)
Here, 0 is ex pli ci tly w ri tte n as the product of three terms. Th e first term d escrib es the screening effec t, where e is th e dielectric cons tant, n , is the refractive index, and (£eff/ £O) is an effecti ve field ratio. The seco n d term is a fun ction of the properties of th e electron when th e incident excited elec tro n with th e initial ve locity v u and with wave ve ctor k., in the conduct ion band is sca ttered to a lower sta te 1with w ave vector k., the electron loses kinetic en ergy Egocor res pon d ing to the energy d ifference between the ground and excited sta tes of th e center. In th e sec on d term, m," is an electro n effec tive mass, and Slurepresents the value of th e o verla p integral of the Bloch functi on for the electron with wave vec tor k, and that for th e elec tro n with k., The third term is th e electr ic d ipole radiative transiti on rate of the center. For cen ters with radiative lifetimes in th e range of 10 us to 1 m s, the cross-section is es tima ted to be 10-18 to 10-20 em", which is too sm a ll to be useful. There is an app a rent di fference between th e nature of the elec tro ns in vacu u m an d in a solid like ZnS. ZnS is a d irect-b andgap se m icond uc tor havin g bo th th e b ottom of the con d u ction band an d th e to p of th e valence band a t th e r p oint k = (000). In vacuum, the veloci ty of ele ctrons increas es m on otonicall y with th e increase in its energy; whereas, in ZnS, it is possible to ha ve electrons w ith h igh ene rgy but low velocity in th e upper minima of the conduction band at th e L [k = (111)] and X [k = (001) ] valleys (see 2.7.2.3). As seen in Eq . 179, if th e ve locity of th e inc iden t electron V u is low w hen it has su fficien t energy Egc for impact exci ta tion, th e exc ita tion cross-section is consi d erably enh anced . Such a si tua tion is realized in ZnS:Mn 2+ when incident electrons in th e X or L va lley collide with Mn2+ centers and are sca ttere d to th e r valley; simultaneously, 3d" elec tro ns of Mn 2+ are excited from the ground to th e exci ted state. Exchange effects are e xp ec ted to dominate because of a resonance be tween th e en er gy sp acin gs of Mn 2+ centers an d th ose w ith in the conducti on ban d . The Born -Bethe treatm ent-i.e., the use of th e Fer mi Gol d en Rul e-is not ap propria te for excha nge processes. A lth o u gh there is n o si m p le approxima tio n to calc u la te ra tes o f exc ha nge p ro cesse s, so me qualitati v e con cl usions can be dra wn from our kn owledge of w ha t hap p ens in the impact exc ita tion of free at oms. The long-range part of th e in terac tion is no longer d om in ant; so in stead of th e p erturbin g H am ilt onian in th e di p ole app roxima tion, one must use the full Co u lom b ter m . Th e exc ha nge interaction is p re do mi na ntly at the short ra nge. H ence, in a crysta l of h igh d ielectr ic constant, th e direct in teraction is screened m uch more effectivel y th an th e exc ha nge in teraction . This is lik el y to be th e cause for th e large cross-section. In ZnS:Mn 2+, the cross -sec tio n fo r the impact excitation is th en doubly enhan ced. One can , therefore, conclude th at ZnS:Mn 2+ is a suitable comb inati on of h ost and cen ter that p roduces efficient EL by th e impact exci ta tion . This is because Zn S satisfies the need for a host in which hot ca rriers ha ve a su itable ene rgy di st ribution, an d M n 2+ has an unusually la rge cross-section near the th reshold. In Figures 84(a ) an d (b), th e e lec tro n e ne rgy di stribution f(£) and exc itation crosssec tion of Mn" centers 0 (£) a re illus tra ted. II Th e en ergy levels of Mn2 + cen ters ar e also sho w n in Figure 84(c) . Th e pea k in f(£) n ear 2 eV impli es th at a large elec tron pop ulation ex ists in th e X an d L va lleys, where elec trons ha ve su fficie n t energy to exci te Mn2 + centers,
Chapter one: Fundamentals of luminescence
127
but have relatively low velocities. The threshold energy for the excitation is a little smaller than the lowest excited state 4T1 of Mn 2+. This results from the br oadening due to the uncertainty principle. As seen from the Figures 84(a) and (b), at electric fields of the order of 1 x 106 V crrr' and larger, a sign ifican t fracti on of th e total electron population exists at energies exceeding the thr esh old excitation energy of 2.1 eV for Mn 2+. A good match of the hot electron distribution with the Mn2+ excitation energy br ings about the relati vel y high EL efficiencies in this system; the efficien cies are of the order of 4 to 6 lm W-l. It was dernon strared v' th at ZnS:Tb 3+0 2-F- thin-film EL devices show efficient green EL with an efficien cy of the order of 1 to 2 lm W-l. It has been show n that in TbOF comp lex centers, Tb3+ substitutes into the Zn 2+ sit e, 0 2- substitutes in to the 52-site, and F- is located at an interstitial site to compensate for th e ch arge difference. Th erefore, TbOF cen ters seem to form centers isoelectronic with ZnS . ZnS:Tb3+F- EL films with a Tb :F ratio of unity, with Tb3+ at Zn 2+ sites and interst itial F- ion s, also form isoele ctric centers that show efficient EL. It is believed that these isoelectronic centers have larger cross-s ections for impact excitation than those for isolat ed rare-earth ions, so th at high EL efficiencies res u lt.
Energy transfer to luminescent centers. In AC powder EL phosphors such as ZnS:Cu,Cl, donor (Cl)- accep tor (Cu) (D-A) pairs are efficien t luminescent centers (see 2.7.4), and the EL emission is caused by the radiative recombinat ion of electron-hole pairs through D-A pairs. Electrons and holes are injected into the ZnS lattice by bipolar field emission. By further inco rp or ating Mn2+cen ters in ZnS:Cu,Cl, yellow emission due to Mn2+ is observed . In this case , the excitation of Mn 2+ cen ters is due to the nonradiative resonant energy tran sfer from D-A pairs to Mn 2+, as illustrat ed in Fig u re 85(c). Jonization of centers and recapture of electrons to produce lum inescence. In thin-film EL of rare-ear th-do ped IIa-VIb comp ound s such as blu e/green-emitting SrS:Ce 3+ and redemitting CaS:Eu 2+, the tran sient beh avior of the EL emission p eaks under pulse excitation exhibits emi ssion peaks when the pulsed voltage is turned on and turned off; in other words, the second peak ap pears when the electric field is reversed in the di rection due to polarization charge trapped in the phosphor-insulator interfaces." The luminescence of these phosphors is due to th e 4/"-15d ~ 4/" transit ion. It is probable that the 4/" groundstate level is located in the forbidden ga p, whil e the 4/"-15d excited state is close to th e bottom of the conduction band. The EL excitation mechanism is as follow s: the luminescence centers are excited by the impact of the electron accel erated by the pulsed voltage and then ioni zed by the applied pulsed field. Electrons released to the cond uction band are captured by traps. Thi s process ha s been exp er imentally confirmed b y m easurements of the excit at ion spectra of the photoinduced conductivities . IS When the voltage is turned back to zero, the trapped electro ns are raised by the reversed field to the cond uction band again and are recaptured by the ion ized centers to produce luminescence.
References 1. Destriau, G., f. Chim. Phys., 33, 620, 1936. 2. Destr iau , G., Phil. Mag., 38, 700, 1947; 38, 774, 1947; 38, 880, 1947. 3. Haynes, J.R. and Briggs, H.B., Phys. Rev., 99, 1892, 1952. 4. Sze, S.M., Physics of Semiconductor Devices, 2nd editio n, John Wiley & Son s, New York, 1981, chap. 5 and 6. 5. Kobayashi, H., OptoelectronicMaterials and Devices, Proc. 3rd Int. School, Cetniewo , 1981, PWNPolish Scien tific Publ ish ers , Warszawa, 1983, chap . 13. 6. Baraff, G.A., Phys. Reo., 133, A26, 1964. 7. Shockley, w., Solid State Electron., 2, 35, 1961.
128
Fundamentals of Phosphors
Wolff, P.A., Phys. Rev., 95, 1415, 1954. Bring uier, E., f. A ppl. Phys., 66, 1314, 1989. Brin gui er, E., f. Appl. Phys., 75, 4291, 1994. Bhattacharyya, K., Goodnick, S.M., and Wager, J.P., f. Appl. Phys., 73, 3390, 1993. Kobayashi , H ., Proc. SPIE, 1910, 15, 1993. Alle n, ].W., Springer Proc. in Physics 38, Proc. 4th Int. Workshop on Electroluminescence, SpringerVerl ag, H eidelberg, 1989, p. 10. 14. Okamoto, K., Yoshimi, T., and Miura, S., Springer Proc. in Physics 38, Proc. 4th Int. Workshop on Electroluminescence, Springer-Verlag, Heidelberg, 1989, p. 139. 15. Tan aka, S., f. Crystal Growth, 101, 958, 1990. 8. 9. 10. 11. 12. 13.
chapter one - section eleven
Fundamentals of luminescence Pieter Dorenbos
Contents 1.11 Lanthanide level loca tio ns and its impact on p hosp hor performance 1.11.1 In troduction 1.11.2 Level position and ph ospho r performance 1.11.3 The free (gaseous) lan tha n ide ions 1.11.4 4f-5d energy d ifferences of lanthanide io ns in compounds 1.11.5 Me thods to determine absolu te level location s 1.11.6 Sys tema tic variatio n in absolu te levellocat ions 1.11.7 Fu ture prospects an d pretailoring phosphor properties References
129 129 130 133 134 137 137 142 142
1.11 Lanthanide level locations and its impact on phosphor performance 1. 11 .1 Introduction The lant h anide ions either in their divalent or trivalent charge state form a ve ry importan t class of lu m in escen ce activa tors in phosphors and single crysta ls .' The fas t 15-60 n s 5d--4f emission of Ce 3 • in compou n ds like LaC l3, LaBr 3, LUzSiO s, and Gd zSiO s is u tilized in scinti llators for "(-ray d etection.' The same emission is utilized in catho de ray tubes and electrol uminescence phosph ors. The photon cas ca de emission involving th e 4? leve ls of Pr 3+ has been in vest igat ed for d ev eloping hi gh qu antum efficiency p hosphors excited by mean s ofaXe d ischarge in the vacuum-Uv' The narrow-line 4£3 transiti ons in Nd 3• are used in laser crys ta ls like Y3AISOJ z:N d l•. Sm 3 ' is u tilized as an efficient elec tro n trap and mu ch research has been devoted to its informa tion storage properties . For examp le, MgS:Ce 3+;Sm 3+ and MgS :Eu z+;S m 3+ were studied for optica l m emory phosp hor applica tion s.' YzSi0 5:Ce 3+ ;Sm 3• was s tu d ied for X-ray imaging p hosphor app lica tio n s," an d LiYSi0 4:Ce3' ;Sm 3• for the rm al neutron imagi ng phosphor applica tio ns .s The famous 5Do-7 7F, 4f6 redline emissions of Eu 3 • an d the blue to red 5d -4f emission of Eu> are both used in d ispl ay and lighting ph osphors.' Th e 4£8 lin e emission of Tb3. is of ten res pons ible for th e green com po ne n t in tricolo r tube lightin g.' D y 3+ p lays an im po rtant rol e in the persistent luminescence phosphor SrAl z04:Eu z+;D y 3+,7.8 Er3' and Tm 3• a re, like Pr 3. , investigated for possible p hoton cas cade em ission p ho sp hor applica tio ns. 129
130
Fundamentals of Phosphors
This bri ef and st ill incomplete summary illustrates the diver sity of applications invol ving the luminescen ce of lanthanide ion s. It also illustrates that w e can distinguish two types of lanthanide luminescent transition s. (1) Transitions between levels of the 4fn configuration . In thi s chap ter, the energy of ea ch 4f'I excited st at e rel ative to the lowest 4f" state will be reg arded as invariant w ith the type of compound . One may then use the Dieke diagram with the extension prov id ed by Wegh et aJ.9 to id entify the many po ssibl e luminescence em ission and optical ab sorption lines. (2) Transitions between the 4f"-1 Sd and the 4f" configurations. The ener gy of Sd levels, contrary to the 4f levels, depends very strongly on the type of compound. For example, the wavelen gth of the Sd--4f emission of Ce 3+ may range from the ultraviolet region in fluorides like that of KMgF 3 to the red region in sulfides lik e th at of Lu 253 . lo In all phosphor applications the color of emission and the quantum efficiency of the luminescence process are of crucial importance as is th e th ermal s tability of the emiss ion in some ap p lica tions . These three asp ects are related to the relative and absolute location of the lanthan ide energy lev els. For example, the position of the ho st-sensitive lowest Sd state relative to the host-inv ariant 4f s tates is important for the quenching beha vior of both Sd--4f an d 4f-4f emissions by multiphonon rel axat ion . The absolute position of the 4f and Sd stat es relative to val enc e band and conduction band states also affects lum inescence qu enching and charge-trappin g phenomena. Although it was realized lon g ago that absolute location is crucial for phosphor performance, the experimental and theoretical understanding of the placement of en erg y levels relativ e to the intrinsic bands of the host ha s been lacking. In th is sec tion, first, a su rvey is provided on ho w rela tiv e and absolute locati ons of lanthanide energy levels a ffect phosphor performance. Next, methods and m odels to determin e relative and ab solute locations are treat ed . After d iscu ssing the en ergy level s of th e free (or gaseous) lanthanide ions, the influen ce of the host compound on the location of th e Sd levels relative to the 4f levels is presented . Next, the influence of the host compound on the absolute location of the lowest 4fn s ta te above the top of the valence band is expl ained. This forms the basis for drawing sche m es for the absolute placement of both the 4f and Sd states of all the divalent and trivalent lanthanide ions.
1.11 .2 Level position and phosphor performance The importance of the relative and absolute po sition s of the energy levels of lanthanide ions is illustrated in Figure 86. We distinguish occupied states that can donate electrons and empty sta tes that can accept electrons. Let us start with the "occu pied states." Figure 86(a) illustrates the downward sh ift of the lowest-energy Sd level when a lanthan ide is brought from the gaseous state (free ion) into the crystalline environment of a com p ound (A). Due to the interaction with th e neighboring anion ligands (the crystal field interaction), the deg en erate Sd levels of th e free ion sp lit (cry stal field splitting), d epending on the site sym me try. In ad d ition, the whole Sd configuration shifts (centroid shift) toward lower ene rgy. The crystal field sp litting comb ined w ith the centroid shift lowers the lowest Sd lev el with an amount kn own as the redshift or d epression D. Clearly the value of 0 det ermines the color of e m issi on and wavelength of ab sorption of the 4f-Sd transitions. Figure 86(b) illustrates the im p or tan ce of lowest-energy Sd level location relati ve to 4£2 lev els in Pr3+ . With th e Sd level above the 150 level of Pr 3+, multiphon on relaxation from th e lowest Sd state to the lower lying 150 lev el takes place. A cascade emission of two photons may result, which lead s to quantum efficienc y larger than 100%. However, with the lowest Sd state bel ow ISO, broad-band Sd--4f emission is observed. Much research is devoted toward th e sear ch for Pr 3+ quantum-splitting phosphors and for finding efficient Sd--4f-emitting Pr 3+-doped mat erials for scin tilla tor applications. Dependin g on the precise
Chapter one:
~)
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Fundamentals of luminescence
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Figure 86 Illustration of influe nce of level location on phosp hor properties: (a) the redshift 0 of the Sd state, (b) photon cascade emission in PrJ+, (c) Sd-4 f em ission quench ing by au toioniza tion, (d) anomalous Sd em ission, (e) thermal quenching by ionization, (f) qu enching b y int erval en ce charge transfer, (g) valence band cha rge tran sfer, (h) charge transfer lum inescenc e, (i) electron trap ping by Sm'", (j) ho le trapping by Ce 1+, (k) electro n tran sfer from Eu 2 + to Srn" , (l) lu min escence quenchi ng by lan thanide to lanthanide charge transfer.
locati on of the lowest Sd sta te in Nd 3+, Eu 2+, an d Sm 2+, either broad-band Sd-4f or narrowline 4f-4f emissions ca n be observ ed ." Figure 86(c), (d ), an d (e) sho w the in terp lay between th e localized Sd elect ron and the delocal ized con d uc tion band sta tes . If th e lowest Sd st at e is above the bottom of the cond uc tio n band as in Figure 86(c), a u toionization occu rs spon taneous ly an d no Sd-4f emission is observe d . This is th e case for LaAI0 3:Ce 3+, rare-earth sesquioxides Ln z0 3:Ce3 +, 1 and also for Eu 2+on trivalent rare-ear th sites in oxid e comp ounds. F Figure 86(d ) illustrates the situa tion wi th Sd ju st below th e con d u ction ban d . The Sd electron d elocalizes but remains in the vicini ty of the hol e left behind . Th e true nature of the s ta te, w hich is som etim es called an impurity trapped exciton sta te, is n ot precisely kn own . The recombinati on of th e electron w ith th e h ole lead s to the so-ca lled an omalous em ission cha racterized by a very large Stokes shift. 13•14 Finally, Fig ure 86(e) sh ows th e situa tion with th e Sd st ate we ll below the conduction band, leading to Sd-4f emission. Th e thermal q uenc hing of th is emission by means of ion izatio n to conduction b and states is con tro lled by th e energy EdC betw een the Sd s tate (d) and th e bottom of the con d u ction b and (C) .13.1 6 A revi ew on th e relationship between EdC for Eu 2 + and th ermal qu enching of its 5d-4f emissi on rec ently appeared ." Knowled ge on such relation ships is im p or tan t for de velopin g temperaturestable Eu -r-d oped light-emitting diod e (LED) phosphors or temperatu re-stable Ce 3+ -doped scin tilla tors. For elec tro lu minescence ap p lica tions, Ed C is an import ant paramete r to di scrim in ate the me chanism of im pac t ion ization agains t th e m echanism of field ionizati on ." Figure 86(f) sh ow s a typical situation for Pr 3 + in a transition m et al complex com p o und like CaTi 0 3 • Th e undesired blue emission from the Pr 3+ 3PO le vel is quenched b y
132
Fundamentals of Phosphors
intervalence charge transfer (IVCT).18 The electron transfers from the 3Po level to the transition metal (Ti4 +). Th e electron is transferred back to the red emi tting Pr J+ 104 level. The position of the 3PO level relative to the transition metal-derived condu ction band controls the quenching process, and th ereby th e color of emission . So far we have di scu ssed examp les of ab solute location of "occupied states." However, a tri valent lanthanide ion may accep t an electro n to form a divalent lanthanide ion. The location of the occupied gro und -state level of a di valent lanthanide ion is therefore the sa me as the unoccupied electron-accepting st at e of the corresponding triv alent lanthanide ion . The accepted electron m ay originate from the va lence band, the con d uc tion band, or ano ther lanthanide ion. Figure 86(g) pertains to a Eu 3 +-doped compound . Eu 3+ introduces an unoccupied Eu ?" state in th e forbidden ga p . The excita tion of an elec tron from the valence band to the unoccupied s tate creates th e groun d state of Eu?". Th is is a dipoleallowed transition that is used, for exampl e, to se nsi tize Yz03:Eu 3+ phosphors to the 254 nrn Hg em ission in tube lighting.' Recombination of the electron with th e valence band hol e leaves the Eu 3+ion in th e 50 0 excited state res ulting in red 4f6-4f6 emi ssion. Figure 86(h) shows a similar situation for Yb3+. In the case of Yb3+ the recombination with the hole in the valence band produces a strong Stokes-shifted charge transfer (CT) luminescence. This type of luminescence gained cons ide rable interest for d eveloping scintilla tors for neutrino detection ." Clearly, the absolute location of the d ivalent lanthanide ground sta te is important for CT exc itation and CT luminescence en ergies. Figure 86(i) shows the trapping of an electron from the conduction band by Sm 3+ to form the gro un d sta te of Sm 2 +. Th e absolute location of an "unoccupied" div alent lanthanid e ground state determines the electron trapp in g depth provided by.the cor resp onding tri valent lanthanide ion . On the other hand, the abs olu te location of an "occup ied " lanthanide gro u nd state determines the valence b and hole trapping depth provided by that lanthanide ion. Figure 86(j) illus trates trapping of a hole from the va lence band by Ce 3+. Th is hole trapping is an important aspect of th e scin tillation mechanism in Ce 3+doped scin tilla tor s. Similarly, Eu-" is an efficient hole trap of importan ce for the X-ray s torage phosphor Baf'Br .Eu >. Phosphor properties become more complicated when we deal with "do uble lanthanid e-d oped systems." Figu re 86(k) shows the situation in Eu?" and Sm 3 +double-doped compounds lik e SrS and MgS that were studied for op tical data storage applicat ion s.v" The ultraviolet write pulse excites an electron from Eu z+ to the conduction band, which is then trapped by Sm 3+. Eu 3+ and Sm z+ are created in the process. An infrared read pulse liberates th e electron again from Srn?". resulting, eventuall y, in Eu?" Sd--4f emission. Similar mechanism s appl y for YzSiOs:Ce3+ ;Sm 3+ and LiYSi04 :Ce 3+;Sm 3+ compounds that were developed for X-ray and th ermal neutron storage phosphor applications, respectively.v' The true m ech anism in the p ersistent luminescence phosphor SrAl z04:Eu 2+;Oy3+ is still disputed. One ne eds to know th e absolu te level en ergy locations to arrive at plau sible mechanisms or to d iscard implausible ones." As a last example, Figure 86(1) sh ow s quenching of emi ssion in Ce 3+ and Eu 3+ co-d oped systems. Th e Ce 3+ electron excited to the lowest 5d state can jump to Eu 3 + when the unoccupied Eu?:' gro und state is locat ed at a lower ene rgy than the occupied lowest Ce3+ Sd excit ed s ta te. After the jump, Eu 2+ and Ce 4 + are formed . The Eu" electron can jump back to Ce 4 + if the unoccupied Ce-'+ ground state is locat ed below the occupied Eu?" ground state. The origi nal s ituation is rest ored without emi ssion of a photon. Similar quenchi ng routes p ertain to Ce 3+in Yb-based com poun d s, and with ap p rop riate level schemes, other "killin g" comb inations can be found as well. The a bove se t of examples shows the importance of ene rgy level locati ons for the performance of phosphors. Thi s im po rtan ce was realized lon g ago, but not until recentl y methods and m od els became av ailabl e that allow the determination of these abso lu te
Chapter one: Fundamentals of luminescence
133
p ositions. In the follow ing sec tions , the h istoric developm ents and current sta tus of absolute level pos ition ing are briefly review ed . Fo r d etail ed in form ation, origin al literature sho u ld be consulted .
1.11 .3 The free (gaseous) lanthanide ions The previous section illu st rat ed the import an ce of lanthan id e level locati ons for p hosph or per forman ce. To un derstan d an d p red ict th ese location s we firs t need to und er stand the properties of the free (gaseous) lanthani de ions. Fig ure 87 shows the d at a avai lab le on the energy (Efd ) needed to exci te an ele ctron from the low est le vel of the 4fnS d ll6s lll config uratio n to the low est level of the 4f"- I Sd 16s lll configura tion in the ga seous free lanthan id e ion s or atoms . The data are fro m Brewer" and M artin" togeth er wi th lat er up d at es." Data are most complete for th e n eutral atoms (m = 2, curve c), the mo n ovalen t lanthan id es (m = I , curve b), an d the div alent lanthani d es (m = 0, curve a). A universal curve, cur ve a in Figure 87, can be cons tr uc ted . By shifting the en er gy of thi s universal curve, the 4f-Sd energies as a fun ction of n can be reprod uce d irrespective of th e charge of th e lanthanide ion (0, +1, +2, or +3) or the number, m, of electrons in 6s (m = 0, I , or 2). This re marka ble phenome n on is d u e to the inner-shell nature of the 4f or bita l. Ap paren tly, the occu p ati on number of electrons in the 6s shell h as no influence on the universal be havior. The main features of th is un iversal va riation have been known for a long tim e and understood in terms of [orgensens spin pairin g th eory for th e bind in g of 4f ele ctron s.F The energy is large w he n the 4f configura tion is half - (11 = 7) or completely (11 = 14) fille d, and the energy is small when it is occu pie d by on e or eig h t electro ns . Figure 88 shows the binding ene rgy (or ioniza tion energy) of the 4f and 5d electrons in the free divalent an d free b-ivalent lanthanide ions wi th m = O. Whe n we ad d the corresponding energies, Efd , from Figure 87 to curv es b and d in Figure 88, w e ob tain the bind in g energies for the 5d electro n (see curves a and c). Th e stronger bindin g of the 4f and 5d electrons in the trivalent lanthani d es than in the d ivalent ones is due to a stron ger Coulomb attrac tion. Clearly, the binding of the 4f electron is resp onsible for th e un iversal behavior in the 4f-5d transitions. The bindin g energy of the Sd electron is ra the r cons tant wi th 11 w hic h indica tes that the na ture of the 5d sta te is relative ly invarian t wi th the typ e of lanthanide ion .
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Fundamentals of Phosphors
134
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1.11 .4 4f-Sd energy differences of lanthanide ions in compounds Figure 87 indica tes that the variation of Efd with n d oes not depen d on the cha rge of the lanthanide ion or on the number of electrons in the 6s orbital. It is also we ll establish ed that the Dieke diagram of 4f ene rgy levels is almost invarian t wi th the type of com po und. The situa tio n is com pletely differen t for the 5d sta tes . Th eir energies are in flue nce d 50 tim es stro ng er b y the ho st com pou nd than those of 4f s tates . Du e to crys tal field sp litt ing o f the 5d s ta tes and a shi ft (cen troi d shift) of th e average energy of the 5d con figuration, the lowest level of the 5d configur at ion decreases in energy as illustrat ed in Figure 89 for ee3+ in LiLu F4 (see also Figure 86(a)). The decrease is kn own as the re ds hift or depression O(n,Q,A ) = O(Q,A ) where n, Q, and A stand for the number of electron s in the 4f" ground sta te, the charge of the lanthanid e ion , and the name of the compo un d, respectivel y. The red shift depend s very strongly on A but ap pears, to good firs t approxim ation , independent of 11, i.e ., the typ e of lanthanide ion . Th is impl ies th at both th e crystal iield splitting and the cen troi d shift of th e 5d levels depen d on the typ e of com p ound but to a good first ap p ro xim ation are th e sam e for each lanthanide ion . Figure 90 shows this principle. It is an inverted Dieke diagram w here the zero of ene rgy is at th e lowes t Sd state of the free trivalent lanthanide ion . When the lanthanide ions are present in a comp ound, one simp ly need s to shif t the Sd leve ls down by the reds hi ft O(3+,A ) to find the ap propria te d iagr am for that com po und . Fig ure 90 illustrates th is for LiLuF 4 • Th e 4f-Sd tr an s ition ene rgy of each lan th anide ion can be read from the diagram . In eq uation form thi s is wri tten as:
Efd (n,3+,A)= Efd (n ,3+, fre e) - 0(3+, A)
(180)
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Chapter one:
Fundamentals of luminescence
135
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Figure 89
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The inverted Dieke d iag ram w here the en ergy of the lowe st Sd level of the free trivalent lanthanide ions are defin ed as the zero of ene rgy. A d ownward shi ft of the Sd level s w ith the red shift value 0 = 1.9 eV pro vides the relative position of the lowest Sd level for the trivalent lan tha n ides in LiLuF 4 •
Figu re 90
136
Fundamentals of Phosphors
5 Oxysulfides sulfides
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Figure 91 The redshift D(3+,A) for trivalent lanthanide ions in compounds. The parameter along
the horizontal axis groups the data depending on the type of compound. related with the shape and size of the first anion coordination polyhedron.Y" The small fluorine and oxygen anions provide the largest values for the crystal field splitting and this is the main reason for the large spread in redshift values for these two types of compounds." With the compiled redshift values, one may predict the 4f-5d transition energies for each of the 13 trivalent lanthanide ions in several hundreds of different compounds using the very simple relationship of Eq. 180. Eq. 180 equally well applies for the 5d-4f emission because the Stokes shift between absorption and emission is to first approximation also independent of the lanthanide ion. For the divalent lanthanide ions in compounds the story is analogous.o-" Again one can introduce a redshift D(2+,A) with a similar relationship Efd(n,2+,A)
= Efd(n,2+,free)-D(2+,A)
(181)
and construct figures like Figure 90 and Figure 91. With all data available on D(3+,A) and D(2+,A), the redshift in divalent lanthanides can be compared with that in the trivalent ones; a roughly linear relationship is found." D(2+,A)
= 0.64D(3+,A)-0.233eV
(182)
Investigations also show a linear relationship between crystal field splitting, centroid shift, and Stokes' shift." Combining Eqs. 180, 181, and 182 with the available data on D(2+,A) and D(3+,A), it is now possible to predict 4f-5d energy differences for all 13 divalent and all 13 trivalent lanthanides in about 500 different compounds, i.e., about 13000 different combinations! Usually, the accuracy is a few 0.1 eV but deviations occur. The work by van Pieterson et al,2Y,.lIJ on the trivalent lanthanides in LiYF4 , YP0 4 , and CaF 2 shows that the crystal field splitting decreases slightly with the smaller size of the lanthanide ion. In these cases the redshift may not be the same for all lanthanide ions. A study on the crystal field splitting in Ce " and Th3+ also revealed deviations of the order of a few tenths of eV from the idealized situation expressed by Eqs. 180, 181, and 182.31
Chapter one:
Fundamentals of luminescence
137
1.11.5 Methods to determine absolute level locations The experimental basis for the results in the previous sections are from the 4f-'5d energy differences, which are easily measured by means of luminescence, luminescence excitation, or optical absorption techniques. We deal with a dipole-allowed transition from a localized ground state to a localized excited state involving one and the same lanthanide ion. Both states have a well-defined energy. To determine the location of energy levels relative to the valence band or to the conduction band is not straightforward. Again one may use information from optical spectroscop y. Figure 86(g) shows the transition of an electron from the top of the valence band (an anion) to Eu 3+. The final state is the 4£7 ground state of Eu 2+. The energy needed for this CT provides then a measure for the energy difference between the valence band and the Eu 2+ ground state. Wong et aP2 and Happek et aP3 assume that the CT energy provides the location of the ground state of the electron-accepting lanthanide relative to the top of the valence band directly. However, this is not so trivial. The transferred electron and the hole left behind are still Coulomb attracted to each other, and this reduces the transition energy by perhaps as much as 0.5 eV. On the other hand Eu 2+ is about 18 pm larger than Eu 3+, and the optical transition ends in a configuration of neighboring anions that is not yet in its lowest-energy state. Both these effects tend to compensate each other, and fortuitously the original assumption by Wong et al., and later by Happek et al., appears quite plausible." The location of occupied 4f states relative to the occupied valence band states can also be probed by X-ray or UV photoelectron spectroscopy (XPS or UPS).35 With the techniques mentioned in the preceding paragraph, the localized level positions of lanthanide ions relative to the valence band states can be probed. The level locations relative to the conduction band can be determined with other techniques. Various methods rely on the ionization of 5d electrons to conduction band states. The thermal quenching of 5d-4f emission in Ce 3+ or Eu 2+ is often due to such ionization processes.lv" By studying the quenching of intensity or the shortening of decay time with temperature, the energy difference, Ed C, between the (lattice relaxed) lowest 5d state and the bottom of the conduction band can be deduced from their Arrhenius behavior." Such studies were done by Lizzo et aP6 for Yb2+ in CaS04 and SrB40 7, by Bessiere et al.'? for Ce 3+ in CaGa 2S 4 , and by Lyu and Harnilton.l'' Also, the absence of Ce 3+ emission due to a situation sketched in Figur e 86(c) or the presence of anomalous emission as in Figure 86(d) provides qualitative information on 5d level iocations.>' One may also interpret the absence or presence of vibronic structures in 5d excitation bands as indicative of 5d states contained within the conduction band." One- or two-step photoconductivity provides information on the location of 4f ground states relative to the bottom of the conduction band. 37--40 Another related technique is the microwave conductivity method developed by Joubert and coworkers that was applied to LU2SiOs:Ce3+41
1.11.6
Systematic variation in absolute level locations
The previous section provides an explanation on the techniques that have been used to obtain information on level positions. But often these techniques were applied to a specific lanthanide ion in a specific compound with the aim of understanding properties of that combination. Furthermore, each of these techniques provides its own source of unknown systema tic errors. These individual studies do not provide us with a broad overview on how level energies change with the type of lanthanide ion and the type of compound. Such an overview is needed to predict phosphor properties and to guide the researcher in the quest for new and better materials.
138
Fundamentals of Phosphors
One of the first systematic approaches wa s by Ped rini et al. who undertook photoconductivity measurements to determine the location of the 4f ground state of divalent lanthanides in the fluorite compounds CaF2, SrF 2, and BaF 2 relative to the bottom of the conduction band.'? They also provide a model to exp lai n the observed variation in 4f gro und-state energy with n. Th e first sys tem atic approach to determine the levels of trivalent lanthanides was undertaken by Thiel and coworkers using XPS.42A1 They stu d ied the trivalent lanthanides in YJAl S0 12 an d determined the 4f ground -st ate energies relative to the valence band of the host cryst al. They also combined th eir find ings w ith the systematic in 4f-5d energy difference found in Ref. 23 to locate th e 5d s tates in the band gap. The absolute energy of the lowest 5d s tate ap pears relatively con stant with the typ e of lanthanide ion. Both XPS and photoconductivity experiments have drawbacks. The oscillator strength for th e transition of the localized 4f ground state to th e delocalized conduction band states is very sma ll and photoconductivity is rarely observed due to such direct transitions. Twostep phot oconductivity is observed more frequently. After a dipole-allowed excitation to the 5d state, it is either followed by autoionization (see Figure 86(c)) or thermally assisted ionization (see Figure 86(e)). For the XPS experiments, h igh Ln-t -concentrated samples are need ed ,42.44 and one has to deal with uncertain final state effects to obtain reliabl e d ata ." At thi s m om ent the amount of information obtained with these two methods is scarce. Although th ey provide us with very valuable id eas and insight on how level energies cha nge with the type of lanthanide ion, there is not eno ug h information to obtain detailed insigh ts int o th e effect of type of com pound . Another m ethod to obtain the systematic va ria tion in level positi on wi th the type of lanthanid e is CT spe ctroscopy. It appears that the energy of CT to Sm 3+ is always (at least in oxide compounds) a fixed amount higher than that for the CT to Eu 3+. The same applies for Tm 3+ and Yb3+. Thi s wa s noticed long ag022A6A7 and recon firm ed by more recent stud ies.4!>-5o An elaborate analysis of data on CT retrieved from th e literature revealed that the sys tem a tic behavior in CT energies holds for all lanthan ides in all typ es of different compounds.>' Figure 92 illustra tes the m ethod to construct diagrams with absolute level location of the di valent lanth anide in CaGa 2S4. The top of the valence band is defined as zero of energy. Th e arro ws numbered 1 through 6 show the observed ene rgies for CT to trivalent lanthanide ion s, and they prov id e us with the location of th e ground state of the corre sponding di valent lanthanides (see Figure 86(g)). Using th ese data we can cons truc t precisely the sa me universal curve, but in an inverted form, as found for the en ergy Efd of 4f-5d transition s in th e free lanthanide ions and atoms of Fig ure 87. Ar row 7 shows the energy of the first 4f-5d tr ansition in Eu 2+. Using Eq. 181, the abso lute locati on of the lowest 5d state for each divalent lanthanide ion can be d rawn in the scheme . It appears constant with n, The universal beh avior in th e energy of the lowest 4f state w ith 11 is determined by the binding of 4f electrons, similar to that depicted in Figure 88, but mod ified by the Madelung potential at the lanth anide si te in the comp ound . Th is Madelung potential incr eases w ith sma ller size of the lanthanide ion due to th e inw ard relaxation of the nei ghboring ne gati vely charge d an ions .14,3.1.39AJ The increase in 5d electron binding ene rgy by 1-2 eV, as observed fo r the free d iv alent lan thanides in Figure 88, is absen t in CaGa 2S4 where the binding of th e 5d electron is found independent of II. Thi s fortuitous situation for CaGa 2S4 , which is also expected for other sulfide compounds, does not apply to oxides and fluorid es. For th ese compoun ds it wa s found that from Eu 2+ to Yb2+ the binding of the levels grad ua lly decrease by abou t 0.5 eV.J.l.34 In other words, the 5d state of Yb2+ is found 0.5 eV closer to th e bottom of the conduction band than that of Eu 2+, w hich is
Chapter one: Fundamentals of luminescence
139
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Figure 92 The location of the lowest 4f and lowest Sd states of the divalent lanthanide ion s in CaGa 2S4 , Arrows 1 through 6 show obs erved energ ies of charge tran sfer to Ln3 + . Arr ow 7 sh ow s the observed energy for the first 4f-Sd tran sition in Eu 2 + ,
consistent wi th the observ ation that Yb 2+ in oxides and fluorides is more suscep tible to anomalous emi ssion than Eu 2 + in these compounds." The unive rsal behavior in both 4f-5d energy d ifferences and CT energies form s the basis for a cons truction method of the d iagrams as seen in Figure 92. Only three ho stdep endent p aram eters, i.e., E CT (6,3+,A), D(2+,A), and the energy E y C (A) between the top of the valence band (V) and the bottom of the conduction band, are ne eded . These parameters are ava ilable for many different compounds.e! Figure 93 shows the energy ECf (6,3+,A ) of CT to Eu 3 + (with n = 6) in com pound (A), and Figure 94 shows the energy of the first excitonic absorption maximum. The mobility band gap , i.e. th e energy of the bottom of the conduction band at Ey C' is assumed to be 8% higher in energy.?' 9
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Fundamentals of Phosphors
140 14
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With the richness of data on ECT (6,3+,A), D(2+,A), D(3+,A), and Eve (A) pertaining to hundreds of different compounds, one may now study the relationship between level location and type of compound in detail. Figure 93 shows already the effect of the type of anion on the position of the Eu 2+ ground state above the top of the valence band. The location is at around 8 eV in fluorides and a clear pattern emerges when the type of anion varies. The energy decreases for the halides from F to I in the sequence F, CI, Br, 1, and for to Se in the sequence 0, S, Se, and presumably Te. This pattern the chalcogenides from is not new and has been interpreted with the Jorgensen model of optical electronegativity.F
°
(183) where T)(X) is the Pauling electronegativity of the anion X and T)opt(Eu) is the optical electronegativity of Eu. a value that must be determined empirically from observed CT energies. With T)opJEu) = 2 the curve through the data in Figure 93 was constructed." The curve reproduces the main trend with the type of anion. It also predicts where we can expect the Eu 2 + ground state in the pnictides; a decrease in the sequence N, 0, As, Sb is expected. However, the wide variation of CT energies within, for example, the oxide compounds is not accounted for by the Jorgensen model. Parameters like lanthanide site size and anion coordination number are also important and need to be considered for a refined interpretation of CT data." An equation similar to Eq. 183 can be introduced for feX to illustrate the main trend in the band gap with the type of aruon. " The band gap follows the same pattern as the energy of CT with changing the type of anion. Interestingly, one may also notice a similar behavior in the values for the redshift in Figure 91 with changing the type of anion. This shows that the parameter values of our model (ECT (6,3+,A), D(2+,A), D(3+,A), and Eve (A)) are not entirely independent from one another. Analogous to the systematic behavior of the lowest 4f energies for divalent lanthanides with changing n, a systematic behavior of the lowest 4f energies for trivalent lanthanides has been proposed." One may then use, in principle, the same method as used for the divalent lanthanides to construct absolute level diagrams for trivalent lanthanide ions.
Chapter one:
Fundamentals of luminescence
141
For the divalent lanthanides, the "anchor point" of construction is the CT energy to Eu 3 + (see Figure 93). Th e CT to Ce4+ might pl ay the rol e of such an anchor p oin t for the tr ivalent lanthan id e level positions. However, infor ma tion on CT to the Ce 4+ ion is only sp ar sely available, insuffi cient to routinely cons truc t level diagrams. We, th erefore, need ano th er anchor point. The ene rgy differen ce EdC (1,3+,A) between the lowest 5d state of Ce3+ an d the bottom of the cond uction band may serve as the required anc hor p oint. Its va lue (see Figure 86(d» can be obtained from tw o-step photoconductivity experime n ts o r from luminescence-quen ching data. Figure 95 demonstrates the lev el positions of both di valent and trivalent lanthanides in the same com pound YP0 4 . The sche me can be compared with that of the free ions in Fig ure 88. N ote th at the binding energy diff erence of m ore than 10 eV between th e free trival ent and free di valent 5d levels is drast ically reduced to abo u t 0.8 eV in YP04 . Energy differ en ces of 0.5-1.0 eV are com mo nly obse rved when con structing diagrams for othe r compounds. The binding en ergy difference of almost 20 eV between the 4f s ta tes of th e free ions is red uced to about 7.5 eV in YP0 4 • This va lue also appears fairly cons tan t for different host m at erials. The fu ll po ten tial of schemes, simi lar to those for YP04 , is d emonstrated by comparing Figure 95 with the situ ation s sk et ched in Figure 86. Actuall y, each of the 12 situa tio ns in Figur e 86 can be found in the sche me of YP0 4 . The arrows m arked 86g, 86h, 86i, 86j, and 861 sho w the same type of trans ition s as in Figure 86(g), (h ), (i), (j), and (1), resp ectively. Vario us other types of transiti on s, quenching routes, and charge-trap p ing d epths can be read di rectly from the di agr am . To nam e a few: (1) Th e low est 5d s ta tes of all th e di valent lanthani d e ions are between p x and the bottom of the co nd uc tio n band. In this situa tio n, the 5d- 4f em ission is alw ays que nche d due to autoionizat ion processes (see Fig ure 86(c) and (dj ). (2) The 5d states of th e trivalent lanthan id es are well below px, and for Ce 3+, Pr 3+, Nd 3+, Er 3+, and Tm" 5d-4f emissions are obser ved .v -" (3) Apart from Eu 3+ , Cd 3+, Yb3+, an d Lu 3 +, all the trivalent lanthanides form va lence band hole trap s. Th e trap is
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The locat ion of the low est 4f (curves b a nd d) an d low est 5d s ta tes (curves a an d c) of the divalen t (curves a and b) and trivalent (cu rves c and d) lanthan id e ion s in YP0 4 • nand n + 1 are the n umber of electrons in the 4£ shell of the trivalen t a nd diva len t lanthan id e ion, respectively. Arrows indi cate sp ecific tran sitions tha t were also discussed in Fig ure 86. The hor izontal d ash ed lin e a t 8.55 eV is E'·'.
Fundamentals of Phosphors
142
deepest for Ce 3+ followed by 1b3+. (4) The groun d-state energies for the divalent lanthanides are high above the top of th e valence band. In practice thi s means that even for Eu and Yb it is not po ssible to stabilize th e divalent state during syn thesis. (5) The trivalent lanthanides crea te st abl e electron traps becau se the ground sta tes of the corresponding d ivalent lanthan id es a re well below the cond uc tion band. (6) Th e gro und states of Sm 2+, Eu 2+, Tm 2+, and Yb2+ are below the 5d sta te of the trivalent lanthanides. This means th at Sm 3 +, Eu 3 +, Tm 3+, and Yb3+ can qu en ch the 5d emission of trivalent lanthanide ions.
1.11.7 Future prospects and pretailoring phosphor properties With th e m ethods described in th is sec tion one can construct level schemes for all the lanthanid e ions w ith few parameters. Th ese parameters are ava ilable for hundred s of d ifferent compounds. At this st age, th e sche mes still contain sys tema tic errors. Often the bottom of the conduction band is not we ll defined or known or levels ma y cha nge due to charge-compensating defects and lattice relaxation wh ich m ay result in (sys tematic) errors th at a re es timated at around 0.5 eY. Such errors are still very importan t for ph osphor performan ce because a few tenths of eV shift of absolute ene rgy level position may cha nge the performan ce of a phosphor from very good to us eless. Th e level schem es are, however, already very p ow erful in predictin g 4f-5d and CT tran sition energies. We may deduce trends in th e energy difference between the lowest 5d state and the bott om of the con d uc tion band, and then u se the se trends to guide the search for find ing better temperature s table phosphors." We ma y deduce tren ds in the absolute location of the lanthanid e gro u nd sta te th at determines its suscep tibili ty to oxid ization or red uc tion." For exa mp le, oxid ation of Eu 2 + is believed to play an important role in the degr ad ation of BaMgAI100 17:Eu 2+ phosphors/54 and kn owledge on level energ ies may p rovid e us ideas to further stab ilize Eu 2 +. The level schemes are p arti cularl y useful when more than one lanthan id e ion is present in th e sa m e compound. CT reactions and pathways from one lanthan id e to the other can be read from the lev el schem es. For perman ent informa tion stor age deep cha rge traps are re quired and for persistent luminescence shallow trap s are need ed . The level schemes provide very clear id eas on wha t combination of lanthanide ions a re need ed to obtain th e desired properties . Per haps eve n more importantl y at this stage is that the level sche mes p rovid e very clear ideas on w ha t combina tion not to choose for a specific applicat ion . Th is cha p ter has surveyed where we are tod ay with our kn owled ge and ex perimental techniques on the prediction and determination of abso lute locati on of lanthan ide ion energy levels in phosphors. Currently we have a basic model, but it need s to be more accura te . Asp ects like lattice relaxa tion, charge-com pe nsating defects, intrinsic defects, the n ature of th e bottom of the cond uc tion band, d yn amic properties involved in charge localization and delocali zati on processes, and th eoretical modeling all ne ed to be considered to improve o u r k now led ge fur the r. It will be th e nex t step on the route for the tailoring of phosphor properties be foreh and.
References 1. Blasse, G., and Gr abmaier, B.C., Luminescent Materials, Spr inger-Verlag, Berlin , 1994. 2. Weber, M.J., Inorgan ic scin tilla tors: Today and tomor row, ]. Lumin., 100/ 35, 2002. 3. van der Kolk, E., et al., Vacuum ultraviolet excitation and emi ssion properties of Pr-" an d Ce 3 '. in MS04 (M = Ba, Sr, and Ca) and pred icting quan tum splitting by Pr 3 + in oxides and fluorides, Phys. Rev., B64, 195129, 2001. 4. C hakraba rti, K ., Ma th ur, Y.K., Rhodes, J.F., and Abb un di, R.J., Stim u lated luminescence in rare -ea rth-doped MgS, ]. Appl. Phys., 64, 1362, 1988.
Chapter one:
Fundamentals of luminescence
143
5. Me ijerink, A., Schipper, w.J ., and Blasse, G., Ph ot ostimulated luminescence and thermall y s tim ulated luminescence of YzSiOs-Ce,Sm, J. Phys. 0 : A ppl. Phys., 24, 997, 1991. 6. Sido ren ko, A.Y.,et al., Storage effect in LiLnSiO. :Ce3 +,Sm 3+,Ln = Y,Lu phosphor, Nu cl. lnstrum . Methods, 537, 81, 2005. 7. Matsu zaw a, T., Aoki, Y, Takeuchi, N., and Murayarn a, Y, A new lon g ph osphorescent ph osphor with high brightness, SrAl z0 4:Eu 2 +,D y3+, f. Electrochem, Soc., 143,2670, 1996. 8. Dorenb os, P , Mechanism of persistent lum inescen ce in Eu> and Dy3+ co-d ope d alu mina te and silicate compounds, f. Elecirochem. Soc., 152, H107, 2005. 9. Wegh, R.T., Meijerink, A., Larnrn inrnak i, R.-J., an d Holsa, J., Extend ing Diek e's d iagr am , J. Lumi n., 87- 89, 1002, 2000. 10. Dorenb os, P., The 5d level po sit ion s of the trivalent lanthanides in inorganic compo un ds, J. Lumin., 91, 155, 2000. 11. Dorenbos, P., f ~ d tran sition energies of divalent lanthanides in inorganic compou nds, f. Phys.: Condens. Matter, 15, 575, 2003. 12. Dorenb os, P., Ene rgy of the first 4P~4f65d tran sition in Eu-'<doped compounds, f. Lumin., 104, 239, 2003. 13. McClur e, D.s . and Pedrini. c., Excitons trapped a t im p urity cen ters in highly ionic cryst als, Phys. Reu., 832, 8465, 1985. 14. Dorenb os, P., Anoma lo us luminescence of Eu 2+ and Yb 2+ in inorganic compounds, J. Phys.: Condens. Matt er, 15 2645, 2003. 15. Lyu, L.-J. and Ha mi lton, D.s., Radiative and nonrad iati ve relaxat ion mea surements in Ce 3+ doped crys tals, J. Lumin., 48&49,251, 1991. 16. Dorenbos. P., Th erm al q uenching of Eu> 5d--4f luminescen ce in inorganic compo unds, J. Phys.: Condens. Matt er, 17, 8103, 2005. 17. Bessiere , A., et al., Spec trosco py and lanthanide impuri ty level locati on s in CaGa zS4:Ln (Ln = Ce, Pr, Tb, Er, Srn), J. Eleci rochem. Soc., 151, H 254, 2004. 18. Boutinaud, P., et al., Making red emitting phosphors wi th PrJ+, Opt. Mater., 28, 9, 2006. 19. Gu erassim ova, N ., et al.. X-ray excited charge transf er lu minescence of ytterbium-containing aluminium ga rne ts. Chern. Phys. Lett., 339, 197,2001. 20. Brewer, L., Systematics and the Properties of the Lanthanides, edi ted by S.P Sinha, D. Reidel Publishing Com pany, Dord recht, The Netherlands, 1983, 17. 21. Martin, W.c., Energy d ifferences between two spectroscop ic sys tems in neutral, singly ion ized, and doubly ionized lanthanide atoms, J. Opt. Soc. A m., 61, 1682, 1971. 22. Jorgensen, C.K., Energy tran sfer spectra of lanthanide comp lexes, Mol. Phys., 5, 271, 1962. 23. Dorenbos. P., The 4f"B 4f"-15d transitions of the triv alen t lanthan ides in halo genides and chalcogenides, J. Lumin., 91, 91, 2000. 24. Andriessen, J., Dorenb os. P , and va n Eijk, C.W.E., Ab ini tio calculati on of the contribution from anion d ipol e po lari za tion and d ynam ic correlation to 4f- 5d exci ta tions of Ce J • in ionic compounds, Phys. Rev., B72, 045129, 2005. 25. Dorenbos, P., 5d- Ievel ene rgies of Ce 3+ and the crys talline environ me n t. 1. Fluoride compounds, Phys. Reu., B62, 15640,2000. 26. Dorenbos, P., 5d- leve l energies of Ce J+ and the crys talline environmen t. IV. Aluminates and simp le oxide s, J. Lumln ., 99, 283, 2002. 27. Dor enbos. P., 5d -level energies of Ce J ' and the cry st alline environmen t. II. Chloride, bromide, and iodid e com po unds , Phys. Reo., 862, 15650, 2000. 28. Dorenbos, P., Rela tion between Eu 2+ and Ce J + f-->d tran sition energ ies in inorganic compounds, f. Phys.: Condens. Matt er, 15, 4797, 2003. 29. van Pieterson , L., et al., 4fJl ~ 4 f" - l 5 d transitions of the light lanthan ides: Experiment an d theory, Phys. Reo. , 8 6, 045113, 2002. 30. van Pieterson, L., Reid, M.F., Burd ick, G.W., and Meijerink , A., 4 fJl ~4f"-15 d tran sitions of the heavy lanthanid es: Exp erim ent an d theor y, Phys. Rev., B65, 045114, 2002. 31. Dorenbos, P , Excha nge and crysta l field effects on the 4f,,-15d levels o f Tb 3+, J. P!lys.: Condens. Matter, 15, 6249, 2003.
144
Fundamentals of Phosphors
32. Wong , We., McClure, OS ., Basun , S.A, and Kokta , M.R , Charge-exchange processes in titanium-doped sapphire crys tals. I. Charge-excha nge energies and titanium-bound excitons, Phys. Rev., B51, 5682, 1995. 33. H appek, U., Choi, J., an d Srivastava, A.M., Observation of cross-ionization in Gd3SczAI30 12:Ce3+, J. Lumin., 94-95, 7, 2001. 34. Do renbos, P., Systematic behaviour in triv alent lanthan die charge tran sfer energies, J. Phys.: Condens. Matter, 15,8417,2003. 35. Sato, S., Optical absorption and X-ray ph otoemission spe ctra of lanth an um and cerium halid es, J. Phys. Soc. [pn., 41, 913, 1976. 36. Lizzo. S., Meijerink, A, and Blasse, G., Luminescence of divalen t ytterbiu m in a lkaline ear th sulpha tes, J. Lumin., 59, 185, 1994. 37. [ia, D., Meltzer, RS. , and Yen, WM., Locat ion of the gro und state of Er3 +in dop ed YzOJ from two-step p ho toco nd uc tivity, Phys. Reo., B65, 235116, 2002. 38. van de r Kolk, E., et al., 5d elec tron de localiza tion of Ce3+ and PrJ+ in YzSiOs and LU zSiOs, Phys. Rev., B7l, 165120, 2005. 39. Ped rini, e., Rogemond, E, and McClure, Os. , Pho toio nization thresholds of rare -earth impurity ion s. Eu 2+ :CaFz, Ce3+;YAG, and SmJ+:CaFz, J. App!. Phys., 59, 1196, 1986. 40. Fuller, RL. an d McC lu re, OS ., Photoionization yields in th e dou bly d op ed SrF2:E u,Sm system, Phys. Rev., B43, 27, 1991. 41. Joubert, M.E, et al., A new microw ave reso nant technique for stu dying rare earth photoionization threshold s in d ielectri c cryst als u nd er laser irradia tion , Opt. Mater., 24, 137, 2003. 42. Thi el, e. W., Systematics of 4f electron energies relative to host ban ds by reson an t photoem ission of rare-ear th ions in aluminum garnets, Phys. Rev., B64, 085107, 2001. 43. Thiel, e.W, Sun, Y, an d Cone, RL., Progress in re lating rare-earth ion 4f and 5d energ y levels to hos t bands in op tical materials for hol e burning, quan tum informa tion and phosphors, J. Mod. Opt., 49, 2399, 2002. 44. Pid ol, L., Viana, B., Ga ltayries, A, and Dorenbos, P , Energy levels of lanth anide ions in a Lu 2Siz0 7:Ln 3 +host, Phys. Reo., B72, 125110, 2005. 45. Poo le, RT. , Leckey, R.e.G., Jenkin, J.G., and Liesegang, J., Electronic structure of the alkalineearth fluorides studied by photoelectron spectroscop y, Phys. Rev., B12, 5872, 1975. 46. Barnes, J.e. an d Pinco tt, H., Electron transfer spec tra of some lanth an ide (lU) comp lexes, J. Chem. Soc. (a), 842, 1966. 47. Blasse. G. and Bril, A , Broad-ba nd UV exci tation of Sm 3+-act iva ted phosph ors, Phys. Leti., 23, 440, 1966. 48. Krupa, [.C; Op tical excitations in lantha nide and actinide com po und s, J. of Alloys and Compounds, 225, 1, 1995. 49. Na kazawa, E., The lowest 4f-to-5d and cha rge-transfer transit ions of rar e ear th ions in YPO. hosts, J. Lumin., 100, 89, 2002. 50. Kru pa, j .C; H igh- energy op tical abso rp tion in f-compoun d s, J. Solid State Chem., 178, 483, 2005. 51. Do renbos, P, Th e Eu'" cha rge tran sfer energy and the relati on with the ba nd gap of compo unds , J. Lumin., 111, 89, 2004. 52. Jorgen sen , e.K., Modem Aspects of ligand Field Theory, No r th-Holland Pub lishing Company, Ams terdam, 1971. 53. Dorenbos. P , Valence s tability of lanthanide ions in inorganic compo unds, Chern . Mater., 17, 2005,6452. 54. H owe, B., and Diaz, A.L., Cha racterization of host-latti ce emission and energy transfer in BaMgA l JO0 17:Eu 2+, J. Lumin ., 109,51,2004.
chapter two - section one
Principal phosphor materials and their optical properties Shinkichi Tanimizu
Contents 2.1 Luminescen ce centers of ns--ryp e ions 2.1.1 Optical spectra of S2 ions in al kali halides 2.1.1.1 Absorpti on spectra 2.1.1.2 Structure of the A and C abso rption bands 2.1.1.3 Temperature dep end ence of the A, B, and C absor p tion bands 2.1.1.4 Emis sion spectra 2.1.2 S2 - Type ion centers in practical phosphors References
2.1
145 145 145 149 151 152 152 155
Luminescence centers of ns 2-typ e ions
Ions with the electronic configuration ns 2 for the grou nd state and nsnp for the first exci ted state (n = 4, 5, 6) are called ns--type ions. Table 1 shows 15 ions w ith the outer electro nic configuration S2. Luminescence from m ost of these ions incorporated in alkali halides and other crystals has been observed. Among these ions , luminescence and related optical p roperties of TI+ in KCI and othe r sim ilar crys tals have been most p recisely stu die d ." > so S2 ions are also called Tlr-like ions. As for powder phosphors, excitati on an d emiss ion spec tra of 5n2+, 5b3+, Tl' , Pb 2+, and Bp· ions int roduced int o various oxygen-d ominated ho st latti ces have been rep orted.v" though the an alys es of these spectra have not yet been completed due to s truc tureless broad-band spe ctra and unknown site symmetries . In this section, therefore, experimental and theoretical works on S2 ion s mainly in alkal i halides will be s umm ar ized .
2.1.1
Optical spectra of S2 ions in alkali halides 2.1.1.1 Absorption spectra
The in trinsic absorp tion edg e of a pure KCl cr yst al is located at about 7.51 eV (165 nm) at room temper ature. Wh en Tl' is in corporated as a substitutional imp u rity in th e crys tal with concen tra tions bel ow 0.01 mol %, four ab sorption bands appear below 7.51 eV, as sho w n in Figu re l (a). Th ey have be en lab eled A, B, C, and D bands in or der of increasing
145
146
Fundamentals of Phosphors
ener gy. Similar bands are ob served by the inc orporation of Pb z+ or Ag- ions , as sho wn in Figures 1(b), (C).8-10 One or tw o D bands lying near the absorption edge are due to charge-transfer transition s from Cl to S2 ions or to perturbed excitons. and are not due to 52 ~ sp transitions. Th e following di scussion will, therefore, be restricted to the A, B, and C bands. First, a model based on free Tl' ions foll owing the original work of Seitz ! will be discu ssed. The 652 ground state is expressed by 15 0 , The 656p first exci ted s ta te con sists of a triplet 3p, and a singlet lP l . The order of these sta tes is 3PO' 3P I , 3P2, and IPI from th e lowenergy side . When a Tl' ion is introduced into an alkali halide h ost an d occupies a cation sit e, it is placed in an octa he d ral (0,,) crys ta l field . The energy levels of the Tl' ion a re lab eled b y the irredu cible representation of the 0 " point group. Th e labeling is made as foll ow s: for the ground sta te 150 ~ IAJ,~' and for th e excited state 3PO ~ 3A l", 3PJ ~ 3Tl l/' 3p z ~ 3E" + 3T zU' and IPI ~ lT l " . Th e lA lg ~ ITI " transition is dipole- and spin-all owed , while the lA lg ~ 3A I II transition is strictl y forbidden. The lA lg ~ IT l lI transition is partiall y allow ed by single t-trip let spi norbit m ixin g, and IA Jg ~ (JE" + 3T z,J is also allowed due to vib ron ic m ixin g of 3E" and 3T 2J1 with 3T l Then, the observed absorp tion bands sh own in Figure 1 can be assign ed as follows: l('
A band s : 1 A lg ~ 3Tl Jl B bands '. l A 19
~
3E + 3 T
C bands .' lA Ig
---7
IT
IJ
111
2u
CSo
~
3Pl )
CSo
~
3PZ )
(1 So
~
IP l )
Focu sin g on the characteri st ics of the A, B, and C abso rp tion bands, the centers of the gravity of th e ene rgies of these bands are given by " :
f A= F -
tj4 -
J(C'+(,/4)2+ (11.(,)2/ 2
~j = F- G + (,/ 2
Here, F an d G are the parameters of Coulomb a nd exchange en er gies as d efined by Condon and Sh ortley." (, is th e spi n -orb it cou pling cons ta n t. A for the A and C bands is called the Kin g-Van Vleck fact or.'? an d is a parameter expressin g the spa tial di fferenc e between the IT I II and 3T I " wavefunc tio ns . The v alues of r, an d A can be obtained fro m the va lues of f Aand f c ex trap ola ted to T = OK, as shown in Fig ure 2.B The oscillator streng th ratio of th e C to A bands is given by!':
wh ere
(La)
147
Chapter two: Principal phosphor materials and their optical properties Table 1 Ion s w ith the 1152 Config ura tion in th e Grou nd State Atomic No.
Elem en t
(ns)(np)
Ion sp ecies
29 30 31 32 33
Cu Zn Ga Ge As
(45)1 (45)2 (45)2(4p) I (45)2(4p)2 (45)2(4p)3
C uZno Ga+ Ge 2 + A s3+
47 48 49 50 51
Ag Cd In Sn Sb
(55)1 (55)2 (5s)2(5p) J (55)2(5p)2 (55)2(5p)3
AgCd o In+ 'Sn 2+ 'Sb3+
79 80 81 82 83
Au Hg
(65)1 (65)2 (6s)2(6p) I (6s)2(6p)2 (6s)2(6p)3
AuHgO i l+ 'P b2 + 'Bi3+
TI Pb Bi
. Luminescence is observed also in powd er phosphors. (See 2.1.2)
( a ) KCI : TI + co ---..... c
~
c-, '-<
C\l
'-< ..... :.0 '-<
C\l
'-"'
( b) KCI
Pb 2 +
>-. ..... sr: c:: 0 ..... c:: c:: 0
.;:: 0.. '-<
0 ..0
{fJ
« 3
4
5
6
7
8
Photon energy (eV) Figure 1 Absorption spectra of (a) TI+, (b) Pb 2 +, and (c) Ag- ions introduced in KCl crystals at 77K. (From Fukuda, A., Science of Light (japan), 13,64, 1964; Klee ma n, w., Z. Physik, 234,362,1970; Kojim a, K., Shiman uki, S., and Kojim a, 1., J. Phys. Soc. Japan , 30, 1380 , 1971. With p ermission.)
and (l b)
Values of importa nt para meters mentioned above ar e list ed in Tabl e 29 for various ns-type ions.
Fundamentals of Phosphors
148
"--
6.34
:> Q)
<;:»
c-,
6.32 6.30
"_,,
KCl : Tl
--"-
x__x
C band
1-0
~
5.04
Q)
c: 5.02 o
~ 5.00 0...
4.98 4.96
o
100 200 Temperature (K)
300
Figure 2 Temp erat ur e d epende nce o f E" and E e for the A and C absorption bands in KCLTI· . (From Hornma, A., Science of Light (Japan), 17, 34, 1968. With perm iss ion .) Table 2
ns 2
Vario us Para me ters Relat ed to the A, B, and C Abso rp tion Bands of ns--Typ e Ions in Alka li H a lid e C rysta ls
Phosp ho rs KC I:Sn 2+ KCl :ln +
Csl.Ag 552
65 2
KCI :AgCdo KI:AgKBr:AgKCl:P b 2+ KCl:Tl· KCl :Au-
Hgo
EA
Ea
G
(eY)
(eV)
Ec (eV)
S
R
1\0
(eV)
(eY)
F (eV)
18 54 360 435 478 525 570
4.36 4.343 2.780 3.100 3.80 2.878 3.005
4.94 4.630 2.865 3.250 3.87 2.981 3.132
5.36 5.409 3.770 4.349 5.41 3.985 4.180
0.599 0.754 0.897 0.575 0.762 0663 0.556
0.527 0.268 0.082 0.147 0.142 0.10 1 0.125
0.316 0.447 0.472 0.585 0.769 0.516 0.554
4.992 4.943 3.296 3.762 4.643 3.457 3.624
4.57 5.031 4.08 4.89
5.86 5.930 4.37 5.11
6.33 6.357 5.44 6.70
1.03 0.984 2.412 0.758
0.951 0.692 0.199 0.529
0.304 0.283 0.540 0.731
5.688 5.867 5.258 5.92
4.2 5.4 14.0 34.2
Note: R: see text, EA , Ell' Ec : The ce nters of g rav ity of the en ergi es of A, B, and C abso rption ba nd s. A.: King -Van Vleck factor, (,: Spin-orbit coupling con stan t, C: Exchan ge energ y, F: Co ulomb ene rgy. From Kleem an, W., Z. Physik, 234, 362, 1970. With permi ssion.
If the IT l lI an d 3T 1u wavefunctions ar e identical, Ie becomes 1. Ass uming that Ie = 1, Eq. 1a becom es: 4 - 2x+ !6 - 2(2x - 1)2 R(x ) = -----'-:=~=~ + 2x 2(2x _1 )2
2
)6-
(2)
This equa tion is known as Sugano 's formula.'! Figure 39 shows a plot of Eq , 2 an d the experime ntal d ata obtained for va rious ns2-typ e ions in alkali h alide crys ta ls. Deviations
Chapter two:
Principal phosphor materials and their optical properties
149
1 000.--.-------------------, KI:Ag KB . A ...,-r . g 500 .KCl :Ag ,. KCl:G a CsI:Ag ."NaCl:Ga KBr:Ga
~
100 f - - -\ --
-
-
-
-
-
-
-
-
-
-
--j
KCI :In • • NaCI: In • KBr: In
50
KI: Au NaCl:Sn ·KCJ:Sn • KCl:Au • • • NaCl i Au .KBr: S n
•
5
KBr: Au
KC1:TI NaCl:T1e. KBr:Tl NaCI: Pb
x= Figure 3 Expe rimen tally obtained R values p lotted agai nst x for va rious n..:;2_typ e ion s in alk ali halide cry s tals. The d raw n curve is Sug anos form ula, Eq. 2. (From Kleem an, w., Z. Physik, 234, 362, 1970. With per mission.)
from th e curve reflect the d eviati on of A. fro m 1. Figure 3 and Table 2 sh ow that th e obs erve d R values for the sa me 52-typ e ion s are nearl y the same m agnitude for di fferent alk ali halide hosts, whereas the va lues for ca tionic 5 2 -type ions and for anioni c 5 2-typ e ions di ffer m arkedly for the same hosts; for exa m p le, R is 5.4 for KCl:TI+, and 435 for KCl:Ag-. In th e case of a nion ic Ag-, the en erg y sepa ra tion bet w een th e A an d B abso rp tio n bands is as small as 0.15 eV, and the ir in tens ities are ab out one -h undred th of th at of the C ba nd because of th e weak spin-orbit in tera ction of Ag. It may be worth mentioning at thi s point th at Sugano's formula w as d erived from mole cul ar or bi tal app roxima tion, but it uses th e experime n tally d et ermined values for both G and S. Th e for mu la shou ld, th erefo re, be co ns id ered as a specia l case of the a tom ic orbital app roximation.
2.1 .1.2 Structure of the A and C absorption bands The C absorp tion band of KCl :Pb 2+ has a triplet s tru cture as sh own in Figure 1(b) . Th is structure is explained as a result of th e sp litting of the excited sta tes due to the in teraction wi th latt ice vibrations , i.e ., due to the d ynamical jahn-Teller effect." The lattice vibra tional modes int eracting with th e excite d st ates of 52 ion s in Oil symm e try consist of A lg, £", an d T2~' Th e sy m me tric triplet structure of the C ban d a p pears w hen the p otential curves of the ground and exci ted st ates in th e co nfig uratio n al coor dinate model h ave the same
Fundam entals of Phosphors
150
6,....---------rr---------,
5
, ,,I
4
I
( "
"
':,, '
3
' '
2
1
-0.4
- 0.2
o
0.2
0.4
Figur e 4 Calculated s pectra of the C absorption band (lA lg ~ JTI ,,) for tw o differ ent (high and low) temperatures. Dotted curves represent symm etric cases. Solid curves re presen t the case that the IT I" excited st at e has the curvature that is half as small as th at of the lA l g gro und state. (Fro m Fu kuda , A., f. Phys. Soc. Japan, 27, 96, 1969. With pe rmi ssion.)
cu r vature w ithin the framework of the Franck-Condon approximation, while the asymm etric triplet structure of the C band a ppears when they have different curvatures. Figure 4 show s examples of calculat ed spectra of the C band for two different temp eratures b y taking account of the T2R interaction mo d e. J5. 16 The parameter c2 app earing in the horizontal axis is that represen ting the co u pling constant between s2-type ions and lattice vibrat ional mod es. The value of c2 becomes sm aller as th e host lattice constant becomes lar ger, and be com es larger if th e charge number of the ion becomes larger in the sa me host lattices . For example, the valu es of c2 are 1.2 eV for NaCl :Tl+, 0.82 eV for KCl:Tl+, and 1.82 eV for KCl:Pb 2+.15 The A band, on the other hand, th eoretic all y ha s a doublet s tru cture, because two com p one n ts cons is ting of the abo ve-mentioned triplet structure have coal esced together due to the in teraction between the A an d B bands . Figure 5 shows an example of the calcu la ted A abs orp tion bands for two d ifferent temperatures 8. Howe ver, it is noted that th e observed A bands shown in Figure 1 have no clea r-cut doubl et structure, in disagreem ent w ith th e calculat ed bands in Figu re 5, and appear as s tr uctur eless bands. It is also noted that th e doublet s tructur e can be observed for KCl:Sn 2 +and KCl:In+ (see p . 836-837 in Reference 4). Define the calculated splittin g energy of the A d oublet band as bA and th at of the C triplet band as be. The ratio of th e tw o is g iven by " :
Chapter two:
Principal phosphor materials and their optical properties
151
2.0 c2
Lj2 K T
f)
= 0.05
Q;"
;:;; <::r>
1.0
Y =
tiw - € ,1/ 3
Figure 5 Calcula ted spec tra of the A absorpti on band (lA 1g ---7 3T 1u) for tw o diffe ren t (h igh and low ) temp eratures. LI is a normalized energy para meter of the ad iaba tic potentials for 3T,u intera ctin g with the Tzg mode. (From Toyozawa, Y. an d Inoue, M., f. Phys. Soc. Japan, 21, 1663, 1966. With permission .)
° joc A
= 0.85 · (R- 2)/( R -l/2)
(3)
where R is the parameter of Eq. 2. It is understood that th e values of 0A are smaller than those of Oc for heavy ion s such as Tl" an d Pb 2 + because of th eir smaller R va lue s, as sho w n in Table 2. This is con sid ered as on e reason that the doublet struc ture of the A band is no t observ ed experimen tally.
2.1.1. 3 Temperature dependence of the A, B, and C absorption bands The intensit y of the C band is rather con stant up to abou t 150K, an d then slig h tly increases between 150 an d 300K. The triplet struc ture of this band h as a tendency to be prominent at higher tempe ratures. As for the B band, the in tens ity increases as temper ature increases, because th e band originates from vib ra tion-allowed transition s. In some cases, temper ature-dependent stru cture is observed in this band , but it is no t p recisely stud ied be cause of the sma ll in tens ity of this band. The intensity of the A band varies with temperatu re similar to the C band . In KCl:TI+, however, th e increase of th e B band inten sity is counterbalanced by the d ecrease of the A ba nd in tensi ty, w hich s uggests a m ixing of the exc ited states of th e A and B bands. The above-mentioned characteristics of the A, B, and C abs orp tion bands are p rom inent featu res of S2 ion s in alka li halid e host lattices. In hosts othe r than alk ali h alides, th ese features are also observed . The a ppeara nce of these fea tures is useful for th e id entificati on of observed abs orp tion an d excit at ion bands.
Fundamentals of Phosphors
152
Wavelength (nm) 300
250 KCl : Tl +
>.
......
. (/J
----(/J ......
-:12K
c·(l) C
c:
------ : 300 K - - - : 80 K
;:l
.- >. C
.- e l-<
o ......
(/J'(/J..D
.-
l-<
E~
ll.J
4.0
4.5
5.0
5.5
Photon energy (eV) Figure 6 Em issio n s pec tra for KCI :Tl+ at 300, 80, and 12K. (From Edgert on , R. and Teegarden, K., Phys. Reo., 129, 169, 1963. With permission. )
2.1.1.4 Emission spectra Figure 6 sh ows em ission spectra of KCl:Tl+ (0.01 mol %) as an exam ple." At 300K (dotted curve), excitation in any of the A, B, or C bands produces the same emi ssion spectrum, i.e., the A emi ssion band pe aking at 4.12 eV (300 nm) and having a width at half-maximum of 0.56 eV (40 run). At low temperatures, excitation in the A absorption band produces the emission at 4.13-4.17 eV, similar to the case at 300K; whereas excitation in the B or C bands produces another emission band located at about 5 eV in addition to the A band. This emission band has a lar ge dip at 5 eV because of the overlap with the A abso rp tion band. The 5-eV emission observed below SOK is assigned to the C emiss ion (lP 1 ~ ISO) ' Although the A emi ssio n band in KCl:TI+ has a simp le s tru ctur e, the A band in most other cases of 5 2-type ion luminescen ce is composed of two bands: the high- energy band labeled A r and the low-en ergy band labeled Ax. Table 3 18 shows energ y positions of the AT and Ax bands for various mon ov alent 52-type ions at temperatures in the range of 4.2 to 20K. In Croup I, AT is much s tronge r than Ax at 4.2K. With increasing temperature, the AT intensity decreases w hile the Ax int en sity increases, maintaining the sum of both intensities as cons tan t. Abo ve 60K, onl y Ax is observed. In Group II, the re is no temperature region in which Ax is mainly observed . In Croup III, the only band observed is assigned to AT' The mechanism th at the A em iss ion band is composed of two bands is ascrib ed to the spin-orbit interaction between the A band emitting state (i.e., the triplet .1 T l l/ sta te) and the upper singlet ITll/ st ate ." This is explained by the configurational coord ina te model as shown in Figure 7.3- 5 If the spin-orbit interaction is strong en ough, the 3T I state and lT 111 states repel each other, so th at th e lower triplet state is deformed to a relaxe d excited state with two minima as shown in Figure 7(b). Thus, the two emi ssion bands are produced from the two minim a T and X. As for decay kin etics of the A emiss ion in KCl:TI+, readers are referred to Reference 19. I/
2.1.2
8 2.
Type ion centers in practical phosphors
Some of the ns--t yp e ion s list ed in Table 1 ha ve long been kn own as luminescence centers of fluorescent lamp phosphors. In oxygen-d om ina ted ho st lattices, th e em issions
Chapter two:
Principal phosphor materials and their optical properties
153
Table 3 Classificati on of the Observed A Emission Peaks a t 4.2- 20K and Their Assignments Group
Ph osphor
AT (eV)
Ax (eV)
KI:Ga+ KBr:Ga + KCI:Ga+ NaCl:Ga+ KI:In+ KI:TI+
2.47 2.74 2.85 3.10 2.81 3.70
2.04 2.24 2.35 2.45 2.20 2.89
II
KBr:In+ KBr:Tl+
2.94 4.02
2.46 3.50
III
KCl:ln+ N aCI:In+ KCl:Tl+
2.95 3.05 4.17
From Fak ud a. A., Phys. Reo.. B1, 4161, 1970 . With pe rm ission.
E
E
111'*ill >
131' iu*>
Q (a)
Q ( b)
Figure 7 Configu rational coo rd ina te model to accou nt for the AT and Ax emission bands: (a) without sp in-orbit int era ction, (b) with spi n- orbit in teraction. (From Farg e, Y and Fon tana , YP., Electronic and Vibratio nal Properties of Point Defects in Ionic Crystals, North -Holland Pub lish ing, Am sterdam , 1974,193; Ranfagni, A., Magnai, D., and Bacci, M., Adv. Phys., 32, 823, 1983; Jacobs, P W.M., J. Phys. Chem. Solids, 52, 35, 1991. With permission .)
from Sn 2+ , Sb 3 +, Tl", Pb 2 +, and Bj3+ are reported . These ions ar e m arked w ith ast er iks in the tabl e. Luminescence features of the above five io ns ar e as foll ows. 1. Th e luminescen ce is due to the A band tr an sition (JP , -7 150 ) , 2. Th e luminescenc e is usually associated w ith a large Stokes' sh ift, and the spectra are consid er ab ly broad, especially in case of Sn 2+ and Sb 3+ . 3. Th e luminescence decay is not ve ry fast and of the order of microseconds. This is because the luminescence transition is spin -for bid de n .
Fundamentals of Phosphors
154
Spectral d ata-? and lie decay times of practical phosphors activated with 52-typ e ion s at room temper ature under 230- 260 nrn excitati on are given below. Sr ZPZ0 7:Sn z+ Excitation bands: Emission band:
(Ref. 21,22) 210, 233, and 250 nm. 464 nm with halfw idth 105 nm.
SrB 6010 :5 0 Z+ Excita tion bands: Emi ssion band: Decay time:
(Ref. 23) 260 and 325 nm. 420 nm with h alfwidth 68 nm. 5 us.
Cas(P04)3F:Sb3+
(Ref. 24, 25) • 175,26 202, 226, 235, 250, and 281 nm for 0 2-compensated samples. • 190,200, 225,246, and 267 nm for Na-compen sated samples. • 480 nm with halfwidth 140 nm. • 400 nm w ith half width 96 nm. • 7.7 us for 480 nm em issi on . • 1.95 us for 400 nm emission.
Excitation b ands:
Emission bands: Decay tim es :
Th e behavior of Sb 3+ in fluorapatite [Ca s(P04hFj ho st lattice is not so sim ple, becau se of the existence of tw o differ ent Ca sit es an d charge com pe nsa tion . Th e low-lying exci ted st ates of Sb 3 + with and w ith ou t O 2 compensation w ere calcu lat ed by a molecular orbital model.> However, the rea son why the d ecay times for 480 and 400 nrn emission bands differ n oticeably ha s not yet been elucid at ed . YP0 4 :Sb 3 + Exci ta tion bands: Emission bands: Decay tim e:
(Ref. 27, 28) 155 nm, 177-202 nm, 230 n rn, and 244 nm. 295 nm with halfwidth 46 run , an d 395 nm w ith h alfwidth 143 nm . Below 1 us .
(Ca,Zn)3(P04)Z:TI+
(Ref. 29) 200 and 240 nm . 310 nm wi th h alfwidth 41 nm.
Excit ation bands: Em ission band :
The em ission p eaks vary w ith Zn contents. BaMgzAI160Z7:Tl+
Excit ation bands: Em ission bands:
Decay times: BaSi 20 S:Pb Z+
Excit ation bands: Emi ssion band :
(Ref. 30) • 200 nm and 245 nm for 1% Tl. • Unkn own for 3 an d 10% 11. • 1% TI: 295 nm with halfwidth 30 nm. • 3% TI: 420 nm with halfwidth 115 nm. • 10% TI: 460 nm with h alfw idth 115 nm. • 0.2 f.1S for 295 nm emission. • 0.6 us for 460 nm emission . (Ref. 31, 32) 187 and 238 nm. 350 nm with halfwidth 39 nm.
Chapter two: Principal phosphor materials and their optical properties
155
In BaO-Si0 2 sys tems, Ba 2Si04, BaSi03, and BaSi3 0 s, a re also kn own. Ba 2Si04:Pb 2+ rev eal s two em issions peaked at 317 an d 370 nm. The excitation bands lie a t 180,202, an d 260 nm . Pb Z+ in another host; SrAl12019:Pb z+ (Ref. 30) Excitat ion bands: Below 200 nm, an d 250 nm for 1% Pb . • Unknown for 25 and 75% Pb . Emission bands: • 1% Pb: 307 nrn w ith h alfwidth 40 nm. • 25% Pb : 307 nm wi th h al fwidth 46.nm, an d 385 nm w ith h al fwidth 75 nm, • 75% Pb : 405 nm with h alfwidth 80 n m , Decay time: • 0.4 us for 307 nm emission . As for the spec tra l d ata and d eca y times of Bi3+ activa ted phosphors, readers are referred to Referen ces 33, 34, 35, a nd 36. YP0 4:Bj3+ (Ref. 33,36) Excitation ban d s: 156, 169, 180, 220, 230, an d 325 nm (for a Bi-Bi p air) Em ission bands: 241 nm Decay time : 0.7 s
References 1. Sei tz, F, J. Chern. Phys., 6, 150, 1938. 2. Fowler, W.B., Electron ic Stat es and Optical Trans itions of Color Centers, in Physics of Color Centers, Fowl e r, WB., Ed ., Academ ic Press, New York, 1968, 133. 3. Farge, Y an d Fon tan a, M.P , Electronic and Vibratio nal Propert ies of Point Defects in Ionic Crystals, Nor th -H olland Pu blish ing Co ., Ams terdam, 1974, 193. 4. Ranfagni, A., Mag nai, D., and Bacc i, M ., Adv. Phys., 32, 823, 1983. 5. Jacobs, PWM., J. Phys. Chern. Solids, 52, 35, 1991. 6. Butl er, K.H ., Fluorescent Lamp Phosphors, Penn sylvania Sta te Unive rsi ty P ress, 1980, 16l. 7. Blasse, G. and Gra bma ie r, B.C., Luminescent Materials, Sp ringer Verlag , Berlin , 1994, 28. 8. Fuk uda, A., Science of Light (Ja pan), 13, 64, 1964. 9. Kleemann, W , Z. Physik, 234, 362, 1970. 10. Kojima, K., Shiman u ki, S., a nd Kojima, T , J. Phys. Soc. japan, 30, 1380, 1971. 11. Condon, E.U. and Shortley, G. H ., The Theory of Atom ic Spectra, Ca mb ridge University Press, Lond on , 1935. 12. King , G.W. and Van Vleck, J.H. , Phys. Reo., 56, 464, 1939. 13. H omma, A, Science of Light (japan), 17, 34, 1968. 14. Suga no, S., J. Chern. Phys., 36, 122, 1962. 15. Toyozawa, Y and moue, M., J. Phys. Soc. japan, 21, ] 663, ] 966; Toyozawa , Y, Optical Processes in Solids, Cambridge Univers ity Press, Lon d on , 53, 2003. ]6 . Fu kud a, A , J. Phys. Soc. japan, 27, 96, 1969. 17. Edgerton, R. a nd Teegard en, K., Phys. Rev., ] 29, 169, 1963. 18. Fukuda, A , Phys. Rev., Bl , 4161, ] 970. 19. H linka, J., Mi hokova , E., and Nikl, M., Phys. Stat. 501. , 166 (b), 503, 1991. 20. See Tabl e 10 and lOa in 5.6.2. 21. Ropp. R C. an d Mooney, RW, J. Electrochem. Soc., 107, 15 1960. 22. Ranby, P W , Mash, D.H., and Henderson, S.T, Br. J. Appl. Phys., Su pp l. 4, S18, 1955. 23. Leskela, M., Kos ken talo. I., a nd Blasse, G ., J. Solid State Chem., 59, 272, 1985. 24. Dav is, T S., Kreid ler, E.R, Parodi, J.A, an d Sou les, I.F , J. Lumi nesc., 4, 48, 1971. 25. Soules, I.E, Davis, I.S., and Kreid ler, E.R, J. Chern. Phys., 55, 1056, 1971; So ules , T F , Bateman, R.L., Hewes, R A., and Kreid ler, E.R., Phys. Reo., B7, 1657, 1973.
156 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.
Fundamentals of Phosphors Tan imizu , S. and Suzu ki, T., Elecirochem. Soc., Extended Abstr., 74-1, No . 96, 236,1 974. Grafmeyer, J., Bourcet , r.c, and [anin, J., J. Luminesc., 11, 369, 1976. Omen, E.W.].L., Srnit , WM .A., an d Blasse, G., Phys. Rev., B37, 18, 1988. Nagy, R , Wollentin, RW, and Lui, CK. J. Electrochem . Soc., 97, 29, 1950. Sornmerdijk, ].L., Verstegen, ].M.P.]., and Bril, A., Philips Res. Repts., 29, 517, 1974. Clapp, R H . and Ginther, R]., f. Opt. Soc. A m., 37, 355, 1947. Butler, K.H ., Trans. Elecirochem. Soc., 91, 265, 1947. Blasse, G. and Bril, A., f. Chern . Phys., 48, 217, 1968. Boulon, G., f. Physique, 32, 333, 1971. Blass e. G ., Prog. Solid State Chem., 18, 79, 1988. ]-Ste!, T., Huppert z, P., Ma yr, W , Wiechert, D.U. J. Lumin., 106,225, 2004.
chapter two - section two
Principal phosphor materials and their optical properties Ma saaki Tamatani
Contents 2.2 Lum inescence cen ters of transition me tal ions 2.2.1 Cr yst al field theory 2.2.1.1 The simples t case: 3d1 elect ron config ura tion 2.2.1.2 Th e cases of m ore than on e d elec tron 2.2.1.3 Tanabe-Sugano diagrams 2.2.1.4 Spin-orb it interaction 2.2.1.5 Int en sities of em ission and abso rp tion band s 2.2.2 Effects of elec tron clou d expans ion 2.2.2.1 Nephelau xeti c effect 2.2.2.2 Ch arge-transfer band 2.2.3 Cr 3+ Phosphors (3d3 ) 2.2.4 Mn4+ Phos phors (3d3 ) : 2.2.5 Mn 2+ Phosphors (3dS) 2.2.5.1 Crystal field 2.2.5.2 Different Mn 2+ sites in cry st als 2.2.5.3 UV absorp tion 2.2.5.4 Lum inescen ce decay tim e 2.2.6 Fe 3+ Phosp hors (3dS) References
. 2.2 2.2.1
157 157 158 161 163 164 164 167 167 168 168 172 173 173 175 176 177 177 178
Luminescence centers of transition metal ions Crystal field iheoru':'
The 3d tran sition me tal ions ut ilized in commercia l powder p hosphors have th ree electro ns (in the case of Cr3 + and Mn 4+) or five elec trons (Mn 2+ and Fe 3 +) occupying th e ou ter most 3d electron orbi tals of the ions. When the 3d ions are in corp orated in to liguids or so lids, spectroscopic prope rties (such as spectral p osition s, wid th s, an d int ensities of lum inescence and absorption bands) are considerably changed from those of gaseous free ion s.
157
Fundamentals of Phosphors
158
These cha nges a re explained in term s of crystalf ield theory, whi ch assumes an ions (ligands) sur ro undi ng the m etal ion as point electric cha rges. When th e theory is exten de d to take into consid eration the overlap of elect ron orbi ta ls of the metal ion and ligands, it is called ligand f ield theoru. In the followin g, th ese theor ies w ill be de scribe d briefly. For m ore d etails, the read er is ref erred to Refer ence 1.
2.2.1.1 The simplest case: 3d J electron conf iguration Firs t, take the case of an ion th at h as the 3d! electron conf igura tion, such as Ti3 + , Table 4 sh ows th e w a vefunctions for the five 3d ele ctron orbitals, and Figure 8 the electron di stributions for these orb itals. For a free ion , the energies of the five 3d orb itals are id entical, a nd are d et ermined by an elec tron kin eti c ener gy and a cen tra l field potential cau sed by th e inner electron she ll." In cases where different orbitals h ave the sa me energy, the orbitals are said to be degen erate. When thi s ion is incorpora ted in a crys ta l, s ur ro un di ng an ions affect it. Consider the case where there are six anions (negative p oint ch arge s) at a d ist ance R from a cen tral ca tion nucleus locat ed a t ±x, ±y, and ±z as show n by ope n circles in Figu re 8. Th is ligand a rr an gement is call ed the oc tahed ra l coord ina tio n. These anio ns in d uce an elec tros tatic p ot ential V on a 3d elec tron of the central cati on , which is exp ressed by ;= 6
Ze2
i
IR,-rl
V=I, -
(4)
Here, R; rep resents a po si tion of the i th ani on, r a position of the 3d electron (coordinates x, y, z), Z a va len cy of an anion, and e an electron ch a rge. When IRi ! ~ Ir , the followin g equa tion is ob ta ine d from Eq. 4 by the expansion on r up to 4 th o rde r.
(5)
Th e effec t of the potential Von the 3d elec tro n or bi tal ene rgy is expressed by th e follow in g int egra tion.
f
",(3d)V", (3d)d, = ( 3dIVI3d)
(6)
The first ter m of Eq. 5 increases the ene rgy of all five orbi tals by th e same amo un t. It ma y be ne gl ected in the field of optical spec tro scopy, where on ly energy differences among ' ele ctron states are meanin gful. From the second term in Eq. 5, the following orbi ta l ene rgies ar e ob tained . (7)
(uIVlu) = (vIVlv) = 6Dq * H ere, the spin-orb it interaction of an elect ron is negl ected.
(8)
159
Chapter two: Principal phosphor materials and their optical properties Table 4
Wave func tions for a 3d Electro n
<Jl" = ) 5/ 16rr R3d (r)(1/ r1)(3 z2
_1' 2 )
l(l ,.
= J5/16rr R3d(r)( 1/r2 )(x2 - !/)
l(l ~
= \(l5j4n H1.,(r)(1/ r2 )yz
l(l 'l
= J15/4rr R3A r) (lj r2 )zx
= J15/41t R3Ar)(1/ 1' 2 )xy
N ote: R,.k ) means the radia l wav efunc tion of a 3d electron . The re are many ways to cons tru ct five wavefunction s for a 3d
electron. He re, they are cons tructed so as to di ago nalize the mat rix for the cub ic crys tal field V; that is, nond iagonal elements of the seq u lar eq uat ion (e.g ., (lIjVlv) )a re eq ua l to zero .
z
z
y
z
y
z
z
Figure 8 Shapes of d or bitals an d ligan d pos itions . 0: Liga nds for octahed ral sym m etry. . : Ligands for tetrahedral symme try.
Here, (9)
(10)
Fundamentals of Phosphors
160
Therefore, the fivefo ld degenera te 3d or bitals split into tripl y d egenerate orbi tals (;,TJ,s) and d oubly degenerate or bit als (u,v). The for me r are called t2 orbitals, and the latter e or bitals.* The ene rg y differen ce between the t2 and e orb itals is 100q.** The splitting origina tes from the fact that u and v orbitals pointing tow ard ani ons in the x, y, and z directions suffer a lar ger elect rost ati c repulsion th an ;, TJ , and S or bitals, w hich poi n t in directions in w hich the anions are absen t. N ext, cons id er the case w here four an ion s at a di stance R from the central cation form a re gular tetrah ed ron (tetrahe dra l coo rd ination) . The electrost ati c poten tial caused by these anions at a 3d ele ctron of the catio n, VI' is exp ressed as follow s.
(11) H ere, T
= 10 fj
Ze
3R 4 4 D =- - 0 I
9
(12)
(13)
Th e sign of the second term in Eg. 11 cha n ges w hen the electron coo rd inates are inverted as x ~ - x, y ~ -y, and z ~ - z/ (tha t is, the ter m has "odd p arit y"), an d the int egrated value of Eg. 6 is zero. Since the thi rd term of Eg. 11 has the same form as th e second term in Eq. 5, values simi lar to Egs . 7 and 8 are ob tained for the 3d elec trons, liftin g the d egen eration. H owever, as sh own by Eg. 13, a t2 or bi tal has a high er energy than an e orbi ta l, and the splitting is sma ller th an that in octa he d ral coordina tion . These results reflect the facts th at the t2 orbitals poi nt tow ard the anion position s and that the n umb er of th e ligands is smaller th an that in the octahedral case. In most crystals, eac h m etal io n is su rro un ded by four or six ligan d s. So, the electrostatic effec t from th e ligands on the cen tra l ca tion (the crystal field) may be approxi ma ted by Egs . 5 or 11, w here all ligand s are ass u med to be locat ed at an eq ua l distance from the centra l ca tion, and to have a geomet ric sy m metry of 0" or Td in notation of the crys tal point group. Th e crystal field with a slightly lower symmetry th an th e 0 " or T d may be treated by a p erturba tion method ap plied to Eg. 5 or 11. The en ergy levels split fur ther in this case. For the above p roced u res, group theor y may be u tilize d based on the symmetry of th e geometric arra ngem ent of the cen tra l ion and li gand s. This is bas ed on the fact th at a cry s tal field h aving a cer tain symmetry is invarian t w hen the coordinat es are transformed by elemental symmetry operations th at belong to a poin t gro up associated wi th the symmet ry; all terms other than th e cry s tal field in th e H amilton ian for electro ns are also not changed in form by the eleme n tal symme try opera tions . In addition, electron wavefu nc tio ns can be used as the basis of a represen ta tive ma trix for the symmetry oper ations, an d th e eigen values (energies) of the H amiltonian can be characte rized by th e red uced representations. Particul arly when th e H amiltonian includes the in ter-elec tron electros ta tic and sp in- orbit interaction s in a multi-elect ron sys tem, gro u p theory is useful for ob tain ing * They are so me times called de a nd dy orbi ta ls in crys ta l field theo ry. No ta tion of /2 and e is gen erally used more in ligand field theor y. .. Th e energy d ifference of 10Dq, a measure of the crystal field , is som etimes rep resented as 1\..
Chapter two: Principal phosphor materials and their optical properties
161
Table 5 Correlati on of Reduced Rep resentation s A1 Az Representa tion
A I' A 2
A 2, 8 8 2 "
A I' 8 1, 8 z Note: Sub scrip t g means eve n pa rity. Odd parity rep resent at ion s, A 1fll A T2 are no t sh ow n . 2 111
3d
•• " ,
1./1
10 Dq,
Figure 9 3d level splitting cause d by the crystal field . energy level sp littin g and w av efunction s, ca lcu lating level energies, an d pred icting th e selecti on rule for transitions between energy levels. Wavefunctions for the tz and e orbitals are the basis for the reduced representations T2g and Eg , respectively, in the 0 h group . When the sy m m etry of the crystal site is lowered from O; to 0 411' one ob tains representa tions in the lower sym me try grou p contained in the original (hi gher) symmetry representati on s fro m a cor relation tabl e of group representations." Tabl e 5 sho ws an example. From thi s tabl e, the number (splitting) and representations of energy levels in th e lower sy m me try can be seen . Fig u re 9 shows the energy level s p litting due to sy m m etry lowering.
2.2.1 .2 The cases of more than one d electron Strong crystal f ield. When there are more th an one electron, th e elec tro ns affect each other electros ta tically through a p otential of
2: ~X ' ' ,I
w h ere rij represents the distance
162
Fundamentals of Phosphors
between the two electrons. When the contribution of the crys tal field is so large th at the electrostatic interaction can be neglected, energies of the states for the dN electron configuration are determined by the number of electrons occupyin g the tz and e orbitals onl y. That is, (N + 1) ene rgy lev els of eN , t2e N -I, .. ., t 2N configurations are produced with energies for 12oe N-o given by: £(n, N-n)=(--4n+6(N -n))Oq
(14)
The en ergy diff eren ce between the nei ghboring two levels is 100q. When the electrostatic interaction is taken into consid eration as a sm all perturbation , the lower symmetry levels split from the levels of these electron confi gurations. Th ey are deri ved from the gro up theoretical concept of prod ucts of representations, ap plied together with the Pauli principle. Th e latter states that only one electron can occupy each electron orbital, inclusive of spin st ate. For example, in the case of dZ (V3 + ion), the following levels can be derived:
Here, each level of 2S+1F, which is (25+ 1)(r) degenerat e, is called a multiplet. 5 stan ds for the total spin angular momentum of the electrons. (T) represents the degen eracy of the reduced representation F: it is 1 for A I' A 2, B1, and B2, 2 for E, and 3 for T 1 and Tz. The energy for a multipl et is obtained as the s um given by Eq. 14 and the expectation value of eZ / r12 (e.g., (t/ 3TJ 1e2/ rl2lt/ 3T1) ) . To di stinguish the parent electron configuration, eac h multiplet is usually exp ressed in the form of Z5+1TUt eN - n ) .
Medium crystal field. When the crystal field stren gth decreases, one can no t neglect the interaction between levels having the same reduced representation but different electron confi gurations; for example, (t/ 3Tlle2/rl2l tze 3T]). This int eraction is called the configurati on interaction. The level energies of the reduced representation are derived from the eigenvalues of a determinant or a secular equation that contain s th e con figu ration interaction.
Weak crystal fi eld. When th e crystal field energy is very small comp ared with that of the confi guration int eraction, tot al angular quantum numbers of Land 5 for orbitals and spins, respectively, determine the energy. In the case of Dq = 0, a level is exp ressed by 25+1 L, with degeneracy of (25+1)(2L+1 ). Symbols 5, P, 0 , F, G, H, ... ha ve been used histori call y, corresponding to L = 0, 1, 2, 3, 4, 5, ... . For the d2 configuration, there exist IS, IG, 3p, 10, and 3F levels. Levels split from these levels by a sm all crystal field perturbation are represented by 25+1Te 5+1L ). In all three cases d escribed abov e, int egr al values for e2 / r J2 can be shown as linear combinatio ns of a se t of parameters: A, B, and C introduced by Racah (Racah parameters). Paramet er A makes a common con trib u tion to energies of all levels. Therefore, level energies are functions of Dq, B, and C for spectroscopic purposes, where the energy d ifference bet ween the le vel s is the meaningful quantity.
Chapter two: Principal phosphor materials and their optical properties
163
2.2.1.3 Tanabe-Sugano diagrams? Each crystal field an d electron confi gurati on in teract ion a ffects the level ene rgies of the 3d transition metal ion s by about 104 em:' . Tanabe and Sug ano" calculated the d eterminants of the electron config ur ation int er act ion described in Section 2.2.1.2 for the dZ to dB configurations in an octahe d ral crystal field . They presented th e solutions of the determinants in so-called Tanabe-Sugano di agram s." Figures 10 to 16 show the dia gr ams for the dZ to dBconf igura tions . These dia grams were prepared for the analysis of optical spectra." The level energ ies (E) fro m the ground level are plotted agai ns t the crystal field energy (Dq), both in units of B. For CIB = 't. valu es of 4.2 to 4.9 ob tai ned from the expe rimen ta l spec tra in free ion s are used. Note th at one can treat the configuration in tera ction for n electrons occupying 10 d orbitals in th e sam e manner as th at for (10-n) holes; the diagram for dn is the sam e as that for dl O-n for Dq = O. In addition, the sign of th e Dq value for electrons becomes opposit e for hol es, so th at the diagram for d10- n in the octa he d ral field is also used for d" in the tetrahedral field. Optical absorption spectra for [M(H zO )61n + complex ion s of 3d met als can be well explained by th e Tanabe-Sugano di agrams con taini ng the tw o emp irical pa ramet ers of Dq and B (about 1000 cm-l ).3 The Dq va lues for metal ion s are in the order:
They are abou t 1000 crrr' for di valent m etals, and abou t 2000 cm' for trivalent metals. For a metal ion , Dq is known to depend on ligand species in the orde r: (16) This ord ering is called the spectrochemical series? Dq va lues in liquid are not so different from those in crystal, but are gov erne d by the ligand ion species directly bound to th e central met al ion. Thus, the spectrochem ical series m ay b e rewritten as ": I < Br < Cl < S < F < 0 < N < C
(17)
Tanabe-Su gano diagrams d emonstrate that th ose con figu rations in which the lowest excited levels (ligh t-em itting levels) are located in the visible spectral reg ion are d1 and d5 . For d3 (Fig u re 11), the light-emitting levels ar e zEe G) and 4Tz(4F) abo ve and below th e crosso ver value of DqI B - 2.2, resp ect ively. As w ill be d escribed later, lum inescence bands from the se tw o levels are obs erved for Cr3 + d ep ending on the crysta l field strength of host mater ials. For d5 (Figure 13), 4T 1(4G) is the lowest excited level, w h ich is located in the visible region at weak crys tal field of DqlB < 1.5. Mn> of thi s con figur ation, hav ing the smallest Dq value among tr an sition metal ion s in Eq. 15, is a sui table activator for green- to red-emitting phosphors . The depend enc e of the zE(ZG) st ates for d3 on Dq is almost parallel to that of the gr ound level. Th is suggests that the waveleng th of the emitted light do es not depend s ign ifican tly on the crys tal field stren gth of different host materials or on the temperature. La ttice vibra tio ns also lead to in st antaneous Dq va riati on, but the emitting level energy is insensitive to th ese variatio ns and , conseq uen tly, the spectr al b and m ay be a sharp lin e. On the other hand, the curves of th e 4T z(4F) for d3 an d 4TI (4G) for d5 have s teep slopes when plotted agains t Do, suggestin g th at the position of • Or gel' also presented d iag ram s of energy levels as a fun ction of Dq for some transition metal ion s s uch as y 3'(d2 ) , N j2 ' ( d~ ), CrJ' (d3 ) , Co2' (d7), and Mn 2' (d' ).
Fundamentals of Phosphors
164
r = 4.42 B= 860 70
60 IS
50
40 30
IE
~=1==~------1 T 2
2
3
Do.' B
Energy lev el diagram for the d 2 configurat ion . (From Kam imura, H., Sug ano, S., and Tanabe, Y, Ligand Field Theory and its Applications, Syokabo, Tokyo, 1969 (in Japanese). With perrni ssion. ) Figure 10
th e em itt ing b ands will depend s tro ngly on ho st m at erials and th at their bandwidths m ay be broad. There have been ex tensive studies on solid-state laser materi als dope d w ith transition m e ta l ions. In p articul ar, for tunable laser s working in the far- red to infra red regions , the op tical properties of various ions-d l (Ti3+ , V4+); d2 (V3+, Cr4+, Mns+, Fe6 +); d3 (V2+, Cr 3+, Mn 4 +); d4 (M n 3 +); dS (Mn 2 +, Fe3+); d7 (C 0 2+); and d8 (N j2+)-have been investi gated in term s of Tanabe-Sugano di agr ams w ith considerable success. In the diagram s for d4, dS, d6, an d d7 config urations, the ground levels are replaced by those of the lower spi n quantum numbers when Dq/ B exceeds 2 to 3. Thi s gives an ap paren t violation of Hund's rule, which states that th e gro und s ta te is the multipl et havin g the maximum o rbita l an gular quantum number among th ose havin g the h igh est sp in quantum number. It is known that th e ion valency is unstable aroun d the Dq /B valu es a t w hic h Hund's rul e s ta rts to brea k down .v"
2.2.1.4 Spin-orbit interaction For 3d transition m et al ions, the contribution from th e sp in-orbit int eraction in electro ns (I Sl, .5,) is as sma ll as 100 em-I, co mpa re d w ith th at due to the cryst al field (- 104 cm').
,
H ith erto, this interaction has been n eglected. Sp in-orbit pl ays a rol e, h ow ever, in determining th e sp litting of sh arp sp ectral lin es and the transiti on probability between the lev els.
2.2.1.5 Intensities of emission and absorption bands The in teraction b et w een an oscillating electr om ag ne tic field of light and an elec tro n b rings ab o u t a transition between di ffere n t ele ctronic sta tes . Sin ce th e elect ric dipole (P)
Chapter two: Principal phosphor materials and their optical properties
165
r = 4.50 8= 918
70
50
40
C:)
t?J"
2
F
I
3 DIL lE
Figure 11 Energy level diagram for the d3 configuration. (From Kamirnura , H ., Sugan o, 5., and Tan ab e, Y, Ligand Field Theory and its Applications, Syok ab o, Tokyo, 1969 (in Jap an ese ). With permi ssion .)
component of the electric field of light has odd parity, and since all wavefunctions of pure d n states ha ve even parity, one obtains (18)
This means that a transition between the states i and f having the same parity is forbidden (Laporte's rule). When a crystal field Vodd has no inversi on symmetry, however, thi s exp ression ma y have small finite value, since wavefunctions having odd parity m ay be admixed with the 3dn wavefunctions accord ing to the followin g expression.
(19)
Here, Ij! II is a wavefunction for an odd parity s ta te lyin g at higher en ergies; the se could be (3d)n-14p states and / or charge-transfer states which will be described la ter. 8.Eu is the energy difference between the 1j!3d" and Ij!" states. Even in the case of 0" having the inversion sym me try, Vodd may be pr oduced instantan eou sly by lattice vibrati on s h aving odd parity, resulting in a slight viol ati on of Laporte's rule. On the other hand , a m agnetic dipole produced by the oscillating ma gnetic field of light ha s even parity, and tran sitions between dn levels are allowed via thi s me chanism. In the abo ve, it is ass umed that multiplets in v olved in the transition have a sam e spin qua n tum number. Tran sition s between d ifferent spin s ta tes are forb idden by or thogonality
166
Fundamentals of Phosphors 'Y =4.61
B=965 E/B 70 IG
60 3P
3F
10 1
2 Dq/B
Figure 12 Ener gy lev el di agram for the d4 configuration. (From Karni mu ra, H ., Sugano, S., and Tanabe, Y. , Ligand Field Theory and its Applications, Syokabo, Tokyo, 1969 (in Jap an ese). With permission.)
of the spin wavefunctions (spin selection rule) . However, in thi s case also , th ey can be partly allowed, sinc e differ ent sp in wavefunctions ma y be sligh tly mixed by m eans of the spin-orbit interaction. Based on the above considerations, int en sities of abs orp tion bands in the visible region for m etal complexes have been evaluated in terms of th eir oscillator strength f. Tabl e 6 shows th e results. * The luminescence deca y time, i.e., the time required for an emissi on from a lev el to reach 1/ e of its initial intensity value after excitation cessation, r (second s), and the oscillator s tren g th, f have the following relations." For electric dipole tr ansiti ons,
)\0- 2
1.51(£"/ £'11 -----'---- -----'-n
f't = -
(20)
and for magnetic dipole tr ansitions, (21)
Here, Ec is th e average electri c field streng th in a cr ystal, £cff is the electric field strength at the ion position, A.o is th e wavelength in vacuum (em) , and n is th e refracti on index. In Table 6, r values estimated from these equations are also shown. • Not e that osc illator s treng th f for tra nsitions allow ed by od d latti ce vibra tions dep end s on tem pe rature as coth (hwj2kT) .Here, tuo mean s ph on on ene rgy.
Chapter two: Principal phosphor materials and their optical properties r=
167
4.48
B = 860
70
25 2F
2C
2H 2 F'
'D
'F 2
CQ '<,
kl
I
40 4D
.I p
4C 30 4
Ts
6A 1
20
4
T1
(/Z3
e 2)
(tz" e)
10
65 0
2
3
Dq /B
Figure 13 Energy level diagram for the d5 configuration. (Fro m Kami rnu ra, H ., Sugano, S., and Tanabe, Y, Ligand Field Theory and its Applications, Syok ab o, Tokyo, 1969 (in Japan ese). With pe rrni ssion. )
2.2.2
Effects of electron cloud expansion 2.2.2.1
Nephelauxetic effect?
The Racah p aram et er s, Band C, in a crystal are consider abl y sma ller th an th ose for a free ion, as sho w n in Tabl es 7, 8, 10, and 11. The reason is as foll ows. Some ele ctro n s of th e ligands m ove into th e orbitals of the central ion and reduce th e cati onic valency. Due to this re d uc tio n, th e d-electron wavefunctions expa nd toward th e ligands to increase th e d ist ances bet w een electrons, reducin g th e interacti on between them. This effect is call ed th e nephelauxeiic effec t. In fact, s ome 3d electro ns ar e know n to exist even at the p osit ion s of the nuclei of the ligands as d et ermined by ESR an d N M R experimen ts. Therefore, the ass um p tion in crystal field th eor y that expa nsio n of the 3d orbitals may be negligibl y sma ll d oes not strictly hold . Th e reducti on of Ba nd C fo r va rious ligands is in the order: F < 0 - N < Cl - C < Br < I - S
(22)
For cen tra l cation s, it is: (23)
This effect may be con sidered to increase with covalency betw een the cation and ligands. Note that the relation in Eq. 22 corresponds to a decreasing order in the elec tronega tiv ity of eleme n ts.
Fundamentals of Phosphors
168 '1=4 .81 B=1065
70
10
2
3
DqlB Figur e 14 Energy level diagram for the d6 configuration. (From Karnimura, H ., Sugano, S., and Tanabe, Y, Ligand Field Theory and its Applications, Syokabo, Tokyo, 1969 (in Japanese). With permission.)
2.2.2.2 Charge-transfer band In crysta l field theory, transitions wi th higher energies than th ose wi thin the do configuration entail do ---7 dn-Is or do ---7 dn-Ip p ro cesses. However, in energy regions (e.g.. 200 to 300 nm for oxides) lower th an these interconfigur a tion al tran siti on s, strong absorption bands if ~ 10-1) , called charge-transfer (CT) (or electron-transfer) b and s, are some times observed.v'? These absorption bands are ascribed classically to electro n transfers from the ligands to a cen tral cation. It is argue d th at (1) the band energy is low er as the electronega tivity of the ligands d ecreases, an d (2) it is reduced as the va lency increases for cations having the same number of electrons.v" Ch a rge-transfer states for 3d ions, however, are no t ful ly und erstood , unlike those for 4d and 5d ion complexes.*
2.2.3 Cr3+ phosphors (3d 3) Luminescence d ue to Cr 3 > is obs erved in the far-red to infrared region, and onl y limi ted ap p lications have been proposed for Cr 3> phosphors." Th is ion has a ttrac ted, however, th e atten tion of spe ctroscop ists since th e 1930s, becau se Cr 3> brings about luminescence wi th an in tere sting lin e stru ctu re in the 680- to nO-nm spectra l region in various ho st materials . In particular, the optical spectra of ruby (A 1203 :C r h ) were fu lly expl ain ed for the first tim e by a p plying crys tal field theory (1958) 13; ruby was ut ilized for the first solid state laser (1960).I'J Figures 17 a nd 18 sh ow the luminescence' > and absorption ! spec tra of ruby crys tals, respectively. The two strong lum inescen ce lin es a t 694.3 nm (= 14399 cm " ) and 692.9 , See 2.4. For rar e-e a rth phosphor s, the e ffect of th e cha rge-tran sfe r bands is inv estigated in co ns ide rable d etail w ith resp ect to the fluorescence proper ties of Fr transit ions ."
Chapter two:
Principal phosphor materials and their optical properties
169
'A z U,"p4) 2A I
10
2
3
Dq/B
Figure 15 Energy level diagram for the d7 configuration. (From Kamirnura, H., Sugano, S., and Tanabe, Y., Ligand Field Theory and its Applications, Syokabo, Tokyo, 1969 (in Japanese). With permission.)
nm (= 14428 em:") with width of ~10 crrr' and decay time of 3.4 ms at room temperature are called R j and R 2 lines. They lie at the same wavelengths as lines observed in the absorption spectrum (zero-phonon lines). These lines correspond to the transition from 2£(t23) ---7 4A2(t23) in Figure 11. The 2£ level splits into two levels due to a combination of the spin-orbit interaction and symmetry reduction in the crystal field from cubic to trigonal.' Two strong absorption bands at ~ 18000 cm' and -25000 cm' correspond to the spin-allowed transitions from the ground level (4A2(t} )) to the "T2(t/ c) and 4T1(t} e) levels, respectively. The spectral band shape differs, depending on the electric field direction of the incident light due to the axial symmetry in the crystal field (dichroism). Many spin doublets originate from the t/e configuration of Cr3+ in addition to the above two spin quartets." Transitions from the ground level (4A2 ) to those spin doublets are spinforbidden, the corresponding absorption bands being very weak to observe." Strong spinallowed absorption bands to those spin doublets, however, are observable from 2£(t23), when a number of Cr 3+ ions are produced by an intense light excitation into this excited state (excited-state absorption)." For 11 multiplet levels, including those obtained through excited-state absorption studies, all the properties of the absorption bands-such as spectral position, absorption intensity, and dependence on the polarized light-have been found to agree very well with those predicted from crystal (ligand) field theory.l-" As shown in Figure 17, with the increase in Cr3+ concentration, additional luminescence lines begin to appear at the longer wavelength side of the R lines, and grow up to be broad bands that become stronger than R lines; this is accompanied by the reduction * In Figure 11, positions for these doublets are not shown clearly. *' In a strong crystal field, two-electron transitions such as I,' --'> 12c' are forbidden.
Fundamentals of Phosphors
170
r=4.71 B=1030
60 IS
50 40 3
l
_~---;>,,-
1
2
E
3
Dq,/B Figure 16 Energy level dia gram for the dB con fig ura tion. (Fro m Kamimura, H ., Sugan o. S., and Tanabe, Y, Ligand Field Theory and its Applications, Syo ka bo, Tokyo, 1969 (in Jap an ese). With perrnission .) Table 6 Oscill at or Strength and Luminescence Decay Time La port e's ru le allow ed
Sp in-a llowed
f t
Spin-forbidd en
f r
Electric dipole
Magnetic dipole
-1 -5 ns 10-2-1 0-3 0.5-5 us
-10-" -1 ms 10-8-10- 9 . 102- 103 ms
Laporte 's rul e forbidde n Electri c di pole Latt ice vibration v" u allowed allowe d _10--1 - 10--1 -50 IlS - 50 us 10-6-10-7 10-6- 10-7 5-50 ms 5-50 ms
Note: 1.fvalu es for the case of sp in-allowed are estimated in Reference 1.fvaJues fo r the case of spin-fo rbidden are assu med to be 10-2-10-3 of those fo r sp in-a llowed. 2. Decay times are calculated from Eqs. 22 and 23, ass uming EJ Eri , = (1/2 + 2)/3 (Lo renz field), /1 = 1.6, and A" = 500 nm.
in the luminescence decay time of R lin es, in the case of Figu re 17, from 3.5 ms to 0.8 ms at room ternp e rature.t '' Additional lin es are attributed to magnetic ally coupled Cr3+-Cr3+ pairs and clu sters. Luminescence lin es are as signed to suc h pair s up to the fourth nearest neighbor; for examp le, the N ] lin e is assig ned to pairin g to the third nearest neighbor, and N 2 to the fourth near est ." In compounds suc h as va rious ga lliu m ga m ets in w hich Cr 3+ ions are locat ed in weak crysta l fields, 4T 2(4F), instead of 2EeG), is th e em itting lev el. 18 As exp ected from Fig ure 11, the luminescen ce spec tru m consists of a broad band in the near-infrared region, i.e., at a longer wavelen gth region than that in th e 2E case. The de cay tim e is as short as - 0.1 ms because the transiti on is spin-allowed . These properties ma ke them promising candi d ates
171
Chapter two: Principal phosphor materials and their optical properties
100 ::0-
C r 20 ~ =
~ u
0.055 %
'"
lI-
o
U
o
700
720
700
740
720
760
800
780
820
A [nm ]
Figure 17 Luminescence spectra in rubies (at 77K). (Figure 1 in the source shows lum inescence spectra and deca y times for rubies containing 0.4, 0.86, 1.5, and 8% concentrations of Cr 203, in add ition to the above two examples.) (From Tolst oi, N.A., Liu, S., and Lapidus, M.E., Opt. Spectrosc., 13, 133, 1962. With p ermission.)
9
8 7 6
5
,, I
3 2 1
o
I
,
I
I
\
I
I
:
I I I
4
E -L C 3 ( 0") E II C 3 ( 7[)
, I I I
(0.28 wt % C r 2 0 3 room temperature)
' I
I I I I
, ,
I
I
I I
\ \
,
'
. 35
40
45
Wave number [x 10 3 em-I] Figure 18 Absorption spectra of a ru by. (Courtesy of A. Misu, unpublish ed. ) E rep resen ts the electric field directio n of an incident ligh t, and C3 do es a three-fold axis di rection of the crystal. Spec trum at higher energies than 35000 crrr ' is for natural light. Absorption lines arou nd 15000 and 20000 cm-1 are shown only in the case of the 0 spectrum, qualit atively w ith respec t to intensi ty and linewid th. (From Karnim ura, H., Suga no, S., and Tanabe, Y., Ligand Field Theory and its Applications, Syokabo, Tokyo, 1969 (in Japa nese) . With p ermis sion .)
for tu nab le solid-sta te laser materials.P'" Th e change of the emitti ng sta te depending on the host materials is a good example of th e importance of the crys tal field in de ter mining the opti cal pro perties of the transition-metal-doped compo unds. Table 7 shows the crystal field parameters ob tained from absorption spectra and luminescence decay times for Cr 3+ in several hosts. Most lumin escen ce bands in 3d ions are caused by electric dipole transi tions. In such materi als as MgA l z0 4 and MgO, in which a metal ion lies in the crystal field wi th the inversion sy m me try, how ever, the R lines occur via a magne tic dipole process-l-" : conseq uen tly, the decay tim es are lon g.
172
Fundamentals of Phosphors Table 7 Cryst al Field Par amet er s for Host
A
Dq
B
(nm)
(em ")
(crrr" )
1630 1630 1825 1660 1750 1725 1471 1508
640 780 700 650 800 640 645 656
1720
765 918
692.9 H
a- A120 3 (ruby) Be3AJ2Si6018 (Emerald) MgA1P 4 MgO LiAlsOs a Y1A1s012 Gd 3GasO '2 Y3Ga50 12
694.3 682.1 679.226 682.2 681 .9 698 27 715.8 701.6 688.7 687.7 745 (broad )" 730 (b roa d)"
Cr (H 2O lo3+ Fre e ion
684.2
Abs. (lG)25
r-h
C (cm" ) 3300 2960 3200 3200 2900 3200
T
(ms) 3
(R)
36.5 (N) 12 (N) 3.7 1.5 0.16 0.24
Ref. 23 23 21 22 24 28 18 18 3 3
Note: A: peak waveleng th of Iuminescence.t: l ie d ecay time ; (R), room tem per ature; (N), 77K . ., Ord e red ty pe b 4T l -4
' A 2 tran si tion, othe rw ise ' F.
-4
.IA, transition.
2.2.4 Mn 4+ phosphors (3d 3) Only 3.5MgO ·0.5Mg F2· Ge0 2 :Mn 4 + is now in practical use amo ng the Mn4+ phosphors, though 6MgO 'As 2 0 s:M n 4 +, which has a performan ce a lmos t e q ua l to that of 3.5MgO ·0.5MgF 2 ·Ge02 :Mn 4+, was used previously." and a number of titanate phosphors were d eveloped between 1940 and 1950.30 Luminescence bands due to Mn4+ exist at 620 to 700 nrn in most ho st mat erials. The spectrum has a structure cons isting of several broad lines orig ina ting from transitions aided by lattice vibra tion . In Al2 0 , and Mg 2Ti04, it rese mbles the R lin es of Cr-" . and is assigned to the 2[ (t / ) -1 4A 2(t 23) transition. Figure 19 shows the luminescence spectra for 3.5Mg O·0.5Mg F2· Ge0 2:Mn 4+. It consists of more than s ix lines a t room temperature; the inten sity of the lin es at the shorter wa velength side d ecreases a t low temperatures. This beh avi or is exp laine d by assuming that thermal eq uilibrium exis ts between two lev els in th e emi tting sta te, and that there ar e more than tw o levels in the gro und state." As for the origin of the emi tting and ground states, diff erent assign me n ts hav e been proposed. Kem en y and Haake assigned the bands to the 4T2(t/ c) -1 4A 2(t 23) tran sition in Figure 11, assuming the Mn4+ site has octahed ral coordination." They propose that the 4T2 level splits into two levels due to the low symmetry field, and that mo re than two vib ronic leve ls accompany the gro un d state. Butler insisted that a (Mn04 )4- complex rep laced (GeO)I-, which is tetrahedrally coo rdina ted .F In this case, the appropriate energy diagr am is Figure 15 instead of Figure 11, and the luminescen ce origin ates from the 2E(e3) -1 4T1(e2t2) tran sition ." The 2[ and 4T1 levels spli t into tw o and three due to the low sym metry field , resp ectively. These proposals, however, could not accoun t for s uch facts as the luminescen ce has a decay time of the order of millis econds; in ad di tion, no visible luminescence has been obse rved due to Mn 4+ in so lid -state materials in which the metal ion s are tetrah edrally coordina ted. Ibuki's group ass igned the lines to tran sitions from tw o excited levels of 2EUl) and 2T 1U?) to the gro u nd s tate 4A 2U/ ) in Figure 11, ass uming Mn 4+ has an octahe d ral coord ination.P Th e main pea k struc tu re in the range 640 to 680 nm at room temperature origina tes from the lattice vibra tion asso ciat ed w ith the 2£ -1 4A2 zero-phonon transition at 640 nm . Blasse explained the s pectral charac teristics by assuming only on e electron ic tran siti on of 2£ -1 4A2 in octahe d ra lly coo rdin ated Mn 4 ' . 34 Both the ground an d excited sta tes are * See 2.2.1.3. The trans ition correspon ds to 2EU / <,)
-4
'T,(t ,V'-) in Figure 15.
Chapter two: Principal phosphor materials and their optical properties
173
, ,,
, I
I, I
I , I I
,
RT 77K
I I
,, I
'•. I
""
600
700 Wavelength (nm)
Figure 19 Lum inesce nce spec tra of 3.5MgO·O.5MgF2. Ge0 2 :Mn4+. (Observed by the au thor.)
coupled wi th special vibra tion modes. The shorter waveleng th pea ks, w hich disap pear at low temperatu res, are ascribed to transitions from an exci ted v ibronic level (an ti-Stok es vibronic transitions). Strong absorp tion bands d ue to Mn 4+ exist, corresp onding to the spin-allowe d transitions of 4A2(t2J ) -7 4T1, 4T2(t/ e) in the visible to near-Uv region, and the bod y color of the phosphor is usu ally yellow. Table 8 shows the crys tal field param eters and luminescence decay times for Mn4+ in several hosts. The larger valency leads to Dq / B values as large as 3, compared with those for Cr3+ ( ~2.5), an d this, in tum, to the absorption ban ds at shorter wa velengths as expected from Figu re 11. The charge-transfer band, on the other hand, lies at longer wavelength (-285 nm in AI20 3) , resulting from the larger valency of Mn4 + . J O~"t'i (See 2.2.2.2.)
2.2.5 Mn 2+ phosphors (3d 5) 2.2.5.1
Crystal field
Luminescence due to Mn 2+ is kn own to occu r in more tha n 500 ino rganic compounds." Of th ese, several are being used widely for flu or escent lam ps and CRTs. The lu minescence spectrum consists of a stru ctur eless b and wi th a h al fw idth of 1000 to 2500 crrr' at peak wa velengths of 490 to 750 nm. Figure 20 shows the lu min escen ce and exci ta tio n spec tra d ue to Mn 2+ in La 203 ·11AI20 3 as an exampl e." The energy level di agram fo r Mn 2+ in bot h oc tahedra l and tetr ah edral coordinations is represented by Fig ure 13. In phosphors, Mn 2+ions are locat ed in the weak crysta l fiel d of Dq/B = 1, an d th e luminescence corresponds to the 4T J (4 G) -7 6A] (6S) transition . When a meta l ion occup ies a cer tain position in a crystal , th e crys tal field strength that affects the ion increa ses as th e space con taining the ion becomes smaller, as expected
Fundamentals of Phosphors
174 Table 8
Crystal Field Parameters for Mrr'!
Dq
Ie (nm)
Host
3Q
676.3 672.6 655.6 653.2 vib 716 702 vib 623-664 str.
a -AI1O J Mg 1Ti0 4 LiA150s a 3.5MgO·0.5 MgF1·Ge01 Mg 6As1Oll
620-665 str.
(crrr')
B (crrr')
C (crrr')
2170 2096 2014 2375
700 848 725 709
2800 3300 2900 3080
(2375)
(709)
(3080)
1065
4919
576.4 (lG)15
Free ion
r
(ms) 0.8 0.5\" 0.2 3.331
(N)
Ref.
(R)
35 36 37 33
2.817 (R)
33
(R)
(N)
3
Note: "A.: peak wa velength of luminescen ce; 1: l i e d ecay time; (R): room temper at ure; (N): 771<; vib: vibration s truc tu re, s tr: s tructu red band .
., Orde red lype.
Wave number [em- I) >-,
3 2 X 10' .~ 2,----,,_---,--------,-- - ----------r----=.-=----------,
3
::: 0,)
g
Excitation spect rum
o
~
'E:::l
1
" A 2 (4 F ) > F) C;; ~O~:;;---====---=>.~:!-::---..L::.::::::::..-==---~:-:-:-C-.:.........0,)
., r, ('
300
400
------"~ ,--------=:::=- --:-!
500
600
Wavelength.(nm) Figure 20
Luminescence and excitati on spec tra of La10J ,llAI10J :Mn2+. (From Tarnatani, M., [pn. f.
Appl. Phys., 13,950,1974. With p ermissi on .)
from Eq. 9. For increases in th e field, the transiti on energy b etween the 4T] and 6A 1 lev els is predicted to decrease (shift to longer w avelengths). (See Figure 13.) In fact, the peak wavelength of the Mn 2+ luminescence b and is known to vary linearly to longer wavelength (547 to 602 nm) with a d ecrea se in Mn-F distance (2.26 to 1.99A) in a group of fluorides already studied, including 10 perovskite lattices of the type A1BIIF 3 , ZnF 2, and MgF 2.41 A similar relationship also h olds for each gro u p of oxo-acid salt phosphors having an analogous crystal structure; th e wavelength is longer when Mn 2+ replaces a smaller cation in each group, as seen in Tabl e 9. On the o th er h and, a larger anion complex makes the cation space shrink, leading to longer-wavelength luminescence. For CalO(P04)6F2:Mnl+, the crystal field at a Mn 2+ ion produ ced b y io ns in eight unit cells around it was calculat ed theoretically. The re sult w as co nsis te n t with the observed luminescence peak sh if t (100 em:") to longer wav elength due to a lattice constant decrease (0.14%) when one Ca in each Ca lO(P04)ifl is repl aced by Cd. 43 In spite of the fact th at the ionic radi us for Zn 2+ (0 .72 A) is smaller than that for Ca 2+
(0.99 A), th e luminescence w avelength in Zn 2Si04:Mn 1+ is shor ter than in CaSi03:Mn1+. This is attributed to a smaller co ordi na tion number (4) in the former as compared with that (6) in the latter. (See Eq . 13.) In m aterials containin g a spinel structure, Mn 1+ can occupy either octahedral or tetrahedral sites . From the fact that the luminescence occurs in th e sh orte r-wavelength (green) region, the tetrahedral si te is expected to be occupied preferentiall y by Mn2+. This is con firmed by ESR44 and ion-exchanges' studies fo r ~ aluminas an d s up p or ted by thermodyn amic data ."
Chapter two: Principal phosphor materials and their optical properties Table 9 Crys tal sy m metry
Host CaF2 ZnF 2 KMgFJ ZnGaP 4 Zn Al204 Zn 2Si0 4 Zn 2Ge04 Ca s(P°4h F Srs(P°4hF rnonocl-CaSir.r, monocl-MgSi0 3 CaS he x- ZnS
0"
°4"
(0,,) 0" 0" C3i C3i CM C6J> C2 CZh Oil
Td
175
Mn 2+Sites and Lu minescen ce Properties Site
Co ord in at ion number
Inv ersion sym me try
Ie (nm)
Ca Zn Mg (A site) (A site) 2Zn 2Zn 2Ca 2Sr 3Ca 64 2M g 65 Ca Zn
8 6 6 (4) (4) 4 4 663 6 6 6 6 4
g g g
495 587 60242 506 513 525 537 570' 558 550 620 660 740 588 591
u u u u
u u u U
g u
r
(rns)
8346 100 10462 4 5 12 10 1466 30 2.2-4. 867 0.25
Note: 1. 2Ca in the site co lumn means exis tence of two different Ca sites. (A site) mea ns larger probability for existence in A sites than for octahed ral B sites. 2. Excep t for those referr ed , crystal symmetries follow those in Reference 61, and luminescence wavel eng ths and de cay times in Reference 5l. 3. In the inversion symme try column, g and u corresp ond to exis tence and nonexistence of a center of sy mmetry, resp ectively. a A valu e ob tained in an Sb-Mn co-doped sample.
In CaF 2:Mn2+, th ou gh Mn 2+ occupies a cubic site w ith hi gh coordination number, Dq is not so large because th e ani on valency of F- is smaller than th at of 0 2- . In additi on, B is large because o f th e s ma lle r nephelauxetic eff ect ." C onsequ ently, thi s com p ou n d yields the shortest luminescen ce wave leng th (-495 nm) observed a mon g Mn -t-doped phosphors ." Since every excited level of d5 is either a spin quartet or a d oublet , all transitions from the ground se xtet to them are sp in-fo rb id d en . Optical ab sorption inten sity is weak, and the phosphors are not colored (i.e., the powder bod y co lo r is white). Th e 4A ) and 4E(4G) levels ha ve th e sa m e energy an d are parallel to the ground level GA l in Fig ure 13. The absorption band corres p ond ing to GAl -7 4A l ,4E(4G) therefore h as a n arrow bandwidth, lying at - 425 nrn , irrespectiv e of th e kind of host material. w' ? One n otices that this band sp lits into more than one lin e when carefu lly investigated. Th e sp litt ing is consid ered to reflect the reduction of the crysta l field symmetry.w" Table 10 shows the cr ystal fiel d parameters for Mn 2+ in rep resentati ve phosphors . Note that Oq / B for th e te trahedral coordination is smaller «1) th an th at (>1) for the octahedral one.
2.2.5.2
Different Mn 2+ sites in crystals
Since the luminescen ce w avelen gth due to Mn2+ is se nsitive to the m agnitud e of the crystal field, several em ission bands are obs erv ed w hen different typ es of Mn2+sites ex is t in a host cr ystal. In SrAI I2 0 19 , th e band s at 515,560, and 590 nm are co nsidere d to orig in a te from Mn 2+ ion s replacing tetrahedrall y coo rd in a ted A J3+, fivefold co o rd ina ted A J3+, and 12-fold coord in a ted Sr 2+, respect ivel y." In lanthanum aluminate, whi ch ha s a la ye r structure of s pinel blocks, a 680-nm band is ob served due to Mn 2+ in oc ta hed ra l coordination, in ad d ition to a green -emi tting band due to tetrahedral coordinati on .v Tw o e m iss ion • The other shortest peak wavelength is at 460 to 470 rim. observed in 5rSb ,O,.,;7 in which Mn> is consid ered to be located in an extraordi nary weak crystal field (5r-0 distance is as large as 2.5 A).
Fundamentals of Phosphors
176 Table 10
Host
Crystal Field Parameters for Mn2 +
A
Dq
(nm)
(crrr")
B (crn')
C (crrr")
Coordination
Ref.
MgGa204 LaAlu018 Zn2Si0 4 Cas (P° 4hF Mg4Ta 209 CaF2 hex·ZnS
504 517 525 572 659 495 59}51
520 543 540 760 425 (2375) 520
624 572 (624) 691 (698) 770 630
3468 3455 (3468) 3841 (3678) 3449 3040
(4) 4 4 6 6 8 4
48 41 48 68 55 46 69
Mn(H2O )62 + Free ion
Abs. 372.5 ClG)l5
1230
860 860
3850 3850
6
3 3
Note: Band C values in parenthesis, which were obtained from other phosphors, are used for calculating Oq values.
bands separated by about 50 nm were recognized long ago in Mn-r-doped alkaline earth siliea tes. 51 Even in the case of the same coordination number, different luminescence bands may come from Mn> ions occupying crystallographically different sites. In CaS(P04)3F, there are principally Ca(I) and Ca(II) sites having different crystallographic symmetries; several additional sites accompany these two main calcium sites. The correspondence between the luminescence bands and the various sites has been investigated by means of polarized light,sZ ESR,s3 and excitation'? spectral studies. In the case of the commercially available phosphor Ca 5(p04h(F,Cl):Sb3+,Mnz+ (for Cool White fluorescent lamps), the Mn?:' band consists of three bands at 585, 584, and 596 nm, originating from Mn> ions replacing Ca(I), Ca(II), and Cl, respectively." Figure 21 shows the spectra in Zn zSi0 4:Mnz+, where two zero-phonon lines are observed at very low temperatures (504.6 and 515.3 nm at 4.2K).5S These lines are assigned to two types of Mn-" differing in their distance to the nearest oxygen; one is 1.90 A and the other is 1.93 A. Since the Oq value depends on the fifth power of the distance (Eqs. 9 and B), a 7% difference in the Oq value is expected between the two types of Mn> sites; this is consistent with the difference estimated by crystal field theory from the observed line positions (2% difference)." The polarization of the luminescence light observed in a single crystal is also related to the site symmetry of Mn 2 ' .% The zero-phonon lines are accompanied by broad bands in the longer wavelength side; these originate from latticeelectron interactions and are known as vibronic sidebands (See Section 1.3.) Multi zerophonon lines resulting from different Mn?" sites are also observed in Mg 4Ta z0 9 ) O and LiAl sOs·37 In ZnS doped with high concentrations of Mn?", although there is only one cation site crystallographically, two zero-phonon lines appear at 558.9 and 562.8 nm at low temperatures. These are ascribed to a single Mn?" ion (1: = 1.65 ms) and a Mn-t-Mn?" pair (1: = 0.33 ms)." In this material, the luminescence band shifts to longer wavelength and is accompanied by a decrease in decay time with increasing Mn> concentration; this is also observed in such hosts as Zn ZSi04/s1 MgGa Z04,ss ZnAl z04,sl CdSi03,sl and ZnF z.Sl Most of these effects are attributed to Mn-t-Mn-" interactions.
2.2.5.3
UVabsorption
Lamp phosphors must absorb the mercury ultraviolet (UV) line at 254 nm. In most cases, Mn> does not have strong absorption bands in this region. To counter the problem, energy-
Chapter two: Principal phosphor materials and their optical properties
177
Wavelength (nm)
560
540
520
500
>-.
+-'
C/l
c:
+-'
c:
B
u
c:
c:
·S ;:l
> +-'
CI:l
~
2.20 Photon energy (eV) Figure 21 Lumin escence spectra of Zn 2Si04:Mn2+. (From Steve ls, A.L.N . and Vink , AT. , f. Luminesc., 8,443, 1974. With per miss ion.)
transfer mechani sm s are utilized to sen sitize Mn2+; transfers are effected th rough th e h ost " or via such ions as 5b 3+, Pb 2+, 5n 2+, Ce 3 +, and Eu 2+, which absorb th e UV efficien tly through allowed transit ion s. These ions are called sensitizers for the Mn 2+ luminescence. In Zn 2Si0 4 , a s tro ng absorp tion band app ea rs at w av elen gths shorter th an 280 nm when doped with M n 2 +. 30 This band is asc ribed to Mn 2 + ~ Mn 3 + ionizati on ' ? or to a d5 ~ d4s transition .Fs?
2.2.5.4 Luminescence decay time The de cay tim e of Mn2+ lumines cence is usuall y in the millisecond ra ng e (Table 9). A shor ter d ecay tim e is expected in th e tetrahedral coordination b ecause it has no cen ter of inv ersio n sy m me try. In most p ractical phosphors, the Mn 2 + sit es are surro un d ed by six oxyg en ion s, but the sy m metry is low er th an octa he dral and th e sites do not have a cen ter of in ver sion . (See Table 9.) It foll ow s th at th e di fferen ce in th e d ecay time between th e phosphors hav ing Mn2 + wi th four and six coordina tion number s is n ot ac tua lly so large. Decay tim es in the fluorid e ph osphors are one order of ma gnitude longer than th ose in the oxo -acid sa lt phosphors. Thi s is thought to be due to the fact th at Laporte's rul e h old s more strictly in the fluorides, since odd states d o not mix into th e d sta tes easily because: (1) Mn 2+ ion s in th e fluorid es are located at a cen ter of inversion sy mmetry, and (2) th e sma ller nephelau xet ic effect m akes th e odd states lie at higher en ergies th an those in oxoacid sa lts.
2.2.6 Fe3+ Phosphors (3d 5) Luminescen ce du e to Fe 3+ lies in the wavelen gth region longer than 680 nm, and on ly LiAI0 2:Fe3+ and LiGa0 2:F e3+ are used for special fluo rescent lamp ap plications." It is easily under st ood fro m Fig ure 13 w hy th e luminescen ce wa velen g ths du e to Fe 3 + are so much • The hos t-absorp tion wave lengt h d oes no t always corresp ond to the bandga p energy of the host mat er ial.
Fundamentals of Phosphors
178
Table 11 Crystal Field Parameters for Fe 3+ Host LiAl.O,
a
~-LiAI02
Ca(P0 3) 2 y-AIF3 Fe(H 2O)62 + Free ion
A
Dq
(nm)
(cm")
680 735 830 735
800 883 1250 1220
Abs. 312 (4G)25
1350
., Ordered structure,
T =
B (crrr')
C (crrr')
644 630
2960 3000
895
3000
820 1015
3878 4800
Coordina tion
Ref.
4 4 6 6
72 73 75 71
6
3 3
7.1 ms.
longer than those due to Mn 2 +, which has the same electronic configuration of 3d5 . That is, the larger valency of Fe3+ brings about the stronger crystal field, reducing the transition energy of 4T](4G) ~ 6A](6S). In fact, as shown in Table 11, Dq/B for Fe3+ is ~1.2, even at tetrahedral sites, and larger than that «1) for Mn2+. The reason for the emission wavelength being as short as 735 nm despite the octahedral coordination in AlF 3 is attributable to the large B value resulting from the small nephelauxetic effect." The absorption (or excitation) spectrum in the visible region due to Fe3 + resembles the shape of that of Mn 2+ . Zero-phonon lines are also observed in LtAl.O, 72 and LiAI0 2 .73 In the UV region, contrary to the Mn2+ case, however, a strong absorption band supposedly caused by charge transfer appears,IO,72,74 and the Fe 3+ phosphors can be excited directly by 254-nm light irradiation without the need of sensitization through other ions.
References 1. Kamirnura, H, Sugano, S., and Tanabe, Y, Ligand Field Theory and Its Applications, Syokabo, Tokyo, 1969 (in Japanese); Sugano, S., Tanabe, Y, and Karnimura, H., Multiplets of Transition. Metal Ions in Crystals, Academic Press, 1970. 2. a) Tanabe, Y and Sugano, S., J. Phys. Soc. [pn., 9, 753, 1954; b). ibid., 766. 3. Orgel, L.E., J. Cl u'/Il. Phys., 23, 1004, 1955. 4. McClure, 05., Electronic spectraof molecules and ions in crystals. Part II. Spectra of ions in crystals, in Solid State Physics, Seitz, F. and Turnbull, D., Eds., 9, 399, Academic Press, 1959. 5. Griffith, j.S, The Theory of Transition Metal Ions, Cambridge Univ. Press, 1964. 6. Ballhausen, CJ., Introduction to Ligand Field Theory, McGraw-Hili, 1962. 7. [orgensen, CK., Absorption Specia and Chemical Bonding in Complexes, Pergamon Press, Elmsford, NY, 1962. 8. Prather, J.L., National Bureau of Standards Monogr., 19, 1, 1961. 9. Di Bartolo, B., Optical Interactions in Solids, John Wiley & Sons, 1968. 10. Tippins, HH., Phys. Rev., B1, 126, 1970. 11. Hoshina. T, Imanaga, S., and Yokono, S., J. Lumincsc., 15,455, 1977. 12. Sluzky, E., Lemoine, M., and Hesse, K., J. Electrochem. Soc., 141,3172, 1994. 13. Sugano, S. and Tsujikawa, I., J. Phys. Soc. [pn., 13, 899, 1958; Sugano, S. and Tanabe, Y, ibid., 880. 14. Mairnan, TH, Nature, 187,493, 1960. 15. Tolstoi, N.A., Liu, S., and Lapidus, M.E., Opt. Spectrosc., 13, 133, 1962. 16. Kushida, T, J. Phys. soe. [pn., 21, 1331, 1966; Shinada, M., Sugano, S., and Kushida. T, ibid., 1342. 17. Powell, RC and Di Bartolo, B., Phys. Status Solidi (a), 10, 315, 1972. 18. Petermann, K. and Huber, G., J. Luminesc., 31&32, 71, 1984. 19. Walling, J.C, Tunable paramagnetic-ion solid-state lasers, in Tunable Lasers, Mollenauer, L.F. and White, j.c.. Ed s., Springer-Verlag, 1987. 20. Moulton, FF., Tun able paramagnetic-ion lasers, in Laser Handbook, Vol. 5, Bass, M. and Stitch, M.L., Eds., Elseviers Science, B. v.. 1985.
Chapter two:
Principal phosphor materials and their optical properties
179
21. Wood , D.L., Imbu sch, G.P., Macfarlane, R M., Kisliuk, P , and Larkin, D.M., f. Chem. Phys., 48,5255, 1968. 22. Macfarl an e, R M., Phys. Reu., ai. 989, 1970. 23. Wood, D.L., Ferg uson, J., Knox, K., and Dillon , Ir., J.P. , j. Chern . Phys., 39, 890, 1963. 24. Pott, G.T and McN icol, B.D., J. SoLid State Chem., 7, 132, 1973. 25. Moore, CE., At omic Energy Leoels, Vol. II, N BS Circular, 1952, 467. 26. Wood, D.L., j. Chern . Phys., 42, 3404, 1965. 27. lmbusch, G.P., Experimenta l sp ectroscop ic techniques for transition metal ions in solids, in Luminescence of Inorganic Solids, Di Bartolo, B., Ed ., Plen um Pre ss, 1978, 135. 28. Sevast'yan ov, v.P, Sviridov, DT , Orekhova, VP, Pas terna k, L.B., Sviridova, R.K., and Veremeichik, TP. , SOV. J. Quant. Eleciron ., 2, 339, 1973. 29. Ouweltjes, J.L., Elenbaas, W , an d Labber te, K.R , Philips Tech. Rev., 13, 109, 1951. 30. Krog er, P.A, Some Aspects of Luminescence of Solids, Elsevier, 1948. 31. Keme ny, G. and Haake, CH, f. Chein. Phys., 33, 783, 1960. 32. Butler, K.H, Proc. Int. Can! Luminesc., Bud apest, 1966, 1313. 33. Ibuk i, S., Awazu , K, and Hata, T , Proc. Int. Can! Luminesc. , Budapest, 1966, 1465. 34. Blasse, C . and Grab maier, B.C , Luminescent Materials, Springer-Verlag, 1994, 128. 35. Geschwind, S., Kisliuk, P , Klein, M.P , Rem eika, J.P., and Wood , D.L., Phys. Rev., 126, 1684, 1962. 36. Stade, J., Hahn , D., an d Dittmann , R , J. Luminesc., 8, 318,1974. 37. McNicol, B.D. and Po tt, CT, J. Luminesc., 6, 320, 1973. 38. Ditt mann, R. and Ha h n, D., Z. Phys., 207, 484,1 967. 39. Trav nicek, M. Kroge r, EA ., Bord en , TI1.P J., and Zahrn, P , Physica, 18, 33, 1952. 40. Data obtained from Chem ical Abstracts in 1948 to 1971, and References 30 and 51. 41. Tarnatani. M., [pn . J. Appl. Phys., 13, 950, 1974. 42. Klasens, H .A , Zahrn, P, an d H uys rnan, P.O., Philips Res. Repts., 8, 441, 1953. 43. Narita, K , j. Phys. Soc. [pn., 16, 99, 1961; ibid., 18, 79, 1963. 44. Antoine, J., Vivien, D., Liva ge, J., Ther y, J., and Collongues, R., Mat. Res. Bull., 10, 865, 1975. 45. Bergstein, A and White, W.B., j. Electrochem. Soc., 118, 1166, 1971. 46. Alonso , PJ. and Alcala, }. Lumlnesc., 22, 321, 1981. 47. Ya ma da, H ., Matsukiyo. H ., Suzuki, T , Yama mo to, H., Okam ura, T , Imai, T , and Morita, M., Electrochem. Soc. Fall Meeting, Abs tr. No. 564, 1988. 48. Palumbo, D.T and Brown , Ir.. J.J., j. Electrochem. Soc., 117, 1184, 1970. 49. Palumbo, D.T and Brown, Jr., J.J., f. Eleetrochem. Soc., 118, 1159, 1971. 50. Stevels, A.L.N., J. Luminesc., 20, 99, 1979. 51. Leverenz, H.W., An introduction to Luminescence of Solids, John Wiley & Sons, New York, 1950; Recent p ublica tions for Zn 2Si0 4 :Mn are Bar thou, C , Benoit, J., Bena lloul, P., and Morell, A., J. Electrochem. Soc., 141, 524, 1994; Rond a, C R , Proc. Znd Int. Display Workshops, Vol. 1, 1995, 69. 52. Ryan, P.M., Ohlman , R.C , and Murp hy, J., Phys. Reu., B2, 2341, 1970. 53. Kasai, P H , J. Phys. Chem. , 66, 674, 1962. 54. Ryan, P.M. and Vodokly s, P.M., j. Elecirochem. Soc., 118, 1814, 1971. 55. Stevels, A.L.N. an d Vink, AT, j. Luminesc., 8, 443, 1974. 56. Bhalla, R.J.R.s . and White, E.W , j. Electrochem. Soc., 119, 740, 1972. 57. Busse, W , Gum lich, H .E., Meissme r, B., a nd Theis, D., J. Luminesc., 12/13, 693, 1976. 58. Brown, Jr., J.}., J. Electrochem. Soc., 114, 245, 1967. 59. Robbin s, D.}., Avo uris, P , Chang, I.P., Dove, 0 .8., Giess, E.A., and Mend ez, E.E., Electrochem. Soc. Spring Meeting, Abstr. No . 513, 1982. 60. Butler, K.H, Fluorescent Lamp Phosphors, Penn sylvania Stat e Universi ty Press, 1980. 61. Wyckoff, RWG ., Crystal Strltctures, Interscience P ublishers, 1965. 62. Van Na y, B.W an d Mik us, P.E, Proc. In/. Can! Luminesc., Bud apest, 1966, 794. 63. Nar ay-Szabo, S., Z. Krisi ., 75, 387, 1930. 64. Tollid ay, J., Nature, 182, 1012, 1958. 65. Morimoto, N., App lem an , D.E., and Eva ns, Jr., E.T , Z. Krisi., 114, 120, 1960. 66. Soule s, T P., Bateman, R.L., H ewes, R.A, and Kreid ler, E.R., Phys. Rev., B7, 1657, 1973. 67. Yamam oto, H ., Megumi, K , Kasano, H, Suzuki, T, Ueno, Y, Morita, Y, and Ishigak i, T, Tech. Digest, Phosphor Res. Soc. 198th Meeting, 1983 (in Japanese).
180 68. 69. 70. 71. 72.
73. 74. 75.
Fundamentals of Phosphors Uehara, Y, Toshiba Rev., 24, 1090, 1969 (in Japanese). Kushida, T., Tanaka, Y, and Oka, Y, Solid State Commun., 14, 617, 1974. Van Broekhoven. L f. lllum. Eng. Soc., 3, 234, 1974. Telfer, D,J. and Walker, G., J. Luminesc., 11,315, 1976. Pott, G.T. and McNicol, B.D., J. Chern. Phys., 56, 5246, 1972. Stork, w.H.J. and Pott, G.T., f. Phys. Chem., 78, 2496, 1974. Tamatani, M. and Tsuda, N., Tech. Digest, Phosphor Res. Soc. 157th Meeting, 1970 (in Japanese). Fox, K.E., Furukawa, T., and White, W.B., J. Am. Cer. Soc., 64, C-42, 1981.
chapter two - section three
Principal phosphor materials and their optical properties Tsuuoshi Kano Contents 2.3 Luminescen ce centers of ra re-ea rth ions 2.3.1 Electronic confi gurati on 2.3.2 Electronic processes leading to luminescenc e 2.3.2.1 4f Energy levels and relaxation 2.3:2.2 4/ '- 1 5d l s tates and charge-transfer s ta tes (CTS) 2.3.2.3 Dival ent and tetravalent cations 2.3.2.4 Energy transfer 2.3.3 Luminescence of specific ions 2.3.3.1 eel + 2.3.3.2 Pr 3 + 2.3.3.3 Nd 3+ 2.3.3.4 Nd 4 + 2.3.3.5 Sm-" 2.3.3.6 Sm 2+ 2.3.3.7 Eu3 + 2.3.3.8 Eu 2+ 2.3.3.9 Gd 3 + 2.3.3.10 Th 3+ 2.3.3.11 D y3+ 2.3.3.12 D y 2+ 2.3.3.13 Dy1+ 2.3.3.14 Ho 3+ 2.3.3.15 Ho 2+ 2.3.3.16 Er:'l + 2.3.3.17 Tm 3 + 2.3.3.18 Yb3+ 2.3.3.19 Yb 2+ References
182 182 183 183 188 189 189 190 190 191 193 193 193 193 194 196 197 198 199 200 200 200 201 201 201 201 201 201
181
182
Fundamentals of Phosphors
2.3 Luminescence of rare earth ionsl - 3 2.3.1 Electronic conf iguration Th e rare-earth elemen ts us ua lly comprise 17 elem ents consisting of the 15 lanthanides fro m La (atomic n umber 57) to Lu (atomic n umber 71), of Sc (a tomic number 21), and of Y (atomic number 39). Th e elec tron ic configura tions of tri val ent ra re-earth ions in the groun d states are shown in Tabl e 12. As shown in the table, 5c3+ is eq uivalen t to Ar, Y 3+ to Kr, and La 3+ to Xe in electronic con figura tio n . Th e lanthanides from Ce 3+ to Lu 3+ have one to fourtee n 4f electro ns ad ded to their inner she ll configur a tion, w hic h is eq uivalent to Xe. Ion s w ith no 4f electro ns, i.e., 5c 3+, Y", La3+, and Lu 3+, ha ve n o elec tronic energy levels th at can in d uce exc ita tion and luminescen ce processes in or near the v isible region. In con tra st, the ions from Ce 3+ to Yb3+, which h ave partially filled 4£ orbitals, have ene rgy lev els characteristic of eac h ion an d show a variety of lu minescence properti es around the v isible region."? Many of th ese ions can be us ed as luminescent ions in phosphors, mostly by rep lacin g y3+, Gd 3+, La 3+, an d Lu 3+ in va rious compo und crystals. Th e azimutal quantum number (I) of 4f or bi ta ls is 3, giving rise to 7 (= 21 + 1) orbi tals, eac h of w hich can accommod at e tw o ele ctron s. In th e groun d s ta te, electro ns are d is tribut ed so as to p ro vid e the ma ximum combined spi n angu la r m om entum (5). The spin angu lar momentum 5 is further combin ed w ith th e orbi tal ang u lar momentu m (L) to give the tot al angular mom entu m (n as follows;
J = L - 5, w he n the number of 4f electro ns is sm aller than 7 J = L + 5, when the number of 4f electrons is lager than 7 An electro nic state is indicated by notation 2S+1L J, w h ere L represents 5, P, D, F, G, H , I, K, L M , .. ., corres p ond ing to L = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ,.., respecti vel y. More accura tely, an ac tual electro nic stat e is ex p ressed as an interm edi at e co up ling sta te, whic h can be d escr ibed as a mi xed s ta te of severa l 2S+1L , states-" combine d by sp in -orb it interaction . For quali tative d iscu ssions, h ow ever, the principal L stat e can be tak en to represent the actua l Table 12
Atomic num ber 21 39 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
Electronic Configu rations of Trivalen t Rare-Earth Ions in the Ground State .L Corresponding S J L(L+S) LS LI Ions element 4f electrons Sc3+
Ar
y 3+
Kr
La3+ Ce3+ p rJ +
Nd 3+ Pm3+ Sm3+ Eu3+ Gd 3+ Tb3+
Dy 3+ H o .!+
Er3+ Tm3+ Vb3+
Lu 3+
Xe Xe Xe Xe Xe Xe Xe Xe Xe Xe Xe Xe Xe Xe
r r r r r r r
0 0 0 1/2
i t
r r r r r t r r r r r r i r r r n r r H H r
H H H H H HH HH H H
H H H II
1
3/2
r r r r r r r
2
r r r r r r
r r
r r
H i H H t r H H H r H H II H
5/2 3 7/2 3 5/2 2 3/2 1
1/2 0
0 0 0 3 5 6 6 5 3 0
0
a 0 5/2 4 9/2 4 5/2 0 7/2
3
6
5
15/2 8 15/ 2 6 7/2 0
6
6 5 3
a
Chapter two: Principal phosphor materials and their optical properties
183
state. Th e m ixing due to spin-or bit inter action is sma ll for the lev els n ear ground s tates, while it is con siderable for excited stat es that have n eighboring s ta tes with sim ilar J numbers. The effect of m ixin g is relatively sm all on th e ene rgy of levels, bu t can be lar ge on their op tical tran sition p rob abilities.
2.3.2
Electronic processes leading to luminescence 2.3.2.1 4£ energy levels and relaxation
The 41 electro nic energy levels of lanthani d e ions ar e characteristic of eac h ion. Th e lev els are not affec ted much by the enviro nmen t because 41 electro ns ar e sh ielde d from ex ternal electr ic fields by the ou ter 5s 2 and 5 p6 electrons . This fea ture is in s tro ng contrast wi th transition metal ions, w hose 3d electr on s, loca ted in a n ou ter or bi t, are hea vily affected by the environm en tal or crystal electric field. The charac ter is tic en ergy levels of 41 elec tro ns of triv alent lanthanide ions have been p recisely investigated by Dieke and co-w or kers." The results are sh ow n in Figu re 22, which is known as a Dieke di agram ." The lev els w ere dete rmined expe rime n ta lly by considering the optical spectra of in d ividual ions incorporated in LaCl3 crys ta ls; th is d iagram is applicable to ions in almost any environmen t because the maximum varia tion of the energy levels is, a t m ost , of the order of severa l hundred crrr' . Each level de sign at ed by the n umber J in Figure 22 is split in to a n u mber of sublevels by the Stark effect due to the crys tal field. The number of sp lit sublev els is, at m ost , (2J + 1) or (J + 1/2) for J of in teger or J of half-integer, respecti vel y. The n u mb er of levels is determ ined by the symme try of the crystal field s urroun ding the rare-earth ion. The wid th of each level shown in Fig ur e 22 in d icat es the ra nge of sp littings wi thi n eac h compone n t. Ligh t-em itting levels are indicat ed by semicircles below th e energy levels. Mos t o f the em ittin g levels are separa ted from th e n ext lower level by at least 2 x 103 crrr' or m ore. This is beca use the exci ted sta tes rela x via two competitive paths: one is by light emiss ion an d the other by phon on emission. The ra te of phon on emission, w, depends on th e n umber of ph on ons emitted sim u ltan eous ly to bridge the energy gap and is expressed as :
w cc ex p ]-k/>;.E/ hv mox )'
(24)
where />;.E is th e energy ga p to th e neares t low er le vel an d hv mox is the ma xim u m energy of phonon s coupled to the em itting sta tes. The phonon emissio n rat e, to, d ecreases rapidly w ith an in crease in />;.E, so tha t the compe titi ve light emission or rad ia tive process becom es dominan t." Th us, the w ell- kn own h igh lu m in escence efficien cies for SO il of Eu 3+ an d 50 4 of Tb 3..- are based on th e la rge energy gap of more th an 10 4 crrr' that n eeds to be bridged to the next low er level of these io ns. The above form ula implies tha t large values of hv mox also quench ligh t emission . Th is is demons tra ted b y th e fac t that lum inescen ce of Eu 3+ in aq ue ous solution is almos t q uenche d, but begi ns to appear if H 20 is replace d by 0 20 .7 Luminescence origina ting from elec tro n ic tran sitions between 41 levels is pred om inantly d ue to electric dipole or magne tic d ip ole interactions. Elec tric d ip ole j-j' transitions in free 4/ions are parity-forbidden, b ut become partia lly allowed by mi xin g with orbi ta ls havin g d ifferen t parity beca use of an odd crystal field com ponent. The se lec tio n ru le in this case is IMI0 6, (excep t for O ~ O, 0~1, 0~3, 0~5) . Typ ical examples of th is mechanism are demonstrated by th e luminescen ce fro m th e 50 1 sta tes of Eu 3+; the intensity of these tran sition s depen d s strongly on the si te sy m metry in a ho st cr yst al (See Section 2.3.3.7 Eu 3+). Magne tic dipole /-/ tran sition s are not affected m uch by the si te sy mmetry beca use they are p arity-allow ed. Th e J se lec tion ru le in this case is /';.f = 0, ± 1 (exce p t for O~O) .
Fundamentals of Phosphors
184 _ J~~ o
40
10' em
1
-
u- 7' -
-
'D
1/2
- . - 1/ 2
u-
-
38
- 'p-' I ,',
"I ;C-
36
C :::::i,ZU N= M-
32
Qp -
-
0-
-
' p L- - " K
W ==
-
V-
J-
1-
' - f)
L
28
,'.1I / -..!
p '-
J /2
s- -
-
Qp- P =T= Q~ :iC
-
./ / -
R_
u. . .
0' -
K2 "T-
26
-
1/ -
[
- -, ' f'
22
_ '_1_
6
--...rpl
1-
--1'0N __
£-
-
f) -
-
M-
J.- . -
-
N M
:1
:J II·:. U ~ G : z
SG'lZ -":":':'1 ~. 1 .1 (;
L ' OI C _'_ II"7/). K- ,
.I D
-.....- l-
L - - r - I I / i!
J-
2 H~ .' 2
J ---.--"j
(;11 . 7.
K .......... J _ _ r.
M--
H- -
c--
~l l ~! - F-
(; ~ :-.
E--
;t.
UC
~~ O
' /'
20
_
' f) f) - - '
K- -
' f) ", Q~' p p- -
1
-
M-
L=
24
-
-
G-
N- -
s- -
-
IJ "
R- -
~ ~J), ii=
K- -
:,:i7. Q ««
-
M-
-
_ _ H-- '·' Q ~-
T
N-
30
-
( 1 --..---
H - -S' '/ A -...-i ' 'J
L_
'p
x- -
-
R_
34
T~ 9 ' Z~'~
- '
¥- -
[. , ~
L_
K --
0(;
t
:!
'I"
E - - . - 7 / :!:
D
B~~ "
~- '
\ (;
18
.\ .':t ,I _~ o
"F,
16 - ' S ,'
14
S ..... ;;,':!
12
1
{; -
~ H ~ :!. H ___.__:.!
-,
1 ."!
J~
,
" ,, - ;'
.
8
·,
:' F -
'I
)
-, -
-
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_
5
!
,·
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~
'F
~ .i . '2
' /I -,
,1_-0
7 :!
R ~·II
v_, s- --" ,
_n ~
- 'JII- ,
-
'I
Q--
"F
B-.-.·,
C-.:..:.:. H-B.-::.!!.' "
(J" F II ..
'C
10
C- -
/)
__ l
,1
R
:.
-
-
-- ,
" V I.Z
A ~1 ~; """':'" ,
' I"
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B 2~ · ~ r~
)' _ _ 1:1, 2
T~
u--
W- -
'. \, - -
11
x- - -
;c _
- -II '
71"
_ _ 0 )( -......... 11 -
-,
U "_
I I ' :Z-
X-
'1 :
-
,'- - ,
I
--,
~ _'_l_" 'h !"'
1'-
-
- 11
,
~' -,.
0
, F, ·z Ce
- -
L-
-
1/1,
4 1g: l
51.,
o/ /;. "
i FI)
Pr
Nd
Pm
Sm
£u
, F.
Cd
Tb
~
Dy
Is
Ho
- ' H6 Tm
Figure 22 Energy levels of trivalen t lanthan id e ion s. (From Dieke. G.H ., Spectra and Energy Levels of Rare Earth Ions in Crystals, Interscien ce, 1968; American institut e of Physics Handbook, 3rd edition, McGraw-Hili , 1972,7-25. With permission .)
Chapter two: Principal phosphor materials and their optical properties
185
Oscillator strengths are of the order of 10-5 to 10-s for partially allowed electric dipole transitions, and lO-s for magnetic dipole transitions. The electric dipole transition probability between 4flevels can be calculated using the Judd-Ofelt theory.s? This theory assumes closure of the wavefunctions mixed to a 4f orbital and takes an average for the energy separation between the allowed states and the 4f levels. In spite of such approximations, the theory gives satisfactory agreement with observed valu es in many cases. The points of the calculation are sketched out as foll ows. The absorp tion coeffi cient, k(A), is experimentally determined; here, k(A) = In(l/lo) / a, where 10 is intensity of incident light, 1 is intensity of transmitted light, and a is sam p le thickness. By using the value of k(A), the line strength S is given by the following formula.
(25)
where p is the density of the lanthanide ion and n is the refractive index. The parameters Q2' Q4' and Q6' giving th e light emission probability, are included in S and are determined by a least square fit of: 2
s= LQII «S,L)! IIU (I) II(S', L' )]' > 1
(26)
1 ~ 2 ,4 , 6
Here, « S,L)! II U'"II (S',L')]'> (t = 2, 4, 6) are reduced matrix elements characteristi c of individual ions and av ailable as a table." Using the parameters Q2' Q4' and Q6 for specific host m aterial, the light emission probability, A, between the levels of interest is calculated as follows:
(27)
The theory contains so me assumptions not strictly valid in actual cases, but still provides useful theoretical explanations for the nature of the luminescence spectra, as well as th e excited-state lifetime, of lanthanide ions.v'? Luminescen ce sp ectra of various trivalent lanthanide ions in YV0 4 (or YPo4 ) ar e shown in Figure 23.12 The luminescence and excita tio n spectra in Y Z03 are sh ow n in Figure 24. 13 Th e luminescence spectra are composed of groups of several sharp lines. Each group corresponds to a transition between an excited and ground state designated by the total ang ular momentum, J. Th e assignment of the transition corresponding to ea ch group of lines can be made on the ba sis of th e energy level diagram shown in Figure 22. The excitation spectra generally con sist of sharp line s due to the 41-4f transition and of broad bands due to the 4f-5 d trans ition and /or char ge -transfer proces ses. Excited st ates giving rise to these broad excitation bands will be di scussed in the n ext subsection. The lifetimes of the luminescence due to 4f ---7 4f transitions are m ostly in the range of m illiseconds bec ause of the forbidden character of the luminescence transition." For luminescence due to a spin-allowed transition between levels having equal spi n multiplicity (e.g., 3PO ---7 3H I of Pr 3+), a relativel y short lifetime of ~10 -5 s is observed .
Fundamentals of Phosphors
186
YVO, : 5m3 •
YVO, :Eu H
I
I
U
L
YPO ., : Tb J >... ...... CIl
= = = cl)
......
A
cl)
\. J )..
U
YVO, : Dy1+
cl)
CIl cl)
=
E ::l
)\
cl)
> .;:::;
J
C';l
YVO, : Ho J+
03 0:::
1.1 YVO, : Er J '
I
J
YVO., : Tm
410
450
500
'
I
!
!
i
550
600
650
700
Wa velength (nm) Figure 23
Em ission s pectra of various trivalent rare-earth ions ill YV04 or YP0 4 ho sts under ca thode-ray excita tion . (From Pa llila, F.e., Elecirochem. Technol., 6, 39, 1968. With permission.)
187
Chapter two: Principal phosphor materials and their optical properties
exci ta tion
Pr
~ missjo~
Sm
Figu re 24 Excitation and emission spec tra of vari ou s trivalent rare-earth ion s in Y203 w ith a conce n tration of 0.1 mol%, excep t for Er3+ (1 mol%). The exci tation spec tra are for the main em ission peak s. Spec tra l dependence of the intensities have not been correc ted . (From Ozawa, R, Bunsekikiki, 6, 108, 1968 (in Jap anese). With pe rmission.)
Q
ex cit ation
emissio n
(3)
....>.. (/)
....~t::
~ Cd
o t::
(/)
.5 ® E ::l
>
(\
Tb
1\ 1\ I
excita tion
emission
";:;
<1;l
350
~
exc ita tion
emis sion
Dy ~
§ -xcitation
emission N co.
« ~
Q ~
~
200
~
.,
,
~.
300
o
.
Ho
L
~~
,"
I
~ Q
~,;;
400
500
600
Tm
®
Er exci tation
emiss ion
r
~
I
~
r:. OJ;} ~.
,
600
700
, 200
Wavelength (nm)
300
400
500
188
Fundamentals of Phosphors
Figure 25 Energies for 4f ---> 5d and CTS transitions of trivalent rare-earth ion s. (From Hoshina. T., Luminescence of Rare Earth [ 011S, Sony Research Center Rep ., 1983 (in Japanese). With permi ssion .)
2.3.2.2 4£"-1 Sd J states and cha rge-transfer states (CI S) In the en ergy region spanned by 4f levels, one finds two additional kinds of electronic states with different characters from th ose level s. They are th e 4fn- 15d ' s ta tes and the chargetransfer sta tes (CTS). In the former, on e of the 4f electron(s) is transferr ed to a 5d orbital and , in the latter case, electrons in the neighboring anions are transferred to a 4f orbital. Both of these processes are allow ed and result in strong op tical absorptions. They are ob served as broadband excitation sp ectra arou nd 300 nm, as is shown in Figure 24. Op tical absorptions due to J-d tran siti on s ar e found for Pr 3+and Tb 3+; those due to a charge-transfer transiti on are found in Eu3+. The broad-band excitation sp ectr a around 230 nm for Sm 3+, D y 3+, and Gd 3+ are due to host absorptions. The energies of the 4f' - 15dl and CTSs are more dependent on their environmen ts than the en ergies of 4f sta tes, but the rel ative order of ene rgies of these states are found to be the same for the whole series of rare-earth ions in any host materials. The tran sition energies from th e grow1d sta tes to these states are shown in Figure 25.2.14111ese energies are ob tained by determining the va lues of parameters so as to agree with absorp tion spectra of trivalent rare-earth oxid es. As show n in the figu re, 4f-5d transitions in Ce 1+, Pr3+, Tb3+, and CTS absorption s in Eu 3+ and Yb3+h av e energies less than ca . 40 x 103 crrr'. They can, therefore, interact with 4f levels, leading to f ---'1 f emiss ions. In case the energy levels of these s tates are low er than tho se of 4flevels, d irect luminescenc e transitions from these lev els are found, such as 5d ---'1 4f tran sitions in Ce 1+, Pr 3+ , and Eu 2 •. Spectra of this luminescence vary as a result of crystal field splitting in host crystals (Section 2.3.3). Luminescence due to the transition from CTS has als o been reported for Yb3+ (See Section 2.3.3.18). By comparing chemical propertie s of tri valent rare-ea rth ions with Fig ure 25, on e can conclude that th ose ions that are easily oxidi zed to th e tetravalent state ha ve lower 4f ---'1 5d transit ion energies, while th ose that are easily reducible to th e divalent stat e have lower CTS transition en ergies. It ha s also been confirmed that 4jO, 4[7, and 4f4 electronic confi gurations are relativel y stable.
Chapter two: Principal phosphor materials and their optical properties
189
2.3.2.3 Divalent and tetravalent cations In appropriate host crystals with d ivalent cons tituen t ions su ch as Ca 2+, Sr 2+, or Ba2+, Sm 2+, Eu 2+, and Yb2+ are stable and can luminesce. The electro nic configurations of these ions are the same as those of Eu 3 +, Cd 3+ , an d Lu 3 +, respectively. The excited states of the d ivalent ions, however, are lowered co m pa red with those of the corresponding tri valent ions, because the divalent ions h ave smaller nuclear ch a rges. The lower 4f-5d transition energy reflects their chemical property of being easily ionized into the trivalent state. All tr ivalent ions , from La3+ to Yb 3., ca n be reduced to the d ivalent s ta te b y y-ra y irradiation when d oped in CaF 2 .15 The ele ctroni c con figurations o f the tetravalent cations, Ce 4+, Pr4+, and Tb4+ a re the same as those of tri valent ions La 3+, Ce 3 +, a nd Cd3+, respectively. Th eir CTS en ergy is low, in accor da nc e with the fact that they are easily reduced. When Ce , Pr, or Tb io ns a re doped in compound oxid e crystals of Zr, Ce , Hf, or Th , the resulting powders show a variety of body col o rs, probably due to th e CTS ab sorption band." Luminescence from th ese CTS s has not been reported.
2.3.2.4 Energy transfer The excitation residing in an ion ca n migrate to ano th er ion of th e sa me s pecies th at is in the ground state as a result of resonant en ergy tr ansfer when th ey ar e located close to each other. Th e io nic se pa ra tion where th e luminescence an d ener gy tran sfer probabilities become comparable is in the vicinity of several An gs tro m s. En ergy mi gration processe s in crease the probability th at the o p tical excitation is trapped at defects or im p ur ity sites, en hanc ing nonradiative rela xation. Thi s causes co n cen tra tion quenching, because an increase in the activator concentrat ion encoura ges su ch nonradiat iv e processes. As a result, th at excitation energy diffuses from ion to ion before it is trapped and leads to e mission . On the o ther hand, a decrea se in the activator concentrati on d ecreases th e energy stored by the ions. Consequently, there is an optimum in the activ a tor conce n tra tion, typically 1 to 5 m ol% for trivalent rare-ea rth ions, resulting from the trade-off of the abo ve tw o factors. In some compounds suc h as Nd P 50 w th e lattice si tes occupied by Nd are se para te d from each other by a relatively la rge di stance (5.6 A), an d a hi gh luminescence efficiency is achiev ed even when all the sites are occu pied by activator ions (Nd) . Such phosphors are called stoichiometric phosphors. The energy transfer between different ion species can take place when th ey ha ve closely matched energy levels. The energy transfer results either in the enhancement (e.g., Ce 3 + ~ Tb 3+) or in the qu enching (e.g., Eu 3 • ~ Nd 3 +) of e mission . The effects of impuriti es on the luminescence intensities of lanthanide ions in Y203 are shown in Figure 26. Energy transfer between 4flevels ha s been shown to orig ina te from the electri c d ipole-electric quadrupole interaction using glass samples.' ? The lumines cence sp ectra of Eu 3+, as well as that of Tb3+, have strong dependence on the concentration. Th is is because at h igher concentrations, th e higher em itting levels, 501 of Eu 3 + and 50 3 o f Tb ' , transfer th eir en erg ies to neighboring ion s of th e same sp ecies by the foll owing cross-rela xa tions; that is:
5 0 3(Tb
3+
) + 7 F6 (Tb 3+) ~ 50 4 (Tb 3+) + 7 Fo(1b 3+)
The en er gy transfer from a h ost crystal to activators leads to host-excit ed luminescence. The type of charge carriers to be captured b y the doped ions, either electrons or h oles, determines th e nature of the va len ce changes in the ions . Fo r a YzOzS h ost , Tb3+ an d Pr3 +
Fundamentals of Phosphors
190
>.. o:
1.0
Q)
0 .1
Tm
11 0
Pr
Co Cd
1.0
~
: Sm , Eu
C
~
Dy
0.1
C Er
Sm
Q)
o
1.0
1.0
0.1
0.1
C
Q) r/l
C
E
:::l
Tm
Eu
1.0
1.0
0.1
0.1
Q)
> ~
C';l
1)
a:::: 10 '
10 '
10 '
10 '
10 '
10 - ,
10 '
Mole fractions of rare ea rth ions in Y203
.-......
>-.
i f)
C
Tb (5 x ] 0 - 6 mole)
1.0
(Nd, Sm, Ho, T rn)
Er , Cd - - - ~--~---
p,
Q)
......
Dy
C
Eu , C e
Q) <.)
C
Q)
if)
Q)
.-E C
0.1 1.0
Dy (9 x 10- 5 mole)
:::l
Q)
.-......>
0.]
lD - 7
10
6
] 0- 5
10. ,
10- J
('j
q)
Mo le fr actions of rare earth ion s in Y20 3
cG
Fig ure 26 Decrease of lumi nescence in tens ities of trivalen t rare-earth ions in Y Z03 due to the ad d ition of other rare-earth ion s. The concen tration of ions is 10-3 mol% excep t for Tb and Dy (see the figure). (From Ozawa, R., Bunseki-kiki, 6, 108, 1968 (in Japanese). With perrni ssion.)
will act as hol e traps, while E1l3+ wi ll act as an electron trap at th e initia l stage of host exci ta tion. In the next stage, these ions w ill cap ture an opposite charge and produce exci tation of 4f levels.u,-211 A similar model has also been applied to Y3AlsOI2:Ce3+,Eu3+,Th3+.21 Energy tran sfer fro m an excited oxy-anion complex to lan than ide ions is resp ons ible for th e luminescence observed in CaW04:Sm3+ ,22 YV0 4: Eu 3 ' ,23,24 an d YzW06:EUJ+.Z5
2.3.3 Luminescence of specific ions 2.3.3.1
ee '
+
Amon g the lanthanid e ions, the 4f ----; Sd transition energy is the lowest in Ce 3+, but the energy gap from th e Sd l s ta tes to th e nearest level (2F 7 / z) below is so large th a t the Sd level serves as an efficien t light-emi ttin g s ta te. The luminescence p hoton energy de pe nds strongly on th e structure of th e hos t crys tal throu gh th e crystal-field splitting of the Sd sta te, as shown in Figure 272 6 (see also Reference 27 and the disc ussion in Section 2.3.3.8
Chapter two: Principal phosphor materials and their optical properties Ce-' : S J
191
exc ita tion energy i · IO" om- l ; w
t-o
c> '"
o
o
I:~C~?{
Ca F,
:
S rF,
:: : : ~()
_- ~ .-
~~
HaF
~ I: ("(\Y1
([ .aA l e e! P .
voc :
I
LaOe( Ca r RO , LaO Rr
I
Oxyhalides
II I I
I
ScBO,
: ~~
r HO ,
:III
LaBO , C. UO,
Fluorides
Ii
r
I
Borates
I I I
: I
II('\; Y, AI, O..
C." MSi, O,
Y, S iO, C. , S iJl; )TO,
LaPO, (el'OI
CdPO ,
~
Aluminates
: III i I I I II
, ;~ ~:" ,/I
Silicates
: . I J II I III I ::1 I III
: I I ( I I: I I
I
"
I
l Phos Phates I
Figure 27 Ene rgies o f Sd excited levels of CeJ + in various hos t crystals. (From Na rita, K. and Taya, A., Tech. Digest, Phosphor Res. Soc. 147th Meeting, 1979 (in Jap an ese). With permission .)
on Eu 2+ described below) and varies from near-ultraviolet to the green region . Typ ical luminescence spectra of some Ce ' t-activated phosphors are shown in Figure 28.28 The two emiss ion peaks are due to the two terminating levels, 2Fs/2 and 2F7I2, of the 4/confi guration of Ce 3.... The d ecay tim e of the Ce 3+emission is 10- 7 to 10-8 s, the shortest in obs erve d lanthanide ions. Th is is due to tw o reasons: the d --7 / tran sition is both parity-allowed and sp inallow ed since 5d1 and 4j1 states are spin doubl ets." By virtue of the short d ecay time, Y2SiO s:Ce3+ and YAl0 3 :Ce3+are us ed for flying spo t scanners or bea m-in d ex typ e cathoderay tubes. Also, Ce 3+ is often used for the sens itizatio n of Tb 3+ luminescence in su ch hosts as CeMgAI I1019 .3o
2.3.3.2 Pr3+ Lum inescence of Pr 3+ cons ists of man y multiplets, as follows: - 515 nm e pa --7 3HJ , - 670 nm CPo --7 3F2) , - 770 nm e po --7 3F4 ) , - 630 n m (lD z --7 3H6 ) , - 410 nm (lSo --7 116 ) , and ultraviolet (5d --7 4f) tran sition s. Th e relativ e in tensities of the p ea ks depend on the host crys tals. As an exa mple, the emissio n sp ectru m of YzOzS:Pr3 + is sho w n in Figure 29. The ra d iative de cay tim e of the 3PO --7 3HJ or 3FJ em issio n is - 10--5 s, w h ich is the sho rtes t lifetime observe d in 4/ --74/ transitions. For example, in YzOzS ho st, d ecay times until 1/10 init ial in tensity are 6.7 us for Pr 3+, 2.7 ms for Th3+, and 0.86 m s for Eu 1 +. 2 The short d ecay time of Pr 3+ is ascribe d to the spin-allowed cha rac ter of the tran sition. Since th e sh or t decay time is fit for fast in forma tion p rocessing, Gd z0 2S(F):Pr3 +,Ce 3+ceramic has been developed for an xray d etecto r in X-ra y comp u ted tomography."
Fundamentals of Phosphors
192
f\' f\
0
I
J\ '1[\
E
c
F
/\ f\
300
400 300
HM
Figure 28 Emission spectra and excitation wavelengths of Ce 3+ in var io us hos ts. (A) YPO" 254-nm exci tation; (B) YPO" 324-nm exci ta tion; (C) GdPO" 280-nm excitati on ; (D) LaPO" 254-nm excitati on; (E)LaPO v 280-run exci ta tion ; (F) YBOJ , 254-nm exci tatio n. (From Bu tler, K.H ., Fluorescent Lamp Phosphors, Technologyand Theory, The Pennsylvan ia State University Press , 1980, 261. With permission.)
>-. .......
Y20 2S : 0.3%Pr RT
(/)
c::
-e-
..,.
....... I:;
k,
::r::
P...
P...
0
P...
0
0
I:;
i
i
i
I:;
M
M
M
u
k.
N
M
M
M
M
6
k.
:::l
::r::
uo
::r::
>
i
~
P...
M
.J ~M 500
M
M
P...
C':
0
P...
0
0
i
i
M
......,
M
M
600
~~
J 700
..
1
800
Wavelength (nm) Figure 29 Emission spec trum of YzOzS:Pr3 + (0.3%) at room tem p erat ur e. (From Hoshina, T., Luminescence of Rare Earth Ions, Sony Resear ch Center Rep., 1983 (in Japan ese). With permission .)
The quantum efficie ncy of m ore th an 1 was reported for P r 3+ lu minescence when excited by 18S-nm ligh t.J2,."I."I The excita tion- relaxa tion p ro cess takes the following p aths: 3H4 ~ 150 (excita tion by 185 nrn ), 150 ~ 1/ 6 (405-nm em issio n), 1[6 ~ JPn (phonon em ission), 3PO ~ 3H4 (484.3-nm emission), JPo ~ JH s (531.9 nm emission), JPo -t ' H6, 3F2 (610.3-nm emiss ion), and 3Po ~ 3F3, 3F4 (704 nm emission). Th e su m of the v isib le ligh t emissions in the above processes was es tima ted to have a quantum efficiency of 1.4,33 In some fluoride crystals, th e 4fSd J sta te was fou nd to be lower than 'So' res ulting in broad- band UV luminescen ce (see Figure 30).
Chapter two: Principal phosphor materials and their optical properties
250
200
193
300
Wavelength (nm) Figure 30 Emission spec tru m of LiYF4 :Pr3+ (1%) usin g 185-nm excita tion. (Fro m Piper, W.w. , Delu ca, I .A., and Ham, F.S., f. Luminesc., 8, 344, 1974. With perrni ssion .)
2.3.3.3 JVd 3 + The four lower-lying levels of N d 3+ provide a condition favorab le to the formation of population inversion . For this reason, Nd 3+ is used as the ac tive ion in m an y high-power, solid -state lasers (at 1.06 urn w av elength); the most common h osts ar e Y3 Als0 12 sing le cry stals (yttrium aluminum ga rnet, YAG) or glass. Th e relati ve emission intensity of Nd 3+ in Y3Als0 12 has been found to be as follows": 4F/ 32
~ 419/ 2
: 0.25
-I
F3/ 2 ~
-I [ 11/ 2
(1.05 - 1.12 um)
: 0.60
4
F3/ 2 ~
4 [13/ 2
(-1.34 urn)
: 0.15
-I F53/ 2 ~ 4[ 9J2
2.3.3.4
(0 .87-0.95 urn]
and othe rs (L = 230
us] :- 0.010
JVd4+
Luminescence in the regi ons -415, 515 , 550, an d -705 nm has been repor te d in CS3NdF7:Nd4+.3s
2.3.3.5 5m3+ Red lum in escen ce at - 610 nm (4G S / 2 ~ 6H7(2) and -650 nm (-IGS/2 ~ 6H9 / 2 ) is ob served in Sm .3+ Hi gh lumin escen ce efficiency in Sm 3+, howev er, h as n ot been reported. Sm 3+ acts as an aux ilia ry activa tor in phot ostimulable SrS:Eu 2+ (Mn 2+ or Ce 3+) phosphors. Und er exc itati on , Sm 3 + ca p tures an electro n, ch angin g to Sm 2+, wh ich in turn produces an excitatio n band peaking a t 1.0 ~m . 36,37 (See 2.6.)
2.3.3.6 5m2+ The 4f5d1 level of Sm 2+ is located below its 4fl evels in CaF 2, resulting in b and lum ine scen ce due to the 5d ~ 4f tran sition (728.6 nm, L - us). In SrF 2 and BaF 2, on the othe r h and, a lin e spectrum due to the 4f ~ 4f Do ~ 7F] tran siti on has been ob ser ved (696 nm, L - ~S) . 38
Fundamentals of Phosphors
194
Y,O , S <>
RT
01 .", Eu
1
'"
, N
0:;
,
~
:" 1
i
1
<>
c>
~
I
<>
J "'" Eu
4 '!:, Eu
500
600
700
Wavelength (o m) Eu J + concentration dependence of the emi ssion spec tru m of YzOZS :Eu3+. (From Hoshina , T., Luminescence of Rare Earth Ions, Sony Research Center Rep.. 1983 (in Japan ese). With per miss ion.)
Figure 31
Also, in BaFCl , line emission at 550 to 850 nm due ag ain to 50 0.1 been repor ted."
--7
7Fo_4 tran sitions has
2.3.3.7 Eu 3 + A number of luminescence lines due to sOJ --7 7Fr of Eu 3+ in Y202S are sho wn in Figure 31. As can be seen , the emissions from 50 2 and 501 ar e quenched, w ith an increase in the Eu 3+ concen tration due to a cro ss- relaxa tio n pro cess, (50[ --7 5 0) --7 (lFo --7 7Fr ), as discu ssed in Section 2.3 .2.4 . The emission in th e vici nity of 600 nm is due to the magneti c dipole transition 50 0 --7 7F 1, which is insensitive to the site symmetry. The emission around 610-630 nrn is due to the electric d ipole transit ion of 50 0 --7 7F2 , ind uced by th e lack of inversi on sy mm etry a t the Eu .1 +si te, and is much s tronger than th at of th e transition to th e 7F 1 state. Luminescen t Eu 3+ions in commercial re d phosphors such as YV0 4, Y203 and Y202S, occupy th e sites th at have no in version synmetry. The strong emission due to the ele ctri c dipole transition is utilized for p rac tical applications, If the Eu 3+ site ha s in version symmetry, as in Ba2GdNbOs, NaLu0 2, 4o an d InB03, 41 the electric d ip ole emission is weak , and the magnetic dipole transiti on becomes relativel y s tro nger and d ominate s, as is sh own in Figure 32. Th e spec tral luminous efficacy as sen sed by th e ey e has its m aximum at 555 nm. In the red region, this sensitivity drops ra p id ly as on e m oves toward longer w avelen gths. Therefore, red luminescence composed of narrow spectra appear brighter to the human ey e than va rious broad red luminescen ces having the sa m e red chroma ticity and emission e ne rgy. For th e red emission of color TV to be used in the NTSC syst em, the red ch romaticity stand ard has been fixed at the coordinat es x = 0.67, Y = 0.33; in 1955, the ideal emis sion spectra w ere proposed as a narrow band aro u n d 610 nm, before the develop me n t of Eu 3+
°
Chapter two:
PrincipaL phosphor materiaLs and their opticaL properties
>-.. ......
5
Do - . 7 F
195
I
(fJ
c
(\) ......
Saz (C d, E u)
c
N a (L u ,
s,» o,
N bO s
(\) C,)
c
(\)
o: (\)
c E :::l
(\)
> ro
'';::; (\)
~
580
60
620 A [nm ]
580
Wavelength
600
620
A [nrn]
Figure 32 Emission sp ectr a of Eu" from the sites h aving the inversio n sy nmetry. (From Blass e, G. and BriJ , A., Philips Tech. Rev., 31, 304, 1970. With perm ission. )
phosphor s.v Thi s proposal was dramatically ful filled for th e firs t time in 1964 b y newly develop ed YV0 4: Eu 3+ .43 Since then, Eu 3+ phosphors have co mple tely replaced b road-band em itting Mn2+phosph ors or (Zn,Cd)S;Ag, which were p redominantly in use a t th at time. Just aft er th e introduction of YV04:Eu 3+, an o th er Eu r'-activated p hosp hor, Y202S;Eu 3+, was de velopedv' an d is in current use due to its better energy efficiency as well as its stability during recyclin g in th e screen ing process of CRT prod uction . The possibili ty of further improveme nt can occu r in m ater ials wi th single-line emission, as in Y2(W04)3:Eu 3+.45 Use of narrow-b and luminescence is also advan tageous in three-band flu orescent lamp applications, where both brig h tness and color reproducibility are req uired . For h igh color ren derin g lam ps, YzO}:Eu 3+has been used as the red-emitting co mpon en t. Th e seque nce of excita tion, relaxation , and em ission processes in YzOzS;Eu 3+is exp lained by the config ur atio na l coordi na te m odel shown in Fig ur e 33 .46 Th e excita tion of Eu 3+ tak es place from the bottom of the 7Fo curve, rising along the stra igh t ve r tica l lin e, until it crosses the charge-transfer sta te (CTS). Relaxation occurs alon g th e CTS curve. Near th e bottom of the CTS curve, the exci ta tion is tran sferred to 50, states. Rela xati on to th e bottom of th e sO, states is followed by light emission down ward to 7F, states. Thi s model can exp lain th e following exp eri me n tal findings. (1) No luminescenc e is fou nd fro m 50 3 in YzOzS:Eu 3+ . (2) The lu minescen ce efficiency is higher for p hosp hors wi th higher CTS ene rgy." (3) Th e quenching temperat ur e of th e luminescence from 50, is higher as J (0,1,2,3) d ecreases. The excited 4f states m ay d issocia te in to an electron-hole p air. This model is supp or ted by the obse rva tion that the excita tion through th e 7Fo ~ 50 Z transiti on of La20 2S:Eu 3+ ca uses en ergy storage th at can be conver ted to luminescen ce by h ea ting. Th e luminescence is th e result of the recomb ina tion of a th erm ally released hol e with an Eu ?" ion.-l8,49 By taking a mod el where CTS is a combin at ion of 4[7 elec tro ns plus a h ole, one finds that th e res ulting sp in m ultip licities sh ou ld b e 7 an d 9. It is the former s tate that affec ts op tical pro perties re lated to th e 7F, s ta te by sp in -restricted covalency." The in ten sity ratio of the lum ine scen ce from 50 0 ~ 7F2 and from SOD ~ 7F1 decreases wi th in creasing CTS energy sequentia lly as ScV0 4, YV0 4, ScP0 4, and YP0 4, a ll of w h ich h ave th e sa me type of zircon stru ctu re .f The above intensity ra tio is small in YF3:Eu3+, ev en th ough Eu 3+ occupies a site without inversion sy m m e try" It is to be n oted that CTSs in flu or ides h ave
Fundamentals of Phosphors
196
45 ,.----.-----r----------,.....-r.r-rJ'l
40
35
'::'
30
I
E u
0
25
~
5Dl 5D,-
>-. 01)
lIj)
20 -
C
.i]
15
'F; 7
r, "'-..
7F," .. 10 - 7FJ'
-r.
7FI -
5
7Fo /
0'--- - - - :::0;::;= =--- - - - - - ----'
Configurational coordinate Figure 33 Configurational coordina te model of Y202S:Eu3+, (From Stru ck, C.W. an d Fong er, W.H., f. Luminesc., 1/2, 456, 1970. With perrnission.) h igher energies th an th ose in oxides. Th ese results s ugges t th at hi gh er CTS energies reduce th e strength of th e elec tric dipole tran sition 'Do -7 7F2 in Eu 3+.
2.3.3.8
Eu2+
The elec tro ni c configu ration of Eu 2+ is 4f and is id entical to that of Cd 3+. The lowest excited state of 4f levels is locat ed at abo u t 28 x 103 crrr' and is higher than th e 4f65d1 level in m ost crystals, so th at Eu 2+ usually gives broad- ban d emi ssion due to f -d transitions The w avelength posi tio ns of the emission band s depen d very much on hosts, changin g from the nearUV to th e red. Thi s depen d en ce is interpre ted as due to the crystal field sp litting of the 5d level, as sh own schem ati cally in figure 34.53 With inc reas ing crystal field stre ng th, the emission bands shift to longer waveleng th. The lu minescenc e peak ene rgy of the 5d-4f transit ion s of Eu 2+ and Ce3+ are affec ted most by crystal parameters de n oting electro nelect ro n rep u lsion; on this basis, a good fit of the energies can be obtained." The near-U 'V lu min escence of Eu 2+ in (Sr,MghP20 7 is used for lamps in copying machines us ing photosensitiv e diazo d yes. Th e blue lu mi nescence in BaMgAllO0 17 is used for th ree- band fluores cent lamps. (See Fig ure 35 .)54 Ba(f,Br): Eu 2+ showing vio le t lu m inescence is used for X-ray d et ect ion th rough phot ost tmul ation.v Red lu minescen ce is observed in Eu- t -activat ed CaSJ6; th e crystal field is stronger in sulfides th an in flu or id es an d oxi des . The lifetime of th e Eu 2+ luminescen ce is 10-5-10-6 s, w h ich is relatively long for an allowed tra nsiti on . This can be explained as follows. The groun d sta te of 4f is 85, an d the multiplicity o f the exci ted s ta te 4j65d1 is 6 or 8; th e se xte t portion of th e excited sta te contributes to the sp in -forbidden character of th e transition." Sha rp-line lumin escen ce a t ~360 n m du e to an f -f transition and havin g a lifetime of milliseconds is ob served when the crystal field is w eak so that the low est excited state of
Chapter two: Principal phosphor materials and their optical properties
hf7-
-
-
-
--r-
-
-
....:::,.,c---
-
-
-
-
-
6p
197
J
u.v.
blu e
- --
ye l l
IJ
--4) 6
Figure 34 Schema tic diagram of th e energies of 4f and 'if5d l levels in Eu 2+ infl uenced by crysta l field ~. (From Blasse . G., Materi al science o f the luminescen ce of inorganic solids , in Luminescence of Inorganic Solids, Diba rtolo, B., Plenum Press, 1978,457. Wi th permission.)
i
,I
.I
I I ttf r.· 'Ii
~:
..
400
500
600
Wavel ength ----- A.
700 (nrn)
Emission spec tra of Eu 2+ in BaMgAl lO0 17 and rela ted com pou nd s usin g 254-nm exci ta tion a t 300K . --- -: Bao.9sEu o o5MgA l lo017 ' _ ._ . -: Ba O.S2, E u o osMgo sAI IOSOI 7.12S' - - -: Ban.75Euo.Il,Mgo.2Allo.sO,7.2' - - : Bao7oEuoll5 AIIl0l7.25' (From Smets. B.M,J. and Verlijsdon k. J.G., Mater. Res. Bull., 21, 1305, 1986. With permission.)
Figure 35
4f(6PJ) is lower than the 4j65d1 sta te, as illus tra ted in Fig ure 34. Th e host crys tals rep orted to produce UV lumin escen ce a re BaAlFs' SrAIF 5 56 (see Fig u re 36), BaM g (S0 4)2P SrBe2Si20 7, 58 and Sr(F,Cl).s9
2.3.3 .9 Cd J + The low es t excited 4f level of C d 3+ (6P 7 / 2 ) gives rise to sharp-line lumin escen ce at -315 nm 60 an d can sensitize the lu minescen ce of other rare-earth ions>' Th e ene rgy levels of the CTS and the 4f65d1 sta tes are th e high est amo ng rare-earth ion s, so tha t C d 3+ causes no quenching in other rare-earth ions . As a consequence, C d 3 + serves, as Y" does, as a good cons tituen t ca tion in host crys tals to be su bs titu ted by luminescent rare-ea r th ion s. For X-ray phosphors, Cd 3+ is be tter sui ted as a constituen t than Y 3+ si nce it has a hi gher absorp tion cross-section due to its larger atomic number.
Fundamentals of Phosphors
198
7
.-c
6
5
SrAIF 5
4
298K
>.
'"-'
sr:
u
C
r:/)
.-S
:
Eu 2 + 0.02
3-
;::l
.-
- 2 ' "-' ro ........
~
1
a
360
370
380
Wavelength (nm) Fig ure 36
Emission spectrum at 298K of SrAlF s:Eu2 + using 254-nm excitation . (From Hews, R.A. and H offman, M.V., f. Luminesc., 3, 261, 1970. With permission.)
2.3.3.10
Tb3 +
Lum inescen ce spectra con sis ting of many lines d ue to 50} -? 7Fr are observe d for 1b3+. As an exa mple, the spec tra of YZOZS :1b 3+ are shown in Figure 37, in w hic h the 1b3+ concentra tion va ries over a wide ra nge. Th e intensit y of the emission s from 50 3 decreases wi th increasing Tb 3 + concentration due to cross-relaxa tion, as dis cussed in Section 2.3.2.4. Am on g the emi ssion lines fro m th e S0 4 s tate, th e 50 4 -? 7F5 emission lin e at ap p roxim ately 550 nm is the stro ngest in n early all host crys tals when the 1b3+ con centra tion is a few mol% or h igh er. Th e reason is tha t this tran sition has the largest p rob ability for bo th electric-dip ole and magnetic-d ip ole induced tra ns itions .' The 1b3+ emission has a broad excitation ban d in the wavelen gth region 220 to 300 nm orig inatin g from the 4j8 -? 4f 5d1 tran siti on. The chr om a ticity due to the 1b3+emission ha s bee n es tima ted by ca lcula tion o f the various transition probab ilit ies.v Th e spec tra l reg ion aroun d 550 nrn is nearl y a t the p eak in the spectra l luminous efficacy; in thi s reg ion, the refore, the b righ tness dep ends only slig htl y on th e waveleng th an d the spectral wid th . Th us, the narrow spec tra l w id th of the 1b3+ emission is not so adva n tageous in ca tho de-ray tube applica tions as comp ared wi th th e case of red Eu 3 + emission previo us ly d escribed . The in tensity ra tio of th e emi ssion from 50 3 to that from 50 4 d ep en d s not only on the 1b3 + conce n tra tion, but also on th e h ost material. In bora te h osts such as ScB03, InB0 3, an d Lu BO", the relative in tensi ty of 50 3 em ission is much w eaker th an in other hosts, such as p hosp ha tes, silicates, and alumina tes.? Figure 38 sh ows emission spectra of a seri es of Ln zOzS:1b 3 +(0.1%) (Ln = La, C d, Y, and Lu) materials ha ving the same crys tal struc tu re.? It is see n that the relative in tensi ty of th e 5D3 emission increa ses drama tically as one pro gresses from La -? C d -? Y -? Lu, w ith the ion ic radii becoming sm aller. In addition to the 1b3+ concentration, one needs to consider tw o ad di tiona l fact ors that help de ter mine the ratio of 50 3 to 5D4 in tens ity. One is the maximum energy of phonons that causes phonon-ind uced relaxation, as disc usse d in Section 2.3.2.1; if the max imum phonon
Chapter two:
Principal phosphor materials and their optical properties
RT
199
Y,O, S 0.1 " " T b -e-
....i
"" 1
<1)
o h
<1)
(/)
<1)
h
.§ ::l
>
4.2 "" Tb
400
500
600
700
Wavelength (nm ) Figure 37 Th3- conce n tra tion depend en ce of emission spe ctra of Y202S:Th3- at room temperature, (From Hoshina, T., Luminescence of Rare Earth Ions, Son y Resear ch Center Rep" 1983 (in Japanese), With permission .)
energy is large, the ratio of 50 3 to 50 4 intensity becomes smaIL The luminescence of Tb3+in borate hosts is explained by this factor, The other factor is the energy position of the 4f 5dJ , level relative to 4f levels, which can be discussed in terms of the configu rat ional coordinat e modeL In this mod el, the potential curve of 4f5d 1 can be drawn just Eke the CTS in Figure 33, If the minimum of the 4f 5d1 curve is fairly low in ene rgy and the Frank-Condon shift is fairly large, there is a po ssib ility that an electron excited to the 4f5d 1 level can relax directly to the 50 4, byp assing the 50 3 an d thus producin g only 50 4 luminescenc e.' The net effect of these two facto rs on the spectra of Ln202S:Tb3+ in Figure 38 is not kn own quantitatively, YV0 4is a good host materi al for various Ln 3+ions, as shown in Figure 23, Ho wev er, Tb3+ do es not luminesce in thi s host. A nonradiative transition via a charge-transfer st ate of Tb4+-02--V4+ has been proposed as a cause.P The transition energy to this proposed state is con sidered to be rela tive ly low because both th e ene rgies of con ver sion from Tb3+ to Tb4+ and that from V5+ to V4+are low, The Frank-Condon sh ift in the tran sition would be so large that th e proposed state would p ro vide a nonradiati ve relaxation path from excited Tb3+ to the ground state, Tb-t-activ ated green phosphors are used in p ractic e in three-band fluorescent lamps, projection TV tub es, and X-ray in tens ifying screens,
2.3.3.11
Dy3+ 61,65
The luminescen ce lines of Oy3+ are in the 470 to SOO-nm region du e to th e 4F9 / 2 ~ 6H15 / 2 transition, and in the 570 to 600-nm reg ion due to the 6F15 /2 -7 6F ll /2 tran sition. Th e color
200
Fundamentals of Phosphors
0.1 % T b
RT
400
70 0
Figure 38 Emission spectra of Lnp 2S:Th3+ (0.1%) (Ln = La, Cd, Y, and Lu) at room temperature. (From Hoshina, T., Luminescence of Rnre Earth Ions, Sony Resear ch Center Rep.. 1983 (in Japanese). With perrni ssion .)
of the luminescence is close to white. In Y(P,V)04' the rel ative intensity of the latter d ecreases with increasing P concentration. Thi s can be understood if one considers that the 6.J = 2 transition probability decreases with a decrease in the p olarity of the neighboring ions as in the case of the 50 0 ~ 7F2 transition of Eu 3+ . The energy of the CTS and 4f5d 1 is relatively large so that direct UV excitation of D y3+ is not effective. The excitation via host complex ions by en ergy transfer can however be effective. The quantum efficiency of UV-excited (250-270 run) luminescence of YV04:Dy3+ has been reported to be as high as 65%.
2.3.3.12
Dy2+ 6(,
Luminescence of Dy2+ has been reported to consist of line spec tra at 2.3-2.7 urn a t 77K and 4.2K in CaF 2, SrF 2, and BaF2 . D y 2+ in these hosts w as prepared by the reduction of D y 3+ th rough y-ray irradiation.
2.3.3.13
Dy4+ 67
Luminescence lin es of Cs 3DyF 7 :Dy4+ a t 525 nm due to 50. j due to 50 4 --? 7F3 transition have be en reported.
2.3.3.14
--?
7F5 transition and a t 630 nm
H 0 3+
Efficient luminescence of H 0 3+ h as rar ely been found due to the cro wded energy level diagram of thi s ion. In LaCI 3, cross-re lax a tion between (552 --? 514) H (518 --? 517) at an
Chapter two: Principal phosphor materials and their optical properties
201
int erioninc dista nce of 7.5 A has been reported. " A g reen luminescence due to the 5F4 , 552 ---j 5[8 transition has been reported in an infrared -to-visible up-conversion phosphor, LiYF4:Yb3+,H o1+.UQ
2.3.3.15
H 0 2+
Infra red luminescen ce of H 0 2+in C aF 2 ap p ea ring aroun d 1.8 urn a t 77K h as been rep or ted.T
2.3.3.16
Er3 +
Green lum inescen ce due to th e 45 3/ 2 ---j 4[15/2 transition of Er 3+h as been reported in infraredto-visibl e up-conver sion phosphors, su ch as LaF 3:Yb3+,Er3+,7! an d NaYF{:Yb3+,Er 3+.72 This luminescen ce was a lso reported in ZnS ,73 Y20 3,7{ and Y202S .75 Th e emiss ion color is a w ell sa turated gre en. Er3+ ions embedded in an optical fiber (sev eral hundreds ppm) function as an optical am p lifier for 1.55 -~m semiconductor laser light. Popul at ion in ver sion is realized between low er sublev els of 4[13/2 and upper sublev els of 4115/2' This technology h as been dev eloped for op tical am p lifica tion in the lon g-d istance op tical fibe r communication systems."
2.3.3.1 7
Tm 3 +
The blue lu minescence of Tm 3+due to the IG4 ---j 3H6 transition h as be en reported in ZnS / 7 as well as in in frared-to-vi sible up-conversion phosphors sensitized by Yb3+ such as YF3:Tm 3+, Yb3+.78 Electrolumin escent ZnS:TmF3 has also been in vest igated as the blue com ponent of multicolor di splays."? Th e efficiency of the blue lumin escence of Tm3+ is low, and is limited by th e com pe titive infrar ed luminescence, which has a hi gh efficien cy.
2.3.3.1 8
Yb3 +
The in frare d absorp tio n band of Yb3+ a t abo u t 1 urn du e to th e 5Fs/2 ---j sF7J2 tr ansition is util ized fo r Er3+-doped in frar ed-to-vis ible up- conversion phosphors as a sensitizer.Fl-" The CTS ene rg y of Yb3+ ions is low next to th e low est of Eu 1+ a mong the tr ivalent lanthanide ion s (see Figure 25). Yb3+has no 4f ene rg y levels inter acting wi th CTS, so that lum inescence due to th e direct transition from CTS to the 4f levels can occur. This luminescence has been observed in phosphates? an d ox ysulfide ho st s." Fig ure 39 shows the excita tion and emi ssion spe ctr a of Y20 2S :Yb3+ and La 202S:Yb3+.81 As se en in Figure 33, CTS is ch ar acterize d by a fa irly la rge Frank-Condon sh ift. As a re sult, the emission sp ectra are com p osed of two fairly bro ad bands terminati ng in 2Fs/2 and 2F7 / 2, as shown in Figure 39.
2.3.3.19
Yb2+
The em ission an d ab sorption of Yb2+ d u e to th e 4f 4 H 4f 35d l transiti on ha ve been report ed. F Em ission p eak s ar e a t 432 nrn in Sr 3(P0 4 h (see Fig ure 40), 505 run in Ca 2P0 4 CI, 560 nm in Srs(P04)3CI, an d 624 nm in BaS(P04)3CI. Th e lifetimes of th e em issio ns are be tw een 1-6 x 10-5 s.
References 1. Blasse, c., Handbook on the Physics and Chemistry of Rare Earths, ed . by Csc hn eid ne r, Jr., K.A. and Eyring, L., Vol. 4, North-Hollan d Pub. 1979, 237. 2. Hoshina, T, Luminescence of Rare Earth Ions, Sony Research Center Rep. (Suppl.) 1983 (in Japanese). 3. Ad achi, C ., Rare Earths-Their Properties and Applications, ed . by Kan o, T and Yan ag ida , H., Ci hodo Pu b. 1980, 1 (in Jap an ese). Kana, T, ibid, 173. 4. Ofelt, c.s, f. Chem. Phys., 38, 2171,1963. 5. Dieke, G.H ., Spectra and Energy Levels of Rare Earth Ions in Crystals, Int erscience, 1968; American Institute of Physics Handbook, 3rd ed ition, McGraw-H ill, 1972, 7-25. 6. Riseber g, L.A. an d Moos, H.W., Phys. Reo., 174, 429, 1968.
202
Fundamentals of Phosphors
Wavelength (nm)
900800700 600 500
::-.
300
250
-
: 80 K --- : 300 K
~
~
- .... -
400
Y 2 0 2 5 : Yb 3 +
~
Ll
:: E
Ll ;::l
~ c ~ c
-:::::::: .....00
:I.e -
0..
Ll'-' >
, ~
Ll
15
10
20
35
30
25
Wave number [ X I0 c m 3
40
l]
Figure 39 Excitation an d emission sp ectra of YZ02S:Yb3+ and La zOzS: Yb3+. (From Naka zawa, E., J. Luminesc., 18/1 9, 272, 1979. With pe rm ission.) (b) Exc itatio n spectrum
D
100
2c:
80
(a) Emiss io n spectrum
'en
u
c:
60
if)
c:
.§
40
:::J
>
~
1i cG
20 0 280
320
360
400 400
440
480
520
Wavelength (nm) Figure 40 Emission (a) an d excita tion (b) s pectra of Sr3(PO.h :Yb2+ at liqu id nitrogen tempe ra ture . (From Pa lilla. F.C, O'Reilly, R E., and Abbrusca to, Vj., J. Electrochem. Soc. , 117, 87, 1970. With permission .) 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Kropp, J.L. and Windsor, M.W., J. Chern. Pltys., 42,1599, 1965. Judd, B.R , Phys. Reo., 127, 750, 1962. Ofelt, C.5. , J. Chem. Phys., 37, 511, 1962. Hirao, K., Rev. Laser Eng., 21, 618, 1993 (in Ja panese). Barasc h, C .E. and Diek e, C .H ., f. Chern. Phus., 43, 988, 1965. Pa llila, F.C, Electrochem. Techno/., 6, 39, 1968. Ozawa, R , Bunseki-kiki, 6, 108, 1968 (in Japa n ese). Joe rgensen, CK., Pa palard o, R , and Rittershaus, E., Z. Naturforschg., 20-a, 54, 1964. McC lure, D.5. and Kiss, 2 ., f. Chern. Phys., 39, 3251, 1963. Hoefd raad , H .E., J. Inorg. Nucl. Chem., 37, 1917,1975. Na kazawa, E. an d Sh ion oya, S., f. Chem. Phys., 47, 3211, 1967. McClure, D.S., The Electrochem. Soc., Extended Abstr., 77-1, 365,1977.
Chapter two: Principal phosphor materials and their optical properties 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68.
203
Ozawa, L., The Electrochem. Soc., Extended Abstr., 78-1, 850, 1978. Yamamoto, H an d Kana, T, ]. Electrochem. Soc., 126, 305, 1979. Robins, D.L Cockayne, S., Glasper, J.L., and Lent, B., J. Electrochem. Soc., 126, 1221, 1979. Borden, Th .r., Philips Res. Rpl s., 6, 425, 1951. Van Uiter t, L.G., Soden, RR, and Linares, R e., J. Chem. Phys., 36, 1793, 1962. Pa lli!a, r.c. Levin, A.K., and Rinke vics, M.,]. Electrochem. Soc., 112, 776, 1965. Blasse , G. and Bril, A , J. Chem . Phys., 51, 3252, 1969. Nar ita. K and Taya, A., Tech . Digest, Phosphor Res. Soc. 147th Meeting, 1979 (in Japanese). Van Uitert, L.G., J. Luminesc., 29, 1, 1984. Bu tler, K H., Fluorescent Lamp Phosphors, Technology and Theory, The Pennsy lvania State University Press, 1980,261. Copyrigh t 1996 by The Pennsylvania State University. Blasse, G., Wanmaker, W.L., Tervrugt, J.W, and Bri!, A., Philips Res. Repi ., 23, 189, 1968. Sommerdijk , J.L. and Verstegen, J.M.P.J., ]. Luminesc., 9, 415, 1974. Yamada, H., Suzuki, A, Uchida, Y, Yoshida, M., Yamamoto, I-1., and Tsuk uda, Y, ]. Electrochem. Soc., 136, 2713, 1989. Sommerdijk, J.L., Brit, A, and de Jager, A W, ]. Luminesc., 8, 341, 1974. Piper, W.W, Deluca , J.A., and Ham, FS., J. Luminesc., 8, 344, 1974. Kushida, T, Marcos, H .M., and Geusic, J.E., Phys. Rev., 167, 289, 1968. Vaga, L.P.,]. Chem . Phys., 49, 4674,1968. Urbach, F., Pea rlman , D., and Hemmend inger, H , ]. Opt. Soc. Am., 36, 372, 1946. Keller, S.P., Mapes, J.E., and Cheroff, G., Phys. Rev., 111, 1533, 1958. Feofilof, r.o. and Kap lyanskii, AA, Opt. Spectrosc., 12, 272, 1962. Mahbub'ul Alarn, AS. and Baldassa re Di Bartolo, B.,]. Chem. Phys., 47, 3790, 1967. Blasse, G. and Bril, A, Philips Tech. Rev., 31, 304, 1970. Avella, F J., Sovers, O,J., and Wiggi ns, e.S., J. Electrochem. Soc., 114, 613, 1967. Bril, A and Klassens, H .A., Philips Res. Rept., 10, 305, 1955. levine, AK and PalliJ a, Appl. Phys. Leii. , 5, 118, 1964. Royce, M.R and Smit h, AL., The Electrochem. Soc., Extended Abstr., 34, 94, 1968. Kano , T, Kinarn eri. K , and Seki, S., ]. Electrochem. Soc., 129,2296, 1982. Struck, e.W an d Fonger, W.H,]. Luminesc., 1&2, 456, 1970. Blasse, G.,]. Chern . Phys., 45, 2356, 1966. Forest, H ., Cocco, A., and Hersh , H ., ]. Luminesc., 3, 25, 1970. Struck, e.W an d Fonger, WH., Phys. Rev., B4, 22, 1971. Hos hina, T, Imanaga, S., and Yokono, S., ]. Luminesc., 15, 455, 1977. Blasse, G. and s-u. A , j. Chem. Phys., 50, 2974, 1969. Blasse, G. and Bri!, A., Philips Res. Rept., 22, 481, 1967. Blasse, G., Material science of the luminescence of ino rganic so lids, in Luminescence of Inorganic Solids, DiBartol o, B., Ed ., Plenum Press, 1978, 457. Sme ts. B.M.J. and Verlijsdo nk, J.G., Mater. Res. Bull., 21, 1305, 1986. Takahashi, K , Kohda , K., Miyahara, J., Kanerni tsu, Y, Amitani, K, and Shionoya, S., j. Luminesc., 31&32, 266, 1984. Hews, R.A. and Hoffman, MV, ]. Luminesc., 3, 261, 1970. Ryan, FM ., Lehmann, W., Feldman, D.W, and Murphy, L j. Electrochem. Soc., 121, 1475, 1974. Verstege n, J.M.P.J. and Sommerdijk , J.L., j. Lum inesc., 9, 297, 1974. Sommerdijk, J.L., Verstegen, J.M.P.J., and Bril, A., J. Luminesc., 8, 502, 1974. Wickersc heim, KA and Lefever, RA, ]. Electrochem. Soc., 111, 47, 1964. D'Silv a, A r. and Passel. VA, j. Luminesc., 8, 375, 1974. Hos hina, T, [pn. j. Appl. Phys., 6, 1203, 1967. Del osh, RG ., Tien, T.Y., Gibbon, F.F., Zacmanidis, PJ., an d Stad ler, H .L., J. Chern. Phys., 53, 681, 1970. Sommerdijik, J.L. and Bri!, A, ]. Electrochem. Soc., 122, 952, 1975. Som merdijik, J.L., Sri l, A. , an d Hoex -Strik, F.M.J.H., Philips Res. Rept., 32, 149, 1977. Kiss, Z.J., Phys. Rco., 137, A1749, 1965. Varga, L.P.,]. Chern . Phys., 53, 3552, 1970. Por ter, Jr., J.F., Phys. Rev., 152, 300, 1966.
sc.
204 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82.
Fundamentals of Phosphors Watts, KK., f. Chern. Phys., 53, 3552, 1970. Weakliem, HA and Kiss, Z.J., Phys. Rev., 157, 277, 1967. Hews, KA and Sarver, J.F, Phys. Reo., 182,427, 1969. Kano, T., Yamamoto, H, and Otomo, Y., f. Electrochem. Soc., 119, 1561, 1972. Larach, S., Shrader, KE., and Yocom, P.N., f. Elecirochem. Soc., 116,47, 1969. Kisliuk, P. and Krupke. W.F, f. Chern. Phys., 40, 3606, 1964. Shrader, KE. and Yocom, PN., f. Luminesc., 1&2,814, 1970. Hagimoto, K Iwatsuki, K, Takada, A, Nakagawa, M., Saruwatari, M., Aida, K, Hakagawa, K, and Horiguchi, M., OFC89 PO-I5, 1989. Shrader, KE., Larach, S., and Yocom, EN., f. Appl. Phys., 42, 4529, 1971. (Erratum: f. Appl. Phys., 43, 2021, 1972.) Geusic, J.E., Ostermayer, FW., Marcos, HM., Van Uitert, L.G., and Van der Ziel, [.P. /. Appl. Phus., 42, 1958, 1971. Kobayashi, H., Tanaka, S., Shanker, v., Shiiki, M., Kunou, T., Mita, J., and Sasakura, H, Physi. Stat. Sol. (a), 88, 713, 1985. Nakazawa, E., Chern. Phys. Lett., 56, 161, 1978. Nakazawa, E., f. Luminesc., 18/19,272, 1979. Palilla, r.c.. O'Reilly, KE., and Abbruscato, vj.. f. Elecirochem. Soc., 117, 87, 1970.
chapter two - section four
Principal phosphor materials and their optical properties Makoto Morita
Contents 2.4 Luminescen ce cen ters of complex ions 2.4.1 Introduction 2.4.2 Scheelite-t yp e compo unds 2.4.2.1 Scheelite comp ounds and their genera l properties 2.4.2.2 Electronic s tru ctures of closed-shell m olecular complex cen ters 2.4.2.3 Luminescence centers of va/"- ion typ e 2.4.2.4 Lumines cence centers of M0042- ion typ e 2.4.2.5 Lum inescence centers of WO/- ion typ e 2.4.2.6 Other closed-shell transition metal com p lex cen ters 2.4.3 Uranyl comp lex centers 2.4.3.1 Electronic structure 2.4.3.2 Lu min escence spectra 2.4.4 Platinum com plex ion centers 2.4.4.1 [Pt(CN) 4F- Complex ions 2.4.4.2 Other platinum complex ions 2.4.5 Other comp lex ion centers 2.4.5.1 WO G6- Ion 2.4.5.2 Perspective of other interesting cen ters References
205 205 206 206 206 207 208 209 210 210 210 210 211 212 213 214 214 214 215
2.4 Luminescence centers of complex ions 2.4.1
Introduction
Ph osphors containing luminescence centers made up of comp lex ion s h av e been well known since 1900s. The specific electronic s tructures are reflected in the spe ctra l band sha pes and tran sit ion energies. These phosphors have been widely used in practical applications. However, in spite of their common usage, it is on ly in the last three decad es that the electronic structures of the complex ions have been exp lained in term s of the
205
Fundamentals of Phosphors
206
crystal field th eory (See 2.2.1); be cau se of this und ers tand ing, new phosphors have been prepa red w ith a variety of colors and w ith high quantum yield s.' Thi s sec tion will firs t focus on the luminescence from co mplex ions wi th closed-she ll electro nic struc tures such as the schee lite compo unds and o thers and describe a variety of ap p lications for these p hos phors. O ther int erest ing luminescence cen ters such as uranyl (II), platinum (II), mixedva lence , and ot her complexes are discussed subsequent ly.
2.4.2 Scheelite-type compounds 2.4.2.1 Scheelite compounds and their general properties Calci um tungstat e (CaW0 4) ha s long been known as a practical p hosp hor, and it is a representa tive scheelite comp oun d. Th e luminescence center is the WO/ - complex ion in w hich the central W met al ion is coordinat ed by four 0 2- ions in tetrah edral symmetry (Td ) . O the r analogous Td complexes are molybdate (MoO l -) and va nada te (VOl") In these three complex ions, the electron ic configuration of the outer-shell is [Xe]4f 4 , [Kr], and [Ar] for WO / -, MoO/ -, an d VOl -, respectively. In general, scheel ite phosphors take the form of A pB04 wi th A standing for a monovalent alka line, diva lent a lkaline earth, or trivalent lanthan id e me tal ion, p for th e number of ion s, and B for W, Mo , V, or P. Brigh t lu min escen ce in th e blue to green spectral regions was observed in the early 20th cen tury. An int roduction to th e elec tronic configurations common to these complex ions is followed by a d iscussion of the res ults of inves tiga tions, refe rri ng to a number of recent review ar ticles on this subject.F
2.4.2.2
Electronic structures of closed-shell molecular complex centers
As a typica l complex ion cen ter, consi der th e elec tronic struc ture of th e Mn0 4- ion. In thi s case, the Mn 7+ ion has a closed-s he ll struc ture wi th no d elec trons. Us ing a on eelec tron transition schem e, consider a one-electron charge transfer process fro m the oxygen 2p orbi tal (tl symme try in T,,) to the 3d orbital (e and t2 symme try) of the Mn 7 + ion . A m olecul ar orbi tal calcula tion- lea d s to e' and a t l sta tes for the lowest unoccup ied m olecul ar orbi tal (LUMO) and the h ighest occupied mol ecul ar orbi tal (HOMO), respectively. By taking the e ~ II transition into account, th e excited electronic states of lI Se electronic configura tion in Td symme try are found to con sist of 3T I ~ 3T 2 < IT [ < IT2 in order of in creasin g energies, the ground s tate being a ]A l sta te . The orbi tal tr iplets (3Tu 3T 2) have degener at e leve ls in the sp ectral reg ion of 250 to 500 nm. By employing more advanced ca lculations (the XCI. m ethod), similar results for th e e f - II transition have been calculated for th e V0 43- io n." Elect ronic structures of the sch eeli te compo unds have in common closed-shell electronic configura tio ns as exp lained for the Mn0 4- ion, and their luminescence and absorption processes are exemplified in the model scheme of the MO,t- complex. Genera lly speaking, th e lu m inescence of MO,n- ion is d ue to the spin-forbidden 3T [ ---c> lAI transition that is made allowed by the spin-orbit interaction. The correspo n d ing 3T I f - JA I absorp tion transi tio n is not easily observed in the excitation spectrum due to th e stro ng spin selection ru le, and the first strong absorption band is assign ed to th e spin-allowed IT I f- IA] tran sit ion . Elec tronic levels and their assignme n ts are given sche ma tica lly in Figure 41 for th e MO/ - ion .? In this model, assu m e an energy level scheme for the MO/ - complex in a tetrah ed ral env ironment. The energy separation between 3TJ and 3T2 has been estima ted to be ab out 500 crrr" for the VOl - complex fro m luminescence exp eriment s." The sp litting of 3TlJ shown in the figure, amoun ts to several tens of crrr' and is due to the lowering of the crystal field symmetry fro m T, and to the in clu sion of the spi n-orbi t interaction. We want to understand changes of spectral properties an d decay tim es of the lu m inescence from these comp lexes at tem peratur es between room temperature and 77K. Then, the
Chapter two: Principal phosphor materials and their optical properties
\
t ,1
207
(3)
'\ I T
z ~ 500 c
,-L .1
./
1
h I/I
~ hl/z
kz
k1
(1) Figure 41 Th ree-level energy scheme for luminescence processes of MO/ - ion in schee lite compounds. It is ne cessary to take int o accoun t of the splittings of the 3T t state to ana lyze cha ng es of emission d ecay times at very low temp eratures. (From Blasse, G., St ructure and Bonding, 42, 1, 1980. With permission.)
simp lified three-level model based on th e two excited sta tes (3T j , 3T2 ) and th e grou n d state JA j is quite sati sfactory. Figure 41 illu strates a sim p le but useful model for the ene rgy levels of ions in sch eelite compounds. If the species of the cen tral metal ion M ar e ch anged, the positi on of the higher exci ted sta tes and the sp litting of these le vels will cha nge considerably .4 H ow ever, the ord ering of the states is rigoro usly observed. H igh er excited sta tes d ue to the t]5 t2 configuration ha ve also been examined theoreticall y.' Excited -sta te ab sor ption from the t]5e to the tiS t2 have been inves tiga ted in CaW04 crys tals ."
2.4.2.3 Luminescence centers of VO/ - ion type Yttriu m vana da te (YV0 4) is a very useful phosphor in use for a lon g time. Thi s com p oun d do es not sh ow luminescen ce a t room temperature; but at temper atures b elow 200K, it shows blue em ission centered at 420 nm, as shown in Figure 42.7 The broad band h as a full width at half maximum (FW H M) of about 5000 cm" , with a d ecay time of severa l milliseconds. Even at 4K, no vibron ic structure is seen . Th e first excit at ion band is located at ab out 330 nm, sep a ra ted by 6000 crrr' from th e em ission band. The emission an d excitati on ar e due to the 3T] H IA] transition, and th e large Stokes' shift is due to th e d isplacem ent bet ween the exci ted - and the ground -sta te potential minima in the configuration al coo rdi na te mod el. In YV0 4, energy migrati on tends to fa vor nonradiative transition processes; because of th is thermal quenchin g, lumine scence is n ot obse rved a t room temper ature. H ow ever, ro om-temp erature luminescence is ob served in YP0 4 :VO/- mi xed crystals. Bright luminescence from V0 1' - ions is com monly ob ser ved in other va nadate complexes such as Mg 3(V 0 4h, LiZnV0 4, LiMgP0 4:VO/ -, and NaCaV0 4 • If trivalent rareea rth ions such as Eu 3 + and D y3 ~ are incorporat ed in to the YV0 4 ho st , bright luminescen ce
208
Fundamentals of Phosphors
1 400 Wavelength (nm ) Figure 42 Emission spectra of YVO, ( - ), CaW04 (- -----), and PbW04 ( _ . _ . _ , +++++) under 250-nm excitation at 771<. Two emission bands of blue and green colors are seen in PbW04 under 313-nm excitation. (From Blasse. G., Radiationless Processes, DiBartolo, B., Ed., Plenum Press, New York, 1980, 287. With perrnission.)
due to th e do pan t ion s is observed beca use of efficien t energy tran sfer processes from the vanada te ion s.
2.4.2.4 Luminescence centers of MoO ;" ion type Many molybd ate phosphors contain ing MoO/- centers are known w ith a general chemical formula MMoO" (where M Z+ = Ca-", Srz" Cd>, Zn > , Ba> , Pbz+, etc.). Luminescen ce propert ies do not depend significan tly on the ion M. In PbMo04, a green emission band d ue to the 3T] -7 IA I transition is observed at around 520 nm at low tem perat ur es (77K), as shown in Figure 43.8 The FWHM of th is broad ban d is abou t 3300 em: ' . The lifeti me is 0.1 ms , sh orter than that of VO,,3- compou nds. The d egr ee of polar ization in luminescence has been measured in some molybdate single crys tals as a func tion of temperatures in the low-temperat ure region." From these stu dies, th e up per tripl et sta te 3Tz separation has been determined to be £-.z = 550 em-I, wi th the tr ipl et 3T j be ing low est. The decay tim e from 3T z to lAI is in th e 1 to 0.1 us range. O range-to-red luminescence is also observ ed in some molybda te complexes in addi tion to the green luminescence. In Ca MoO",1° for example, gree n emission ap pea rs under UVlight excita tion (250-310 nm) , but the orange emission at 580 nm is observable only if the excita tion light of wavelengths longer than 320 nm is used. Orange emission was thus observed unde r excitati on just below the op tica l ban d gap . The inte nsi ty of the ora nge emis sion decreases or increases whe n CaMoO" is d op ed wi th Y3+ or Nat ions .' Therefore, this ora nge emission is ascribed to latti ce defec ts. In th e case of PbMoO",8 red emi ssion (centered at 620 nm) is also observed und er photo excitation at 360 nm at room temperature, as shown in Figu re 43. Deep -red emissio n can be seen under 410-nm excitation at 77K. These bands are thou ght to be due to defe ct centers of MoOl- ion s coupled to ion vacancies. Thermol uminescence of MoO}- salts" ha s been investigated to clarify the electron ic struc ture of the defect centers and im purities in these materials. Stud ies of the luminescence of moly bda te compounds containi ng triva len t rare-earth ions as act iva tors, such as Gd z(MoO"hEr3+ (abbreviated as GMO:Er3+),I ZNa sEu(MoO")4' and KLa(Mo0 4)z:Er3+, have been
oz-
209
Chapter two: Principal phosphor materials and their optical properties
r.
77K, / / 370 nm _ _.. ., , , Excitation
t. , 360 nm Excitation
/'.. , \ y'-'
I
/
I'
. \
\
~.
500
77K, 410nm Excitation
\f
/ I /,.,.'
~. 400
\
/
/
I
1
\
\
\\
,
600
" 700
800
Wav elength (nm) Figure 43 Spectral features of emission from PbMo04 . Orange-to-red emission (_ ._. _) is observed at room temperature unde r photoexcitation at 360 nrn. This emission is compared with the deepred one ( - ) at 77K under 410-nm exci tation and the green one (------) under 370-nm excitation also at 77K. (From Bernhardt, H,J., Phys. Stat. Sol. (a), 91, 643, 1985. With perm ission .)
reported . Strong, sharp luminescence due to rare-earth ions has been reported in the visible and the nea r-infrared spe ctral regions due to efficient energy tran sfer from the MoO/- ion .
2.4.2.5 Luminescence cen ters of WO/- ion type There are man y blue phosphors of in terest in th e metal tun gstat e series of compl exes having the chemical form ula MW 0 4 (Mz+ = alk aline ea r th meta l ion). The splittin g ~ l of the 3T j sta te, sho w n in Figure 41, is abo u t 20 crrr' for th e WO I ~ - ion cen ter. Th e spin-or bi t interaction in the M0 4 n- io n becomes s tronger w ith increasing ato m ic numbers of th e metal; thus, VO}- < MoO/- < WO /-. In ord er of increasing L-S cou p ling , the spi nforbidden 3T] H ]Aj transit ion p rob ab ilit y is enhanced and the emission lifetime d ecreases corres pond ingly. The lifetime of the blue emission fro m the WO / - io n is as short as 10 us: this is 100 times shorter th an th at of the VOl - ion . A rep resen tative tu ngst at e p hosp h or is CaW0 4; this material emits a br ig h t blue emissio n in a broad band (cen te red at 420 nm) wi th FW HM of about 5000 crrr'. The mixed crys tal (Ca,Pb)W0 4 prod uces a very strong gr een emission with hi gh q ua n tum yields reaching 75%.7 The blue emission spectra of Ca W0 4 and PbW0 4 under 250-n m excitation are shown in Figur e 42. In Ca W0 4 , th ere is a weak em ission band at Y530 nm superimposed on the longer waveleng th tail of th e blue emission . Pb W0 4 man ifest s th e presence of the or an ge band und er 313-n m exci tation. The ora nge luminescence was interp reted as being due to imp uri ty ions or to Scho ttky d efec ts. In d ecay tim e measurements of CaW04, 13 the fast decay comp onen t of abo u t 30 us was found at temperatures betw een 1.5 and 5.0K, which canno t be explained as being d ue to the crystal field splitting of the emi tting leveI 3T]. It has also been confirmed by stu d ies of th e emission and exci ta tion spec tra that onl y a sing le, bro ad b lue emission band exis ts in pure single crystals of CdW0 4 and Zn W04 . J4 Ba, W03 F4 ha s a crystal s truct ure s imi lar to MgW 0 4 and this structure is consi de red to be most favorabl e to rea lize a high quan tu m efficiency. This is because a su bs titution
Fundamentals of Phosphors
210
of the F- ion for 0 2- see ms to reduce the magnitude of the phonon ene rgy and this in turn quenches nonradiative transition processes in the [W03FJ- tetrahedron . The emission process was analyzed using the configurational coo rd ina te diagram. " a nd quantum yields of 75% have been rep orted in this material. "
2.4.2.6 Other closed-shell transition metal complex centers There are other interesting e m iss ion centers w ith clo sed-shell confi gurations besides V041- , MoO/ -, and WO/ - ions .? Th ey form a se ries of phosphors of the [MO/-J type, where M = Ti4+, Cr 6+ , Zr 4-, Nb5+, Hf4+, and Ta5+. These com p lexes have been in ves tig ated extensive ly as possible new media for solid-state lasers. " The luminescence spectra from KVO F4, K2NbOFs,18 and 5i02 glass.Cr'" 19,20 have been re ported recently as new co m p lex centers pos sessing this electronic con fig uration .
2.4,3
Uranyl complex centers
2.4.3.1
Electronic structure
Th e uranyl ion is a linear triatomic ion with a chemica l formula [O=U=O]2+ (0'011 symmetr y). The stro n g, sharp lin e luminescence from thi s center has been kn own for more than half a cen tury. Iorgensen an d Reisfeld?' have th o roughly discussed the hi st ori cal backgro un d an d theoretical as pec ts of the luminescen ce of th ese centers . The electro nic structu re of uranyl ions is particularly interesting. As for the excited sta tes o f uranyl ions, first consid er the charge-tran sfer p rocess of an ele ctro n from 0 2+- to U 6+. Th e resulting U 5+ (Sf ) ion has the follo wing a tomic orbitals: au (5f o)' rru (SIt I)' s, (5f±2)' lJ>u (~rd · The electronic levels, 2F7/2 and 2F512, consis t of several s ta tes ha vin g total angula r m om entum Q I = 1/2, 3 /2, 5/2,7/2 in 0 =" sym me try. On the other h and, 0l - has molecul ar o rbital configurations, (rru 4 au) and (rr} a }) . A combin ation of th ese states gives total angular momentum Q 2 = 1/2, 3/2. From vector co u p ling of Q l an d Q 2,21.22 the U0 22< ion can be expressed as p ossessing total angular m omentum of Q = 0, 1, 2, 3, 4/ 5.21.22 On the basis of investigations of th e p olarized abso rp tion and the isotope effects, Denning et aP ' h ave determined th at th e lowest excited s ta te is Q = 1 CnJ;' a )lu) (au and Ou stand fo r th e electronic states of 0l - and the 5J1 ion , respectively ), a:, shown in Figure 44. The luminescence o f UOl Tcorresponds to a Ing~ [LJ.;'· (0 =,,) m agnetic dipole-all owed transition . M ore p re cise molecul ar or bita l calculations>' an d abso rp tion exp eriments in CS2U0 2Cl4_xBrx mixed crystals" con firm the (auoJ state as th e lowest exc ite d sta te . The sta tes arising fro m the (rr} oJ configuration must be taken into acc oun t to consid er the higher ele ctronic exc ited states. Until th e nature of the excited elec tron ic state of Q = 1 eng, auo u) wa s fin all y clarified in 1976, the od d pa ri ty state Jl u was thought to be the lowest excited sta te. Th er efore, reports on urany l ions published befo re 1976 must be read w ith this rese rv a tio n in mind . Figure 44 sh ows assign m en ts and positions (in units of crrr") of electronic levels of uranyl ions as d eterm in ed from the absorp tion spectra of Cs 2 U0 2 Cl,. 2.1
2.4.3.2 Luminescence spectra A luminescen ce sp ectr um from a Cs 2U0 2Cl4 single crystal at 13K, accompanied by vib ronic structure du e to Morita and Shoki." is shown in Fig ure 45. The Frank-Condon p attern shows v ibron ic progressions of th e fund amental vibrations, V s = 837 cm' and v." = 916 crrr', of th e UO/+ ion. By applying th e con fig u ration al coord ina te m odel to Cs 2U0 2Cl4, th e nuclear d ispl acement i1Q is es timated to be 0.094 A for th e two p otential minima of the IEgc ng) exc ited sta te and the IA ,g(l L g+) grou nd state in 0 411 (0_,,) symmetry." Emi ssion peaks w ith sy m bo l * in the figure are due to traps, and these peaks disappear above 20K. The fin e s truc tu res se en in th e vi bronic progressions are electri c dipole-allowed tr an sitions due to co u p ling with odd-p arity lattice vibrations.
Chapter two: Principal phosphor materials and their optical properties
I 27700 cm- 1 39 crn "
1 60b c
Q =3
22600
Q =2
340 22000
6 000 cm- J
Q=3
50
,
Q=4
!
26200 50
.dq
211
20350
Q =2
904 20 100
ll)=*=~~=:::::::::::= 1.6
Q =
1
Q =
0
liv 500nm
.--...---~
_ _ _...1..-
Figure 44 Ene rg y levels and their assig nmen ts of UOl ' ion in O'olt symmet ry. Emission is d ue to the magnetic dip ole-allowed J n ~ ---7 l ~g+ (O_J,) transition . (From Denning, R.G., Snellgrove, T.R., and Wood wark, D.R., Molec. Pilys., 32, 419, 1976. With p er rnission .)
Flin t and Tann er" ha ve in vest igated the luminescence of various other uranyl complexes, of the series A zUOzCl4 ·nH 20 (A = Rb' , Cs", K+, (CH3) lN ') They found good agreement between the molecular vibr ations observed in th e luminescenc e spec tra and tho se reported in infr ared and Ram an spe ctra. Dynamic aspects of luminescen ce of [U0 2Cl4 ]2phosphors have also proved to be of interest. Krol" has investigat ed the decay of the luminescen ce of Cs zUOzCl4 at l.5K under strong laser irradiation and ob tain ed nonexp onential decays; these decays are thou ght to be due to the presence of bie xcitons associated with inter ionic inte ractions . Localization of excitons ha s al so been reported in CsUO Z(N03 ) 3 .Z9 Excitation energy transfer to trap s has been studied in CszUOzBr} Oin the temperature range between 1.5 and 2SK and compa red with a diffusion-limited transfer model. There are additional spectral features in ur anyl compounds. For example, opticall y active single crystals of N aUO z(CH 3COOh exhibit" a series of complicated vibronic Jines due to the p resen ce of two emi ssion centers, which are resolved by the difference of the degree of circul ar polarization in luminescence. Decay times of the luminescence of uranyl ~-diketonato complexes" in liquid solven ts ha ve been found to be in th e 1 to SOO-ns range; the drastic va riations are understood in terms of changes in the nonradiative rate cons ta nts correlated to the energy position of the zero-phonon emis sion lin e.
2.4.4 Platinum complex ion centers Platinum(II) and mi xed-valence platinum(ll, IV) com plex ions have also been investigated exten sively. The best known platinum(II) complex is a yellow -green comp oun d , ba rium tetracynoplatinate (II) Ba[Pt(CN) 4] ·4H zO (abbrev ia ted BCP), w h ich p ossesses a linear chain
Fundamentals of Phosphors
212
* Trap >-.. ......
'(jj
c
<1,) ...... c: c:
0
"-,
(j)
o:
"- -,
E
u..:l
10
I
Vo
20
•
19
Wave numb er
18
17
[ 10 3 em -I J
Figure 45 Emission spe ctra of Cs 2UOlCI4 at 13K, showing the vibron ic progressions. Insert ed figure shows de tails of vibro ni c s tru ctures and trap centers are denoted by the symbol ". (From Morita , M. and Shoki. T., J. Lumincsc. 38/ 39, 678, 1987 and u np ub lish ed resul ts. With permission .)
struc ture . Mixed-valen ce complexes such as the b rom id e-d oped potassi um tet racyanop la tina te K 2[Pt (CN )4J BrO.1 ,3H20 (KCP :Br) an d Wolfram 's red salt (WRS sa lt), i.e., [P t(Il)L4J [P t(lV)L4 X2J X4,2H 20 (L = e thy la mine C 2 H sN H 1 ; X = CI, Br) h ave also been studied comprehensive ly. With reference to earlier rev iew articles,' " the elec tron ic str uct ure of one -dimens iona l pl at inum (II) com p lexes is described below an d alon g with the unique spectroscopic cha rac ter of these co m plexes.
2.4.4.1 [Pt(CN )4j2- Complex ions The very stron g green lu m in escence of the anisotrop ic p latin um(II) co mp lex BCP has been known for more th an 65 years , The [Pt(CN)4F- ion forms a fla t tetragon al pla ne, wi th th e Pt 2+ being located in the cen ter; in BCP, th e Pt 2+ forms a lin ear cha in as shown in Fig ure 46. X-ray d iffract ion ana lysis" confi r ms a lin ea r cha in s tructure of p lana r [Pt (CN ).F- along th e c-ax is, Since th e Pt- r-to- Pt'" di sta nce in BCP is as short as 0.327 nm , the direct overla p of the 5d/ orbi ta ls is p ossib le. Mon reau-Colin-" h as tab u lat ed op tical proper ties of these mat erials obta ine d from s tudies of the reflec tion, ab sor p tion , and emission spectra for a series of pl at inu m com p lexes of th e general form: M[Pt(CN)lJ'nH 20 (w he re M = M g l ' , Ba 2+, Ca 2+, Lil + an d K } +), Lar ge spectral shifts of the emi ssion ban d s are observed w ith changes in the M ion. It ha s been es tab lishe d that the se shif ts a re cor related to the Pt-t-to-P t> in terac tion along the one-d imension al platinum (II) chai n. From molecular orb ita l calculat ions." 5d and 6p orbitals of Pt(II) can couple wi th the n' orbital of the CN- ion to form a l h (5d/) HOMO and a2u ' (6pJ LUMO or bi tals. The emission in BCP is d ue to the a2u ' ~ a 1g tran sition in 0 4" sy m metry and shows a stro ng p olarizat ion
Chapter two: Principal phosphor materials and their optical properties
( b)
( a)
213
C
BaPt (CN) 44H2 0 [ BCP ]
E
~j
20 o
y ,\------'---------'--- -'-------25
-
20
15
Excitation photon energy [103 cm : 1)
Figure 46 (a) Schema tic s tructu re of the one-d imensio nal pla tin um(1 I) com pl ex Ba[P t(CN LJ-4HzO (BCP). (b) Abnormal shifts of emission band peeks of plat in um (lI) comp lexes, BCP (do tted line) and K2[Pt(CN) ;]·3H zO (solid line), with changi ng exc itation pho ton ene rgy at 4.2K. (From Murat a, K. and Morita, M., Tech . Rep. Scikci Unio., 18, 1383, 1974 and unpublis hed results. With permi ssion .)
dependence along the c-axis (Pt 2+-Pt 2+ chain ). Th e room -temperature emission of BCP is cen tered at 520 nm ; the em ission sho ws a large blu e sh ift to 440 n m when Ba2 + is repl aced by K+. The emi ssion process is in ter p reted as b ein g due to Frenkel exc itons." Th is is becau se the position of the em ission band shows a red shift proporti on al to R-3, where R is the Pt-Pt di stance an d is a fun ction of ionic radius of the M ion . The emission band position of BCP at 4.2K shifts con tin uously to lower energies as the w aveleng th of the excita tion light is va ried from the ultraviolet to visible spectral region ." Figure 46 sho ws this phe nomenon, one th at cannot be interpreted by presen t th eoret ical underst and ing. This effect is likely related to the d yn am ic relaxat ion of exciton s in one-d imensiona l pJat inum(II) chai ns .
2.4.4.2
Other platinum complex ions
There are many ad ditional Juminescent materials con taining [Pt(CN)4F- complexes besid es those mentioned above. Luminescen ce of [Pt(CN) 4F- is observed in mixed crystals]" of K2[Pt(CN )]6:K2[ Pt(CN )4] (1:1) u nd er nitrogen laser exci ta tion arising fro m intervalenc e transitions between pt (rV) and Pt (ll ) ion s. When the Ba ion in BCP is replace d by rare-ea rth(lII) ions, new complexes Ln 2[Pt (CN Lh ·nH 20 (Ln = Eu 3+, Sm 3+, Er" . e tc.) can be form ed ." Sharp line luminescen ce of Ln(III) ions is normally observed in the se mat eri als du e to energy tran sfer from the [Pt(CN)4F- complex. Extensive inves tigations hav e been cond uc ted at low temperatu re using high pressure and strong magnetic field s.:1:\·w A mixed- valen ce platinum complex is KCP :Br, with a val en cy egu al to 2.3. Th is compound is metallic in appearance and do es not flu oresce. A typi cal examp le of a lu minescent mixed complex is the WRS sa lt mentioned previously; it contains Pt(Il) and Pt (IV) ions. The structure of the WRS sal t consis ts of an alternative stack of a tetragonal pl ane unit [Pt(II)L4] and an octahed ra l unit with six coordinat ion atoms [Pt(IV)L4X2] . These units form a quas i-one-di me nsio nal linea r chain of [X-Pt(IV)-X--Pt(II)--X] br id ged by halogen s. The
214
Fundamentals of Phosphors
WRS com p lex has been known sin ce the late 19th century; it w as determined by Day37 to be a typicallow-climensional compound. Tanino and Kobayashi" first reported reson ance Raman scattering and NIR luminescence at about 1 11m in WRS salts at 4.2K. A sim ilar mixed-valence complex, (Pt(en)z][Pt(en) zIz](Cl04) 4 (en = NH zCH 2CH2NH z), was found to show a luminescence band a t 1 urn, with a lifetime of ab out 200 ps a t 2K.4ZThe origin of this lum inescence band is ascribe d to self-trapped -exciton ic (STE) sta tes."
2.4.5 Other complex ion centers 2.4.5.1 WOl- Ion In BizW06;13 the emission cen ter is identifiabl e as W0 6&- in a cubic crys tal field. Red emission is obse rved at 4.2K, w ith a peak a t 600 nm an d a FWHM of about 1500 crrr'. The excit at ion sp ect ru m con sists of a band a t 390 nm with an appar ent Stokes' sh ift of 9000 crrr' . Acco rding to se lf-cons isten t field m olecular orbital calcul ations." the emitting levels in WOl - are due to tw o 3'[1u states. By em p loying Figure 41, the emission and absorption can be assigned to 3T 111 ~ All' and ITl u H lA l g transitions, resp ectively. Emi ssi on centers of the W0 66- ion w ere also reported in many compounds w ith the perovskit e structure A zBW0 6 (A = Ca > , Sr2+ , Ba2+; B = Mgz+, Ca z+, Sr z+, Ba2 +). Wolf and Karnml erSack" reported infrared emission of rare-earth ion s incorporated into a ve ry com p licated com po und 18R-Ba6BizW3018' In this case, there are three W06&- ion sites in the compound w ith an hexag onal closed-packed p olymorphic struc tu re . The emission spectra consist of two bands at 21700 and 17000 crrr? due to two 6c sites and one 3a site, respectively. The corresp on d ing excita tion bands a re at 36000 cm' (6c) an d 29000 crrr-' (3a), respectively. The luminescence of W0 6&- ion s can also be seen in othe r materials su ch as Li6W0 6, 12RBazLazMgWzOlz, a nd Ca 3La zWZ0 12•
2.4.5.2 Perspective of other interesting centers The above-mentioned W06&-lumin escence center is on e of the closed-sh ell transition metal co mp lex ion s, ge ne rally expressed as (Mo6 ]n- (where M = Ti, Mo. Nb, Zr, Ta, an d W). Two papers-w on th e luminescence p roperties of MoO/- and MoO,,"- complexes have been published. Rec ently, luminescence from a e ur op ium octamolybd at e pol ymer, Euz(HzO)dMosOd 6Hz047 and the p icos econd de cay of the transient ab so rbance of (WlOOd 4- in ace toni trile" ha ve been reported. The lumin escence of ur anat e (U06&-) cent ers in solid s ha ve been review ed by Bleijenberg ,"? Thus far, thi s disc uss ion of luminescence centers of com p lex ions focuse d on practical phosphors. H owever, under the ca teg or y of complex ion s, a more general survey is p ossible. Compl ex com p o un d s consis t of a central met al ion and surround in g an ions or organic ligands . In th ese compounds, there are-in princi pl e-four possible luminescence processes that origi nat e from the cen tra l metal ion, from th e ligand, from ligand-to-metal cha rge-transfer (LMCT), and from metal-to-ligand ch ar ge-tran sfer (MLCT) tr an siti ons. Due to the se different tran sition processes, the luminescen ce fro m complex ions can eith er be sharp or broad, a nd can occur in a bro ad sp ectral region . Ur anyl complexes luminescing of green-yellow color are examples of cen tr al metal ion transitions. Eu(III) ~-diketonato complex, a typical NMR shift reagent, also shows bright an d sh arp red lumine scence due to the central Eu (III) ion. For more th an half a century, the luminescence of the Zinc( II) 8hydroxyquinolinat o com p lex ha s been sh own to be due to the aroma tic organic ligands. Emission transition s due to the LMCT sche m e is found in sche elite compounds. Phosphorescence due to MLCT tran sitions is predominant in com p lexes su ch as ruthenium(II) trisb ip yridyl ((Ru (bpYhF+), metal-phthal ocy anines (e.g., Cu-Pc, a fam ous pigment), and metallo p orphyrins (e.g., Mg-TPP). The latter two complexes are usually considered as organic phosphors because of Io -memberedn-ring structures.
Chapter two:
Principal phosphor materials and their optical properties
215
In the future, one will be able to design new phosphors of complex ion types that can be excited by va rious excitation sources such as hi gh electron beams, X-ray lasers, and NIR-Iaser di odes. Phosphors of complex ions will con tin ue to play a useful role in luminescence applications.
References 1. Morita, M., MoO/ -, WO/ - compounds, and on e-dimen sional com po un ds, in Hikaribussei Handbook (Handbook of Optical Properties of Solids), Shionoya. S., Toyoza w a, Y, Kod a, 1., and Kukirnoto. H. , Eds., Asa kura Shoten, Tokyo, 1984, ch ap . 2. 12. 6 an d 2. 19. 2. (in Jap anese) . 2. Blasse, G., Structure and Bonding, 42, 1, 1980. 3. Ballhau sen , C.]. a nd Liehr, A.D., J. Mol. Spectrosc., 4, 190, 1960. 4. Ziegler, 1., Rank, A, a nd Baerends, E.]., Chem. Pliys., 16,209, 1976. 5. Keb ab cioglu, R and Mueller, A., Chern. Phys. Leii., 8, 59, 1971. 6. Koepke', C , Wojtowica, A]., and Le rnpicki. A., f. Luminesc., 54, 345, 1993. 7. Blasse, G., Radi ati on less processes in luminescent materials, in Radiationless Processes, DiBartolo , B., Ed ., Plenum Pre ss, Ne w York, 1980, 287. 8. Bernh ardt, H.J., Phys. Stat. Sol.ta), 91, 643, 1985. 9. Rent, E.G., Opt. Spectrosc. (USS R), 57, 90, 1985. 10. Cr oen ink, I.A., H akfoort, C, and Blasse, G., Phys. Stat. Sol.ia), 54, 329, 1979. 11. Bohm , M ., Erb , 0 ., a nd Scharrnan , A. , f. Luminesc., 33, 315, 1985. 12. Herren, M. and Mor ita, M .,]. Luminesc., 66/67, 268, 1996. 13. Blasse, G. and Bokkers, G.,]. Solid. State. Chem., 49, 126, 1983. 14. Shi rak awa, Y, Tak ah ar a. T , and Ni sh imura, T , Tech. Digest, Phosphor Res. Soc. Meeting, 206, 15,1985. 15. Tew s, W , Herzog, G., and Roth , 1., Z. Phys. Chern . Leipzig, 266, 989, 1985. 16. Blasse, G., Verhaar, H.CG., Lammers, M.J.L Win gelfeld, G., H oppe, R, an d De Maayer, P., f. Luminesc., 29,497, 1984. 17. Koep ke, c., Wojtow icz, A .]., a nd Lernpicki, A , IEEE f. Quant . u«. 31, 1554, 1995. 18. H az enkarnp, M .F., Strijbosch, AW P.M., an d Blasse, G., f. Solid State Chent., 97, 115, 1992. 19. H erren, M., Ni shiuchi, H., and Morita, M., ]. Chem. Phys., 101,4461, 1<)<)4. 20. H er ren , M., Yamanaka, K, and Morita, M., Tech. Rep. Seikei Uniu.. 32, 61, 1995. 21. jorgensen , CK and Reisfeld, R , Structure and Bonding, 50, 122, ] 982. 22. Denning, K G., Fos ter, D.N .P., Snellgrove, TR, and Woodw ark, D.K, Moiec. Phys., 37, 1089 and 1109, 1979. 23. Denning, K G., Snellgrove, 1.K, and Wood w ar k, D.K, Molee. Phys., 32, 419, 1976. 24. Dekock, RL., Baerends, E,J., Boerrig ter, FM ., and Snijders, I.G ., Chern . Phys. Lett., 105, 308, 1984. 25. Denning, KG., N orris, J.O.W , and Laing, P.J ., Malee. Phys., 54, 713, 1985. 26. Morita , M . a nd Sho ki, 1. , J. Luminesc., 38 /39, 678, 1987 and un published resu lts. 27. Flint, S.D. an d Tanne r, FA, Malec. Phys., 44, 411, 1981. 28. Kro l, D.M., Chem. Phys. Lett., 74, 515, 1980. 29. Th orne, J.KG . a nd Denning, K G., Malee. Phys., 54, 701, 1985. 30. Krol, D.M. and Roes. A , Phys. Rcu., 23, 2135, 1981. 31. Mu rata , K. and Mo rita, M., f. Luminesc., 29, 381, 1984. 32. Yaya rnura, 1., Iw at a,S., Iw a mura, S., and Torn iyasu , H. ,]. Cliem . Soc. Faraday Trans., 90, 3253, 1994. 33. Gliemann, G. an d Yersin. H. , St ructure and Bonding, 62, 89, 1985. 34. Krogmann, K., Allgew. Cliem., 81, 10, 1969. 35. Monreau -Colin, M.L., St ructure and Bonding, 10, 167, 1972. 36. Mon cuit, S. a nd Poulet, H. , J. Phys. Radium, 23, 353, 1962. 37. Day, F, Coll ective sta tes in sing le and mi xed va lence metal cha in co mpo unds , in Chemistry and Physics of One-Dimensional Metals, Keller, H .J., Ed., N ATO-ASI Ser ies B25, Plenum Press, N ew York, 1976, 197. 38. Murata, K an d Morita, M ., Tech. Rep. Seikei Unio., 18, 1383, 1974 and unpublished results.
216 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49.
Fundamentals of Phosphors Wisw ar ath, A.K. , Smith, W.L., an d Patterson, HH., Chern, Phys. Leu., 87, 612, 1982. Yersin, H. and Stock, M., J. Chetn . Phys., 76, 2136, 1982. Tanino, H. and Koba yash i, K., J. Phys. Soc. Japan, 52, 1446, 1983. Wada, Y, Lemmer, U', Gobel, E.O., Yamashita, N., and Toriumi, K., Phys. Rco., 55, 8276,1 995. Blasse, G. and Dirkson, G.]., Chern . Phys. Lett., 85, 150, 1982. Van Oosternhout. A.B., J. Chem. Phys., 67, 2412, 1977. Wolf, D. an d Kemmler-Sack. S., Phys. Stat. Sol. (a), 86, 685, 1984. Wiegel, K. and Blasse, G., J. Solid State oe«. 99, 388, 1992. Yarnase, T. and Naruke. H, J. Chem. Soc. Dalton Tran s., 1991, 285. Duncan , D.C., Netzel, T.L., and Hill , c.i.. ln org. Chem., 34, 4640,1995. Bleijenberg, K.c. , Structure and Bonding, 50, 97, 1983.
chapter two - section five
Principal phosphor materials and their optical properties Shigeo Shionoya
Contents 2.5 Ia-VIIb compounds 2.5.1 Introduction 2.5.2 Intrinsic optical properties 3.5.2.1 Band stru ctur e and exciton 3.5.2.2 Self-trapping of excitons and intrinsic luminescence 2.5.3 Color center s 2.5.4 Luminescence centers of ns2-type ions 2.5.5 Lumin escen ce of isoelectronic traps References
217 217 218 218 218 218 219 220 220
2.5 Ia- VIIb compounds 2.5.1
Introduction
Ia-VIlli compounds-that is, alk al i halides-are prototypical co lorless io nic cry s tals. Their crysta l s tru cture is of the rock- salt type except for three com pounds (CsC !, CsBr, and CsI) that po ssess the ces iu m chloride structu re. Melting points are gen era lly in the 620-990°C range, which is relati vely low. Research on th e lumin escen ce of alka li halides has a lon g history. Since the 1920s, luminescence studies on ns--typ e (Tlt -type) ions incorporated in alkali halides have been active ly pursued (See 2.1). In the ba sic studies of those early d ays , alkali halides were used as hosts for variou s luminescence centers, because as nearl y ideal ionic crystals theoretical treatments of the observa tions were possible. Sin ce th e 1940s, the optical spectroscopy, including luminescence of color centers, has bec ome an ac tive a rea of s tudy. It was discovered in the lat e 1950s th at excitons in pure alk ali halid es are self-trap p ed and ca n themselves produce luminescen ce. Although alkali halide s are im p ortant from the poin t of v iew o f basic research as mentioned above, the ir luminescence is rarely utili zed in pract ical app lications. This is beca use alkali halides are wa ter so lub le and ha ve low melt in g poin ts, so that they are
217
Fundamentals of Phosphors
218
unsuitable as hosts of practical phosphors. Alkali halide phosphors pr esently in common use include N aI:TlTand CsI:Na T only as cliscussed below.
2.5.2
Intrinsic optical properties 2.5.2.1 Band structure and exciton
All alkali halides have band structures of th e direct transition type. Both the bottom of the conduction band and the top of the valence band a re located at the k = 0 p oint (T p oint) in k-space. The bandgap energies Eg are as follows. The largest is 13.6 eV for LiF, and the smalles t is 6.30 eV for KI with RbI and CsI ha ving almost the same value. For N aC!, a rep rese n ta tive alk ali halide, Eg is 8.77 eV. E; decreases with incre asing atomic number of the ca tions or anions making up the alk ali halides. A little bel ow the fundamental absorption ed ge, sharp absorp tion lines du e to exciton s are ob served. The va lence band is composed of p electro ns of th e halogen ion s, an d it is split into two compo un ds, with the inner quantum numbers representing total an gular m omentum j = 3/ 2 and 1/ 2. Corresponding to this splitting, two sharp exciton absorp tion lin es are observed . The binding en ergy of excitons is 1.5 eV (NaF) at its largest and 0.28 eV (NaI) at its sma lles t; it is 0.81 eV in NaC!. I
2.5.2.2 Self-trapping of excitons and intrinsic luminescence' Excitons in all alkali halid es except iodides d o not move aroun d in the crystal, unlike excitons in IIb-VIb and Illb-Vb compounds; these excitons are self-tra pped immed iatel y a fter their creation as a result of very strong electron-lattice coupling . Self-trapped excitons emit luminescen ce called intrinsic luminescence. In iodides, excitons can mo ve freely for som e di stance before becoming se lf-trapped and emitting their intrinsic luminescence. Before discussing the self-trapp ing of excitons, let us conside r the self-trapping of po sitive holes. In alkali hal ides, holes do not move freel y, but are sel f-trapped and form VK cen ters. As show n in Figure 47, the V K center is a state in which tw o nearest -neighbor an ions ar e attrac ted to each other by trapping a positi ve hole between them , so that it tak es the form of a m olecular ion denoted as X2- . Th e self-trapped exciton is a state in which an ad d itional electron is trapped by the VK center. The spectral position of the intrinsic luminescence is shifted considerabl y tow ard lower en ergies from the exciton ab sorption . Thi s is because an exciton undergoes a large lattice relaxation, emitting phonons to reac h a self-tra pped state. Emission spectra are composed of tw o broad bands in most cases. The luminescence of the short-waveleng th band, call ed a-luminescence, is polarized parallel, w hile that of the long-w avelength band, called n-lu m ine scenc e. is polarized perpendicular to the mol ecular axis of VK centers. In NaC!, the peaks of these two types of luminescence are a : 5.47 eV and rt : 3.47 eV. 0'luminescen ce is due to an allowed transition from the singlet excited state, while nluminescence is due to a forbidden transition from th e triplet excited stat e. As a result, the d ecay time of the former is short (ab out 10 ns). while th at of the latter is relatively long (abou t 100 us).
2.5. 3 Color centers' In alka li halides, lattice defects th at trap an electron or a h ole ha ve abs orption bands in the visibl e region, and hence color th e host crysta ls. Therefore, such d efect s are called color centers. Figure 47(a) illu st rates the principal electron-trapping centers F, FAt F', M, and MI. Principal hole-trapping centers VKJ VKA' H, an d H A are illustra ted in Figu re 47(b). Most of color cen ters emit luminescence. The F cen ter ha s an electronic energy s truc tur e ana logo us to the h ydrogen atom . It sh ows strong absorp tion and em ission due to the s H p transition.
Chapter two: Principal phosphor materials and their optical properties
219
€) Cation OAnion •
Ot her al kal i metal ion (a)
(b)
Figure 47 (a) Pr incipal elec tro n trapp ing centers and (b) principal posi tive hole trapping cen ters in alkali halides.
In Na C!, the absorption is a t 2.75 eV an d the emi ssion a t 0.98 eY. A lka li h alide crystals con tain ing some kind of color centers, typically an FA center, ar e used as materials for tunable so lid-sta te lasers ope rating in th e n ear -in frared region .
2.5.4 Luminescence centers of ns2- type ions For some time, ns--type (Tit-ty pe) ion cen ters in alkali h alides have bee n inves tiga ted in de tail from both experim en tal and th eoretical points of view as being a typical example of an impuri ty cen ter in ionic crystals (See 2.1). Almos t all ns--type ions, i.e., Cat, In ', Sn 2+, Pb 2+, Sb 3+, Bj3+, Cu-, Ag, an d Ale, h ave been studied; Tl" has been studied in signi fican t de tail. For the se ion centers, the absorption and emission due to the S2 H sp transition, their sp ectral shapes, the polariza tion correla tion be tween the absorp tion and th e emission, and th e d ynamica l Jahn-Teller e ffect in excited states d ue to electron- la ttice int eractions have been in vesti gated tho rou ghl y. The range of phenom ena have been w ell elu cida ted in th e litera tu re an d are an example of the remarkabl e con tribu tions that op tica l spectroscopic studies h ave ma d e to our understan ding o f impurity centers. H ow ever, the lumin escence in alka li h alides is almost w or th less from a p ractical point of view of application, so that fur ther detailed d escrip tion w ill no t be prov id ed her e. Th e only exampl e of practical use of these ma ter ials is N aI:Tl+a nd CsI:Tl+ single-crystal p h osphors (near-ul traviolet to blue emitting) , which h ave bee n used as scin tilla tors. For alkali halides such as Na I and CsI, it is easy to grow large single crys ta ls so tha t they are suitable for these applications in p art icle and high-energy rad iation detection.
Fundamentals of Phosphors
220
( b)
550
500
450
400
350
300 Wavelength (nrn)
260
220
Figure 48 (a) Emi ssion sp ectru m (300K) an d (b) excitatio n spe ctru m of CsI:Na+. (Fro m Hsu, O.L. an d Bate s, C. W., Phys. Rev., B15, 5821, 1977. With permission .)
2.5.5 Luminescence of isoelectronic traps An exa m p le of an isoelectron ic trap (See 1.4.4) show ing luminescence is CsI:Na+. It emi ts bLue luminescen ce with high efficiency wh en excited by h igh-energy radiation. ' Presentl y, CsI:Na+film s prepared by a va por d eposition m ethod are used for X-ray image in tensi fiers. The concentratio n of Na' is very low, 6 ppm being th e optimum value. Emission and excit ation spectra of Cs I:Na+ are show n in Figure 48.4 The pe ak of the excita tion spe ctrum agrees well wit h the calcul at ed value of an exciton bound to an isoelec tro nic trap Na ' . The luminescenc e is considered to arise fro m th e relaxed exciton s tate of this bou nd exciton, w h ich is assumed to be a VK A cen ter trapping an electron. If so, the lumin escence sho u ld be polarized parallel or perpendicular to the molecul a r axis of the VK A cen ter. How ever, no polari zat ion was observed in expe rimen ts, leaving the s truc ture of the em itting sta te undet ermin ed as yet.
References 1. Review articles : (a) Song, K.5. and Willia ms, R.T., Self-Trapped Exciions, (Springe r Series in Solid -State Sciences 105), Springer-Verlag, Berl in, 1993. (b) Kan 'no K., Tanak a, K., an d Hayashi, T., Rev. Solid State Sci , 4(2/3), 383, 1990. 2. Review articles: (a) Sch u lma n, J.H . an d Comp ton, W.D., ColorCenters in Solids, Pergamon Press, Oxford , 1963. (b ) Fowler, W.B., Ed ., Physics of Color Centers, Academ ic Press, New York, 1968. 3. Brinckman n, P., Phys. Lett., 15, 305, 1965. 4. Hs u, O.L. and Bates, C.W., Phys. Rev., B15, 5821, 1977.
chapter two - section six
Principal phosphor materials and their optical properties Hajim e Yamam oto
Contents 2.6 IIa-VIb compounds 2.6.1 Int ro du ction 2.6.2 Fundamental physical prop erties 2.6.2.1 Crystal structures 2.6.2.2 Band structures 2.6.2.3 Phonon energies and dielec tric cons tants 2.6.3 Over view of activa tors 2.6.4 Typica l examples of applications 2.6.4.1 Storage and stim ulation 2.6.4.2 Cathode-ray tubes 2.6.4.3 Electro lu minescen ce (EL) 2.6.5 Host exci ta tion process of luminesc ence 2.6.6 Prepara tio n methods of p hosphors 2.6.6.1 Sulf id es 2.6.6.2. Selenid es Referen ces
221 221 222 222 223 224 224 228 228 229 230 231 232 232 234 234
2.6 IIa- Vlb compounds 2.6.1
Introduction
Ph osp hors based on alk aline ear th chalcogenid es, mostly sulfid es or selenides, are one of the oldes t classes of p hosphors. Many investigations we re made on these p h osphors from the end of 19th cen tury to the beginning of 20th cen tury, particu larly by Len ard an d coworkers as can be seen in Reference 1. For th is reason, these phosphors a re s till ca lled
Lenard phosphors. Even with their long his tory, progress in und ers tanding th e fundamental ph ysical properties for these phospho rs has been qui te s low. There are good reasons for th is: these materia ls are h ygroscopic and produce toxic H 2S or H 2Se w hen p laced in con tac t
221
Funda mentals of Phosphors
222 Tabl e 13
Co mpounds
The Lat tice Con stant, Dielectr ic Constants, an d Pho no n Freq uen cies of Ila-Vlb Compounds" Lattice cons tan t (n m)
MgOa CaO CaS CaSe SrO SrS SrSe BaO BaS BaSe
0.4204 0.4812 0.5697 0.5927 0.5160 0.6019 0.6237 0.5524 0.6384 0.6600
Dielectric cons ta nts Ell
9.64
n.i-. 11.6' 9.3 7.8 13.Ib, 14.7' 9.4 8.5 32.8' 11.3 10.7
E.. d
2.94 3.33b, 3.27' 4.15d 4.52d 3.46d 4.06d 4.24d 3.61b ,3.56J 4.21d 4.41d
Phono n frequ ency (crrr")
COra
(J)w
401 295b, 311c 229 168 231b, 229' 185 141 146' 150 100
725 577b, 585' 342 220 487', 472' 282 201 440' 246 156
No te: The nota tion E" and E". ind icate s tati c and optical dielectric con stan t, and
(J)ro
and
(J) LO
the
freq uency of the tran sve rse and longitud inal optical ph ono n, resp ectivel y. ., From Reference 18. b
c d
From Refere nce 19. From Referen ce 20. From Reference 21.
with m oisture. Further, their luminescence proper ties are sensitive to imp uri ties and n on stoichi omet ry. Such p roblems make it d ifficult to obtain controlled reprod ucibi lity in perfor m ance w he n technologies for ambiance co n tro l and ma terial p uri fica tion we re in sufficient. In th e 1930s and 1940s, research on th is fam ily of mat erials was carried o ut active ly to mee t de ma nd s for mil itary uses, m ostly for the d etection of infrared light by the photostimulation effect. After thi s pe riod, the se m at erials we re ign ored for man y years, until arou nd 1970 w he n Lehmann>" d emon st rat ed th at the al kaline earth chalcogen id es could be synthesized rep rod ucibly. He als o sh owed that an excep tio nally la rge nu mber of ac tiva tors can be in troduced into CaS, man y of wh ich exhi bit hi gh lu min escence efficiency. H e showed th at the se features of CaS phosph ors we re attracti ve to ap plications and seem to compen sat e for th e d rawb ack cause d by thei r hygroscopi c natu re. Accordingl y, Leh m ann 's work rev ive d research in teres t in these materials and this activity has continued through th e ye ars. In the 1980s, so me attempts we re made to app ly CaS p hosphors to CRTs,?-JOAlso in thi s pe riod , single crys tal s w ere grown for many of th e IIa-VIb compou nds .t' <'? The band structure was investigated an d the basic optical parame ters were ob taine d in these single cr ys tals.l l - H .I ?
2.6.2 Fundamental physical properties 2.6.2.1 Crystal structures Mo st of the IIa-VIb family have Na Cl-type struc tures, excep t for MgTe, w hich crystallizes in the zinc-blende struc tu re, and BeQ , w hich favo rs the w ur tzite lat tice." The lat tice param ete rs are sho w n in Table 13. The compo unds in the N aCl-type struc ture can form solid so lutions in a w ide ran ge of compos itio n. As a consequence, the emission color can be varied by changin g the host composition as we ll as the ac tiva tor species and conc en tra tion. Such d iversity is one of the adva ntageo us fea tures of the IJa-VIb compoun d s.
Chapter two:
...... -
Principal phosphor materials and their optical properties
223
~
.
0.2
c;:l
o
E
0 ...... ro
<;:»
c-,
co l-<
0
3
(J)
c
~
15
-0.2
-0.4 L
11
r
15
x
K
r
Wave vector k Figure 49 The ban d str ucture of SrS calcu lated by the self-consis ten t APW (augmented plane wave) method. The energy scale is show n by the a tomic unit (= Ryd berg cons tant x 2 = 13.6 x 2 eV). N ote that energy valu es ob tained in the figure (e.g.. band gap energies) do not necessarily ag ree w ith expe rime ntally obt ained values. (From Ha segawa, A. and Yan ase, A. , J. Phl/s. C, 13, 1995, 1980. With pe rmission.)
2.6.2.2
Band structure
The band structure calculated for SrS is shown by Figure 49. 16 The first thing on e should notice is that the absorp tion edge is of the indirect transition type,13.16 with the va lenc e band ma ximum at the Tpoint (k = 000), an d the condu ction band minimum at the X point (k = 100). However, the op tical transition corr es po n d ing to thi s edge is forbidden because phonons having the momentum and p arity required to induce a phonon-assisted direct tran sition are n ot av ailable in the NaCI-type structure . The lowest direct transit ion occurs at the X point of the valence band and not at the Tpoint. In SrS, the con d u ction band is composed mainly of 5s and 4d orb itals of the Sr atom, and the Tpoint ha s m ainly 55 cha racter while the X p oint possesses 4d character. The 55 and 4d or bitals are clos e to each other in en ergy, allowing the X point to be located a t lower en ergy th an the T point when the SrS crystal is formed. The sa m e feature is also
Fundamentals of Phosphors
224
Table 14 Th e Obs erved Band gap Energie s of IIa-VIb Compounds (in eV)
Compound CaO CaS CaSe SrO SrS SrSe BaO BaS BaSe
Eg(X c - r Jb
Bandg ap energies at 2Kd Gap at X point Gap at r point
4.434 3.85 4.32 3.8 13 3.806 3.421
6.875 5.343 4.898 5.793 4.831 4.475 3.985 3.941 3.658
5.80 6.08 5.387 4.570 8.3 5.229 4.556
Absorption edge at 300Kc 4.20
4.12 3.73 3.49 3.20
• Dat a from Kane ko, Y. and Kod a, T.. f. Crystal Growth, 86, 72, 1988. b Th e ga p between X point in the conduction band and Tpo int in the va lence band. c Dat a from Mori mot o, K., Masters Thesis, The Uni versi ty of Toky o, 1982 (in Jap anese).
found in Ca and Sa chalcogenides. In contrast to this, Mg at oms have 3d orbitals that lie som e 40 eV hi gher than 3s; as a consequence, MgO has a direct bandgap . The band gap energies mea sured a t 2K13 and th e ab sorption ed ge energies at 300K17 for IIa-VIb compounds are shown in Table 14. The exciton binding energi es of about 40 to 70 meV are obtained a t Fpoint. 13 The spectral shape of optical absorp tion near the edge follows Urbach 's rule qu ite we11. 13•22 Th at is, the absor p tion coefficient a is expressed as a function of photon en ergy E by the following formula. (28)
where av, c. and En are material constants. For SrS, av = 4 x 107 crrr', (J = 1.07, and Eo = 4.6 eV, which is nearly equal to the lowest exciton energy at the X point.'? Th is fact in d ica tes that th e ab sorption tail at room temperature ap pears as a result of int er acti on between excitons and phonons a t the X poin t. Th e absor p tion edge due to th e forb idden indirect tr ansition is masked by the absorption tail of the direct tr an sition. At sufficien tly low temperatures, however, th e ind irect absorp tion ap pea rs and reveals a sp ectral shape cha rac terized by a oc (Eo - £)2.
2.6.2.3 Phonon energies and dielectric constants Th e dielectric constants an d optical phonon energies obtained from in frar ed reflection spectra of sin gle crystals" are given in Table 13.
2.6.3
Overvi ew of activators
Activa tors that can be introduced into CaS and their m ain luminescence properties ar e su m marize d in Tabl es 15 and 16.4 Elements n ot appearing in these tabl es w ere found to be nonluminescent.' However, radioactiv e elements except for U and Th, the platinumgroup elements, an d Hg and TI have not been exam ined . As for th e description of lumine scence p roperties, it is noted that the tables present only typical examples sin ce the luminescence spectra an d deca y chara cteristics depend on the activa tor concentration. As an example of the activa tors listed in Table 15, the luminescen ce and absorption spectra of Bi3 + in CaS due to 6s2 ~ 6s6p transitions ar e sh own in Figure 50.6
n ;:,,-
'"
~
'" '"
Table 15
Acti va tors
Coactiv ators-
Lum inescen ce color
Luminescen ce sp ec tr um"
0
Nothing ct, Br cr. Br, Li Nothing Cu. Ag F, u. Na, Rb, P, Y, As No thing, or Cu , Ag F, ci Br F, ci. Sr ci Sr, u. N a No th ing Na, K F, ci Br N othing, or u . Na, K c i Sr, I u. K, ci I F, ct. Br, I, P, As, Li u. Na , K, Rb
Bluish- green Yellow Yellowish- green Yellow Red to IR Violet to blue Or an ge, red, an d yellow Yellowi sh- oran ge Bluish-white Violet UV to IR Orange Green Yellowish-green Bluish-white Blue to bluish- green UV Blue
Band Band Band N arrow band Broad band Two bands Broad band Band Broad band Band Very broad band Broad band Band Band Broad band Two bands Narrow band Narrow band
P Sc Mn Ni Cu Ga As Y Ag Cd In Sn Sb La Au Pb Bi
8'~
Activ at ors and Coactiva tors in CaS and Luminescen ce Properti es
Note: The rare-earth elements are listed in Table 16.
Peak (eV) 2.53 2.13 2.18 2.10 -
2.10 -
2.00 2.8
Typ e of d eca y cur ve
Deca y time cons tant"
Exponential H yp erb olic
6.5 us -500 IlS 4 ms
-
3.40 2.77
'" '" s
"'::S ;:,,0
-
50
us
:::,
;;;-
-200 us -1 ms
S ' Vi :::,
~
..... ;:,,-
-
2.3 2.27 2.55
:::..
"'::S 0
Expon ential
Hype rbol ic Exponential Hyperbolic Hyperbolic Hyperbolic H ype rbolic
'" S· ..;::;'"' . ;:,,-
-
Hyperb olic Hyp erb olic H yp erb olic
'U
- 500 IlS 0.8 IlS - 200 J.lS - 10 J.lS -1 us - 1 us
::;. '" -g.....
;::;. :2..
"'::S
C3
'1:3
'" ::t
B;'
• Efficient co-activators are shown in bold letters. b A spectrum changes depending on a co-activator. The period when the luminescence intensity falls to l ie times the initial value. From Lehmann, W., J. LUlIlil1csc., 5, 87, 1972. With permission. C
N N
CJl
N N C\
Table 16
Ion Ce 3+ Py3+ (Nd 3+) ' Sm 3+ Sm 2+ Eu 2• Gd 3• Th3+
a
Rare Ear th Activa tors in CaS and Luminescence Prop erties
Luminescence color
Luminescence spe ctrum
Type of decay curve
Decay time constan t"
Green Pink to green
Two bands, peaked at 2.10, 2.37 eV Lines , green, red , and IR
Hyperbolic G reen: expo ne n tial
-IllS 260 us
Yellow Deep re d (low temperatur e) Red
Lines, yellow, red , and IR Lines, green, red , an d IR Narrow band, peaked at 1.90 eV Lines, UV Lines, UV to red Line s, yello w, bluish-gree n, and IR Lines, blue to IR Lines, UV, gree n, and IR Line s, blu e and red Lines , IR Band, peaked at 1.66 eV
Yellow: expo nen tial
5 IlS
Dy' · H 0 3. Er3+ Tm 3+
Green Yellow & bluish-green Gre en ish-white Green Blue wit h some red
Yb 3. Yb z.
Deep red
Hyp erb olic Exponentia l Gree n: expon ential Yellow : (1+t/ ,)-l Gree n: (1+t/1:)-l Gree n : (1+t/ , )-1 Blue: exp on ential
-I lls 1.5 ms 1.8 ms 150 lls 150 IlS 370 us 1.05 ms
Hyperb olic
- 10 us
Lum inescen ce of N d" is not ide ntified .
b The peri od wh en the lum inescence intensity falls to 1 I e time s the ini tial value. For the type expressed by (1 +t !t)-l, the time cons tan t mean s "C.
From Lehmann, \Al.,
J.
Lumines c., 5, 87, 1972. With permission.
~ ;:s i=:l.. :::>
s
~
~
:::>
Vi'"
~
~
oVl
"1::l ;:s-
a
;;i
Chapter two: Principal phosphor materials and their optical properties
227
>
Q)
r-r--
>
Q)
...... 100 V)
o o
100
(V\
C ::J
",
.0 I.-
rv
c
50
0
.... ------- ---,_ \
V) V)
E l..W
I
Ca S Pure
0 2. 0
,. tI
3.0
4.0
/
I
I
...
~
/
c 0
ASS.
50 C5.. I.-
0
Vl
.Cl
«
a 5.0
Quantum En ergy, eV Figure 50 Lum inescen ce an d a bsorp tion sp ectra of CaS:Bi3+ (0.01%),K+ a t room temperature. Th e hatched zone ind icates the luminescence spectrum. Th e solid line sho ws the absorp tion spectrum of CaS:Bi3+,K+, and the broken line that of pure CaS. (From Lehmann, W., Gordon Research Conference Report, July 1971. With perm ission .)
One significant difference between IIa-VIb compounds and IIb -VIb compounds is that the concept of the donor or acc eptor in the latter is not app licable in the former. For example, donor-acceptor pair luminescence is not observed for IIa-VIb compounds doped with (Cu ' , Cl") or (Cu '. A ]3+ ) pairs. Another example is that a lka li ions act as "co-activators" of Ag or Au activ ators, ju st as halogen ions do. As thi s exa m p le indicates, many activator / co-activator com bina tion s violate the charge compensation ru le in an ionic cry stal. In fact, the co-activators d o not playa donor role similar to that of h alogen ions in IIb-VIb compounds; inst ead, these ions help the activator diffuse into th e ho st lattice by creating lattice defects. Th ese obse rva tions are presumably relat ed to th e s tron ge r ionicity of IIa-VIb compounds comp a red to IIb -VIb compounds . As is the case wi th alkali halides, luminescence centers are localized in IIa -VIb compounds. As described p reviously, many materials of thi s g ro up have h igh luminescence efficien cy. Cathodoluminescence e fficie ncies for variou s phosphors are sho w n in Table 17.5-7 Ab ove all, CaS:Ce3 + sh ows an efficiency nearly as hi gh as ZnS:Ag,Cl or ZnS:Cu,Al, which a re the m ost efficien t cathod e-ray phosphors. The e fficiencies of CaS:Eu 2+,Ce3+ and MgS :Eu 2+ are much h igher than th e efficiency of Y202S:Eu 3+ (abou t 13% in energy efficienc y) and very go od red-emitting phosphors. (Here, Ce 3+ acts as a sen sitizer of Eu 2+luminescence .) Luminance of CaS:Eu 2+,Ce3+ is, however, o nly 80% that of Y202S:Eu3+, because the emission peak of CaS:Eu 2+,Ce3+ is at 650 nrn . while th at of Y202S :Eu 3+ is at 627 om. It should be noted that the d at a lis ted in Table 17 were measured a t low electro n-beam current density. It was found th at for the Eu 2+, Mn 2+ , and Ce 3+ em issions , the efficiency de creases with increasing cu rren t d en sity. This sa tur a tion in lumin escen ce intensity can be reduced by increasing the ac tiva to r concen tra tion, but not elimi na ted; th e reason for the sa tura tion is not clear as yet. As show n in Table 18, the luminescence peak shifts to sho r te r wavelengths in Eu 2+ and Ce 3 + (f-d transitions) and in Mn 2+ (d-d transition) when th e host latti ce is varied from CaS to SrS to BaS. This sh ift is re asonable from theoretical p oints of view because the
Fundamentals of Phosphors
228 Table 17
Ca tho d olu m inesce nce Efficien cy of Alkali Earth Chalcogen id e Phosphors Ene rgy efficiency (%)
Phosphors
Lu m inescen ce color
16 16
MgS:Eu Z+ Ca S:Mn2 + CaS:Cu CaS:Sb CaS:Ce3+ CaS:Eu Z. CaS:Eu 2+,Ce3 + CaS:Sm 3+ CaS :Pb 2• CaO :Mn 2+ CaO:Pb2+
Orange-red Yellow Blue-violet Yellow-green Gr een Red Red Yellow
18 18
22 10 16 12 (+IR) 17
UV
5 10
Yellow
UV
Note: The efficiency was measu red at room tem perature relati ve to a standard materia l. For MgS:Eu 2+, excitation w as made a t an accelera ting vo ltage of 18 kV and a curren t de ns ity of 10-7 A / cm 2• The s ta nda rd was Y20 2S:Eu 3+. For o ther phosp hors, excitation was mad e at 8 kV and 1 ~ A /cm 2 or less. TIle s ta nd a rd wa s P-1, P-22, or MgWO•. The meas urem en t erro r is ±1O%. From Leh mann, W.,]. Elect rochem. Soc., 118, 1164, 1971; Lehmann, W., Gordon Research Conjereucc Report, July 1971; Kasano. H., Megu mi, K., and Yamamoto, H ., Abstr. [pn. Soc. App/. Phys. 42nd Meeting, No. 8P-Q-11, 1981. With perm iss ion. .
Table 18
H ost Ca S SrS BaS
Sp ect ral Peak Shi ft by a Host Material for Eu 2+, Ce 3+, and Mn 2+ Activation
Pea k wa vel en gth (run) Ce 3, Eu 2• Mn2' 0.1% 0.04% 0.2% 651
616 572
520 503 482
585 - 550 - 541
Dist anc e between the near est n eighb or ions (nm) 0.285 0.301 0.319
From Kasano, H., Megum i, K., and Yamamoto, H. , Abstr. [ pn. Soc. Appl. Phys. 42nd Meeting, No . 8P-Q-ll, 1981. With pe rmission.
crystal field par ameter (10Dq) (see 2.2.1) de cre ases in th e ab ove order. See also 2.2.5 for Mn 2+ luminescence and 2.3.3 for Eu 2+ and Ce3 +luminescence.
2.6.4 Typical examples of applications 2.6.4.1
Storage and stimulation
It is an othe r remarkable fea ture of the ll a-Vlb com pounds that they show var ious phenomena relat ed to traps, e.g., storage, photostimulati on (infrared stim ulation), and photoqu enching (see 1.7). Cao7Sro3S:Bi3+,Cu w as d eveloped as a particul arl y efficien t s torage mat eri al." This mat erial doped w ith Bi3+show s bluish- violet emi ssion due to the 6s6p ~ 6p2 tran sition of Bi3+; the ad di tion of Cu shifts th e emission toward longer w avelen gths and im proves its luminance (see 12.2). CaS:Bi 3+, repor ted as early as in 1928 by Lenard,' is one of the best kno wn of all CaS p hosp hors. Thi s phosphor requires the Bj3+luminescent cen ters to be co-activated by an
Chapter two: Principal phosphor materials and their optical properties
•
•
.
Conduction band IR light
1.3 eV
UV
I ight
~~
229
I.......~----
•
!:l.!!r1 in-
(Sm 3 +) <e"
esenc e
- - t-
t-
Ce 3 + Ce4+ :/:;,/ _.( v / / / / / /./ »:>; ' /// /
,
F 7I 2
2 F S12
- -,-(i- - IS 0
//~ ~/:; :,r-::
.r >: »:
2
»:
0
Schema tic d iagram of infrared st imulati on mechanism of SrS:Ce h ,Sm 3+ The ori gin al figu re has been sim p lified . (From Keller, S.P. and Pett it, G.D ., Phys. Reo., 111, 1533, 1958. With perrn ission .)
Figure 51
alkali metal ion . The luminescence and absorption spec tra of CaS:Bi 3+,K+ are show n in Figure 50.4 Photostimulat ion a ttrac ted atte n tion as a means to d et ect infrared light. During World War II, an enormou s volume of research w as car ried out on th is p heno menon in [apan -? and the U.S. for mi litary purposes. Some of the materials developed in this period includ es (Ca,Sr)S:Ce 3 ',BP+,23SrS:Eu 2+ ,Sm 3+ and SrS:Ce 1 ·,Sm 3+ .24 In these co mpositio ns, the primary activa tor (i.e., Ce" or Eu 2 +) de ter m ines the luminescence spectrum, while the auxiliary activator (i.e., Bi3 +or Sm 3+) for ms the necessa ry trap s th at de ter m ine th e sti m ulable wavelength in the in frared . These waveleng ths range fro m 0.8 to 1.4 urn for Sm 3+ and 0.5 to 1.0 urn for Bi3+. The stim ula tion mechanism propose d for SrS:Ce 3 +,Sm 3 + is sche ma tica lly show n in Figure 5J.25 He re, one assum es tha t Sm 3+ forms electron trap s and Ce 3+ forms hole traps. Electrons trap ped by Sm 3 ' ions are released to the conduction band by absorption of infrared light of energy corresponding to the trap depth. After migrati on throu gh the lattice, some of the electrons are retrap ped by Ce1 . , which already ha s trapped ho les. The electron and the hole recombine in Ce'" , releasing ene rgy characteristi c of the Ce3 + lu m inescence. In SrS:Eu 2 +,Sm 3 +, it is thou ght that Eu 2 +plays a similar role as Ce3+. The concept of an activa tor ion working either as an electron or a hole trap is also ap plica ble to othe r m at er ials wi th localized luminescence cen ters .
2.6.4.2 Cathode-ray tubes Applica tion of CaS ph osphors applied to CRTs have attracted atte n tion because of their high efficiency and diversi ty in emission colors." Green-emi tting CaS:Ce 3 • with w eak temp eratu re gu en ching has bee n tes ted for ap plica tion in h eavily load ed projec tion tubes.s" However, CaS:Ce 3+ was foun d to be not sa tisfacto ry for projection tube uses becau se it shows serious luminan ce satura tion at high current d en sit y. Amber-emitting (Ca,Mg)S:Mn 2+ has been tested in termin al display tubes.!? The commercial applica tion of this phosphor was also abandoned beca use its efficiency and persisten ce were found to be unsatisfactory. Screen ing technolo gy for this family of phosphors has improved, bu t evolution of toxic H 2S gas in th e m anu facturing and reclaiming processes remains a serious impediment for the widesp read application of these phosphors.
Fundamentals of Phosphors
230 Tabl e 19
Lum inescence color
Phosphors CaS:Ce CaS:Erl, CaS:Th3+ CaS:Eu3+ SrS:Ce3+ SrS:Mnl + SrS:Cu,Na 1•
a
(a) Color and Lu minance of DC Electrolum inescence of CaS and SrS Po wder Phosphors
Green Green Green Red Bluish-green Green Green
Lu mina nce (cd zm-) (Applied voltage is shown in paren thesis) Continuous drive Pu lse d ri ve" 1700 (70 V) 300 (80 V) 17 (80 V) 100 (50 V) 400 (70 V) 270 (120 V) 270 (80 V)
600 (110 V) 85 (120 V) 50 (120 V) 17 (120 V) 200 (110 V ) 17 (J20 V)
Pulse w id th is 10-20 us. D uty is 1- 1'/, 'X,.
From Vech t, A., J. Crystal Growth, 59, 81, 1982. With pe rm iss ion .
Tabl e 19
(b) Properties of Thi n-Film AC Electro luminescence Devices Using CaS or SrS Phosp hors
Che m ical composition CaS:Eu l + CaS:Ce3+ SrS:Ce3 +
Maximu m luminan ce (cd Zrn-)
Ma ximum efficiency (1m/W)
Lum inescence color
200 150 900
0.05 0.1 0.44
Red Green Greenish-blu e
Note: The drivi ng voltage has a frequency of 1. kHz. From Ono. YA ., Elcctrotuminceccncc Di,;play,;, Series for Information Displ ay, Vol. 1, World Scie n tific Publishing, Singapore, p. 84, 1995. With permission.
2.6.4.3
Electroluminescence (EL)
Phosp ho rs based on CaS and other IIa-VIb compounds are important electro lumines cent mat erial s because th ey can provide colors other than the orange color p rovid ed by ZnS:Mn2+(See 1.10). Table 19(a) shows the properties of the EL cells made of fine p hosphor p art icles manu factured by Phosphor Products CO.27 (See also [Sections 2.6.6.1). Am on g the m at erials list ed in this table, CaS:CeJ~ is nearly as bright as Zn S:Mn 2+. Altho ug h the d egradat ion has been improved, the lifetime of this material is s till at an impradica l leve l. In th in -film elec tro lum ine scen t devices, CaS and SrS phosphors p rovid e luminances th at are high er th an ZnS phosphors in the red an d green-blue region s. The luminan ce an d lumino us efficiency of th e three primar y colors obtained by IIa-VIb compo unds are shown in Tabl e 19(b).28 As SrS:Ce EL w as developed in 198429, formatio n p rocess of SrS:Ce thin-films has been in vesti gat ed by vario us techniqu es, e.g., electro n-bea m deposition , sputtering, hot-wall d eposit ion, and molecu lar bea m epi taxy. A recent study has shown that thin-film formation in excess sulfur promot es introduction of Ce 3+ ion s in SrS latti ce b y crea ting Sr vaca ncies". By th is optimi zati on , lumin ou s efficiency higher than 1 lm / W was achieved at 1 KHz d rivi ng freq uency" H ow ever, the emission color of SrS:Ce is not sa tu rated blue, but greenish-b lue w ith color coord ina tes x =0.30 and y=0.52 in the above case-", Lu minescence of more satur ated blue wi th x=0.18 and y =0.34 can be ach ieve d either by codoping of Rb as a charge com pensa to r-" or by su p p ly ing H 20 vapo r d u ring SrS:Ce film depositio n" . A th in- film of SrS:Cu shows better performan ce of blue EL33. Mod erat ely high lu minou s efficien cy of 0.22 Im /W was obtained a t 60 H z driving with sa tura ted blue of color coo rd ina tes, x=0.15 and y=0 .23.
231
Chapter two: Principal phosphor materials and their optical properties
Ad d ition of Ag to Cu fu r ther improves color coor d in a tes to x=0.17 and y=O.13, which is close to the prim ary blue in CRTs. Lumin ous efficiency of SrS:Cu, Ag was re ported to be 0.15 lm/W at 60 H z driv ing. Photoluminescen ce stud ies ha ve shown tha t a SrS:C u thin film h as th e e m iss ion peak at aro un d 480 nrn, while CaS:C u shows th e pea k a t arou nd 420 n m . Accord ingly, th e emission peak can be tuned by th e forma tion of the so lid so lu tio n (Sr, Ca) :Cu 34 . However, EL by the so lid so lution has not been ob tained ye t, though EL of CaS:C u thin film s wa s reported rece n tly> ,
2.6.5 Host excitation process of luminescence Fig ure 52 shows a luminescen ce sp ectrum of SrSe containing trace amounts of Ba 2+ . 13 Th e line at 3.74 eV can be assigned to a free exci ton a t th e ind irect bandgap (i.e., an indirec t excito n), w hile th e broa d band at low er energy arises fro m th e reco mb in at ion of a local ized indirect exciton tra pped by the short-ra nge potentia l of Ba2+ ac ting as an isoelec tro n ic impuri ty. Presumably, the loca lized indirect exci tons a lso prod uce lu m in escen ce by other types of ac tiva tors. It was observed that th e exci ta tion spectrum of CaS:E u 2 + in the vacuum-UV region shows tha t the lumin escence efficiency inc reases w ith a s tep -like shape a t the ene rgy position twice that of the direct exciton transition (Figure 53).22 Thi s fac t shows tha t an exci ted elec tron can efficien tly create two direct exci tons th rough an Auger process. Direct excitons thus created are sca ttered an d transfor m ed to ind irect exci tons. w hic h even tu all y tra nsfer energy to an imp ur ity producing lum in escence. These ex perimen tal res u lts p rovide ev idence th at exci ta tion energy given by the ba nd- to-band transition is efficien tly transferred to ac tiva tor s via exci tons in alkali earth su lfid es and selen ides.
I
I I I I , . -'
f, ,
,, / I" I I/
01
_
I ....
"-
I
I
I I I
(X - r)!I I
I I I I
o
L-..L_
3.2
_
-----'----_
_
- ' - -_ _...l..--_ _-'------_-------'=--------U-..-- - L_
3.4
3.6
_---'
3.8
Photon energy (eV ) Figure 52 A lumin escence (a solid line) and an excitation spectrum (a broken line) of SrSe con taining a trace of Ba 2+ at 2K. The no tation (X-r) indicates a recom bin ation transi tion of an exciton from the X point of the cond uction band to the T point of the valence band. The ver tical lines above the s pectra show phonon structures. The broad emis sion band is due to local ized exci tons at the isoe lectronic Ba cent er. (From Kaneko, Y. and Koda . T., f. Crystal Grototh, 86, 72, 1988. With permission.)
232
Fundamentals of Phosphors
CaS:Eu2 + (O.lmoL%)
o
10
20
30
40
Photon Energy (eV) Figure 53 An excitation spectrum of CaS:Eu z+ (0.1 mol%) in the vacuum UV region at 77.1<. (From Kaneko, Y, Ph.D. Thesis, The University of Tokyo, 1984 (in Japanese). With permission.
2.6.6 Preparation methods of phosphors 2.6.6.1
Sulfides
The preparation of sulfide phosphors can be classified into two methods; one entails the sulfurization of alkaline earth oxides or carbonates and the other involves the reduction of sulfates. The following agents are known to sulfurize or reduce the starting materials; the sulfurizing agents are HzS, CS z' S+C (in many cases, starch or sucrose are used as the source of carbon), and Na ZC03+S (or Na.S). and the reducing agents are Hz and C. In addition to these agents, fluxes are often added to the starting materials at the level of several to 10 wt°!<). Typical fluxes are alkali carbonates, alkali sulfates, and NH 4Cl. Lithium compounds are particularly effective in promoting crystal growth and diffusion of activator ions into the sulfide lattice. This is probably because Li', which has a small ionic radius, enters interstitial sites and generates cation vacancies; ionic diffusion is accelerated through these means. Fluxes that promote crystal growth and ion diffusion effects remarkably, however, may have a side effect to degrade luminescence efficiency because the constituent ions of the fluxes (e.g., Li' F-) are likely to remain in the phosphor lattice as impurities. The material and the quantity of a flux are selected by considering these two kinds of effect. Typical preparation methods are shown below for CaS:Ce 1 • .36
(29)3
1200°C ) CaS: Ce 3+ 2 hr
Chapter two: Principal phosphor materials and their optical properties
233
(30)37
(31)38,39
Ca C0 3 + Ce0 2
1300°C
- - - - - - - 7)
2 hr
CaO : Ce 3+ (32)40
CaO : Ce 3+
H 2 S + PC I3 1200°C, 2 h r
)
CaS : Ce 3+
(33)
When the sulfurizin g or red ucing agen ts ar e in the solid or liquid st at e, the reac tion can be perfor med in an encapsula ted crucible. When th e agen ts are in th e gas p hase, how ever, the reaction must be d on e in a quartz tube th at allows a gas flow. In th is rev iew, the form er will be called the crucible method and the latter the gas-flow method. These two meth od s are described below .
The crucible method. Exa mples of this method are given in the secon d reaction of Eq. 29, and Eqs. 30 and 31. By selec ting an ap p ropria te flux, th is me thod provid es particles of fairly large size and good dispersion cha racteristics. On the other hand, con tact wi th the flux can in trod uce impurities into the phosphor, res ulting in degraded efficie ncy. Insufficient su lfur ization or p art ial oxida tion may also occur by exposure to oxygen, since qu antities of a flu x generally used are ins ufficien t to cover all the particle surfaces. When firin g m us t be carried out for man y hou rs at high tem perature, a do u ble-crucible configuration is used; one crucibl e nestles in the other wi th carbon between, thus p reven ting ph osp hors from oxid ation. The gas-flow method. Examples are giv en in the first reaction of Eq. 29, and Eqs. 32 an d 33. Alkali compou nds, wh ich are used as flu xes in the cr ucible method, cannot be used in th is case becau se they can vaporize and react with th e quartz tube during th e firing. As a resu lt, this method provides sm aller particles w ith poor d ispersion ch aracterist ics. Imp rovem en t is obtained in some cases if a small amo un t of PCl 3 gas is s upp lied for a pe riod of tim e, as in the method for Eq. 32. On the other hand, the gas- flow method can give h igh lu minescen ce efficien cy because con tam ination of phos phors wi th impuri ties are less probab le than in th e crucib le me thod and also beca use stoichiome try m ay be con trolled th rou gh adj us tme n t of th e composi tions and flow ra tes of the gases. In th is case , firing may be repea ted an d different prep ar ati on
Fundamentals of Phosphors
234
m ethods can be experim en ted w ith and / or co mbin ed , if necessary. The method in Eq. 33 is us ed to obtain fine p ar ticles for elec tro lu mi nescen t p o wder phosphors. Prep a rati on methods ca n ha ve considera ble effects on luminescen ce properties. The lum in escence peak positions of CaS:Ce3 + p rep ared by th e flu x method sh ow a p eak th at is blu e-shifted by abo u t 600 cm' relative to th e position s ob tained in phosphors prep ared by th e ga s-flow m et h od. Diffe rences ar e als o observed in th e exc ita tion spectra an d tempe rature d ependencies of the lu minescen ce in tens ity." Sulfides o the r th an CaS ca n essen tia lly be obtain ed by the same p rocess. H ow ever, p olysul fid es ar e form ed more easi ly from sulfides of the heavie r cation elements. In other word s, the sulfurizing reaction proceed s more slow ly for th e sulfid es of lig h ter elements. It h as been reported th at the reduction of sulfa tes is a better way fo r th e sy n thes is of SrS and BaS. Fo r example, the foll ow in g reactions ha ve been emp loy ed :" SrS0 4 +S +starch(o r sucrose)
-------7
SrS+(C0 2, CO, H 20, S0 2)
(34)
The sy n thes is of MgS b y red uctio n of MgO or MgS0 4 requires rep ealed reac tio ns to com p lete su lfuriza tio n ." Ano the r me th od for MgS syn thesis st arting w ith Mg metal and employing CS2, a more powerful s u furizin g ag ent, has als o been used. This latter method is report ed to be p art icularly effective in producin g MgS .23 Mg powder + S
-------7
MgS (an ex plosive reaction) (35)
2Mg O + CS 2
2.6.6.2
-------7
2MgS + CO 2
Seienides"
Selen ides a re sy n thesized by m ethods similar to th ose to for m sulfid es using elemen tal Se or H 2Se ins tea d . Th e follow ing reac tio n can be used to prep ar e CaSe: CaO + am id ol (a w eak organ ic base)
-------7
CaSe +(C0 2 , CO, NH 3 , H p , Se0z}
(36)
SrSe and BaSe are ob tain ed by firin g Sr or Ba nitrates with Se and st arch. Th e fired products in cl ud e Se and p olyselenides, w hich ar e th en vaporized by annealing in va cu um a t about 600°C. After annea lin g, a sing le phase o f SrSe or BaSe is ob tai ned .
References 1. Len ard , P , Schmidt, F., an d Tornasch eck. R., Handb. Exp. Phys., Vol. 23, Akadem . Verlagsges, Leip zig, 1928. 2. Lehm ann , W., J. Electrochem. Soc., 117, 1389, 1970. 3. Lehma nn , W. an d Ryan , EM. , J. Elect rochem. Soc., 118, 477, 1971. 4. Lehm a nn , W., J. Liuninesc., 5, 87, 1972. 5. Lehmann . w.. f. Electrochem. Soc., 118, 1164, 1971. 6. Lehm ann, W., Gordon Research Conference Report, Ju ly 1971. 7. Kasano, H., Megu m i, K., and Yamamo to, H., J. Elect rochem. Soc. , 131, 1953,1 984. 8. Kaneh isa, 0 ., Meg um i, K., Kasa no , H ., and Ya mamoto, H. , Abstr. Jpn. Soc. Appl. Phy" 42nd Meetillg, No. 8P-Q -ll, 1981. 9. Ts uda, N ., Tarn at an i, M ., and Sato, T., Tech. Digest, Phosphor Res. Soc. 199th Meeting, 1984 (in Jap an ese). 10. Yam am ot o, H., Meg u mi , K., Kasa no. H ., Kaneh isa, 0 ., Uehara, Y., and Mor ita, Y, f. Electrochem . Soc., 134, 2620, 1987.
Chapter two: Principal phosphor materials and their optical properties 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.
235
Kaneko, Y, Mor imo to, K , and Kod a, T, J. Phys. Soc. Japan, 51, 2247, 1982. Kaneko, Y, Morimoto, K , and Kod a. T , J. Phys. Soc. Japan, 52, 4385, 1983. Kan eko, Y and Koda . T , J. Crustal Growth, 86, 72, 1988. Kaneko, Y, Morimot o, K , and Kod a, T , Oyo Buiuri, 50, 289, 1981 (in Jap anese). Krebs, H ., Fundamentals of Inorganic Crystal Chemistry, McGr aw-Hill, Lon d on, 1968, 159 and 163. Hasegawa, A . and Yanase , A , f. Phys. C, 13, 1995, 1980. Morimoto, K, Masters Thes is, The University of Tokyo, 1982 (in Japanese). [asp erse, J.R , Kah an, A , Plendel, J.N., and Mitra, S:S., Phys. Rev, 146,526, 1966. Jacobsen, J.L. and Nixon, E.R, J. Phys. O em. Solids, 29, 967, 1968. Galtier, M., Mon tane r, A, and Vidal, G., J. Phys. Chent. Solids, 33, 2295, 1972. Boswarva, I.M., Phys. Rev. 81, 1698, 1970. Kaneko, Y, Ph .D. Thesis, The Universit y of Tokyo, 1984 (in Ja pan ese). Kameyarna, N ., Theory and Applications of Phosphors, Mar uze n, Tokyo, 1960 (in Ja panese). Keller, S.P., Ma pes , J.F., and Che roff, G., Phys. Rev., 108, 663, 1958. Keller, S.P. and Pettit, G.D., Phys. Rev., 111 , 1533, 1958. Japanese Patent Publication (Kokoku) 47-38747, 1972. Vecht, A., f. Crystal Growth, 59, 81, ]982. Ono, Y.A, Electroluminescent Displays, Series for Inf or mati on Displays, Vol. I, Worl d Scientific Publi shin g, Singapore, 1995, 84. Barrow WA, Coovert R E., and King, CN., Digest of Technical Papers, 1984 SID Inti. Symp. 249, 1984. Ohrni, K, Fukud a, H ., Tokud a, N., Sakurai, D., Kimura, T , Tan aka, S., and Kob ayash i, H., Proc. 21sf Inti. Display Research Conj., (Nogoua ), 1131, 2001. Fukada, H., Sasaku ra, A , Sugio, Y, Kimura, T , Ohrn i, K. , Tanak a, S. and Kobayas hi, H ., lpn. f. Appl. Phys., 41 L94] , 2002. Takasu, K., Usu i, S., Ok a, H., O hrni, K, Tanaka, S., and Kobayashi, H, Proc. lOtll Inti. Display Workshop, (Hiroshima), 1117, 2003. Sun, S.5., Dickey, E., Kan e, J., an d Yocom, r .N ., Proc. ]7t11 Inti. Display Research Calif , (Toront o), 301, 1997. Eha ra, M., Hakam at a, S., Fu kad a, H., Ohrni. K., Kom ina rni, H ., Naka n ish, Y, and Hatanaka, Y, [pn. Appl. J. Phys. 43, 7120-7] 24, 2004. Hakarnata, S., Ehara . M., Fu kuda , H ., Kominami, H., N akanishi , Y, and Hatanaka, Y, Appl. Phys. Leit., 85, 3729-3730, 2004. Ok amoto, F. and Kato, K , Tech. Digest, Phosphor Res. Soc. 196th Meeting, 1983 (in Jap anese). Vij, D.R and Mathur, Y. K., Illd. f. Pure Appl. Phys., 6, 67, 1968a . Okamoto, F. and Kat o. K, f. Electrochem. Soc., 130, 432, ] 983. Kato , K and Oka mot o, F., Jpll. J. Appl. Phys., 22, 76, 1983. Yama moto, H., Man abe. T , Kasano, H., Su zuki, T , Kane hisa , 0 ., Ueh ar a, Y, Mori ta, Y, an d Watanabe, N ., Electrochem. Soc. Meeting, Extended Abstracts, No. 496, 1982. Kane hisa, 0 ., Ya mamoto, H., Okam ura , T , and Morita, M., J. Electrochem. Soc., 141, 3188, 1994.
chapter two - section seven
Principal phosphor materials and their optical properties Shigeo Shionoya
Contents 2.7 IIb-VIb com po unds 2.7.1 Introduction 2.7.2 Fundamental intrin sic properties 2.7.2.1 Crystal structure 2.7.2.2 Melting point and crystal grovv th 2.7.2.3 Band str uctu re 2.7.2.4 Exciton 2.7.2.5 Type of cond uc tivity and its contro l... 2.7.3 Luminescen ce of sha llow donors and acceptor s 2.7.4 ZnS-type phosphors 2.7.4.1 Luminescence of deep donors and accep tors 2.7.4.2 Lu minescen ce of transition metal ions 2.7.4.3 Luminescence of rare-earth ions 2.7.5 ZnO Ph osphors References
237 237 238 238 238 238 241 242 242 244 244 258 260 260 261
2.7 IIb- Vlb compounds 2.7.1
Introduction
lfb-VIb compounds includ e the oxides, sulfides, seleni des, and tellu rides of zin c, cadmium, and mercury. Among th ese compound s, those with bandgap energ ies (Ex) larger than 2 eV (i.e., ZnO, ZnS, ZnSe, Zn'Ie. and CdS) are candidate m at erials for phosphors that emit visible luminescence; ZnS is th e m ost important in this sen se. In. this section, the fund ament al op tical p rop erties and luminescence characteristics and m echanisms of this class of phosphors w ill be exp la ine d .' The term IIb-VIb com p oun ds used below will be limited to the above-m ention ed compounds. ZnS-typ e phosphors are presently very import ant as cathode-ra y tube (CRT) phosph or s. These phosphors h a ve a long his tory, d atin g ba ck a bo u t 130 years. At the
237
Fundamentals of Phosphors
238
International Conference on Luminescence held in 1966 in Budapest, a presentation titled "The Century of th e Discovery of Luminescent Zin c Sulfide" w as giv en / in which the hi story of luminescent ZnS wa s d iscussed . In 1866, a you ng French chemist, Th eod ore Sidot, su cceed ed in g row ing tin y ZnS cryst als by a sublimation m ethod. Althou gh h is origin al purpose was to stu d y cryst al growth, the cryst als grown exhi bited phosphorescence in th e d ark. Th e exp erimen ts were repeated, th e ob ser vat ions con firme d, and a note to th e Aca d emy of Sciences of Paris w as presented. This note was published by Becq ue re l.' These ph osphorescent ZnS (zinc-blende) cr ystals we re thereafter called Sidor's blende . From p resent knowl edge, one ca n conclude that Sidots blende contained a small quantity of co p per as an impurity res po ns ible for the ph osphorescen ce. The h ist ori cal p rocesses of th e ev olu tion of Sid or's biende to th e present ZnS phosphors are describe d in 2.7.4.
2.7.2 Fundamental intrinsic properties Important p hysical prop erties of IIb-VIb compounds rel at ed to luminescence are show n in Table 20.
2.7.2.1 Crystal structure lIb -VIb com po unds crystalliz e either in th e cu bic zin c b lend e (ZB) stru ctur e or in the hexagonal wurtzite (W) s tru ctu re; ZnO, CdS, an d CdSe crys tallize in the W s truc tur e, while ZnSe, Zn'Ie, and CdTe in the ZB structure . ZnS crys tallizes into both the W type (tradition ally called a-ZnS) and the ZB typ e ( ~-ZnS). Th e ZB structure corresp onds to the low-temper ature phase; th e ZB ~ W tran siti on tem per ature is known to be abou t 1020°C.
2.7.2.2 Melting point and crystal growth In lIb- VI com po unds, the sublima tion pressure is very hi gh . As a result, the comp ounds, w ith the exception of th e tellurides, d o not m elt at atm osph eric pressure. They do me lt at pressures of several ten s of atm osp heres of argon, but the m elting points are pr ett y high : 1975°C for ZnO, 1830 ± 20°C for ZnS, and about 1600°C for Zn Se. Zn S powder phosphors ar e p rep ared by firing Zn S powders a t 900 to 1200°C. Ph osphor p ar ticles fired at relatively low temperatu re (be low abo u t 1000°C) are of the ZB structure, while those fired at temperatures ab ove 1000°C are of the W s tru ctur e. In the past, sing le cryst als of IIb-VIb compou nds w ith high su blima tion pressure were grown b y the sublim ation-recrystalliza tio n method , the vapor pha se reaction method , the va por phase chem ical tran sp ort m ethod, or by th e high-pressure melt gro w th method. Recently, variou s epitaxia l grow th m ethods, such as molecular beam epitaxy (MBE), me talorgan ic chemical vapor d eposition (MOCVD), and atomic layer epitaxy (ALE), have been ac tively d eveloped, es pecia lly for Zn Se and Zn S (see 3.7.6). As a result, thin singlecrystal films w ith very hi gh purity and h igh cryst all inity are presen tly available for the se tw o com pou n ds .
2.7.2.3
Band structure
In the IIb-VIb compou nds treat ed in this section, the cond uction band has the character of the 5 orbi tal of th e ca tions, while th e valence band has the character of the p orbital of the anions . These com p ou nds are all di rec t-trans ition type semiconductors, and both the bottom of the cond uction band and the top of th e va lence ban d are located at the r p oint [k = (000)] in k- space. Thi s is si mp ly sho w n in Figures 7(a) and (b) in 1.2 for the ZB an d W s truc tur e. TIle band s tructure of Zn Se of the ZB struc tur e obtaine d by non local pseudopotential calcula tions is sh ow n in Fig ur e 54.4
Q
is
""<::l
;:; ....
....
~
'V
~.
r;
~.
Table 20
Crystal struct ure ZnO ZnS
W W
ZnSe ZnTe
ZB ZB ZB
CdS
W
CdSe Cd Te
W
ZB
Lattice cons tan t (A) a c a c
= 3.2403 = 5.1955 = 3.820 = 6.260 5.4093 5.6687 6.1037
a c a c
;:.
Important Physical Propert ies of Ilb-Vlb Compounds Relat ed to Luminescence
= 4.1368 = 6.7163 = 4.30 = 7.02 6.4818
Sta tic d ielectric cons tan t c II 8.8 c 1.. 8.5 8.6 8.3 8.1 10.1 c 1110.3 c L 9.35 c 1110.65 c L 9.70 10.2
Bandgap ene rgy (eV) RT 4K 3.436
3.2
Exciton ene rgy,4 K (eV)
Exciton bin d ing energy (meV)
3.375
59
Effective mass m' / l11o Hole Electron 0.28
0.59
""<::l
[
~
a....
-;....:;
~
3.911
3.8
3.871
40
0.28
3.84 2.819 2.391
3.7 2.72 2.25
3.799 2.802 2.381
36 17 11
0.39 0.16 0.09
2.582
2.53
2.552
28
0.2
1.840 1.606
1.74 1.53
1.823 1.596
15 10
0.11 2 0.096
c II 1.4 c 1.. 0.49
.... . ;:; V; :;:,
;:s l:l..
0.75 Heavy: 0.6 Light : 0.16 c II 5.0 c 1.. 0.7 c II 2.5 c 1.. 0.45 Heavy: 1.0 Ligh t: 0.1
;:;:. ~.
....
~
[
""<::l
d
""<::l
.... '""
~.
N
CJJ
'-0
Fundamentals of Phosphors
240
8 L•. .5 L.
6 4
----> 1)
'--'
o
L.
L'.3
>.. -2 L.
en ......
1)
C
LU
- 4
-6
L.
- 8 - 10
L6
- 12 A
r
X
U,K
Wave vector Figure 54 Band struc tur e of ZnSe. (From Chelikowsky, 1976. With permi ssion.)
J.R.
and Co he n, M.L., Phys. Rev., B14, 556,
As shown in Fig ur e 7(a) of 1.2, the valence band in th e Z8 struc ture is sp lit by th e spin-orbit interaction into a hi gher ly ing r s(A) s ta te (in w hi ch the orbital s tate is doubly degenerate) and a nondegenerat e r 7 (8) s ta te. In th e IN struc ture as shown in Figure 7(b), on the other h and, all the orbi ta l d egeneracy is lift ed by the spi n-orbi t in teraction and the aniosotrop y of crystal field , and the split st at es are r 9(A), r 7(8), an d r 7 (C) in descending order of en er gy. Th e case of ZnO is an exception: L9(A) and 1 7(8 ) ar e reversed, so that the order is L7(A), 19(8 ), and r 7(C) instead . Th is origina tes from th e fact that in ZnO the sp litting b y the spin-orbi t in teraction is negative an d sma lle r th an th at due to the crys tal field an isot rop y, unlike o the r IIb-VIb compounds . Th e negati ve spin-orbit splittin g ari ses be cau se of mixin g of th e d orbitals of Zn with th e va lence band . In MX-type compound se micon du ctors, the bandgap ene rgy Eg usually increa ses if M or X is replaced by a hea vier element. Looking at E~ values in Tabl e 20, it can be noted that this general rule is us uall y obse rved, except in the case of ZnO, where the Eg value is smaller than that of ZnS . Th is is also caused by the mixing of the Zn d orbital w ith the valence band. It is seen in th e band s tr uc ture of ZnSe, sh ow n in Figure 54, that in the cond uction band there are two minima in u pper en ergy regions at the L [k = (111)] and the X [k = (100)] points with energies o f 1.2 and 1.8 eV ab ove th e bottom of th e con d uc tion b and , res pec tiv ely. The co nd uc tio n band s truc ture of ZnS is very sim ilar, having th e tw o upper minima at the sa me p oints. Th e exis ten ce of these tw o upper m in ima plays an important role in th e exci ta tio n process of high-field, thin-film electro lumi nesc en ce in ZnS (See 1.10). The fact th at IIb -VIb compou nd s are direct-gap semiconductors means that they are appropriate ho st m at erials for phosphors. If one compares the radiati ve recombina tion coefficient of electrons an d h oles for direct and indirect transitions, the value for the form er is four orders of magn itude lar ger. In practical phosphors, the radiat ive emission is not caused by direct recombinati on , but by transitions taking place via energy levels of activators introduced as impurities. For imp ur ities as donors or acceptors, their ene rgy levels
Chapter two:
Principal phosphor materials and their optical properties
241
4.2K
~
Ci3
z
~
E-<
60K
x l
Z .......
Ex
150K Ex
325
330 335 340 WAVELENGTH (n m)
Figure 55 Exciton lu minescence sp ectra of ZnS (ZB type) at vari ous temp eratures. (Fro m Na ka mura, 5., Sakashita, T., Yosh imura, K. , Yamada, Y, and Taguchi, [pn. f. App !. Plzys., 36, L491, 1997. With permi ssion .)
are generated b y perturbation on the conduction or valence band. The refo re, the impu rity ene rgy levels take on the same character as their parent bands, and th e ra d ia tive recombination p rocesse s and rates in these levels are similar to those in the pure ho st mate rial. ZnS:Cu,Al an d ZnS :Ag,Cl phosphors, which are very important as p hosphors for ca tho de ray tub es (CRT), are typical examples of this type of phosphor, as w ill be exp lained in 2.7.4. This is the rea son why direct-gap type materials are most favorabl e as phosphor hosts.
2.7.2.4 Exciton In Ilb-Vlb comp ounds, the exciton structure is clearly obs erved at low temperature in abso rp tion and reflection spectra near the fundamental absorpti on edge. Ab sorpti on spec tra of CdS shown in Figure 11 of 1.2 are a typical exam ple . Th e exci to n energy and its bind ing energy are sh ow n in Table 20. An exciton is anni hi lated, emitting a photon by th e recombinati on of th e cons tituent electro n and hole pair. Figure 55 show s exciton luminescen ce spectra from a high-q ua lity epitaxia l layer of ZB-type Zn S grow n by MOCVD at vari ous temperatures." At 4.2K, th e Ex line from intrinsic free excitons at 326.27 nm is th e stronges t. The line Ex-I LO is the free exciton line acco mpa n ied b y the simultaneous em issio n of one longi tud inal op tical (LO) ph on on. Even in very pure crystals of Ilb- Vlb compounds, a trace amoun t of impurities, at concentrat ion levels of 1014 to 1015 cm-', are found. Exciton s are bound to th ese imp urities, and lum inescence from these bound exciton s is also observed a t low temper ature. Lines (D°, X) and (AD, X) are from excitons bound to neutral d on ors (DO) and ne u tral accep tors (AD). These bound exciton line s are customar ily d enoted as 12 and 11' respec tive ly. Frequen tly,
Fundame ntals of Phosphors
242
100 r - - - - , - - - - - - " r - - r - - - - , - - - , - - - - , , , - - - - - - , - - - - - , r - - - , ,I
>-
90
I" \ MERCURY
80
, I
o 70 w
,
w
,
ct:
z 60
w
ARC
, I I I , I I I
I
, I I , I ,
50
> 40 ~
--' 30 w ct: 20
I I
I
I I I I
I
I
I
I
I I
I
I
I
I I
1
10
I
o'-__
..c......l_
_
-----L _ _
- L- l -_
---L_
-----...I.- L------=""'--'=="'--_
50005100 5200 530 0 540 0
.-J
5 500 5600 5700
WAVELENGTH (A) Figure 56 Spectrum of the edge emission of CdS at 4K; dashed line rep rese nts spectrometer window. (From Klick, c.c. J. Opt. Soc. AIIl ., 37, 939, 1947. With perrn ission .)
an 13 line d ue to excitons bound to io nized d onors is obse rve d at waveleng th a little shorter than 12, With increasing tem pera ture from 4.2K, bound excitons are released from impurities, so tha t only the lu min escen ce line due to free excitons is observed as can be seen in the figure . The binding energy of the exciton in ZB-type ZnS is 36 meV, so that exciton lu minescence is obs er ved up to room temperature. The exci ton binding energy in ZnO is as large as 59 meV and is the largest among IIb-VIb comp ound s. Lu m inescence of free exciton s is observed a t 385 nm at room temp erat ur e in p ure ZnO. Th is ult raviolet luminescence was fou nd as early as the 1940s} but it was not recogn ized a t that tim e that th is lu m in escen ce origi na tes from excitons . Th is lu minescen ce persists up to fairly high temperatures; it is still obse rved at temperatures as high as 770K ?
2.7.2.5
Type of conductivity and its con trol
As-grow n single crysta ls of ZnO, ZnS, ZnSL', and Cd S are usua lly n-type in conductiv ity, while th ose of ZnTe are p-ty pe. The cond u ctiv ity co ntrol of Ilb -VIb compou nd s, es peciall y for ZnSe, has made remarkable progress recen tly. This progress is due to the demand to develo p blue and bl ue-g reen light-emitting diodes and semicond uc tor lasers. Th e p rep aration of p-ty pe ZnSe wi th high conductivi ty has been a fund amental problem , w hich was solved rece n tly b y in trod uci ng nitrogen accep tors usin g nitrogen plasma (See 2.7.6). Prese n tly, it is possible to contro l the type of conductivity in mos t llb -Vlb co mpounds.
2.7.3 Luminescence of shallow donors and acceptors Sin ce th e 1940s, it has been kn ow n that pure Cd S crys tals show luminesce nce at low room tem perat ur e wi th a characteris tic sp ectra l structure on the low-en ergy side of the fundam en tal absorption ed ge. Th is luminescence was called edge emission. Its spectru m is shown in Fig ur e 56.8 It has been es tablished th at the characteristic edge emi ssion is ob served in all IIb-Vlb compounds except for ZnO.
Chapter two: Principal phosphor materials and their optical properties
243
The lines in Figure 56 are equally spaced, with an interval of about 40 meV, which is equal to the energy of longitudinal optical (LO) phonons in CdS. The halfwidth of the lines is approximately 5 meV. The relative intensities of the lines in the figure (numbered n = 0, 1, 2, ... from the short wavelength side) decrease toward longer wavelengths with ratios of 1.00:0.87:0.38:0.12:0.030:0.015. This ratio exactly obeys the Poisson distribution In = e-sSn/n' with S = 0.87. The n = 0 line is known as the zero-phonon line, while lines of n = 1, 2, ... are caused by simultaneous emission of 1, 2, ... LO phonons. It has been established that the characteristics of the edge emission are satisfactorily interpreted in terms of donor(D)-acceptor(A) pair luminescence (see 1.4.4). The transition energy E of this luminescence is a function of the distance r between 0 and A in a pair, and is given by: (37) where ED and E A are the ionization energies of a neutral donor and acceptor, respectively, and £. is the static dielectric constant. The transition probability W also depends on rand is expressed by (38) where r B is Bohr radius of the donor electron and W o is a constant related to the D-A pair. The mechanism for donor-acceptor pair luminescence was first verified in the edge emission in GaP doped with S donors and Si acceptors (see 1.4.4 and 2.8).9 The intra-pair distance r is distributed discretely, so that a spectrum consisting of discrete lines is expected. In GaP:Si,S, a great number of sharp lines were observed adjacent to the highenergy tail of the n = 0 line, and the value of r for each line was determined. On this basis, the main part of the n = 0 line is thought to be composed of a large number of unresolved pair lines for pairs with relatively large r values. A great number of sharp lines were also observed in the edge emission of CdS 10 ,11 and ZnSe. 12,n These facts present clear evidence as to the origin of the edge emission. In ZnSe, the identification of each line has been made in analogy to the GaP case; in CdS, the analysis is not easy to make since the spectra are much more complicated because of the W structure. Eqs. 37 and 38 indicate that the pair emission energy shifts to lower energies and the decay time becomes longer with increasing r values. Then one expects that in the timeresolved spectra of the edge emission, the peaks of the lines composed of unresolved pair lines should shift to lower energies as a function of time after pulse excitation. This has been observed in CdS,14 and presents further evidence for the pair emission mechanism in the edge emission. The fact that the relative intensity ratio of the edge emission lines obeys a Poisson distribution indicates that the configurational coordinate model (see 1.3.2) is applicable to each pair center with a different r value. The donors and acceptors participating in the edge emission are shallow. In these cases, the constant S appearing in the Poisson distribution, which is called the Huang-Rhys-Pekar factor and a measure of the strength of the electron-phonon interaction, is small, of the order of 1 or less; the phonon coupled to the center is the LO phonon of the entire lattice, but not a local mode phonon. The depths of donor and acceptor levels (ED and E A ) in IIb-VIb compounds are determined from bound exciton emission lines, edge emission spectra, or absorption spectra between a donor or acceptor level and the band. The available data on levels are
Fundamentals of Phosphors
244 Table 21
Depths of Donor and Acceptor Levels, ED and E A (meV) in IIb-VIb Compounds Lia (a) Donor Br B Al Ga In F Cl Eo, calc
Zns
110 29±2
ZnSe ZnTe CdS CdSe a
25,6
100 25,6 18.5
33,9 20±2
27.2
28,2
28,2 35,1
26,2 20,1 32.7
33,1
33,8
32,5
32,1
21 28
Interstitial Li.
(b) Acceptor ZnS ZnSe ZnTe CdS CdSe
EA, cd ," 108 62
Li
Na
Cu
Ag
Au
N
P
As
150 114 60.5 165 109
190 102 62,8 169
1250 650 148 1100
720 430 121 260
1200 550 277
110
85,500 63.5 120,600
110 79 750
Note: Calculated values by the effective mass approximation' S, ED, . "k and E", cok' are also shown,
shown in Table 21. Calculated values of ED E A by the effective mass approximation are also shown."
2.7.4 ZnS-type phosphors 2.7.4.1
Luminescence of deep donors and acceptors
ZnS type phosphors such as the green-emitting ZnS:Cu,Al and the blue-emitting ZnS:Ag,Cl are very important from a practical point of view, especially as phosphors for cathode-ray tubes. Luminescence centers in these phosphors are formed from deep donors or deep acceptors, or by their association at the nearest-neighbor sites. In this subsection, a brief history of the development of these phosphors will be given first, and then the characteristics and the mechanisms of their luminescence will be explained,
(a) History, After the research by Sidot described in 2.7,1, it became gradually clear that when ZnS powders are fired with the addition of a small amount of metallic salt, luminescence characteristic of that metal is produced. In the 1920s, it was established that a small amount of copper produces green luminescence, while silver produces blue luminescence. In this sense, copper and silver were called activators of luminescence, The firing is made at 900 to 1200°C with the addition of halides (such as NaCl) with low melting points as fluxes, It was found that if the firing is made without the addition of activators but with a halide flux, blue luminescence is produced, Thus, this type of blue luminescence was called self-activated luminescence, In the 1930s and 1940s, research on ZnS-type phosphors was very active, Results of the research are described in detail in a book by Leverenz." published in 1950, In this book, the emission spectra of a great number of phosphors in ZnS, (Zn,Cd)S, or Zn(S,Se) hosts activated with Cu or Ag are shown. The spectral data shown in the book are still ver y useful. It should be noted that this book was written before the concept of the co-activator was conceived, so that chemical formulas of some phosphors given in the book are always not appropriate, and care must be exercised, For example, a phosphor written as ZnS:Ag[NaCl] should be written as ZnS:Ag,Cl according to the rule in use today The ZnS self-activate phosphor is shown as ZnS:[Zn] in the book, but should be ZnS:Cl(orAl) instead, as will be explained.
Chapter two:
Principal phosphor materials and their optical properties
245
In the lat e 1940s, Krog er and co-workers v" demonstrated that halide flu x ad ded in the fir ing p rocess to ZnS phosphors not only p romote s cryst al grow th, but introduces ha lid e ion s (VIlli gro up ani ons) into the ZnS latti ce, and that th ese hal ide ion s participate in the formation of luminescen ce cen ters, Kroger et al, ass u med th at the copper or silver activators are in the m on ovalen t state and subs titu te for Zn 2+ ion s, and that charge com pens ation for the m onova len t activators is accom pl ishe d by introducin g VIlli gro up anio ns subs titu ting for 51- ions, It w as sup po sed that charge com p ens ati on should occu r not only w ith VII grou p an ion s, b ut also with IIIb gro up ca tions, such as AP+, substituting for Zn 2 + ions, Kroger's group 19 clearly sho we d that if AP+ions are introdu ced w ithou t using halid e flu xes, simi lar kinds of lumi nescenc e are produced, and thus ev ide nce d the above ass u mption , The VIIb or Illb ions were called co-activators , These ions are ind isp ensabl e for th e formatio n of luminescen ce centers, but the luminescen ce spec tr um is d eter mined only by the kin d of lb ion activ ators and is almos t indepe nd ent of th e kind o f co-ac tiva tors. Thi s is the reason for the naming of co-ac tiva tors . In those d ays, the natur e of the electroni c tran sitions respo ns ible for th e lu m in escen ce in ZnS p hospho rs wa s active ly di scussed. The so-called Scho n- Klase ns m odel, first proposed by Schon and then dis cussed in detail b y Klasen s." ga ined ge ne ra l acceptance. Thi s mode l assu mes th at th e luminescen ce is caus ed by the recombination of an electron in th e cond uction band, wi th a hol e located in a level a little above the val en ce band . Prencer and Willia ms ?' pointed ou t th at Ib gro up activator s and VlI b or IIIb gro up co-ac tivators should be recogni zed , res pectively, as the accep tor s an d th e d on ors. It wa s assumed that donors and acceptors are sp ati all y associa ted in some way; then it was p roposed that the luminescence tak es place in cen ters of p airs of d onors (co-ac tiva tors) and accepto rs (ac tiva tors) associa ted at the secon d and th ird n eare st-neighbor site, and that the luminescence tra nsit ion occ urs from the excited sta te of d onors to the ground state of accep tors. This w as the first proposal for the d on or-acceptor pair luminescen ce concep t, w hic h was later recognized as a basis for underst and ing se m icond uc tor lu m inescence as mention ed in 2.7.3. The above narrati on tou ches upon the esse n tia l point s of the progr ess in resea rch in this area up until the 1950s. This resea rch was ac tive ly p ursued in the 1960s. As a result, the lumine scen ce mech an ism of ZnS-typ e phosphors using activ ators of Ib eleme n ts has been elucida ted qu ite thorou ghly. Th is will be d escrib ed below.
(b) Ctassifica tion and emission spectra. The luminescence of ZnS- ty pe phosphor s us ing Ib grou p activators (Cu, Ag ) and lIIb (AI, Ga, In ) or VIlli (CI, Br, I) group co-ac tiva tors can be classi fied in to five kin ds, de pending on th e relative ra tio of th e concentration s of activators (X) and co-ac tiva tors (Y). This condi tion is shown in Figure 57.22 The rang e of conc entrations for X and Y is 10-6 to 10~ mol /mol. Th e labe ls of th e lu m ine scen ce in th e figure or igin ate from the emission color in the case that th e activa tor is Cu; that is, G = green, B = blue, and R = red. R-Cu,In ap pears onl y when the co-act iva tor is a IIIb gro up element. SA me an s the self-activated blue luminescen ce. Fig ure 58 de picts the em ission spectra of these five kinds of phosph or s at room temperature and at 4.2K.23 As sh ow n in Tabl e 20, the bandgap ene rgy E ,~ of Zn S is 0.08 eV larger for the W structu re tha n for the ZB structure. Corresponding to 'this, the emiss ion peaks of ph osphors wi th the W s tructur e are sh ifte d by almos t this amo u nt toward shorter wavelength. In ge neral, ph osphors p rep ared b y firing above 1000°C have the W struc tu re, whil e those below this ha ve the ZB structure. Emissio n peaks at room temper ature ar e located at lon ger wa velengths than th ose a t 4.2K, excep t in the case of th e SA luminescen ce. The lon g waveleng th shift is almost p roportional to that of Eg . The SA luminescen ce shows the in verse beh avior; that is, the peak at room temperature is locat ed at shorter
Fundamentals of Phosphors
246
eo
o
t ,;/
/
/
/
Equal cone . line
- - - log [ YJ
Figure 57 Five kinds of luminescence in ZnS phosphors classified from the point of view of the relative ratio of the conce n trations of activa tors (X) and co-activators (Y). G-Cu: gre en Cu. B-Cu: blue Cu . R-Cu : red Cu, R-Cu, In: red Cu . In, SA: self -acti vated blue. (Fro m van Goal, w., Philips Res. Repi. Suppl., 3, 1, 1961. With permission. )
wavelengths. These emission spectra a re almost independent of the kind of co-activators, except for the case of the SA luminescence. The G-Cu emission spe ctra of ZnS :Cu ,AI shown in Figure 58 are almost the sa m e as th ose of ZnS:Cu,Cl . In the case of the SA luminescenc e, th e spectra of ZnS:CI an d ZnS:AI are a little different. The spectrum of ZnS:AI is slightly shifted to longer wa velengths. If the activator is changed from Cu to Ag, the emi ss ion peaks a re shifted by 0.4 to 0.5 eV to sh orter w avelengths. Th e blue luminescence of ZnS :Ag,C1 (peak at 45 nrn , W typ e) corres pond s to the G-Cu luminescence. Au , a Ib group element, also acts as an acti vator. The luminescence of ZnS:Au,Al corresponding to the G-Cu luminescence has its p eak a t 550 run in the ZB structure, which is shifted slightly to longer wavelengths relative to ZnS:Cu,AI. ZnS an d CdS, and also ZnS and ZnSe, form binar y allo ys (solid solutions) with relatively sim p le properties . Eg cha nge s almost in proportion to the compositi on . For example, E, in (Zn,Cd)S (W) changes from 3.91 eV for ZnS to 2.58 eV in CdS, almost in proport ion to the ratio of Cd. The five kinds of luminescence discussed above also appear in the allo ye d materials an d have similar properties. In Zn,Cd 1_xS:Ag,C1 (W), the emi ssion peak ch anges almost p roportionally to Eg , i.e., change s from 435 nm for x = 1 to 635 nm for x = 0.4 continuously. It is possible to obtain a d esired luminescen ce color from blue to red by sim p ly adjusting the composition . In ZnS ,Se1_,:Ag ,CI, the si tua tion is similar, but th e change of the em ission peak is n ot alw ays proportional to Eg and sometimes a we ak subband appears. Among the five kinds of luminescen ce discussed abo ve, the important on e for practi cal use is the G-Cu luminescence, which is produced when the conc en tra tions of the activat or and co-activator are ne arl y equal; in thi s case, charge compensation is readily and simply attained. ZnS:Cu,Al (green-emitting) an d ZnS:Ag,Cl (blue-emittin g) phospho rs are
Chapter two:
Principal phosphor materials and their optical properties
247
Wavel en gth (nm) 800 900 1 000
Figure 58 Spe ctra of the five kinds of lu m inescence in znS phosp hors a t room tem perature (so lid line) an d 4.2K (do tted line). Activ at ors and co-activ at ors of the se ph osphors and th e crystal struc ture are shown below. (Shion oya, S., Kod a, T., Era, K , an d Iujiwara, H ., I. Pilys. Soc. Japan , 19, 1157, 1964. With pe rmission .) Luminesce nce G-CLI B-CLI SA R-Cu R-Cu,ln
Activa tor
Co -activator
Crystal s truc ture
Cu Cu
AI
W ZB ZB W ZB
I
Cl Cu C ll
In
extrem ely im p o rt ant in CRT app lica tions. ZnS:C u,Au,AI (g reen-e mi tting) phosph or s in wh ich both Cu and A u are used as ac tiva tors a lso fin d usage in th is area . The excitation sp ectra for these five kinds of luminescen ce consis t of tw o band s in all cases. The fir st one, h aving th e peak a t 325 to 340 rim, correspon d s to the fund am ental absorp tion edge (or th e exc iton posi tion ) of th e ZnS host crystal . and is called th e host exci ta tion ban d . Th e se con d , h avin g th e p eak at 360 to 400 nm in the longer w avelen gth region , is characteris tic of the luminescence cen ter, an d is ca lled the ch aracteristic exci tation ba nd . This band is p rod uced by th e transition fro m th e g ro un d s ta te of th e ce n ter (corresp on d in g to th e acce p tor level ) to the excited sta te of the ce n ter (corresponding to th e donor level) or to the conducti on b and. As an example, the excita tio n sp ectra for th e SA lu minescence in a ZnS:CI single crysta l (Z8) are show n in Figure 59(a) .I«.24 An absorp tion spec trum of the crys ta l and the absorp tio n band of the SA center a re show n in Figure 59(b) for com parison. (c) A tomic structure of luminescence centers and luminescence transitions. T he a to m ic s tr uc tur e of the luminescence centers and the nature of luminescence tran sitions for th e five kin ds of luminescen ce mention ed above were eluci da ted in 1960s, mostly by th e resea rch of Shio n oy a and co-workers, as wi ll b e describ ed below . Experim ental tools th at pl ayed important roles in clarifying these s ubjects in clu ded measurements of th e polariza tion of luminescence light using phosphor si ng le crysta ls grown by melti ng p owder p hospho rs under high argon p ress u re" an d m easurem ents of time-resol ved e m ission spectra . The essen tial ch aracteristics deri ved fro m the results of these m easu rements, as well as th e
Fundamentals of Phosphors
248 Wavelength (nm)
300
320
340
360 380 400
10 -
440 ( a)
C
' Vi
c<]) ...., c c 0
(/) (/)
5
E
Cl.l
N
:::: I
E
u
C---J N
:::: 1::$
0 14 12 10 8 64 20
4.0
-14 12 - 10 , 8 Eo 6
C---J
(b)
4.0
3.5
3.0
4
~
2 0
Phot on energy (cV) Figure 59 Excita tion spec tra (a) for the SA lumin escence at various tem p er atu res and an absorpt ion spectrum (b) a t 91K in a Zn S:CI single crys tal. In (b), curve 1 is the absorption spec trum plotted as 0. 1 /2 vs. E (a = absorption coefficien t, E = pho ton energy), and cur ve 2 is the a bsorption band of the SA cen ter obtained from curve 1 an d p lotte d as a. vs . E. (Fro m Kod a, T. and Shionoya, S., Phys. Rev. Lett., 11,77,1963; Koda, T. and Shionoya, S., Phys. Rev., 136, A541, 1964; Shionoya , S., in Luminescence of Inorgan ic Solids, Go ldberg, P., Ed ., Acad em ic Press, New York, 1966, cha p. 4. With perrni ssion .)
a tomic s truc ture o f the luminescence cen ters an d th e n ature of the lu m in escence transitions ob taine d fro m th ese res ults, are summarized in Table 22. (i) Polarization of luminescence. Luminescence light from cen ters locat ed wi thin the host crystals with uniaxial sy mm etry, like the w ur tzi te structure crystals, shows polarizat ion d ue to the anisot rop ic crystal field s. In the case of isotrop ic crys tals, if th e lu minescence cen te r is a spa tia lly assoc iated center, incl ud ing an activa tor and/ or a co-activator with the si te symmetry whi ch is char acterist ic of the center and is lower than the sym metry of th e ho st cryst al, th en th e polarizat ion of luminescence light results when po lar ized excitati on light is used. The refore, one can d etermine th e na ture of the site symme try of the lumin escence center by observing the d ifference in th e po lar iza tion of luminescenc e light w hen polarized and un polari zed light are used for exci tat ion. Among th e five kin ds of luminescen ce of ZnS phosphors, p olariza tio n m easurements were firs t cond uc ted for the SA lu m inescen ce of a ZnS :CI sin gle cry stal." This crystal h a d th e cubi c ZB s truc tu re, but conta in e d a co nsiderable amou nt of stackin g d isord er, leading to sma ll volumes in which hexagonal st ru cture occurred . The hexagonal reg ions have th e c-axis defined b y th e [111L axis of the Z B struc tur e. In polarization
n
S
"'"<:::
;;-
...
~ ~
'""d ...S;.
"
~.
Table 22
Lum inescence
Phosphor
G-Cu
ZnS:Cu ,AI(CI)
SA
ZnS:CI(AI)
Polari zation of lumi nescence ZB:no po l. W :pol. .L c Charac t. pol.
B-Cu
ZnS :ClI,I(Cl)
Charact. pol.
R-Cu
ZnS:ClI
Ch aract. pol.
Symmetry of center No charact. symm . Charact. symm . ZnS :Cl, C3, ZnS:AI, c, Ch aract. symm. C3v Charact. sy mm.
Shift of emission pe ak
Typ e of luminescen ce tran siti on
Ob served
D-A pair
Observed (ZnS:Cl)
D-A pair
No shift
Int ra-center
No shift
Intr a-center
Zn S:Cu,ln
Cha ract. pol.
Ch aract. symm. C,
No shift
Int ra-center
"'"<::: ;:,-
o
Structure of center CU\ ub and AI\ lIb(CI-. ub) (rando m distrib.) V(Zn2+)-Cl-'"b (AP+s"b) (assoc ia ted)
V>
"'"<::: ;:,-
...
Q
3
'";...;£;.
[j}
'"
;:,:
"'-
C3v
R-ClI,In
i'2..
Characteristics of the Five Kind s of Luminescen ce in ZnS-type Phosphors
CU\ ub-Cl+;n, (associated ) V(S2-)-CU\ ub (assoc iated) CU\u b-In3\ub (associated)
s:
::<. '"
o ~
[
"'"<:::
d
"'"<:::
Note: The po lariza tion of luminescen ce, the symm etry of the lu mine scence cen ter ob tained from the characteristics of the polarization, the spectral shift of em ission peak in time-resolv ed emission spectra. th e type of lumin escence tran sition inferr ed from the spectral shift, and the ato mic st ruct ure of the center. (V = v acancy; sub = subs titu tiona l; in t = inter stit ial).
;::;. '"
~.
N
>;>..
<.0
250
Fundamentals of Phosphors
m easurements, one of the cry stal s ur faces was irradiated perpendicularly by polarized excitatio n light, and th e polar ization wa s measured for the luminescen ce light em itted fro m the opposi te surfaces. The measurements were m ade for (110), (112), and (111) planes. Excitation was made with 340-nm light belon ging to the host excita tion band (H ) and with 365-n m light belonging to th e charac ter istic excitation band (C). Both polarized and unpolarized light was used. The results were expressed in terms of the d egre e of pola rization observed, i.e., p(e) = [(III - 1.1)/(1 11 + 1.1)]8' where Inand 1.1 are the em issio n in tensi ties measured wi th the ana lyzer p arallel and perp end icular, respectively, to the po larize r, and e is th e an gle between the optical ax is of the polar izer and a particular crystal axis . In the case of unpolar ized excita tion, e is th e an gle bet w een the op tical axis of the an alyzer and a particular crysta l axis. The p( e) cu rv es measured a t 77K a re shown in Figu re 60.24 In the case of the characteri st ic (C) excita tion, the p (e) curves d epend critically on w hether the excitation light is p olarized or unpolarized ; under polarized excita tion, p(e) sho ws specific ang ular dep end en cies th at vary for d ifferen t crys tal pl anes. In th e case of ho st (H ) excita tion, on the othe r hand , th e p (e) curves are quite ind ependen t of the polarization of excitation light, and are the same as th ose obtain ed under the unpolari zed C excita tion. The results observed under th e p olar ized cha rac teristic exci tatio n clearly indicat e that th e cen ter has the characteristic sym metry, which is lower th an th at of the ho st latti ce. Pren er and Williarns-" p rop ose d a model for the struc tu re of the SA cen ter, which ass umes that the cen ter consists of a Zn 2 + vaca ncy [V(Zn 2+)] an d one of the cha rge-compensa ting co-activator s ass ociated at one of the nearest subs titu tional sites, i.e., a Cl co-activator at th e neares t 52-s ite or an AP ' co-ac tiva tor at the ne ares t Zn 2+ si te. Figure 61 show s a mode l of th e SA cen ter in ZnS:CI. The results of polariza tion m easurem ents were an alyzed assu ming thi s mod el. According to th is model, th e SA cen ter in ZnS :Cl h as C3 \, symmetry (in the case of ZnS:Al, C, sy mme try). Dip ole transitions that are allo w ed in a C31> cen ter are those due to a a -dipol e perpe ndicular to an d a rc-dipole parallel to the sym me try ax is. The ang ular d ep endence of the p olarization of luminescence d ue to a a- or a n-dipole was calcu lat ed assu ming th at the symmetry axe s of the cen ters are di st ributed u ni formly along the va rio us di rection s of the Zn-S bonding axes. The results of the calculation are re presen ted in Figure 60 by the th in solid lines. Co mp aring the experimentall y observed p(e) curves und er polarized C excita tion wi th those calculat ed, it can be con cluded that the observed an isotrop y of the lu minescence resu lts fro m the a-d ipole. Thus, the Prener and Williams model for the atomic structure of the SA center was co nfirme d b y th ese observa tions . The p olarizat ion of the lu minescence perp endicul ar to th e c-axis observed under unpolarized C excitation an d under polari zed and u npolarized H excitation is ascribed to the crystal str ucture inclusive of the hexagon al d om a ins arising from the stacking d isorder. Measu remen ts of the p olarizat ion of luminescence we re also mad e for the SA luminescen ce in a ZnS:Al cryst al." The observed p( e) curves showed charac teris tic ang ular d ep end en cies under p olarized C exci tation similar to th e SA lu m inescence in ZnS :CL The results were analyzed assuming C, sym metry for the cen ter as in the Pre ne r and Willia ms m od el, and were we ll explained by ascribing the luminescence to a d ipole lying in the mi rror plane of the cen ter (II dipole). Results of measurements of th e pol arizati on for the five kinds of luminescen ce are s um marize d in Tabl e 22. B-Cu ,2HR-Cu,29an d R-Cu J n 3o lumine scen ce showed character istic polari zations under C excitat ion, in d icating that the luminescence centers are some kinds of spa tia lly associa ted cen ters. Th e sy mme tries of the cen ters d etermin ed by ana lysis are a lso shown in the table.
Chapter two: Principal phosphor materials and their optical properties
tr
(ITO) (112)
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251
POLARIZED EXCITATION
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-
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Figure 60 The degree of polarization prO) for the SA lumin escence in a ZnS:CI single crysta l at 77K under polari zed and un polarized excitation. C and H denote, respectively, the characteristic and host excitation. 0 is the angle between the optical axis of the polarizer (or the ana lyzer in the case of unpalarized excitation) and parti cular crystal axis [111]cfor the surface (11 0) and (112), and [112] for the surface (111)(" Open circles, closed circles, and crosses show experimental results. The curv es dra wn with thin solid lines are P(O) curves calculated assuming C5- or "It-dip ole. (From Koda. T. and Shionoya, S., Phys. Rev. Leti., 11, 77, 1963; Phys. Rev., 136, A541, 1964. With permission.) The G-Cu luminescence does n ot show charact eristic p olari zation differently from the o the r four kinds of luminescence." In a ZnS: Cu,Al crystal of the W struc ture, th e polariza tion of luminescence p erpendicula r to the c-axis was observed in dep en den tly of whether the exc itation w as due to the Cor H band and w as a lso independent of w heth e r th e excita tio n was polari zed or unpolarized. In the case of a crystal of the ZB structur e, no p olari zati on was ob served . Thes e fac ts indicat e that ac tiva tors and co -activa tors forming the G-Cu cen ters are not associated wi th each o the r spatia lly, but are randomly d istributed in a crystal occupying re sp ective lattice sites .
252
Fundamentals of Phosphors
..
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Model of the SA lu minescence center in ZnS:Cl p hosphors.
(ii) Time-resolved emission spectra. In D-A pa ir luminescence, sp ectral lines or bands are com posed o f a large num ber of unresolved pair lin es. In tim e resolved emission spectra, as explained in 2.7.3, th e peak of the lines or bands should sh ift to low er energies with the lap se of time. Fig ure 62 shows time-resolved spec tra of the green lu minescenc e of Zn S:Cu ,A f.3 2 It is clearly see n that th e peak shifts dram at icall y to low er energies as a func tion of time. If the excita tio n in tensi ty is inc reased over a w ide ran ge, the D-A pair lum inescence peak sho uld sh ift to hi gher en ergies under cer tain con d itions . With su fficient intensity, the lines th at originate from pairs w ith larger in tra-pa ir d istan ces, and hen ce have lon g lifetimes, can be sa tu ra ted , thus leading to th e shift. Figure 63 shows changes in the gre en luminescen ce sp ectra of ZnS :Cu, AI ob served with cha nging excitati on intensity ov er a ra ng e of five orders of ma gnitude." The peak is see n to shift to high er energies with excita tion intensity. Also for th e blue luminescence of Zn S:Ag,At very similar sp ectra l pe ak shifts w ere observed , both as a function of time and with increasing excitation in tensity.v In Table 22, s hif ts of emission peaks obs erved in the tim e-resol ved sp ectra are listed for the five kinds of luminescenc e.
At omic structure of various centers and luminescence transitions. Th e at omic structure of lum inescence centers and th e nature of luminescence tran sit ions d edu ced from a bove-men tio ne d ex perimen ta l observ atio ns and their ana lys is are su mm arized in Table 22 for the fiv e kind s of luminescence. Th e luminescence for wh ich peak shifts ar e observed is th ou ght to be ca used by D-A pair typ e tr ans iti ons, w h ile th e lu m in escence no t sh owing p eak s hi fts is s urely due to intra-center transit ions. G-Cu center-This cen te r is fo rme d b y ac tiv a to rs (A) (C u, Ag , or Au) and coac tiva to rs (D) (AI or Cl. Br) introduced with n early eq ual concen tra tions an d d istributed randoml y in th e lattice occupying the ap p rop ria te lattice sites. Lu m inesce nce transiti on ta kes pl ace from th e D level to th e A level in va rious D-A pairs ha ving di ffe rent intra-pa ir di stanc es . Emission sp ectra consist of broad bands th at ar e quite different fro m those o f th e ed ge em ission. In th e G-Cu lu m inescence, th e A level is much d eeper than th ose levels inv o lved in th e edg e em ission . Th e electron-ph onon interaction for the accep tors is mu ch stron ger, res u lting in a Huang-Rhys-P eka r factor (i ii)
Chapter two: Principal phosphor materials and their optical properties
253
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Photon energy (eV) Figure 62 Time-resolved sp ectra of the gree n lum inescence of ZnS:Cu,A I phos p hor a t 4.2K. (From Era, K., Shionoya , S., an d Washizaw a. T., J. Phys . G em. Solids, 29, 1827, 1968; Era, K., Shionoya, S., Washizawa, Y, and Ohrn atsu, H., f. Phys. Chem. Solids, 29, 1843, 1968. With perrni ssion .)
S mu ch larger th an 1 in th e configurational coordinat e model. Spectra for S = 20 are broad Gaussian s. Th e spectra of th e G-C u luminescence ca n be interpreted in this w ay. Th e energy levels of Zn S:Cu,AI a re shown in Fig ur e 64.33 Before exci tation, Cu is monovalent (1+), wh ile Al is trivalent (3+), so that ch ar ge compensa tion is rea lized in th e lattice. Absorp tion A of th e fig ur e located at ab out 400 nm gives th e characteristic excitati on band of the cen ter. When exci ted, Cu an d AI become d ival ent (2+). Th e levels of Cu 2 + (301 9 con fig uration) are split by the crysta l field into 2T2 and 2£ s ta tes, with 2T2 ly ing higher in the ZB str u cture. Under excitation, th ree in duced ab sorption bands 8, C a nd 0 a re obse rve d , as sh own in Figure 65.34 Th e C abs orption tak ing place ins id e C u 2+ states h as a sha rp zero-p hono n line at 1.44 urn . Luminescence of Cu 2+ due to th e downward transi tion of C is observe d .
254
Fundamentals of Phosphors 10~
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U tiliz ing th e ind uced absorp tion bands due to Cu 2+ (C band) and to A(2+ (B ban d), direct evidence for the D-A pa ir emission mechanism of th e G-Cu lumine scenc e can be ob tained ." If the green luminescence of ZnS :Cu,Al or iginates fro m Cu-AI p airs, the dec ay ra te of th e lu min escence Rill m mu s t be correla ted wi th the deca y ra tes of the in tens ities of the Cu- and Al-in d uced absorption, R eu an d RAJ ' Res ults of st udies of the luminescenc e deca y and the decays of Cu and A l ab so rption inten sities are sh ow n in Figure 66.33 It is seen in th e figure th at RC ll an d RAI ar e equal to each other, and both are always equal to th e h alf va lu e of R' um during d eca y, namely Rill m = Reu + Rt\1. Thi s experimenta lly observed relat ion presents very clear and di rect evi d ence for th e C u-AI pair me ch anism . SA cen ter - Th is cen ter is form ed by th e sp a tia l associa tion of a Zn 2 + vaca n cy wi th a co-ac tiva tor CI- (or AP +) a t the nearest-neig hbor s ite. An emission peak sh ift is observed as shown in Tab les 22, so tha t the ini tial state of this emission is n ot a level in the associated cen ter, b u t is considered to be the level of an iso lated Cl (AI) d onor. A Zn 2+ vacancy needs tw o Cl" ions for charge compensa tion; on e of the tw o Cl: ions forms the associa ted cen ter, w hi le the o ther is isolated and is resp onsib le for the initial sta te of th e emission . The pola rization of lum inescen ce is de termined by the symmetry of the surroundings of the hol e a t a Zn 2 + vacancy.
Chapter two:
255
Principal phosphor materials and their optical properties
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Figure 64 Energy leve ls an d a bsorp tion tran sitions of ZnS :Cu,Al phosphor before excitation (a) and during excitation (b). (From Suzuki, A. and Shionoya , S., [. Phys. Soc. Japa n, 31,1455, 1971. With permission. ) ,---, ~
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Figure 65 Spec tru m of induced a bso rp tion (solid line) of a ZnS:Cu,AI sing le crystal under excitati on at 77K, and abso rp tion spec trum before exci tation (dashed line). (Fro m Suzuki, A. and Shionoya, S., l. Phys. Soc. Japan, 31, 1455, 1971. With permission. )
R-Cu cen ter- The polarization characteristics of this lum inescence can be interpreted by assum ing C3 \ sy m me try and by taking into acco unt the cry sta l field spl itting of the Cu 2+ 3d orb ital in this sy mmetry." It has been con cluded that this cen ter is formed by the spatial associ at ion of a substitutional Cu" an d a SZ- vaca ncy at the nearest-neighbor sites. Therefore, this center is one in which the relati on between activator and co-activato r is just rever sed from that of the SA center. In th is sense, thi s center can be a lso called the self-coac tiva ted
256
Fundamentals of Phosphors
• : Lumi nesence 0: AI absorption x : Cu absorption
E
w 1,_ _J - - - - - - 1 _ - - L _ - - - ' 40 20 o
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60
Time (min) Figure 66 Decay curves of gre en luminescen ce an d ind uced absorp tion in tensities due to Cu and Al in a ZnS:C u,Al single crystal a t abou t 10K. (From Suzuki, A. an d Shionoya, S., ]. Phys. Soc. Japan, 31, 1455, 1971. With p erm ission .)
center. N o spectr al shift is ob served, so that this luminescence is due to intra-center transitions and the initial sta te is th e level of the 9- vac ancy. R-Cu.1n ce nter-The polari zati on characteristics of this lumine scence can be in terpreted by assuming C, symme try and b y taking into account the sp litting of the Cu 2 + 3d orbital .P There is no spectral sh ift. Therefore, this cen ter is formed by the spa tial association of a substitu tiona l Cu" and 1n3+ a t the nearest neighbor si tes, and the luminescence is due to int ra-center transitions from the In d onor level to th e Cu acceptor level. B-Cu center-The polarization characteristics of this luminescence can be explained if one assumes either C3/J or C, symmetry for the center." A model of this cen ter ha s been prop osed, which suggests that the center is formed by the sp atial association of a substitutional Cu' an d an interstitial Cu· .3S Such a proposed center would have C3,>sy mmetry. In ZnSe, the cen ter cor res pond in g to the B-Cu center in ZnS sho ws gr een luminescence. Measurements of op tically detected electron spin resonance indicates that the center has th e stru cture of a pair composed of a s ubstitu tional Cu" and an interst itial CU+.36 It is clear th at the B-Cu center in ZnS h as th e sa m e typ e of structure and ha s C31' sym me try. Th e B-Cu luminescen ce shows no sp ectra l shift, indicating th at it is du e to an in tra-cen ter transitions. H ow ever, th e nature of the initial state of the transition is not clear.
(d) Other luminescence characteristics (i) Stimulation and quenching. It has be en known sin ce th e beginning of this cen tu ry that Cu-activated gree n ZnS phosphors show di stinct s tim ulation or quenching of the lum ines cenc e if irradiated b y red or near-infrared (NIR) light while under ultraviolet excitation or during the luminescent decay following excitation. Whether stimulation or quenching occurs depends on the conditions of ob servation, i.e., the ratio of the intensity of the excitati on light to the red or NIR light and on the temperature. Exposure with red or NIR light during de cay usually results in stimulation first, changing to quenching as time
Chapter two:
Principal phosphor materials and their optical properties
257
progresses. The spec tra of the light p ro ducing stim ula tion and qu en chin g h ave t w o p eak s at 0.6- 0.8 and 1.3 11m. It is seen from Figures 64 an d 65 that these two pe aks corresp ond to th e ind uced absorption bands C and Of indica ting th at absorp tion b y excited Cu activa tors (Cu 2+ ion s) is res po ns ible for these phenomena. By measuring time-resol ved emission spec tra an d phot oconductivity under irr adiation with red to NIR light, the m echanisms for s tim u la tion and quenching ha ve been es tab lishe d ." H oles are cre a ted in th e va lence band by in d uce d absorp tion of th e C and D bands (in th e case of the C ba n d , onl y at room temper ature). These holes m ove in the va lence band and a re tra p ped b y unexcited Cu activa tors . If Al (or CI) co-activ ators exis t in the vi cinity an d h ave tr apped electrons, g reen luminescence is produced im med iat ely. In O-A type lum in escence, th e tran sition probab ility is larg er when th e in tra pair di stance is smaller, so th at this imme d ia te luminescen ce p rocess occ urs. Und er the s tim ula tion irr ad iat ion of red to NIR ligh t with ultravi olet excitat ion, th e average value of th e intra-pair di st ance for excited Cu-AI pa irs becom es sho rte r, increasing the average tra ns ition p roba b ility and producing stim ula ted luminesce nce . On the ot her han d, qu enc hing is cau sed by a p rocess w here holes crea ted in th e va lence ba nd are trapped by vario us nonrad iative recomb in ation cen ters, thus de creasin g the effective number of hol es .
(ii) Kille r effect. It h as also been kn own since the 1920s th at the lumine scen ce in tensity of ZnS p hosphors is greatl y reduce d by con tamination with very sm all amounts of th e iro n gro up elemen ts Fe, Ni , and Co . Beca use of it, the iron gro up eleme n ts were called killers of lu m inescen ce. It follows that it is very important to re move iro n gro up eleme n ts in th e manu facturing processes of Zn S phosph ors. Figu re 67 sho ws how con tamina tion by Fe 2+, N i2+, and C 0 2+ reduces the in tens ity of the green lumine scen ce in Zn S:Cu ,Ap a The res ult s for Mn 2+, w hich also bel ongs to the iron gro up , are also sho wn . In th is case, an or ange luminescen ce of Mn 2 + is p roduced , as will be de scribed in 2.7.4.2; the Tv1n2+ in tens ity is also shown in the figure. The iron gro up ions have absorp tion band s in the visibl e region, an d their spectra ove rla p the G-Cu luminescen ce spectru m. It is th ou ght th at resonance en ergy tr ansfer from excited G-Cu centers to iron grou p ion s takes place, redu cing the G-Cu lumine scence intensity and caus ing the killer effect. The overlap betw een the lu minescen ce an d th e absorp tion spectra of iron gr oup ion s ob tained us ing sing le crys tals, an d the d ecrease in the green lu minescen ce intensity ca used by energy tran sfer w ere calcul at ed. The results are sh own in Figure 67(b) by th e d otted lin es. The de crea se of the luminescen ce inten sity in Mn 2+ is we ll de scribed b y this en ergy transfer effec t. In the case of Fe 2+, Ni2+, an d C 0 2+, the actual in ten sity decrease is considerably more th an th at w hic h was calc ula ted . As the reas on for th is disagree men t, it is sugges ted th at iron gro up ion s create d eep levels, and electrons and hol es recombine nonradiatively via the se levels. (iii) Concentration quenching and luminescence saturation. In ZnS :Cu,AI phosphors, if the concen tra tion of activat ors and co-acti va tors is increased to obtain br igh ter phosphors, concen tration qu enching results. Th e op tim um con centration is Cu : 1.2 x 10-4 mol/mol and AI: 2 x 10-4. The se phosphors sh ow, w hen used in CRTs, th e phenomenon of lu minescence intensity satura tion when the curren t d en sit y is raised, as show n in Figure 68.39 A very sim ilar ph enomen on occurs in ZnS:Ag,AI.4o Luminescen ce satu rati on phenomena p resen t serious p robl ems in the practical use of these p hosphors for CRT p urposes. The cause of the concen tration qu enching and luminescence saturation is not we ll understood , b u t the n onradiat ive A uger effec t is thou ght to pl ay an importan t ro le." In th is effect, excited Cu-AI pa irs are annihilated n onradiatively, th eir en ergy is tr an sfe rred
258
Fundamentals of Phosphors
1.0.,.----.--------....,..,.-....,,..---------------,
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C
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Figure 67 Dep en d ence of the G-C u lu minesc ence intensity (solid curves) in ZnS:Cu,Al(M) phosph ors on the concen tration of Mn2+ (Mn2+, Fc ~ ' , N i2+, and C 0 2,) at (a) room temperature and (b) 4.2K. In the case of Mn 2+, the in tens ity of the Mn 2+ orange luminescence and the total of the inten sities of the G-Cu and Mn2+ lu mi nescence are also shown. Dotted curves in (b) are calcula ted ass um ing the killer effect due to reso na nce ene rgy transfer. (From Tabei, M., Sh ionoya, S., and Ohma tsu, H., lpn. f. Appl. Phys., 14, 240, 1975. With permission.)
to unexcited Cu acti vators, and electrons are raised into the cond uc tion ban d . In th is way, the excita tion energy migrates in the lattice, and is dissipated in no nr adi at ive recom bina tion trap s.
2.7.4.2
Luminescence of transition metal ions
Divalent tran sition metal ions with a 3d n electron configurations have ion ic rad ii close to th ose of th e ca tions of IIb-VIb compounds. Therefore, these transition met al ions can be easily in tro d uced into IIb-VIb compounds; the optical properties of such ions have be en inv es tiga ted in d eta il. Optical transitions taking place within th e ions can be int erpreted by crys tal field the ory (see 2.2), assuming Td symmetry in ZB la ttices. Besides in tra -ion abso rp tion, va rious charg e-tr ansfer absorptions, such as th e valence band ~ ion and ion ~ the conduction band, are observed. Luminescence from cha rge -trans fer transition s is also observed in some cases . The absorption band A of Zn S:Cu,AI in Fig ure 64 is an exa m ple o f th is kind of absorption . Mn 2+- Th e ora nge luminescen ce of ZnS:Mn 2+ ha s be en known since earl y d ays, and is important in ap p lica tions in electroluminescenc e. The em issio n. spec trum has a peak at 585 nm at roo m temperature w ith a halfwidth of 0.2 eV. Absorption spec tra of a ZnS:Mn 2+ single cr ystal are sho w n in Fig ur e 69.41 The gr ou nd sta te of Mn 2+ is 6A ] (orig inating from the free ion sta te 6 5). Abso rp tion band s at 535, 495, 460, 425, and 385 nm correspo nd to transitions from the grou nd s tate to 4Tj(4G) , 4T2(4G) , (4A j,4 £WG ), 4T2(4 0 ), and 4£(4 0 ) exci ted sta tes, respecti vely Luminescenc e is produced from
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259
Principal phosphor materials and their optical properties
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Figure 68
Current density dependence of the green luminescence of three kinds of ZnS:Cu,Al (W type) phosphors at room temperature. Excitation was made by a pulsed 15-kV electron beam with 5-fls duration and a repetition frequency of 30 Hz. (From Kawai, H., Kuboniwa, S., and Hoshina, T., Jpn. J. Appl. Phys., 13, 1593, 1974. With permission.)
the lowest excited state 4Tj (4G), and a sharp zero-phonon line common to the absorption and the emission is observed at 558.94 nm." The location of the Mn 2 + energy levels in relation to the energy bands of the ZnS host is important in discussing the excitation mechanism for Mn 2 +. From measurements of X-ray photoelectron spectroscopy, it has been found that the Mn?" ground state is located 3 eV below the top of the ZnS valence band.P In more recent measurements using resonant synchrotron radiation, the locations of the Mn> ground state in ZnS, ZnSe, and ZnTe are 3.5, 3.6, and 3.8 eV, respectively, below the top of the valence band." &Z± and Fe 3+ - Iron in ZnS is usually divalent. Two kinds of infrared luminescence due to intra-ion transitions of Fe Z+ are known. One appears at 3.3 to 4.2 urn and is due to the transition from the lowest excited state 5Tz(50) to the ground state 5E(50) of Fe Z > .45 The other is composed of two bands at 971 nm and 1.43 urn, which are due to transitions from a higher excited state 3T1(3H) to the ground state 5E and the lowest excited state 5Tz, respectively." In the killer effect of Fez+ mentioned in Section 3.7.4.1, excitation energy is transferred from the G-Cu center to Fe 2+ and is down-converted to infrared luminescence or is dissipated nonradiatively. It has also been known since early days that iron in ZnS emits red luminescence at 660 nm. This luminescence is due to a pair-type transition, and is considered to be caused by a transition from a CP- state (CI- donor trapping and electron) to an Fe3+ state
260
Fundamentals of Phosphors
.~ 1.5 c
"'0
c;:; o
1.0
P..
o
77K
005
402K 400
450
500
550
600
Wavelength (nm) Figure 69 Absorption spectra of a ZnS:Mn 2 + (4.02 mo!'!'a) (ZB type) single crystal with 1.34-mm thickness at 4.2 and 17K. (From McClure, D.s., f. Chem. Phys., 39, 2850, 1963. With permission.)
(photoionized Fe 2+).47 In this process, lwninescence at 980 nm is also produced from intraion transitions in F e :J+ .48 Cl!!+-Copper in ZnS is usually monovalent. As mentioned in 2.7.4.1(c)(iii), when ZnS:Cu,AI phosphors are excited, an ionization process Cu" ~ Cu 2 + takes place and luminescence at 1.44 urn, due to an intra-ion transition from 2T2e O) to 2EeO) of Cu 2+, is produced (see Figure 64). ZnO:Cu exhibits green luminescence in a band spectrum with the peak at 510 nm and a broad halfwidth of 0.4 eV The zero-phonon line was observed at 2.8590 eV49 In ZnO the Cu 2 + level is located 0.2 eV below the bottom of the conduction band, which is different from the case of ZnSo Luminescence is considered to be produced by the recombination of an electron trapped at the Cu 2+ level (Cu' state) with a hole in the valence band.
2.7.4.3
Luminescence of rare-earth ions
The luminescence of trivalent rare-earth ions in Ilb-VTh compounds has also been investigated in fair detail. The introduction of trivalent rare-earth ions with high concentrations into Ilb-Vlb compounds is difficult, because the valence is different from the host cations and the chemical properties are also quite different. This is one of the reasons that bright luminescence from trivalent rare-earth ions in ZnS is rather difficult to obtain. Exceptionally bright and highly efficient luminescence has been observed in ZnS:Tm3+."'J This phosphor shows bright blue luminescence under cathode-ray excitation. The spectrum is shown in Figure 70; the strongest line at 487 nm is due to the lG4 ~ 3I-I6 transition (see 2.3).51 Beside this line, there are weak lines at 645 nm (lG4 ~ 3F4 ) and 775-800 nm (lG 4 ~ 3H5 ) . In the figure, the spectra of commercial ZnS:Ag,Cl phosphors (P22 and Pll) used in color CRTs are also shown for comparison. The energy efficiency of ZnS:Tm3+ under cathode-ray excitation is 0.216 W /W and is very high; this value should be compared with 0.23 W /W obtained in ZnS:Ag,CI (P22).
2.705
2nO phosphors
It has been known since the 1940s that by firing pure ZnO powders in a reducing atmosphere, a phosphor showing bright white-green luminescence can be prepared.s The
Chapter two: Principal phosphor materials and their optical properties
261
~+- Band path , 0.3nm '"'-ZnS : Tm 3 1
450
400
500
550
Wavelength (nm)
S:
(b)
400
450
500
550
Wavelength (nm )
Figure 70 Emission spec tra of a ZnS:Tm J + ph ospho r under cathode-ray excitation. Spectra of ZnS:Ag,CI ph osphors (pn , PH ) are also show n for compa riso n. (From Shr ad er, R.E., Lar ach, 5., and Yocom, P N., ]. Appl. Phys., 42, 4529, 1971. With permission.) chemi ca l formul a of thi s phosphor is written cu stomarily as ZnO:Zn because it is obtained in a reducing atmosphere without adding acti vators. The emission spectrum has a p eak at 495 nm and is very broad , having a h alfwidth of 0.4 eV This phosphor shows conductivity, and hence is an excee d ingly valuable phosphor for applications to vacuum fluorescent di spla ys and field emission di splays. The or igin of the luminescen ce center and the luminescence mechanism of ZnO:Zn phosphors are barely under st ood. Due to firing in reducing atmospheres, these phosphors con tain excess zin c; its amo u n t can be determined by colorimetric analysis and has been foun d to b e in th e 5- 15 ppm range. This amount of excess zinc is almost proportional to the inten sity of the white-gr een luminescence of these phosphors." Therefore, it is clear th at intertitial zinc or oxygen va cancy participates in the formation of the luminescence cen ter, but n othing h as been established with certainty. The decay constant of the emission is abo u t 'l us: the decay becomes fast er with increased excitation intensity.P From this, the lumine scenc e is th ought to be due to a bimolecular type recombination.
References 1. Review of lum inescence of IIb-VIb comp ou nd s: a. Sh ionoya, 5., Luminescen ce of latt ices of the ZnS type, in Luminescence of Inorganic Solids, Goldberg, P , Ed ., Acad emic Press, New York, 1966, cha p. 4.
262
Fundamentals of Phosphors
2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.
b. Shionoya, S., ll-VI Semiconducting Compounds, 1967 International Conference, Thomas, D.G., Ed., W A. Benjamin, Inc., 1967, 1. c. Shionoya, S., f. Luminesc., 1/2, 17, 1970. d. Curie, D. and Prener, JS., Deep Center Luminescence, in Physics and Cemistrun of II- VI Compounds, Aven, M. and Prener, JS., Ed., North-Holland Pub. Co., Amsterdam, 1967, chap. 4. Arpiarian, N., Proc. Int. Con! Luminesc., Budapest, 1966, Szigeti, G., Ed., Akaderniai Kiado, Budapest, 1968,903. Becquerel, E., Compt. Rend. Acad. Sci., LXIII, 188, 1866. Chelikowsky, J.R and Cohen, M.L., Phys. Reo., B14, 556, 1976. Nakamura, S., Sakashita, I., Yoshimura, K, Yamada, Y, and Taguchi, I., [pn. f. App!. Phys., 36, L491, 1997. Shrader, R.E. and Leverenz, HW, f. Opt. Soc. Am., 37, 939, 1947. Miyamoto, S., [pn. f. Appl. Phys., 17, 1129, 1978. Klick, c.c. f. Opt. Soc. Am., 37, 939, 1947. Thomas, D.G., Gershenzon, M., and Trumbore, F.A., Phys. Rev., 133, A269, 1964. Henry, C.H., Faulkner, RA., and Nassau, K, Phys. Rev., 183, 708, 1968. Reynolds, D.C. and Collins, r.c. Phys. Reo., 188, 1267, 1969. Dean, P.J. and Merz, J.L., Phys. Rev., 178, 1310, 1969. Merz, J.L., Nassau, K, and Siever, J.W, Phys. Rev., B8, 1444, 1973. Colbow, K, Phys. Rev., 141,742,1966. Bhargava, R.N., f. Cryst. Growth, 59, 15, 1982. Leverenz, HW., An Introduction to Luminescenceof Solids, John Wiley & Sons, New York, 1950. Kroger, F.A. and Hellingrnan, J.E., Trans. Electrochem. Soc., 95, 68, 1949. Kroger, F.A., Hellingman, J.E., and Smit, N.W., Phusica, 15, 990, 1949. Kroger, F.A. and Dikhoff, JAM., Physica, 16,297, 1950. Klasens, HA., J. Electrochem. Soc., 100,72, 1953. Prener, JS. and Williams, F.E., f. Electrochem. Soc., 103, 342, 1956. van Gool, w., Philips Res. Rept. Suppl., 3, 1, 1961. Shionoya, S., Koda, I., Era, K, and Fujiwara, H., f. Phys. Soc. Japan, 19, 1157, 1964. Koda, I. and Shionoya. S., Phys. Rev. Leii., 11,77, 1963; Phys. Rev., 136, A541, 1964. Kukimoto, H, Shionoya, S., Koda, I., and Hioki, R, f. Phys. Chern. Solids, 29, 935, 1968. Prener, J.5. and Williams, F.E., J. Chern. Phys., 25, 361, 1956. Urabe, K and Shionoya, S., J. Phys. Soc. Japan, 24, 543, 1968. Urabe. K, Shionoya, S., and Suzuki, A., f. Phys. Soc. Japan, 25, 1611, 1968. Shionoya. S., Urabe, K, Koda, I., Era, K, and Fujiwara, H, f. Phys. Chern. Solids, 27, 865, 1966. Suzuki, A. and Shionoya, S., f. Phys. Soc. Japan, 31, 1719, 1971. Shionoya, S., Kobayashi, Y, and Koda, I., f. Phys. Soc. Japan, 20, 2046, 1965; Suzuki, A. and Shionoya, S., f. Phys. Soc. Japan, 31, 1462, 1971. Era, K, Shionoya, S., and Washizawa, Y, f. Phys. Chern. Solids, 29, 1827, 1968; Era, K, Shionoya, S., Washizawa, Y, and Ohrnatsu, H, f. Phys. Chern. Solids, 29, 1843, 1968. Suzuki, A. and Shionoya. S., f. Phys. Soc. Japan, 31, 1455, 1971. Broser, 1., Maier, H and Schultz, HJ., Phys. Rev., 140, A2135, 1965. Blicks, H, Riehl, N., and Sizrnann, R, Z Phys., 163, 594, 1961. Patel, J.L., Davies, J.J., and Nicholls, J.E., f. Phys., 04, 5545,1981. Tabei, M. and Shionoya, S., f. Luminesc., 15, 201, 1977. Tabei, M., Shionoya, S., and Ohrnatsu, H, [pn. f. Appl. Phys., 14,240, 1975. Kawai, H, Kuboniwa, S., and Hoshina, I., [pn. f. App!. Phys., 13, 1593, 1974. Raue, R, Shiiki. M., Matsukiyo, H, Toyama, H, and Yamamoto, H., f. App!. Phys., 75, 481,1994. McClure, 0.5., J. Chem. Phys., 39, 2850, 1963. Langer, D. and Ibuki, S., Phys. Rev., 138, A809, 1965. Langer, D., Helmer, r.c. and Weichert, N.H, J. Luminesc., 1/2,341, 1970. Weidemann, R, Cumlich, H-E., Kupsch. M., and Middelmann, H-V., Phys. Rev., B45, 1172, 1992.
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263
Slack, G.A. and O'M eara, B.M., Phys. Rev., 163, 335, 1967. Skow ronski , M. and Lire, D., f. Lumin esc., 24/25, 253, 1981. [aszczyn-Kopec, P an d Lam bert, B., f. Lum inesc., 10, 243, 1975. Nelkowski, H., Pfut zenreuter, O. and Schrittenlacher, w., J. Luminesc., 20, 403,1979. Dingle, K , Phys. Rev. Leti., 23, 579, 1969. Sh rade r, K E., Larach . S., an d Yocom, PN., J. Appl. Phys., 42, 4529, 1971. Ch arreire, Y. and Parche, P., J. Electrochem. Soc., 130, 175, 1983. Harad a, T. and Shionoya, S., Tech. Digest, Phosphor Res. Soc., 174th Meeting, February 1979 (in Japanese). 53. Pfanel, A., J. Electrochem. Soc., 109,502, 1962.
45. 46. 47. 48. 49. 50. 51. 52.
chapter two - section eight
Principal phosphor materials and their optical properties Toshiya Yokogawa Contents 2.8 ZnSe 2.8.1 2.8.2 2.8.3
and related luminescent materials MOVPE MBE II-Type doping 2.8.4 p-Typ e doping 2.8.5 ZnSe-based blue-green laser diodes 2.8.6 ZnSe-based light-emitting diodes Referen ces
265 265 266 267 268 268 270 271
2.8 ZnSe and related luminescent materials The wide-bandgap ZnSe and related luminescent materials have attrac ted consid erabl e recent a ttention becau se of the ad ven t of blue-green lasers and light em itt ing di od es (LEDs). Since ZnSe is nearly lattice matched to GaAs (which is a high-quality substra te material), high-quality ZnSe can be grown. Furthermore, addition of 5, Mg. or Cd to ZnSe leads to a tern ary or quaternary alloy with a higher or lower bandgap, a property wh ich is needed to fabri cat e hetero structure devices. Advanced crystal growth techniques such as metal orga n ic vapor ph ase epitaxy (MOVPE) and molecular beam ep itaxy (MBE) have mad e it p ossible to grow not only high-quality ZnSe but also lattice-matched ternary and qu aternary alloys on (100) GaAs substrates. The success of these growth techniques at low temperatu res ha s resulted in limiting the concentration of background impurities. The reduction of background impurities has allowed us to control th e con d uctivi ty by the incor pora tion of sha llow accep tors and donors. In this section, crystal g row th techniques for ZnSe and related luminescent materials will be described first, and then app lica tion for ZnSe-b ased laser di od es will be discussed.
2.8.1 MOVPE MOVPE gro w th of ZnSe invol ves the pyrol ysis of a vapor-phase mixture of HzSe and, most commo nly, dimeth ylzin c (DMZn) or diethylzinc (DEZn). Free Zn atoms and Se
265
266
Fundamentals of Phosphors
molecules are formed and these species recombine on the hot substra te surface in an irrev er sible reaction to form ZnSe. Growth is ca rr ied out in a cold-wall rea ctor in flowing H 2 a t a tm os p he ric or low p ressure. The s ubs tra te is heated to temper atures of 300 to 400°C, typicall y by radio frequenc y (RF) heating . Transp ort of the metal-or gan ics to the growth zone is ac h ieved by bubbling H 2 through th e liquid so urces, which are h eld in temperaturecontroll ed containers. Wh en DMZn and H 2Se ar e used for the MOVPE growth, a prem ature reaction tak es pl ace, resulting in poor uniformity and poor surface morphology of th e ZnSe epitaxial layers. This has been solved by using dialkyl zincs an d dialkyl se leni des (DMSe or DESe).1 With th ese source materials, uniform growth of ZnSe epitaxial layer s with sm ooth sur face morphology has been ach ieved. However, the growth temperature h as to be increased abo ve 500°C for Zn Se. Recently, it ha s be en est abli shed that th e grow th at relatively lower temperature and th e use of h igh-purity sou rce materials ar e required to obtain high-quality ZnSe films. It ha s also b een reported th at photo-assisted MOVPE growth dramaticall y enh ances the gr owth rate of ZnSe at temper atures as low as 350°C, reducing th e op tim um grow th temper a tu re.2 It wa s generally th ought th at the source materi als o f DMZn or DEZn typically conta in 10 to 100 ppm chlorin e impurity. Th e Zn Se ep itax ial la yers gro w n using such sources show s tro ng bound-exciton emissi on due to chlo rin e donor impurities. On the other hand , ZnSe la yers gro w n using high-purity DMZn showed dom inant free-exciton emissi on in the low-temperature photoluminescen ce spectru m , as shown in Figure 71.2 The DMZn u sed was puri fied so that the chlorine content was below the detection limit of 5 ppm. Numer ous attempts h ave been m ade to grow p-type ZnSe crystals usin g gro u p I and V elements as accep tor dopants. H owever, there h ave been only a few a ttem p ts for the MOVPE grow th of p-type Z nSe. With Li doping by MOVPE , hi gh p-type cond uctivity (50 Q - I em:") m aterials with ca rr ier co nce ntra tions up to lOIScrrr' have been dem on strated, although the ve ry fast diffu si on rat e of Li dopants in ZnSe crystal results in poor con trollabili ty of ca rrier concentrati on s.' N itrogen is thought to be a stable acceptor impurity. How ev er, nitrogen doping in th e MOVPE resulted in hi ghly resistive ZnSe film s. The nitrogen d oping in MOVPE s till experiences a problem with the low acti vati on efficiency of acceptors due to hydrogen p as sivat ion. I..; n-type d opin g elements for th e MOVPE g row th also h as been in vesti gated u sin g Al and Ga to subs titute for th e Zn site a nd Cl, Br, and I for the Se site in th e ZnSe la ttice. It ha s be en reported tha t iodine d op ing w ith ethyliodide or n-butyliodide re sults in a good co n tro llab ility of ca rri er concentrat ion s, w hi ch r an ge from 1015 to 1019 cm-3 .6,7
2.8.2 MBE Molecular be am ep itaxy (MBE) is th e g row th of semiconductor film s su ch as Zn Se by the impingement of directed atomic or m olecular be ams on a crystalline sur face under ultrahigh-vacuum (U H V) condition . Molecul ar beams of Zn an d Se are ge nera ted in a resistivel y healed Knudsen cell wi thin the g ro wth cha m ber. Ga As subs trates are usu ally used for th e Z nSe g row th . Modern IT-VI M BE sy stems are genera lly a multichamber app aratu s co m pr ising a fast en try load -lock, a preparation ch amber, an d two growth cha mbers for II-VI and III-V films. Systems are of st ainless steel cons tr uction pumped to UHV con ditions. Base pressures of 10-n to 10- 1tI Torr are normally a ttai ned. A major attraction of MBE is that th e u se of UHV conditions enab les the incorporat ion of hi gh- va cuum-based surface analytical an d d iagnostic techniques. Reflection hi gh-energy elec tr on diffraction (RHE ED) is commonly em ployed to exa mi ne th e subs tra te a nd the ac tua l epita xial film during
Chapter two:
Principal phosphor materials and their optical properties
267
Ix Ex
>~
conventional DMZn
CJ)
z
UJ ~
Z
...J
0W
>
Ex
~
...J
ur
a:
2.76
high purity DMZn
2.78 PHOTON
ENERGY
2.80 (eV)
Figure 71 Pho toluminescence spectra at 10K of ZnSe films grown using a conventi ona l DMZn source in whi ch about 15 ppm chlorine impurities are involved (upper trace), and using a purified source for which the ch lorine content is below the detection limit of 5 ppm. The em ission line label ed Ex is du e to free excitons, and emission lines 12 and I, are due to exciton s bound to neutral accep tors . (From Kukirnoto, H., J. Crystal Growth, 101, 953, 1990. With perrnission.)
grow th . A (quadrupole) m ass spectrometer is essential for monitoring the gas composition in the MBE gro w th cha mber. Early ZnSe-based laser diodes show room-tempera hire and con tin uous wave (RTCW) lifetimes of the order of a minute because of degradation cau sed by extende d crystalline defects such as s tacking faults . Transmission electron microscopy (TEM) im aging indicates that the degradation or iginates from di slocation networks that dev elop ed in the quantum well region during lasing. The dislocation networks w ere produ ced by th e stacking faults nucleated at the II-VI/GaAs interface and extending into th e II-VI lay er. To reduce the stacking fault den sity, incorporation of GaAs and ZnS e buffer lay ers and Zn treatment of the II-VI/GaAs interface were employed." The low est defect density film s were reported to be obtained wh en the (2X4) As-stabilized GaAs su rface was exposed to a Zn flu x, which resulted in (2X4) to (l X4) surface reconstructions. Thi s was then followed by the epita xial gro w th of ZnSe. Stacking fault densities of 103 crrr? or less were ach ieved und er th is grow th condition.
2.8.3 n-Type doping Group III atoms such as Al and Ga substituting in Zn sites and G roup VII atoms such as CI and I in Se sites are typical impurities producin g n-ty pe carr iers in ZnSe crys tals.
268
Fundamentals of Phosphors
n-Type doping in ZnSe during MBE growth h as been extensively stu di ed . Ga impurities ha ve often been used as a donor dopant altho ugh ma ximum ca rrie r concentrations are limited to approximatel y 1017 crrr-'. The photoluminescence (PL) properties of Al- or Gadoped la yers shows a remarkable degradation of the band-edge em ission when the carrier co ncen tra tion exceeds 10 17 crrr-'.? Cl impurities w er e also studied as a d onor dopant at the Se site.'? In Cl doping, n-type carrier concentra.tion increases with th e temperature of th e Zn Cl 2 cell. The maximum ca rrier concentration has been estimated to be 10 19 crrr' , resulting in a sm all resistivity of 10 °' Dcm. Lately, Cl im p u ri ties have been used to fabricate bluegreen laser diodes and light-emitting diodes because of advantages presented by controllab ilit y and crystallin e quality.
2.8.4
p- Type doping
Group I atoms such as Li and Na at Zn sites and gr oup V atoms such as N, P, and As at Se sites are typical impurities us ed to produce p-type ca rr iers in ZnSe crystals , Net acceptor concentrations (N,\-N o) of 10 1( cm-' have be en ac hiev ed using Li d oping at a growth temperature of 300°C. 1l Capacitan ce-voltage (C- V) profil ing is usually used to measure N A-N o, which implies uncompensated acceptor concen tra tion. When th e Li impurity concentration (N A) exceeds 10 17 crrr', N ,\-N o decreases due to increased com pensa tion . This com p ensa tion is thought to origin a te from in creased concentration (N o) of Li interstitial donors in heavily doped ZnSe. Lithium doping is also problematic in that lithium atoms ca n ea sily diffuse within th e epitaxial layer. Hi ghly resistive ZnSe films h av e been grown w ith As an d P doping . A first principles total energy calculation suggests th at two neutral accep tors co mbi ne to form a new deep state that results in the high resist ivity of As- and P-doped ZnSe.12 Exp erimental results, which show p-type conduction is difficult in As- o r P-d oped ZnSe, are con sist ent with this proposed model. An important breakthrough came with the deve lopment of a N 2 plasma so ur ce for MBE.13·14 Th is technique empl oy s a small helical-coil RF plasma chamber re p lacing the Knudsen cell in the MBE chamber, The active nitrogen species is thought to be eith er neutral, mon oatomic N free radicals, or neutral, excit ed N 2 mol ecules. Th is techn ique ha s been u sed to ach ieve N A-N o = 3.4 x 1017 crrr' and blue emission in LEOs. Thi s ad va nce was rapidly followed by the first Z nSe-based laser. Ma ximum net acceptor con centration ha s been limited to around 1 x 10 18 em>' in ZnSe. At present, ho w ever, the nitrogen -p Jasma doping is the best wa y available to a. chieve p-type ZnSe an d has been most frequently used to grow p-n junctions by MBE and to fabricate ZnSe-b ased laser diodes. N incorporation d ep ends on the growth temperature and the plasma power. Increased N incorporation is found with low growth temper ature and hi gh RF p ower. Photoluminescence (PL) spectra in lightly N-doped ZnSI' la yer s w ith co nce n tra tions less than 10 17 crrr-' show a neutral acc ep to r bound-exciton em iss ion and a weak em ission due to donor-a cceptor pair (OA P) recombination. With in creasing N con centration , up to 10 1 ~ crrr', OAP em iss ion became dominant in the PL spectrum. This highly N-dop ed ZnSe sho ws a p-type con d uction as confirm ed by capacitance-voltage and Van de r Pau measurements. From PL an a lys es of th e exci ton ic and DAP e miss ion s, th e N- acceptor ionization en ergy w as estimated to b e ab out 100 meV, which is in good agreemen t w ith the result calculated with an effective ma ss approximation .
2.8.5 ZnS e-based blue-green laser diodes ZnSe-based blue-green laser diodes have been s tud ied int en sively to be applied in nextgenera tion, high-d ensity optical di sk m emories and la ser printers. Since the first demon-
Chapter two:
Principal phosphor materials and their optical properties
269
stration of II-VI blue-green laser diodes ." further improvemen ts in mat erials qua lity coupled with the use of w id e ba ndgap ZnMgSSe qua ternary alloys for im p ro ved elec trical as well as optical confinement and th e developmen t of oh m ic con tac ts to p-type laye rs have led to room-tem p era tu re (RT) CW opera tion of ZnSe-ba sed laser di odes with very reduced thr esh old currents and voltages has bee n ach ieved ." Th e firs t elec trica lly injected ZnSe-based laser was obtained using ZnSSe cladd in g layer s latt ice-m a tched to the GaAs subs tra te an d a ZnCdSe single quantum we ll surro un de d by Z nSe w aveg uide layers . The b and structure in the strain ed-l a ye r ZI1n H1CdolsSe/ZnSe sys tem wa s th ou ght to be a type I quantum well s truct ure wi th cond uction an d va len ce ban d offse ts of L\E c = 230 meV and L\E v = 50 meV, resp ectively. Acco rd ing to a com mon anio n ru le, th e con duc tion band offset is relatively la rger than that of the va lence band in th is sys tem. Op tical an d electrical confinement in th is pro totyp ical laser structure is quite w eak d ue to th e cons train t in the de vice d esign by the la rge lattice m ism a tch between ZnSe and CdSe. The use of the latti ce-m at ch ed qua ternary ZnMgSSe allow s grea ter refrac tive index and ba n dgap differ en ces to be realized . Th e inco rporation of Mg in to th e cladding layer impro ves th e confi nemen t factor, resulting in the RTCW op era tion of the II-VI lasers. Shor ter-wavelength lasers with a ZnSe ac tive layer have also been mad e possible. A typical s truct ure of the ZnCdSe /ZnSSe /ZnMgSSe sepa ra te-con fine me n t he terostructure (SCH) lase r is sho w n schema tically in Figure 72.16 Th e incorpora tion of GaA s :Si and ZnSe:Cl buffer layers and the Zn beam ex pos ure on a n As -s tabi lized surface of the GaA;' b uffer laye r were employed to red uce s tack ing fault d en sity. The s tack ing fau lt d en sit y of the laser s tructure was es tima ted to be 3 x 103 cm-2 . For th e p- and t l-: Zn 1_, Mg,SySe 1_y cladd ing layers, d esigned for optical confi nemen t, the Mg concen tra tio n "vas nominally x = 0.1 and the sul fur concen tra tion y = 0.15. The Cd comp osi tion of 0.35 in th e 'ZnCdSe active layer results in lasing w ave length A = 514.7 nm. Low -re sista nce quasi-ohmi c con tac t to p-ZnSe :N is usually achieved usin g heavily p-doped ZnTe:N and ZnSe /ZnTe m u ltiquantum w ells as an intermediate layer. The threshold current under CW operation wa s found to be 32 m A, corresponding to a th reshold cur ren t densi ty of 533 A crrr-', for a laser d iode with a stripe area of 600 11m x 10 urn an d 70 /95°ft) h igh reflectiv e coa tin g. The th reshold voltage was 11 V. Currently, th e life time of laser diodes op er ating at a tem p era tu re of 20°C has been reported to be 101.5 hours, the longest for ZnSe -based Jaser diodes .'? The spectacu lar progress in edge-emitting laser s h as stimula ted exploration of more ad vanced designs such as the ve rtica l-cav ity sur face -emitting lasers (VCSELs) opera ting in the blue-gre en region. VCSELs ha ve recentl y attracted m u ch atten tion becau se of thei r sur face-norma l op eration, potential for extremely low thr eshold curren ts, and th e eas e with which they ma y be fabricated in closely spaced and tw o-d imension al arra ys. Th ese lasers ar e ideal for integration wi th othe r devices suc h as transist ors for ph ot on ic sw itch ing app lications. Outpu t character istics s uch as n a rrow diver genc e beams and opera tion in a single longitud inal mode, d ue to th e large mod e spacing of a short cavity, are ad di tion al adva ntages . Blue-green VCSELs have experience d significan t progress recently. For example, electrical pumped ope ra tion has bee n demo ns tra ted a t 77K.ISThe VCSEL s truc tu res used we re con sistent w ith a CdZnSe /ZnSe multiqu antu m- w ell (MQW) active layer, 11- and p-ZnSe cladding layers, and two Si0 2 / Ti0 2 d istribu ted Bragg re flec tors (DBRs), as shown in Figur e 73. The reflec tivi ty of the Si0 2 / Ti0 2 dielectric mirrors was grea ter th an 99%. Th e VCSEL devices were cha racterized at 77K under pulsed operation. A very low th reshold current of 3 rnA was obtained in th e VCSEL. Sin gle lon gi tu d inal mode opera tio n was obtained a t the lasin g wavelength of 484 nm. Above th e thr eshold , the far-fie ld ra d iation angle was as narrow as 7°, which indicated the spa tial coherence exp ected for VCSEL
270
Fundamentals of Phosphors Pd I Pti A u ele ct rode insulator r-/'''----7I'-------"""L-
Z nT e :N
tL<...-e-~.,.,---Z nS e : N I ~+..<................."<'"t"-ZnSe :N
P~~---i.::.:..:C~~
ZnSSe:N
(30 n m) ZnTe:N MOW (0 1 j1 m) (03j1m )
Zn xM91 _xS ySe 1-y:N (0 .7j1m)
ZnSS e :N ZnC d Se Z nSSe :CI Zn xM91.xSySe 1-y:CI (1.0 j1 m )
ZnSSe :CI (140 nm) ZnS e :CI (30 nm) n-G aAs buffer la y e r ( O.25fi m)
n-G aA s s ub .
In electrode Figu re 72 Schematic s truc ture of Z nCd Se /ZnSSe / ZnMgSSe SCH laser s . (Fro m Itoh , S., Na kayama, N., Matsum oto, S., et al., [pn . f. Appl. Phys., 33, L938, 1994. With pe rmission.)
emission . Th is blu e VeSEL op en s the d oor for a bro ad range of new device applicati on s for II-VI m at erials.
2.8.6 ZnSe-based light-emitting diodes Further improved performance of Zn Se-b ased light-emitting di od es (LEOs) has also been d emon stra ted since the first demon s tration of II-VI bl ue-green laser dio des . Recently, hi gh br ightness ZnSe-based LEOs operating a t p eak wavelengths in the 489- to 514-nm ran ge hav e been reported.' ? The LED cons isted of a 3-11m th ick n-type ZnSe:Cl layer, a 50- to 100-nm green-em ittin g active region of ZnTe o1Se o9, and a J-um th ick p-typ e Zn Se:N layer gro w n by M BE on the Zn Se subs tra tes . The d evices produced 1.3 m W (10 rnA , 3.2 V), pe ak in g a t 512 om w ith an ex terna l efficien cy of 5.3%. Th e em issio n spectru m of the LED was relat ively broad (50 nm) due to emission from the ZnTeSe active region . The luminous per for ma nc e of the de vice was 171m W-1 at 10 rrtA, which is com parable to the performance of s uper-b righ t red LEO s (650 nm) based on AIGaAs double het er ostructures. The lifetime of the LEOs at RT ha s been report ed to exceed 2000 hours, wh ich is the longest lifeti me of an y ZnSe-LEO device.
Chapter two: Principal phosphor materials and their optical properties
271
Au/Pd contact Insulator . . . . . . . ... . ... ....
..
.
p-ZnSe
n-ZnSe
DBR Mirror
n-GaAs sub.
In/Sn contact
Laser Emission Figure 73 Schem ati c s tru ct ure of CdZnSe / Zn Se blu e-green vertica l cavi ty s urface em itting lasers. (From Yokogawa, 1., Yoshii, S., Tsujimura, A., Sasa i, Y, an d Mer z, J.L., [pn. ]. App!. Pllys., 34, L751, 1995. With per mission .)
High-brightness LEDs wi th a Zn CdS e quantum well have bee n d em on st rated .P' The LED consists of a Zn CdSejZnSSe multiquantum w ell and ZnMgSSe claddin g laye rs grown on GaAs subs tra tes . Th e d evices produced 2.1 mW (20 m A, 3.9 V), and peake d at 512 nm wi th an external efficiency of 4.3'10. A na rro w sp ectral output of 10 nm h as been obt ain ed in the devices.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Mitsuhash i, H ., Mitsuishi, 1., Mizuta, M., an d Ku kim oto, H , [ pn. J. App!. Phys., 24, L578, 1985. Kukim oto. H., ]. Cryst. Growth, 101, 953, 1990. Yasud a, 1., Mitsuishi, 1., and Kuki rnoto. H ., App!. Phys. Lctt ., 52, 57, 1988. Kam at a, A., Mi tsuhash i, H., and Fu jita, H , App!. Phys. Leii., 63, 3353, 1993. Wolk, J.A , Ager, III, j .w, Duxstad, K., Haller, J.E.E., Task er, N .R, Dorm a n, D.R, and Olego, D.J., App!. Phys. Leti. , 63, 2756, 1993. Shiba ta, N ., Ohki, A , and Katsui, A , f. Cryst. Growth, 93, 703, 1988. Yasu da , T , Mitsu ishi, I., an d Ku kim oto, H ., App!' Pliys. Leit ., 52, 57, 1988. Cunsho r, R L., Kolodzi ejski, L.A, Malloch. M.R, Vaziri, M., Cho i, C , and O tsu ka, N., App!. Pllys. Lett, 50, 200, 1987. Ni ina . 1., M inato, T , an d Yoneda, K., [pn . ]. App!. Phys., 21, L387, 1982. Ohkawa, K., Mi tsuy u, T , an d Yamazaki, 0 ., ]. App!. Phys., 62, 3216, 1987. DePu ydt, J.M., Hasse, M.A, Cheng, H ., an d Po tts, J.E., App!. Phys. Lett. , 55, 1103, 1989. Cha d i, DJ and Chang, K.L., App!. Phys. Leti ., 55, 575, 1989. Pa rk, RM., Troffer, M.B., Roul eau, C M., DePuyd t, J.M., and H aase, M.A, App!. Phys. Lcit ., 57, 2127, 1990. Ohkawa, K., Karasawa, 1., and Mit su yu. T , ipn. f. App!. Pl1ys. , 30, L152, 1991. Ha sse , M.A., Qiu, L DeP uyd t, J.M., and Cheng, H ., App!. Pl1ys. Lett., 59, 1272, 1991.
Fundamentals of Phosphors
272
16. Itoh, S., Nakayama, N ., Matsumoto, S., Nagai, M., Nakano, K., Ozawa, M., Okuyama, H., Torniya. S., Oha ra, T, Iked a, M., Ishib ashi, A., an d Mori, Y, [ pn. j. Appl. Phys., 33, L938, 1994. 17. Taniguchi, S., Hino , T, Itoh, S., Nak ano, K., Nakayama, N ., Ish ibashi, A., and Iked a, M., Electron. Leii ., 32, 552, 1996. 18. Yokogaw a, T, Yoshii, S., Tsujimura, A., Sasai , Y, and Merz, J.L., [p n. j. App l. Phys., 34, L751, 1995. 19. Easo n, O.B., Yu, Z., H ug hes , wc.. Roland, WH., Boney, Cook, Jr., J.W, Sche tzina, J.F., Cantwell, G., and Harsch, We., A~"PI. Phys. Leii ., 66, 115, 1995. 20. N ikkei Electronic s, 614, 20, 1994 (in Japa nese) .
c.
chapter two - section nine
Principal phosphor materials and their optical properties Hiroshi Kukimoto Contents 2.9
Illb-Vb compo un ds 2.9.1 Gen eral overview 2.9.2 Ga P as lum inescence mat er ial 2.9.2.1 Energy band struc ture 2.9.2.2 Isoelect ron ic trap s 2.9.2.3 Don or-acceptor pair emission 2.9.2.4 Applicat ion for light-emitting diodes References
273 273 276 276 276 278 280 281
2.9 IIIb- Vb compounds 2.9.1
General overview
IIIb-Vb compo u nds , w hic h cons is t of the group Illb and Vb elemen ts of the periodic table, include man y important semicon ductors such as GaP, GaAs, Ga N, and InF. These materials are not used fo r ph osph or s in a p olycrystalline form as is the case of IIb-VIb compounds, but they are utilized for many op toelectronic devices such as light-emitting diodes, semicond uctor lasers, and ph otodiodes in a sing le crystalline form of th in film s. IIIb-Vb compounds are to some ex ten t sim ilar to IIb-VIb com po unds in terms of the nature of the atomic bond; more precisely, from a v iewpoin t of the nature of their ionic and covalent bonding, the y are well situated between grou p IV ele me n tal semicond ucto rs and IIb-VIb compound semico nd uc tors . Therefore, many sim ilari ties in the op tical properties can be seen between these classes of compounds. In some cases, the optical prop erties due to impurities can be m ore clearly observed in IIIb-Vb com po u nds th an in IIb- VIb com pound s. Before moving on to the op tical properties of typicallIIb-Vb co mpo u nds, an overvie w of the composition of this group is presented. Typical characteristi cs of IIIb- Vb compounds are show n in Table 23. The materials composed of lighter elemen ts tend to be more io nic th an th ose com posed of heavier elem ents. Th is trend is reflected in their crystal structure and energy gap; the wurtzite
273
N
Table 23 Cr ys tal Ma ter ia l str uc tu re"
La ttice ca ns t. (A)
a
c
ZB H
3.615 2.51
6.69
BP BAs
ZB ZB
4.538 4.777
BSb AIN AlP
ZB W ZB
3.111 5.467
AlA s
ZB
A lSb
BN
-
-
Densit y (g crrr' )
Melting poin t
Band
(0C)
str uctur e" ID
3.49 2.26 2.97 5.22
>2000
Bandgap' (eV)
D
7.2 3.8
ID ID
2.0 1.6
Effecti ve m as s" Electro n
1.2
3.26 2.40
1525
D ID
6.20 2,45
0.29
-
5.662
-
3.60
1740
ID
2.15
ZB
6.136
-
4.26
1080
10
1.63
GaN
W
3.189
D
3.39
0.5 1.56 (11), 0.19 (.i) 0.39 1.64 (II), 0.23 (.i) 0.22
GaP
ZB
5.451
-
4.13
1465
10
2.27
0.25
GaAs
ZB
5.653
-
5.32
1238
D
1.43
0.0665
GaS b
ZB
6.096
-
5.61
712
D
0.70
0.042
InN In P
W ZB
3.533 5.869
InAs
ZB
6.058
4.980
5.185
5.693
6.10
'J
Pro p erties o f HIb-Vb Compounds
6.88 4.79
- 1200 1070
D D
0.6-0.7 1.34
0.04 0.079
5.67
943
D
0.35
0.023
H ole
>j::,.
Mobili ty (ern? V-I S-I) Elect ron
H o le
Die lec tri c constan t Sta tic £0
Optica l e;
7.1 6.85 (Lc), 5.09 (lie) 11
4.5 4.95 (Lc), 4.10 (llc) 8.2 10.2
0.51 (h), 0.2 (I)
0.63 (h), 0.20 (I) 0.5 (h ), 0.26 (I) 0.5 (h) , 0.11 (I) 0.8 0.67 (h) , 0.17 (1 ) 0.475 (h), 0.087 (I) 0.32 (h), 0.045 (I) 0.45 (h ), 0.12 (I) 0.41 (h), 0.025 (1 )
80
Refractive index" 2.12 (0.589) 2.20 (0.05) 3.0 - 3.5
8.5 9.8
4.8 7.5
2.25 (0.4) 2.99 (0.5)
180
290
10.1
8.2
3.2 (0.56)
200
400
12.0
10.2
3.45 (1.1)
5.4
2.00 (0.58)
300
100
9.5 (ole), 10.4 (lIe) 11.0
9.1
5.19 (0.344)
8500
400
12.9
10.9
3.66 (0.8)
4000
1400
15.7
14.4
3.82 (1.8)
1200
"'i'"j
:::
5.
:::. ~
~
S-
4000 4600
650
15.0 12.6
6.3 9.6
3.33 (1.0)
33000
460
15.2
12.3
3.52 (3.74)
Vi' ~ 'V
~
0
VI
"'l::l
5"
Vi
Tabl e 23 La tti ce ca ns t. (A) C rys ta l Ma te ria l struct u re" a c InSb
ZB
6.479
-
Den si ty (g crrr') 5.78
Melting p oint (OC ) 525
Band Bandgap ' structure" (eY) D
0.18
, Z B: zin c-bien de , H: hexagonal , W: wurtzite . 0: direct type, 10: ind irect ty p e. , At 300K.
b
d
II, .1: pa ra llel and
perpend icular to th e principa l ax is; h and I: heavy an d light holes .
" Wave lengt h urn in paren thesis.
n
Prop er ties of Illb-Vb Com pou nds (continue d) Effec tive mass:' Elec tron 0.014
Mobi lity (cm2 Y-1 S-l)
i5 -;:;. Di electric co nsta nt
H ole
Electron
H ole
Static
0.40 (h) , 0.016 (I)
78000
750
16.8
Eo
Optica l E••
Re frac tive index"
15.7
4.00 (7.87)
'"-e
~ ~
"0
"" ' 3 " -B' ;'2,.
""1:::!
;:,<::) OJ)
""1:::! ;:,<::)
s""
~
'" "" ' !i5 Vl
'"
;: ::L.
;:;. ::;. '" <::)
-;:;.
;:).
;'2,. ""1:::!
c;
""1:::!
'";::;.
~.
N 'J
en
Fundamentals of Phosphors
276
struc tu re and wider gaps are prevalent in lighter m at erials, while the zinc-blende structure and n arrower bandgaps occur in h eavier materials. Furthermore, one should note that optical properties of these materials largely depend on the type of energy band stru ctur e, direct (0) or in d irect (ID). Light material s, inclu d ing BN, BP, AIN, and GaN, h ave high melting points and w ide bandgaps. In general, conductivity control of these m at erials has not been so easy. Recently, GaN and related alloys have become important mat er ials for blue light emi ssion as is described in Section 2.8.5. In contrast, he avy materi als including AISb, GaSb, InSb, and InAs ha ve low melting points and narrow bandgap s. In add ition, the y features high mobility. The se properti es are suited for light-em itting de vices and photodete ctors ope rating in the infrared region. AlP, A lAs , GaP, C a As, a nd InP are locat ed b etw een the above two extrem es. Th eir bandgap s range from the near-infrared to th e visi ble light region, and these m aterials and related alloys of AIGaAs, GaPAs, GaInP, GalnAs, an d GaAIPAs are key materials for the optoelect ronic ap p lica tions , which are de scrib ed in Section 2.8.3.
2.9.2 GaP as luminescence material 2.9.2.1
Energy band structure
GaP is an indirect ga p se mic ond uctor with an energy band s truct ure sim ila r to that of Si, as illustrated in Fig ur e 74. Th e bottom of the conducti on band is located near the X point in momentum or w a ve ve ctor (k ) space, while the top of the va lence band is found at the I point. Before and af ter the even t of optical transition , momentum mu st be con served for light and elec tro n alike . Becau se the momentum of light is negligibl y s mall, direct electron transition s must take pl ace bet ween bands a t the sa me k va lues. Therefore , the probability o f an intrinsic optical transition across the ban dgap in Ga P, i.e.. between the X and I points, is inherentl y ve ry low unless phon ons p a rticip at e in the transition. For the same rea son, th e op tical tran sition probability associated wi th shallow donors and accep tors is also sma ll. Never the less, GaP is an important materi al for practical lightemitting diod es. Th e reason for th is is to be found in the following sec tion .
2.9.2.2 lsoelectron ic traps The description of iso electronic tra ps given in this section can also be fou nd in 1.4 dealing with the fundamentals of luminescence in semiconductors. Co nsi d ering nit rogen-doped GaP, the nitrogen (N) at om enter s a t the phosphorous (P) site o f the Ga P lattice. Nand P a toms are isoelectronic with each othe r s ince the y belon g to the sa me Vb column of the pe riod ic table and ha ve the same number of valence electrons . Th ere fore, the N impurity in GaP would appear to be unable to bind ele ctrons or holes to itse lf as d o other common d on or or acceptor impurities in se m icon d uc tors. Yet, th e N a tom in Ga P does bind an elec tron because N is m ore a tt rac tive to electrons than P, owi ng to the nuclear charge of N being more exp ose d , i.e.. du e to la rge difference of electron nega tivi ty between Nand p. Sim ilarly, a Bi at om in Ga P can b ind a hol e. These impurities in GaP are ca lled isoelectronic traps (or centers j.? Since the tr ap p ed elec tro n is localized around the N at om in rea l space, its wavefunction is spread consider ably in m omentum space. Such a si tua tion is show n in Figure 75, where the electron d en sity of an isoelectronic trap with a b ind ing energy of 10 meV is com p ared to that of a sha llow d on or with a binding en e rgy of 100 meY. On e can clearly see that the amplitud e o f the elec tro n w avefunction at th e r point for the isoelectronic trap is about three orders of magnitu de lar ger than that for the sha llow d on or. Once an ele ctron is bound to N by a shor t range potential, a hole ca n also be bound to the negativel y charge d center by the Coulombic p ot enti al, res u lting in the formation
277
Chapter two: Principal phosphor materials and their optical properties
4
;> v
'-"
>-. on
.... v
~
r 0
L3
-2 L
r Wave vector
x k
Figure 74 Energy band structure of GaP. Ener gies at OK ar e: X I - 1 8 = (2.339 + 0.002) eV, 11 - I~ = (2.878 + 0.002) eV. (From Cohen, M.L. and Bergstresser, T.K., Phys. Reo., 141, 789, 1966. With perrni ssion.)
100 ~
v > .;:;
10
ro
a:J ....
'-"
10meV
~
Isoelectric trap
(f)
c
-0 ~
C .... ti v
W
0.01
0.001
0.0001
r
X
Wave vector k
Figure 75 Electron densi ty d istr ibutions fo r a l O-meV isoelectro nic trap and a lOO-meV shallow donor in GaP. (From Dean , P.]., f. Lum inesc., 1/ 2, 398, 1970. With permission.)
Fundamentals of Phosphors
278
of an exciton bound to N . Thus, the radiative recombination of excitons bound to N in GaP takes pl ace with high probability. The high efficien cy of bound exciton recombination at N centers is further promoted by the fact that Auger recombination due to a third particle (electron o r hole) canno t tak e place as it does in the case of exciton recombination at neutral donor s or acceptors. Thus, N is responsible for th e efficient luminescence obse rved in gree n GaP light-emitting diodes. At high N concentrations, the luminescence due to exciton s p referably bound to N-N pairs is observed .' The pair distribution in Ga P occu rs with d ifferent distances and lead s to the spectrum sho w n in Figu re 76 . This type of luminescence is used for yellow-green GaP light-emitting diodes. Slig h tly m ore com plica ted isoelectro nic traps in GaP consist of nearest-neighbor donor-acceptor co m p lexes of 2n-0, Cd-O, o r M g-O, and a triplex of Li-0-Li .5.6 Each of th es e complexes can be regarded as being isoelectronic with one GaP mol ecule where eig h t valence elec tro ns reside. Because of the hi ghly locali zed nature of the 0 potential, these com p lexes ca n bind ele ctrons and form bound excitons as is the case of N. The lumin escence due to excitons bound to 2n-0 is ut iliz ed for red GaP light-emitting d iodes.
2.9.2.3
Donor-acceptor pair emission
A general conc ept and nature of d onor-acceptor pair em ission is desc ribed in 1.4. The important equations for the emission are repeated here . The transition energy E(R) of a di screte p air wi th separation R is g ive n by: (39) where Ex is bandgap ene rgy, ED and E,\ are the d onor and acceptor binding ene rgie s, res pe ctively, e is the electronic charge, and £ is th e static dielectric constant. On the othe r hand, the transition p robability W(R ) between a tightly bound electr on (or hol e) and a loosely bound hol e (or electron ) is ap p roximately giv en by:
(40)
w he re Wo is a constant and R B is th e Boh r radius of a loosely bound electron (or hole). Since GaP is an ind irect gap semiconductor with a low transition probability, em ission from the remote pair can be easily satura ted under high excitation cond itions. This situa tion results in the observati on of well-resolved, fine line structure in the luminescence spec tra correspond ing to va rious donor-acceptor pairs with d iscrete values of R. The spectrum as sh own in Figure 24 in 1.4 is for the em ission taking place between S donors substituting into the P sites and Si accep tors substituting into the Ga sites. For this type of emi ssion (type f) in a zinc-blende structure, R can be expressed in terms of a she ll number m as R = (m/2) 1/2 ao, where m =1= 14, 30, 46,.... From this relation, it is po ssible to assign spe cific lin es w ith corr espond ing R values. O nce the value of R is determined , the observed energy ca n be pl otted ag ain st R. Then, with extrap olation to R = DO , Eg - (ED + EA ) can be determined. Fits of the simp le expression Eq. 39 to so me observed values are show n in Figure 77 as exampl es. If either ED or EA is known through other experiments, the oth er is determined . The results ob ta ined in thi s m anner for emi ssion spectr a aris ing from vario us pair combinat ions of d on or s and acceptors in GaP are shown in Table 24.
9 ~
~ ~
~ ABSORPTION
::,0
~ I
NN.3
(al GaP CRYSTAL UBNC 458
NN 7
r
I
100 .
I
t ll t
115
~ o
I
Jtf\~
u,
20f-
...L
NN/ A
..
S;
2i
......., '"s· ;::,
~
4 .ZoK
t\
M
;:;-
S-
~NN" NN,
40
-l
~
~
\
~
NN 3
NN2
0'
NN 10
\
::;. '"
NN4
~
NN 5
w
::J
1-
\
GaP CRYSTAL UBNC 45A
60
111
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280
Fundamentals of Phosphors 1 .n
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2.9.2.4 Applicationfor light-emitting diodes The charac ter istics of practical GaP ligh t-emittin g diodes are summ arized in Tabl e 25. One sh ou ld note again th at iso elec tro nic impurities of N, N-N pairs, and Zn-O are u tilized for gr een, yellow, and red ligh t-emitt ing diodes, respectively. Another thing to be no ted is th at p ure-green diodes have also become ava ilable, where GaP without isoelectronic impurities is used. Thi s has become possible by im proving cry stal quality in terms of de creasing the defects that ac t as n onrad iative recom bination centers. The em ission is ascribe d to the transition associa ted with free hol es a nd donor bound electro ns.
281
Chapter two: Principal phosphor materials and their optical properties Table 24
Ener gy Depths of Donors and Acceptors in GaP
EA
ED Donor
(meV)
Acceptor
(meV )
Li(int. A) Sn(Ga) Si(Ga) Li(int . B) Te(P) Se(P) S(P) Ge(Ga) O(P)
58 69 82.1 88.3 89.8 102.6 104.2 201.5 896.0
C(P) Be(Ga) Mg(Ga ) Zn (Ga) Cd(Ga) Si(P) Ge(P)
46.4 48.7 52.0 61.7 94.3 202 257
Note: P o r Ca in pa rentheses in d icates the lattice sites to be substituted . Li occu pies tw o different interstitial sites A and B.
Table 25
Prope rties of GaP Light-Emitting Diodes
Mat erial s
Emissi on color
Peak wav elen gth (nm )
GaP :Zn,O GaP:NN Ga P:N GaP
Red Yellow Green Pure green
700 590 565 555
Ext. quantum efficien cy
Lum inous efficienc y
(%)
(lm /W)
4 0.2 0.3 0.2
0.8 0.9 1.8 1.4
References 1. 2. 3. 4. 5. 6. 7.
Cohen, M.L. and Bergstresser, T.K., Phys. Rev., 141, 789, 1966. Thomas, D.G. and Hop field, J.J., Phys. Rev., 150, 680, 1966. Dean, F J., f. Luminesc., 1/ 2, 398, 1970. Thomas, D.G. et al.. Phys. Rev. Leu., 15, 857, 1965. Henry, CH., Dea n, FJ ., and Cuthbert , J.D., Phys. Reu., 166, 754, 1968. Dean, P.J. and Illegems, M., f. Lum inesc., 4, 201, 1971. Dean, P.J., Frosch, CJ., an d Henry, CH., f. Appl. Phys., 39, 5631, 1968.
chapter two - section ten
Principal phosphor materials and their optical properties Gen-ichi Hatakoshi Contents 2.10 (AI,Ga,ln)(P,As) alloys emitti ng visible lumin escence 2.10.1 Band gap energy 2.10.2 Crystal growth 2.10.3 Characteristics of InG aAIP crys tals gro wn b y MOCYD 2.10.4 Light-emitting devices References
2.10 2.10.1
(Al,Ga,In)(~As)
283 283 284 285 288 290
alloys emitting visible luminescence
Bandgap energy
Ga AIAs, GaA sP, InGaAsP, and lnGaAIP are Illb-Vb com pou nd semiconductor materials used for dev ices in the visibl e wave leng th region. Tabl e 26 shows th e compositional dependence of the bandgap en ergy Eg,H I where Egr, E/ and EgL correspond to the di st ance between va lence-band edge and cond uction-band ed ge for r, X, and L vall eys, respectively. Emission by di rect transition occurs in a composition region, where the E/ value is smaller than that for Eg x and Eg L. Lattice con stants of alloys are d etermined by their composition and genera lly vary depending on the composition rat io. Therefore, the lattice constant of ternary alloys such as Ga AlAs and GaAsP is determined uniquely by th e bandgap energy value. In the case of GaAIAs, the composit ional dependen ce of th e latti ce constant a is very sm all: for example, a = 5.653 A for GaAs and a = 5.661 A for AIAs Y The refore, epitaxial layers of GaAIAs can be grow n using a GaAs substrate . The cha nge in th e lattice con stant of GaAsP is compa ratively large; in this case, GaAs or GaP is used as a substrat e, d epending on th e composition of the epitaxia l layer. In qu aternary alloys such as lnGaAsP and lnGaAIP, th e bandgap energy can be varied without altering the value of the lattice con stan t. Th e Eg value for InG aAlp9-11 in Table 26 correspond s to the case wh ere the alloy is lattice-matched to GaAs. Thi s means that GaAs can be used as a substrate for crys tal gr owth of InGaAlP allo ys . InGaAsP can als o be lattice-matched to GaAs, and visible light em ission is obtained for th is case . Such lattice 283
284
Fundamentals of Phosphors Tabl e 26
Bandgap energy (eV)
Ma terial system GaH Al"As
GaAs1_,P ,
In I_,Ga, AsyP1_y
Ir105(Ga1_.AlJo_,P
Co mpositio na l Depen d e nce of the Bandgap Energy
E/ = 1.425 + 1.155x + 0.370x2 = 1.911 + 0.005x + 0.245x 2 E gt = 1.734 + 0.574x + 0.055x" E/ = 1.424 + 1.150x + 0.176x 2 E/ = 1.907 + O.l44x + 0.211x2 E/ = 1.514 + 1.l74x + 0.186x2 (771<) Er,x = 1.977 + 0.144x + 0.211x" (77K) E/ = 1.802 + 0.770x + 0.160x 2 (771<) E/ = 1.35 + 0.668x- 1.068y + 0.758x2 + 0.078y 2 - 0.069xy - 0.322x"y + 0.03xy 2 E/ = 1.91 + 0.59x E/ = 2.26 + 0.09x
E/
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ma tching w ith GaAs can be reali zed by selecting the composition rati o acco rd ing to y - 0.5 for the In l_y(Ga l_,AlJyP system and x - (1 + y) /2.08 for the In l_xGaxAsrPl_y system," respectively. All rna terials d escribed here h ave the zinc -blen de str ucture. Ban d structures for Ga j_xAlxAs, Ino.s(Gal_xAUo,sP, and GaAsJ _xP x var y between d irect transit ion and ind irect transition types. In gen er al, direct transit ion -type crys tals have the adva n tages of hi gh radia tive efficiency and narrow emiss ion spec trum .
2.10.2 Crystal growth Thin-film crysta ls for optical devices usin g the afore me n tioned co mpo und semi cond uctors are grown by liqu id phase epitaxy (LPE), va por p hase ep itaxy (VPE), and mo lecular beam epi taxy (MBE). LPE utilizes the recryst allization of the so lu te from a supersaturated so lution . Co n ventional h alogen-transport VPE is class ified into hydride VPE and chloride VPE. Metalorgani c chemical va por d eposition (MOCV O), an other VPE method , uses metal organic compo unds, suc h as trimethylgallium (TMGa) and trim ethylin d ium (TMIn), as so urce gases for Gro up III ma terials. MBE is a type of ultra-high- vacuum d epo sition, w here molecules or atoms of th e consti tuen t elements are sup p lied from solid sour ces or gas so urces. Attainable device s tructures for light-emitting d iodes (LEOs) and semicon d uctor lasers de pend on th e me thod of crystal growth . For example, the grow th aspect on a stepped or grooved subs tra te varies, d ep ending on the method. In the LPE method, crys tal grow th proceed s so as to embed and level th e groove. Suc h a cha rac teristic fea ture has been u tilized to obtain va rio us struc tures of GaAlAs semicond uctor lasers for practical use .12•13 fnGaAsP crys tal ca n also grown by LPE. Transverse-mode sta bilized s tru ctu res for In GaAsP / GaAIAs se m icond uctor laser s oscilla ting in the O.6-llm wavelength ran ge have been gr ow n by the LPE me thod ." Th e problems w ith th e LPE m ethod ar ise from the difficul ty to grow ultra-thin layers and to con tro l the compositio n of ep itaxial layers for so me ma teria l sys tems. For exam pl e, th e segrega tion coeffic ient (d efined as the ratio of atom s incorporated from the liqu id so lution to th ose in the so lid crystal) of Al is relat ively large in the case of LPE grow th
Chapter two:
285
Principal phosphor materials and their optical properties
for GaAIA s. This ca uses a gra d ual decrease in the Al amoun t in the solution, resulting in a graded composition structu re for thick GaAIAs growth. Th e problem of segregation is even more serious for InG aAIP g row th . It w as very d ifficult to obtain high-qual ity InGaAIP crystals by the LPE methods because of the extremely lar ge seg reg ation coefficient of Al.1 5 The development of MBE and MOCVD techniques has enabled the production of hi ghquality, thin-f ilm crystals for the InGaAlP systerns .v'<" Th e MBE and MOCVD method s ha ve an advan tage in that con trolled ultra-th in lay ers, which can be applied to form multiquantum well (MQW) struc tures for light-emitting devi ces, can easily be obtained. In orde r to realize a d ouble het erostructure for semicond ucto r laser s and LEOs, p-type and n-typ e semi conductor crystals are required, In gene ra l, Group VI elements, such as Se and S, act as donors for th e Ill-V sys tem and thus are used as n-type dopants. Group 11 elem en ts su ch as Zn, Mg, and Be behave as acceptors and ar e used as p-ty pe dopants. Group IV dopants such as Si and Ge are am pho teric impurities. For exa mple, w hen Si is substituted for a Group III site at om , it acts as a d onor. On the contrar y, it acts as an accep tor when substituted for a Group V sit e atom. The su bstitu tion site depends on the grow th cond ition.
2.10.3 Characteristics of InGaAlP crystals grown by MOCVD An attractive technique for MOCYO growth of InGaAlP material sys tems is gro wth on an off-angle substrate. This proc ess is related to the formation of a natural superlattice.t"" In the InGaAlP sys tem, the bandgap energy valu e depends on the growth condition, for example, on the gro w th tempera hire. This is attributed to the dependen ce of atomic ordering on the grow th temperature. An orde red structure of an InG aAIP alloy is produced by the form ation of a natural supe rlattice, where the Group III atoms are arranged systematically. It is known that a disord ered alloy has a larger bandgap energy than that of an orde red
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286
Funda mentals of Phosphors
allo y, and that the dis ordered state is enhanced by u sing an intent ionally misoriented substra te. It follows that th e ban dgap ene rgy of th e InGaAIP crys tal grow n on a mi soriented subs trate has a la rger ban dgap en ergy tha n that grown on a (lOO)-oriented substra te. Figure 78 shows th e d ependen ce of th e band gap energy, obtained by photoluminescence measu re ment, of InGaAIP alloys on the substrate orien ta tion." As shown in the fig u re, the ban d gap en ergy inc reases w ith inc reasing substrate tilt angle aw ay from the (100) p lan e to w ard the [011] direction ." Thi s is considered to be due to the supp ression of crysta l or d ering. The off-angle subs tra te technique is ut ilized to fabrica te sho rt-waveleng th InGaAIP lasers. In general, shortening of the osc illa ting wa veleng th or, eq u iva len tly, an increase in the ban d gap energy is obtained b y increasing the Al composition of the alloy as shown in Table 26. Introd u cti on of off-a ngle substra tes has the advan tage of wavelength shortening w hi le usin g a smaller Al conten t in the active layer. This is preferable because forma tion of un d esirab le n onradiat ive recomb ination cen ters arising from inco rpora tion of oxygen imp ur ities (w h ich in crease w ith increasing Al composi tion) is reduced . The
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a cr ys tal pla ne norma l to the [100] d irect ion.
Chapter two: Principal phosphor materials and their optical properties
287
bandgap of the cladding layers can also be increased by using off-angle substrates. Thus, electron overflow can be effectively suppressed by creating a larger bandgap difference between the active and cladding layers in this way. Another effect of off-angle substrate s is to increase accep tor concentration in p-type lay ers. As sh own in Figure 79, Zn in corporation and the net acceptor concentration strong ly depend on the tilt angle of the substrate.F Thi s dependence is similar to that for the PL peak ene rgy shown in Figure 78. Both the Zn concentration and th e net acceptor concen tration inc rease with increasing tilt angle from (100) toward the [all] d irection. High acceptor conc entrations are preferable for p-type cladding layers because of their effect in reducing electron overflow from the active la yer to the p-cladding layer,23,24 due to the increase in the conduction-band heterobarrier height at the interface between the ac tive and the p-cladding layers. Electri cal acti vity of p-type dopants dep ends on th e effects of residual impurities such as h ydrogen and oxy gen and also upon grow th cond ition s. Oxygen inc orporation into InGaAIP crys tals results in the electrical compensation of Zn accep tors. It also ca uses a nonradiative center due to the formation of deep levels. These phenomen a are se rious problems for light-emitting devices. Ox ygen incorporati on ca n be reduced by inc reas ing the V:III ratio in MOCVD growth 25 and by the utilizati on of the off-angle technique." An exa mple of expe rime ntal results is show n in Figure 80.26 The effect on oxygen reduction in off-angle s ubs tra te is remarkabl e, especially for high Al composition cryst als, and is very useful for p roducing highly d oped p-type cladding layer s. High acceptor concen trations exceed ing 1 x 101 s cm ? hav e been reported for InAlP crys tals fabri cat ed by MOCVD growth on off-ang le substrates." Experimental results showing imp ro vemen ts in the hetero-interface properties of quantum w ells gro w n on misoriented substrates have been reported ." Full width at half maximum (FWHM) value of the PL spectra for InG aP / InGaAlP sin gle quan tum wells shows a stro ng dependence on the subs trate misorient ation. The FWHM value is found
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288
Fundamentals of Phosphors
to decrease wi th increasing misorienta tion from the (100) toward the [all ] direction . This resu lt ind icates that the int erface sm oothness and ab ruptness are impro ved by emp loying off-angle subs tra tes. A rem arkable im p rovemen t in the temp erature charact eristics of InGaAIP lasers has been ach ieved by employing an off-angle techni qu e. Shor t-waveleng th and high-tempera ture operation ha ve bee n reported for InGaAIP lasers grown on m isor iented subs tra tes.
2.10.4
Light-emitting devices
Sem iconductor lasers and LEDs in the visible waveleng th region are obtained using GaA1As, GaAsP, In GaAsP, and In GaAI P system s. Fig ure 81 show s the avai lable wave( n rn )
Wavelen g t h
500
600
700
800
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Figure 81 Ava ilable w avelen gt h range for sem icond uctor lasers and LEDs. Con stituen t alloy system s are ind icate d by D / B or A/ C / B, where D, A, C, and B de note the mater ial systems for the doub le h eterost ru cture, the active lay er, the claddin g layer, and the subs trate, resp ectively.
Chapter two:
289
Principal phosphor materials and their optical properties
length range for semiconductor lasers and LEDs. Th e wavelength range is restricted in the case of semiconductor lasers because the active layer is required to have a d irecttransition-type band stru cture; here also, cladding la yer s with bandgap ene rgies greater than that of the active layer are required in order to confine the injected ca rrie r within the active layer. It is d ifficult, in general, to ob tain shorter wavelength semiconductor lasers for a gi ven material sy stem because the bandgap differ enc e between the ac tive and the cladding layers d ecreases with shorten in g oscillation w av elength, resulting in a sign ifican t carri er overflow from the active laye r. Visible-light os cillations in the O.6-llm wavele n gth region h ave been realiz ed fo r InGaAlP IG aA s, 16-18 GaAIAs / G aA s,29 InG aAsP I GaAlAs I GaA s,!4 and InGaAsP I InGaP I GaAsp 30 sys tem s. As for LEDs, indirect-transition- type alloys can also be us ed for emi ssion layers, and cladding layers are not necessaril y required . Therefore, the p ossible wa velen gth range for LEOs is larger than that for semicond uctor lasers. In gene ral, high-brightness characteristics are obtained by using direct-t ransition alloys an d by introducing a double heterostructure. The isoelectronic trap technique, which is effective in improving the emi ssion efficiency of GaP LEDs, is also applicable to the GaAsP systems-s-" in the indirect tran sition region. Nitrogen is used as the isoelectronic impurity. GaAsP :N LEDs show electroluminescence efficiencies of an orde r of magnitude higher than tho se without nitrogen doping." Exam ples of emi ssion spectra for v isible-ligh t LEDs are show n in Figure 82. GaAIAs32 and InG aAlp33 alloys have direct transition band structures and thus the LEDs with these alloys ha ve higher bri ghtness and narrower emission spectra, as shown in the figure. Light-extraction efficiency of LEDs is affected by various factors, which can be controlled by device de sign. 34.35 Remarkable enha ncem en t of light-extraction efficien cy has been rep orted for InG aAIP LEDs by introducin g current-spreading and current-bl ocking Iayers." >' Introduction of DBR rnirror'" is effective for LEOs w ith absorbing subs tra tes . Hi gh-pow er InGaAlP I Gap LEDs with chip reshaping)? ha ve also been reported . Other
InGaAIP
GaAsP
GaAIAs
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500
600 Wavelength
Figure 82 Electrolum inescence spec tra for visibl e-light LEDs.
700
en rn)
290
Fundamentals of Phosphors
ap p roach es such as surface texture, resonant cavi ty s tru ctur e, and photonic crystals h ave been investigated for improv ing the LED efficiency."
References 1. Madelung, 0., Ed., Landolt-Bornstein, Numerical Data and Functional Relationships in Science and Technology, III, 17 and 22a, Sprin ger-Verlag, Berlin, 1982. 2. Casey, HC, Jr. and Panish, M.B., Heterostructure Lasers, Academic Press, New York, 1978. 3. Lee, HJ., [uravel, L.Y, Woolley, J.C , an d SpringThorpe, A.J., Phys. Rev. B, 21, 659, 1980. 4. Thompson, AG., Card on a, M., Shakl ee, KL., and Woolley, J.C, Phys. Rev., 146, 601, 1966. 5. Craford, M.G., Shaw, R.W , Herzog, AH, and Groves, WO., J. Appl. Phys., 43, 4075, 1972. 6. Holonyak, N ., [r., Nelson, KJ., Colema n, J,J., Wright, PD., Fin, D., Groves, WO., an d Keune, D.L., J. Appl. Phys., 48, 1963, 1977. 7. Kuphal, E.,]. Cryst . Growth, 67, 441, 1984. 8. Adachi, S., J. Appl. Phys., 53, 8775, 1982. 9. Asahi, H ., Kaw amura, Y, and Nagai, H , J. Appl. Phys., 53, 4928, 1982. 10. Honda, M., Iked a, M., Mori, Y, Kaneko, K , and Watanabe, N., ]pn. J. Appl. Phys., 24, U87, 1985. 11. Watanab e, M.O. and Ohba, Y, Appl. Phys. Lett., 50, 906, 1987. 12. Aiki, K., Nakamura, M., Kurod a, T , Umeda, J., Ito, K , Chinone, N., an d Maed a, M., IEEE J. Quantum Electron., QE-14, 89, 1978. 13. Yamam oto, S., Hayashi , H., Yano, S., Sakur ai, T , and Hijika ta, T , Appl. Phys. Leu ., 40, 372, 1982. 14. Chong, T and Kishino, K., IEEE Photonics Tech. Lett., 2, 91, 1990. 15. Kazurnura, M., Ohta, 1., and Ter arnoto, 1., ]pn. J. Appl. Phys., 22, 654, 1983. 16. Kobayashi, K., Kawa ta, S., Go myo, A, Hino, 1., and Suz u ki, T , Electron. Lett., 21, 931, 1985. 17. Ikeda, M., Mori, Y, Sato, H, Kan eko, K., and Watana be, N ., Appl. Phys. Lett., 47, 1027, 1985. 18. Ishikawa, M., Ohba, Y, Sugawara, H , Yamamoto, M , and Nakanisi, T, Appl. Phys. Len; 48,207, 1986. 19. Suzuki, T , Go myo, A, Iijim a. S., Kobayashi, K., Kawa ta, S., Hino, I., and Yuasa, T , [pn. J. App!. Phys., 27, 2098, 1988. 20. Nozaki, C , Ohba, Y, Suga wa ra, H., Yasuami, S., and Nakani si, T , J. Crystal Growth, 93, 406, 1988. 21. Ueda, 0 ., Tak echi, M., and Korneno, J., Appl. Phys. Leii., 54, 2312, 1989. 22. Suzuki, M., Nish ikaw a, Y., Ishikawa, M., and Kokubun, Y, J. Crystal Growth, 113, 127, 1991. 23. Hatakoshi , G., Itaya, K., Ishikawa, M., Okajima. M., and Uem atsu . Y, IEEE J. Quantum Electron., 27, 1476, 1991. 24. Ha tak oshi, G., N itta, K., Itaya, K , Nishikawa, Y, Ishikawa, M., and Okajirna, M., ]pn. J. Appl. Phys., 31, 501, 1992. 25. N ishikawa, Y, Suz u ki, M., an d Okajirna, M., ]pn. J. A ppl. Phys., 32, 498, 1993. 26. Suzuki, M., Itaya, K., Ni shikawa, Y, Sugawara, H ., and Ok ajirna, M., J. Crystal Growth, 133, 303, 1993. 27. Su zu ki, M., Itaya, K., an d Okajim a. M., ]pn. J. Appl. Phys., 33, 749, 1994. 28. Watan abe , M., Rennie, J., Okajima, M., and Ha tako sh i, G., Electron. Leit., 29, 250, 1993. 29. Yamamoto, S., Hayashi , H., Ha yakaw a, T , Miyauchi. N., Yano, S., and Hijikata , T , Appl. Phys. Lett., 41, 796, 1982. 30. Usu i, A , Matsumoto, T , Inai, M., Mito, 1., Kobayashi , K., and Watan ab e, H., ]pn. ]. Appl. Phys., 24, L163, 1985. 31. Cr aford, M.G. an d Groves, WO., Proc. IEEE, 61, 862, 1973. 32. Ishiguro. H, Sawa, K , Nagao, S., Yamanaka, H ., an d Koike, S.,Appl. Phys. Leu., 43,1034,1 983. 33. Sug aw ara, H, Itay a, K, Nozaki, H, and H atakosh i, G., Appl. Phys. u u.. 61, 1775, 1992. 34. Hatak osh i, G. an d Sugawara, H, Display and Imaging, 5, 101, 1997. 35. H atakoshi , G., 10th Int. Display Workshop (IDW '03), Fukuoka, 1125, 2003. 36. Suga wara, H ., Itaya, K, and Hatakoshi, G., J. Appl. Phys., 74, 3189, 1993. 37. Kram es, M.K , Och iai-H olcomb, M., Hofler, G.B., Car ter-Coman. C , Chen, E.I., Tan, I.-H., Grillot, P., Gard ne r, N .F., Chui, HC, Huang, J.-W , Stockm an , S.A, Kish , F.A., and Craford, M.G ., App l. Phys. Lett., 75, 2365, 1999. 38. Issue on Hig h-Efficiency Light-Emitting Diodes, IEEE ]. Sel. Top. Quantum Electron; 8, No. 2, 2002.
chapter two - section eleven
Principal phosphor materials and their optical properties K enichi Iga
Contents (AI,Ga,I n)(P,As) alloy s emi tting infrared luminescence 2.11.1 Co mpound semiconductors based on InP 2.11.2 Det erm in ation of GalnAsP f lnP solid compositions 2.11.3 Cryst al gro w th 2.11.4 Applied devices References
2.11
2.11 2.11.1
291 291 293 294 295 295
(Al,Ga,In)(P,As ) alloys emitting infrared luminescence Compound semiconductors based on InP
Semiconductors for w hic h bandga ps correspond to a lon g wavelength spec tra l region (1 to 1.6 urn) are importan t for op tical fiber comm unica tion us ing silica fibers exhibiting extremely low loss and low dis persion, infrared imaging, lightw ave sensing, etc. Figure 83 depicts a diagram of lattice constan t vs . bandgap of several comp ound se mico nd uc tors based on InP, In As, Ga As, GaN an d AlAs, which can emi t ligh t in th is infrared region.
Semiconductor crystals for 1 to Lb-tnn wavelength emission. Terna ry or q ua ternary semicond uctor crys tals are used since binary semiconductor crystals w ith 1 to Lo-urn ba ndgaps are no t av ailab le. Ma tch ing of lattice constan ts to substrates in crysta l gro w th processes is importan t for fabrica ting semiconductor d evices such as semiconductor laser s and ligh t-em itting d iod es (LEDs) with high current injection levels (>5 kA cnr? urrr" ) or a high -outpu t power d en sity (> 1 m W urrr") or for p ho todiodes used for low- n oise d etection of very weak op tica l sig nals. The bandgap of a specific qua ternary crys tal can be varied widely while com pletely maintaining the lattic e match to a binary crys tal used as a subs tra te, as show n in Figure 83. An example is Ga) nl_xAsyPt_y, wh ich uti lizes InP (a = 5.8696 A) as a substra te; the ban d gap can be changed in the region of 0.7 :S Eg:S 1.35 eV when the composi tion is adj usted along the vertical line. The correspondi ng emission wavelength ra nges from 0.92 to 1.67 urn . The ternar y ma teria ls lattice-ma tche d to the InP substrate are Alo.47Ino53As and Gao47Il\Js3As. 291
292
Fundamentals of Phosphors 2.5,-----------------------,
AlAs
?-----,
2.0
,
s~ UJl:n
1.5
>. Cl
:;;
c:
W
a.
'" c: CD '"
Cl "0
1.0
~,
GaNAs
GainNAs
"
GalnAs
0.5 InAs
a 5.5
5.6
5.7
5.8
5.9
6.0
6.1
Lattice Constant a (Al
Figure 83
Diagram of lattice constant vs. bandgap for several compound semiconductors.
Possible compound crystals corresponding to light emission of 0.8 to 211mare as follows: 1. 2. 3. 4. 5. 6.
Ga)n1_xAsyP1_y (InP): (Ga 1_xAIJ)n1_yAs(lnP): Gal_xAlxAsySb1_y(GaSb): Ga)nJ_xAsySb1_/lnAs): Ga)nl_xAsySbJ_y(GaSb): Ga)nl_xNxAsl_x(GaAs):
0.92 < Ag < 01.67 (11m) 0.83 < Ag < 1.55 (11m) 0.8 < Ag < 1.7 (11m) 1.68 < Ag < 2 (11m) 1.8 < Ag < 2 (11m) 1.1 < Ag < 1.6 (urn)
The binaries in the parentheses indicate the substrates to be used. Crystal growth of these materials is possible with a lattice mismatch ±0.1 % or less. Among these, the heterostructure composed of Ga)nl_xAsyPl_x and InP has been widely employed as a material for semiconductor lasers or photodiodes for lightwave systems. The relationship between x, y, and the bandgap energy associated with Ga)nl_xAsyPl_Yf which are lattice-matched to InP, can be expressed as follows.
x=
0.466y (0 :s; x :s; 1) 1.03 - 0.03y
(41)
(42) which was phenomenologically obtained by Nahory et al.' The values of x and yare no longer independent of one another, since the lattice constant must be adjusted so as to be matched to that of the InP substrate, 5.86875 A. Consequently, the bandgap energy can be expressed by specifying the Ga or As contents. The band-structure parameters of GalnAsP IInP are summarized in Table 27.2
Longer-wavelength materials. Fluoride glass fibers have found use in long-distance optical communication in the 2- to 4-l1m wavelength range. Signal loss in fluoride glass fibers is predicted to be one or two orders of magnitude lower than that for silica fibers. Also, this spectral band is important for LIDAR (Light Detection and Ranging) and optical
Chapter two: Principal phosphor materials and their optical properties
293
Table 27 The Band Structure Parameters of Ga)n1 _,AsrP1 _yll nP
Parameter
Dependence on the mole fractions x and y
Energy gap at zero dopin g Heavy-hole mass Light-hole mass Dielectric constant Spin-orbit splitting Condu ction-band mass
Eg leV] = 1.35 - O.72y + 0.12y2 m hJ1I m o = (1-y)[0.79x + 0.45(l -x)] + y[0.45x + O.4(l -x)] m;h I m o = (1-y)[0.14x + 0.12(1- x)] + y[0.082x + 0.0261(1- x)] e = (l -y)[8.4x + 9.6(l- x)] + y[13.1x + 12.2(l-x)] 6 leV] = 0.11- 0.31y + 0.09x2
rn, I rna = 0.080 - 0.039y
From Ag raw al, G.P. and Dutta, N .K., Long-uxnielengt): Semiconductor La, crs, Van Nostran d Reinhold, Ne w York , 1986, 85. With permi ssion .
sensing. A potential material system to cover the wavelength range from 1.7 to 5 urn is GalnAsSb l AlGaAsSb.
2.11.2
Determination of GalnAsP/InP solid compositions
First, a review of the general concep ts of crystal preparation for GalnAsP latticematched to InP, which ha s been co m m o n ly u s ed in light-emitting devices. Ga)nl_xAsyPJ_r contains two controllable parameters, enabling independent adjustment of the lattice constant and the bandgap energy. The lattice constant a(x,y) of Ga)nl_xAsll_y is g iven as follows:
a(x, y) = a(GaAs)xy + a(GaP)x(l- y) + a(InAs)(l- x )y + a(InP)(l- x)(l- y)
(43)
According to measurements by Nahory et al..' the binary lattice constants are: a(GaAs) = 5.653 A, a(GaP) = 5.4512 A, a(InAs) = 6.0590 A, and a(InP) = 5.8696 A. The following equation is obtained by inserting this data into Eq. 43:
a(x, y) = 0.1894y - 0.4184x + O.013xy + 5.8696
(A)
(44)
The relation between x and y, therefore, is given by the followin g equation, when th e a(x,y ) coincides with the lattice constant of InP :
0.1894y - O.4184x + 0.0130xy
=0
(45)
Usually, Eq. 45 is approximated as:
x = 0.467y
(46)
According to the theory by Moon et aJ.3 an d experimental res u lts, the relation between the bandgap energy and compositions x an d y is given by:
Eg (x, y) = 1.35 + 0.672x -1.091y + 0.758x 2 + 0.101y 2
(47)
-O.157xy - 0.312x 2y + O.109xy 2 The bandgap energy calcu la ted in terms of x an d y using Eq . 47 agrees with the phenomenological results of N ahory et al.'
Fundamentals of Phosphors
294 3.0 Indirect Gap Surface
Direct Gap Surfac e
-:
2.0
1.0
2.0
- x____ >
~
Indirect Gap Region
\ , /Lattice
OJ
W
" InAs
Lattice Match to GaAs
Match to InP GaAs
Figure 84 Bandgap energy V S, compos ition s x and y in Ga)nl_xAsI'PI _y' (From Casey, H.C. and Panish , M.B., Heterostructure Lasers, Part B, Academic Press, New York, 1978. With permission .)
The bandgap energy vs . composi tions x and y is illus tra ted in Figure 84.4 With the aid of this fig ure , one can obtain th e band s truc tur e of Ga inAsP la ttice-matched to InP for the en tire set of allowed compositions of y. The bandgap of GainAsP in the vicinity of GaP is seen to be ind irect in th e figure.
2.11.3 Crystal growth Liquid phase epitaxy (LPE). In the case of liquid phase epi taxy, on e has to determine the liqu id com position of an In-rich melt in thermal eq uilib rium wi th the solid phase of th e desired x an d y com position s for Ga)nl _xAsyPl_jr The As comp osition y in th e Ga)n J_xASyPl_y solid of the desired bandgap energy is given by Eq. 42 when its lattic e cons tan t is eq ual to that of In P. The Ga composition x is ob tained by Eq. 46. In this way, the atomic frac tions of Ga, As, and P in the In-r ich m elt that exis ts in equi librium with the desired Gaxlnl_,AsyP I_y solid can be obtained . The actua l weights of InP, In As, and GaAs per gram of In can be es tima ted . The degree of la ttice m isma tching !Lia/aI can be examined by X-ray diffraction and shou ld be less th an 0.05%. Metal-organic chemical vapor deposi tion (MOCV D). In th e meta l-o rg anic chemical va po r dep osit ion (MOCY D) method , gas so urces are used for growth of the structu res.' To sa tisfy the latt ice-ma tch cond ition, the flow rates of trim ethylin d ium and ars ine (AsH 3 ) are fixed and the triethylgallium flow ra te is adjus ted . The phosphin e (PH 3) flow rate is varied to obtai n different compositions . Grow th rat es of InP and qua ternary ma terials are abo ut 2 urn Zh, differing sligh tly for d ifferent alloy composi tion s. The compositions are calcu lated from the wave leng th of the pho tolu minescence spectral peak int ensities. Chemical beam epitaxy (CBE). Trimeth ylindium and triethylgalliu m with H 2 carrie r gas are used as Gro up III sources in chemical bea m epitaxy (CBE) de posi tion." Group Y sou rces are pure AsH 3 and PH3, w hich are precracked at 1000°C by a high -tem perature
Chapter two:
Principal phosphor materials and their optical properties
295
crac king cell. Solid Si and Be are used as n-type an d p-typ e d op ants, resp ectively. The typi cal grow th temperature is 500°C, which m us t be calibrat ed, for example, using the melting point of InSb (525°C). Typical growth ra tes for InP, Ga lnAsP (J"g = 1.3 urn) , and GaInAsP (A g = 1.55 urn) are 1.5, 3.8, and 4.2 um / h, respectively. Impurity d oping control ov er w id e ranges is one of the m ost important issu es in the fabrica tion of op toe lectronic devices. The adv an tages of using Be ar e that it is a well-be have d accep tor p roducing a shallow level above the va lence band, an d it can be incorporat ed into GalnAsP at a relati vely high level (on the order of 1019 em >'). The impurity lev els of GalnAs grown by var ious ep itaxial techn iques are 3 x 1015 crrr? by MBE, 8 x 1015 crrr' by MOCVD, an d 5 x 1014 crrr? by CBE.
2.11.4
Applied devices
Semiconductor lasers emitting 1 to 1.6-J.1n1 wavelength. The op tical fiber made of silica glass exhib its a very low transmission loss, i.e.. 0.154 dB/km a t 1.55 urn. Th e ma terial dis pe rsion of retractiv e index is minimum at the w avelength of 1.3 urn. These are ad va n tageou s for long-distance optical comm un ica tions . Semiconductor las er s emitting l .3-~m w avelen gth using lattice-matched Ga lnAs P I InP h ave been de veloped having low th resholds of ab out 10 rnA and very long d evice lifetimes. The l .3-~m waveleng th sy st em h as been used since 1980 in public telephon e networks a nd undersea cable syste ms. In th e 1990s, the 1.55-flm system wa s realized b y tak in g the advantage of the min imum tran smission loss . In th is case, the linewidth of th e light sour ce must be very small, since the d ispersion of th e silica fiber is rela tively lar ge com pa red to that at 1.3 urn . Figure 85 exh ibit an exa mp le of a single-mode laser struc ture that p rovides n ar row lin ew idth ev en w hen modulated a t high speed -signa ls." High -p ow er semicon d uc tor lasers em ittin g at 1.48 urn are employed as a pumping so urce for Er-dop ed op tical fiber amplifier (EDFA). A su rface-em itt ing laser operatin g a t thi s w avelength is sho w n in Figure 86 an d is expected to be us ed in long-w avel en gth netw orks and optical int erconnects." For th e purpose of subs tan tially improv ing laser performance, qua n tu m wells h ave been consi de red for use as the active region of se micon d uc tor las er s. Figure 87 giv es an exa m p le of quantum wire lasers employ in g a GaInAs / GaInAsP sys tem that em its a t 1.55 um .? Other optoelectronic devices. The coun terp ar t of se micond u ctor laser s is a photod etector that receives the tran smitted op tical signa l. Photodiodes h avin g high qu an tum efficiencies in wavelength 1.3 to 1.6 urn band employ th e GalnAs ternary se micon duc tors latti ce-m atched to InP as w ell. Th is sys tem p rovid es low-noise and high-sp eed photodiodes, i.e., PIN di odes and avalan che photodiodes (APDs). Infra red (IR) detectors an d CCDs are important for infrared imaging. Illuminat ion by IR LEDs a re useful for imagi ng as w ell. Eye-safe radiation in th e 1.3- to 1.5 5-~m ra nge is another im po rtant issue in IR imaging.
References 1. Nahory, R.E., Pollack, M.A., Johnst one, W.O., and Barn es, RL., Appl. Phys. Leit., 33, 659, 1978. 2. Agra wal, G.P. an d Dutta, N.K., Long-Wavelength Semiconductor Lasers, Van Nostran d Rein hold , New York, 1986, 85. 3. Moon, R L., An typas, G.A., and Jam es, t. w, J. Electron. Mater., 3, 635, 1974. 4. Casey, H.C. and Pani sh, M.B., Heterostructure Lasers, Part B, Acad emic Press, New York, 1978. 5. Man asevit, H.M., Appl. Phys. Lett., 12, 156, 1968. 6. Tsang, w.I., IEEE ]. Quant. Electron., QE-23, 936, 1987.
N '-0
.>
0'\
Si0 2 p-InP n-GalnAsP (Blocking) p-InP p-InP elee trode As ymmetric Gratings
Waveguide Structure n-lnP
-120nm
Ga O.dnO.5 3As
active tc 1 '" 30 crn'
passive tc 2"'" 200 cm- 1
(A~:l~t; ~ m)
=I~::m =*= 8nm
- - f,@//P// //%/.1
<
WiM wN#
I
'"1"j ~
;:s
~
200nm
~
:::: '"
B' ti)
p-lnP
~
;'2
Figure 85 An examp le of single-mode laser. (From Shi m, J.I., Komori , K., A ra i, S., Suema tsu, Y, and Somcha i, R., IEEE] . QUlln t. Electron., QE-27, 1736, 1991. With permission .)
~;::o
;;;
Chapter two: Principal phosphor materials and their optical properties
297
p-side Mirror
p-side AUlZn/Auffi/Au/Ni/Au Electrode Si02 Insulator
~~~~~~!~~~~~~£.. :::::=~
p-GalnAsP Cap
p-lnP Blocking Cladding n-lnP ~ p-lnP Blocking t=====~§~§§~~~~F===9~ n-lnP Cladding n-GalnAsP Etch Stop ~ n-lnP Substrate Light Output
<, n-side AuGeffVAu Electrode n-side Mirror
Figure 86 An exam ple of 1.48 urn surface emitting lase r. (From Baba, T , Yogo, Y , Suzuki, T , Koyam a, F., and Iga, K., [EICE Trans. Electronics., E76-C, 1423, 1993. With pe rmiss ion.) 7. Shim , J.I., Komori , K., Arai . S., Suematsu, Y , and Som chai, K , IEEE J. Quan/. Eleciron., QE27, 1736, 1991. 8. Baba, T., Yoga, Y, Suzuki, T, Koyama, E, and Iga, K., IEICE Tran s. Electronics., E76-C, 1423, 1993. 9. Kud o, K., N agashima, Y. Tamura, S., Arai, S., Hu an g, Y., and Suerna tsu, Y, IEEE Photon. Teehnol. Lett., 5, 864, 1993.
298
Fundamentals of Phosphors Au/So p-InP
Si0 2
SEM-View
n-GalnAsP
n-GaInAsP(GRIN-OCL : O.2!1-m) A=70nm .
k
>\
GaInAsP Barner (12nm)
~
Ga••Jn.,.As W ell (120m) InP i-GaIoAsP(GRIN-OCL : O.2I-tm)
Figure 87
An exa mple of quantum wire lasers employing GalnAs /Ga [nAsP system to emit 1.55 urn wavelen gth. (From Kud o, K., Naga shima, Y. Tamura,S., Arai, 5., Huang, Y, and Suematsu, Y, IEEE Photon. Technol. Lett., 5, 864, 1993. With permission.)
chapter two - section twelve
Principal phosphor materials and their optical properties Shuji Nakamura
Contents 2.12 GaN and related luminescence materials 2.12.1 Introduction 2.12.2 n-Type GaN : 2.12.3 p-Type GaN 2.12.4 GaInN 2.12.5 GalnN / AIGaN LED 2.12.6 GaInN sin gle-quantum well (SQW) LEOs 2.12.7 GalnN multiquantum well (MQW) LOs 2.12.8 Summary References
299 299 300 300 301 302 303 307 311 311
2.12 GaN and related luminescence materials 2.12.1
Introduction
GaN and related materials such as AIGaInN are III-V nitride compound semiconductors with the wurtzite crystal structur e and an energy band structure that allow direct interband transitions which are suitable for light-emitting devices (LEOs). The bandgap en ergy of AIGaInN varies between 6.2 and 1.95 eV at room temperature, depending on its composition. Therefore, these III-V semiconductors are useful for light-emitting devices, especially in the short-wavelength regions. Among the AIGaInN systems, GaN ha s been most intensively studied. GaN has a bandgap energy of 3.4 eV at room temperature. Recent research on III-V nitrides has pa ved the way for the realization of high-quality crystals of GaN, AIGaN, and GaInN, and of p-type conduction in GaN and AIGaN,u The mechanism of acceptor-compensation, which prevents obtaining low-resistivity p-type GaN and AIGaN, has been elucidated.' In Mg-doped p-type GaN, Mg acceptors are de activated by atomic hydrogen that is produced from NH3 gas used to provide nitrogen during GaN growth. After growth, thermal annealing in N 2 ambience can reactivate the Mg acceptors by removing the atomic hydrogen from the Mg-hydrogen complexes.' High-brightness blue GaInN / AlGaN LEOs have been fabr icated on the basis of these results, and luminous
299
300
Fundamentals of Phosphors
intensities over 2 cd have been achieved.v" Also, blue/green GaInN single-quantum-well (SQW) LEDs with a narrow spectrum width have been developed .Y These LEDs are now commercially available. Furthermore, recently, bluish-purple laser light emission at roomtemperature (RT) in GaInN/GaNI AlGaN-based heterostructure laser diodes (LDs) under pulsed currents'r-" or continuous-wave (CW) operation was demonstrated.f>" Recent studies of (Al,Ga,In)N compound semiconductors are described in this section.
2.12.2 n-Type CaN GaN films are usually grown on a sapphire substrate with (0001) orientation (c face) at temperatures around 1000°C by the metal-organic chemical vapor deposition (MOCVD) method . Trimethylgallium (TMG) and ammonia are used as Ca and N sources, respectively. The lattice constants along the a-axis of the sapphire and CaN are 4.758 and 3.189 A, respectively. Therefore, the lattice-mismatch between the sapphire and the GaN is very large. The lattice constant along the a-axis of 6H-SiC is 3.08 A, which is relatively close to that of CaN. However, the price of a SiC substrate is extraordinarily expensive to use for the practical growth of CaN. Therefore, at present, there are no alternative substrates to sapphire from considerations of price and high-temperature properties, even as the lattice mismatch is large. Grown GaN layers usually show n-type conduction without any intentional doping. The donors are probably native defects or residual impurities such as nitrogen vacancies or residual oxygen. Recently, remarkable progress has been achieved in the crystal quality of GaN films by employing a new growth method using buffer layers. Carrier concentration and Hall mobility, with values of 1 x 10 16 crrr-' and 600 ern? VS-I at room temperature, respectively, have been obtained by deposition of a thin GaN or AlN layer as a buffer before the growth of a GaN film." In order to obtain n-type GaN with high carrier concentrations, Si or Ge is doped into GaN.19 The carrier concentration can be varied between 1 x 1017 and 1 x 1020crrr-' by Si doping. Figure 88 shows a typical photoluminescence (PL) spectra of Si-doped GaN films. In the spectra, relatively strong deep-level (DL) emission around 560 nm and the band-edge (BE) emission around 380 nm are observed. The intensity of DL emissions is always stronger than that of BE emissions in this range of Si concentrations.
2.12.3 p-Type CaN Formerly, it was impossible to obtain a p-type GaN film due to the poor crystal quality of CaN films. Recently, Amano et aJ.1 succeeded in obtaining p-type GaN films by means of Mg doping and low-energy electron-beam irradiation (LEEBI) treatment after growth. In 1992, Nakamura et apo found that low-resistivity p-type GaN films are also obtained by post-thermal annealing in N 2 ambience of Mg-doped GaN films. The resistivity of asgrown films is 1 x 106 Q.cm. When the temperature is raised to 400°C in a N 2 ambience for annealing, resistivity begins to decrease suddenly. After annealing at 700°C, the resistivity, hole carrier concentration and hole mobility become 2 n·cm, 3 x 1017 crrr? and 10 em? V'S-l, respectively. These changes of the resistivity of Mg-doped CaN films are explained by the hydogenation process model in which atomic hydrogen produced from NH 3 during the growth is assumed to be the origin of the acceptor compensation. If low-resistivity p-type GaN films, which are obtained by Ny-ambient thermal annealing or LEEBI treatment, are thermally annealed in NH 3 ambience at temperatures above 400°C, they show a resistivity as high as 1 x 106 n·cm. This resistivity is almost the same as that of as-grown Mg-doped GaN films . Therefore, these results indicate that the abrupt resistivity increase in NH 3ambient thermal annealing at temperatures above 400°C is caused by the NH 3 gas itself.
Chapter two:
Principal phosphor materials and their optical properties
100
Si doping
301
(a) N= 4 X IOI8/ em3
r-..
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d)
:s
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~ ...... ...... ro ......
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500
600
700
800
Wavelength (nm) Figure 88 Room-temperature PL spectra of Si-doped CaN films. Both samples were grown under the same growth conditions but changing the flow rate of SiH 4 . The carrier concentrations are , (a) 4 x 1018 crrr? and (b) 2 x 1019 crrr '. (From Nakamura, S., Mukai, T., and Senoh, M., [pn . J. Appl . Phys., 31, 2883, 1992. With permission.)
Atomic hydrogen produced by the NH 3 dissociation at temperatures above 400°C is considered to be related to the acceptor compensation mechanism. A hydrogenation process whereby acceptor-H neutral complexes are formed during the growth of p-type GaN films has been proposed." The formation of these complexes during film growth causes acceptor compensation. The Nz-ambient thermal annealing or LEEBI treatment after growth can reactivate the acceptors by removing atomic hydrogen from the neutral complexes. As a result, noncompensated acceptors are formed and low-resistivity p-type GaN films are obtained .
2.12.4
GalnN
The ternary Ill-V semiconductor compound, GalnN, is one of the candidates for blue to blue-green emitting LEDs, because its bandgap varies from 1.95 to 3.4 eV depending on the indium mole fraction. It was very difficult to grow high-quality single crystal GalnN films due to the high dissociation pressure of GaInN at the growth temperature. Recently, this difficulty has been overcome by means of the two-flow (TF)-MOCVD method." and high-quality GalnN films have been obtained. Figure 89 shows the results of roomtemperature PL measurements of high-quality GalnN films grown by this method. A strong sharp peak is observed at 400 nm in (a) and at 438 nm in (b). These spectra are due to BE emission of GalnN films because they have a very narrow halfwidth (about 70 meV) . Figure 90 shows the bandgap energy (Eg(X)) of Ga(l_x)InxN films estimated from PL spectra at room temperature as a function of the indium mole fraction X.22 The indium mole fraction of the GaInN films was determined by the measurements of the difference of the X-ray diffraction peak positions between GalnN and GaN films . Osamura et a1. 23 showed that Eg(X) in ternary alloys of Ga(l_x)InxN has the following parabolic dependence on the molar fraction X:
Fundamentals of Phosphors
302
100 -
-
Undoped GalnN
(a)
(a) T=830 °C
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Wavelength (run) Figure 89
Room-temperature PL spectra of the CalnN films. Both samples were grown on CaN films under the same growth conditions but changing the growth temperature: (a) 830°C and (b) 780°C. (From Nakamura, S. and Mukai, T., [pn . f. Appl . Phys., 31, Ll457, 1992. With permission .)
Eg (X) = (1- X)Eg,GaN + XEg'lnN - bX(l- X)
(48)
where Eg•GaN is 3.40 eV, Eg,lnN is 1.95 eV, and the bowing parameter b is 1.00 eV. The calculated curve is shown by the solid line in the figure. Here, the bowing parameter, which is also called nonlinear parameter, shows downward deviation of the bandgap energy of ternary compounds compared to the linear relation between the bandgap energy of binary compounds, that is, from (l-X)E g,GaN + XEg,lnN' Figure 91 shows a typical room-temperature PL spectrum of a Zn-doped GalnN film. 22 It has two peaks. The shorter wavelength peak is due to BE emission of GalnN, and the longer wavelength peak is due to a Zn-related emission and has a large halfwidth (about 70 nm, i.e., about 430 meV).
2.12.5
GalnNIAIGaN LED
Figure 92 shows the structure of a GaInN / AIGaN double-heterostructure (DH) LED fabricated by Nakamura et al.4-6,22 In this LED, Si and Zn are co-doped into the GaInN active layer in order to obtain a high output power. Zn-doped GalnN is used as the active layer to obtain strong blue emission, as shown in Figure 91. Mg-doped GaInN does not show strong blue emission, in contrast to the Zn-doped films . Figure 93 shows the electroluminescence (EL) spectra of this system with forward currents of 0.1, 1, and 20 mA. 4-6,22The typical peak wavelength and halfwidth are 450 and 70 nm, respectively, at 20 rnA. The peak wavelength shifts to shorter wavelengths with increasing forward current. This blue shift suggests that the luminescence is dominated by the donor-acceptor (DA) pair recombination mechanism in the GalnN active layer codoped with Si and Zn. At 20 m A, a narrower, higher-energy peak emerges around 385 nm . This peak is due to band-to-band recombination in the GalnN active layer. This peak is
Chapter two: Principal phosphor materials and their optical properties
303
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Indium Mole Fraction X Bandgap energy of Ga(l_x)ln xN films as a function of the indium mole fraction X. (From Nakamura, S., Jpn . J. Opl., 23, 701, 1994. With permission.)
Figure 90
resolved at injection levels where the intensity of impurity-related recombination luminescence is saturated. The output power of the GafnN/ AIGaN OH blue LEOs is 1.5 mW at 10 mA, 3 mW at 20 rrtA, and 4.8 mW at 40 rnA . The external quantum efficiency is 5.4% at 20 mA. 22 The typical on-axis luminous intensity with 15° conical viewing angle is 2.5 cd at 20 rnA when the forward voltage is 3.6 V at 20 rnA .
2.12.6
GalnN single-quan tum-well (SQW) LEOs
High-brightness blue and blue-green GalnN / AIGaN OH LEOs with a luminous intensity of 2 cd have been fabricated and are now commercially available, as mentioned above. 4 - 6 ,22 In order to obtain blue and blue-green emission centers in these GaloN/ AIGaN OH LEOs, the GalnN active layer was doped with Zn . Although these GaInN / AIGaN OH LEOs produced high-power light output in the blue and blue-green regions with a broad emission spectrum (FWHM = 70 nm), green or yellow LEOs with peak wavelengths longer than 500 nm have not been fabricated." The longest peak wavelength of the EL of GalnN/ AIGaN OH LEOs achieved thus far has been observed at 500 nm (blue-green) because the crystal quality of the Galnl-I active layer of OH LEOs deteriorates when the indium mole fraction is increased to obtain green band-edge
Fundamentals of Phosphors
304
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~
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.........
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350
450
500
550
600
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Wavelength (nm) Figure 91 Room-temperature PL spectrum of a Zn-doped Gao.9sITIo. osN film . (From Nakamura,S.,
[pn . J. Opt., 23, 701, 1994. With permission.)
p-Electrode ~ I
I
p-GaN p-Alo.lsGaO.85N Gao.94Ino.o6N n-Alo.lsGaO.85N n-EleJrod e n-GaN GaN Buffer Laver Sapphire Substarte
Figure 92 Structure of the GaInN/ AIGaN double-heterostructu.re blue LED. (From Nakamura,S., J. Opt., 23, 701, 1994. With permission.)
[pn.
emission." Quantum-well (QW) LEDs with thin GaInN active layers (about 30 A) fabricated to obtain high-power emission from blue to yellow with a narrow emission spectrum/" are described below. The green GalnN SQW LED device structures (Figure 94) consist of a 300-A GaN buffer layer grown at low temperature (550°C), a 4-11m-thick layer of n-type GaN:Si, a 30A-thick active layer of undoped Gao5sIna4sN,a lOoo-A-thick layer of p-type Alo2Gao8N :Mg, and a 0.5-11m-thick layer of p-type GaN:Mg. This is the SQW structure. Figure 95 shows the typical EL of the blue, green, and yellow SQW LEDs containing different indium mole fractions of the GalnN layer, all at a forward current of 20 mA. The
Chapter two:
Principal phosphor materials and their optical properties
305
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400
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500
550
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Wavelength (nm) Figure 93 Electrolumi nesce nce sp ectra of a GaInN / AIGaN double-heterostructure blu e LED. (From Nakamura, S., Jpn . J. Opi., 23, 701, 1994. With permission .)
GuInNgreen SQW LEDs Single-Quantum-Well Structure (SQW) p-Alo.2Gao.8N
•
p-electrode p-GaN p-Alo.2Gao.8N Gao.ssIno.4sN n·GaN -
6 Gao.5sIno.4sN
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GaN buffer layer Sapphire substrate -
Figure 94 The struc tur e of green SQW LED. (From N ak amu ra, S., Senoh , M ., Iw asa, N., Nagah ama, S., Yamad a, T., and Mukai, T., [pn . J. App l. Phys. Lett ., 34, L1332, 1995. With perrnission.)
306
Fundamentals of Phosphors
(a) Blue ,.-.,
100
(b) Green
(c) Yellow
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450
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Wavelength (nm) Figure 95
Electroluminescence of (a) blue, (b) green, and (c) yellow SQW LEDs at a forward current of 20 mA. (From Nakamura,S., Senoh. M., Iwasa, N., and Nagahama, 5., [pn. J. App!. Phys., 34, L797, 1995. With permission.)
peak wavelength and the FWHM of the typical blue SQW LEOs are 450 and 20 nm, respectively; of the green 525 and 30 nm; and of the yellow 600 and 50 rim, respectively. When th e peak wavelength becomes longer, the FWHM of the EL spectra increases, probably due to the inhomogeneities in the GaInN layer or due to strain between well and barrier layers of the SQW caused by lattice mismatch and differences in the thermal expansion coefficients. At 20 mA, the output power and the external quantum efficienc y of the blue SQW LEOs are 5 mWand 9.1%, respectively. Those of the green SQW LEOs are 3 m Wand 6.3%, respectively. A typical on-a xis luminous intensity of the green SQW LEOs with a 10° cone viewing angle is 10 cd at 20 rnA. These values of output power, external quantum efficiency, and luminous intensity of blue and green SQW LEOs are more than 100 times higher than those of conven tional blue SiC and green GaP LEOs. By combining these highpower and high-brightness blue GaInN SQW, green GaInN SQW, and red AlGaA s LEOs, many kinds of applications such as LED full -color displays and LED white lamps for use in place of light bulbs or fluorescent lamps are now possible. These devices have the characteristics of high reliability, high durability, and low energy consumption. Figure 96 is a chromaticity diagram in which the positions of the blue and green GaInN SQW LEOs are shown. The chromaticity coordinates of commercially available green GaP LEOs, green AIGalnP LEOs, and red AlGaAs LEOs are also shown. The color range of light emitted by a full-color LED lamp in the chromaticity diagram is shown as the region inside each triangle, which is drawn by connecting the positions of three primary color LED lamps. Three color ranges (triangles) are shown for differences only in the green LED (green GalnN SQW, green GaP, and green AIGalnP LEOs). In this figure, the color range of lamps composed of a blue GaInN SQW LED, a green GaInN SQW LED, and a red AlGa As LED is the widest. This means that the GaInN blue and green SQW LEOs show much better color an d color purity in comparison with other blue and green LEOs. Using these blue and green SQW LEOs together with LEOs made of AlGaAs, more realistic LED full color displays have been demonstrated.
Chapter two:
Principal phosphor materials and their optical properties
307
0.9 , - - - - - - - - - - - - - - - - - - - ,
0.8
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0.2 0.1 Blue GaInN
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0.1
0.2
0.3
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0.5
0.6
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x Figure 96 Chromaticity diagram in which blue GalnN SQW LED, green GaInN SQW LED, green GaP LED, green AIGalnP LED, and red AIGaAs LED are shown. (From N akamura, S., Senoh, M., Iwa sa, N., Nagaharna. S., Yamad a, T., and Mukai, T., [pn . ]. App/. Phys. Leu., 34, L1332, 1995. With permission.)
2.12.7
GalnN multiquantum-well (MQW) LDs
The structure of the GalnN MQW LOs is shown in Figure 97. The GalnN MQW LD device consists of a 300-A-thick GaN buffer layer grown at a low temperature of 550°C, a 3-llmthick layer of n-type GaN:Si, a O.l-l1m-thick layer of n-type Gao9SlnOosN:Si, a O.5-l1m-thick layer of n-type AloosGao92N :Si, and a O.l-l1m-thick layer of n-type GaN:Si. At this point, the MQW structure consists of four 35-A-th ick undoped GaOSSInalSN well layers by 70-Athick undoped Ga a.9sIn0.Q2N barrier layers. The four well layers form the gain medium. The het erostructure is then capped with a 200-A-thick layer of p-type AlozGa osN :Mg, a (Ll-um-thick layer of p-type GaN:Mg, a O.5-llm-thick layer of p-type AloosGao92N:Mg, and a O.5-llm-thick layer of p-type GaN:Mg. The n-type and p-type GaN layers are used for light-guiding, while the n-type and p-type AlaosGa on N layers act as cladding for confinement of the carriers and the light from the active region. Figure 98 shows typical voltage-current (V-I) characteristics and the light output power (L) per coated facet of the LD as a function of the forward OC current at RT. No stimulated emission was observed up to a threshold current of 80 rnA, corresponding to a current density of 3.6 kA crrr-', as shown in Figure 98. The operating voltage at the threshold wa s 5.5 V.
Fundamentals of Phosphors
308
Ridge-waveguide purplish-blue InGaN MQW LDs Mult-Quantum-Well Structure (MQW) p-Alo.osGao.nN
p-electrode p-GaN p-Alo.osGao.nN p-GaN p-Alo.2Gao.sN -=~iiiiii~ GaInNMQWn-GaN n-Alo.osGao.92N n-GaN n-Gao.9sIno.osN
x=O.02 .--_...... x=O. IS
n-GaN
n.AI"."GaM'~ x
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Energy
GaN buffer layer F=============~ (0001) sapphire substrate
Figure 97 The structure of the Ga lnN MQ W LOs. (Fro m Na kam ura,S., Seno h, M., Naga hama, 5., Iwasa, N., Yamada, T , Ma tsus hi ta, T, Sugimo to, Y., an d Kiyo ku, H ., Presen ted at the 9th Annual
Meeting of IEEE Lasers and Electro-Optics Society, Boston , POU, Nov. 18-21, 1996. With permission.)
10
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60
80
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Typical light outp ut power (L)-current (1) and vo ltage (V)-curren t (J) characteris tics of GalnN MQW l Os measured under CW operation at RT (From Naka m u ra,S ., Seno h, M., Nagahama, S., Iwasa , N ., Yamada, T, Matsushita, T, Sugim oto, Y., and Kiyok u. H., Presen ted at the 9th Annual Meeting of IEEE Lasers and Electro-Optics Society, Boston, POU, Nov. 18-21, 1996. With permission .)
Chapter two: Principal phosphor materials and their optical properties 300
309
r-------------------, CW 1.5mW APC
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Time (hours) Figure 99 Ope rating curren t as a fu nction of time for GalnN MQW LOs under a constan t o utp ut power of 1.5 mW per facet con tro lled usin g an autopower con troller. The LD was oper ated under DC at RT. (From Na kamura, S., p resented a t Materials Research Society Fall Meeting, Boston , N1.1, Dec. 2-6, 1996. With perrnission.)
Figure 99 shows the results of a lifetime test of CW-operated LDs ca rr ied out at RT, in which the op erat ing current is sh own as a function of time, keeping output p ower constant at 1.5 mW per facet using an a u to pow e r controller (APC). Th e opera ting cu rren t grad u ally increases due to th e in crease in the thresh old current from its initial value; the current then increases sh arp ly aft er 35 h ours. This short lifetime is probably due to heating resulting fro m the high operating currents and vo ltages . Short-circu iting of the LDs occurred after the 35 hours, as mentioned ab ove. Figure 100 shows em ission spectra of GaIn N MQW LDs with various operating cur ren t under RT CW operation . Th e threshold cur rent and volt age of this LD were 160 rnA and 6.7 V, res pective ly. The thresh old current d en sit y w as 7.3 kA em> . At a current of 156 rnA , many lon gitudinal modes ar e observed w ith a mode sep a ra tio n of 0.042 n m: thi s se pa ra tion is smaller than th e ca lculated va lu e of 0.05 nm, probably due to refractive index chan ges from th e value used (2.54) in the calc ul ati on. Peri odic subband emissions are ob served with a p ea k sepa rat ion of about 0.025 nm (tiE = 2 m eV). The ori gin of these subbands has not yet been identified. On in creasing th e forwa rd curr en t from 156 to 186 m A, th e lase r em ission becomes single mod e an d shows mode hopping of th e peak w avelength toward hi gher ene rgy; th e peak emission is at th e cen ter of ea ch subband em ission. Figure 101 shows th e peak w avelength of the laser emi ssion as a function of the operating current under RT CW operation. A gradual inc rease of th e peak wa velength is o bserved , p robabl y due to bandgap n arrowing of the active layer cau sed by the temperature increase. At certain cur ren ts, large mode hopping of the peak w avelength toward h igher energy is observed with in crea sing op er ating current. The delay time of the laser em ission of the LOs as a fun ction of th e op era ting curren t w as m ea sured und er pulsed current modulation usin g the method des cr ib ed in Reference 14 in order to estimate the ca rrier lifetime (1). From this measurement, 't , was estimated to be 10 ns, which is larger th an previou s estimates of 3.2 ns.!' Th e threshold
Fundamentals of Phosphors
310
156 rnA
166 rnA
176 rnA
186 rnA
399
400 401 Wavelength (om)
402
Figure 100 Emi ssion spectra of GalnN MQW LOs with various operating currents under RT CW operation . (From Nakamura, S., presented at Mat erials Research Socieh) Fall Meeting, Bost on, N1.1, Dec. 2-6, 1996. With permission.)
carrier density (I\ h) was estimated to be 2 x 1020 crrr-' for a threshold current density of 3.6 kA cm- 2, and an active layer thickness of 140 A.14 The thickness of the active layer was determined as 140 A, assuming that the injected carriers were confined in the GalnN well layers. Other typical values are 'ts = 3 ns, Jth = 1 kA crrr-', and I\h = 2 X 1018 crrr-' for AIGaAs lasers and nth = 1 x 1018 crrr? for AIGaInP lasers. In comparison with other more conventional lasers, nth in our stru cture is relatively large (two orders of magnitude higher), probably due to the large density of states of carriers resulting from thei.r large effective ma sses." The Stokes' shift or energy differences between excitation and emission in GalnN MQW LDs can be as large as 100 to 250 meV at RT.24-26 Thi s means that the energy depth of th e localized state of the carriers is 100 to 250 meV in thes e devices. Both the sp ontaneous emission and the stimulated emission of the LOs orig ina tes from the se deep localized energy states. 24- 26 Using high-resolution, cross-sectional transmission electron microscopy (TEM), a periodic indium composition fluctuati on wa s observed in the LOs, probably caused by Galn'N phase separation during growth.25•26 Based on these results, the laser emi ssion is thought to originate from GaInN quantum dot-like states formed in these structures . The many periodic subban d emissions obse rv ed probably result from transitions between the subband energy levels of the GaInN quantum dots formed from
Chapter two: Principal phosphor materials and their optical properties
-=s --=
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180
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Current (rnA) Figure 101 Peak wav elen gth of the emi ssion sp ectr a of the GaffiN MQW LDs as a function of the op eratin g current under RT CW op eration. (From Nakamura,S., Cha racteris tics of GaInN multiqu antum-well- structure laser diodes, presented at Materials Research Society Fall Meeting, Boston, NU , Dec. 2-6, 1996. With permission. ) In-rich regions in th e GaInN well layers. Th e size of the GaInN dots is estimated to be approximately 35 A from th e high-resolution, cross-sectional TEM pictures. 25,26 It is d ifficult to con trol the siz e of GalnN dots that form in adjacent In-rich and -poor regi ons. The en ergy separation of each su bband emission in Fig ur e 100 is onl y abou t 2 m eV, which is cons ide red to be relatively small in comp a rison with the ene rgy difference between the n = 1 and n = 2 subba nd ene rgy transition s of oth er more con trolled quantum dot s. These periodi c subband energy levels are probabl y caused between n = 1 subband levels of qu antum dots with different dot sizes.
2.12,8
Summary
Superb right blue and gre en GaInN SQW LEOs ha ve been de veloped and comm ercialize d. By combining high-power, high-brightness blue GaInN SQW LEOs, green GalnN SQW LEOs, and red AIGaAs LEOs, many kinds of applications, such as LED full-color displ ays and LED white lamps for use in place of light bulbs or fluorescent lamps, are now possible. The se devices have the cha ra cteristics of high reliability, high durabil ity, and low energy consumption. RT CW operation of bluish-purple GalnN MQW LDs has been demonstrat ed recently with a lifetime of 35 hours. The carrier lifetime an d the threshold car rier den sity were estimated to be 10 ns and 2 x 1020 cm-', res pec tively. Th e em ission spectra of GalnN MQW LOs under CW ope ra tion a t RT showed periodic su bban d emis sions w ith an ene rgy separa tion of 2 meV. These periodi c subband emissions are probabl y due to the transiti ons between the subband energy levels of quantum dots formed from In-rich regions in the GaInN well layers. Further improvement in the lifetime of the LOs can be obtained by reducing the threshold current and voltage. The advances in thi s technology ha ve been rapid in the past decade. Prog ress attained has been reviewed in a number of places.F
References 1. Ama no, H., Kito, M., H irarnatsu , K., and Akasaki, I., Jpn. ]. Appl. Phys" 28, L2112, 1989. 2. Stri te, S., Lin, M.E., and Morkoc, H., Thin Solid Films, 231,197,1993.
312
Fundamentals of Phosphors 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16.
17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
27.
Nakamura, S., Iwasa, N ., Senoh, M., and Mukai, T., [pn. f. Appl. Phus., 31, 1258, 1992. Nakamura, S., Nikkei Electronics Asia, 3, 65,1 994. Nakamura, S., Mukai, T., and Sen oh , M., Appl. Phys. un .. 64, 1687, 1994. Nakamura, S., Mukai, T., and Senoh, M., f. Appl. Phys., 76, 8189, 1994. Nakamura, S., Senoh, M ., Iw asa, N., and Na gahama, S., [pn, f. App!. Phys., 34, L797, 1995. Nakamura, S., Senoh, M., lwasa, N., Nagahama, S., Yamada, T., and Mukai, T., [pn. f. Appl. Phys. uu .. 34, L1332, 1995. Nakamura, S., Senoh, M., Nag ahama, S., Iwasa, N., Yamada, T., Mat sushita, T., Kiyoku. H., and Sugimoto, Y., [pn. f. Appl. Phvs., 35, L74, 1996. Nakamura, S., Senoh, M., Nagahama, S., Iwasa, N., Yamada, T., Matsushita, T., Kiyoku, H., and Su gimoto, Y., [pn. f. Appl. Phys., 35, L217, 1996. Nakamura, S., Senoh, M., Nagahama, S., Iwasa, N ., Yamada, T., Matsushita, T., Kiyoku , H., and Sugimoto, Y., Appl. Phys. Lett., 68, 2105, 1996. Nakamura, S., Senoh. M., Na gah ama, S., Iw asa, N ., Yamada, T., Matsu shita, T., Kiyoku, H., and Sugimoto, Y, App!. Phys. Lett., 68, 3269, 1996. Nakamura, S., Senoh, M., Nagah ama, S., Iwa sa, N ., Yamada, T., Mat su shita, T., Sugimoto, Y, and Kiyoku, H ., Appl. Phys. Leti., 69, 1477, 1996. Nakamura, S., Senoh, M., Na gahama, S., Iwa sa, N ., Yamada, T., Mat sushita, T., Sugimoto, Y, and Kiyoku, H ., Appl. Phys. Leit., 69, 1568, 1996. Nakamura, S., Senoh, M., Nagahama, S., Iwasa, N., Yamada, T., Mat sushita, T., Sugimoto, Y, and Kiyoku, H ., Appl. Phys. Lett., 69, 3034, 1996. Nakamura, S., Senoh, M., Na gah ama, S., Iwasa, N ., Yamada, T., Mat su shita, T., Sugim oto, Y, and Kiyoku, H ., First room-temper ature continuous-w ave operati on of GaInN multi-quantum-well-structure laser diodes, presented at 9th A nnual Meeting of IEEE Lasers and ElectroOptics Society, Boston, PDl.l , Nov. 18-21, 1996. Nakamura, S., Characteristics of GaInN multi-quantum-well-structure laser diodes, presented at Materials Research Society Fall Meeting, Boston, N1.1, Dec. 2-6, 1996. Nakamura, S., [pn. f. App!. Phys., 30, L1705, 1991. Nakamura, S., Mukai, T., and Sen oh , M., [pn. f. Appl. Phys., 31, 2883,1 992. N akamura,S., Mukai, T., Senoh, M., and Iwasa. N ., [pn. f. Appl. Phys., 31, L139, 1992. Nakamura, S. and Mukai, T., [pn. I. Appl. Phys., 31, L1457, 1992. Nakamura, S., [pn. f. Opt., 23, 701, 1994. Osarnura. K., Naka, S., and Murakami, Y,/. App!. Phys., 46, 3432, 1975. Chichibu, S., Azuhata, T., Sota, T., and Nakamura, S., Appl. Phys. Lett ., 69, 4188, 1996. Narukawa, Y., Kaw akami, Y, Fuzita, Sz., Fujita, Sg ., and Nakamura,S., Phys. Rev., B55,1938R, 1997. Narukawa, Y, Kawakami, Y, Funat o, M., Fujit a, Sz., Fujita, Sg., and Nakamura, S., Role of self-formed GaffiN quantum d ots for the exciton localization in the purple laser diodes emitting at 420 nm, App!. Phys. Lett., 70, 981, 1996. Nakamura, S., and Pasol, G., The Bille Laser Diode, Springer - Verlag, Berlin, 2000.
chapter two - section thirteen
Fundamentals of luminescence Hiroyuki Matsunami
Contents 2.13 Silicon 2.13.1 2.13.2 2.13.3
carbide (SiC) as a luminescence materiaL Poly types Band structure and optical absorption Luminescence 2.13.3.1 Luminescence from excitons 2.13.3.2 Luminescence from donor-acceptor pairs 2.13.3.3 Other luminescence centers 2.13.4 Crystal growth and doping 2.13.5 Light-emitting diodes References
313 313 314 314 314 316 318 319 319 319
2.13 Silicon carbide (SiC) as a luminescence material 2.13.1
Polytypes
Silicon carbide (SiC) is the oldest semiconductor known as a luminescence material. This material shows polytypism arising from different stacking possibilities. In he xagonal close packing of the Si-C pa ir, the positions of the pair in the first and second layers are uniquely determined (A and B) as shown in Figure 102(a). However, in the third layer, there are two possibilities, either A or C as shown. In the former case, the stacking order becomes ABAB... , giving a wurtzite (hexagonal) str uctu re, and the latter becomes ABCABC. .., giving a zinc-blende (cubic) structure. In SiC crystals, there can exist various combinations of these two structures, which give different stacking orders called polytypes. Among the many polytypes, 3C-, 6H-, and 4H-SiC appear frequently: these structures are shown in Figures 102(b)-(d) together with 2H-SiC (Figure 102(e» . Here, the number indicates the period of stacking order and the letter gives its crystal structure: C = cubic, H = hexagonal, R = rhombohedral. Since the position of each atom has a different configuration of nearest-neighbor atoms, the sites are crys tallographically different; that is, they have cubic or hexagonal site symmetry. Hence, when an impurity atom substitutes into the position of Si or C, it gives rise to different en ergy levels depending upon th e number of inequivalent sites present in the material. In 3C-SiC and 2H-SiC, only one cubic or one hexagonal site exists, respectively, 313
Fundamentals of Phosphors
314
(b)3C-SiC
(c)6H-SiC
0.
c-axis (a)Close packing
(d)4H-SiC
(e)2H-SiC
Figure 102 Position of Si-C pair in typical SiC pol y typ es. (a) Close packing of equal spheres (SiC pair), (b) 3C-SiC, (c) 6H-Si C, (d) 4H-SiC, and (e) 2H-SiC.
whereas in 6H-SiC there exist one hexagonal and two cubic sites and in 4H-SiC one he xagonal and on e cubic sites.
2.13.2
Band st ructure and optical absorption
Figure 103 shows the ab sorption spectra of d ifferent polytypes of SiC at 4.2K. l The spectra contain shoulder features related to phonon-assisted transitions, which are characteristics of indirect band structures. In the figure, the positions of the exciton bandgaps are shown. In Table 28, the values of exciton bandgaps and exciton binding energies are tabulated .' The characteristics near the fundamental absorption edge have quite similar structure for all the polytypes excep t 2H-SiC. This is due to the similarity of the phonons involved in optical absorption in the different polytyp es .
2.13.3
Luminescence'?
Since SiC has an indirect band structure, strong luminescence can be expected from the recombination of either bound excitons or donor-acceptor pairs.
2.13 .3.1
Luminescence from excitons
Figure 104 depicts the photoluminescence sp ectrum from excitons bound at N donors in 3C-SiC. 1 From the energy d ifference between the exciton bandgap and the peak energy
Chapter two:
C\l <, .-l I
315
Fundam entals of luminescence
Wavelength (nm) 400
510
360
380
8
o
C'J
<, .-l
~
~
3C
Q) ~
o
~
Cf-< Cf-< Q)
0
o ~
0 .,..., ~
0.
~
0 (J)
,D
2.40
3.00
3.20
3.10
3 .30
3.40
3.50
Photon energy (eV) Figure 103 Absorption spectra for typical SiC polytypes. Exciton band gap is s how n for each p olytyp e. (From Choyke, w.J., Mater. Res. Bull., 4, S141-S152, 1969. With permi ssion .)
Table 28
Bandgap Energies in Typic al Pol ytypes of S:C
3C (Zinc-blende ) 6H 4H 2H (Wur tzite)
Ee x (eV) 4.2K
Em (meV)
Conduction ban d m inimum
2.390 (ID)"
13.5b
X"
3.023 (ID)" 3.265 (ID)" 3.330 (ID)"
20d
78c
?
Note: Eex: Excito n bandgap, Ee• c : Exciton bind in g ene rgy. 10 : indirect band structure. X, U , M , K: p osition in Brillouin zo ne . a Choyke, w.J., M ater. Res. Bull., 4, 5141, 1969. b Ned zvetski i, OS. et al ., Sou. Phys. - Semicon., 2, 914, 1969. c Sankin, Vl ., SOD. Phys. Solid State., 17, 1191, 1975. d Dubrovs kii, G.B. e t al., Sou. Phys. Solid Siaie., 17, 1847, 1976. e Herman , F. e t al ., Mater. Res. Bull., 4, 5167, 1969. ( Choy ke, w.J., unpublished result, 1995. g Pa trick, L. e t a l., Phys. Reo., 137, A1515, 1965.
Fundamentals of Phosphors
316
3C-SiC: N
LA TO
6K +->
~
l::
La
TA
1000
a
:;j
..0 l-< C(j
>.
+->
100
~
Ul
l:: (l)
+->
l::
~
+->
.c ,,..,bD
10
....J
2.40
2.30
Photon energy (eV) Figure 104 Pho toluminescence spec tru m of excitons bo und at N don ors in 3C-SiC. Ecx ind icates the exci ton bandgap. (0: zero p honon; TA: tra nsverse aco ustic; LA: longitu d inal acou stic; TO: tran sverse optic; and LO: lon gitu dinal optic). (From Choyke. w.y., Mater. Res. Bull., 4, S141-S152, 1969. With permission .)
corresponding to th e zero-pho non line, th e exciton b indi ng energy for N donors is estima ted to be 10 m eV. Since the resolution of peak energ ies is much bet ter than that in the absorption spectra, th e exac t value of phonon energies can be obtai ned from the pho tolumines cence spectra . In the pho toluminescence spectr um of 6H-SiC, there exis ts a zero-p honon pea k du e to the recom bination of exci tons bound at N do nors su bstituted in to hexa gon al C sites and tw o zero-p honon p eaks due to those located in cub ic C sites.' Since the energy levels of N do nors in inequivalent (hexagonal, cubic) sites are different, the pho tolumi nescence peak s have differe n t energies.
2.13.3.2 Luminescencefrom donor-acceptor pairs In SiC, N atoms belongi ng to the fifth colum n of the period ic table work as do nors, and B, AI, and Ga in the th ird column work as acceptors. When donors an d acceptors are sim ultaneously inco rporated in a crystal, electrons bound at donors and holes at accep tors can create a pair due to the Coulombic force between electrons and ho les. Th is in terac tion leads to strong p ho toluminescence through recombina tion and is known as don or-acceptor pair luminescen ce. Fig ure 105 shows th e photoluminescence spec trum from N- Al donor-acceptor pa ir recombination in 3C-SiC at 1.8K.4 This gives a pec uliar structure showing the recombination of ele ctro ns and holes in d on or-acceptor pairs of type 2 w ith N donors replaci ng C and Al rep lacing Si. From a de tailed ana lysis of th is pec uliar structure, the va lue of 310 meV is ob tained for the sum of ED(N) and EA(AI), where ED(N ) is the N-don or level
Chapter two: Fundamentals of luminescence
317
Wavelength (nm) 540
550
560
<J ''';
c
;:l
19 17 20 18
.......
3C-SiC:N.Al ~1
16
10
1.8K
8
~ 15
12 13
<J
..c::: be
j 2.20
2.24
2.28
2.32
Photon energy (eV) Figure 105 Ph otoluminescence s pectru m of N donor-Al acceptor pair recombination in 3C-SiC. The number for each peak ind ica tes the order of di stance between donor and acceptor. (From Choyke, W.I. and Patrick, L., Phys Rev., B2, 4959-4965,1970. With permission.)
and EA(AI) the AI-acceptor level. At 77K or higher, the spectrum changes to that due to the recombination of free electrons and holes bound at Al-acceptors (free-to-accep tor recombination) because of thermal excitation of electrons bound at N-donors to the conduction band. From the spectrum, the value of EA (AI) can be determined precisely. Based on the se studies, the values of EA(AI) := 257 meV and ED(N) := 53 m eV were obt ained." From a sim ilar analysis, the B- and Ga-acceptor level s can also be determined. In most SiC polytypes, except for 3C-SiC and 2H-SiC, th ere are inequivalent sites, and impurities substituting into those sites give rise to different energy levels. Thu s, spectra of d on or-ac ceptor pair recombination and free-to-acceptor recombination can become complicated . As examples, donor-acceptor pair recombination spectra in 6H-SiC at 4.2K are sh own in Figure 106(a) and free-to- acceptor recombination sp ectra at 77K in Figure 106(b).s Although the energy levels are different for different acceptors (B, AI, and Ga), the shap es of spe ctra are quite sim ila r when the abscissa is sh ifted by an energy of the order of 0.05 eV, as shown in the figure . The B series (peaks denoted as B) in the sp ectra show donor-acceptor pair luminescence for N donors in hexagonal C sites and Al acceptors, and the C series (pe aks denoted as C) arising from N donors in cub ic C sites and Al acceptors. Here, the en ergy level s of Al acceptors are thought to be very sim ilar, whether they are in hexagonal or cubic Si site s. The subscripts in the figure are defined as follows: 0 im plies a zero-phonon peak and LO implies peaks involving longitudinal optical phonons. Peaks A indicate free-toacceptor recombination : Nand Ab are due to accep tor s substituting in to he xagonal and cubic Si sites , respectively. Since there are three different sites for donors and acceptors, respectively, in 6H-SiC, anal ysis for the p eculiar s tructure observed in the sp ect ra bec omes ve ry difficult. The photon energ y, hv(R) , from donor-a cceptor pair lumin escence is gi ven b y hv(R) -R6exp(-4ncR3/3), where R is the distance between a donor and an acceptor and c is larger of the don or or acceptor conc entrations. By curve fitting of the ab ove relati on to the spectra, the value of ED+ EAcan be obtained," Since the value of EA is calculated from free-to-acceptor recombination as in Figure 106(b), EDcan also be determined. Although
Fundamentals of Phosphors
318
;:r------------::--=------, CLO BLO §
6H-SiC
B2LO C2LO
4.2K
•
• •
~
.gL.._...L-__...L---...L.--...I...---..Jo-..::-----' .,.., ...J
2.0
1.9
2.1
2.2
Photon energy (eV)
6H-SiC
17K
~
.g ......__
---1._ _---''--_ _''--_ _......._ _-&-......
.,.., 1. 9 ...J
2.0
2.1
2.3
2.4
Photon energy (eV) Pho tolum inescence spec tra of (a) d on or-acceptor pair recombination at 4.2K and (b) freeto-acceptor reco mbination at 77K in 6H-SiC d op ed with 8, AI, an d Ga. A o: free-te-Al accep tor peak, (b) 8 0 : N- d on or(hexagonal site)- Al acceptor, (c) Co: N- d on or(cubic site)-Al accep tor. LO ind icates longitudinal phonon. (From Ikeda , M., Mat sunami, H., and Tana ka, T., Phys. Reo., 822, 2842-2854, 1980. With p erm ission. )
Figu re 106
on e hexagon al site and tw o cu bic sites exis t in 6H- SiC, the difference between the energies for the tw o cubic sit es seems to be very sm all. Curv e fitting wa s car rie d out b y ass uming that the luminescence in tens ity related to cubic sites is two tim es larger than that related to hexagonal sites. The calcul at ed en ergy levels of impurities are given in Table 29. In the tabl e, the results of d ifferen t poly ty pes are a lso shown ." In each polytyp e, the ra tio between th e accepto r ene rgy levels for cu bic and hexagon al sites is very sma ll, whereas tha t of donor energy levels is large.
2.13.3.3
Other lu minescence centers
In ad d ition to the above lu minescen ce cen ters, lu minescen ce due to d efects p roduce d by ion implan tat ion ? and due to th e localized cen ters such as Ti2 have been reported.
Chapter two: Fundamentals of luminescence Table 29
Polytype 3C-SiC 6H-SiC 4H-SiC
319
Energy Levels of Donor and Acceptors
Site
Don or N
C C H C H
56.5 155 100 124 66
Energy level (meV) Acceptor Al Ca
B
254 249 239
343 333 317
735 723 698
191
267
647
From Ikeda , M., et al., Phys. R£'V., 822,2842,1980. With permission .
2.13.4 Crystal growth and doping Crystals of SiC have been grown by the so-called Acheson method, in which a mixture of Si02 and C is heated to ab out 2000°C. To grow pure single crystals, the powdered SiC crystal mixture is sublimed in a specially designed crucible by the Lely method . Recent large-diameter (approximately 2-inch diameter) single crystal boules have been produced by a modified Lely method ut ilizing a SiC seed in the sublimation growth. On those single crys tals, epitaxial growth has been carried out by either liquid phase epitaxy (LPE) or vapor phase epitaxy (VPE). In LPE, molten Si in a graphite crucible is used as a melt in which a SiC substrate is dipped into.s In VPE, chemical vapor deposit ion (CVD) with SiH4 and C3H8 has been widely used. To get a high-quality epitaxial layer at low temperatures, step-con trolled epitaxy is used, which utilizes step-flow growth on offoriented SiC substrates'? Doping with third column elements as donors or fifth column elements as acceptors can be done easily through both in LPE and VPE.
2.13.5 Light-emitting diodes Earlier, yellow light-emitting diodes (LEOs) of 6H-SiC utilizing N-B donor-acceptor pair luminescence wer e demonstrated; they were later replaced by GaAs 1_xPx:Nyellow LEOs. Blue LEOs of 6H-SiC p-n junction utilizing N-AI donor-acceptor pair luminescence are usually mad e by LPE6 or VPF methods. The mechanism for electro luminescence through injection of carriers was clarified by Ikeda et a1. 8 A typical spectrum of blue LEOs is show n in Figure 107.9 The spectral peak is located at 470 nrn with a width of 70 nm for a forward current IF of 20 rnA (0.3 x 0.3 rnm-). The diode consists of LPE-grown AJ-doped p-SiC/N-doped nSiC/n-6H-SiC substrate. LEOs are fabricated w ith a p-side down structure, and the light comes through the n-SiC. The maximum external quantum efficiency is 0.023% (IF = 5 rnA). Since the blue LEOs utilize N-Al donor-acceptor pair luminescence in n-type epila yers , the brightness increases with incorporation of AI, and it exceed s 20 mCd (IF = 20 rnA) .
References 1. Choyke, W.J., Mater. Res. Bull., 4, SI41-S152, 1969. 2. Choyke, w.J . an d Pat rick, L., Silicon Carbide-1973, Mar shall, RC., Faust, j.W., and Ryan, C.E., Eds., Unive rsity of South Carolina Press, 1974, 261-283. 3. Choyke, wj. and Pa trick, L., Phys. Rev., 127, 1868-1877, 1962. 4. Choyke, W.J. and Patrick, L., Phys. Rev., B2, 4959-4965, 1970. 5. Iked a, M., Matsunami , H., and Tanaka, T., Phys. Rev., B22, 2842-2854, 1980. 6. Matsunam i, H., Ikeda, M., Suzuki, A., and Tanak a, T., IEEE Trans. Elee. Devices, ED-24, 958961, 1977.
Fundamentals of Phosphors
320
Room temperature
6H-SiC IF = 20 rnA
400
500 Wavelength (nm)
600
Figure 107 A typical spectrum of bright blue LEOs of 6H-SiC. (From Mat sushita, Y., Koga, K., Veda, Y, and Yamaguchi, T., Oyobuturi , 60, 159-162, 1991 (in Japanese).)
7. Shibahara, K., Kuroda, N ., Nishino, S., and Matsunami, H., Jpn . J. Appl. Phys., 26, U815U 817, 1987. 8. Ikeda, M., Ha yak awa, T., Yamagiwa, S., Matsunami, H., and Tan aka , T., J. Appl. Phus., 50, 8215-8225, 1979. 9. Matsushita, Y, Koga, K., Veda, Y, and Yamaguchi, T., Ouobuturi, 60, 159-162, 1991, (in Japanese).
chapter two - section fourteen
Fundamentals of luminescence Rong-Jun Xie, Naoto Hirosaki, and Mamoru Mitomo
Contents 2.14 Oxynitride phosphors 321 2.14.1 Introduction 321 2.14.2 Overview of oxynitride phosphors 322 323 2.14.3 Characteristics of typi cal oxyn itride phosphors 2.14.3.1 LaAl(Si6-zAlz)N lo-zO z:Ce3+ (z = 1) 323 2.14.3.2 ~-SiAION :Eu 2+ • • •• •• • •• ••• • •••••••• • • ••• •••• • • ••• • •• •• •• •• • • • • •• • • •• • • • • • • • • •• • • •• • • • •• • • •• •• •• • • 323 2.14.3.3 MSiP2N2:Eu2+ (M = Ca, Sr, Ba) 324 2.14.3.4 a -SiAION:Eu 2+ .• . .. •• ••. ••••.• ••.•• ••.• •• •. ••••. •••• •••• •••••. •. •.• .• .• .•.• . . . .• . . . . . . . . . . . . . . . . 325 2.14.3.5 M 2SisNs:Eu 2+ (M = Ca, Sr, Ba) 325 2.14.3.6 CaAl SiN 3:Eu 2+ . .. •. • .•• . . .• • •• ••. •. • • • • • • •• .• • . • • ••• • • . ••• .• • • • . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 2.13.4 Ap p lica tions of oxynitrid e phosphor s 327 Refer ences 328
2.14 Oxynitride phosphors 2.14.1 Introduction Inor ganic phosphors are composed of a host lattice d op ed w ith a sm all amo unt of im p ur ity ion s th at activate luminescence. Most of these materi als are oxid es, s ulfides, fluorid es, halides, and oxysulfides doped with transition metal ion s or ra re-earth ion s. Recentl y, with th e advent of solid-state lighting technologies as w ell as the d evel opment of plas ma an d field emission display panels, a great number of trad itional phosphor s can no t m eet the requirem ents for new applications, for example: (1) excita tion by near-ultraviolet (UV) or visible light; (2) efficient emission of appropriat e co lors; a nd (3) surviva l a t ad verse en viron m en ts. Therefore, novel phosphors with superior luminescent p ro pe rties are being sou gh t using new host materials. The integration of nitrogen (N) in sili cates or al u min osilica tes p roduces a wide ra ng e of com p lex stru ctur es with increased flexibility com pared to the oxos ilica tes . a nd thus a n ew class of materials, nitridosilicates, nit ridoaluminosilicat es, a nd s ialons, a re obtai n ed .' Th ese novel luminescent materials-the oxyn itride phosphors-ha ve been sy n th esized by d op in g w ith app ro pria te amounts of rare-earth ac tiva to rs .v -" The rare earths d op ed in the oxy n itrid e phosphors usually en ter into in ters titial sites a nd a re coor di na ted by (0 , N) 321
322
Fundamentals of Phosphors
ions locat ed at various di stances. For tho se rare ea rths (i.e., Eu 2 + and Ce 3+) emi tting from their Sd excited state, w h ich is strongly affected by the crysta l-field envi ronme n t (e.g., covalency, coo rd ina tion, bond len gth, crys ta l-field stren gth), ap prop ria te e mission colors can be obta ined by carefully se lecting th e host lattice. Du e to a high er cha rge of N' compared w ith that of 0 2 - an d because of the nephelau xet ic effect (high cov alency), the crys tal-field sp li tting of th e Sd levels of rare ea rths is larger and the center of gravity of the Sd sta tes is shi fted to low en ergy (i.e., lon ger wav eleng th) in these oxynitride comp ounds. Furthermore, the Sto kes sh ift becomes smaller in a more rig id lattice, which results w he n more N 3- is inco rpo ra ted . This will result in more versa tile luminescent properties of oxyn itri de p hosp hors, incr easin g their range of applica tions. In this section, the characte ristic features and po ten tial applications of rare -earth-d oped nitrid e phosphors are d escribed .
2.14.2
Overview of oxynitride phosphors
Table 30 list s oxy n itr ide p hosp h ors reported in the literature in recen t yea rs. Th e host lattice of the se phosphors is based on nitridosilicates, oxonitridosilica tes, or oxo n itridoaluminosilicates, wh ich are derived from silica tes b y form al exc ha nges of a nd Si by N and AI, respectively. The struc ture of these host lattices is bui lt on highly conde nse d networks constructed from the co rner-sha ring (Si, Al)-(O, N) tet rah edra . Th e d egree of condensation of the net w or k s tr uc tures (i.e.. the molar rati o Si:X > 1:2, w ith X = 0 , N ) is higher than the max im u m va lue for oxosili cates (l :4:S; Si:O:S; 1:2).2\ Co nse que n tly, these highl y condensed materials e xh ibit high chem ica l and ther m al s tabilities . Moreov er, the str uctu ra l va riab ilities o f this class of mat er ials provide a significan t exte nsion of con ventional silica te che mistry, form in g a large famil y of Si-AI-O-N multiternary com poun d s.
°
Tabl e 30
Emissio n Co lor and Cr ys ta l Structure of Oxynitride Phosphors
Phosphor
Em iss ion color
Y-Si-O-N:Ce 3+
Blue
BaAI UO J6N:Eu z+
Blue
~-Alumi n a
[2,4J
JEM:Ce 3+
Blue
Ortho rho m bic
[1 9]
SrSiAl z03N 2:Eu +
Blue -green
Ortho rhomb ic
[1 4J
:Eu 2+
Blue-g reen
O r thorho m bic
[14]
BaSiz0 2 N 2 :Eu 2 +
Blue-g reen
Mo no clinic
[18]
a -SiA ION :Yb2+
G reen
H exagon a l
[15]
~-SiAI ON :Eu z+
Green
H exagona l
[17]
MYSi4N 7:Eu + (M = Sr, Ba)
Green
He xagonal
[12]
MSizOzN 2:Eu 2 + (M = Ca . Sr)
Gree n-yello w
Monoclinic
[18]
a -SiA ION:Eu 2+
Yello w- ora nge
Hexagonal
[7,8,10,11]
LaSi,No:Euz+
Red
Orthorhombic
[6]
LaEuSi 2N,Oz
Red
Orthorhombic
[6J
Ca zSisN s:Eu 2 •
Red
Monoclin ic
[5]
M 2Si sN s:Eu 2+ (M = Sr, Ba)
Red
O rthorhombic
[5]
Red
Ortho rho m bic
[20]
2
SrSisAI0 2N 7
2
Ca AISiN 3 :Eu
2+
Crystal structure
Refere nces
[3J
323
Chapter two: Fundamentals of luminescence
The most u sual ap proach es for synthesizin g oxynitr ide phosphors are so lid -s ta te reactions and gas-red uc tion-n itrid ation . The solid-sta te reac tion involves th e reac tion am ong ch emical com ponents including metals, n itride, and ox id e starting pow d er s a t high tem p era tures (1400-2000°C) under an N 2 atmosph ere. The nitridation reaction is ge nerally p erformed in an alumina boa t co n ta ining th e oxi de prec ursor powder load ed in side an alumina /quartz tu be th rough which N H ) or NH 3-CH 4 gas flows at appropri at e rates a t high temper atures (600- 1500°C) . The NH, or N H 3-CH 4 gas ac ts as both a redu cing and nit rid at ion agen t.
2.14.3
Characteristics of typical cximitride phosphors
2.14. 3.1 LaAI(Sit>_:Alz)NJO_zOz:Ce3+(z = 1) Crystal structure. The LaAI(Si6_zAlz)NIO_zOz (JEM) p hase was iden tified in th e prepara tio n of La-stabili zed a -SiAIO N materials.Alt h as an or thorho m bic structure (space g ro up Pbcn) with a = 9.4303, b = 9.7689, and c = 8.938 6 A. The Al a tom s and the (Si, AI) a to ms ar e tetrahed rall y coordi na ted by the (N, 0) a to ms, yield ing an AI(Si, AI)6(N, 0 )103- netwo rk. Th e La atoms a re locat ed in the tunnels ex te ndi ng alon g the [001] direction and are irreg ularly coord ina ted by seven (N , 0 ) atoms a t an average distance of 2.70 A. Luminescence characteristics. As show n in Figure 108, the emission sp ec tru m of JEM:C e 3 + disp lays a broad band w ith the p ea k located a t 475 nm und er 368-n m exci ta tio n ." The em ission effic iency (ex terna l q ua n tum efficien cy) is a bo u t 55% w hen exci ted at 368 nm. This blu e phosp ho r ha s a broad exc ita tion spec tru m , ex ten di ng from the U V to the visible ran ge. Wh en the concentration of Ce 3+ or th e 2 value increases, both th e exci ta tio n and emission spec tr a are red shifted. Preparation. The starting materials for JEM are Si3N 4 , AIN, AI20 y La 203, a nd Ce02 . The po w der phosph or is synthesized by heat ing th e powder mixtu re a t 1800-1 900°C for 2 h un d er 1.0 MPa N 2 .
2.14.3.2 !J SiAION:Eu 2+ Crystal structure. Th e struc ture of
~-S iAI ON is derived from ~-Si3N4 by su bstitution of AI-D by Si- N , and its che m ical composi tion ca n be w r itt en as Si(r.zAlzOzN : (2 re p resen ts
EX
EM
::i
.i C
'iii c Q)
.S --l Q..
350 400 450
500
550
600
650
Wavelength (nm)
Figure 108
Emiss ion and excit ati on of LaAl
700
200
250
300
350
Wavelength (nm) (Si (~ , A lJ N l o-: 0z : C e3+
(z = 1).
400
450
Fundamentals of Phosphors
324
EX
EM
:i
~
z-
'(ij
c
QJ
.~ -'
Q..
450
500
550
600
650
700
200 250 300 350 400 450 500 550 600
Wave length (nm)
Figure 109
Emission and excitation of
Wavelength (nm) ~ -SiA1 0N : Eu 2+ .
the number of Al-O pairs substituting for Si- N pa irs and 0 < z ::; 4.2).Z3 ~-Si AI ON has a hexagonal crys tal struc tur e and the P63 space gr oup. In this structure, there are con tin uous channels parall el to the c direction. Luminescence characteristics. The ~-SiAION:Eu z+ phosphor gives intense green emission with th e peak located at 538 nm;" as seen in Figure 109. The broad emi ss ion spectrum has a fuU width of half maximum of 55 nrn . Two w ell-res olved broad bands cen tered at 303 and 400 nm are observ ed in the excitation spe ctru m. The broad excitation ran ge enables the ~-SiAION :Euz+ ph osphor to emit s trong ly under near UV (390-410 nm) or blu e-light excitation (450-470 nm) . This green phosphor has a chrom a ticity coord inates of x = 0.31 and y = 0.60. Th e ex tern al quantum efficien cy is abo u t 41% when excited at 405 nm. Preparation. Sta rting from Si3N4, AIN , Al z0 3, and Eu zOy the ~-SiAION:Euz+ ph osphor is synthesized at 1800-2000°C for 2 h under 1.0 MPa N z. An Eu concentration of <1.0% is used.
2.14.3.3 M Si202N2:Eu 2+ (M
=
c« 51', Ea)
Crystal structure. All MSizOzNz compounds crystaU ize in a monoclinic lattice with different space groups and lattice parameters for M = Ca. Sr, Ba:CaSizOzNz, P2 1 / C, a = 15.036, b = 15.450, c = 6.851 A; SrSizOzNz, P21 1M, a = 11.320, b = 14.107, c = 7.736 A; BaSizOzNz' PzI M, a = 14.070, b = 7.276, c = 13.181 A.18.Z4 A nitrogen-rich phase MSi zOZ_8Nl ~ !\J (M = Ca. Sr, (j> 0) has been identified, s ugges ting that some modifications of MSizOzNz (M = Ca. Sr) exist depending on the syn thesis temperature. Luminescence characteristics. All MSizOzNz:Eu z+ph osphors have a broad-ba nd emission sp ec tr u m w ith d iffe rent full widths a t h alf maxi m u m : CaSiZ01N:Eu z+ , 97 n m ; SrSizOzN :Eu z+ , 82 nm; and BaSizOzN:Eu z+, 35 nm (see Figure 110). CaSi zOzN:6%Eu 2+ sh ow s a yellow ish em ission w ith a maximum at 562 nrn. SrSizOzN:6%Eu z+ emits gre en color with a ma ximum at 543 run , an d BaSi20 zN:6%Eu z+ yield s a blue-green emission with a peak at 491 nm. The excita tio n spectrum of CaSi zOzN:6%Eu z+ shows a flat broad band coverin g the 300-450 nm ran ge, wh ereas two well-resolved bro ad bands centered at 300 and 450 nm are seen in SrSizOzN:6%Eu z+ and BaSizOzN:6%Eu z+, resp ectively. Preparation. The MS izOzNz:Euz+ phosphors are synthesized by heating the powder mixture of Si3N4, sto, and alka line-ear th carbonates a t 1600°C under 0.5 MPa N z.
325
Chapter two: Fundamentals of luminescence
EM ,, , , , , , ,, ,, ,, ,, ,
EX
,r , r I I I I
, ,
I I
, ,
I
I
1
I
, , ,,
I I I
1
, ~ ,
I
,
.I I ','
CaSi2 0 2 N2 SrSi 2 0 2 N2 BaSi 202N 2 400
450
500
550
600
650
700
200
Wave length (nm)
300
350
400
450
500
550
Wavelength (nm)
Figure 110 Emission and excitation of MSiP 1N1:Eu 1+ (M
2.14.3.4
25 0
= Ca. Sr, Ba).
a-SiAION:Eu 2+
Crystal structure. ex-SiAlON is isostructural to ex-Si3N4 . It has a hexagonal crystal s tructure and the P31 c space group. The ex-SiAION unit cell con tent, consis ting of four "Si3N 4 " units, can be give n in a general formula MxSi t2_m_nAI/1I+"0,,N16_" (x is the solubility of the m etal M).2,.2(' In the ex-SiAION structure, 711+n (Si-N) bonds are rep laced by m (AI-N) bond s and n (AI- O) bon ds; the charge discrepancy cau sed by the subs titution is com pe nsa ted by the in trod uction of M cations includ in g Li'. M g 2+, Ca 2 ' , Y" . and lanthan id es. The M ca tions occupy the interstiti al sites in the ex-SiAION latti ce and are coordi na ted by seven (N, 0 ) an ions at thr ee different M-(N, 0 ) di stances. Luminescence characteristics. ex-SiAION: Eu 2 + phosphors give green-yello w, ye llow, or yellow -orange emissions w ith peaks located in the ran ge of 565-603 n m/ ,S,lO,!1 as sh own in Figure 111. The broad -band emi ssion spec tru m covers from 500 to 750 nm with the full wid th of half maximum of 94 nm . The excitation spectrum of Eu 2+ in ex-SiAlON has two broad bands wi th peaks a t 300 and 420 nm . respectively. The extern al quantu m efficiency of the ex-SiAION:Eu2+ ph osphor wi th optimal composition is ab ou t 58% when excited at 450 nm . By tailo ring the composition of the ho st latti ce and contro lling the concen tra tion of Eu 2+, the em ission color of ex-SiAION can be tuned th rou gh a w ide range. Preparation. The Ca-ex-SiAIO N:Eu 2 + phosphor is syn thesized by so lid -state reac tions. The p owder mixt ure of Si3 N 4 , AIN, CaC0 3, an d EU2 0 3 is fired at 1600- 1800°C for 2 h under 0.5 MPa N 2 . The gas-red uction- n itridation method is also used to prep are ex-SiAION :Eu 2+ phosphor." It is syn thesized from the CaO-AI 203-Si0 2 sys tem, by usin g an N H 3-CH 4 gas mixtu re as a reduction-nitridation agent. The Eu con centration in ex-SiAlON phosphors var ies from 0.5 to 10%.
2.14.3.5 M 2Si sN s:Eu2+ (M
= Cal
Sr, Ba)
Crystal structure. Ca 2S isN s has a monoclinic crystal sys tem w ith the space gro up of Cc, whereas both Sr2Si sNs and Ba 2SisNs have an orthorh ombic latt ice wi th the spa ce gro u p of Pmn2 1.27,2S The local coord ination in the str uctu res is qu ite similar for these ternary alka lineea rth silicon nit rid es, half of the nitrogen atom s connec ting two Si neighbors and th e o the r half ha ve three Si neighb ors. Each Ca atom in Ca 2SisN s is coordinated by sev en nitrogen
326
Fundamentals of Phosphors
--c:J
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?:'
' (jj
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OJ
C
!
-l
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1
450
500
550
600
650
700
750
200
250
Wave length (nm)
Figu re 111
300
350
400
450
500
550
Wavelength (nm)
Emission and excita tio n of cx-SiA10 N :Eu 2 + .
atoms, whereas Sr in Sr 2Si sN 8 and Ba in Ba2Si sN s are coordinated by eig h t or nine ni trogen atoms. Luminescence characteristics. M 2Si sNs:Eu 2+ (M = Ca, Sr, Ba) phosp hors gi ve ora nge-red or red emission, as shown in Figure 112. A sing le, broad emi ssion ba nd is cen tered at 623, 640, and 650 nm for Ca 2SiSNg, Sr2SiSN g, and Ba2SisN 8, respectively. A red shif t in the emission waveleng th is observed with increasin g the ioni c size of alka line-ear th meta ls. Th e exci ta tion spectrum resembles eac h othe r, in dicat ing the chemical env ironmen t of Eu 2+ in these ma terials is very sim ilar. The exci tation spectrum ex tensively shifts to longer wavelengths, w ith the peak located at 450 nm for all samples . EM
:i ~
z(jj
c
,
.S
/~
OJ
-l
0...
.
-. _.- Ca2SisNs - - - - Sr2SisNs - - Ba2SisNs
550
600
650
700
750
Wavelength (nm)
Figu re 112
800
850
200 250 300 350 400 450 500 550 600 Wavelength (nm)
Emission a nd excita tion of M 2S isN H:Eu 2+ (M = Ca . Sf, Ba).
327
Chapter two: Fundamentals of luminescence
EX
EM
550
600
650
700
750
800
850
200 250 300 350 400 450 500 550 600
Wavelength (nm)
Figure 113
Wavelen gth (nm)
Emission and excita tion of CaA1SiN 3 :Eu 2+ .
Prepara tion. Th e tern ary alkaline-earth silicon nitr ides a re either sy n thesized by firing the powder mi xture of Si3Noj , M 3N 2, and Eu N a t 1600-1 800°C under 0.5 M Pa N 2 or p repared by the react ion s of metalli c alka line -ear ths wi th silicon d iimide at 1550-1 650°C un der nit rogen atmos phere.5.27.2R
2.14.3.6 CaAlS iN 3 : Eu2+ Crystal structure. Ca AJSiN 3 has an orthorhom bic crys tal s tru cture and th e space gro up of Cmcs. , the unit cell parameter being a = 9.8007, b = 5.6497, and c = 5.0627 A.20 Th e Ca atoms are found in the tunnels surrounded by s ix corner-sha ring tetrahedra of (AI, Si)N.j' Luminescence characteristics. CaAlSiN 3 :Eu 2 + is a red phosphor. The luminescen ce spectra are given in Fig ure 113. The excitation spectru m is ex tre me ly broad , rangi ng from 250 to 550 nm . Again a broa d emission band cen tere d at 650 nm is ob se rved w hen exci ted at 450 nm. The chro ma ticity coo rd ina tes of red p hos p hor a re x = 0.66 and y = 0.33. Th is p hos phor has an ex terna l qu antum efficien cy as high as 86% under 450 n m excita tion. The emission spec trum is red shifted with inc reasing Eu 2+concen trations. Preparation. Th e CaA lSiN 3 :Eu 2+ phosphor was syn thes ize d by firing a powder m ixture of Si3N.j, AIN, Ca 3N 2, and EuN at 1600-1800°C for 2 h und er 0.5 MPa N 2.
2.14.4 Applications of oxynitride phosphors As shown in the previous sec tion, oxynitride phosph or s emit efficien tly under UV and visible-light irrad iat ion . This co rrela tes well with the emission waveleng ths of th e UV ch ips or blue light-em itt ing di od e (LED) chips, m aking their use as d own-con version phosphors in w hi te LEOs feasible. We have prop osed that yellow a-SiAION p hosphors co uld be used to generat e warm w hi te light when com bined with a blue LED . The firs t w hi te LED lamp was rep orted b y Sakuma et al. usin g an ora nge- yellow a-SiAION: Eu 2+an d a blue LED chip." It emits w arm w hite light with the color temperature of 2800 K. To ob tain wh ite LED lamps with h igh color rendering index, ad di tiona l phosphors such as green and red phosphors are used . Sakuma et al. have rep ort ed white LEOs with va riou s co lor temper atures and a colo r rend ering index of >80 using ~-SiAION:Eu2+ (green) , Cf.-SiA ION:E u 2+ (ye llow), and
328
Fundamentals of Phosphors
CaA1SiN3:Eu Z+ (red) phosphors.P Mueller-Mach et al. have used (Ca,Sr,Ba)SizOzNz:Euz+ (yellow-gre en) and (Ca,Sr,BahSisNs:Euz+ (orange-red) phosphors to fabricate highly efficient white LEDs.3\
References 1. Schn ick, W., Inter. f. lnorg. Mater., 3, 1267, 2001. 2. Jan sen, S.R , de H ann , J.W., van d e Yen, LJ .M., Hanssen, R , Hintzen , H .T., and Metselaa r, R , Chern. Mater., 9, 1516,1997. 3. van Krevel, J.W H ., Hintzen, H.T., Metselaar, R , and Meijerink, A., f. Alloy Compd, 268, 272, 1998. 4. Jan sen, S.R., Migchel, J.M., H in tzen , H .T., and Metselaar, R., f. Electrochem. Soc., 146, 800,1 999. 5. Hoppe, B .A., Lut z, H., Morys, P., Schn ick, W, and Seilmeier, A., I. Phys. Chem . Solids, 61, 2001, 2000. 6. Uhed a, K , Takizaw a, H., Endo, T , Ya ma ne, H., Shima d a, M., Wanf, C M., and Mitom e, M., I. Lum., 87-89, 867, 2000. 7. van Krevel, J.WH., va n Ru tten, J.W.T , Mandal, H., Hintzen , H .T., and Metselaar, R , I. Solid State Chem., 165, 19,2002. 8. Xie, R.-J., Mitom e. M., Uheda, K , Xu, FF, and Akirnune, Y, I. A m. Ceram. Soc., 85, 1229, 2002. 9. Xie, R.-J., Hirosak i, N ., Mitorno, M., Yam am oto , Y, Sueh iro, T, and Oha shi , N., f. Am. Ceram. Soc., 87, 1368, 2004. 10. Xie, R-J ., Hirosaki, N., Mitorno, M., Yam am oto , Y., Suehiro. T , and Sakuma, K., I. Phys. Chern. 8, 108, 12027, 2004. 11. Xie, R.-J., Hirosaki . N., Sak um a, K., Yamam oto, Y, and Mitomo, M., A pp. Phys. Lell., 84,5404, 2004. 12. Li, YQ., Fan g, C M., d e With, G., and Hint zen , H.T., I. Solid Slate Chem ., 177, 4687, 2004. 13. Xie, R.-J., Hir osak i, N., Mit om e, M., Suehiro, T., Xin. X., an d Tan aka, H., I. Am. Ceram. Soc., 88, 2883, 2005. 14. Xie, R.-J., Hirosa ki, N., Ya ma mo to, Y, Suehiro . T, Mitom e . M., and Sakuma, K., [pn. I. Ceram. Soc., 113, 462, 2005. 15. Xie, R.-J., H irosaki, N., Mit om e, M., Uhe da, K, Suehir o. T , Xin, X., Yamamoto, Y, and Sekigu ch i, T , I. Phys. Chern . B, 109, 9490, 2005. 16. Sue hiro. T , Hi rosaki, N ., Xie, R-J., and Mitom e, M., Chem. Maler., 17, 308, 2005. 17. Hirosaki, N ., Xie, R.-J., Kimo to, K., Sekiguchi, T , Yamam o to, Y, Suehiro, T., and Mitome, M., Ap p. Phys. Lett ., 86, 211905, 2005. 18. Li, YQ., Delsin g, CA., de With, G., and Hintzen, H.T., Chem. Mater. , 17, 3242,2005. 19. Hiro saki , N., Xie, R.-J., Yama mo to, Y., and Suehi ro, T , Presented at the 66/11 A utumn An nual Meeting of the Japan Society of Applied Physics (Abstr act No. 7ak6), Tokusim a, Sep t. 7- 11, 2005. 20. Uh ed a, K , Hirosaki, N., Yamamoto, H., Yamane, H ., Yam am oto, Y, Inarni, w., and Tsuda, K., Presented at the 206t11 Ann ual Meeting of the Electrochemical Society (Abstract No. 2073), Honolulu, Oct. 3-8, 2004. 21. Schnick, Wand Huppert z, H., Chem. Eur. i.. 3, 679, 1997. 22. Grins, J., Shen, Z., Nygren, M., and Esk rtorn, T , f. Mater. Chem., 5, 2001, 1995. 23. Oyama, Y, and Kamigaito, 0 ., [pn . I. Appl. Phys., 10, 1637, 1971. 24. Hoppe, H .A., Stadler, F, Oeckl er, 0 ., an d Sch nick, W , A ngew. Chem. Int. Ed., 43, 5540, 2004. 25. Hampshire, S., Park, H .K., Thomp son , D.P., and Jack, K H., Nature (London) , 274, 31, 1978. 26. Cao, G.Z. and Metselaar, R., Chern . Mater., 3, 242, 1991. 27, Schlieper, T and Sch nick, W , Z. Allorg. A Lig. Chent., 621, 1037, 1995. 28. Schlieper, T and Schnick, W , Z. Allorg. A llg. Chem., 621, 1380, 1995. 29, Sakuma, K., Omichi, K, Kimu ra, N., Ohas hi, M., Tana ka, D., Hirosaki. N., Yama mo to, Y, Xie, R.-J., and Sueh iro, T , Opl. Lett., 29, 2001, 2004. 30. Sakuma, K, Hirosaki, N., Kimu ra, N., Ohas hi, M., Xie, R.-J., Yama mo to, Y, Sue hiro, T , Asa no, K., and Tanaka, D., [EICE Tran s. Electron., Vol.E88-c' 2005 (in press). 31. Mueller-Mach, R , Mu eller, G., Krarnes, M.R, Hoppe, B .A., Stad ler, F, Schnic k. W., [uestel , T, and Schmidt, P., Phys. Stat. Sol. (a) 202, 1727,2005.
Index
Subject A Absorption, 2,18,22,36,52,54,65,86,93,97-98, 153, 218, 223, 258, 314 coefficient, in crystals, 3-4, 8, 19-21, 24, 185, 224, 248 cross-section, 3, 7-9, 91, 197 intensity of, 6, 19, 98, 151, 169, 171, 175 of light, 1,3-6,8,12, 19,40 spectrum, 23, 31, 41, 53, 55-56, 63-64, 68, 169, 227, 247-248, 255 Acceptors, 40-41,43-44,52,90, 92, 124,227,243, 247,285,287,295,300-301,317-318 Adiabatic approximation, 28 After-glow, 73-74, 76-81, 83-86, 84-86 (AI,Ga,In)(P,As) alloys emitting infrared luminescence applied devices, 295, 305 compound semiconductors based on lnP, 291-293 crystal growth, 284-285, 294-295 determination of GalnAsP /InP solid compositions, 293-294 emitting visible luminescence bandgap energy, 283-284 characteristics of InGaAIP crystals grown by MOCVD, 285-288 crystal growth, 284-285 light-emitting devices, 288-290 Anomalous emission, 131, 137-139, 139 Anthracene, 54-57
B Back-scattering factor, 103, 108 Bandgap, 11, 13, 15, 18-20, 18-21, 40, 43, 60, 62-63, 63, 108, 112, 118, 126, 208, 231, 287,289,291,294,309,314 energy, 43, 62, 106-108, 108, 124,240, 245, 278, 283,285-286,292-294,299,301-302 Band theory, 11-18, 11-19, 22 Bethe's formula, 104 Biexcitons, 24, 68 Bloch function, 13, 17, 126
Bloch's theorem, 13 Bohr radius, 40, 45, 64, 66-67, 92, 243, 278 Boltzmann distribution, 6, 31, 118 Born-Bethe treatment, 125-126 Bragg-condition, 14-15 Branching ratio, 75 Breathing mode and configurational coordinate model, 26-30
c Carbostyryls,59 Cathode-ray tubes (CRT), 102, 124, 229-230, 244 phosphors, 89, 229 Cathodoluminescence, excitation mechanism of, 101 Charge-transfer (CT), 168 state (CTS), 75-76, 165, 168, 188, 195, 199 Charge-transfer band, 168 Concentration-quenching processes, 56 Condon approximation, 30, 150 Conduction bands, 11-12, 18-19, 124 Configurational coordinates, 26 model, 26-30, 32, 34, 37, 48, 75, 78, 87, 149-150, 152, 195, 199, 207, 210, 243 Configuration interaction, 162 Cooperative optical phenomena, luminescence, 97-99 Correlation energy, 67 Coulomb attraction, 63, 133 Coulomb force, 24, 43 Coulomb interaction, in resonant energy transfer process, 90 Coulomb potential, 15, 22, 124 Coumarins, 56, 59 CRT, see Cathode-ray tubes Crystal lattice, 11-12 Crystal potential, 14-15 Crystal structures, type of rock-salt, 11-12,217 wurtzeite, 11-12 zinc-blende, 11-12, 18-19, 23, 222, 278, 284, 313
329
Fundamentals of Phosphors
330
o Dead vo ltage, 105 Delta functi on , 63 Density of states, 18, 62-63 , 79, 124, 310 Dexte r mechanism, 56 Dexter 's theo ry of reso nan t energy tran sfer 90 Dieke dia gr am , 130, 134-135, 183 ' Dip ole mom ent, 4, 6-9, 7, 9 Dipole-quadrupole int eraction , 91 Dip ole transition , 8-9, 91, 185, 250 Direct ga p mater ial, 20, 24 Direct tran siti on, 18- 22, 24,41, 62,201, 223-224, 283 type semicond uc tors, 41, 43, 62, 238, 284 Distributed Bragg reflector s (DBR), 269, 279 Donor-acceptor pair (DAP), 43-46, 74, 122, 124, 227,243,268,278-280, 314, 316 Donors, 39-43, 94-95, 124,240-245,285, 300, 314, 316-317, 319
E Effective mass tensor, 17-18 Eigenvalue equation, 14 Einstein's B-coefficient of optical absorp tion 6 Electric dipole ' moment, 4, 6, 8-9 oscillator, electromagnetic radiation from , 5 transfers energy, 4 transition probability, 6-7, 30 transitions, 62, 98 Electro luminescence (EL), 52, 72, 111, 131, " 230-231, 240, 289,302,305,319 morgaruc, see In organic electrolum inescen ce qua ntum efficiency, 72 Electro mag ne tic rad iation from electric dipole osci llator, 5 Elec tronic ener gy bands, 11 Electro nic transitions, in organic molecules, 52-53 Electron orbital, spa tial distribution of, 27 Electro n-phonon int eraction, 31, 34, 37, 243, 252 Emission spec tra, 31, 53-55, 95, 145, 152, 194, 201, 212, 289, 309, 311, 323 Empty states, 12,22, 130 Ene rgy band , qualitati ve int erpretation of, 14 conse rva tion, 19 eigenva lues, 14, 162 Energy levels for electrons and holes, 61-62 of free exciton, 22 Excim ers , 54 Excitation energy transfer, 89-90 concen tration quenching of luminescence, 96-97 diffu sion of excitation, 94-95 reso na n t energy transfer, theory of, 90-9 3 exchange interaction, 92-93 multi polar int eraction, 90-92
ph on on-a ssisted energy tran sfer, 93-9 4 se ns itiza tion of luminescen ce, 95-96 Excitation migration , 94, 96 Excitonic molecul e, energy of, 24, 68 Excitons, 23-24, 41--43, 64-68, 89, 146, 213, 217-218, 224, 231, 241-242, 267, 278, 314,316
F Ferrn ion s. 12 First-order reaction type, chemical reaction kine tics, 84 Flu orescence , 51, 54-59, 73-76, 74, 81 lifetim e, of transition-metal ion, 36 mol ecules containing heteroatoms, 56 qu antum yield , 56-59 Forster mechanism, 55-56 Fourier coefficients, 14 Four ier ser ies, 13 Franck-Condo n coefficient, 36 Franck -Condo n factor, 30 Franck-Condo n prin cipl e, 27, 30 Free excitons, 40--41 , 241-242, 267 Frequ ency factor, 28, 75, 82
G GaAs qu antum wells, 64 GaN and rela ted lumin escence materials, 299-300 Ga lnN ,301-302 Ga ln N / AIGaN LED, 302-303 GaInN multiqu antum-well (MQW) LD, 307-311 GalnN sing le-qua ntu m-we ll (SQW) LEDs, 303-307 n-t yp e GaN, 300, 304, 307 p-type GaN, 300-301, 307 Ga ussian shape, 30-31, 34 Gian t oscillator stre ngt h effect, 41 Glow curve, 80-84 y-rays, ene rgy dissip ati on , 105
H H armon ic oscillatio n, 29 Harmonic osci llator, wave function of 30-3 1 H igh- en ergy electro n, excitation proc~sses by, 89, 106 Hi gh est occu pied m olecul ar orbita l (HO MO), 52-53, 206, 212 Host sensitization, 107 Ho t elec tro n, 119, 121, 124, 127 Hua ng-Rhys -Pekar factor, 31, 243, 252-253
IC p rocesses, see In ternal conversion pro cesses Image force, 114-115
Index: Subject
331
Impact ioni zation, 119- 121, 124-125, 131 Impurity trapped exciton state , 131 Indirect gap materials, 21 Indirect transition, 18-22, 24 type semico nd uctors , 41, 43 Inhomogeneous broadening, 36 Inorganic electroluminescence, 111 high-field EL, 111-11 4 electron ene rgy dis tribu tion in high electric field , 118- 122 excitation mechanism o f lum inescence centers, 122-1 27 injection of carriers, 114-118 injection EL, 111-112, 114 Intern al conve rsio n (IC) p rocesses, 53-54 Inte rsystem cross ing (ISC), 53-54 Inter valen ce cha rge tran sfer (!VCT), 131-132 Isoelectronic traps, 43, 107, 112, 220, 276, 278 Iv'Cl', see Inte rv alence cha rge transfer
J Jahn-Teller effect, 29, 34, 149, 219 IT-coupling scheme, 10 Jorgensen model of optical electronegativ ity, 140
K Killer effect, 257-259 Killer ions, 95 King-Van Vleck factor, 146
L Lagu erre's polyno mial fun ction s, 30 Lambert's law, 3 Lanthani de level locations and its impact on phosphor performance, 129- 130 absolute level locations, systematic variati on in,137-142 4f- 5d energy differences of lanthan ide ions in compounds, 134-136 free (gaseo us) lanthanide ions, 133 future prospects and pretailorin g ph osp hor pro perties, 142 level positio n and phosphor perform an ce, 130-133 me thods to determine abso lu te level locations, 137 Latt ice vector, 12- 13 LED, see Light-em itting diodes Ligand field theory, 158, 169, 171 Light , absorp tion and emission of, 1 in crysta ls absorptio n coefficient, 3 optical constan t and complex dielectric constan t, 2-3 reflectivity, 3-4 transmissivity, 3-4 by impuri ty atoms
classica l harmonic oscilla tor model of op tical cen ters , 4-5 electric d ipole transition probability, 6-7 electro nic tran sition in an atom, 5-6 forbidden tran siti on , 9 impur ity atoms in crys tals, 9 int ensity of light emission and absorption , 7-8 osci llator stre ng th, 8 selectio n ru le, 9-10, 20, 73-74 Light-emitting d iodes (LED), 72, 111-11 2 application for, 280-281 ph osphors, 131 Linear combina tion of ato mic or bital method (LCAO me tho d), 15-1 7 Local ized center, classifica tion of, 25-26 Low-dimension al sys tems, 61- 72 Lowest unoccu pi ed mo lecular orbital (LUMO), 52-53, 206, 212 LS-coupling schem e, 10 Lucky electron mod el, 120-121 Luminescence configurational coordina te mod el and clas sical mod el, 26-28 quantum mechani cs and, 28-30 D-A pair luminescence, 44-46 decay of, 73-76 fluorescen ce, 74-76 qu asistable state and phosphorescence, 76-77 trap s and phosphorescence, 77-80 of d onor-acceptor pairs and semicond uc tors, 43-46 exci tation mechanism of, by catho de-ray and ionizing radiation, 101 collision of primary electrons with solid surfaces, 101-103 energy transfer to luminescen ce centers, 107 ioni zation processes, 105-1 07 luminescence efficien cy, 107- 108 pe ne tra tion of primar y electrons in toa solid, 103-105 of exci tons bound to im purities and se mico nd uctors, 40-43 fun d am entals, electronic sta les an d op tical tra nsi tion of so lid crys tals absorp tion, d irect tran sition , and indirec t transition, 18-22 band theory, 11-19 exciton, 22-24 of isoelec lronic traps and semiconductors, 43 of localized center, 25-26 of low-d imensional systems, 61-72 nonr ad iati ve transitions, 36-37 of orga nic com pound s electronically excited states of organic molecules and their photoluminesce nce, 51-54 fluorescence of organic molecul es in a so lid sta le, 54-56
332 orga nic fluorescent and ph osphorescence comp ounds with high qu antum yields, 56--59 origin, 51- 52 quantum y ield of fluorescen ce, 56 ph otost irnul at ion and pho toq uenc hing, 85-87 polarization of, 33, 127, 248 of semiconductor microcrystal lites, 68 sensitization of, 89, 95-96 , 107 spectral shapes, 30-34 line broadening by tim e-d ep endent perturbation, 34-36 line broadening by time-indep end ent perturbation, 36 thermal quenching of, 26, 37 thermoluminescence and, 80-85 Luminescence centers of com plex ions , 205-206 comp lex ion centers perspective of oth er in teresting cen ters, 214-21 5 W06 &- ion , 214 plat inum complex ion centers, 211-212 oth er platinum comp lex ions, 213- 214 [Pt(CN )4F- com p lex ions, 212-213 Scheelite-typ e com po unds electronic struc tures of close d -shell mol ecul ar com plex cen ters, 206--207 general prop erties, 206 luminescen ce cent ers of MoO/ - ion type, 208-209 luminescen ce centers of V04 3-- ion type, 207-208 luminescenc e centers of WO/- ion type, 209-210 other closed- sh ell tran sition metal complex cen ters, 210 uranyl complex centers electronic struc ture , 210 luminescence s pec tra, 210-211 Lum inescen ce centers of ns--type ion s centers in practical phosphors, 152-1 55 op tica l spec tra of, in alka li halides absorption spec tra , 145-149 emiss ion spectra, 152 str ucture of the A an d C absorp tion ban d s, 149-1 51 temperatu re d ep end ence of the A, B, and C absorp tion bands, 151 Luminescen ce cen ters of rare-earth ions electronic config ura tion, 182-183 electro nic pr ocesses leading to luminescence d ivalent an d tet ravalent cations, 189 ene rgy tra nsfer, 189-190 4f ene rgy levels and relaxa tion, 183-1 88 4fo-l 5d 1 states and cha rge- trans fer states (CTS), 188 specific ion s Cel" 190-191 Dy2+, 200 Dyl" 199-200 Efl ', 201
Fundamentals oj Phosphors Eu2+, 196--197 Eu3" 194-196 Cd 3' , 197-198 H o z',201 Nd 3' , 193 N d 4+, 193 PrJ' , 191-1 93 Sm2+, 193-194 Sm3', 193 Tb3"198-1 99 Tm3+, 201 YbZ" 201 Yb3" 201 Luminescen ce cen ters of tran sition metal ions CrJ' phosphors (3d3 ) , 168- 172 crystal field theory, 157-164 cases of m ore than one d electron, 161-162 3d l electron configuration, 158-161 int ensities of emission and absorption band s, 164-167 spin-orbit int eraction, 164 Tanabe-Sugan o diagrams, 163-164 electron cloud expansio n, effect of charge-tra nsfer band , 168 n ephelau xetic effect, 167-168 Fe3, phosp hors (3d 5), 177- 178 Mn 2+ ph osph ors (3dS ) crysta l field, 173-1 75 different Mn> sites in crys tals, 175-1 76 luminescence decay time, 177 UV absorption, 176--177 Mn4+ phospho rs (3d3 ) , 172-173 Lum inescen ce material, silicon carbide (SiC) as band str ucture an d optical absorption, 314 crys tal grow th an d doping, 319 light- em itting d iodes, 319 lumin escence from d on or-acceptor pairs, 316--318 from excitons, 314-3 16 po lytypes, 313- 314 Lu minesce nce transi tions, atomi c struct ure of various cen ters and, 253-255 Lu minescen t tran siti on, ang ular freque ncy of, 29- 30
M Mesop ic visio n, 51 Metal orga nic vapor pha se ep itaxy (MOVPE), 265-266 Metal-t o-ligand cha rge transfer (MLCT), 59, 214 Molecular bea m epitaxy (MBE), 230, 238, 265-267, 284 Mome ntu m conserva tion , 19 selec tion ru le, 20 Mott transi tion, 24 Mult iplet, 162, 164-1 65 Multi qu antum-w ell (MQW) active laye r, 269 Mul tiqu antum-well (MQW) LD, Ca ln N, 307-311
Index:
333
Subject N
Nanometer-size semicond uc tor microcrys talli tes, 65 Naphthalimid es, 59 Naphtholylene benzimdazoles, 59 Natural lifetime, 7-8 Nephelau xetic effect, 167, 175, 177- 178, 322 Nonrad iative multiphonon transition pro bability, 76 No nr adia tive re laxation probability, 36-37 Nonrad iat ive tran siti on , 28, 54, 56, 74-76 , ] 99, 207,210 luminescence, 36-37 probabi lity by thermal activa tion, 75 ns2-type ions, luminescenc e cen ters of cent ers in practical phosphors, 152-]55 opti cal spec tra of, in alkali halides absorp tion spe ctra, 145-149 emission spec tra, 152 structure of the a and c absorption bands, 149-151 temperatu re dep endence of the A, B, and C abso rp tion bands, 151
o Octahedral coord ination, 158, 160, 172, 175, 178 Ia-VIIb compound s, 217-218 color cen ters, 218-219 intrinsic op tical properties band structure and exciton , 218 self-trappi ng of excitons and intrinsic lum inescence, 218 luminescence centers of ns--typ e ions, 219-2 20 lum inescence of isoe lectro nic traps, 220 Optica l abso rp tion spectrum, 63- 64 transition probability of, 6-7 Op tical centers classical harmonic oscillator model of, 4-5 oscillator strength of, 8 Op tical cons tant, 2-3 Organic compo u nds , luminescence of electron ically excited states of organic molecules and their pho tolumi nesce nce, 51-54 fluorescence of org anic molecules in a solid state, 54-5 6 organ ic fluo rescent and p hosphorescence com pounds with high qu ant um yield s, 56-59 origin, 51-52 qUil ntum yield of flu orescen ce, 56 Organic fluorescent mo lecules, classification, 56-57 Organ ic solids , fluorescence in, 54-56 Organic thin-film electro lumines cent devices, 59 Overlap integr al, 16, 30, 37, 126 Oxynitride phosphors, 321- 322 app lications of, 327- 328
characteristics of ~-SiAlON : E u 2 + , 323-324 LaAl (Si(,-zAl z)N ]o_p z:Ce3 +, 323 M2S isNs:Eu 2+, 325-327 MSi20 2N 2 :Eu 2+, 324 a- SiAlON :Eu 2+, 325 overview of, 322-323
p
Pauling electronegativity, 139-140 n-electron sys tem s, 51 Perrin 's mod el, 93 Pe rylene, 54, 57 Phon on (s), 31, 47, 76, 94, 106, 121 emission, 21 en ergy, 21 longitud inal op tical (LO), 42, 107 number, 32 number, and op tical transition, 32 Ram an scattering of, 36 Phosphor(s) applicati on s of, 129-130, 132, 327- 328 lanthanid e level locations and perf orm an ce of, see Lanthan id e level locati on s an d its impact on ph osphor per form ance localized lu minescen t cen ters, 25-26 luminescence centers of Cr3+, 168-1 72 Fe)' , 177-1 78 Mn 2+, 173- 177 Mn <+, ]72- 173 oxy nitride , see Oxy nitrid e phosphors ZnO pho sp hors, 260-261 ZnS-ty pe p hospho rs luminescen ce of deep d onors and accep tors, 244-258 lu minescen ce of rare -ear th ion s, 260 lum inescen ce of tran siti on me tal ions, 258-260 Ph osphorescence, 51, 56, 59, 73-74, 76-77, 84-85, 214 Pho toe xcitation, 46, 51, 55, 106, 208 Pho ton(s), 5-8, 18-21, 23-24, 36, 56, 108, 129, 131-132 Photostim ulat ion caused by rad iati ve recomb inat ion of the electro ns (ho les), 86 spe ctra, 86-87 Plan k's equation, 6 Plasm ons, 89, 105-106 Po isson d ist ribution, 243 Polarizati on of luminescence, 247- 248,
250-251, 254 Posit ive ho le, 12, 218 Primar y electrons, 101-104, 108 Pseu d opotential, 15-1 7, 238 Pseudopotential method , 15 Pu re semicond uc tors, 40
334
Fundamentals of Phosphors Q
Qu antum Quantum Qu antum Quantum
confinement effect, 66, 68 mechanical resonance, 89 mechanics, 5, 12, 19, 28, 61 numbers, 10, 13,28, 162, 164,218
R Radi ativ e lifetime, 5, 7-9, 64, 68, 91, 126 Radi oluminescen ce, excitation mechanism of, 101, 107 Raman scattering of phon on s, 36 Rare-earth ions, luminescen ce centers of elect ronic configura tion, 182-1 83 electronic pr ocesses leading to luminescen ce d ivalent and tetravalent ca tions, 189 en ergy tran sfer, 189-1 90 4/ energy levels and relaxation, 183-188 4fl-l 5d1 sta tes an d charge-tr ansfer states (CTS),188 specific ion s Ce 3+ , 190-1 91 Dy 2+, 200 Dy3+, 199-200 ErJ+, 201 Eu 2., 196-197 Eu 3., 194-1 95 Cd 3+, 197- 198 Ho] ·, 200-201 Nd 3. , 193 Nd 4+, 193 PrJ+, 191- 193 Sm 2+, 193-1 94 Sm 3. , 193 Tb3+, 198-1 99 Tm 3. , 201 Yb2+, 201 Yb3., 201 Reciprocal wa ve vec tors, 15 Redshifts,56 Rehactiveindex, 2, 91, 126, 185, 269, 309 Rydberg en ergy, 63-6 4
s Schottky barrier, 114, 116-117 Schrodinger equa tion, 12, 29 Secondary yield, 102-103 Second-order reaction type, chemical reaction kinetics, 79-80 Semiconducto r(s) crystals, 43 deep levels and, 46-49 impurities in, 39-40 luminescenc e of donor-acceptor pa irs, 43-46 excitons bound to impurities, 40-43 isoelectronic traps and , 43 Semiconductor microcr ystallit es, luminescen ce of, 68
Sensitizati on , of luminescence, 89, 95-96, 107 Sensitizers, 90, 95, 177 Silicon carbide (SiC) band struc ture and optical absorption, 314 crys tal grow th and doping, 319 light-emitting d iod es, 319 luminescence from d onor-acceptor pair s, 316-318 from exciton s, 314-3 16 polytypes, 313-314 Solid crystals, electronic states and opti cal tran sition of, absorp tion, direct tran sit ion, and indirect tran siti on , 18-22 band theory, 11-19 exciton, 22-24 Spectrochem ical seri es, 163 Spin-or bit int era ction , 10, 18, 149, 152-153, 160, 164-1 66,169,182-183,206,209,240 Star k effect, 42-43, 183 Stickin g potential, 102 Stoke s' law, 26 Stoke s' shift, 26-27, 34, 54, 136, 153, 207, 214, 310 Stokes-shifted charge transfer (CT) luminescence, 132 Structure of A and C absor p tion bands, 149-151
T Tanabe-Sugano di ag ram s, 163-164 Taylor series, 29 Tetracene emi ssion spectrum, 55 Thermal acti vat ion ener gy, 49, 75, 86 Thermalized electrons and holes, 107-108 The rmall y stim ulated luminescence, 80 Thermal qu enching of lum in escence, 26, 27, 207 Thermal rela xation, 75 Thermoluminescence, 52, 80-85, 208 lIIb-Vb compounds, 273-276 CaP as luminescen ce ma terial donor-acceptor pair emiss ion, 278- 280 energy band structure, 276 isoelectronic traps , 276-27 8 light-emitting diodes, ap plication for, 280-281 Thomson-Whiddington's formula , 104 Tl+-like ions, 145 T2 orbitals, 160 Total energy of ground state, and coord ina te model, 26-27 Transition metal ions, luminescence cent ers of Cr 3+ phosphors (3d 3) , 168-172 crystal field theory, 157-1 58 cases of more than on e d electron , 161-1 62 3d l electron configu ra tion, 158-1 61 intensities of emission and abso rpt ion bands, 164-16 7 sp in-orbit interaction, 164 Tanab e-Sugan o diagram s, 163-164 electron cloud expansion, effect of charge-transfer band, 168
Index:
Subject
nephelauxetic effect, 167-168 Fe3 + phosphors (3d5) , 177-178 Mn 2 + phosphors (3d5) crystal field, 173-175 different Mn 2+ sites in crystals, 175-176 luminescence decay time , 177 UV absorption, 176-177 Mn4+ phosphors (3d 3) , 172-173 Transition moment, 9 Transition probability of optical absorption, 6 Trap, 43-44, 47, 73, 77-81, 84, 86, 129, 132, 141-142 lIa-Vlb compounds, 221-222 activators, overview of, 224-228 applications of cathode-ray tubes, 229-230 electroluminescence (EL), 230-231 storage and stimulation, 228-229 fundamental physical properties band structure, 223-224 crystal structures, 222-223 phonon energies and dielectric constants, 224 luminescence, host excitation process of, 231-232 phosphors, preparation methods of selenides, 234 sulfides, 232-234 Ilb-Vlb compounds, 237-238 fundamental intrinsic properties band structure, 238--241 crystal structure, 238 exciton , 241-242 melting point and crystal growth, 238 type of conductivity and its control, 242 luminescence of shallow donors and acceptors, 242-244 ZnO phosphors, 260-261
335 ZnS-type phosphors luminescence of deep donors and acceptors, 244-258 luminescence of rare-earth ions, 260 luminescence of transition metal ions, 258--260
v Valence bands, 11-12, 18-1 9,46,77 VeSEL, see Verti cal-cavity surface-emitting lasers Vertical-cavity surface-emitting lasers (VeSEL) , 269-270
w Wavefunction, 7, 9, 11,14-17, 27-31, 34, 37, 43--44, 46,61,64, 90-92,124 Wavefunction of ha rm onic oscilla tor, 30-31 y YP0 4 single crysta l, optical spectra, 33
z Zero-point vibr ation, 31 ZnSe and related luminescent materials, 265 metal organ ic vapor phase epitaxy (MOVPE), 265-266 molecular beam epitaxy (MBE), 266-267 n-Type doping, 267-268 p-Type doping, 268 ZnSe-based blue -green laser diodes, 268-270 ZnSe-based light-emitting diodes, 270-271
43676 ISBN 1-4200-4367 -6 90000
9 781420 043679
