Nanocrystalline Silicon: Electron Spin Resonance

Encyclopedia of Nanoscience and Nanotechnology

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Nanocrystalline Silicon: Electron Spin Resonance Takashi Ehara Ishinomaki Senshu University, Minamisakai, Ishinomaki, Miyagi, Japan

CONTENTS 1. Introduction 2. Basics of Electron Spin Resonance 3. n-Type Nanocrystalline Silicon 4. p-Type Nanocrystalline Silicon 5. Light-Induced Electron Spin Resonance 6. Dangling Bond Defects 7. Other Detection Method of Spin Resonance 8. Summary Glossary References

1. INTRODUCTION Since the report of deposition using chemical transport method by Veprek and Marecek [1], hydrogenated nanocrystalline silicon (nc-Si:H), usually called hydrogenated microcrystalline silicon (
c-Si:H), has gathered much interest. The primary preparation method of nc-Si:H is plasmaenhanced chemical vapor deposition (PECVD) using highly diluted SiH4 in H2 [2–5]. In this method, dangling bond (DB) defects are decreased due to termination by hydrogen atoms. In addition, impurity doping can be done using phosphine gas or diborane gas as phosphorus or boron source, as well as in the case of hydrogenated amorphous silicon (a-Si:H) [6]. Moreover, doping efficiency of nc-Si:H is higher than that in a-Si:H. Dark conductivity of 10−1 cm−1 has been achieved by doping [7–9]. Due to the properties shown here, nc-Si:H has been a promising thin-film material for some optical devices such as solar cells [10–15] or thin solid transistors. As highly diluted SiH4 gas has been used as a silicon source, the deposition rate of nc-Si:H was extremely low; however, high-rate deposition of nc-Si:H films at low temperature has been reported [16]. This kind of work is thought to enhance the use of nc-Si:H thin films for devices. Recently, as another kind of low-temperature deposition

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method, hot-wire chemical vapor deposition (HWCVD) has been extensively studied. In the method, a thermal and catalytic reaction between deposition source gas and tungsten hot-wire catalyzer that causes the film deposition instead of plasma decomposition of SiH4 is used for deposition of films [17–19]. Although the growth mechanism of nc-Si:H by the CVD method is still under discussion [4, 20–22], the structure of nc-Si:H has been studied and reported. The nc-Si:H is a heterogeneous material consisting of small crystallites with size on the order of 5 to 30 nm which are embedded in columns structure that is parallel to the film growth axis with a diameter of 50 to 200 nm [23, 24]. In such mixed phase structure, electron spin resonance (ESR) spectroscopy is an important method to study a particular electronic state, because the techniques are sensitive to the microscopic environment of this state. In this chapter, ESR studies of nc-Si:H are reviewed. After the section of basics of ESR, the signal of conduction electrons, conduction holes, light-induced ESR, and dangling bond defects signals will be reviewed.

2. BASICS OF ELECTRON SPIN RESONANCE ESR spectroscopy is a very important method to study the characteristics of paramagnetic species. In the case of nc-Si:H, ESR has been used for the study of silicon DB defects and carriers, mainly conduction electron (CE) in n-type materials. In the study of nanocrystalline or related material such as a-Si:H, ESR spectroscopy has been used in a great deal of work to estimate spin densities by intensity of corresponding signals. However, the ESR spectroscopy can also be used for the research of paramagnetic characteristics of the paramagnetic species in the material. The important information of ESR spectroscopy to clarify the characteristics of paramagnetic species in nc-Si:H are g-value, spinlattice relaxation time, and line width. In this section, basic theories of these important data are explained.

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 6: Pages (495–503)

496

Nanocrystalline Silicon: Electron Spin Resonance

The g-value is the magnitude of the electron Zeeman factor for the paramagnetic species considered. The g-value can be determined by the equation

=

E = g
B B In it, E is energy of microwave,
B is Bohr magnetron, and B is magnetic field. g-value becomes 2.0023 in the case of free electron. In the case of actual paramagnetic species, the value becomes different because of the effect of local magnetic field. In the case of DB defects in a-Si:H or nc-Si:H, the effect of spin-orbit coupling is important. The spin-orbit coupling is caused by local magnetic field due to movement of electrons in its orbit. Thus, the g-value determined by spin-orbit coupling depends on the structure of the orbit. The effect of spin-orbit coupling is anisotropic; thus, it also depends on axis determined by magnetic field. Here, we assume the spin-orbit coupling of DB with a state in silicon. If we take a DB orbital along the z-axis, the anisotropic g-values are given by g
= gz = ge


g⊥ = g x = g y = g e

 DB Li n n Lj DB  ij − En −E DB n=xyz

lorentzian line shape. The actual homogeneous linewidth  is determined by the equation



where ge  , and E DB are the g-value of free electron, the wave function of DB, and the energy of the DB, respectively. n and En stand for the wavefunction and the energy of a coupling state.  is a coupling constant of the spin-orbit interaction, which is 149 cm−1 in the case of silicon. The g-value observed in the spectra is  g = gx2 + gy2 + gz2 Spin-lattice relaxation time is also one of the important characteristics for paramagnetic species. When ESR happens, the spin system absorbs the microwave with appropriate energy. Hence, the spin system can be thought as with high energy compared with its surroundings. The actual spin system undergoes interactions with the surroundings until its energy eventually is restored to the same as that of the surroundings through contact with it. As with any sufficiently simple thermodynamic system that receives as extra energy U0 at t = t0 , it loses this excess energy to its surroundings with an exponential decay, described as U = U0 exp−t − t0 /T1

where U0 is the excess energy at time t = t0 , and T1 is the characteristics time called relaxation time that corresponds to the energy flow rate magnitude from the spin system to the surroundings. Relaxation time T1 reflects the degree to which the spin system is connected to its surroundings. Finally, we mention the line width of ESR signals. The spectral lines are classified into those that are homogeneously broadened and those that are inhomogeneously broadened. Homogeneous line broadening for a set of spins occurs when all the spins are in the same magnetic condition and have the same spin Hamiltonian parameters. This means all the spins have the same line shape of the signal. The line shapes of homogeneous broadening have a

1 + e2 B12 T1 T2 1/2 1 + 2T1 T2

In the equation, T1 and T2 are the relaxation time of spins, B1 is oscillating magnetic field, and e corresponds to e =

g e
B


The line width of homogeneously broadened lines depends on the relaxation time of the spins. The inhomogeneous broadening mechanism distributes the resonance magnetic field frequencies without line broadening due to individual equivalent spins. The observed signal is a superposition of a large number of individual spin packets, all of which have slightly different g-values from each other. Even if the outer magnetic field is homogeneous, the inhomogeneous broadening is caused by various factors. With an anisotropic interaction in a randomly oriented system in a solid, the distribution of local magnetic field resulting from the anisotropic g-tensor gives rise to the inhomogeneous broadening. This corresponds to asymmetrical line shape in many cases. The DBs in the silicon are included in this case. Another factor that induces the inhomogeneous line shape is the unresolved hyperfine structure. This occurs when the number of hyperfine components located near nuclei is so great that no clear hyperfine structure is observed. In the hydrogen in the nc-Si:H, the line broadening due to the hyperfine structure by the hydrogen atoms may cause this broadening.

3. n-TYPE NANOCRYSTALLINE SILICON In this section, ESR studies of doped n-type nc-Si:H are described. In the beginning of ESR studies of nc-Si:H, most of the studies published were devoted to n-type materials. In the n-type materials, the spectra are dominated by CE signal at g = 1.997–1.999 rather than silicon DB defects signals. The studies of the CE signal are described in this section. Before the first report of ESR study of n-type nc-Si:H, many works of related material such as n-type a-Si:H, crystalline silicon, and polycrystalline-silicon (poly-Si) have been reported. The first report of CE ESR signals in crystalline silicon was done by Portis and co-workers [25]. They described a lorentzian line of CE that has dependence of line width and reciprocal paramagnetic susceptibility on temperature. The ESR study of CE in crystalline silicon has been followed by a great deal of work [26–30]. For example, Feher reported a g-value of the CE of 1.99865–1.99885. Kodera reported the CE signals at averaged g-value of 1.99869 and hyperfine splitting of 41.7 G due to doped phosphorus atoms. In the case of doped poly-Si, ESR study of CE has been reported by Hasegawa et al. in 1979 [32]. In their work, the DB signal observed for phosphorus-doped samples disappeared by annealing at above 700–750  C, where the sample crystallized, and a new signal that has similar g-value as CE in crystalline silicon has been observed at g = 1.997–1.999. ESR spectra of phosphorus-doped a-Si:H have also been reported by various workers [33–36]. For example,

Nanocrystalline Silicon: Electron Spin Resonance

Dersch observed a new signal at g = 2.0043 beside the DB signal at g = 2.0055 in the case of highly phosphorus-doped sample. The result is different from that in crystalline silicon or poly-Si. The signal broadens with increasing phosphorus doping due to hyperfine interaction with the phosphorus nuclei. A new signal with g = 2 0043 is suggested to be due to electrons in localized conduction band tail states. As an early ESR work of nc-Si:H, Hasegawa et al. have also reported the ESR spectra of undoped and phosphorusdoped nc-Si:H [37]. They observed the signals due to DB with g-value of 2.0049 when the doping level, gas flow rate ratio of SiH4 and PH3 , is below 10−4 . In Figure 1, ESR spectra of phosphorus-doped nc-Si:H with various doping levels are shown. The CE signal at g = 1 9970 has been observed in the sample at the doping level of 10−3 . Other signals were observed at g = 2 0043 in more highly doped samples than a doping level of 10−2 with the decrease of crystalline fraction volume ratio observed by Raman spectra. The g-value of the signal is similar to that in localized electron near the tail of the conduction band in the a-Si:H. The DB signals observed in undoped or slightly doped samples have not been observed in phosphorus-doped sample with doping level more than 10−3 . In the work, Hasegawa et al. [37] described the dependence of the ESR signals with g = 2 0049, 2.0043, and 1.997 on the doping level and rf power for films preparation. The result indicates that spin density of the signal with g = 1.997 due to CE does not increase appreciably with increasing doping ratio above 10−3 . The result is different from that in phosphorus-doped poly-Si in which CE signal increases with doping level.

Figure 1. ESR spectra measured at 77 K for nc-Si:H films deposited at various doping levels. Reprinted with permission from [37], S. Hasegawa et al., Phil. Mag. B 48, 431 (1983). © 1983, Taylor & Francis Ltd.

497 Finger and co-workers have reported the temperature dependence of nc-Si:H ESR spectra [38]. In Figure 2, ESR spectra of undoped and phosphorus-doped nc-Si:H at various temperatures from 10 K to 300 K are shown. Both samples show a resonance of DB at g = 2.0049–2.0055 with spin density of 1–2 × 1017 . As the temperature is decreased, resonance of CE appears at g = 1.9981–1.9985 with characteristic asymmetric Dysonian line shape [27]. The signal due to CE becomes stronger with respect to the DB signal with a decrease of measurement temperature. The signal intensity of the CE follows the Curie law, that the signal intensity is proportional to T−1 , down to 10 K at below 200 K. The CE peak is not detectable above 200 K because of a strong line broadening. Finger et al. reported details of their ESR work of nc-Si:H in 1998 [39]. They have reported measurement of ESR spectra of the samples prepared with different n- and p-type doping at 40 K. Over a wide range of conductivities, they observed the DB and CE simultaneously in n-type nc-Si:H. The intensity of the CE signal, calculated from a gaussian line shape, shows a clear correlation with conductivity of the samples. This correlation supports the attribution of the signal at 1.998 as a CE. Müller and co-workers also have reported the ESR of doped nc-Si:H [40]. In highly doped n-type sample (doping level of 17 ppm) at 20 K with high microwave power of 25 mW where all signal beside CE are saturated, the signal of hyperfine interaction of 31 P has been observed as a shoulder of CE signals. The hyperfine splitting is about 110 G and the intensity ratio of hyperfine signal with respect to total CE signal is about 10%. In the report, they studied the dependency of g-value and line width of the CE signals on doping level. At higher doping level, the CE signal intensity significantly increases, whereas the DB signals are almost independent of doping condition. Therefore, the resonance of the DB signals becomes less visible with increase of doping level. They also have investigated dependence of g-value on conductivity. For the doped samples, the g-value of the CE signals is around 1.9980 at doping levels below 33 ppm and decreases continuously for

Figure 2. ESR signals of nc-Si:H at different temperatures. The spectra are normalized to similar peak-to-peak height. The g-value of the free electron (g = 1 9983) and the dangling bond (g = 2 0052) are indicated. Reprinted with permission from [38], F. Finger et al., Phil. Mag. B 70, 247 (1994). © 1994, Taylor & Francis Ltd.

498 higher doping level to 1.9972 at 133 ppm. The undoped samples have g-value of 1.9980–1.9985 with a large scatter due to the overlap with DB signals. The g-value also shows dependence on the measurement temperature. The g-value decreases with rising temperature between 4.5 and 300 K. The g-values observed in nc-Si:H are generally lower than what is reported for crystalline silicon (g = 1.9985–1.9995) [25–30] or poly-Si (g = 1.997–1.999) [32]. The peak-topeak line width also shows dependence on the measurement temperature. Above 30 K, all samples with various doping levels from 1 to 133 ppm show a large increase in the CE line width with rising temperature. All samples exhibit line width of 9 G at 30 K and increased to 27–34 G at 300 K. They attributed the strong increase in line broadening with rising temperature for T > 60 K to the corresponding decrease in spin-lattice relaxation time, because of their previous report of a steep decrease of T1 in nc-Si:H [41]. In their measurement, the minimum line width has not been observed at the lowest temperature but at 30 K. The line width increased again at the temperature lower than 30 K as well as in slightly doped crystalline Si [42–45]. In the case of crystalline silicon, Maekawa and Kinoshita explained the increase of line width by decrease of temperature by a change in the correlation frequency of the exchange interaction due to reduction of electron hopping rate between neighboring sites at lower temperature [42]. Maruyama and co-workers explained this phenomenon by condensation of the donor electrons into singlet ground states of donor cluster by coupling each other by antiferromagnetic exchange interaction [43]. Malten and Finger and co-workers have reported pulsedESR study of nc-Si:H [39, 41]. In the pulsed-ESR spectrum of n-type nc-Si:H, CE resonance dominates and DB signal is only visible as a shoulder. In addition, two hyperfine lines are observed as a shoulder with a hyperfine splitting of about 110 G. [They assigned as hyperfine] due to 31 P nuclei, by comparison with phosphorus-doped a-Si:H and crystalline silicon samples. They also estimated spin-lattice relaxation time, T1 , by inversion recovery curves. The shape of recovery curve observed in nc-Si:H sample measurements is not simple single exponential form, because of overlapping with other resonance curves or distribution of relaxation time due to the structural disorder of the samples. They fit the recovery curve with stretched exponentials, which was reported by Durny and co-workers [46] and actually applied in the case of pulsed-ESR study of a-Si:H [47]. The fitting of the curve is in the form    

t It = I0 exp − −1 T1 In this equation, the dispersion parameter  represents a distribution of relaxation time T1 . Obtained T1 , relaxation time of CE, is faster than that in DB by two or three orders at the temperature lower than 100 K. Lips, Fuhs, and co-workers reported ESR study of band tail states and free electron in phosphorus-doped nc-Si:H in a study of metal-insulator transaction (MIT) [48–50]. In the plot susceptibility of the CE line as a function of doping level, the susceptibility increases linearly with doping level below 3 × 1018 cm−3 . In contrast, susceptibility increases sublinearly above doping level of 4 × 1018 cm−3 . At the MIT,

Nanocrystalline Silicon: Electron Spin Resonance

the paramagnetic susceptibility changes from Curie-type to Pauli-type [51]. This MIT occurs at a similar doping concentration as crystalline silicon, indicating that the donors are distributed homogeneously in the crystalline phase of the films. In degenerately doped films, they detect a Curie contribution to CE signal at spin density of 4–8 × 1018 cm−3 . They assign this signal to tail states originating from disorder at the grain boundary. In the case of a-Si:H, nitrogen-doping effect has been studied by Morimoto, Zhou, and co-workers [52, 53]. They found similarity between the nitrogen doping and that of phosphorus doping found in conductivity, and the photoconductivity decay indicates that nitrogen can act as donor in a-Si:H. Although the nitrogen doping is also of substitutional type, the solid phase doping efficiency of nitrogen is estimated to be three orders of magnitude lower than that of phosphorus. In the case of nc-Si:H, nitrogen-doping effect has been studied by Ehara and co-workers [54–56]. In the nitrogen-doped nc-Si:H, conductivity was not increased, but rather decreased at higher doping level of 10−1 because of structural change to amorphous. We also have studied ESR of nitrogen-doped nc-Si:H [55, 57]. Figure 3 shows the temperature dependence of the ESR spectra of undoped and nitrogen-doped nc-Si:H. As shown in Figure 3, both silicon DB and CE peaks have been observed at g = 2.005–2.006 and 1.9975–1.9985, respectively. In the undoped sample, signal intensity of the CE showed a maximum at 40 K and decreased at lower temperature. This may be interpreted as the interruption of the thermal excitation of electron. In the doped sample, the CE signal intensity exhibited a maximum at 20 K. The difference in the temperature dependence suggests that the doped samples have shallower donor levels. The result indicates that the nitrogen atoms work as an electron donor in the nc-Si:H, although the efficiency is far less than that of phosphorus as well as in the case of nitrogendoped amorphous silicon.

Figure 3. Temperature dependence of the ESR spectra of undoped and nitrogen-doped nc-Si:H. Reprinted with permission from [57], T. Ehara et al., Jpn. J. Appl. Phys. 39, 31 (2000). © 2000, Institute of Pure and Applied Physics.

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Nanocrystalline Silicon: Electron Spin Resonance

The ESR signals of CE in another formed nanocrystalline silicon have also been reported. Young and co-workers have reported the ESR study of porous silicon (PS) at liquid helium temperature [58, 59]. They observed a new isotropic resonance center at g = 1.9995 in both p-type and n-type PS. The paramagnetic center observed was identified due to the conduction band electrons in silicon nanocrystals by comparing its g-value with those of shallow donors in bulk silicon. Bardeleben and co-workers also reported n+ -, p+ -, and p-type substrate supported and freestanding PS [60]. Signal of CE has been observed in n+ -type substrate supported PS as well as previous works at 40 K. However, the CE signal has not been observed in freestanding PS in spite of their careful search in the temperature range 4–300 K. In their possible explanation, they have considered that the strong hydrogen contamination range might have led to the passivation of the phosphorus donor.

4. p-TYPE NANOCRYSTALLINE SILICON ESR studies of p-type, boron-doped a-Si:H, as well as phosphorus-doped a-Si:H, have been reported by various workers [33–36]. Boron-doped a-Si:H shows a broad line at g = 2.01 with line width of 20 G, beside the DB line. The signal attribution and line width have been assigned as holes in localized valence band tail states, and due to g-distribution, respectively. In contrast, ESR spectra of p-type, borondoped nc-Si:H have not been well studied. Possible signals to be observed beside the DB are due to holes; however, it has been difficult in the p-type samples using continuous wave (CW) ESR measurement. Finger et al. reported absence of CE signal by CW-ESR spectrum of a boron-doped sample with doping level of 23 ppm [39]. This result is consistent with the doping effect. However, any peaks corresponding to holes have not been observed clearly. The signals of holes induced by boron doping have been observed by pulsed ESR spectroscopy using an electron spin echo field sweep (ESE-FS) method [41, 61]. This measurement mode is preferable for measurement of broad signals, because it suppresses base line contributions. At lower temperature than 40 K, the spectra of p-type sample show a very broad line at around g = 2.1 with peak-to-peak width of 500 G in addition to DB signal. In the case of a-Si:H, a possible origin of the peak, resonance of holes trapped in valence band tail states, shows different parameters (g = 2.01 and width of 20 G). In crystalline silicon, the resonance of holes is only observed under external strain [62, 63] or extremely pure materials [64]. However, the observed signals under those conditions have some similarity with the broad peak of their work. Therefore, they tentatively attribute this broad resonance to hole states in the crystalline area of the material. Another group also observed signal of holes by pulsed ESR spectroscopy in p-type nc-Si:H sample [65]. Kanschat et al. observed a broad structure of conduction holes (CH) at g = 2.08 which they identify with holes trapped in valence band tail states. It is shown that the CH state behaves very similarly on illumination as the CE center.

5. LIGHT-INDUCED ELECTRON SPIN RESONANCE The light-induced ESR (LESR) is an effective method to study the transport mechanism or band structure of the material. In the case of a-Si:H, LESR signals of g = 2.004 and g = 2.01 are observed at low temperature beside the DB signal at g = 2.0055 observed in dark. These signals have been tentatively attributed to conduction band tail electron (g = 2.004) and valence band tail holes (g = 2.01) based on the results of doping and photoluminescence experiments [33, 66–69]. The LESR spectrum of nc-Si:H was first reported in 1994 [38]. The light illumination by a 100-W halogen lamp enhances the free electron resonance at g = 1.9983 in ESR of undoped nc-Si:H at 40 K. The signal of CE increased by a factor of about 3. Finger and co-workers reported details of LESR of nc-Si:H in [39]. In Figure 4, the LESR spectrum of an n-type nc-Si:H is shown with its dark ESR spectrum. In the figure, dark and LESR of a-Si:H are also shown for comparison. As shown in the figure, increase of both DB signal and CE signal is observed. The LESR-CE line is assigned to photoexcited CEs and increased DB signal can be due to newly appeared neutral DB changed from negatively charged DB by photoexcitation. The result observed in LESR of nc-Si:H is significantly different from the LESR of a-Si:H as shown in the figure. Finger and co-workers also have studied the various kinds of LESR transient of nc-Si:H using white light and infrared light source. The LESR of nc-Si:H shows different transient characteristics from that in a-Si:H. After switching off of white light, decay of LESR signal has not been observed by infrared light illumination. The result is different from that in the case of a-Si:H. They have studied various transient characteristics of LESR intensity using sub-bandgap illumination. Lima and co-workers have also reported decay of LESR after illumination is discontinued [70]. In their case, the spin signal does not go to zero even after a few thousand seconds after the switch off of lamp, remaining around 20% of the original intensity. Although the time scales of the samples they measured are different from each other, the decay curve can be fitted using a model by Yan et al. [71] for

Figure 4. LESR together with dark ESR spectra of an n-type nc-Si:H sample, in comparison with a high-quality a-Si:H sample. Reprinted with permission from [39], F. Finger et al., Phil. Mag. B. 77, 805 (1998). © 1998, Taylor & Francis Ltd.

500 distant-pair recombination via tunneling between localized states. Kondo and co-workers have reported light-induced phenomena in nc-Si:H [72]. They have measured excitation photon energy dependence of LESR response in nc-Si:H using lamp with monochrometer at 5 K. The intensity of LESR decreases at 1.1 eV with decrease of the excitation energy, and this energy coincides with the crystalline silicon bandgap energy. They also reported details of temperature dependence of LESR signal intensity and concluded that the LESR signal does not arise from a mobile electron but from a localized electron trapped at a shallow level center close to the conduction band edge. They assigned the measured activation energy of 11 meV to the binding energy of the center. These shallow states that provide electrons probably originated from the conduction band tail caused by the structural disorder. They also have studied increase in the neutral DB density due to light exposure and suggested the defect formation correlates with the surface oxidation and surface band-bending model due to the oxidation. The assignment of the peak at g = 1.998 to shallow localized states close to the conduction band is also reported by Kanschat and co-workers [73]. They studied ESR, photoconductivity, and electrically detected magnetic resonance (EDMR) [74]. At low temperature, they have plotted increased density of CE centers by illumination and photoconductivity as a function of generation rate. If the CE resonance were due to free electrons, a linear relationship between increased CE spin density and photoconductivity would be expected. However, they observed almost the same dependence as that in a-Si:H [75]. Therefore, they assigned the CE states monitored at T = 30 K to shallow localized states below the conduction band edge.

6. DANGLING BOND DEFECTS Before the review of the ESR study of DB in nc-Si:H, ESR studies of DB in nc-Si:H related materials are reviewed for comparison. DB defects in crystalline silicon surface or Si/SiO2 interface, Pb center, which gives g-value of g
= 2.001–2.002, g⊥ = 2.008–2.009, have been reported by some workers [76, 77]. ESR studies of DB in a-Si:H have also been reported. The asymmetric ESR signal of DB in a-Si:H is observed at g = 2.0055 with peak-to-peak width of 5–7 G [66, 78]. Calculation of the g-value of aSi:H DB using an interactive extended Hückel theory has been reported and the g-value of 2.0055 was explained by threefold-coordinated silicon atoms [79, 80]. Complicated ESR spectra of dislocations in crystalline silicon have been reported by a few groups [81, 82]. Hasegawa and co-workers have reported ESR spectra of poly-Si prepared by annealing of amorphous silicon films by low-pressure chemical vapor deposition (LPCVD) [83]. In the case of poly-Si, g-value of DB in the poly-Si depends on the annealing condition. The g-value observed depends on annealing time rather than temperature. The sample annealed for 24 h shows g-value of 2.0048 that is lower than the sample annealed for 2 h that has g-value of 2.0055–2.0057. The same group also reported ESR spectra of poly-Si prepared by PECVD [84]. The g-value and the stress strongly depended on the rf power and the thickness. In addition, the g-value of the DB

Nanocrystalline Silicon: Electron Spin Resonance

Figure 5. Dark CW ESR spectrum of nc-Si:H at 40 K. Included are numerical deconvolutions. Reprinted with permission from [40], J. Müller et al., Phys. Rev. B 60, 11666 (1999). © 1999, American Physical Society.

decreased from 2.0054 to 2.0042 with H2 plasma annealing time [85]. These results are examined in terms of formation of different types of grain boundary that can be associated with a lattice deformation around the DB. In the 1980s, a few groups reported ESR studies of nc-Si:H. Veprek and co-workers reported the ESR spectra of nc-Si:H sample prepared by the chemical transport method [86]. The spectra showed the peak of DB defect at g = 2.0055–2.0059 with the peak-to-peak width of 9 to 11 G. They analyzed that most of the samples have a line shape that could be approximated by a gaussian. However, line shape was not a simple gaussian or lorentzian function and appeared to be possibly a superposition of plural signals in some cases. The observed g-value of silicon DB is similar to that in a-Si:H (g = 2.0055). However, the peak width was larger than that in a-Si:H (5–9 G). They determined the defects density in the films as between 5 × 1017 and 2 × 1018 by the spectra. However, they did not discuss the detail of the DB defect using the spectra. As far as the author knows, it is the first report of ESR study of DB in nc-Si:H. Hasegawa and co-workers reported ESR of undoped nc-Si:H with a preferential orientation [87]. They prepared the undoped nc-Si:H sample at various substrate temperatures and rf powers. They reported that the increase in substrate temperature and decrease in rf power induce the change of orientation of nanocrystalline silicon from (111) to (220). g-value of ESR increased from 2.0048 to 2.0056 with increase in substrate temperature, and decreased from 2.0056 to 2.0049 by increase in rf power. This means change in orientation from (111) to (220) causes the increase in g-value. They explain the shift of g-value by change in morphology around the spin. Finger et al. reported the temperature dependence of undoped and doped nc-Si:H [38, 39]. They observed asymmetric DB signal at g = 2.0049–2.0055 with spin density of 1017 . They observed saturation effect in the DB signal at low temperature as a result of the increase in spin-lattice relaxation time. Later, they determined the spin-lattice relaxation time as 10−4 –10−2 s at the temperature of 20–100 K by measurement of pulsed ESR spectra. In the nitrogendoped nc-Si:H, similar asymmetric signal of DB has been observed [55, 57]. Although the change in signal shape has been observed, it has been caused by structural change from nanocrystalline to amorphous, not by doped nitrogen.

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Nanocrystalline Silicon: Electron Spin Resonance

In most of the ESR studies of nc-Si:H, the DB signals have been observed as an asymmetric line at around g = 2.005. Müller and co-workers reported the numerical fits for two-defect resonance of the asymmetric DB signal [40, 61]. The first center is supposed to be a silicon DB at g = 2.0052. The second center is signal at 2.0043. The origin of the second center was initially attributed to electron trapped in the conduction band tail, since g-value of this center is similar to that in n-type a-Si:H [33–36] and LESR of undoped a-Si:H [66–69]. However, this assignment is considered to be ambiguous, because the center is also observed in p-type nc-Si:H. Therefore, another explanation of the second center at g = 2.0043 is needed. Müller suggested the attribution of the second center as the DB states in Si-rich Si-O layers as an attribution of the peak [88], because an increase of the signal at g = 2.0044 in nc-Si:H samples was observed after exposure to air for several weeks [89]. Pulsed-ESR study of nc-Si:H, reported by Kanschat and co-workers, supported the existence of two centers [65]. They identified two DBlike structures at g = 2.0052 and g = 2.0043. Whereas, the g = 2.0052 signal is evenly distributed in the gap, the signal at g = 2.0043 states is found to be localized in the lower part of the gap. Kondo and co-workers studied nc-Si:H DB to investigate its microscopic structure [90]. They have suggested DB at grain boundary with g-tensor of g
= 2 0022 and g⊥ = 2 0078 similar to the Pb center. They have simulated the DB signal in Q-band ESR spectra well by assuming randomly oriented grain boundary defects and DB in amorphous fraction. Lima and co-workers reported the simulation of asymmetric line shape of DB assuming a powder pattern of single center with g
= 2 0096 and g⊥ = 2 0031, and a gaussian broadening of 3.2 G. As another approach of ESR study of DB in nc-Si:H, our group studied X- and Q-band ESR spectra of various kinds of nanocrystalline silicon [91]. We have prepared unhydrogenated nanocrystalline silicon (nc-Si) [92, 93] and nanocrystalline silicon embedded in SiO2 (nc-Si in SiO2 [94–96] and compared their ESR spectra with that of nc-Si:H. ESR spectra of nc-Si at room temperature shows asymmetric DB signal at g = 2.0055 with peak-to-peak width of 10–11 G. Although the peak width is larger, that g-value is similar to that in unhydrogenated amorphous silicon (a-Si) prepared by low-pressure Ar sputtering. However, in the Q-band ESR measurement, spectra of nc-Si show larger g-value (g = 2.006) and significantly asymmetric signal compared with that in a-Si. In the case of the nc-Si in SiO2 , X-band ESR shows signal of DB at g = 2.005–2.006 with peak-to-peak width of 13 G. In Q-band ESR spectra, the signal of silicon DB has been observed at g = 2.006. In Figure 6, Q-band ESR spectra of silicon DB signal in nc-Si:H, nc-Si, nc-Si in SiO2 , and a-Si are shown. As shown in the figure, spectra of nc-Si:H and nc-Si have a similar shape beside the intensity of sharp signal of carbon contamination observed at g = 2.0026. We have simulated the broad DB signal in the Q-band ESR spectrum of nc-Si in SiO2 , using anisotropic distribution of g-tensor that g⊥ has larger distribution than g
, with g-value of g
= 2 0022 and g⊥ = 2 0078. We suggested the DB structure corresponding to this simulation as distribution in C3V symmetry. Another kind of nanocrystalline silicon defect, DB in PS, has also been studied. Bhat and co-workers reported the

Q-band

g = 2.006 g = 2.0026 µc-Si

µc-Si:H

µc-Si in SiO 2

a-Si 30 G g = 2.0055 2.020 2.015 2.010 2.005 2.000 1.995 1.990 g value Figure 6. Q-band ESR spectra of unhydrogenated nanocrystalline silicon (nc-Si), hydrogenated nanocrystalline silicon (nc-Si:H), and nanocrystalline silicon embedded in SiO2 (nc-Si in SiO2 and a-Si).

ESR spectra of PS for various angles [97]. They observed the narrow signal of DB at 2.0052. Pokes and co-workers determined the DB density in the PS layer as roughly 3 × 1016 cm−3 by measurement of ESR [98]. They observed anisotropic ESR spectra in the PS prepared using 0.1- cm Si wafers. Rong and co-workers reported detail of g-value dependence on measurement axis and hyperfine by 29 Si and attributed the defect in the PS as Pb -like centers [99]. Some groups reported the annealing effect of ESR spectra that induces the broadening of spectra by the increase in the strain of the back bonds. Meyer and co-workers reported the effect of rapid oxidation of PS to ESR spectra [100]. They observed two different types of defects after oxidation. One is isotropic and very similar to the DB in a-Si:H; the other is Pb center-like defects. Two signals show difference in the rotation pattern in the ESR signals. As a similar oxidation, heat treatment effect in air to ESR spectra has also been reported by Laiho et al. [101]. Plasma nitridation effect to ESR spectra of PS has also been reported by Yokomichi and co-workers [102]. They observed increase of Pb center density and decrease of PL intensity by plasma treatment.

7. OTHER DETECTION METHOD OF SPIN RESONANCE Optically detected magnetic resonance (ODMR) study of nc-Si:H has been reported by Boulitrop and co-workers [103]. In their work, they have monitored the defect-related luminescence bands at 0.75 and 0.85 eV at 5 K. They detected quenching resonance at g = 2.0043 and enhancing resonance at g = 1.9997 and g = 2.016. The signals at g = 2.0043 and g = 1.9997 are similar to the DB signal and CE

502 signal observed by conventional ESR spectroscopy. The signal at g = 2.016 has a very large line width of about 100 G. As mentioned in Section 5, EDMR studies of nc-Si:H have been reported [49, 89]. Lips and co-workers have reported the bandtail states in phosphorus-doped nc-Si:H using ESR and EDMR [104]. They have shown that the CE states originate from both delocalized electrons in the impurity and conduction band as well as from trapped electron in bandtail states at grain boundary. Bronner and co-workers reported EDMR study of nc-Si:H before and after the electron irradiation [105]. Their measurement of EDMR spectra in the photocurrent mode shows an increased DB contribution and change in recombination path with sample irradiation. They find an EDMR signal attribute to recombination of CE in shallow traps and DB defects.

8. SUMMARY In ESR spectra of nc-Si:H, signals of carrier (electrons or holes) and DB defect have been observed. Signals of the CE have been observed in undoped, phosphorus-doped, and nitrogen-doped samples at g = 1.997–1.999. The CE signal is sharp and has a faster spin-lattice relaxation time than the DB signal. In the case of p-type nc-Si:H, it has been difficult to observe the signal of the conduction holes, because of the very broad line shape. However, the signals of holes can be observed at g = 2.01 with line with of 500 G by pulsed-ESR spectroscopy. In the light-induced ESR spectra, enhancement of CE peak has been observed in undoped and phosphorus-doped samples. The ESR signals of DB defects have been observed at g = 2.005–2.006. Several authors have attributed the asymmetric shape of the DB signal to the existence of two centers, silicon DB at g = 2.0052 and oxygenrelated center at g = 2.0043.

GLOSSARY Hydrogenated amorphous silicon (a-Si:H) The amorphous silicon in which some of the dangling bond defects are terminated by hydrogen atoms. Conduction electron (CE) The electrons conduct in the Si. Dangling bond defect (DB) The unpaired electrons of Si atoms that do not participate in the formation of Si-Si covalent bonds. Electron spin resonance (ESR) One of the magnetic resonance techniques. ESR absorption corresponds to the energy difference of spin states in magnetic field. Light-induced electron spin resonance (LESR) ESR under light illumination. Hydrogenated nanocrystalline silicon (nc-Si:H) The material that contains nanocrystalline grain with the size of a few tens to a few hundred nanometers. In the material, some of the dangling bond defects are terminated by hydrogen atoms. Pulsed ESR Electron spin resonance measurement using microwave pulse instead of continuous wave macrowave.

Nanocrystalline Silicon: Electron Spin Resonance

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