Advances in Electronics and Electron Physics, Volume 42

ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS

VOLUME 42

CONTRIBUTORS TO THISVOLUME

D. Berenyi J. J. DeCorpo

S. B. Karmohapatro Herbert F. Matare E. S. Rittner F. E. Saalfeld J. R. Wyatt

Advances in

Electronics and Electron Physics EDITEDBY L. MARTON Stnithsoniar~Institution, Wu.shington,D.C. Assistant Ediror CLAIRF MARTON

EDITORIAL BOARD E. R . Piore T. E. Allibone M . Ponte H . €3. G. Casimir W. G. Dow A. Rose L. P. Smith A. 0. C. Nier F. K . Willenbrock

VOLUME 42

1076

ACADEMIC PRESS

New York San Francisco London

A Subsidiary of Harcourt Brace Jovanovich. Publishers

COPYRIGHT 0 1976, BY ACADEMIC PRESS,INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

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CONTENTS CONTRIBUTORS TO VOLUME 42 . FORE w ORD . . . . . .

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vii ix

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Mass Spectroscopy F. E. SAALFELD. J . J . DECORPO.AND J . R . WYATT

I. I1 . 111. IV . V. VI . VII .

Introduction . . . . . . . Instrumental Design and Techniques Surface Studies . . . . . . Ionization Processes . . . . . Ion-Molecule Studies . . . . High Temperature Studies . . . Sampling of Reactive Species . . References . . . . . . . .

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Recent Advances in Silicon Solar Cells for Space Use E. S. RITTNER

I . introduction .

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I1. Violet Cell . . . . . . . . . . . . . . . . . . 111. Nonreflective Cell . . . . . . . . . . . . . . . IV . Summary . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

41 42 50 53 54

Recent Applications of Electron Spectroscopy D . BERBNYI

I . Introduction . . . . . . . . . . . . . I1 . Instrumental Techniques . . . . . . . . . 111. Observables in Electron Spectroscopy Measurements . IV . Main Fields of Application . . . . . . . . . V . Electron Spectroscopy in Solution of Practical Problems V1. Conclusions and Perspectives . . . . . . . . References . . . . . . . . . . . . . .

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Laboratory Isotope Separators and Their Applications S. B. KARMOHAPATRO I . Introduction . . . . I1. Production and Formation 111. Acceleration of Ions . . IV . Mass Analyzers . . . V . Performance . . . . VI . Collection .

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of Intense Ion Beams .

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113 118 135 137 146 152

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VII . Applications . VIII . Conclusions . References . .

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157 168 169

Light-Emitting Devices. Part I: Methods HERBERT F. MA TAR^ 1. Introduction . . . . . . . . . . . 2. Radiative and Nonradiative Recombination . . 3. Radiative Recombination and the Injection Process References for Sections 1-3 . . . . . . . 4. Materials for Light Emitters . . . . . . . References for Section 4 . . . . . . . . 5. The Heterojunction . . . . . . . . . References for Section 5 . . . . . . . . Appendix to Section 5 . . . . . . . . References for Appendix . . . . . . . . 6. Methods ofJunction Formation . . . . . References for Section 6 . . . . . . . .

AUTHORINDEX . SUBJECT INDEX .

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179 185 189 195 195 221 222 242 243 246 246 278 281 291

CONTRIBUTORS TO VOLUME 42 Numbers in parentheses indicate the pages on which the authors’ contributions begin.

D. BERBNYI,Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen, Hungary ( 5 5 ) J. J. DECORPO, Physical Chemistry Branch, Naval Research Laboratory, Washington, D.C. (1)

S. B. KARMOHAPATRO, Saha Institute of Nuclear Physics, Calcutta, India (113) HERBERT F. MATARB,ISSEC International Solid State Electronics Consultants, Los Angeles, California (179)

E. S. RITTNER,COMSAT Laboratories, Clarksburg, Maryland (41) F. E. SAALFELD, Physical Chemistry Branch, Naval Research Laboratory, Washington, D.C. (1) J. R. WYATT,Physical Chemistry Branch, Naval Research Laboratory, Washington, D.C. (1)

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FOREWORD Mass spectroscopy is the first item in the present volume. It has been a long time since we published reviews on this subject, the previous ones having appeared in Volumes 1 (1948), 8 (1956), and 27 (1969). In reviewing the present-day status of the subject, F. E. Saalfeld, J. J. DeCorpo, and J. R. Wyatt cover new aspects of instrument design and techniques, with a thorough survey of many applications such as surface studies, ionization processes, ion-molecule studies, the effect of high temperatures, and the sampling of reactive species. In a short review E. S. Rittner acquaints us with recent improvements in silicon solar cells. Their efficiency has been increased by 50% due to the introduction of new technologies, which are discussed in detail. Electron spectroscopy, reviewed here by D. Berenyi, has lately become an important analytical tool. The theoretical aspects of photoelectron spectroscopy were covered by S. T. Manson in Volume 41. In this volume we have a very general presentation of many instrumental and observational facets of electron spectroscopy, including many applications. To round off the subject, we expect to publish in the near future the second part of Manson's review. S. B. Karmohapatro reviews recent advances in laboratory isotope separators and their applications. This subject is rather close to mass spectroscopy, since the laboratory instruments used for isotope separation are essentially a subclass of spectrometers. However, as the author points out, these separators can be used not only for the separation of isotopes, but also as low-energy accelerators or as high-intensity mass spectrometers. This aspect offers a broad field of applications in atomic, solid state, and nuclear physics. The last review is the first installment of a two-part review on lightemitting devices, written by H. F. Matare. After an examination of the physical background, a survey of the materials used is followed by a description of heterojunctions and by methods employed for junction formation. Following a long established practice we would like to list here the reviews, with their authors, which we expect to publish in later volumes: Time Measurements on Radiation Detector Signals In Siru Electron Microscopy of Thin Films Charged Particles as a Tool for Surface Research ix

S. Cova A. Barna, P. B. Barna, J. P. P k z a . and 1. Pozsgai J. Vennik and L. Fiermans

FOREWORD

X

Electron Micrograph Analysis by Optical Transforms X-Ray Image Intensifiers Electron Bombardment Semiconductor Devices Atomic Photoelectron Spectroscopy. I1 Recent Advances in Electron Beam-Addressed Memories Light-Emitting Devices. 11: Applications Nonlinear Atomic Processes High Injection in a Two-Dimensional Transistor Semiconductor Microwave Power Devices. I1 Basic Concepts of Minicomputers Physics of Ion Beams from a Discharge Source Physics of Ion Source Discharges Auger Electron Spectroscopy High Power Electron Beams as Power Tools Terminology and Classification of Particle Beams On Teaching of Electronics Wave Propagation and Instability in Thin Film Semiconductor Structures The Gunn-Hilson Effect A Review of Applications of Superconductivity Minicomputer Technology Digital Filters Physical Electronics and Modeling of MOS Devices Measurement and Application of Precise Time Thin Film Electronics Technology Characterization of MOSFETs Operating in Weak Inversion Electron Impact Processes Sonar Microchannel Electron Multipliers The Negative Hydrogen Ion Electron Attachment and Detachment Noise in Solid State Devices Radar Signal Processing Electron Beam Controlled Lasers Amorphous Semiconductors Electron Beams in Microfabrication. I and I1 Photoacoustic Spectroscopy Design Automation of Digital Systems. I and I1

Supplementary Volumes Sequency Theory

G. Donelli and L. Paoletti J. M. Houston, K. H. Vosburgh, and R. K. Swank D. J. Bates, R. Knight, and S. Spinella S. T. Manson

J. Kelly H. F. Matare J. Bakos W. L. Engl S. Teszner and J. L. Teszner L. Kusak G. Gautherin and C. Lejeune G . Gautherin and C. Lejeune N. C. Macdonald and P. W. Palmberg B. W. Schumacher B. W. Schumacher and J. H. Fink H. E. Bergeson and G. Cassidy A. A. Barybin M. P. Shaw W. B. Fowler C. W. Rose S. A. White J. N. Churchill, T. W. Collins, and F. E. Holmstrom G. M. R. Winkler T. P. Brody

R. J. Van Overstraeten S. Chung F. N. Spiess R. F. Potter R. Geballe R. S. Berry E. R. Chenette and A. van der Ziel Merrill 1. Skolnik Charles Cason H. Scher and G. Pfister P. R. Thornton A. Rosencwaig W. G. Magnuson and Robert J. Smith

H. F. Harmuth

xi

FORE WORD

Computer Techniques for Image Processing in Electron Microscopy High Voltage and High Power Applications of Thyristors

W. 0. Saxton

G. Karady

Throughout the years we have enjoyed the wholehearted cooperation of many friends. Our warmest thanks go to them for the help they have given us. We would like to invite, as in the past, comments on the published volumes and suggestions for future ones.

L. MARTON C . MARTON

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Mass Spectroscopy F. E. SAALFELD, J. J. DECORPO, AND J. R. WYATT Physical Chemistry Branch Naval Research Laboratory Washington, D.C.

I. Introduction ............................................................................... 11. Instrumental Design and Techniques.. .................................................. 111. Surface Studies ............................................................................ A. Introduction ........................................................................... B. Ion Scattering Spectrometry (ISS) ................................................... C. Ionized Neutral Mass Spectrometry (INMS) ....................................... D. Secondary Ion Mass Spectrometry (SIMS) ................................ E. Electron Spectroscopy for Chemical Analysis (ESCA) ............................. F. Auger Electron Spectroscopy (AES) ...................................... IV. Ionization Processes ...................................................................... A. Introduction .....................................................

2 2 5 5 6 7

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C. Chemical Ionization (CI) E. F. G. H. I. J.

Field Ionization (FI) ....................................... Field Desorption Ionization ............................................ Surface Ionization (SI) ............................................................... Penning Ionization ........................................ Other Ionization Processes.. . . . . . . . . . . . Shapes of Ionization Efficiency Curves

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A. Introduction

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C. Flow Systems ......................................................................... 21 .......... 22 D. Low Pressure, Single Collision Techniques E. Other Systems ......................................................................... 23 VI. High Temperatu ........................................... 25 A. Introduction ........................................... 25 B. Knudsen Cel ................................................................. 25 VII. Sampling of Reactive Species ............................................................ 28 A. Introduction. .... .... ...... 28 B. Sampling of Plasmas ................................................................. 28 C. Sampling of Combustion. ....................... 30 D. Sampling of Radicals ................................................................. 33 References ................................................................................. 35 1

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J. J. DECORPO, AND J. R. WYATT

I. INTRODUCTION

Three mass spectroscopy reviews appeared in earlier volumes of Advances in Electronics and Electron Physics. “ Modern Mass Spectrometry ” by Mark Inghram was published in 1948 as part of Volume 1 of this series; “ Mass Spectroscopy by Larkin Kervin appeared in Volume 3, and in 1969 (Volume 27) P. H. Dawson and N. R. Whetten reviewed “Mass Spectroscopy using RF Quadrupole Fields.” In addition, many topics closely related to mass spectrometry have also been covered. For example, Eldon Ferguson reported on “ Thermal Energy Ion-Molecule Reactions ” in Volume 24, and Michael T. Bowers and Timothy Su reviewed the same subject in Volume 34. Since mass spectroscopy has received substantial coverage in this series, the reader may well ask why the Editor would wish to publish another review on the subject. The answer is straightforward; mass spectroscopy has and is undergoing a rapid and remarkable expansion which has not been slowed or stopped by inflation, recession, or other social or scientific upheavals. An indication of mass spectrometry growth is the number of published volumes on mass spectrometry and the number of mass spectrometry meetings being held every year throughout the world. For example, more than 20,000 mass spectroscopy papers were published in 1972 and 1973 (Burlingame et al., 1974). Mass spectrometry has been applied to almost every area of research being pursued today. Studies of diverse subjects such as cancer research, identification of drugs, forensic analysis, atmospheric end water environmental analysis, combustion, and lasers have benefited from mass spectrometry. In some of these studies, the mass spectrometer is used both as a chemical reactor and as an analytical instrument. Because of these diverse applications, no person or group can completely review the field of mass spectroscopy; we must therefore limit the scope of this review. This review covers instrumental designs and techniques, surface studies, ionization processes, ion-molecule reactions, high temperature systems, and sampling of reactive species. The subjects of organic structure, biological studies, and environmental analyses are not covered since these subjects were adequately reviewed by Burlingame et al. (1974). ”

11. INSTRUMENTAL DESIGNAND TECHNIQUES Any mass spectroscope has four basic components: 1. a system by which the sample to be studied is introduced into the instrument; 2. an ion source where ions (either positive or negative) that are characteristic of the sample are produced;

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3. an analyzer region where the ion beam is sorted into its various massto-charge ratios (rn/e); 4. a detector system where the separated ion beams are collected and, by some method, rendered observable. There are many different approaches to accomplishing each task outlined in these four steps. We will be concerned only with a general description of the major components. Many variations have been reported in each of these components. These changes are limited only by the experimenter’s ingenuity and the current technology. Some of these approaches are discussed later in this chapter. Some of the methods of sample introduction being used today are the molecular ieak, viscous leak, gas chromatography, solid sample electrodes, direct insertion probes, and heated filaments. Gaseous samples are normally introduced into the mass spectrometer through an inlet system that incorporates a molecular leak. A molecular leak is characterized by molecular flow of the molecules in the system. This means that the rate of flow is limited by wall collisions rather than molecular collisions, i.e., the mean free path of the molecules in the sample reservoir must be greater than the diameter of the leak. This inlet is normally used for quantitative gas analysis and is designed so that the composition of gases in the ion source is the same as that in the sample reservoir. It should be noted, however, that the composition in the reservoir changes with time. A viscous-leak inlet is employed to investigate the properties of a single gas (e.g., isotope ratios). In this inlet, which is usually a long capillary tube, the gas flow is limited by the viscosity of the gas. Therefore, the gas flow will depend on the nature of the mixture in the inlet. With this inlet, the composition of the gas in the reservoir does not change with time, but the composition of the gas in the ion source is not the same as that in the reservoir. A viscous leak is used to sample high pressure systems ( - 1 atm) permitting continuous analyses of various chemical processes. The gas chromatograph inlet, with the various pressure reducing devices, is the most important tool for the identification of the components of complex mixtures such as flavor extracts and enclosed environmental atmospheres. These inlets and the application of gas chromatography have been adequately reviewed (Burlingame, 1970; McFadden, 1973). Spark source mass spectrometers are used for the direct analysis of trace impurities (at the parts-per-billion level) in solids. The sample under study is shaped into a solid electrode and placed in the ion source. A radio-frequency discharge is initiated between the electrodes to vaporize and ionize the sample. This source produces ions that have a considerable energy spread; therefore, this source is used exclusively in double focusing mass spectroscopes.

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The direct-insertion probe has greatly extended the applications of mass spectrometry in organic chemistry. The sample is placed in a capillary tube, which is then directly inserted into the ion source of the mass spectrometer through a vacuum lock. The sample is heated and vaporizes directly into the ionizing electron beam. Thus, many materials with insufficient vapor pressure to be sampled with a molecular or viscous leak can be investigated. In addition, the direct-insertion inlet may also be operated at cryogenic temperatures, enabling mass spectrometric studies of unstable compounds to be carried out. Several laboratories have used these cryogenic probes with notable success (e.g., McGee et al., 1966). In the heated filament inlet, also known as surface ionization, a solution of the sample is painted on the filament and allowed to dry. The filament is then introduced into the ion source and the source evacuated. The filament is heated to vaporize the sample. A proportion of the sample vaporizes as ions and these ions are mass analyzed. The exact proportion of ions produced in the vaporization process is dependent on the ionization potential of the sample and work function of the filament. Measurement of isotope ratios of the alkali and alkali-earth metals is the chief use of this type of inlet system. There is no universal ion source for all mass spectrometric applications. Accordingly, various sources have been developed for specific applications, and each source has several variations. The most widely used source today is the electron impact type; it is sold by every commercial manufacturer. Ionization in this source is produced by the bombardment of the gaseous molecules with controlled, low energy (0 to 100-V) electrons. There are many other types of sources that are gaining in popularity. These include the photoionization, field ionization, surface ionization, chemical ionization, discharge, and laser sources. A discussion of some of these esoteric sources is given in Section IV. The methods of mass sorting the ion beam produced in the various ion sources are numerous. There are three basic methods to accomplish this mass sorting, or ion focusing: direction focusing, where ions of the same mass and velocity, but different initial direction, are focused on the detector; velocity focusing, where ions of homogeneous velocity and direction are focused; and double focusing, where ions of the same mass, but with varying velocities and directions, are focused. The three measurable quantities of a moving electrical charge are velocity, momentum, and energy. If two of these quantities are specified, the mass-to-charge ratio of the ion can be determined. The more common types of mass analyzers in use today are direction focusing 60°,90",and 180" magnetic fields; velocity focusing linear time-of-flight, radio frequency, quadrupoles, etc. ;and double focusing. This last type of instrument employs an energy seiector (an electrostatic analyzer)

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and a momentum selector (a magnetic field). The two most widely used configurations are the Mattauch-Herzog (a 31"50' energy selector and a 90" momentum selector) and the Nier-Johnson (a 90" energy selector and a 90" or 60" momentum selector). There are presently two principal methods of ion detection: the photoplate (a mass spectrograph) and electrical detection (a mass spectrometer). The photoplate detector has advantages when employed for exact mass measurements and for the integration of the ion signal, such as in a spark source mass spectrograph and quantitative analysis when a direct-insertion probe is used. An electrical detector usually consists either of a Faraday cup and/or an electron multiplier, followed by signal amplification. This is the most widely used method ofdetection and is employed for precision measurement of ion abundances.

STUDIES 111. SURFACE A . Introduction

The ability to characterize surfaces has increased dramatically due to combining of mass spectrometry with scattering and sputtering techniques. Several techniques are discussed in this section: ion scattering spectrometry (ISS), ionized neutral mass spectrometry (INMS), secondary ion mass spectrometry (SIMS). In addition, two other surface techniques where mass spectrometry has recently been incorporated, electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES), are discussed. We discuss principles of operation, capabilities, and limitations of these techniques. Except for ISS, these techniques utilize some type of emission (photons, electrons, atoms, molecules, or ions) from the studied surface. This emission results from surface bombardment. In general, the emitted photons, electrons, and ions are analyzed according to their energy. The ions and neutral particles are also mass analyzed. The mechanisms of ion scattering on surfaces were reviewed by Kaminsky (1964) and those of ion sputtering of surfaces by Carter and Colligan (1968). Therefore, our discussions of these mechanisms are brief. The concentration profile below the surface of a particular species is important. The techniques discussed in this section can, with various degrees of success, provide an in-depth concentration profile. This is accomplished by bombarding the surface with energetic particles that sputter away layer after layer, thus exposing new layers deep in the solid. The newly exposed surface layers can then be analyzed.

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B. Ion Scattering Spectrometry (ZSS) ISS uses ion scattering in which the energy of the reflected beam can be used to determine the mass of the interacting surface species. At a scattering angle of 90°, the scattering equation for an elastic collision is given by

( M , - MlMM2 + Ml) (1) where E , and E , are the initial and scattered ion energies, respectively, M , and M, are the masses of the primary ion and scattering atom, respectively. From Eq. (1) it is clear that scattering occurs only when M , > M , . Thus, it is convenient to use the 4He ion as the primary beam. A typical commercial instrument (Goff, 1973) has a primary ion source, a sample holder and manipulator, an electrostatic energy analyzer, and an electron multiplier. These components are all housed under ultrahigh vacuum. The primary ion beam is focused onto a 1-mm diameter area of the sample. The electrostatic analyzer accepts the ions scattered through a 90" angle. Generally, inelastic processes can be neglected if the primary ion beam has an energy less than X keV, where X is the atomic mass of the primary ions. During a typical analysis the primary ion beam (4He+jwith an initial energy of 1-3 keV is kept constant and the voltage on the electrostatic energy analyzer is varied. The instrument thus sweeps through the El / E , ratio in Eq. (1). Therefore, the output signal represents the concentration of the scattering atom on the surface having mass M 2 . ISS has the unique ability to analyze species exclusively from the surface. In comparison, sputtering techniques analyze species between the surface and a depth of 10-60 A. Besides providing a surface analysis, ISS has the advantages of uniform sensitivity to most elements and low sample consumption, which is essential for thin film work. ISS cannot accurately provide a depth concentration profile because some of the ions electrostatically analyzed come from regions near the crater wall that are produced by the primary ion beam. To reduce this problem the entrance slit of the electrostatic analyzer can be made smaller. However, this has the effect of lowering the detection sensitivity. Another limitation of ISS is the dependence of the scattered particle intensity on a scattering cross section, neutralization probability, and a geometric factor. Bingham (1966) has tabulated the scattering cross sections for most elements; however, there is little information on the other two parameters. Therefore, standards are required to determine quantitatively the chemical composition of a surface. A combined ISS and SIMS instrument has recently been marketed. This type of instrumentation is discussed later in Section II1,D on SIMS. More sophisticated ISS instrumentation has been described (Suumeijer and Boers, 1971; Wheatley and Caldwell, 1973). These instruments utilize mass&/EO

=

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analyzed primary beams, and an adjustable scattering angle. Suumeijer and Boers’s instrument also has a second mass filter in series with the electrostatic analyzer, which permits any secondary sputtered ions to be identified. C. Ionized Neutral Mass Spectrometry ( I N M S )

The sputtering of neutral atoms from the surface is the most probable emission process. Primary ions with energies up to 1 keV have a sputtering coefficient (number of atoms emitted per primary ion) of unity (Carter and Colligan, 1968). Other processes have a coefficient of Many experiments use various modfied conventional mass spectrometers (Honig, 1959) to detect the large number of neutrals liberated from the surface. Most modifications are designed to collimate the neutrals through an ionization chamber. The resulting ions are mass analyzed (Bradley and Ruedl, 1962). These initial experiments were severely limited by sensitivity problems. This was due to the fact that the sputtered neutrals have an energy of a few electron volts; thus, their probability for ionization is extremely small. Woodyard and Cooper (1964) developed the first practical INMS. Their apparatus used a conventional Nier-type electron impact ion source with the ion repeller replaced by a copper plate-the material being studied. The ion repeller was negatively biased with respect to the ion source. When argon was added to the ion source to form a low pressure discharge, a fraction of the ions from the discharge bombarded the sample material. The sputtered neutral particles were ionized in the argon discharge. These new ions were then extracted and mass analyzed. The actual mechanism for ionization was not clear. Woodyard and Cooper (1964) claimed that about one atom in lo4 surface neutrals could be detected. Coburn and Kay (1971; Coburn e f al., 1973) refined the above experiment and concluded that the ionization process was of the Penning type (to be discissed in a later section). Coburn and Kay constructed an rf sputtering system with the cathode made of the material of interest. As in the previous experiment when argon was added to the system, a discharge formed and the ions bombarded the cathode material. The sputtered neutrals ionized in the discharge and were subsequently mass analyzed. The rf sputtering system enhanced the ionization of the sputtered neutrals by several orders of magnitude. A lower pressure, but otherwise similar rf discharge arrangement has been reported (Oechsner and Gehard, 1972). Typical operating conditions of INMS produce sputtering rates from 1 to 200 monolayers/min, and the neutrals are representative of the lattice material (Honig, 1973). Penning ionization cross sections of various atoms are known, so that the relative detection sensitivity can be calculated. The primary ions have energies of about 200 eV, causing sputtering to be limited

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F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

to a few layers. The major limitation of INMS is that the whole sample must be sputtered; thus, there is no opportunity to examine the lateral profile of the surface.

D. Secondary Ion Mass Spectrometry ( S I M S ) SIMS uses sufficiently high energy ions to bombard the surface in order to liberate positive and negative ions, which are characteristic of the chemical composition of a surface. The emission of these secondary ions is well known (Sloan and Press, 1938; Herzog and Viehboek, 1949).The sample is bombarded in the ion source of a mass spectrometer with a beam of primary ions. The resulting secondary ions are mass analyzed and in some cases also energy analyzed. High primary ion densities were used in SIMS during the past decade (e.g., Liebl and Herzog, 1963; Fogel, 1972). It was previously thought that SIMS would be unusable for the analysis of individual monolayers since the surface is rapidly removed during the intense bombardment. Recently, Benninghoven (1973) has used low primary ion currents to analyze individual monolayers. This SIMS approach is known as the “static method.” With static SIMS, the primary ion current is low, so that an individual monolayer may last many hours. The necessary sensitivity is achieved by bombardment of a large area of the sample (0.1 cm2) and the use of ion counting equipment. Secondary ion emission is a complex interaction between the primary ion and a limited area of the solid surface. As the primary ion penetrates the solid it loses energy. Part of this energy apparently returns to the surface and is transmitted to a surface particle. If the transmitted energy is high enough, the particle is liberated. Due to complex ionization processes which are not understood (Benninghoven, 1973), some of these particles leave the surface as ions. The surface particles may be emitted as molecular ions or as fragment ions. Secondary ions formed with a primary beam in the kiloelectron volt region will have several electron volts of energy when they leave the surface. This low energy ensures that the ion originates from the surface. After the primary ion bombards the surface atom, it will be backscattered. The energy analysis of the backscattered primary beam allows the atomic mass of its impact partner to be calculated. Readers interested in a theoretical treatment of ion emission from surfaces are referred to a number of works: Andersen and Hinthorne (1972), Jurela (1970), Beske (1967). SIMS can identify many species at 1-ppm concentrations. However, Honig (1959) and Andersen (1973) showed that the magnitude of the secondary ion signal is a complex function of the state of the surface and the concentration of the particular constituent in the sample. Some typical SIMS investigations are the change of chemical composition of the surface

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monolayers induced by ion bombardment, the changes in catalysts after use, the reactions of metals with gases, and studies of adsorption layers. Evans (1972) reviewed two additional types of SIMS instruments: first, the direct-imaging analyzer which was developed by Castaing and Slodzian (1962) and more recently made available commercially (Morabito and Lewis, 1973). The sample is bombarded with a primary ion beam about 300 pm in diameter. The secondary ions are extracted from the sample chamber and are mass analyzed in a stigmatic magnetic prism. The ions are then reflected and energy analyzed by an electrostatic mirror lens, further mass analyzed, passed through a projection lens, and displayed on a scintillator. The image on the scintillator represents the distribution of a particular element in the sampled area. The second instrument described by Evans (1972) is the secondary ion microprobe mass spectrometer which was originally designed by Liebl and Herzog (1963). This instrument is known as the ion microprobe mass analyzer (IMMA). In this technique, the mass spectrometer is tuned to one particular mass and the primary ion beam of about I-pm diameter scans the surface across an area 300 x 300 pm. The output current of the multiplier modulates the brightness of a synchronized oscilloscope beam. In this way the topographical distribution of a particular element is displayed. Since the direct-imaging analyzer records all information simultaneously, it yields the information in less time than the IMMA system. Liebl(l972, 1974) recently constructed an ion-electron microprobe. The instrument has the acronym UMPA (universal microprobe analyzer) and permits ion and electron bombardment, separately or simultaneously, by use of a new lens design. This instrument is reported to have better resolution and sensitivity than that of the IMMA design (Liebl, 1974). The SIMS technique is useful for qualitative analysis of surface compositions and depth distributions. Using an empirical approach Andersen and Hinthorne (1972) showed that secondary-ion currents can give semiquantitative information. In addition, a complicated calibration technique does exist (Wermer, 1972) for certain systems, such as the oxides. The SIMS approach, like other techniques, has advantages and limitations. For example, the direct-imaging analyzer and IMMA use intense high energy primary beams, which can change the characteristics of the surface. Static SIMS uses a large target area, which gives poor lateral concentration profiles. On the other hand, the direct-imaging analyzer and the ion microprobe can provide lateral elemental distribution (1-2 pm). Static SIMS provides information from a monolayer, whereas the other SIMS techniques provide in-depth concentration profiles. Secondary ion techniques are sensitive (sometimes at the expense of the sample). Benninghoven (1973) claims a SIMS detection limit below 1 ppm

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F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

of a monolayer for many elements and compounds. Unfortunately, all secondary ion techniques suffer from a matrix effect,” i.e., certain elements affect the total secondary ion emission. For instance, the secondary ion yield enhancement by oxygen and other electronegative species limits the usefulness of SIMS information by making it difficult to obtain a true concentration profile. On the other hand, hydrogen can be detected, which is difficult by other techniques. It is difficult to interpret secondary ion mass spectra because the ionization and fragmentation mechanisms producing the observed ions are not well known. The interpretation is further complicated by the uncertainty as to whether or not an observed ion corresponds to a similar surface compound. The field of SIMS is still in its infancy. This is evidenced by the rapid development of new instrumentation designed to overcome some of the present deficiencies. For example, Bakale et al. (1975) reported a high mass resolution ion microprobe mass spectrometer. The exact mass measurements enable the identification of the chemical composition of the secondary ions. An instrument that allows simultaneous operation of ion scattering spectrometry (ISS) and SIMS is commercially available. Three spectra are recorded: the ISS spectrum and the positive and negative SIMS spectra. The latter two are complementary in their sensitivity variation; the positive SIMS spectra are extremely sensitive to elements on the left-hand side of the periodic table, while the negative SIMS spectra favor the right-hand side. The three spectra taken together are claimed to represent a complete surface analysis. “

E . Electron Spectroscopy for Chemical Analysis (ESCA) ESCA is a surface analysis technique that is still in its initial stages of development. It uses photoexcitation to stimulate electron emission. The electron energy distribution analysis is used (Siegbahn et al., 1967) to determine chemical structure. Within the past few years virtually all photoexcitation has been accomplished by an X-ray source. ESCA’s most important advantage is its ability to determine the chemical environment of a surface atom. ESCA’s limitations are: it provides data from the first ten monolayers; it has a detection limit of approximately 1% of a monolayer; and it cannot detect hydrogen. A more fundamental problem with ESCA is that the energy of the emitted electron must be corrected for the surface work function, electric charging, and recoil effects. Mass spectrometers have recently been combined with ESCA for mass analysis of any sputtered particle. Evans (1975a) reported the combining of ESCA with ion sputtering to determine depth profiles. For this combination

MASS SPECTROSCOPY

11

one problem that has not been addressed is the possibility of sample damage by the sputtering process which destroys the chemical bonding. In this case ESCA is only an elemental analysis technique. A thorough discussion of ESCA is given in a treatise on electron spectroscopy edited by Shirley (1972). F . Auger Electron Spectroscopy ( A E S )

AES has developed rapidly over the past several years as a powerful method for chemical surface analysis. AES originated when Lander (1953) noted that electron bombardment of a material resulted in the emission of secondary electrons with a certain energy distribution. He was able to relate the distribution to Auger transitions of atoms on the surface. The technique was advanced by Harris (1968) who demonstrated that the Auger peaks were enhanced by differentiating the distribution. AES is accomplished by bombarding the surface with a primary electron beam having an energy of 1-10 keV while an energy analysis is performed on the emitted electrons. A small number of these electrons are due to Ailger transitions. The Auger electron energy is related to the core levels of the parent atom. The Auger transitions are well known and tabulated as a function of atomic number (Shirley, 1972). Therefore, the atomic number of the atom on the surface can be determined. The chief limitations of AES are: it cannot detect hydrogen and helium (they do not have core electrons); it is primarily an elemental technique (identification of a chemical compound is rarely successful); and its electron bombardment may change the surface composition (e.g., desorption). In addition, if the Auger electrons undergo any inelastic collisions, they lose part of their original energy and are not characteristic of the emitting atom. These scattered electrons and other secondary electrons form a severe background. Current instruments employ differentiation to overcome these problems (Evans, 1975b). Holloway (1975) has recently reported combining AES with sputtering techniques. His in-depth profiling approach yields qualitative and quantitative information. Holloway’s AES in-depth profiling has a significant advantage over other techniques using only sputtering since only AES and ISS results reflect the actual concentrations at or near the surface (Honig, 1973) at a given moment. However, as mentioned previously, ISS suffers from geometric and topographic effects and can examine only the surface monolayer. Haque (1973) has successfully combined mass spectrometry with AES in an investigation of metal contact phenomena. The combination was used in an ultrahigh vacuum system with facilities for electron and ion bombardment and gas admission. Komiya et al. (1975) and Narusawa et al. (1975) have also combined AES with mass spectrometry. A more complete discussion of AES is given in review articles by Morabito and Lewis (1973) and Chang (1974).

12

F. E. SAALFELD, J. J. DECORPO, A N D J. R. W Y A m

TABLE I SUMMARY OF MAJORADVANTAGESAND LIMITATIONS OF SURFACE TECHNIQUES ~

Technique

Limitations

Advantages

ISS

Outer monolayer analysis Uniform sensitivity for most elements

Low sensitivity Poor lateral resolution Slow profiling

INMS

Uniform sensitivity

Low sensitivity Poor lateral resolution

SIMS

PPM detection limits for many elements Microanalysis Good mass resolution

Quantitation and matrix effects Difficult to interpret secondary ion spectra

ESCA

Chemical information

Poor lateral resolution Slow profiling Poor sensitivity

AES

Microanalysis Minimal matrix effects

Quantitation Poor sensitivity, elemental only

Major advantages and limitations of the various surface techniques are summarized in Table I.

IV. IONIZATION PROCESSES A. Introduction Often the success of a mass spectrometric study depends on the source of ions. For instance, a molecular structure determination might require a different method of ion production than the determination of an ionization potential. Therefore, new ionization methods are incorporated into mass spectrometry. In this section we briefly review the most common method, electron impact, and recent ionization methods.

B. Electron Impact ( E l ) The majority of mass spectrometers use an EI ion source. A beam of electrons is produced by heating a filament. The emitted electrons accelerate through an electric field between the filament and ionization chamber. The electrons are directed into the ionization chamber and their energy varied by changing the potential between the filament and the ionization chamber.

MASS SPECTROSCOPY

13

EI results in ionization (positive and negative), excitation, and fragmentation of gas molecules. The amount of fragmentation has been the subject of a great number of theoretical papers (e.g., Wahrhaftig, 1972). However, the theoretical understanding of mass spectra has been successful in only a few cases. Some important advantages of EI are that most molecules have large ionization cross sections, that the ionization produces ions with fragmentation, and that the intensity and type of fragment ions can be changed by varying the electron energy. The EI mass spectra obtained give considerable information on the structure and fragmentation of the bombarded molecules. However, the fragmentation mechanisms may be difficult to interpret (Muccino and Djerassi, 1973; Tomer et al., 1972). The theoretical aspects of ionization by EI have been treated in a number of reviews. An excellent comprehensive treatise covering both the theoretical and experimental aspects of EI ionization was written by Massey and Burhop (1969). An important aspect of EI ionization is the relationship between the ion intensity ( I + ) and the electron energy (E). This relationship is expressed by the equation I + = I,( 1 - exp{ - a,(E)L(E)[M])) At low pressure, Eq. (2) reduces to

(2)

I + = Z,o(E)L(E)[M] (3) The electron beam current I, and the concentration of molecules in the source [MI are independent of energy. The path length of the electrons Land the ionization cross section a,, depend on electron energy (Massey and Burhop, 1969). Because a magnetic field is usually employed to collimate the electron beam, the electrons transverse the ionization region in a helical path. The actual change in L over the usual range of electron energies may be neglected. The dependence of the ionization cross section on energy is zero for energies equal to or less than some critical energy E, and is approximately a linear function of the excess energy above E, . Such a threshold law applies only a few volts greater than E, .The ionization cross section reaches a maximum at 20-50 V greater than E , , then gradually decreases with increasing electron energy. The energy at which E = E, for a given ion is called the appearance potential of the ion. The initial shape of the ionization efficiency curve is of particular importance in determining appearance potentials. Major disadvantages of an EI source are that the energy spread (1-2 eV) of the electron beam prevents accurate appearance potential measurements, and that the heated filament causes decomposition of the gas molecules. Negative ions are formed by resonance capture, dissociative capture, or

14

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

pair production (Melton, 1970). Because of smaller ionization cross sections and experimental difficulties, negative ions are not as valuable as positive ions in analytical studies. However, they are of great interest in ion chemistry. Dillard (1973) comprehensively reviewed this subject. C . Chemical Ionization (CZ)

CI mass spectrometry is an outgrowth of the ion-molecule reaction studies of Munson and Field (1966). In a review of CI Field (1968) defined chemical ionization as a mass spectrometric technique in which the ionization of a molecule is the result of ion-molecule reactions. The ions produced by ion-molecule reactions make up the CI mass spectrum of the compound being studied. Chemical ionization is not to be confused with chemiionization in which a charged species is produced by the reaction of neutral species (Fontijn, 1974). CI requires a reagent gas in the ion source at approximately l-torr pressure. The primary ionization of this reagent gas occurs by electron impact. The primary ions react with other reagent gas molecules. The resulting ions have two important properties: they react with the compound of interest, which has been added to the ion source in a much smaller quantity ( - 0.1%) (this compound is known as the additive); and they do not react further with reagent molecules. Methane and isobutane are the most widely used reagent gases. As an illustration of the CI method, methane undergoes the usual electron impact reactions to form primary ions: e-

+ CH, -+CH:,

CH;, CH;, C H t , C+

(4)

At high ion source pressures ( - 1 torr) these ions undergo the following major secondary reactions: +CH,

(5)

+ CH,-+C,Hi + H,

(6)

CH; +CH,+CH: CH;

The secondary ions, which comprise approximately 90% of the total ionization, are inert to further reactions with methane. However, upon collisions with additive molecules (AH), they react in the following manner: CH: C,H: C,H:

+ AH +AH: + CH, + AH +AH: + C,H, + AH + A + + C,H,

(proton transfer)

(7)

(proton transfer)

(8)

(hydride transfer)

(9)

Therefore, no molecular ion, M’, is expected from CI, but a “quasi-parent ion, (M - H)+ or (M + H)’, is observed. CI ionization is more “gentle” than EI ionization. Hence, this technique is an excellent method of determining the molecular weight of the compound when the EI method fails.

”

MASS SPECTROSCOPY

15

CI advances have resulted from the use of different reagent gases. For example, charge exchange followed by fragmentation occurs when helium or argon, which have high ionization potentials, are used as the reagent gas. Hydrogen as a reagent gas produces both the quasi-parent ion and fragment ions. Use of these reagent gases allows the CI technique to be used for structural analysis (Foltz et al., 1973), kinetic measurements (Field, 1969), and equilibrium studies (Field, 1972). Negative chemical ionization was studied by Dougherty et al. (1972, 1973) and is a technique of adding nonreactive gases to enhance negative ion production. CI is important in the study of compounds that do not have an EI molecular ion. Often CI and EI data supplement each other in chemical analysis (Fales, 1971). One caveat concerning CI is that, unlike low pressure ionization, the ions produced are not isolated. Thus, collisions between the CI ions and other molecules affect the mass spectra. D. Photoionization ( P I )

The energy necessary to excite or ionize an atom or molecule can be obtained from a collision between the molecule with a photon as well as an electron. The design of a photon impact ion source is similar to EI except that the photon beam replaces the electron beam. Experimentally, the chief problems with PI are the generation and transmission of the photon beam. Since vacuum ultraviolet energies are needed to ionize molecules, there are few “windows” suitable for PI (LiF and aluminum windows have limited use; Gorden et al., 1969).Therefore, windowless systems are used to transmit the photon beam into the ion source. The windowless systems consist of a vacuum monochromator, a differentially pumped photon source, and ac ion source (e.g., Rowe et al., 1973; Dibeler, 1970; Chupka, 1972). Two common types of photon sources used are the low pressure spark and rare gas discharge (Marr, 1967; Cairns et al., 1973).In the low pressure spark photons are produced repetitively in a ceramic capillary. The radiation has intense emission lines between 1350 and 400 di (9.2-31 eV). The radiation is passed through a vacuum monochromator where the desired energy is selected. Typical resolution for this source is 0.02 eV. The rare-gas discharge method has the advantage that it is easy to construct a rare-gas discharge lamp. The resonance radiation produced in the discharge lamp depends on the particular gas. The usefulness of this type of photon source is limited because it produces photons at few energies. Another less commonly used photon source is synchrotron radiation (Parr and Taylor, 1973).In a synchrotron an electron storage ring accelerates electrons to relativistic velocities in a circular orbit. These orbiting electrons emit an intense continuum of radiation. A vacuum monochromator is used to select the desired energy. The sychro-

16

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

tron’s high photon flux and available energy range make it an ideal photon source. PI sources operate at lower temperatures than EI sources because they do not use hot filaments. This is clearly an advantage when studying thermally sensitive compounds. The well-known and controlled amount of energy of the photon beam is an important characteristic of photoionization. This permits very accurate energetic measurements, but PI sensitivity is orders of magnitude lower than that of EI. PI is principally of interest in research problems (e.g., Berkowitz et al., 1973; Diebler, 1970). Danby and Eland (1972) showed that photoelectron and PI coincidence studies can be used to study ions having known internal energies. Sieck and Ausloos (1972) use PI to study ion-molecule reactions and to determine the structure of reactant ions (Sieck and Gorden, 1973).PI was used to test quasi-equilibrium theory (Rosenstock et al., 1973).

E . Field Ionization ( F I ) Atoms or molecules can be ionized by high electric fields of 107-108 V/cm. This type of ionization has been termed field ionization (FI). The high potential is generated between a surface and a sharp point or edge (known as the FI tip). FI occurs by the tunneling of an electron from a molecule to the surface. FI and its applications in mass spectrometry originated with the field-emission work of Muller and Bahadur (1956) and Inghram and Gomer (1955).Reviews covering the physical theory of FI have been published (Beckey, 1971 ; Robertson, 1972). Field ions are formed with very little internal energy. Therefore, there is a high probability of their being detected as molecular ions. In addition, at low field strengths, reactions take place at the surface of the thin film surrounding the FI tip. Here, intermolecular processes are observed and the spectra resemble those obtained from CI. The feature of producing an intense molecular mass + 1 ion makes FI a powerful analytical tool. In theory, FI provides mass spectra with a minimum of fragmentation and isotope scrambling. This information is critical for the analysis of complex mixtures and isotope dilution analysis. FI has been combined with conventional mass spectrometers in such a fashion that EI and FI techniques can be applied sequentially. Comparison of the spectra obtained from the use of such a dual ion source was used in structural elucidation of organic compounds (Fales et al., 1975). A FI source has focusing lenses common to all mass spectrometers. However, the design and the materials used in the construction of the FI tip are very critical. The earlier FI tips were fragile and subject to arching. A recent design (Brown et al., 1973; Anbar and Aberth, 1974) of a multipoint

MASS SPECTROSCOPY

17

-

FI source appears to overcome these drawbacks. Their FI source consists of lo00 metal points spaced 25 pm apart over a 2-mm2 area. an array of FI is a valuable tool for the determination of the molecular weight but has been limited by a number of problems. For instance, understanding of the FI fragmentation and correlating it with molecular structure are more difficult than with EI since catalytic and ion-molecule processes are involved. FI appears to be an excellent technique for studying catalytic effects of the tip material; however, it is difficult to extrapolate to field-free conditions. Instrumental problems have also limited the usefulness of FI. Even though Beckey (1971) increased the intensity of field ions by careful preparation of the tip, the FI sensitivity was an order of magnitude lower than that of EI. Commercially available FI instruments are not satisfactory as received from the manufacturer (Fales, 1971), and it is left to the researcher to make the necessary improvements.

F . Field Desorption Ionization Beckey (1971) showed that a FI tip coated with a sample and placed in a high field desorbs sample ions. Presumably the ions are formed on the surface of the emitter. The mass spectra produced by field desorption consist of molecular ions and molecular mass plus one ions. This technique permits generation of mass spectra of thermally unstable nonvolatile compounds. The key to the success of this technique is the preparation of the tip. The larger the surface of the tip, the greater the amount of sample ions desorbed. Field desorption has great utility for molecular weight determinations, and, when used with other mass spectral data, is valuable for structure determination. G . Surface Ionization (SZ) At high temperatures a fraction of atoms or molecules adsorbed on a metal surface vaporize as ions. The ratio of the number of neutrals (NO) to ions ( N + ) vaporized is given by the Saha-Langmuir equation:

N + / N 0 = exp[(lP - q5)/kT]

(10)

where IP is the ionization potential of the neutral species, T the surface temperature, and q5 the work function of the metal. For efficient surface ionization, the metal should have a high work function, the neutral species a low ionization potential, and the surface a high temperature. Tungsten oxide ribbon, which has a high work function, has been successfully used in surface ionization. Investigations of negative ion formation by surface ionization have been reported (Zandberg and Paleev, 1972). Surface ionization has the advantage of selectivity for the species ad-

18

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

sorbed on the surface; thus background gases present little problem. In addition, the energy spread of the resulting ion beam is small because the ions are formed in a field-free region. It is a sensitive technique-as little as 10-l4 gm can be detected. The principal use of SI is the determination of isotope ratios of elements with IP less than approximately 9 eV.

H . Penning Ionization Penning ionization can b e understood in terms of the following: A + Bf + e Penning ionization A*

(11)

+- B< +e

(12) If the energy of A*, frequently helium metastable, exceeds the ionization potential of B, then Penning ionization can occur. The cross section at thermal energies is larger than a gas-kinetic cross section. Important uses of Penning ionization are the reduction of starting AC discharge potentials, a probe to the study fundamental chemical processes, efficient ionization of a gas, and a source of lasing action (Silfvast, 1971). Penning ionization has been used in INMS (see Section 111). ABf

associative Penning ionization

I . Other Ionization Processes

New ionization methods are continually being developed. Two recent examples are discussed. An electrojet method (Dole et al., 1973) overcomes the inability to volatize high molecular weight species. Dole et al. showed that when a compound dissolved in a solvent is sprayed from a negatively charged jet, negative charge accumulates on the droplets. If the concentration is adjusted correctly, then each droplet contains only one solute molecule. These solvated ions are analyzed by a mass spectrometer. Cold electron sources such as 63Ni, '08Po, and 'H have been used in the construction of durable ion sources. (There is no risk of burning out a filament at high pressures.) The plasma chromatograph (Cohen and Karesek, 1970) and other instruments operating at atmospheric pressure (McKeown and Siegel, 1975) employ such an ion source. A disadvantage of this ion source is the reduction of ionization caused by the accumulation of any organic film on the radioactive material. J . Shapes of Ionization Eficiency Curves The ionization efficiency curves of the processes discussed in this section have different shapes. Some of these shapes are shown in Fig. 1. A theoretical treatment of ionization has been discussed by several authors (Massey

19

MASS SPECTROSCOPY

tt 0

1

2

1 1:::

Theory

Experimental

Typical process

iq;;Ti:::e He + hs + He*

(1) Photoionization He hv He+ + e - (+hv') (2) Electron excitation He e - +He* + e ( 3 ) Associative ionization He* e He e He* He -+ He: e(4) Penning ionization He $. eHe* + e He* A r - + A r +t He + e -

ii f/

+ +

-+

+ + +

-+

+

+

-+

( 1 ) Electron ionization He + e - + He+ + 2e-

FIG.1. Dependence of threshold law on the type of process.

and Burhop, 1969; Reed, 1962). The probability of ionization ( P ( E ) )is approximated by the expression P ( E ) tl ! 6 ( E - E,) dF

(13)

where 6 is a delta function centered on E , . If q = 1, then P ( E ) = ( E - EC)'-'/(q - l ) !

(14)

where n is a freedom factor expressing the number of electrons leaving the collision complex. Thus, for EI, tj = 2, and P ( E ) is a first-order function of excess energy. For PI, 9 = 1, and P ( E ) is a zero-order function (i.e., a step function). V. ION-MOLECULE STUDIES A . Introduction

The study of ion-molecule reactions is an intimate part of mass spectrometry. All mass spectrometers convert a fraction of the neutral molecules to ions. These ions can react with neutral molecules. In most mass spectrometers these reactions are eliminated by operating at low pressures. However, for certain applications it is desirable to have a large number of

20

F. E. SAALFELD, J. J. DECORPO, A N D J. R. W Y A ' l T

ion-molecule reactions occurring. Several commercially available mass spectrometers have a high-pressure ion source for this purpose. Ion-molecule reactions play an important role in many chemical processes and are the subject of much investigation. They can be studied under a variety of conditions, from single collisions to equilibrium conditions requiring thousands of collisions.The ionic products of these reactions are mass analyzed. One can truly state that the study of mass spectrometry and ion-molecule reactions are symbiotically related. Virtually all aspects of ion-molecule reaction studies have been extensively reviewed previously, most recently by Ferguson (1975).A two-volume series edited by Franklin (1972) covers thoroughly the entire field. McDaniel et al. (1970) wrote a book discussing ion-molecule reactions from a physicist's viewpoint, providing a good theoretical treatment of the subject. A large amount of data about ion-molecule reactions at thermal collision energies has been measured using the ion cyclotron resonance technique (ICR). Baldeschwieder and Woodgate (197 1) reviewed the technique. Bowers and Su (1973) discussed in detail thermal energy rate constants obtained using ICR. Another technique for studying ion-molecule reactions, ion kinetic energy spectroscopy, is discussed in an article by two of the principal investigators in this new area (Beynon and Cooks, 1974). Ferguson (1973) compiled a review of ion-molecule reaction rates at thermal energies. A broader collection of rate data was compiled by Sinnott (1973). This section concentrates upon ion-molecule studies reported over the years 1970-1975. Particular emphasis is placed upon areas that the authors feel are evolving rapidly. Studies of ion-molecule reactions can be divided into several areas based upon the experimental conditions employed. Each area can provide particular types of information, such as kinematics, rates, and thermochemistry. B. Static Systems In static systems ions are produced and undergo reactions at pressures greater than about 1W3 torr. They range from simple, closed ion sources to elaborate sampling systems operating at atmospheric pressure. Much of the work studying static systems involves the use of a pulsed ion source which can operate at pressures up to several torr (Franklin, 1972). Although this technique has several drawbacks, such as the lack of selectivity in ion formation, it still is widely used. Much work continues in this area (Beggs and Field, 1971; Rhyne and Dillard, 1971; Chong and Franklin, 1973; Lifschitz and Tassa, 1973; Cheng and Lampe, 1973; Schnitzer and Klein, 1975). An outgrowth of such studies has been the development of a different ionization technique for mass spectrometry. The technique of chemical ionization is

MASS SPECTROSCOPY

21

now widely available as an option to some commercial mass spectrometers as an aid for the analysis of high molecular weight compounds; see Section IV. Another approach is to use photons instead of electrons to produce ionization. Most workers have used resonance lamps as photon sources since they are simple and yield adequate photon fluxes. The lamps cannot be rapidly pulsed, therefore it is not possible to vary the time between ion formation and extraction. A description of this technique is given by Sieck et al. (1971). In order to study ion-molecule reactions that become significant at pressures higher than a few torr, a somewhat different ion source is required. In general, electrons from a radioactive material undergoing beta decay are used to produce ionization. After passing through several stages of pressure reduction, the ions are mass analyzed. Such an instrument was constructed by Kebarle and Haynes (1967). This technique is particularly applicable to clustering reactions, for example, H,O+(H,O),

+ H20$H,0+(H,0),+,

Recently ion sources operating at atmospheric pressure have been investigated as an analytical tool (McKeown and Siegel, 1975). Ionization is produced using electrons from a radioactive source. Due to the large number of collisions, the ionization is transferred to species having lower ionization potentials, for example, water clusters. Such a technique was shown to give detectability limits for certain compounds in air of less the one part per trillion (French and Reid, 1975). Interpretation of the mass spectra from such a system is different from conventional mass spectral interpretation and requires an understanding of ion-molecule chemistry. C. Flow Systems

Flow systems have been used for many years to study kinetics. The application of flow systems to the study of ion-molecule reactions is more recent. Flow systems used to study ion-molecule reactions operate at pressures on the order of 1 torr and flow rates of 10-100 m/sec, hence high speed pumping is required. Using electron impact or electrical discharge, ionization is produced in the carrier gas at the upstream end of the flow tube. Various gases are added along the tube to produce ions via charge exchange. These ions can be titrated with another gas, yielding information about the kinetics of the reaction. In some cases equilibrium occurs between the various species allowing direct determination of the heats of formation of the ions.

22

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

Thermal equilibrium data of ion-molecule chemistry can be obtained using only a flow system. This is the only system in which ion-molecule reactions can be studied in a field-free region and at pressures sufficient to obtain equilibration. Flow reactors can operate at temperatures approaching 1000°K (Fehsenfeld, 1975). In order to study ion-molecule reactions at higher energies in a flow system, electric fields have been imposed parallel to the direction of flow. The following interesting result has been obtained using such a “ flowing drift ” technique (Fehsenfeld, 1975).The observed rate of an ion-molecule reaction depends upon the buffer gas. Although the ion kinetic energy was the same for both helium and argon buffer gases, the rate of translational to vibrational energy transfer with helium was slower. The rate was sufficiently slow that the vibrational temperature of the ion depends upon the buffer gas. This experiment emphasizes the importance of knowing the temperature of a system to obtain true rate constants. The major limitation of the flow technique is the inability to study systems at kinetic energies higher than 1-2 eV. In order to obtain such ion kinetic energies the pressure must be reduced. Therefore, the mean free path of the ions approaches the size of the tube diameter where they are neutralized by wall collisions. Methods for studying higher energy systems are discussed in the following section.

D. Low Pressure, Single Collision Techniques Over the past decade a large amount of information concerning ionmolecule reactions in the 1-100 eV energy range has been obtained using “beam” instruments. In this type of instrument a monoenergetic ion beam is formed. The beam collides with target molecules, yielding ionic products. The products are formed as the result of a single collision. The energy and angular dependence of the product molecules can be measured. This information is used to understand the mechanism of the individual reaction on a molecular level. Beam machines can be divided into two classes: crossed beam and collision chamber. In the crossed-beam apparatus the ion beam collides with a beam of neutral molecules. An example of such an instrument was reported by Herman et al. (1969b). Since both the direction and speed of the ion and target molecule can be defined, studies with this instrument can yield more detail about the kinematics of the collision than with other types of instruments. Additionally, reactions involving little momentum transfer, such as proton transfer from the reactant ion to the neutral molecule, can be studied because the product ion has the velocity of the neutral precursor. The initial velocity of the neutral is necessary to propel the product ion to the detector. The power of the technique is demonstrated by a study of Herman et a/.

MASS SPECTROSCOPY

23

(1969a) of the reaction of ethylene ion with ethylene. The authors showed how the ratio of various products strongly depends upon the relative energy of the reactants. A similar type of instrument is one using a collision chamber to contain the target gas, for example, the instrument reported by Hied et al. (1973). There are several advantages to using a collision chamber containing target gas at a pressure of about torr vs. a beam of target gas. The number density of the target gas is easily measured, permitting the determination of absolute reaction cross sections. The apparatus is simpler than the crossedbeam apparatus. The consumption of the target gas is orders of magnitude smaller, permitting the use of an expensive target gas, e.g., CH,D, . The major beam technique limitation is that the lower energy limit for an ion beam having a usable intensity is about 1 eV. Only for reactions between a heavy ion and a light molecule, for example, Ar+

+ H, -,ArH' + H

(16)

does the center-of-mass energy approach thermal energies. Another difficulty is that the ions are not in the ground state since they are formed using electron impact. This problem has been attacked by producing the primary ions in an ion source at pressures of about 1 torr. This technique reduces the excited ions to their ground state by collisions (e.g., Leventhal and Friedman, 1969).

E . Other Systems Several groups have studied ion-molecule reactions at low pressures using other techniques, a prominent example being the ion cyclotron resonance technique ICR (Bowers and Su, 1973). Ions are produced in the ICR cell and their disappearance is measured along with the appearance of product ions. Since ions can be contained in an ICR cell for several minutes, reactions having extremely small cross sections can be studied. Because the ICR cell operates at pressures < lo-' torr, the reactant ions are in various excited states. Smith and Futrell(l974) have attacked this problem by producing ions external to the ICR cell in a higher pressure source. These ions are mass analyzed and then injected into the ICR cell. The effect of varying the reactant ion internal energy was studied by varying the pressure in the region of ion formation. ICR is useful for the study of the interaction of photons with ions (Richardson, 1975). Because of the long residence time in the ICR cell, there is a greater probability of the ions interacting with a given photon flux than in other techniques. Kramer and Dunbar (1972) reported the effect of photoexcitation of the reactant ion using ICR.

24

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

The effect of reactant internal energy upon the rate of an ion-molecule reaction was studied by Chupka and Russell (1969). They used a photoionization mass spectrometer having a tunable photon source. Hydrogen ions could be selectively prepared in various excited vibrational states. They found that the vibrational state of the hydrogen molecular ion had a significant effect upon the rate of the ion-molecule reaction Hl

+ H e - + H e H ++ H

(17) The quadrupole trap is a new technique for studying ion-molecule reactions. Ions are trapped in a three-dimensional quadrupole field. Such traps can contain ions for periods of days (Walls and Dunn, 1974). This requires the use of ultrahigh vacuum torr). Thus, this technique has potential for studying extremely slow ion-molecule reactions. So far we have discussed techniques for studying ion-molecule reactions over the 0.02-100-eV kinetic energy region. This region is of most interest because it covers the energy range of chemical bonds. Surprisingly much can be learned about the decomposition of ions by studying ion-molecule reactions at higher energies. Of particular note is the technique of ion kinetic energy spectroscopy (IKES) (Beynon and Cooks, 1974), which uses ion beams of several kiloelectron volts. Initial IKES experiments used a conventional double-focusing mass spectrometer with a detector placed at the focus of the electric sector. The electric sector was used to energy analyze ions that decomposed in the field-free region after they were accelerated. Recently Ast et al. (1972) placed a small collision chamber at the focus of the electric sector of a doublefocusing mass spectrometer. The magnetic sector was used in conjunction with the electric sector to examine a doubly charged ion produced by the reaction M+ + N + M + +

+ N +e-

(18)

This type of reaction was used to deduce second ionization potentials (Ast et al., 1972). Another interesting reaction studied by IKES is M + ++ N + M +

+ N'

When M + + is a rare-gas ion having known single and double ionization potentials, the primary and excited ionization potentials of N can be determined from the decrease in kinetic energy of the ions M. This type of reaction was recently observed at lower energies by Hierl and Cole (1975). A variation of this technique is to reverse the position of the electric and magnetic fields. In this way kinetic energy analyses of the reaction products of mass-selected ions can be made. Such an instrument has been constructed

MASS SPECTROSCOPY

25

by Beynon et al. (1973). In some respects this instrument is similar to the “beam machines discussed earlier, except it operates at much higher energies. It is quite useful for elucidating ion decomposition pathways, yielding information on molecular structure from fragmentation patterns. ”

VI. HIGHTEMPERATURE STUDIES A . Introduction

Mass spectrometry is a powerful tool for the study of high temperature systems. By high temperature systems we mean systems at equilibrium above 1000°K. No other technique provides qualitative and quantitative information about high temperature species. High temperature mass spectrometry has been reviewed previously (Inghram and Drowart, 1960; Grimley, 1967; Margrave, 1968; Rapp, 1970; Gingerich, 1972). Therefore, only areas of high temperature mass spectrometry research emphasizing advances that utilize current technology will be covered. B. Knudsen Cell Studies

The principal technique for studying high temperature systems mass spectrometrically is a Knudsen effusion cell. This technique was developed by Chupka and Inghram (1953). A Knudsen cell is placed such that effusing species, presumably from an equilibrium condition, pass directly into the mass spectrometer ion source. The species form a molecular beam which is usually mechanically chopped to distinguish beam species from the background. The most studied system over the past 20 years is the vaporization of carbon. The thermodynamic properties of the vapor species C,-C, were determined by Drowart et al. (1959). The pressures were measured over a range of five orders of magnitude. Due to the importance of graphite as a high temperature material, studies continue of its vaporization. Groups that have recently studied the vaporization of carbon include Wachi and Gilmartin (1972), Meyer and Lynch (1973), Zavitsanos and Carlson (1973), Milne et al. (1973), and Steele and Bourgelas (1973). Since oxides are an important class of refractory materials, their vaporization has been studied by many mass spectrometry groups (Balducci et al., 1971; Wu and Wahlbeck, 1972; Trevisan and Depaus, 1973; Akerman and Rauh, 1973; Bennett et al., 1974; Hildebrand and Murad, 1974). Gingerich and co-workers have concentrated on systems producing various binary and tertiary mixed-metal vapor species and carbides (e.g., Cocke

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et al., 1973). Carbides also have been studied by Stearns and Kohl (1973, 1974). High temperature species that produce negative ions have been studied (Franklin et al., 1974; Petty et al., 1973). These ions are produced via electron impact in the ion source. Negative ions produced directly from an alumina system have been studied (Srivastava et al., 1972). The Knudsen cell technique can be modified by adding a gas inlet to the cell. In this way various pressures of a reacting gas can be added to the system. If the gas-solid reactions are sufficiently fast compared to effusion of species from the cell, equilibrium is obtained. This technique is necessary when there is no convenient solid source of the element required, such as various nitrides which can be used as a source for nitrogen. Using a gas-inlet Knudsen cell, Wyatt and Stafford (1972) measured the thermodynamic properties of various carbon-hydrogen and carbon-nitrogen species. Often various factors limit the accuracy of vapor pressure determinations to the extent that thermodynamic values are useless. This is particularly true for determining alloy activities. One method for correcting this problem is to incorporate an internal standard into the system. Because of possible reactions between the sample and the standard, it is often not desirable to add a standard to the sample; therefore a Knudsen cell having two or more compartments is used (Hackworth et al., 1971). The sample to be measured is placed in one compartment and the standard in another. Such a technique was used by Jones e t a / .(1970)to obtain activities accurate to better than 2%. An alternative method is to measure the ratios of ion intensities for all the components throughout a composition range and convert these to activities using the Gibbs-Duhem equation (Sodeck et al., 1970). The principal goal of the research discussed in the above sections was to obtain thermodynamic data about high temperature species. To do this, it is necessary to sample a system at equilibrium. In many instances it is difficult to determine whether equilibrium exists. There are many other factors that limit the reliability of the data; these are discussed later. However, thermodynamic data also can be obtained by measuring the bond energy of the vapor species. The use of photoelectron and photoionization spectrometry is particularly promising in this regard (Berkowitz, 197 1). Bond energies can be determined to 0.01 eV by this technique under the appropriate conditions. An example of this work for gallium and indium halides is given by Dehmer et al. (1974). There are several limitations to using photon techniques. The ionization produced is orders of magnitude less than with electron impact sources. Therefore, only the most abundant species can be studied. At higher temperatures a larger fraction of the rotational, vibrational, and low-lying electronic

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states of the molecules are populated. These cause a smearing of the spectra making assignment of the appearance potentials more difficult. Finally, tunable photon sources are expensive and complicated. Although mass spectrometry is a powerful tool for the study of high temperature systems, it has limitations (Stafford, 1971). The most difficult problem is the lack of cross-section data for ionization of a given species. The cross section is the most uncertain factor when ion intensity is converted to number density of the parent molecule in the molecular beam. Other uncertainties such as instrumental ion transmission efficiency and multiplier gain must also be determined. Molecular cross sections can be crudely estimated by summing the cross sections of the atoms. Atomic cross sections were calculated by Lin and Stafford (1968) and by Mann (1970).Stafford (1971) believes these values are likely to be accurate within a factor of two. Unfortunately an estimate of the ionization cross section is useless unless the fragmentation pattern is known. Often various fragment ions correspond to parent ions of other species in the system adding more uncertainty. Using lower energy electrons (15 vs. 70 eV) usually reduces the amount of fragmentation, but also lowers the sensitivity. Additionally, calculated atomic cross sections are for 70-eV electrons, and therefore cannot be used with confidence at lower energies. One problem that has plagued those studying high temperature systems is the lack of satisfactory container materials. Often the crucible reacts with the system under study. Additionally, the melting point of the material often limits the highest temperature that can be used. A solution is to use the material being studied as its own container. An example of this technique is the work of Lincoln and Covington (1975) using high intensity laser pulses to produce rapid local heating. They studied the vaporization of graphite and alumina using irradiation levels of 100-600 kW cm-2. With this system they obtained readily temperatures of 3000-4000"K. They tested several models to obtain a value for the surface temperature of the material. The model assuming that the vapor species leave the material with velocities corresponding to an adiabatic free-jet expansion gave the best results. In their instrument they are able to measure the velocity of the vaporizing species as well as the mass. Despite the fact that the quadrupole mass spectrometer mass discriminates more than does a sector instrument, more researchers are using quadrupoles for high temperature research. This is due to the high ion transmission efficiency and relatively small size of quadrupole. Vasile er al. (1975) discuss an analyzer system that uses a quadrupole mass spectrometer and incorporates several ciever design features. The mass spectrometer is placed in a differentially pumped chamber between two inlet systems. One inlet system contains a Knudsen cell oven assembly close to

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F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

the ion source. This is for high sensitivity. The second inlet system contains an identical Knudsen cell oven assembly that is separated from the ion source by a distance of 75 cm. The region through which the beam passes contains a beam modulator and a set of electrostatic quadrupole deflection rods. A stopwire can be placed in the system between the oven and the deflection area such that beam species cannot enter directly into the ion source. When a suitable electric potential is placed on the rods, species having a dipole moment are refocused into the ion source. In this way, information can be obtained about the structure of the effusing species. Additionally, by measuring the phase shift in the ion signal, the flight time of the neutral progenitor of the ions can be determined. If the species are effusing at low pressure and a known temperature, their approximate mass can be calculated. Vasile et al. (1975) found at source pressures greater than about 0.1 torr the velocity of the effusing species was pressure dependent, implying noneffusive flow. By examining the phase shift and the effect of electrostatic deflection of various ions, insight can be obtained about their neutral progenitors. This system, we believe, represents the state of the art in high temperature beam sampling mass spectrometry.

VII.

SAMPLING OF

REACTIVESPECIES

A . Introduction

Mass spectrometers are frequently used for chemical analysis because of their exceptional versatility, sensitivity, and universal detectivity. For any successful analysis a representative sample must be introduced into the instrument. This requirement is not difficult to meet for routine gas analyses ; however, for the analyses of combustion, lasers, spark discharges, radicals, and certain solid materials special sampling procedures must be developed. Frequently, the development of such procedures constitutes difficult research programs which are either totally ignored or inadequately described when the research results are reported. In an effort to correct this situation, the American Society for Mass Spectrometry held a symposium at its 1974 annual meeting in Philadelphia on sampling reactive systems. The papers in this symposium were published in the January, 1975 issue of Volume 16 of the International Journal of Mass Spectrometry and Ion Physics. B. Sampling of Plasmas

Hasted (1975) thoroughly reviewed the mass spectrometric monitoring of ions in plasmas and swarms. He points out that the purpose of such a monitoring system is to produce an ion signal that is proportional to the ion

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species density in the plasma. Since this goal is difficult to prove experimentally, an understanding of the basic physics is needed in order to assure correct sampling. Hasted also points out that although the discharge plasma can be sampled in a manner such that the observed ion count is proportional to the plasma’s ion density, the proportionality constant is not known. In addition, he notes that the ion count obtained in flame sampling is not proportional to the ion concentration in the flame because the flame cools during expansion into the mass spectrometer. This expansion freezes the ion population at temperatures lower than in the flame and allows collisions to occur during the sampling. Hasted (1974) stated that a system suitable for plasma monitoring must have minimal ion discrimination, high source pressure capability, large dynamic range, and an absence of critical injection geometry. Most plasma analyzer systems use a metal sampling orifice, the potential of which can be varied and across which there is a large pressure differential. Ion optics are usually installed behind the orifice to focus the extracted ions into the mass spectrometer. Normally high capacity pumping is employed both on the lens system and mass spectrometer. This pumping permits the system to operate at a pressure low enough so that the ion mean free path is greater than the dimension of the mass spectrometer. Actual pumping capacity varies depending on the pressure of the plasma and orifice size. Hasted (1974) reviewed the theory of ion optics and the characteristics of biased monitoring orifices. He concludes that, because of collisional effects, biased orifices should be avoided. And even with unbiased orifices the ion accelerating system should always be tuned for maximum signal. Vasile and co-workers (1974; Smolinsky and Vasile, 1975), described in detail their plasma monitor which incorporates the four basic requirements cited by Hasted (1974). Since Vasile et al. used a quadrupole, their system has some dependence on the ion injection geometry; however, this type of mass spectrometer is currently being accepted by most workers for plasma monitoring. An important aspect of Vasile’s apparatus is the sampling orifice. This orifice is a 12-pm diameter hole drilled with a laser in an aluminum plate (Vasile and Smolinsky, 1974).The laser-drilling details were not reported. The potentials of the outer support cylinder and the sampling orifice were electrically floated so that they attained the wall potential. The potentials on the cylinder lenses were adjusted for maximum signal in agreement with Hasted’s recommended procedure (Hasted, 1974). The species produced in a rf discharge in methane have been reported (Smolinsky and Vasile, 1975).These authors identified a variety ofspecies in a CH, discharge. They concluded that the higher homologous hydrocarbon ions were produced by condensation reactions of C: with C , and C ; with C, and C,. They suggest that radical processes are responsible for the production of the neutral species.

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F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

Another interesting type of plasma sampling, reported by Lincoln and his co-workers (Lincoln and Covington, 1975), is the dynamic sampling of laser-induced vapor clouds during the submillisecond expansion after a material has been exposed to laser radiation. This method of sampling makes it possible to use a high power laser as a convenient tool for heating refractory materials to very high temperatures (see the previous section). Typical laser conditions were bursts of 1.06-pm radiation for approximately 0.5 msec producing surface radiation levels of 100-600 kW ern-,. A major problem is the temperature existing at the solid-vapor interface. This problem is compounded by the irregular exposure time-intensity distribution of the laser energy. This complication might be eliminated by operating the laser in the Q-switched mode. However, Lincoln preferred his approach to obtain heating of the bulk material to simulate the heating of a planetary probe. Lincoln’s system consists of a large vacuum chamber containing a nude ion source for a Bendix Model MA-2 T-O-F mass spectrometer. This instrument takes 50 spectra per millisecond and can time-resolve the species produced by a single laser “shot.” The laser beam is split; one portion is directed to a detector which measures an amount of energy proportional to the energy that irradiates the sample. The remaining portion of the beam radiates the sample normal to the surface. The surface is 45” to the center axis of the ion source. Only vapors leaving the surface along this axis enter the system. To measure ions generated from the surface, Lincoln turns off the mass spectrometer filament; to measure neutrals, an ion deflector in front of the ion source and the mass spectrometer filament are turned on. Using this instrumentation, Lincoln and Covington (1975) investigated laser plumes from graphites, alumina, and silica. They identified both the stable and transient species in these plumes. By sampling graphite plumes, they showed that triatomic carbon is the predominant species. Hydrogen is not produced; acetylene and carbon monoxide always have small, but equal concentrations. The major vapor components observed from alumina were Al’, Al, 0, , 0, and A1,O. From silica, the major species observed were O,, SiO,, and traces of Si. No Si’ was observed.

C . Sampling of Combustion The chemistry occurring in combustion is extremely complex and reactive. For example, Fristrom (1975) reported that reactive species such as radicals, ions, and excited molecules (and atoms) exist in permixed laminar flames in mole fraction concentrations ranging from lo-’ to lo-”. Analysis of these species, especially the radicals, is critical to understanding combustion chemistry. However, the quantitative detection of these species is ex-

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tremely difficult. Therefore, considerable effort has been devoted to the development of sophisticated techniques for sampling and analyzing combustion species. Methods used for the detection of stable molecules are well covered in standard mass spectrometry textbooks, such as the one by Kiser (1965) and therefore will not be described here. Our main concern in this section is the analysis of the unstable species in combustion. In general to analyze a combustion system, three important steps must be carried out: the combustion must be probed by some method; the material removed from the combustion must be transferred to the analytical instrument; and the analysis must be made. Each of these steps is discussed below. A fundamental problem in probing combustion is obtaining a representative sample without disturbing the chemistry. While flames can be probed many ways, sampling probes are usually divided into two general categories: isokinetic and sonic. Isokinetic probes remove a combustion sample at the velocity of the gas stream. The quenching of the reactive species is not effective by this approach. Therefore, this method of sampling is used for the analyses of nonreactive species. The principal advantage of this probing method is that disturbance of the flow is minimized. Sonic probes, on the other hand, disturb the combustion in the probe area. However, the use of sonic probes permits the combustion to be rapidly quenched by adiabatic expansion. This expansion lowers both the temperature and the pressure of the reacting system. Fristrom and Westenberg (1965) have published an excellent description and theory of these probes; thus only a brief description will be presented. Most sonic probes are made of quartz with tapers between 15" and 45". It is the taper of the probe, not the tube size, that determines the amount of combustion disturbance caused by the sampling. The orifice of the probe is designed to give a five- to tenfold pressure reduction, and usually has a 10-100-pm diameter. The tubing that connects the probe to the rest of the analysis system is not critical to the probing procedure. But the tubing may cause adsorption problems in the sample transfer. Because adsorption is a serious transfer problem for polar molecules such as water, several approaches have been used to eliminate this problem. A simple method is a flow system. Here, the combustion is sampled for a time sufficient for the adsorbed species to equilibrate with the transfer line walls. The time required for this process varies according to the wall material: (for water vapor) 30 min on stainless steel, 5 min on glass, and f min on Teflon (Fristrom, 1975). There are several types of flows including molecular and sonic. The results obtained from a molecular flow system are the easiest to interpret. This method was described by Pertel (1975). With the molecular

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F. E. SAALFELD, J. J. DECORPO, A N D J. R. W Y A R

flow system, the partial pressure of the species in the analytical instrument is proportional to the species concentration in the combustion. Unlike molecular flow, sonic flow uses a continuous input, and there are significant variations between the species concentration and their concentration in the analytical instrument. The chief advantages of sonic flow systems are large samples (thereby permitting the detection of small concentrations) and rapid equilibrium with the walls. Neither of the methods described above successfully transfers reactive species, such as radicals. In order to detect radicals, “collisionless inlet systems such as that discussed by Foner (1966) must be used. Pertel (1975) discussed the theory of these inlet systems. There are two major types of molecular beam inlets: effusive and supersonic. The effusive molecular beam system samples the boundary layer of the combustion. In low temperature combustion studies such as those reported by Wyatt et al. (1975) this type of system is used effectively. Wyatt et al. showed that the combustion being sampled depended on the wall temperature for its existence; the products in the boundary layer were the same as those in the continuum. This observation is not true for “hot ” flames. Wyatt et al. passed their reaction tube directly through the ion source of a mass spectrometer, and the combustion products effused into the ion source, which was less than 1 cm from the reactor. To sample the continuum of a flame requires a supersonic molecular beam inlet. With this inlet, many of the reactive species are frozen in excited states (Pertel, 1975). A problem using mass spectrometry is that the excited species frequently have unusual fragmentation patterns. It is possible, although difficult, to assume an effective temperature and calculate the change in the fragmentation pattern. These spectra are then compared to the literature pattern. Another problem frequently encountered in supersonic molecular beam inlet systems is the formation of cluster species such as water and argon. There are many methods for analyzing the combustion species. The principal purpose of this report is to discuss mass spectrometric methods, but the reader should be aware of other useful analytical methods. The most promising ones are optical methods which eliminate sampling. One of these is coherent, anti-Stokes, Raman spectroscopy (CARS) (Begley et al., 1974). The chief advantages of this method are high sensitivity (105-109 times the sensitivity of commercial laser Raman methods) and the laserlike” CARS beam. Laser-induced fluorescence spectroscopy is another promising method, but is useful only for selected species that fluoresce. Many older techniques such as Raman and infrared spectroscopy, gas chromatography, and the Orsat (pressure-volume) analysis have been used with varying degrees of success to analyze combustion (Fristrom and Westenberg, 1965). Mass spectrometry is the most useful of all the analytical techniques ”

“

MASS SPECTROSCOPY

33

because of its versatility. Mass spectrometry can, in principle, detect all molecules, radicals, and atoms. It thus has no “blind spots.” Mass spectrometry is extremely sensitive and has a large dynamic range. Therefore, the instrument can simultaneously detect the trace constituents and the major products of combustion. Finally, most mass spectrometers have extremely rapid response times and can be computer controlled. Thus, the analyses can be carried out in “real time.”

D. Sampling of Radicals The study of radicals with mass spectrometers has evolved in two directions. The first deals with the study of the properties of the radicals themselves, such as ionization potential, heat of formation, and bond energies. The second focuses on a reacting system; the purposes of these studies are to identify the radicals present and to define the influence of the radicals on the system. Simple inlets are employed in the first type of radical study. These inlets usually consist of a heated quartz tube positioned near the ionization chamber. In the second, the important conditions discussed in the combustion section must be satisfied. Thus, these inlets are more complex. Both types of radical studies evolved from the classic work of Eltenton (1942, 1947, 1948). He employed three different types of reactors attached to a mass spectrometer to study low pressure (-4 torr) thermal decomposition, low pressure flames, and higher pressure ( - 100 torr) thermal decompositions. In all cases, Eltenton’s reactors were separated from the ion source by a 0.008-in. thick gold diaphragm. The gold diaphragm contained a hole with a diameter less than 0.001 in. This hole permitted the decomposition products to effuse into the ion source. Eltenton also showed that by oscillation of a flame on the high pressure side of the diaphragm, he could determine not only the origin of a given species but also study its chemical sequence. Lossing (1971, 1972; Lossing and Semeluk, 1970) extended Eltenton’s method of determining radical properties to an amazing degree. He improved not only the sampling method, but also the mass spectrometry. Others have applied the approaches of Lossing and Eltenton (e.g., Fisher and Henderson, 1967) to various systems. In these studies minor modifications have been incorporated, but the basic concepts employed are not fundamentally different from the approach used by Eltenton and Lossing. A type of radical study that differs from the classic work of Eltenton is the study of radicals produced by electron impact. In these studies a molecule ionizes and fragments in a conventional mass spectrometer ion

34

F. E. SAALFELD, J. J. DECORPO, A N D J. R. WYATT

source. The ions are removed and the neutral fragments (radicals) diffuse into a second ion source, where they are studied by mass spectrometric techniques. Beck and Osberghaus (1960) pioneered these radical studies which have been continued by other workers (Niehaus, 1967; Lampe and Niehaus, 1968; Melton, 1966, 1968; Saunders et al., 1969; Preston et al., 1969). The technique was further refined by Reeher (1974) who employed a quadrupole mass spectrometer with a dual ion source in which the operation of the source and the recording of the radical spectra and appearance potential were automatic. Reeher was able to correlate the radical spectra with the positive ion spectra. Eltenton’s classical flame studies were extended and greatly improved by Foner and Hudson (1953, 1962). These workers developed a collision-free molecular beam sampling technique such as the system described in the preceeding section. In order to prevent loss of the radicals at the walls and to reduce the background in their mass spectrometer, they improved the Eltenton oscillation technique by employing a three-stage chopped molecular beam system. The molecular beam was chopped at 170 Hz by a vibrating reed, and phase-sensitive detection was employed to eliminate background signals. This approach was refined further by Fite (1975). In fact, the phasesensitive detection method is now offered commercially. The normal instrumentation used for phase-sensitive detection consists of a system to reduce the pressure and to provide a collision-free beam. This consists of one or more differentially pumped chambers each containing an skimmer orifice. The size of the orifice is determined by the pressure reduction needed. The collimated beam is mechanically chopped and then ionized and mass analyzed. Only the mass spectra signals in phase with the chopper are detected. Another method of radical study is the shock tube experiments performed by Bradley and Kistiakowsky (1961) using the time-of-flight (TOF) mass spectrometer constructed by Kistiakowsky and Kydd (1957). In these experiments, the shock wave chamber is attached directly to the backing plate of the mass spectrometer ion source. The backing plate, which is only 0.3 cm from the ionizing electron beam, is 0.001 in. thick and contains a 0.005-in. diameter orifice. Bradley and Kistiakowsky used the TOF because of its good time resolution (50 psec) and its ability to follow a large number of ions simultaneously. For example, in their N,O studies, they detected the presence of oxygen atoms. Finally, several studies of radicals produced by heterogeneous reactions have been reported. For example, Martin and Rummel (1964) designed a means for producing radicals from a palladium catalyst to study hydrogenation-dehydrogenation processes. A palladium tube (0.25-cm i.d. with wall thickness of 0.008 cm) is inserted into the ion source directly over,

MASS SPECTROSCOPY

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and parallel to, the ionizing electron beam. The gas to be studied is passed directly over the catalyst. The catalyst temperature was 40°C due to radiation from the mass spectrometer filament. However, Martin and Rummel had provisions for heating the catalyst to 450°C. They observed methyl radicals from methane. In these studies Martin and Rummel reported ionmolecule reactions in the gas phase. Their data, however, did not prove that these homogeneous reactions were independent of the catalyst position. DeCorpo et al. (1974) carried out a similar catalytic study on various hydrocarbons using a platinum catalyst. These workers, who used an instrument design similar to that used by Martin and Rummel, found that the chemical composition of the ions produced was temperature and pressure dependent. At temperatures less than 750°C the radicals formed polymers or clusters; at higher temperatures, the radicals produced were similar to the ones produced in flames. These two studies show that mass spectrometry can be used to study radicals produced by a hetergeneous mechanism. REFERENCES Akerman, R. J., and Rauh, E. G. (1973). High Temp. Sci. 5, 463. Anbar. M., and Aberth, W. H. (1974). Anal. Chem. 46, 59A. Andersen, C. A., and Hinthorne, J. R. (1972).Science 175, 853. Ast, T., Beynon, J. H., and Cooks, R. G . (1972). J. Am. Chem. SOC.94, 661 1. Bakale, D. K., Colby, B. N., and Evans, C. A., Jr. (1975). Anal. Chem. 47, 1532. Baldeschwieder, J . D., and Woodgate, S. S. (1971). Acc. Chem. Res. 4, 114. Balducci, G., &Maria, G., Guido, M., and Piacente, V. (1971).J. Chem. Phys. 55, 2596. Beck, D., and Osberghaus, 0. (1960). Z. Phys. 160, 406. Beckey, H. D. (1971). Field ionization mass spectrometry, I n “International Series of Monographs in Analytical Chemistry,’’ Vol. 42. Pergamon Press, Oxford, New York, Toronto, and Sydney. Beggs. D. P., and Field, F. H. (1971). J . Am. Chem. Sor. 93, 1567. Begley, R. F.. Harvey, A. B., Byer, R. L., and Hudson, B. S. (1974). Am. Lab. 6, 1 1 . Bennett, S. L., Lin, S. S., and Gilles, P. G. (1974). J . Phys. Chem. 78, 266. Benninghoven, A. (1973). SurJ Sci. 35, 427. Berkowitz, J. (1971). Ado. High Temp. Chem. 3, 126. Berkowitz, J., Appleman, E. H., and Chupka, W. A. (1973). J . Chem. Phys. 58, 1950. Beske, H. E.. (1967). Z. Narurforsch.. Teil A 22, 459. Beynon, J. H., and Cooks, R. G. (1974). J . Phys. E 7, 10. Beynon, J. H., Amy, J. W.. Baitinger, W. E., Ridley, T., and Cooks, R. G. (1973). Anal. Chem. 45, 1023A. Bingham. F. W. (1966).Sandia Research Report SC-RR-66-506, TID-4500 Physics. “Tabulation of Atomic Scattering Parameters Calculated Classically from a Screened Coulomb Potential,” Sandia Research Report SC-RR-66-506, TID-4500 Physics. Sandia Lab., Albuquerque, New Mexico. Bowers, M. T., and Su, T. (1973). Adv. Electron. Electron Phys. 34, 223-279. Bradley, J. N., and Kistiakowsky, G. B. (1961). J . Chem. Phys. 35, 256. Bradley, R . C., and Ruedl, E. (1962). Proc. Inr. Con/: Ioniz. Phenom. Gases, Srh, 1961 Vol. 1, p. 150.

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Brown, H. L., Aberth, W., and Anbar, M. (1973). Proc. 21st Annu. Con$ Mass Spectrom. Allied Top. p. 456. Burlingame, A. L. (1970). “Topics in Organic Mass Spectrometry.” Wiley (Interscience), New York. Burlingame, A. L., Cox, R. E., and Covrick, P. J. (1974). Anal. Chem. 46,2483. Cairns, R. B., Harrison, H, and Schaen, R. I. (1973). Ado. A t . M o l . Phys. 8, 131. Carter, G., and Colligon, J. (1968). “Ion Bombardment of Solids.” McGraw-Hill, New York. Castaing, R., and Slodzian, G. S. (1962). J . Microsc. (Paris) 1, 395. Chang, C. C. (1974). I n “Characterization of Solid Surfaces” (P. F. Kane and G. B. Larrabee, eds.), p. 509. Plenum, New York. Cheng, T. M. H., and Lampe, F. W. (1973). J. Phys. Chem. 77, 2841. Chong, S. L., and Franklin, J. L. (1973). J . Am. Chem. SOC. 94, 6630. Chupka, W. A. (1972). I n ‘‘Ion-Molecule Reactions” (J. L. Franklin, ed.), p. 34. Plenum, New York. Chupka, W. A., and Inghram, M. G. (1953). J . Chem. Phys. 21, 371. Chupka, W. A, and Russell, M. E. (1969). J . Chem. Phys. 49, 5426. Coburn, J. W., and Kay, E. (1971). Appl. Phys. Lett. 19, 435. Coburn, J. W., Taglauer, E., and Kay, E. (1973). Proc. 21st Annu. Con$ Mass Spectrom. Allied Top. p. 534. Cocke, D. L., Gingerich, K. A., and Kordis, J. (1973). J. High Temp. Sci. 5, 474. Cohen, M. J., and Karesek, F. W. (1970). J . Chromutogr. Sci. 8, 330. Danby, C. J., and Eland, J. H. D. (1972). Int. J . Mass Spectrom. Ion Phys. 8, 153. DeCorpo, J. J., McDowell, M. V., Johnson, J. E., and Saalfeld, F. E. (1974). J . Catal. 34, 61. Dehmer, J. L., Berkowitz, J., Cusacks, L. C., and Aldrich, H. S. (1974). J. Chem. Phys. 61,594. Dibeler, V. H. (1970). I n “Recent Developments in Mass Spectroscopy” (K. Ogata and T. Nayakawa, eds.), p. 731. Univ. Park Press, Baltimore, Maryland. Dillard, J. G. (1973). Chem. Rev. 73, 589. Dole, M., Gunies, J., Blickensderfer, R. P., and Teer, D. (1973). Proc. 2 l s t Annu. ConJ M a s s Spectrom. Allied Top. p. 362. Dougherty, R. C., Dalton, J., and Blair, F. J. (1972). Org. Mass Spectrum. 6, 1171. Dougherty, R. C., Tannenbaum, H. P., and Roberts, J. D. (1973). Proc. 2 f s r Annu. ConJ Mass Spectrom. Allied T o p . p. 242. Drowart, J., Burns, R. P., DeMaria, G., and Inghram, M. G. (1959). J . Chem. Phys. 39, 2299. Eltenton, G. C. (1942). J. Chem. Phys. 10, 403. Eltenton, G. C. (1947). J . Chem. Phys. 15, 455. Eltenton, G. C. (1948). J . Phys. Colloid. Chem. 52, 463. Evam, C. A., Jr. (1972). Anal. Chem. 44,67A. Evans, C. A., Jr. (1975a). Anal. Chem. 47, 818.4. Evans, C. A., Jr. (1975b). Anal. Chem. 47, 855A. Fales, H. M. (1971). I n “Mass Spectrometry Techniques and Application: Newer Ionization Techniques” (G. W. A. Milne, ed.), p. 179. Wiley (Interscience), New York. Fales, H. M., Milne, G. W. A,, Winkler, H. I., Beckey, H. D., Damico, J. W., and Barron, R. (1975). Anal. Chem. 47, 207. Fehsenfeld, F. C. (1975). Int. J . Mass. Spectrom. Ion Phys. 61, 151. Ferguson, E. E. (1973). At. Data Nucl. Data Tables 12, 159. Ferguson, E. E. (1975). Annu. Rev. Phys. Chem. 26, 17. Field, F. H. (1968). Acc. Chem. Res. 1, 42. Field, F. H. (1969). J . Am. Chem. SOC.91, 2827. Field, F. H. (1972). Phys. Chem. Ser. One 5, 133. Fisher, I. P., and Henderson, E. (1967). Trans. Faraday SOC.63, 1342. Fite, W. L. (1975). Inr. J . Mass Specrrom. Ion Phys. 16, 109.

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Fogel, Y. M. (1972). Int. J. Mass Spectrom. lon Phys. 9, 109. Foltz, R. L., Fintiman, A. F., Jr., Mitscher, L. A., and Showalter, H. D. H. (1973). Chem. Commun. p. 872. Foner, S. N. (1966). Adu. Am. Mol. Phys. 2. 385. Foner, S. N., and Hudson, R. L. (1953). J. Chem. Phys. 21, 1374. Foner, S. N., and Hudson, R. L. (1962). Adu. Chem. Ser. 36, 34. Fontijn, A. (1974). Pure Appl. Chem. 39, 287. Franklin, J. L. (1972). “ Ion-Molecule Reactions.” Plenum, New York. Franklin, J. L., Wong, J. L. F., Bennett, S. L., Harland, P. W., and Mangrave, J. L. (1974). Adu. Mass Spectrom. 6, 3 19. French, J. B., and Reid, N. M. (1975). Proc. 23rd Annu. Conf: Mass Spectrom. Allied T o p . p. 566. Fristrom, R. M. (1975). I n t . J. Mass Spectrom. Ion Phys. 16, 15. Fristrom, R. M., and Westenberg, A. A. (1965). “Flame Structure.” McGraw-Hill, New York. Gingerich, K. A. (1972). Chimia 26, 619. Goff, R. F. (1973). J . Vac. Sci. Technol. 10, 355. Tech. Note 4%. Gorden, R., Rebbert, R. E., and Ausloos, P. V. (1969). Natl. Bur. Stand. (US.), Grimley, R. T. (1967). I n “ Mass Spectrometry in the Characterization of High Temperature Vapors” (J. Margrave, ed.), pp. 195-243. Wiley, New York. Hackworth, J. V., Hoch, M., and Gegel, H. L. (1971). M e t . Trans. 2, 1799. Haque, C. A. (1973). I E E E Trans. Parts, Hybrids, Packag. 9, 58. Harris, L. A. (1968). J . Appl. Phys. 39, 1419. Hasted, J. B. (1974). Ado. Mass Spectrom. 6, 901. Hasted, J. B. (1975). Int. J. Mass Spectrom. Ion Phys. 16, 3. Herman, Z., Hiesl, P. M., Lee, A,, and Wolfgang, R. (1969a). J . Chem. Phys. 51, 454. Herman, Z., Kerstetter, J. D., Rose, T. L., and Wolfgang, R. (1969b). Rev. Sci. Instrum. 40,538. Herzog, R. F. K., and Viehboek, R. P. (1949). Phys. Rev. 76, 855. Hierl, P. M., and Cole, C. (1975). Proc. 23rd Annu. Conf: Mass Spectrom. Allied T o p . p. 296. Hierl, P. M., Stratten, L. W., and Wyatt, J. R. (1973). Inr. J . Mass. Spectrom. Ion Phys. 10, 385. Hildebrand, D. L., and Murad, E. (1974). Chem. Phys. 61, 1232. Holloway, D. M. (1975). J. Vac. Sci. Technol. 12, 392. Honig, R. E. (1959). Adu. Mass Spectrom. 1, 162. Honig, R. E. (1973). Adu. Mass Spectrom. 6, 337. Inghram, M. G., and Drowart, J. (1960). Proc. Inr. Symp. High Temp. Technol., 1959. Inghram, M. G., and Comer, R. (1955). Z. Naturforsch., Teil A 10, 863. Jones. R. W., Stafford, F. E., and Whitmore, D. H. (1970). M e t . Trans. 1, 403. Jurela, 2. (1970). I n “Atomic Collision Phenomena in Solids” (D. W. Palmer, M. W. Thompson, and P. D. Townsend, eds.), p. 339. North-Holland Publ., Amsterdam. Kaminsky, M. (1964). “Atomic and Ionic Impact Phenomena on Metal Surfaces.” Academic Press, New York. Kebarle, P., and Haynes, R. M. (1967). J. Chem. Phys. 47, 1676. Kiser, R. W. (1965). ”Introduction to Mass Spectrometry: Instruments and Techniques.” Prentice-Hall, Englewood Cliffs, New Jersey. Kistiakowski, G . B., and Kydd, P. A. (1957). J. Am. Chem. SOC.79, 4825. Komiya, S., Narusawa, T., and Satake, T. (1975). J . Vac. Sci. Technol. 12, 361. Kramer, J. M., and Dunbar, R. C. (1972). J. Am. Chem. SOC.94, 4346. Lampe, F. W., and Niehaus, A. (1968). J . Chem. Phys. 49, 2949. Lander, J. J. (1953). Phys. Rev. 91, 1382. Leventhal, J. J.. and Friedman, L. (1969). J . Chem. Phys. 50, 2928. Liebl, H. J. (1972). Mestechnik (Braunschweig) 358. Liebl. H . J. (1974). Anal. Chem. 46, 22. Liebl, H. J., and Herzog, R. F. K. (1963). J . Appl. Phys. 34, 2893.

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F. E. SAALFELD, J. J. DECORPO, AND J. R. WYATT

Lifschitz, C., and Tassa, R. (1973). Int. J. Mass Spectrom. Ion Phys. 12, 433. Lin, S. S., and Stafford, F. E. (1968). J. Chem. Phys. 48, 3885. Lincoln, K. A., and Covington, M. A. (1975). Int. J. Mass Spectrom. Ion Phys. 16, 191. Lossing, F. P. (1971). Can. J. Chem. 49, 357. Lossing, F. P. (1972). Can. J . Chem. 50, 3973. Lossing, F. P., and Semeluk, G. P. (1970). Can. J . Chem. 48, 955. McDaniel, E. W., Cermak, V., Dalgarno, A., Ferguson, E. E., and Friedman, L. (1970). “IonMolecule Reactions.” Wiley, New York. McFadden, W. H. (1973). “Techniques of Gas Chromatography-Mass Spectrometry Applications in Organic Chemistry.’’ Wiley (Interscience), New York. McGee, H. A., Malone, T. J., and Martin, J. (1966). Rev. Sci. Instrum. 37, 561. McKeown, M. and Siegel, M. W. (1975). Amer. Lab. 7, 89. Mann, J. B. (1970). In “Recent Developments in Mass Spectrometry” (K. Ogata and T. Hayakawa, eds.). Univ. Park Press, Baltimore, Maryland. Margrave, J. L., ed. (1968). “Mass Spectrometry in Inorganic Chemistry.” Adv. Chem. Ser. No. 72. Adv. Chem. Ser., Washington, D.C. Marr, G. V. (1967). “Photoionization Process in Gases.” Academic Press, New York. Martin, T. W., and Rumrnel, R. E. (1964). Science 143, 797. Massey, H. S. W., and Burhop, E. H. S. (1969). “Electronic and Ionic Impact Phenomina,” Int. Ser. Monogr. Phys., Vol. 1. Oxford Univ. Press (Clarendon), London and New York. Melton, C. E. (1966). J. Sci. Instrum. 43, 927. Melton, C. E. (1968). Int. J. Mass Spectrom. Ion Phys. 1, 353. Melton, C. E. (1970). “Principles of Mass Spectrometry and Negative Ions.” Dekker, New York. Meyer, R. T., and Lynch, A. W. (1973). High Temp. Sci. 5, 192. Milne, T. A., Beachey, J. E., and Greene, F. T. (1973). N A S A S T A R 11, 1055. Morabito, J. M., and Lewis, R. K. (1973). Anal. Chem. 45, 869. Muccino, R. P., and Djerassi, C. (1973). J . Am. Chem. SOC.95, 8726. Miiller, E. W., and Bahadur, K. (1956). Phys. Reo. 102, 624. Munson, M. S. B., and Field, F. H. (1966). J. Am. Chem. SOC.88, 2621. Narusawa, T., Satake, T., and Komiya, S. (1975). Natl. Symp. Am. Vac. SOC.,22nd Y3. Nichaus, A. (1967). Z. Naturforsch A 22, 690. Oechsner, H., and Gerhard, W. (1972). Phys. Lett. A 40,221. Pan, G. R., and Taylor, W. (1973). Rev. Sci. Instrum. 44, 1578. Pertel, R. (1975). Int. J. Mass Spectrom. Ion Phys. 16, 39. Petty, F., Wang, J. L.-F., Steiger, R. P., Harland, P. W., Franklin, J. L., and Margrave, J. L. (1973). High Temp. Sci. 5, 25. Preston, F. J., Tsuchiya, M., and Svec, H.J. (1969). Int. J. Mass Spectrom. Ion Phys. 3, 323. Rapp, R. A,, ed. (1970). “ Physicochemical Measurements in Metal Research,” Part 1. Wiley (Interscience), New York. Reed, R. I. (1962). “Ion Production by Electron Impact.” Academic Press, New York. Reeher, J. R. (1974). Ph.D. Thesis, Iowa State University, Ames (unpublished). Rhyne, T. C., and Dillard, J. G. (1971). Inorg. Chem. 10, 730. Richardson, J. H. (1975). Am. Lab. 7, 15. Robertson, A. J. B. (1972). Phys. Chem. Ser. One 5, 103. Rosenstock, H. M., Larkins, J. T., and Walker, J. A. (1973). Int. J. Mass Spectrom. Ioti Phys. 11, 309. Rowe, J. E., Christman, S. B., and Chaban, E. E. (1973). Rev. Sci. Instrum. 44, 1675. Saunders, R. A,, Larkins, J. T., and Saalfeld, F. E. (1969). Int. J. Muss. Spectrom. Ion Phys. 3, 203. Schnitzer, R., and Klein, F. S. (1975). Int. J. Muss Spectrom. Ion Phys. 16, 377.

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Shirley, D. A., ed. (1972). “Electron Spectroscopy.” Am. Elsevier, New York. Sieck, L. W., and Ausloos, P. (1972). J . Res. Natl. Bur. Stand., Sect. A 76, 253. Sieck, L. W., and Gorden, R., Jr. (1973). Chem. Phys. Left. 19, 509. Sieck, L. W., Searles, S. K., and Ausloos, P. (1971). J . Res. Nail. Bur. Stand., Sect. A 75, 147. Siegbahn, K., Nordling, C., Fahlrnan, A., Nordberg, R., Hedrnan, K., Johansson, G., Bergmark, T., Karlsson, S. E.. Lindgren, I., and Lindberg, B. (1967). “ ESCA-Atomic, Molecules and Solid State Structure Studied by Means of Electron-Spectroscopy.” Alrnquist and Wiksells, Uppsala. Silfvast, W. T. (1971). Phys. Rev. Lett. 27, 1489. Sinnott, G. A. (1973). Natl. Bur. Stand. ( U S . ) , Spec. Publ. 381, T7402478. Sloan, R . H., and Press, P. (1938). Proc. R . SOC.,London, Ser. A 168, 283. Smith, D. L., and Futrell, J. H. (1974). Chem. Phys. Len.24, 611. Srnolinsky, G., and Vasile, M. J. (1975). I n t . J . Mass Spectrom. Ion Phys. 16, 137. Sodeck, G., Entner, P., and Neckel, A. (1970). High Temp. Sci. 2, 311. Srivastava, R. D., Vy, 0. M., and Farber, M. (1972).J . Chem. SOC.,Faraday Trans. 2 68, 1388. Stafford, F. E. (1971). High Temp.-High Pressures 3, 213. Steams, C. A., and Kohl, F. J. (1973).J . Phys. Chem.77, 136. Stearns, C. A., and Kohl, F. J. (1974). N A S A Star 12, 1390. Steele, W . C., and Bourgelas, F. N. (1973). N A S A Star 11, 1465. Suumeijer, E. D. T.M., and Boers, A. L. (1971). J . Phys. E. 4, 663. Torner, K. B., Turk, J., and Shapiro, R. H. (1972). Org. Mass Spectrom. 6, 235. Trevisan, G., and Depaus, R. (1973). Z. Naturforsch., A 28, 37. Vasile, M. J., and Srnolinsky, G. (1974). Ado. Mass Spectrom. 6, 743. Vasile, M. J., Stevie, F. A,, and Falconer, W. E. (1975). Int. J . Mass Specrrom. Ion Phys. 17,195. Wachi, F. M. and Gilmartin, D. E. (1972). High Temp. Sci. 4, 473. Wahrhaftig, A. L. (1972). Phys. Chem. Ser. One 5, 262. Walls, F. L., and Dunn, G. H. (1974). Phys. Today 27, 30. Werrner, H. W. (1972). Vacuum 22, 613. Wheatley, G. H., and Caldwell, C. W. Jr. (1973). Rev. Sci. Insrrum. 44, 744. Woodyard, J. R., and Cooper, B. C. (1964). J. Appl. Phys. 35, 1107. Wu, H. Y., and Wahlbeck, P. G. (1972). J . Chem. Phys. 56, 4534. Wyatt, J. R., and Stafford, F. E. (1972).J . Phys. Chem. 76, 1913. Wyatt, J. R., DeCorpo, J. J., McDowell, M. V., and Saalfeld, F. E. (1975). Int. J . Mass Spectrom. Ion Phys. 16, 33. Zandberg, E. Ya., and Paleev, V. I. (1972). Sou. Phys.-Tech. Phys. 17,665. Zavitsanos, P. D., and Carlson, G. A. (1973).J. Chem. Phys. 59, 2966.

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Recent Advances in Silicon Solar Cells for Space Use* E. S. RITTNER COMSAT Laboratories Clarksburg, Maryland I. Introduction . . . . . .

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11. Violet Cell . . . . . . . . . ........................ ............................... 111. Nonreflective Cell .............................. ......................................... IV. Summary ........................ ........................................................

References ...................................................................................

41 50 53 54

I. INTRODUCTION Arrays of silicon solar cells have heretofore served quite satisfactorily as the prime power source in satellites and are likely to be employed in this application for many years. The silicon solar cell was invented by Chapin, Fuller, and Pearson in 1954 at Bell Telephone Laboratories. In its original form, the cell consisted of a thin, highly doped p-region formed on an nregion base with ohmic contacts to each region and with sunlight incident on the p-face. Hole-electron pairs are created in the silicon by absorption of photons of sufficient energy to bridge the band gap. In each region minority carriers created within a diffusion length of the junction are collected at the junction, thus producing a photocurrent as well as useful power in an external load. The efficiency in early units was only a few percent, but this was rapidly improved with the introduction of gridded contacts to the front surface to reduce series resistance and of an antireflection coating to minimize reflection losses from the silicon. Use of the cell in space applications revealed rapid degradation in power output as a result of radiation damage, particularly by low energy protons. This problem was solved by the addition of a thin cover slide of fused silica, which also has the merit of a high thermal emissivity to facilitate radiation cooling. Another substantial improvement in radiation resistance resulted from a switch from the original p/n structure to an n/p structure. This is primarily the result of a higher diffusion constant of electrons relative to holes in the substrate region, wherein most of the collected photocurrent originates. *This paper is based upon work performed in COMSAT Laboratories under the sponsorship of the Communications Satellite Corporation. 41

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E. S. RITTNER

After intensive development efforts, the efficiency of typical cells for space use had reached about 10% by 1970, and the international solar cell manufacturing community was not optimistic about the prospects for further major improvement. However, the fact that the theoretical limit (Rittner, 1954) on the efficiency was higher by a factor of about two prompted the initiation of a research and development program at COMSAT Laboratories. This program, guided by an understanding of the physics of the solar cell, set out systematically to eliminate insofar as possible the major sources of loss in the cell. Two generations of improved silicon solar cells have resulted thus far, designated the violet cell and the COMSAT nonreflective (CNR) cell. The former cell produces 30% more power and the latter cell 50% more power than the conventional cell. The remainder of this article provides detailed information concerning the physical basis for these improvements and the performance characteristics of the improved cells. 11. VIOLETCELL

The improvement in efficiency exhibited by the violet cell (Lindmayer and Allison, 1972, 1973) is a direct result of the elimination of the dead layer at the surface of the cell; this layer previously suppressed the response in the blue and violet portion of the solar spectrum. The enhanced response of the new cell in this spectral region may be understood with reference to Fig. 1. This figure shows schematically the penetration depth for different wavelengths of light in both conventional and violet cells. Since the optical absorption coefficient in silicon is extremely high in the short wavelength end of the solar spectrum and decreases progressively with increasing wavelength, red light penetrates very deeply, orange and green light somewhat less, whereas blue and violet light scarcely penetrate beyond the surface layer (see Fig. 2). In the conventional cell the surface has been rendered n-type by a hightemperature diffusion process that dissolves a high concentration of phosphorus in the silicon. This disrupts the silicon lattice and produces a high concentration of recombination centers. As a result, light absorbed in the surface region produces photoexcited carrier pairs which tend to recombine quickly and thus not contribute to the photocurrent. In the violet cell the n-region is markedly thinner and the concentration of phosphorus is also reduced. Thus the unresponsive “dead” layer is not present, and blue and violet light can now contribute to the photocurrent. More details concerning the n-surface layer may be seen in Fig. 3, which shows diffusion profiles for phosphorus in silicon for three progressively increasing junction depths. The highest of these curves is representative of the conventional solar cell. The depth of the junction is about 4000 A, and for a distance of about 1500 A there is a very high concentration of dissolved

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JUNCTION DEPTH

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FIG. 1. Light penetration depth for different wavelengths in conventional and violet cells. From Rittner (1976).

phosphorus together with a silicon phosphide phase accompanied by the introduction of a large concentration of edge dislocations which serve as recombination sites. The combination of a high concentration of recombination centers together with a high doping density leads to an extremely low recombination lifetime and thus to the establishment of the dead layer. As may be seen from the dashed line in Fig. 2, a dead layer 0.15-pmthick filters out radiation of wavelength below about 0.4 pm. This dead layer diminishes in thickness as the diffusion depth becomes shallower (middle curve of Fig. 3). For the lowest curve in Fig. 3, which is representative of the violet cell, the dead layer is negligibly thin on the scale of the drawing and a field-aided region resulting from the phosphorus spatial distribution extends

44

RITTNER E. S. RITTNER

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0.7

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to the near vicinity of the surface. As a result, the violet cell is responsive to the blue and violet portion of the solar spectrum, whereas the response of the conventional cell is deficient in this spectral region. This may be seen in Fig. 4, which compares the measured spectral quantum yield of conventional and violet cells and also shows the spectral distribution of space sunlight in order to indicate the importance of capturing the blue and violet end of the spectrum. The open-circuit voltage of the violet cell has been improved relative to that of the conventional cell by about 40 mV by decreasing the substrate

45

SILICON SOLAR CELLS FOR SPACE USE 102

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resistivity from 10 to 2 i2 cm. Although the higher substrate doping results in a higher damage coefficient for space particulate radiation, only a minor sacrifice in radiation hardness results, as will be shown below. The higher open-circuit voltage, in turn, produces an improvement in the curve sharpness or “fill factor.” Decreasing the thickness of the n-type surface layer introduces a new problem in that the lateral electrical conductance through the thin layer is no longer adequate. The solar cell is a low-impedance device, and a small series resistance can degrade the output significantly. It is possible to over-

46

E. S. RITTNER I

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FIG.4. Spectral distribution of space sunlight and spectral quantum yield of conventional and violet cells. From Rittner (1976).

come this difficulty by using more closely spaced contacting grids. In the conventional cell, three collecting coarse grid lines per centimeter are used to contact the surface. Reduction in grid spacing by a factor of about three leads to a reduction in series resistance to a value on the order of that in the

47

SILICON SOLAR CELLS FOR SPACE USE

conventional cell, and a tenfold reduction in spacing produces a series resistance of only a few hundredths of an ohm, a factor of 10 lower than that in the conventional cell. Since the series resistance associated with the n-layer varies inversely with the square of the number ofcontacting lines, there may be a small residual series resistance in the violet cell associated with the metal to semiconductor contacts or with the small size of the metal lines. Improvement in fineness of line is required to prevent increased light obstruction by the larger number ofgrids. In this respect also a net improvement relative to the conventional cell has been achieved. The lower series resistance also contributes to a higher fill factor. Extension of the sensitivity of the cell to the blue and violet region also raised another problem, that of the antireflection coating. None of the previously employed commercial antireflection coatings for solar cells (SiO,, TiO,) are transparent in the blue and violet end of the spectrum. The situation for SiO, is illustrated by curve 4 of Fig. 5 ; the strong absorption in the blue would filter out the improvement in blue-violet response of the 1W

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48

E. S. RITTNER

violet cell. It thus became necessary to develop new antireflection coatings of high transparency over the entire solar spectrum, satisfying optical matching conditions and exhibiting the required stability in a radiation environment. This problem has been solved (Revesz, 1973; Revesz et al., 1976) with the introduction of vitreous antireflection coatings of T a 2 0 5 or Nb20, prepared by thermal oxidation of the corresponding vacuum deposited metals. The greatly improved transparency vs. wavelength of such films is shown in Fig. 5 by curves 1 and 2, respectively. The behavior of vacuum deposited tantalum oxide (Ta,O,) coatings is not nearly as ideal (see curve 3 of Fig. 5). The high transparency of the new coatings down to short wavelengths minimizes the absorption of ultraviolet radiation which might cause darkening. Experimental studies (Fogdall and Connaday, 1975) of T a 2 0 5coatings under a simulated space radiation environment (ultraviolet, protons, and electrons) indicate a high degree of stability. The requirements for best optical matching dictate a coating refractive index equal to the geometric mean of that of the silicon and of the cover slide. (The index of the adhesive matches that of the cover slide.) Since the index of silicon at the wavelength at which the solar spectrum peaks is about 4 and the index of the SiO, cover slide is 1.46, the matching index of the coating is 2.4. Properly prepared films of Ta,O, and Nb205exhibit a refractive index of 2.24 and 2.40, respectively. The index of vacuum-deposited Ta,O, is lower (2.14) and therefore results in somewhat poorer matching. The index of SiO, (x 2 1) is only 1.9, which is close to the optimal matching condition between silicon and air. As a result, cell response decreases on affixing the cover slide if SiO, is used and increases if either Ta,O, or Nb,05 is employed as the antireflection coating. The ohmic back surface contact in conventional cells represents an excellent sink for photoelectrons produced in the substrate and a correspondingly good source of thermally generated electrons contributing to the dark saturation current I , , if the substrate thickness W is not large compared to the diffusion length L, in the substrate. Because of the wish to minimize the mass of solar cells for space use, the condition W 9 L,, is usually not fulfilled at beginning of life. Therefore, some loss in sensitivity to deeply penetrating red and near infrared radiation and some degradation in open-circuit voltage result. Replacement of the ohmic contact by a player much more highly doped than the substrate introduces a retarding field at the back contact, which tends to “reflect ” photoelectrons to the junction for more complete collection (Wolf, 1970) and also improves the open-circuit voltage (Iles, 1972) by reducing the value of I , . Such a back contact with aluminum doping is employed in the violet cell. Typical current-voltage characteristics of the violet cell are contrasted with those of the conventional cell in Fig. 6 (middle and lowest curves,

SILICON SOLAR CELLS FOR SPACE USE

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\,

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1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 VOLTAGE (mV)

FIG.6. Current-voltage characteristics of CNR, violet, and conventional cells under AM0 illumination. Cell area, 4 an2.From Rittner (1976).

respectively). The short-circuit current, the open-circuit voltage, and the fill factor are all higher in the violet cell, so that the starting output power and efficiency are higher by about 30%. Another improvement in the cell has been the use of new contact materials (Cr-Au-Ag) with low moisture sensitivity. None of the percent gains of the violet cell relative to the conventional cell are lost on exposure to space radiation (Arndt, 1974).This may be seen with reference to the dashed spectral quantum yield curve in Fig. 4 and to the middle and lowest curves of Fig. 7, which show the maximum power output as a function of 1-MeV electron fluence. Degradation in power output occurs owing to a decrease in minority carrier lifetime in the base of the cell, which prevents photocarriers produced by deeply penetrating light from reaching the junction. This occurs in the conventional cell as well. The rate of falloff with fluence of the output power of the violet cell is perceptibly higher than that of the conventional cell owing to the lower substrate resistivity. Nevertheless, the efficiency advantage of the violet cell relative to the conventional cell persists throughout the cell lifetime for typical communications satellite missions. Two independent satellite test flight results (Statler and Treble, 1974; Goldhammer, 1975) have confirmed the high efficiency of the violet cell under space conditions over a period of the order of a year.

E. S. RITTNER Ib

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The violet cell has been fabricated in substantial quantities, extensively tested, space qualified, and licensed* for manufacture in the United States. The impact of this new technology will be felt in satellite programs of the near future.

Although the antireflection coatings developed for the violet cell represent a substantial improvement over conventional coatings for use with cells of wide spectral sensitivity, they are still of the quarter-wavelength tuned variety. Hence, on both sides of the tuned minimum the reflectance rises rapidly with increasing and decreasing wavelength as shown by the dashed curve in Fig. 8. Correspondingly, the reflection loss averaged over the entire spectral region to which the violet cell is sensitive is about 7%. This reflection loss has been virtually eliminated in the secondgeneration improved silicon solar cell, the COMSAT nonreflective (CNR) cell (Haynos et al., 1974). This has been accomplished by sculpturing the surface of the silicon via selective etching, which results in microscopic tetrahedra of high density and uniform distribution, as shown in the scan-

* OCLI, Photoelectronics Group, 15251 E. Don Julian Way, City of Industry, California 9 1746.

SILICON SOLAR CELLS FOR SPACE USE 80

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FIG. 8. Reflection as a function of incident light wavelength for covered violet cell and CNR cell. From Rittner (1976).

ning electron photomicrograph of Fig. 9. Since the reflection coefficient is less than unity, the resulting multiple interactions of light with the surface produce a reflectance that is low and independent of wavelength over the entire region of cell response, as shown by the solid line in Fig. 8. (Indeed, nearly all of the observed reflectance is attributable to the refractive index mismatch of the magnesium fluoride on the top surface of the cover slide and to the metallic grid contact to the silicon.) There are other benefits deriving from the surface preparation (Arndt et al., 1975; Meulenberg et al., 1975), which may be seen by referring to the schematic of Fig. 10. In this figure an incident light ray normal to the plane of the surface is traced through two reflections and refractions. In addition to the reflection suppression, the light is bent far from the incident direction, thus lengthening the light path and increasing the effective optical absorption coefficient. In addition, photocarriers are generated closer to the junction. These effects result in reduced transmission losses and decreased radiation degradation. The current-voltage characteristics under simulated space sunlight of CNR cells at the present state of development are shown as the uppermost

52

E. S. RITTNER

FIG.9. SEM photomicrograph normal to CNR surface.

curve of Fig. 6. The maximum power output is 85 mW, corresponding to an efficiency of 15.6% (AM0 standard spectrum; total area, 4 cm’). The maximum power output as a function of 1-MeV electron fluence is shown as the uppermost curve in Fig. 7. The substantial power output advantage of the CNR cell relative to the violet and conventional cells persists over the entire fluence range studied.

SILICON SOLAR CELLS FOR SPACE USE

53

FIG. 10. Optical path in selectively etched bare silicon surface.

A flight test of the CNR cell is scheduled for December 1976 aboard the Navigational Technology Satellite, NTS-2; and an array for a later flight test on UK-6 is in preparation. IV. SUMMARY A major reduction in the junction depth of the n / p silicon solar cell has resulted in a cell with enhanced blue-violet response. This has necessitated changes in the contacting grid geometry to reduce contact resistance and development of a new antireflection coating with higher index of refraction, less absorption, and radiation-resistant properties. The combination of these innovations together with a fivefold higher doping level in the substrate and the use of a highly doped p-type back contact resulted in the violet cell with a 30 7; improvement over typical satellite solar cells. Subsequently, the development of a reproducible chemical etching technique produced a nonreflecting surface comprising a uniform distribution of microscopic tetrahedra of high density. The combination of this surface preparation together with elements of the violet cell technology provided the basis for the development of the nonreflective cell, which exhibits a 50 % improvement in power over the conventional satellite cell, In both of these improved cells the gains relative to conventional cells are not lost to ultraviolet or penetrating space radiation. All of the above new technologies are rapidly being assimilated into the practices of the major solar cell manufacturers. ACKNOWLEDGMENTS The writer wishes to thank Messrs. .I. F. Allison, R. A. Arndt, and A. Meulenberg, Jr. for their help and T. D. Kirkendall for providing the SEM photomicrograph of the surface of the CNR Cell.

54

E. S. RITTNER

REFERENCES Arndt, R. A. (1974). COMSAT Tech. Rev. 4, 41-52. Amdt, R. A., Allison, J. F., Haynos, J., and Meulenberg, A. (1975). Con/: Rec., I E E E Phorouoltaic Spec. Con$, 11 th, 1975 pp. 40-43. Chapin, D. M., Fuller, C. S., and Pearson, G . L. (1954). J. Appl. Phys. 25, 676. Fogdall, L. B., and Connaday, S. S. (1975). “Irradiation of Thermal Coatings,” Final Report, INTELSAT Contract CSC-IS-556. Boeing Aerospace Company, Seattle, Washington. Goldhammer, L. J. (1975). IEEE Trans. Aerosp. Electron. Syst. aes-11, 1170. Haynos, J., Allison, J., Arndt, R., and Meulenberg, A. (1974). Con/: Proc. Int. Con/: Photouoltaic Power Generation, 1974 pp. 487-500. Iles, P. (1972). ConJ Rec., I E E E Photouoltaic Spec. Conf., 9th, 1972 p. 1. Lindmayer, J.,and Allison, J. F. (1972).Con/: Rec., I E E E Photouoltaic Spec. Con$, 9th. 1972 p. 83. Lindmayer, J., and Allison, J. F. (1973). COMSAT Tech. Reo. 3, 1-21. Meulenberg, A., Arndt, R. A., Allison, J. F., and Haynos, J. (1975). Con$ Rec., I E E E Photouoltaic Spec. ConJ, 11th. 1975 pp. 20&208. Revesz, A. G. (1973). COMSAT Tech. Rev. 3, 449-452. Revesz, A. G., Allison, J. F., and Reynolds, J. H. (1976). C O M S A T Tech. Rev. 6, 57-69. Rittner, E. S. (1954). Phys. Reu. 96, 1708. Rittner. E. S. (1976). I n “Astronautical Research 1974.”Pergamon, Oxford. Statler, R. L., and Treble, F. C. (1974). Conf. Proc. Int. Con$ Photouoltaic Power Generation, 1974 pp. 369-377. Wolf, M. (1970). ConJ Rec., I E E E Photouoltaic Spec. Con/:, 8th, 1970 p. 360.

Recent Applications of Electron Spectroscopy D. BERENYI lnstitute of Nuclear Research ofthe Hungarian Academy of Sciences Debrecen, Hungary

I. Introduction,. ............................................... ..................... A. The Present Scope of Electron Spectroscopy B. Origin and Development ..................... ............................... C. Basic Concepts ........................................................... D. Classification of the Field ....... ............................................ 11. Instrumental Techniques ............................................. A. General Arrangement ................................................................. ........................ B. The Electron Spectrometer.. .............................. C. Samples and Irradiation.. ................................. D. Control and Data Handling .... .............................. E. Calibration ................. ............................... F. Sampling Depth ........... ................................................... ........................ 111. Observables in Electron Spectr

................................................................... B. Line Shift, Linewidth, and Line Shape ............................................... C. Line Splitting and Satellite Structure.. ............................................... D. Angular Distribution .................................................................. E. Photoionization Cross Section.. ...................................................... IV. Main Fields of Application ............................................................... A. Molecular Orbitals .................................................................... B. Solid State Structure ..................................................................

................................................................... D. Structure of Chemical Compounds and Biological Systems ........................ E. Other Fields of Application ............................................. V. Electron Spectroscopy in Solution of Practical Problems .............................. VI. Conclusions and Perspectives ............................................ References ..................................................................................

55 56 58 60 62 67 67 69 70 70 71 74 77 78 80 80 87 90 94 101

105

I. INTRODUCTION A . The Present Scope of Electron Spectroscopy

Spectroscopic study of the electrons emitted in radioactive decay was initiated at the beginning of this century. In past decades a vast amount of data has been collected on the continuous (beta) and monoenergetic (internal conversion) electron spectra of radioactive nuclei; this has helped, pri55

56

D.

BERBNYI

marily, to clarify the details of their decay schemes. The instrumental techniques employed in the actual measurements were very different, but the main equipment was usually some sort of magnetic spectrometer. The energy range of these electrons extends generally from several tens of keV to several MeV. In the past decade and especially in recent years there has come into prominence the investigation of lower energy electrons in the region from several keV down to several tens of MeV. These electrons have very different origins: Auger electrons, X-ray and uv ejected photoelectrons, electrons generated by electron irradiation, etc. The goal of these studies at first was to obtain information about atoms, molecules, solid state structure, and surfaces of various materials. The most important equipment for these different studies is the electrostatic electron spectrometer. Today the scope of electron spectroscopy is much broader than it was in the past. From a special branch of nuclear physics it has become a rather general and useful method, not only in different branches of science but also in solving numerous tasks and problems in everyday life, e.g., in connection with catalysis in industry, with corrosion, with environmental pollution, with the production of new medicines, etc. The application of electron spectroscopy in newer fields and in the solution of new problems is a continuous and broadening process even today.

B. Origin and Development We have seen the broadening and the interdisciplinary character of the present scope of electron spectroscopy. As the scope has broadened, the number of published works in relevant fields has continually increased and has now attained the rate of several hundred papers per year. Since October 1972 a new periodical has been devoted to electron spectroscopy in the broad sense discussed above, entitled Journal of Electron Spectroscopy and Related Phenomena (Elsevier, Amsterdam). Originally published bimonthly, this periodical has been published monthly since October 1973. Analytical Chemistry publishes a fundamental reviews issue every two years. Electron spectroscopy appeared for the first time in the special 1972 issue, in two parts, with references to 325 papers altogether. The reviews section in the 1974 issue contains 840 references. Numerous international conferences on the applications of electron spectroscopy have been held : London, England, 1969; Uppsala, Sweden, 1970; Oxford, England, 1969; Asilomar, USA, 1971 ; Brighton, England, 1972; Namur, Belgium, 1974; and scientific meetings in other fields have dealt partly with these topics. The above facts illustrate the rapid development of the field in recent

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

57

years. In this connection, a question emerges concerning the origin of this so successful method. Effects from the chemical composition of the target appearing as an energy shift of the photoelectrons ejected by X rays were first observed by Siegbahn and his group in Uppsala (Siegbahn et al., 1958; Nordling ef al., 1958; Sokolowski er al., 1958). Such observations were made possible due to the development of high precision beta-ray spectroscopy (10- precision, resolution by this group in the 1950s). In addition to developing the techniques of precision beta-ray spectroscopy, a very important step was made toward different applications of electron spectroscopy by the observation that a fraction of the photoelectrons of several keV could come out of the target material without any energy loss and produce a sharp line in the electron spectrum superimposed on the continuous background. Before finding the chemical effect, the Uppsala group had obtained precise data on atomic binding energies on the basis of the above techniques using observations of the sharp photoelectron lines. Another source of current applications of electron spectroscopy is found in the ionization studies of the outermost electrons of atoms and molecules by electron impact and photoionization started by Turner and Al-Joboury (1962), Al-Joboury and Turner (1963), and Terenin and co-workers (Vilesov et al., 1961), independently. C. Basic Concepts

The conceptual basis for the application of electron spectroscopy in diverse fields and problems rests on the changes induced by the atomic environment of an atom, i.e., the molecular, solid state, etc. environment, which changes affect the different valence states of the atom in question, etc., and thereby the binding energy of the electrons in every shell to some extent. This in turn alters the kinetic energy of the electrons ejected from the atomic shells by external irradiation of X rays, ultraviolet light, or charged particles (electrons). The change in the binding energy is very small (at most about 10 eV, and in general only several eV), and that is why only the advent of high precision electron spectroscopy made the observation of such effects, and so the broad application of electron spectroscopy, possible. One should emphasize here that a change in the atomic environment changes not only the binding energy of the electronic shells in the atom concerned, but also the lifetime of the electronic states of the atom as well as the angular distribution (correlation)and the spin polarization of the ejected electron. Not only are these effects expected in principle but they are also useful in concrete applications (see Sections I1 and 111). An overwhelming majority of the applications, however, use the shift in the atomic binding

58

D. BERbNYI

energies and so-in the case of X-ray or ultraviolet irradiation-the equation in these applications is the photoelectric equation

basic

where Ekinis the kinetic energy of the ejected photoelectron, Eirrthe energy of the irradiating X-ray or ultraviolet quantum, and Ebinthe binding energy of the electronic shell of the atom concerned. In the case of a gas or vapor sample and X-ray irradiation, the above equation is valid without any corrections. If, however, such a sample is irradiated by ultraviolet light, Ebininvolves at least two parts, namely the ionization potential at the nth molecular orbital I , and an Evibterm, characterizing the vibrational energies of the ion. With a solid electrical conductor sample, an additional -A+ term is included in the equation in addition to the Ebinterm, due to the presence of the work function of the ejected electron from the solid sample. This A$ term is constant for a given sample in a spectrometer. The presence of the -A+ term necessitates a calibration procedure in the actual measurements, or the measurement of a relative shift instead of the binding energy. If the solid sample is an insulator, charging-up effects also appear. All these problems are discussed in detail in Section I1,E. In the applications treated above the most important and central feature of the whole apparatus is the high precision attainable with electrostatic electron spectrometers operating in the energy range from several eV to several keV. A high vacuum of at least 10-7-10-8 torr is maintained inside the spectrometer. The sample can be in gaseous, vapor, or solid state, and recently in liquid form as well. It is irradiated by X rays from an appropriate X-ray tube, by ultraviolet light from a discharge lamp, or by electrons from an electron gun. The electrons ejected from the sample are detected by an electron multiplier after energy analysis and focusing. The whole experimental arrangement involves data handling, scan control systems, and data output units (cf. Section 11,D).

D. ClassiJication of the Field As has been mentioned, the present scope of applications of electron spectroscopy is very broad with regard not only to the different types of problems considered (e.g., structural chemistry, surface studies, molecular orbitals, etc.) but also to applied procedures and techniques. Nowadays the ESCA (electron spectroscopy in chemical analysis or application) abbreviation is used more and more to designate the whole broad field of the applications of electron spectroscopy. It follows that a

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

59

possibly more correct new interpretation of the ESCA abbreviation (introduced by Siegbahn and associates in 1967) would be electron spectroscopy concerned with applications. Originally, ESCA meant the spectroscopy of photoelectrons ejected by X rays and it is used with this meaning frequently even now. Throughout this paper the abbreviation ESCA is used with the broadest meaning. The specific abbreviation in the literature, however, for the spectroscopy of X-ray ejected photoelectrons is XPS (X-ray photoelectron spectroscopy) or more rarely XRPS or X-ray PES. Similarly, for the spectroscopy of electrons ejected by ultraviolet light, the most usual designation is UPS (ultraviolet photoelectron spectroscopy) or, more rarely UV-PES, VUV-PES (vacuum ultraviolet photoelectron spectroscopy), or simply PES. There are some other abbreviations that have been introduced but they are used very rarely or not at all, as, e.g., IEE (induced electron emission) or PESIS (photoelectron spectroscopy of inner shells), chiefly for X-ray ejected photoelectron spectroscopy, and PESOS (photoelectron spectroscopy of outer shells) or MPS (molecular photoelectron spectroscopy). The present scope of the applications of electron spectroscopy, however, in a broad sense contains not only XPS and UPS but also Auger electron spectroscopy (AES) and different types of secondary electron spectroscopic methods, e.g., ELS (energy loss spectroscopy), REEL (reflection-electron energy loss spectroscopy),TEEL (transmission-electron energy loss spectroscopy), etc. It is worth mentioning that there are complete instruments with which the XPS, UPS, AES, and secondary electron spectroscopy studies can be carried out after some change, e.g., in the irradiation unit. To conduct XPS and UPS investigations with the same electron spectrometer i s quite common. As regards the Auger electron lines produced by filling vacancies after photoelectron ejection, they appear in the XPS spectrum among the photoelectron lines. Thus it is quite understandable that the review on XPS in the special review issue of Analytical Chemistry mentioned above (Section 1,B) also contains a survey of Auger electron spectroscopy (Hercules and Carver, 1974). Nowadays, however, it is sometimes helpful to differentiate between X-ray excited Auger electron spectroscopy and that excited by electron irradiation, both of which are generally referred to by the abbreviation AES. The potentialities, techniques, and evaluaticns are somewhat different in the two kinds of AES. In recent years many excellent surveys have been published on the whole ESCA field or some part of it (e.g., Siegbahn er al., 1967, 1968; Turner et al., 1970; Baker and Betteridge, 1972; Carlson, 1972; Siegbahn, 1972a,b, 1974; Bremser, 1973; Swartz, 1973; Holm, 1973a; Hagstrom and Fadley, 1974;

60

D. BERENYI

Price, 1974; Drummond et al., 1974; Berenyi, 1974). For further information on AES one can suggest the surveys of Harris (1968a,b, 1974), Ramsey (1971), Burhop and Asaad (1972), Ignatiev and Rhodin (1973), Chang (1975); and with regard to the secondary electron spectroscopic methods the following papers: Daniels et al. (1970), Hengehold and Pedrotti (1972a,b), Bauer (1972), and Luth and Jussell (1974). The goal of the present paper is to show the most important and substantial results and potentialities of electron spectroscopy in its varied applications with special attention to recent achievements in applications and techniques. It is also an important task here to point out trends and the expected direction of future developments. Even under such limitations it seems nearly impossible to treat every part of the whole field with equal emphasis. That is why XPS will be the main subject of the survey, including UPS, but with only occasional references to AES and to secondary electron spectroscopic methods. As for the actual organization of the subject matter following this introductory section, the instrumental techniques and the experimentally observable parameters in the course of applications of electron spectroscopy are summarized in Sections I1 and 111. In Section IV the main fields of application (molecular orbitals, band structure of solids, surface research, structure of chemical compounds, and biological systems, varia) are surveyed. A separate section is devoted to practical problems (environmental studies, catalysis, etc.) solved by ESCA, and finally conclusions are drawn in Section VI. 11. INSTRUMENTAL TECHNIQUES A . General Arrangement

In Section 1,C the main components of an ESCA apparatus (ESCA used here in its most general meaning) have been mentioned. For proper orientation in this section on instrumentation, refer to the schematic block diagram of the apparatus given in Fig. 1. As can be seen, the electron spectrometer (usually of the high precision electrostatic type) is the most important component, and various additional units are peripherally attached to it. It also has been mentioned that a vacuum of 10-6-10-8 torr is needed in the vacuum chamber of the spectrometer itself. A much higher vacuum here would be unnecessary because there is little effect on the focused electrons by collisions with the residual molecules of air even a t these values of the vacuum when the real size of the spectrometer is taken into consideration. However, for physical effects on the sample-especially in surface investigations-a much higher vacuum is desirable. Sometimes even a

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

CHAMBER FOR SAMPLE PREPARATION

k

SAMRE

ELECTRON

61

ELECTRON ETECTOR

CHAM-

BER

-%?RADIAION JNI T

-1-

-PUMPING SYSTEM

1

[OUTPUT

FIG. 1. A block scheme of ESCA equipment.

lO-’-lO- lo-torr value is not enough, and especially the partial pressure for the so-called active gases (e.g., CO, 0, , H,O) must be very low (10- 12-1014 torr). On the other hand, when a gaseous or vapor sample is used, the vacuum in the vicinity of the sample will be much lower than necessary in the spectrometer. That is why, in many cases, a separate sample chamber is constructed in ESCA equipment using differential pumping. The only connection between the sample chamber and the vacuum chamber of the spectrometer is via the entrance slits, which let in the electrons to be focused. To transport the electrons from the sample to the slits and to improve the resolution, an electron optical system is needed in some cases. Sometimes it is also useful to have another separate vacuum chamber for preparation of samples by, e.g., vacuum evaporation, or for cleaning the surface of the sample by argon ion bombardment. At the same time this also offers the possibility of storing some sample standard surfaces in this chamber. It should be emphasized here that the actual arrangement of the sample chamber or the so-called test chamber is different in various instruments. There are cases in which there is only one differentially pumped sample chamber where all the preparing and purifying tasks are carried out. In other cases a separate chamber of the spectrometer is added for the preparation and treatment of the sample, but during the actual measurement the sample is transported into the spectrometer chamber itself. Finally, it is possible to have three separate chambers as is schematically indicated in Fig. 1. The pumping system of an ESCA apparatus may employ one ion getter pump or turbomolecular pump, a cryopump, and the necessary forepumps.

62

D. BERENYI

The main additional components of the ESCA apparatus (as indicated in Fig. 1) will be discussed in separate subsections in this section, namely the electron spectrometer, the sample preparation and irradiation units, and the control and data handling units. Similarly, some fundamental problems of ESCA techniques, such as calibration, sample charging-up, and escape depth, will be surveyed separately. The whole of Section I11 will be devoted to a discussion of the observables, i.e., the measurable parameters, in ESCA. Finally, one must point out that special review papers have been published on instrumentation and experimental techniques in ESCA, such as Bank et al. (1972), Wannberg et al. (1974), and Gelius (1974). In addition, the general survey papers referred to above also contain instrumental sections. B. The Electron Spectrometer

Electron spectrometers with an iron core were used in the ESCA field only in exceptional cases. The inhomogeneities in the core make it unsuitable for work in the energy region relevant to ESCA measurements (several tens of eV at ultraviolet excitation and several keV at X-ray excitation). Magnetic spectrometers without iron, however, played a very important role in the development of the ESCA method and are still used in a limited number of cases even now [an example of a modern magnetic spectrometer for ESCA applications is that of Fadley et al. (1969a, 1972)]. The disadvantage of magnetic spectrometers is their high sensitivity to the earth’s magnetic field and to any external magnetic disturbance. These interfering and disturbing magnetic fields can be compensated for by a rather complicated Helmholtz coil system. Magnetic shielding of the electrostatic spectrometers is a rather simple task which can be solved by using soft magnetic materials around the spectrometer. This is certainly one of the main reasons for the increasing significance, and exclusive use, of electrostatic spectrometers in ESCA. Incidentally, it is worth remarking that all the commercial equipmentwithout exception-is of the electrostatic type. There are many versions of electrostatic electron spectrometers and numerous combinations of the various versions, about which we cannot go into detail here. One can differentiate between two large classes of electrostatic spectrometers, namely energy dispersive and nondispersive spectrometers. In the nondispersive type instrument the energy of the electrons is determined by changing the value of a retarding field produced by a system of electrodes and grids. In the group ofenergy dispersive systems, the two most characteristic types are the spherical condenser and the cylindrical mirror (or coaxial cylinder) spectrometers. They are schematically shown in Fig. 2. In the case of the simple retarding field spectrometer the luminosity

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

(a)

(b)

63

(C)

FIG.2. Schematic drawings of the three main types ofelectrostatic electron spectrometers: (a) nondisperslve retarding field spectrometer, (b) spherical condenser; (c) cylindrical mirror or coaxial cylinder spectrometer.

(efficiency) is very high, but the operation is an integral one and the energy resolution is moderate. In some recent versions (e.g., J. D. Lee, 1972, 1973; Lindau et al., 1973), however, about I-eV linewidth was attained at a very high luminosity, i.e., a rapid collection of signals. The best precision and resolution are provided by the spherical condenser type electron spectrometer. Hemispherical, spherical sector, and cylindrical sector spectrometers all are versions of this type. However, manufacturing these spectrometers is rather difficult since one must produce spherical surfaces with a very high precision. The problem mentioned above is much reduced in the case of cylindrical mirror type spectrometers. At the same time their luminosity is much higher than that of spherical type instruments. Here, however, a focal plane does not exist as in spherical spectrometers, permitting the simultaneous detection of the electrons in a broader energy range using a suitable detector system. The detector in electrostatic electron spectrometers is usually a so-called channel multiplier or channeltron. If the spectrometer has a focal plane, then a multichannel detector system (the so-called multichannel plate detector) can be used to detect a region of the spectrum at the same time. Recently, numerous papers on new designs for spectrometers of both spherical and cylindrical mirror types have been published, e.g., Citrin et al. (1972), Brundle et al. (1974), Betteridge et al. (1974), and Palmberg (1975). A very important improvement introduced for dispersive electrostatic spectrometers is preretardation of the electrons before entering the spectrometer field, suggested by Helmer and Weichert (1968). Specifically, in electrostatic spectrometers the A E / E ratio is constant; and so, if the analyzing voltage E is lower at the spectrometer, the linewidth (FWHM, full width at half maximum) A E will also be smaller. In ESCA measurements the

64

D. BERENYI

absolute width of the lines is very important. It follows from the above that it is possible to operate the spectrometer at a relatively low and constant voltage and to use a changing retarding voltage to take the whole spectrum. Figure 3 shows the effect of various retarding and analyzing voltages on the linewidth. An additional advantage in using the above preretardation is that there is no need to correct for the efficiency of the detector as a function of the electron energy because the spectrometer is operated at a constant voltage and the electrons always enter the detector with the same energy. For some spectrometer types, preretardation increases the luminosity while keeping AE constant. I

1

1

I

I I

I

I

I

I

I

I

1

I

I

-

I

!

I 71 I\

,

I

26.5

27.0

26.5

27.0 E (eV)

26.5

27.0

'27.5

FIG. 3. The half-width (FWHM, full width at half-maximum) of a line in the electron spectrum, i.e., the absolute resolution of an electrostatic spectrometer for three different retarding and analyzing voltages for electrons of the same energy. The half-width is the greater the higher is the analyzing voltage (and the retardation correspondingly smaller). From Risley (1972).

C. Samples and Irradiation

It has been mentioned that gaseous, solid, and liquid samples can be studied by the ESCA method. Previously liquid samples were examined only by vaporizing or freezing. Recently, new methods have been worked out for the examination of a liquid beam and a liquid layer on the surface of a moving wire (Siegbahn and Siegbahn, 1973; Siegbahn, 1974). Material to be investigated in powder form can be made to adhere to the surface of cellophane adhesive tape or pressed into a metal mesh or simply onto the surface of a softer backing material. It is also possible to dissolve the sample material to be studied in a suitable solvent and place drops on a backing, and after drying to put this into the spectrometer.

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

65

For surface studies, the sample must be very pure. That is why the most suitable procedure is to produce the sample inside the vacuum of the ESCA apparatus. In these studies ultrahigh vacuum is also indispensable (see Section 11,A). For example, according to measurements carried out by the McPherson Instrument Corporation a carbon monolayer is formed on the surface of the sample after 16 hr at lO-"-torr partial pressure of carboncontaining gases. The surface of the sample can be purified by ion irradiation from an argon ion gun. As samples, gases and vapors are also used, as well as atomic and molecular beams. The cooling and heating of the sample can also be important. The temperature of the sample ranges from room temperature to several hundred degrees Celsius, in general. However, much higher sample temperatures have been produced, and cooling to the temperature of liquid nitrogen and lower has been attained (Bauer and Spicer, 1972; Brundle and Roberts, 1973). Ultraviolet light irradiation is performed using Geissler discharge tubes. The most frequently used ultraviolet exciting radiations are the two resonance lines of He, but sometimes other resonance lines are used as necessary (a group of these resonance lines is exhibited in Table 1). TABLE I RESONANCEEMISSIONLINESUSEDFOR IRRADIATION IN UPS Gas Hg Xe Xe Kr

H, (Lyman-a) Kr Ar Ar Ne Ne He He

Wavelength (A)

Energy (eV)

2537 1470 1296 1236 1215 1165 1067 1048 136 744 584 304

4.89 8.4 9.6 10.0 10.2 10.6 11.6 11.8 16.8 16.7 2 1.22 40.8 1

For X-ray irradiation the MgKcr and the AIKcr lines are generally used, but sometimes some other X-ray lines are also used (see Table 11). Yates and associates (1973) worked out a dual anode X-ray source, permitting either MgKa or AlKa irradiation by switching from outside the vacuum.

66

D. BERENYI

TABLE 11 X-RAY LINES FOR IRRADIATIONIN

XPS Line

Energy (eV)

YM, ZrM, MgKa AlKa Cr Ka CuKa

132.3 151.4 1253.6 1486.6 5415 8048

A problem with the ESCA method is the existence of an energy gap between the ultraviolet and X-ray exciting radiations used. Synchroton radiation may be used to fill this gap (cf.,e.g., Siegbahn, 1974, Chapter 6 ; Gelius, 1974). The energy resolution of an ESCA apparatus usually depends on three factors: (1) the natural width of the exciting radiation AE,; (2) the natural width of the level out of which the photoelectron is ejected AEl ;and (3) the instrumental resolution (linewidth) of the spectrometer AEi . Of these factors the first is normally the largest and primarily determines the final resolution of the apparatus (AE,) according to the formula

AE, = AEf

+ AEf + AE:.

It is for this reason that the resolution (linewidth) of UPS is about two orders of magnitude better (smaller) than that of XPS (= 0.01-1 eV). One must note, however, that UPS can be used only at the outer levels because of the limited excitation energy, and so this ultraresolution can be used in rather special cases. The outer atomic levels due to molecular or solid state effects are broadened in general, and the width of the bands are wider than the UPS resolution. To improve the resolution of XPS and to reduce in this area the disturbing background from continuous X radiation and X-ray satellites, monochromatization of the exciting X rays was introduced by Siegbahn and co-workers (1972). Currently, several different techniques for monochromatization are used (Siegbahn, 1972a). By monochromatization the energy width of the exciting radiation is reduced by a factor of about thrce, and ,o the total width of the XPS line becomes about half of that without monochromatization. With the best of such techniques the absolute width of the ESCA line could be reduced to 0.4-0.5eV. One should mention here that there are increasingly more such ESCA

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

67

apparatuses in which ultraviolet, X-ray, and electron irradiation can all be performed with the same equipment. Several studies (e.g., Kramer and Klein, 1972; Siegbahn, 1974) have been carried out on the effect of the exciting radiation on the sample. According to Siegbahn, a 20-hr X-ray exposure caused no change in the electron spectrum of Na,S,O, . The Kramer and Klein investigation proved, in the case of nonheme iron protein, that the X-ray dose necessary in XPS measurements is about an order of magnitude smaller than that needed for the decomposition of the compound concerned. D. Control and Data Handling

A special unit is needed to control the scan in the desired spectrum region according to a preset (interval between the spectrum points to be measured, measuring time at the individual points, repetition, etc.). Similarly, the signals from the channeltron or multichannel plate detector are to be stored, evaluated, and read out. Data smoothing, determination of the position and intensities of the lines, and deconvolution are the tasks here. A11 the above tasks for acquiring and processing data can be handled most suitably and flexibly with a small computer. Using a computer it is much easier, e.g., to measure for different times at the individual measuring points, to use adifferentdensity of measuring points, etc. during the automatic measuring cycle controlled by the computer according to a prescribed program. E . Calibration In using an electron spectrometer for ESCA measurements, the calibration of the spectrometer has emerged as one of the major problems. Although the customary practice is to calibrate the instrument using standard lines, an absolute calibration is also feasible (e.g., Johansson et al., 1973; Brundle et al., 1974). A tabulation of lines suitable for calibration by solid samples and another one by gaseous samples are given in Tables 111 and IV, from the paper of Siegbahn (1972b). Calibration, the determination of the actual shift values in metallic, and, even more readily, in gaseous samples, is relatively simple in principle. If the sample is an insulator, however, a charging-up occurs, which gives rise to a shift of the line. To eliminate or correct for this disturbing effect different procedures have been used. One of them is to deposit noble metals, especially gold, on the sample surface in vacuo. Another possibility is to use the carbon contamination deposited from the vacuum system onto the

68

D. BERENYI

TABLE 111 BINDINGENERGIES OF SOMELEVELS SUITABLE FOR CALIBRATION USINGSOLIDSAMPLES' Level

Binding energy (eV)b

932.8 (2) 573.0 (3) 368.2 (2) 335.2 (2) 284.3 (3) 122.9 (2) 83.8 (2) 71.0 (2) 0.0 (1)

' From Sieghahn (1972b).

* Relative to the Fermi level. TABLE IV BINDINGENERGIES OF SOMELEVELS SLJITABLE FOR CALIBRATION USING GASEOUS SAMPLES' Level

Binding Energy (eV)

Ne 1s F Is (CF,) 0 1s (CO,) N 1s (N2) c 1s (CO,) Ar 2P3/2 ~ P W Kr 3 4 0 Ne 2s Ne 2p Ar 3p

870.37 (9) 695.52 (14) 541.28 (12) 409.93 (10) 297.69 (14) 248.62 (8) 214.55 (15) 92.80 (10) 48.47 21.59b 15.8Ib

From Siegbahn (1972b). Weighted mean value.

sample surface. Swartz et al. (1972) proved experimentally that a third method, internal mixing of the sample with an appropriate standard, is as effective as the deposition of gold on the surface. Recently, however, some doubts have emerged in connection with the

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

69

application of deposited noble metals on the sample surface on the basis of detailed microscopic investigations of the surfaces in these cases (Betteridge et al., 1973; Ebel and Ebel, 1974). With regard to the use of carbon contamination as a standard, it seems to be problematic for a different reason, i.e., because of its not uniquely defined chemical state. Thus, the use of an internal standard within the sample itself when it contains such a shifted component (line) seems to be the most advisable correction technique at present. F . Sampling Depth

One of the important issues in the ESCA study ofsolid samples is that of the sampling or escape depth. Mention has already been made (Section 1,C) of how important the escape of some photoelectrons without energy loss from a solid target was to the development of the ESCA method. Currently, the actual question is how deep is the layer from which ESCA information, i.e., photoelectrons, can come. As early as in the great ESCA book of Siegbahn and associates (1967), an experiment is described where iodine double layers are taken on a surface successively. According to this experiment, three double layers give a two times higher iodine 3d,,, photopeak intensity than one double layer and ten of them only three times higher. For quantitative characterization of sampling or escape depth in ESCA studies, the parameter A (escape depth) is defined according to the equation

where I , and I d are the intensities of an actual photoelectron line in the spectrum in the cases of an infinitely thick layer and one of thickness d, respectively. Thus, if the irradiated (by X rays or photons) layer thickness d is equal to A, then the intensity of the line in question ( I , ) is about twothirds of I = . As can be seen in Fig. 4 the escape depth A is the greatest a t the smallest energies (several eV) and it shows a minimum at about 100 eV. It approximates again the values that it has at several eV at the high electron energies of several keV (e.g., Holm, 1973b). The actual value of the escape depth depends on the energy of the electrons and the atomic number of the layers concerned. Many investigations of this phenomenon have been carried out recently. Figure 5 shows the importance of A at different photoelectron energies for gold and the fit to the data using the above exponential expression.

70

D. BERENYI

10

100

eV

1000

FIG.4. The approximate form of the function of the escape depth vs. electron energy.

0

A

€KIN 0

x

0

10

20

30

LO

50

9 ~ 0 e v 19 X 1403eV 26 8, 2671 eV 368

60

70

80 d ( 8 )

FIG.5. The intensity of a photoelectron line of gold as a function of the thickness of gold layer for three electron energies. From Klasson et al. (1972).

111. OBSERVABLES IN ELECTRON SPECTROSCOPY MEASUREMENTS

A. General Outline

First, we shall enumerate the most important measurable parameters from which information can be derived for various fields. As is well known, the basic types of spectroscopic measurements are those of the energy and angular distribution as well as some kind of time measurement, and of the spin state.

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

71

In fact, the area encompassed by measurable quantities in electron spectroscopy is much broader but it is derived from the above four basic types. Some of the most important measurable quantities are: Determination of the energy (or energy shift) of the individual lines in the electron spectrum Relative intensity of the lines in the spectrum; observation of the relative intensity of the spectrum lines as a function of time The linewidth and line shape; splitting of the lines of the electron spectrum due to different effects (e.g., multiplet splitting, electrostatic splitting, etc.) Satellite structure of the lines; angular distribution of the electrons ejected from the sample; variation of the photoionization cross section as a function of the excitation energy Spin-polarized states of the electrons The above list is not complete, of course. There are several further possibilities. As regards electron spectroscopic measurements in which time determination has a role (coincidences between radiations, time spectra), one can mention, e.g., the use of the coincidences between scattered and ejected electrons in electron energy loss spectroscopy (Van der Wiel and Brion, 1973) or the electron spectrometers applying time-of-flight techniques (e.g., Bachrach et al., 1975). While enumerating the observables, it is very important to point out that in most of the applications of electron spectroscopy to date measurement of the energy of the lines in the electron spectrum has been used (energy shift, determination of the ionization potential) even if other parameters were also studied. In this section, most of the above listed measurable quantities will be dealt with in separate sections: line shift and linewidth, as well as shape, line splitting and satellite structure, angular distribution, and the photoionization cross section. To close this section we mention that the utilization of measurements of the spin-polarized state of photoelectrons (e.g., Busch et al., 1970, 1972; Siegmann, 1975) as well as of the linear polarization ofthe exciting synchrotron radiation (e.g., Siegbahn, 1974, Chapter 6) has been started. B. Line Shift, Linewidth, and Line Shape In determining the energy of the bands in UPS spectra, the ionization potentials of several of the highest occupied levels are obtained and not only the least bound one as in the case of conventional methods (see Section IV,A). In the case of XPS and, in general, in that of solid samples, usually the

72

D. BERENYI

shift of the photoelectron lines (and not their absolute location in energy) due to the effect of the chemical (molecular) environment is measured because of the additive term in the photoelectric equation due to the different work functions of the sample and spectrometer, respectively. The actual value of the line shifts usually amounts to a maximum of 10- 12 eV (from the least measurable shift of approximately 0. I eV) in different compounds (Swartz, 1973). The region in which the shift occurs varies from element to element. The elements most appropriate for XPS studies are those in which this region of shift is the largest (e.g., C, F, S, Cl, etc.). In the survey paper of Carlson (1972) a table is shown where, among other items, the observed maximum shift is indicated. The interpretation of the shifts is one of the best methods for clarfication of the relevant chemical structure as well as for a check on the theoretical models for the molecular structure (cf. Section IV,B). The width of the line in the electron spectrum depends (Bremser, 1973) on the inherent energy spread of the exciting radiation (AEJ, on the natural width of the level involved (AE,), on solid state effects (AEJ, and on the instrumental resolution of the spectrometer (AEsp), namely approximately 6E2 = AE:

+ AE; + AE: + AW,Z,

In most cases the dominating factor is the inherent width of the exciting radiation. The actual value of this for XPS is 0.9 eV for AlKa and 0.8 eV for MgKa radiation, respectively. This is the reason linewidths in UPS are one or two orders of magnitude smaller than those in XPS (in practice, this very good resolution is attainable only with the most simple molecules, and above all with inert gas atoms because of the density and overlapping of the bands of the molecule). The most important reason for monochromatization of the exciting X radiation also arises here (see Fig. 6 ) . In the case of Auger spectra, the linewidth depends on the natural width of the levels involved. An increase in the linewidth and line shape in the course of the study of an actual sample can be, e.g., an indication for a small shift or line splitting (see below). The width of a line however, is related to the lifetime of the state out of which the photoelectron has been ejected (Allan and Siegbahn, 1971). Therefore, the measured linewidth contains strong direct information about the lifetime of the level concerned. It can be readily determined if the width of the exciting X ray and the instrumental resolution are accounted for using the above equation. Shaw and Thomas (1972) interpreted the changes in the measured linewidth of the 1s hole states for C, N, 0 as changes in the lifetimes of the states concerned caused by changes in the chemical environment. In a similar way Yin and Adler (1974) and other investigators also demonstrated the quasi-atomic behavior of atoms in solids (medium-Z elements). Recently,

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

73

Ag 3 d 3

Al K d FWHM= tM) eV

.

MgKd. FWHM=O,@,eV

NaKd FWHM.076 eV

MonochromatizedA l k . FWHM.0.6eV

I

l

l

I

I

I

I

I

-

kpacirg between lines-6e

K INETlCENE_RGY

FIG.6. The width of Ag 3d lines in the XPS spectrum using different exciting radiations. From Drummond et a / . (1974).

Citrin et al.’s measurements (1974) indicate that there are also significant nonlifetime contributions to intrinsic linewidth. The shape of the photoelectron lines can also be the subject of research. Hufner and Wertheim (1975b) observed, for example, the systematics of core line asymmetries in the core level spectra of Ni. Finally, we note that the different line-splitting mechanisms (see the next section) can also result in broadening of the electron spectrum line.

74

D. BERkNYI

C . Line Splitting and Satellite Structure The chemical environment can cause a shift in the binding energies of the electrons in the individual atomic levels and correspondingly a shift will appear in the photoelectron spectrum (see the earlier subsection). If the atoms of an element are in different atomic environments in the sample, a “multiline ” structure will come about due to the different shifts of the original line. There are Fweral other causes for multiline structure in the photoelectron spectrum; but in contrast to the shift, which is approximately equal at the different levels, other types of line “multiplication” are limited to one or several levels or at least markedly different at the different atomic and molecular levels. In considering the multiplication of lines, we d o not here treat such effects as, e.g., photoelectron peaks from satellite X rays or discrete inelastic processes, which are regarded as instrumental effects. In this section we intend to consider primarily two phenomena resulting in multiplication of the photoelectron lines, namely splitting and “ shake up satellite structures. Before going into the details of the two effects mentioned above, we briefly note certain other effects, namely, the vibrational structure of UPS lines (bands) characteristic of the level involved (see, e.g., Siegbahn, 1972b) and the so-called Jahn-Teller splitting of the molecular orbitals. In this latter case the effect is due to the induced geometrical instability of the molecule and the resulting distorted configuration when ionization occurs (see, e.g., in Jonas et al., 1972). Hiifner and Wertheim (1975a) suggested an interesting two-hole mechanism for the explanation of a specific satellite in Ni and some other metals. Line splitting due to isotope effects has also been shown, e.g., by Delwiche ec al. (1973) and Bergmark and associates (1974). The common basis for the different splittings of core levels to be dealt with is the photoelectric equation where the binding energy is equal to the difference between the energy of the initial ( E i )and final states (I&). The final state is an atom with a hole in one of the core levels, and the different possible couplings of the spin and angular momenta result in level degeneracy and consequent splitting of the line. One of the simplest splittings of this type is the well-known spin-orbit splitting when the spin of the hole can couple with its angular momentum in two possible ways. Such types of splitting were observable for the outermost d and p levels, primarily in the heavier elements since the size of the splitting approximately increases with atomic number. The observed value of the splitting is about 1-2 eV (Ley et al., 1974). The familiar form of splitting is multiplet or exchange splitting. In this ”

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

15

case the angular momentum of the unpaired electron or unfilled orbital ofa paramagnetic molecule (or to be more general, a many-electron system with open-shell configuration) splits the hole state depending on the coupling between this angular momentum and that of the hole state. In the simplest case the hole is in an s level, and so there will be only two final states according to spin-up or spin-down coupling (Hedman et al., 1969; Fadley, 1972). I f the hole is located in other than the s shell (p, d, etc.), a multiple splitting of the line occurs, and the situation becomes much more complicated. This can be analyzed by means of configuration interaction calculations because the final state is reached by coupling of different subshell configurations. Such an effect was observed, e.g., in the 3p shell of MnF, and FeF, (Fadley et al., 1969b). Multiplet splitting was first observed in paramagnetic gas molecules (02, NO) by Siegbahn and associates (1969). Figure 7 shows the corresponding lines in the electron spectrum. Since that time numerous multiplet

I

eV

Ok

545

N's

54l 415

L10

I

405

BINDING ENERGY

Flti. 7 . Magnetic splitting in the paramagnetic gas molecules. From Siegbahn rt a/. (1969).

76

D. BERENYI

splittings have been observed, not only for paramagnetic gas molecules, but for transition metals and rare-earth metals and their compounds. The actual value of the splitting amounts to 6-8 eV (see Wertheim et al., 1972; Berenyi, 1974; Hedman, 1974). Even for a particular type of atom the extent of splitting depends on the degree of delocalization of the unpaired electrons. That is why the splitting is different for the same atom and level in different compounds. Thus, splitting can give information on covalency in the actual bonding. Multiplet (or magnetic) splitting can be observed only in molecules with unpaired electrons or, in general, in many-electron systems with open-shell configurations. Novakov and Hollander (1968), however, found splitting (from 3 to 16 eV) for 5p,,, shells of various compounds of some heavy atoms (Au, Th, U, Pu), where the initial state has a zero spin, i.e., closed-shell, not open-shell configuration. (Such splitting was not observed for the neighboring 4f and 5d shells.) The reason for the phenomenon is the presence of a nonspherically symmetric electrostatic environment, i.e., crystal-field splitting (called electrostatic or quadrupole splitting), but an accurate quantitative analysis is currently lacking (Hollander and Shirley, 1970; Fadley, 1972). In the photoelectric process there is some probability for the simultaneous promotion of a second electron to an unfilled orbital (shake up) or to the continuum (shake off) together with the ejection of the “primary” photoelectron (Rosencwaig er al., 197 1 ;Allan and Siegbahn, 1971).In the case of shake up the energy of the photoelectron is given by the equation

E,

=

Ex = Eb - Ex

where Ex = Ebl - Eb2,the energy difference between the two levels involved for the second electron. As a result of the above phenomenon, a satellite line will be located on the low kinetic energy side of the “main” line (Fig. 8). Such satellite structures were found and studied in the noble gases (Siegbahn et al., 1569), for simple gas molecules (Allan er al., 1972), and for solid state samples involving possible transitions of the second electron between the valence and the unfilled conduction bands (Novakov and Prins, 1972; Okusawa er al., 1974). The intensity of the shake up satellite line relative to the main line seems to be greater for solid samples, amounting to 30% and more. The theoretical description of the phenomenon (energy intensity) is rather adequate, especially for the case of rare gases (e.g., Allan and Siegbahn, 1971; Carlson and Nestor, 1973).With respect to molecules and solid state effects the interpretation seems to be much less clear (Allan and Siegbahn, 1971; Okusawa et al., 1974; Hedman, 1974; Hillier and Kendrick, 1975).

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

77

c /800s

A

co, -01s

10000

300G1z % . J L (

1200s

CS,-Cls

4000

2000 1000

ci18CQs -

3000 -

cos- s2p

2 000 1000

0 - 20

-10

0

RELATIVE KINETIC ENERGY (eV)

FIG.8. Shake-up satellite lines in an XPS spectrum. The “main” line is located at 0 eV in the figure. From Allan et al. (1972).

The study of the shake up and shake off satellite structures yields important information about unfilled molecular orbitals and solid state band structures (Siegbahn, 1972b; Novakov and Prins, 1972).

D. Angular Distribution Measurement of electron spectra for different angles between the direction of the photon and the photoelectron also yields important information about molecular orbital or solid state structures. In some of the measurements the distribution of the photoelectrons emitted at a given angle with respect to the polarization direction of the exciting ultraviolet light was also measured. In other cases the spectra are taken simply as a function of the angle between the direction of the incoming photon and the outgoing electron.

78

D. BERENYI

There is currently increasing activity in this area, especially in the study of gas molecules using ultraviolet excitation photoelectron spectroscopy (Grimm, 1972; Carlson et al., 1972; Betteridge and Williams, 1974). X-Ray ejected photoelectron spectroscopy for solid samples coupled with photoelectron angular distributions or the measurement of angular correlations is much less frequently encountered. Comparison of the experimental angular distribution and the corresponding calculation offers a method for identifying the initial molecular orbital and for understanding the nature of the molecular interactions. Unfortunately there are very few calculations available to allow such comparisons (Berry, 1969; Carlson et al., 1972). Dill (1972) and Manson (1973) recently have done such calculations. Also recently, the electronic structure of the chemisorbed system of hydrogen on tungsten was studied by measuring the angular dependence of photoelectron spectra. O n the basis of this investigation, there is some indication of a two-dimensional band structure in the energy levels induced by hydrogen (Egelhoff and Perry, 1975). Another important utilization of measurements of electron spectra as a function of the angle between the photon and the electron in research of the outermost orbitals is that it helps in differentiating overlapping bands. If the shape of a band changes strongly when taken at different angles, it may indicate overlapping of different electronic bands (Carlson, 1972). The study of the solid state structure, or to be more precise, the crystal structure of a sample by measuring the distribution from a single crystal was first reported by Siegbahn and associates (1970) in the case of NaCl. Single crystals of metallic gold have been investigated by this method (Fadley and Bergstrom, 1972). Single crystals of GaAs were also studied by angular distribution of ultraviolet photon-ejected electrons (Wooten et al., 1972). E. Photoionization Cross Section

In a similar fashion as in dealing with the angular dependence of photoelectron spectra (treated in the previous subsection), measurement of spectra as a function of the exciting energy, i.e., determination of the photoelectric cross section at different energies also yields valuable information. In this way, one can not only separate the overlapping bands in the study of the outermost bands but can contribute to the clarification of the nature of the individual orbitals (Eastman and Kuznietz, 1971). Figure 9 shows that the cross section for s-type orbitals is high if the energy of the exciting energy is high. For p-type orbitals, the situation is reversed (Price ef al., 1972). The reason for this is clear on the basis of the energy dependence of the cross section for the different shells as shown in Fig. 10, measured by synchrotron radiation (Siegbahn, 1974).

2P

‘2

Ne

u PS

2s

A

I

Ne

x PS

I

I

100

130

I

I

30

10

i2’

eV

C2H6

2P I

I\

1230

I

1200

I

I

14EO

1460

UPS

I

1480

1460 eV

)”‘

H2S

01 b2 01 I

a1

1

I

~

I

30

I

10 eV

9s

x PS I

1

I

I

1240

1220 -ENERGY

I

I

1240

OF PHOTOELECTRON

1220 eV -

FIG.9. The photoelectron spectrum of various compounds in the valence region at different exciting radiations. From Price et a!. (1972). 10-24 rn2

1

He1

hu-

l hv=Al Kd I

I

10

20

50

100

loo0

m

PHOTON ENERGY (eV)

FIG.10. Calculated values of the photoionization cross section for the different shells as a function of the energy of the exciting photons. From Siegbahn (1974).

80

D. BERENYI

Knowledge of the photoionization cross section is also indispensable for the intensity evaluation of the spectra and for any quantitative analysis using electron spectroscopy (see Section IV,E). Combination of the ionization cross section and the angular dependence of the photoelectric cross section offers a powerful tool for investigation of molecular orbitals (Siegbahn, 1974). Activity is rather intensive now in this field, but at present both theoretical calculations and experimental data (especially at higher exciting energies) are still insufficient. Relevant theoretical studies were carried out recently by Lohr (1972) and Itikawa (1973) and experimental ones by Nefedov et al. (1975) in the X-ray region and by Schweig and Thiel (1973, 1974) in the ultraviolet region.

Iv. MAINFIELDS OF APPLICATION In this section we intend to survey the most important fields of application of electron spectroscopy. Naturally, such a survey cannot be either complete or precise with regard to classification of the different fields. In most cases the borderline is not a t all sharp, and different fields and experimental results overlap a great deal. At the same time, the perspectives of various classifications can also overlap. For example, while speaking about surface studies, chemical structure information is also automatically included; or as another example, data on the structure of chemical compounds are linked to information on molecular orbitals and to solid state bands. Separate subsections will be devoted to molecular orbitals, to the band structures of solids, to surface studies, to the structure of chemical compounds and biological systems, and finally to some other applications. A survey of some practical applications in environmental studies, catalysis, agrobiology, etc. will be the subject of a short separate section. The organizational pattern of this survey of the various fields will begin with a general characterization followed by some typical examples, but by no means will this be a complete summary or enumeration of all published papers. A . Molecular Orbitals

In the study of molecular orbitals by the method of electron spectroscopy, the most important information to be obtained is the determination of the binding energies (ionization potentials). The great advantage of photoelectron spectroscopy here is the possibility for determining the ionization potential for more than just the least bound occupied molecular levels,

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

81

which is the case in most of the corresponding conventional methods. The orbitals can be followed down to where the binding energy corresponds to the energy of the exciting ultraviolet radiation and even further down to the atomic levels when X rays are used. The fact that we can reach to the deeper molecular levels using X-ray excitation of molecular orbitals is only one of the advantages. The s-type orbitals are excited with a higher probability by X rays than the p-type ones. The situation is reversed in the case of uv excitation (see Section 111,E). In this way, the data obtained from X-ray and uv excitations are complementary to research on the character of the orbitals, due to the energy dependence of the photoionization cross section. The interpretation of the shift for inner shells observed in X-ray excited electron spectra (which is so important from the point of view of chemical structure investigations) also gives information about molecular orbitals. Calculation of the observed shift is possible on the basis of the bonding model assumed for the case at hand. To understand the nature of the individual orbitals one must analyze the information relating to a system of lines including relative intensities, location-distance, and their succession, etc. The group of lines in Fig. 11

t

21

20

19

18

17

16

15

14

13

12 eV

FIG. I t . The UPS spectrum of 0, excited by the 21.2-eV radiation of He. From Turner (1968).

represents vibrational structures belonging to individual orbitals. The width ofthe line or bands also gives us information on the nature and lifetime of the level. The very narrow bands, such as those on the left side of Fig. 11, correspond to nonbonding or very weakly bonding or antibonding orbitals (“lone pair” orbital). The Jahn-Teller splitting of the band shows the geometrical distortion of the molecule, while the multiplet splitting of the core lines is connected to

82

D.

BERBNYI

covalency and to the extent of delocalization and pairing of the unpaired electron. The angular dependence of the electron spectrum helps to deconvolute the overlapping bands, and comparison of the experimentally determined angular distribution pattern with the theoretically calculated one also elucidates the nature of the orbital. However, while using electron spectroscopy, we acquire information that is not only about filled molecular orbitals. In fact, in analyzing the satellite structure at the core levels from shake up and shake off phenomena, the characteristics of unfilled molecular levels are studied. As can be seen, the possibilities for investigation of the molecular orbitals and their characteristics are very rich within the framework of electron spectroscopy. These possibilities are very important from the point of view of quantum chemistry because they permit experimental testing of calculations. That is why there is a rather strong interaction at present between calculations in quantum chemistry and experimental studies in electron spectroscopy. They mutually stimulate one another. There are several good papers summarizing the results and possibilities of electron spectroscopy in research of molecular structure including relevant theory, e.g., Turner et al. (1970), Baker and Betteridge (1972), Worley (1971), and Allan and Siegbahn (1971). Bock and Ramsey’s (1973) and Siegbahn’s (1975) papers are devoted to summarizing theories for the interpretation of photoelectron spectra in their entirety. Some other papers deal only in part with a survey of the theoretical models concerned, e.g., Siegbahn (1972a,b, 1974), Albridge (1973), and Holm (1973a). To interpret the measured data (ionization potentials, intensity relations, linewidths, etc.), sometimes semiempirical calculations (as extended Huckel, CNDO, INDO), and sometimes ab initio LCAO Hartree-Fock formalism are used. In the latter case we cannot identify the H-F eigenvalues with the measured ionization energy (such an identification would be based on Koopmans’ theorem). In the exact theory calculations must be performed both for the molecule and for the ion using the photoelectric process, and the difference between the corresponding energies will give the value to be compared with experimental data for the ionization potential. In addition, however, for the most accurate calculations the electron correlation (configuration interaction) should be taken into account. Because of the time- and money-consuming character of the ab initio calculations (large computers, long computational time periods) a new approximation, the so-called Xa method, has been worked out; it is more precise than the above-mentioned semiempirical calculations (cf. Fig. 12), while the claimed computational time is about two orders of magnitude less than that for the ab initio calculations; the effect of the ionic state after the electron ejection as well as the electron correlation is taken into considera-

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

SCF-U-SW

ESCA

CNDO

83

XF-LCAO

20 -

30 -

50

FIG. 12. Comparison of the experimental values of the ionization energies of the orbitals for SF, with the results of different theoretical calculations. From Siegbahn (1972a).

tion in the approximation. With the Xu method, calculations for more complex molecules seem to be feasible, while these are practically impossible using the ah initio approach. Recently, numerous calculations have been performed for concrete cases (e.g., Klasson and Manne, 1972; Kosmus er al., 1973; Hopfgarten, 1973; Almhof, 1973; Worley et al., 1973; Kellerer et al., 1974; Levy et al., 1974; Bodor et al., 1974; Adams, 1974; Baerends et al., 1975). Most of the calculations, however, refer to the determination of the ionization potential, and there are many calculations missing (see Section 111) for other measurable parameters, such as, e.g., intensity relations, photoionization cross sections, angular distribution patterns, satellite intensities. There are, however, some calculations of this type too (e.g., Gelius, 1972; Huang et a/., 1974; Hillier and Kendrick, 1975). As for experimental results, a lot of measurements and comparison of them with the relevant theoretical calculations have been carried out in recent years using the techniques of both XPS and UPS, primarily for determination of ionization potentials of molecules.

84

D.BERBNYI

Even recently there have been studies done on simple molecules (e.g., on NO,, Natalis et al., 1971;on carbon suboxide, Rabalais et a!., 1972;on HBr and DRB, Delwiche et al., 1973;on XeOF,, Brundle and Jones, 1973;on some relatively simple silicon compounds, Perry and Jolly, 1974),but the trend is toward more intensive investigation of complex molecules and especially of series of related compounds. By this means, the analysis of the spectra, i.e., the identification of the individual bands, is easier, more reliable, and straightforward. Some recent studies of the molecular structure of complex and polyatomic compounds are: for perfluorodiazines, by Suffolk (1974);for 9,lOdihaloanthracenes, by Streets and Williams (1974);for tricyclooctenes, by Bishof et af. (1974);for acetaldehyde and acetyl halides, by

W I-

4

[L

k-

z 3

8 W

2

c 4

10

12

14

16

18

20

ELECTRON VOLTS

FIG. 13. UPS spectra of some related carbonyl compounds. From McGlynn and Meeks (1975).

85

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

Chadwick and Katrib (1974); for fluorine-substituted diabensenes, by van der Ham et af. (1974), for amine N-oxides, by Wieczorek et al. (1975); for butenyne and monomethyl-substituted butenynes, by van Hoorn (1975); for 2-phosphanaphthalene, by Schafer et al. (1975); and for 1,2-dithiole-3thiones, by Gonbeau et al. (1975). Figure 13 shows the photoelectron spectra of some related compounds, and Fig. 14 shows schematically how the analysis of the spectra (the identification of the bands) takes place. Finally, it should be mentioned that studies of the core and valency levels are frequently carried out at the same time by XPS examinations (e.g., Clark et al., 1972). Research on the orbital structure of ions, radicals, and other transient species is gaining more and more ground. Jonathan et al. (1972) among others summarized their results at the Asilomar conference on transient species, on atomic oxygen, nitrogen, hydrogen, and halogens. More recent papers on atomic and ionic oxygen, respectively, were published by Samson and Petrosky (1974) and by Mirza and Hurt (1974). The works of Dixon et al. (1971), Morishima et al. (1972), and Golob et al. (1973) dealt with different radicals, while Matsuzawa (1974) investigated the long-lived excited states of atoms. Finally, it should be mentioned that measurements for the other measurable parameters (see Section 111) also have been carried out, and such studies

9

10

11

12

13

14

15

16

17

18

19 eV

FIG. 14. Schematic pattern and interpretation of U P S spectra of trimethylsilylacetylene M ( H ) and four trimethylsilylhaloacetylenesM(X) with X = F, CI, Br, I. From Bieri et al. (1972).

86

D. BERENYI

are more and more utilized for elucidation of molecular orbitals. There is an especially large number of works on the angular distribution of photoelec. numerous measurements have been trons (references in Section 111,D)Also, made of photoionization cross sections, as well as of satellite and multiple line structure (refer also to the appropriate parts of Section HI).Figure 15 shows a characteristic example of how to obtain information about delocali-

I

i;

8

40 I 35eV

35

0

Z

s W

LI)

20 30f

* BINDING ENERGY (eV) FIG.15. Cr 3s lines in the XPS spectrum showing the extent of multiplet (spin) splitting depending on the actual compound in which the Cr occurs. From Carver et nl. (1972).

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

87

zation and the degree of covalency by multiplet splitting. Here it can be seen qualitatively that the separation between the two multiplet peaks becomes smaller if the ligand participates more intensively in the covalent bonding with the metal. In the case of K,Cr/CN/, no splitting can be observed because of a large degree of localization of valence electrons (Carver et al., 1972). B. Solid State Structure

We have seen above that when studying the valence region of the photoelectron spectra in gas or molecular beam samples (Khodeyev er al., 1972), information is obtained on the molecular orbitals, i.e., on the molecular structure. The valence region of solids corresponds to the solid state band structure, and so when examining the valence region of photoelectron spectra in the case of solid samples the information obtained refers to the band structure of solids. This field of research is a very active branch of the applications of photoelectron spectroscopy. Several recent survey papers (Leckey, 1972; Trautwein and Keune, 1972; Hedman, 1974) deal with the results of investigations in solid states using electron spectroscopy, and during the Asilomar conference a separate session was devoted to this subject. In the investigation of solids by electron spectroscopy, information can be obtained primarily on the band structure of solids, or to be more precise, on the density of states of occupied valence levels. In this respect electron spectroscopy is the most direct and reliable experimental technique, and for this research ultraviolet and X-ray excitations supplement each other. In the case of UPS the resolution (smaller linewidth) is better, but here measurements must be carried out at several different energies because the final state is in the immediate neighborhood of the Fermi level (few keV). Therefore the energy distribution will be strongly dependent on the exciting energy because of the density conditions of occupied and nonoccupied states and some selection rules due to crystal momentum conservation (see Fig. 16). If we use monochromatized X rays in these investigations, the resolution of XPS will be nearly comparable to that of UPS. Then, the use of monochromatization is very fruitful and even necessary here. The valence bands of numerous metals, alloys, and metal compounds have been investigated since the late 1960s and especially in the 1970s (see Baer er al., 1970; Hedman et al., 1971; Eastman and Kuznietz, 1971; Eastman, 1972; Hagstrom, 1972) by both UPS and XPS techniques. This research has been continuing recently as well, using both techniques, although using mostly monochromatized excitation in the case of XPS. For metals, experimental electron distribution data are compared with the corre-

88

D. BERkNYI

8

7

6

5

L

3

2

tO=E,r

INITIAL ENERGY (eV)

FIG.16. The valence band of Ag in the UPS spectrum at different exciting radiations. From Eastman (1972).

sponding theory calculated on the basis of assumed density of states. A comparison of the theoretical and experimental density-of-states curves for the valence band of thorium metal is shown in Fig. 17. In the investigation of the 4f and valence bands of Gd, Tb, and Dy, it was found that the peak near the Fermi level predicted by APW theory is present only in the spectrum of Gd, while there is no definite peak in the Tb and Dy spectra in spite of the theoretical predictions (McFeely et al., 1973). The study of a series of metals, alloys, or metal compounds and an intercomparison of them are very fruitful in general. Therefore thorium and uranium as well as their oxides were examined in two experiments (Veal and Lam, 1974a,b).The d bands of various metals are the subject of the following papers: Poole er al. (1973a) for Zn and Cd; Poole et al. (1973b) for 3d, 4d, and 5d bands in the region 2 = 29-93; Hufner and Wertheim (1974) for 3d metals from Mn to Cu; Fabian et al. (1974) for rare earths. Shepherd and

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

I

I

---

I

I

89

1

THEORETICAL EXPERIMENTAL

-4

-3

-2

-1

EF

1

E (eV)

FIG. 17. Comparison of the theoretical density-of-states curve with the XPS spectrum in the valence region. From Veal and Lam (1974b).

Williams (1974) dealt with the band structures of transition metal (groups VA and VIA) dichalcogenides, while Abbati et al. (1974) with the bandstructural similarity of SnTe to PbTe and PbSe, and Shevchick et al. (1973) with the effects of amorphous and crystalline states of Se and Te samples for valence and core level spectra. Valence band electron spectra of alkali halides (LiF, NaF, NaCl, NaBr, NaI, KF, KCl, KBr, KI) were observed and interpreted by Kowalczyk el al. (1974). Dobbyn and associates. (1974), however, compared the results of XPS for the valence band of Pt with the corresponding one from SXS (soft X-ray emission spectroscopy) measurements. Nemoshkalenko et al. (1975) consider the theoretical interpretation of the electron spectra of Cu, Ag, and Pd in the valence region. Studying valence spectra of alloys, one can trace changes in the density-of-states structure as a function of the change in the components of the alloy. A recent paper reporting on work of this type is that of Friedman et al. (1973). Some recent work was done on nonmetallic materials, e.g., on the valence band of graphite. In this area, XPS and SXS were used in the same study (Kieser, 1974). Research in the band structure of solids by measuring and analyzing the energy distribution of the photoelectrons (including the analysis of the widths of the bands) has been covered in more or less detail above. However, as mentioned in Section 111, there are also other measurable parameters that can be utilized in investigations of the band structure and other features of solids. In the following, we point out briefly the results and possibilities using these latter techniques. In Section III,D some measurements of Siegbahn, Fadley, and others on the angular dependence of photoelectron spectra were described in which

90

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the crystal structure of solids is reflected fairly directly. This leads to a possible experimental technique that has not been utilized sufficiently until now, A recent measurement of this type, however, for Mg(OH), was carried out by Freund and Scharpen (1974). The utilization of the spin polarization of photoelectrons in investigations of the magnetic structure of matter as well as electrostatic splitting due to the anisotropic charge distribution in the crystal geometry also has been mentioned (Sections III,A and C, respectively). The application of the cooling of samples offers a further (but until now scarcely used) possibility for research on the nature of bands in solids. The left-hand part of the band in Fig. 18 shows no temperature dependence, and so it can be assigned to “pure” states, while the other part seems to be assignable to hybridized states (Bauer and Spicer, 1972).

I

l

Ev+ 7.0

l

1

8.0

1

1

1

9.0

1

1

10.0

1

1

11.0

ELECTRON ENERGY, E(eV)

FIG. 18. Electron energy distribution excited by 11.4-eV photons from AgBr at various temperatures from 80 to 285°K (E, is the valence-band maximum). From Bauer and Spicer (1972).

C . Surface Studies One of the most dynamically developing fields of application of electron spectroscopy is the study of surfaces. At the Asilomar conference (1971) there was no separate session devoted to surface studies using electron spectroscopy, and there were exceedingly few papers even dealing with investiga-

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

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tion of surfaces. Since that time, however, the number of papers on this subject in various periodicals has continually increased, numerous surveys have been published (Todd, 1973; Holm, 1974; Brundle, 1974,1975; Shirley, 1975), and at the Namur conference (1974) surface studies were treated in detail. At the same time, recent conferences on the surfaces of materials also have dealt with the role of electron spectroscopy in this field (e.g., the Conference on Surface Properties of Materials, Rolla, Missouri, 24-27 June 1974; Surface Science Symposium of the 21st National Symposium of the American Vacuum Society, 8-1 1 October 1974). Certainly, it cannot be claimed that electron spectroscopy is only a surface method. As we have seen (Section II,C), the sample can be gas, vapor, atomic or molecular beam, solid material, and, recently, liquid too. At the same time, the sampling depth in electron spectroscopy is several tens of angstroms, which generally means a surface layer, but relative to the monolayer it is rather thick (one monolayer is approximately 3 8, thick). Nevertheless, many experts share the opinion that perhaps electron spectroscopy has its greatest potential utility as a technique for the study of surfaces. There are numerous new methods now for the study of surfaces. We cannot review and compare all of them here, and excellent papers do so in detail for their respective fields (e.g., Holm, 1973b; Benninghoven, 1973; Riach and Goff, 1974; Rynd and Rastogi, 1975). Table V exhibits the rich research potentialities of these relatively new techniques. Each method has its special advantages and disadvantages. For example, one of the most sensitive methods for detecting surface impurities is SIMS (secondary ion mass spectroscopy) (10-l3 gm cm-’). At the same time, however, the SIMS method destructively removes the surface layer, and its sensitivity varies for the different elements over a three orders of magnitude range) (qualitative method). LEED gives information about the geometrical arrangement of the atoms on the surface (but says nothing on elemental composition), while XD gives similar information about deeper layers, and so on. In fact, the various methods supplement each other from different points of view. The XPS and in part the AES methods are mainly useful in obtaining information about the chemical structure of the surface, and about the chemical state of the atoms on the surface. UPS finds relatively few but increasing applications in surface studies (Brundle and Roberts, 1972; Yu et al., 1975). Recently the angular distribution of ultraviolet excited photoelectrons was used for the investigation of surface oxygen absorption (Waclawski et a/., 1975) and for the elucidation of other surface problems (Traum et al., 1975). The role of AES and its relation to XPS should be noted here briefly. AES in its “classic” form (ie., with electron excitation) has a fairly good sensitivity in element identification analysis, but it is rather destructive of the

92

D. BERENYI

TABLE V SCHEMATIC REVIEWOF

NET METHODSFOR STUDYOF SURFACES"

THE

Excitation Ejection

Photon

Electron

Photon

XRF XD

EMP

Electron

XPS UPS

AES ELS LEED RHEED

Ion

LMA

EID

Ion IEX

INS

SIMS RS ISS IMMA

XRF, X-ray fluorescence; XD, X-ray diffraction; EMP, electron microprobe; IEX, ion excited X rays; XPS, X-ray photoelectron spectroscopy; UPS, ultraviolet photoelectron spectroscopy; AES, Auger electron spectroscopy; ELS,energy loss spectroscopy; LEED, low energy electron diffraction; RHEED, reflected high energy electron diffraction; INS, ion neutralization spectroscopy; LMA, laser microprobe analyzer; EID, electron induced desorption; SIMS, secondary ion mass spectroscopy; RS, Rutherford scattering; ISS, ion scattering spectroscopy; IMMA, ion microprobe mass analyzer.

surface of the sample and its energy resolution is moderate so that practically no chemical shift can be observed. Nowadays, however, AES by X-ray excitation (e.g., Wagner, 1972b; Berthou and Jorgensen, 1974; Fuggle et al., 1975) is coming into use, and sometimes it gives a better resolution and more clearly shows chemical shifts then XPS does (e.g., in the examination of Cu, CuO, Cu,O, and Cu,S thin films by Larson, 1974). Nevertheless, AES in general can be used most suitably in the case of light and medium elements because of the variation of fluorescence efficiency as a function of atomic number. With regard to XPS applications in surface studies, some remarks should be made about instrumentation (cf. Section 11). The most striking difference here as compared to other fields of application of electron spectroscopy is the need for an ultrahigh vacuum. In contrast to 10-6-10-8 torr used in other fields, here a minimum of 10torr is necessary with a very low

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

93

(10- 2-10- l 4 t o n ) partial pressure of active gases. This requires a sample chamber with very effective differential pumping and, furthermore, apparatus to clean the surface with an argon ion pump, to prepare samples in situ in a high vacuum, and to cool down or to warm up the samples (or sometimes the whole sample chamber)-to mention only some of the most important requirements. In surveying the most important research trends and results using electron spectroscopy (primarily with XPS) let us begin with the study of surface oxidation processes. It is a very remarkable fact that on the basis of XPS spectra one can differentiate between the oxygen in oxides and in adsorbed form on a surface (see Fig. 19). Kim and Davis (1973) studied the nickel-

'

CPS

I

1

1

I

535

530

1

1000 -

500 -

01

'

540

I

BINDJNG ENERGY(&

FIG. 19. 0 Is line in an XPS spectrum from an oxidized nickel sample: ( 1 ) at 25"C, (2) increasing the temperature to 300'C in vacuo, ( 3 ) exposed again to atmosphere at 25°C. The left peaks are due to adsorbed oxygen on the metallic nickel surface. From Schon and Lundin (1972).

oxygen system, and Roberts (1974), in turn, studied copper oxide surfaces treated by benzotriazole. Nishijima and associates (1974) dealt with oxidized states on Ni and Cu surfaces. Barrie (1973) detected the properties of in siru evaporated and oxidized A1 films. The surfaces of chrome steel (Coad and

94

D. BERENYI

Cunningham, 1974) and oxide films produced by anodic oxidation of tungsten (Carlson and McGuire, 1972) as well as of gold and platinum (Dickinson et al., 1975) have been examined. Another group of measurements deals with the adsorption, absorption, and chemisorption of elements and compounds (other than oxygen) by surfaces as well as with catalytic processes. The chemisorption of carbon monoxide by Mo and W was examined by Atkinson et al. (1973). The absorption of mercury by platinum black (Affrossman and Fuggle, 1974) and a series of gases including oxygen (CO, C,H4, 0, ,H,S, HzO, CO,) on Ni (Page et al., 1974) were the subject of some recent measurements. In a work of Menzel (1975) a comparison of UPS and XPS with other methods in the investigation of adsorption on metal surfaces is given. Some recent examples occur in the study of Grimblot er al. (1975) and Escard et al. (1975). In an earlier work a detailed survey is given on XPS as a tool for research in catalysis (Deglass et al., 1970). Electron spectroscopy has some significance as a method for element identification analysis of surfaces. An accepted figure for the sensitivity is approximately 1 % of monolayer atoms. Brundle and Roberts (1973), however, detected 0.2% of a monolayer of mercury absorbed by a gold substrate. Recent developments in quantitative surface analysis using XPS are reported by Swingle (1975). Research on surfaces is very important for a series ofpractical problems, such as corrosion, catalysis, properties of optical and electrical thin films, thermionic conversion of energy, etc. Therefore the emergence and application of such a powerful method as electron spectroscopy are very useful, and further efforts and endeavor in this direction are indispensable.

D . Structure of Chemical Compounds and Biological Systems In most cases the basis for information about the structure of chemical compounds is the shift of the core levels (and corresponding electron spectrum lines) due to the change in the chemical environment, i.e., to change in the actual state of the valence electrons, namely the oxidation state, degree of covalency, electronegativity, coordination, etc. If the outermost electrons are completely or partly removed from the atom in the molecular structure, the electron density around the atom and consequently the shielding of the nuclear charge are decreased. This means an increase in the binding energy of the core levels. The shift in the core level of the same kind of atom can be different in a molecule if the atoms of the same element occupy different positions there. In the case of biological structures (enzymes, proteins, etc.), the shift of some active ions (metals) in the biomolecule gives valuable information (e.g., Kramer and Klein, 1972; Millard and Masri, 1974;

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

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Burness et al., 1974). The principles of and some information about shifts were already mentioned above in Sections 1,C and II1,B. Considerable data have been published on the observed shifts of the individual core levels in different compounds. Some summarization of these results has also been done (e.g., Holm, 1972; Swartz, 1973; Bremser, 1973), but a comprehensive and up-to-date tabulation of shift data is lacking as yet. Observed shift values for atoms in different compounds usually are arranged in a plot attempting to correlate the observed shifts with some simplified chemical work concept (e.g., oxidation number, electronegativity, degree of ionicity, atomic charges, etc.). Interpretation of shifts is also sometimes attempted by using some semiquantum-chemical calculation, as extended Hiickel, CNDO, INDO, etc., or precise ab initio theories (see Section IV,A).

I

0

I

I

1

10 PAULING CHARGE (qp)

FIG.20. Shifts of C 1s binding energies for halomethanes as a function of Pauling charges. From Allison et a!. (1973).

By making a plot similar to that given in Fig. 20, one can draw conclusions about the chemical structure of a compound with previously unknown structure. In certain cases, the measurement of shifts in several strongly related compounds is necessary to clarify the chemical structure. Sometimes the investigation of two related compounds is sufficient to settle a problem in connection with the chemical structure. Comparing the observed shifts with various theoretical calculations, we can obtain information on molecular orbitals and the relevant theories in a similar fashion to the study of valence levels (Section IV,A). We shall now discuss some important models relating experimental shift values to molecular structure (cf. Schwartz et al., 1972; Jolly, 1972a,b; Sieg-

96

D.

BERBNYI

bahn, 1972a; Albridge, 1973; Holm, 1973a). In the precise ab initio HartreeFock formalism the binding energies are calculated as the difference between the total energies of the initial ( E J and final ( E , ) molecular system. Different approximations are generally used here, e.g., the neglecting of relativistic effects and electron correlation effects, and using the Xu method (see Section IV,A) or the pseudopotential model. One of the well-known approximations here is to take the calculated eigenvalue as the energy of the ionized level in question without taking into consideration the reorganization of the molecule after ionization (Koopman’s theorem). Another much more simple interpretation of shifts is possible using the potential model. Here the shift is regarded as resulting from the change of the charge of the atom concerned in the valence region due to bonding in the molecule. The electrostatic potential from the charges of the other atoms in the molecule are also taken into account. The charges in question are often obtained from semiempirical models (extended Huckel, CNDO, INDO, etc.) or from Pauling electronegativities (Pauling charges) and sometimes from a6 initio H-F calculations. The assumption of a partial charge on the atom explains the shift by “concentrating” all the change due to the molecular environment on the change in the charge of the valence electrons of the atom being considered. This has a rather limited applicability in the case of elements from the same group in the periodic system. The basic assumption of the thermodynamic model is that atoms having cores with equal charge are equivalent chemically. In this model the observed shifts are directly correlated to measurable thermochemical quantities. In Table VI a short review of the above-mentioned theoretical interpretations is given (using the data of Holm, 1973b) for orientation. Some relevant theoretical calculations published quite recently are Clark and Adams (1973), Adams and Clark (1973), Davis and Shirley (1974), Bagus and Wahlgren (1974), Viinikka and o h r n (1974), Moddeman and Cothern (19759, and Baerends et al. (1975). To show the power of core level shift measurements using electron spectroscopy in elucidating chemical structure we do not want to quote the numerous “classic” examples recalled already many times; but we do direct the reader above all to the classic work of Siegbahn et al. (1967) and certain other survey papers (e.g., Siegbahn et al., 1968; Allan and Siegbahn, 1971; Nordling, 1972; Holm, 1973a). However, we shall discuss some recent results in this field. Figure 21 shows the shift of C Is peak characterizing the different positions of the carbon atoms in CF,CH for acetylenic and fluorocarbon bonding (Cavell, 1975). According to another study, the significant shifts from the values for the unsubstituted carbonyl observed in some substituted chromium carbonyls indicate a moderate increase in electron density of the axial CO relative to that of the four equatorial ligands; the Cr 2p shift is due to a

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

97

TABLE VI

VARIOUS THEORIESFOR

INTERPRETATION OF CHEMICALSHIFT SPECTROSCOPY" Agreement with experiment (ev)

Method HF ah initio Complete Koopmans

Potential model Partial atomic charge

a

Remarks

k0.5

Relativistic and electron correlation effect usually neglected.

21

Large errors with regard to ionization energies, but fairly good shift values.

+I

Charges derived from different mostly semiempirical calculations. Limited applicability for elements From the same group of the periodic chart.

changing

Potential for direct comparison with thermochemical data.

k0.S

Thermodynamic model

CORE ELECTRON

IN

From data of Holm (1973b).

6000

1000

CF3 CCH

i 4

1

1

302 I

952

1

1

1

I

95L

. -1

1

1

1

1

1

1

298 296 294 292 BINDING ENERGY (corr)

300 I

1

I

1

956

I

1

958

I

i

960

I

'

962

290(eV) I

'

% (eV)

KINETIC ENERGY

FIG. 21. Binding energies of the C Is level in CF,CCH depending on the actual atomic environment of the carbon atom in the molecule. From Cave11 (1975).

98

D.

BERBNYI

change in the potential from the surrounding atoms on substitution (Barber et al., 1972a,b). Swartz and Alfonso’s (1974) investigation excluded the possibility of the presence of quaternary nitrogen atoms in transition metal biguanide complexes and decided between two possible variants of the structure. Another core level shift study established the existence of a highly charged silver ion in the ethylenebis/biguanide/silver(III) ion (Zatko and

FIG.22. Series of UPS spectra of cyclopentanediones and the corresponding schemes of structure. From Houk et a/. (1973). Reprinted with permission from Journal of the American Chemical Sociery. Vol. 95, p. 8366 (1973). Copyright by The American Chemical Society.

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

99

Prather, 1973). The amount of charge transfer between tetrathiofulvalene and tetracyanoquinodimetan stacks was determined by XPS studies on S 2p and N 1s levels (Ginnard et al., 1975). The above results are only a few examples. Many other measurements have been published recently, e.g., Burger and Fluck’s (1974) work on the solvation of SbCl,, that of Clark et al. (1975) on chlorobenzenes, and Nefedov and associates’ (1975) study on the bonding of Cu, Zn, and Ga. We should also like to mention here in passing that the study of line splitting using XPS can also give some information on chemical structure (Siegbahn, 1972b; Carver et al., 1972). Information about chemical structure can also be obtained using UPS techniques, although the main field of UPS is in research on molecular orbitals. In many cases, however, the borderline is not very sharp between chemical structure determination and molecular orbital studies. If we desire structural information using UPS, the main practical method is the study of a series of related compounds and comparison of their spectra tracing the changes in the number and intensity of bands. Figure 22 shows how a part of the UPS spectra changes as the structure of cyclopentanediones varies. Similar studies done on related molecules are, e.g., Rabalais and Colton’s (1972) work on the phenyl group and its unsaturated substituents, Spanget-Larsen’s study (1974) on azan phthalenes, that of Tam et al. (1974) on some aldehydes and ketones, Allan and associates’ (1975) investigation of benzologue tropones, and the examinations of carbonyls by McGlynn and Meeks (1975). In the case of nonbonding or weakly bonding orbitals (narrow peaks in the UPS spectra, see Section IV,A) there is a similar relationship between the ionization potential (the position of narrow bands) and the electronegativity such as that for the inner shell lines using XPS (e.g., in Chau and McDowell, 1975). A fairly good example of how it is possible to elucidate the structure of concrete compounds with UPS using only comparison of two spectra is the case of (a-naphthy1)-(CH,),-(a-naphthyl) vapor. This investigation demonstrated that the open chain structure rather than the cyclic is the valid structure (Berkowitz et al., 1973, 1974).

E . Other Fields of Application

There are several other fields of application of electron spectroscopy in addition to those dealt with earlier in this section, e.g., determination of the binding energies of electrons in atoms (e.g., Krause and Wuilleumier, 1972), chemical analysis, and X-ray spectroscopy by means of photoelectrons. In the next subsections the latter two fields will be briefly reviewed.

100

D.

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1. Qualitative and Quantitative Analysis It is quite obvious that the inner shell photoelectron lines are characteristic for the atom and element from which they are ejected. That is why the word analysis was originally included in the name of the whole method, i.e., ESCA (electron spectroscopy for chemical analysis). In time, however, it became apparent that other applications of electron spectroscopy, e.g., elucidation of the structure of chemical compounds, research on molecular orbitals, surface studies, etc., are much more important. Currently, the application of electron spectroscopy in chemical analysis in general as a nondestructive method has a solid basis, but its significance and field are limited relative to other fields of application surveyed in the earlier sections above. We have considered some applications of electron spectroscopy (including Auger spectroscopy) in surface analysis. The potentialities in the use of electron spectroscopy in chemical analysis, however, are broader. It is useful not only because of its capabilities in gas analysis, but also in other areas when, e.g., one can assume and/or prove the same bulk and surface composition. With regard to qualitative analysis using electron spectroscopy certain difficulties are present and corrections are necessary, e.g., 2-dependence of the photoelectric cross section, absorption effects, surface contaminations, etc. Wagner’s (1972a) work showed, however, that the sensitivity for the detection of elements by XPS, i.e., the X-ray photoelectric cross section, varies as a function of atomic number by no more than one order of magnitude, and the change is rather regular. These are fairly minor problems compared to those elicited by other powerful methods used in the field, e.g., SIMS. One of the well-known results from chemical analysis using electron spectroscopy is the development of a procedure for bulk quantitative analysis of Moo,-Moo, mixtures, with a relative standard deviation of 2%, by Swartz and Hercules (1971). Previously no other nondestructive technique was available, and such an analysis could be accomplished only by using destructive and time-consuming wet methods of analytical chemistry. An important step in this development was combining the surface analytical examination with electrochemical deposition, which made possible quantitative trace analysis by electron spectroscopy to the parts per billion level or lower (Brinen and McClure, 1972, 1974; Siegbahn, 1974). Potential analytical applications using XPS are more obvious than those using UPS. UPS applications are possible, however, partly based on the sharp peaks in the UPS spectrum due to nonbonding, very weak bonding, antibonding, or lone pair” electrons (e.g., in chlorine, bromine, iodine, “

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

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fluorine), and partly using a curve-fitting computer program with standard spectra of the components to be analyzed. These techniques are especially useful in the analysis of mixtures of gases or vapors (Betteridge and Baker, 1970; Betteridge et al., 1975). Finally, one can use electron spectroscopy (UPS) to characterize a complex compound by the “pattern of the corresponding photoelectron spectrum considered as a “fingerprint.” ”

2. X-Ray Spectroscopy by Means of Photoelectrons If we choose an appropriate converter, where the binding energy is well known, it is possible to use XPS to determine the energy distribution of X rays on the basis of the photoelectric equation. The name of this method of X-ray spectroscopy is PAX, i.e., photoelectron spectrometry for the analysis of X rays (Krause, 1974). PAX can be used in a broad energy range (from about 20 to 5000 eV) along with the advantageous features of the other methods of X-ray spectroscopy (crystal diffraction spectrometer, semiconductor detector, and proportional counter). Its energy resolution is comparable to that of crystal diffraction spectrometers, while PAX is superior for intensity determinations. While it surpasses semiconductor and proportional spectrometers in resolution, its detection efficiency is lower. It is obvious that PAX is a useful supplement to the other methods of X-ray spectrometry (Krause and Wuillemier, 1973). The most customary converters used are some noble gases, namely He, Ne, Ar (Krause, 1974). Numerous studies of X-ray spectra have been carried out using PAX (see Krause and Wuilleumier, 1972; Krause, 1972, 1974; recently in the ultrasoft region of X rays from a tungsten anode, Keski-Rahkonen, 1975), and wider applications are to be expected in the fuuture. V. ELECTRON SPECTROSCOPY IN SOLUTION OF PRACTICAL PROBLEMS

This section is devoted to applications of electron spectroscopy in studies relating to practical problems, i.e., to industrial production or everyday life. Such a separate section might not be justified by the quantity of relevant published material, but it is certainly justified by the importance of this field of applications. Another fact that has to be emphasized here is the lack of sharp borders between practical and other applications. Some results, e.g., in surface studies or structural chemistry, fall under the heading of this section or, more frequently, will have practical applications sooner or later. The significance of the study of giant organic molecules, e.g., is quite

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D. BERENYI

clear for the pharmaceutical industry. It was not only by chance that a researcher from Pharmacia AB took part in the pioneering ESCA investigations of the Uppsala group (Siegbahn et al., 1967) and that the first extensive studies included such samples, among many others, as insulin, vitamin B,, , phthalyl-sulfothiazole (Hagstrom et al., 1964; Siegbahn et al., 1967). It is also obvious that the use of electron spectroscopy to give a “fingerprint” (see Section IV,E,l) characterization of drugs, similar to X-ray diffraction, has significance in identifying new medications. Studies of the oxidation of surfaces as well as of the phenomena of adsorption and absorption on surfaces (much work in this area has been done, see, e.g., the relevant section in the survey paper of Hercules and Carver, 1974) are important for the detailed clarification of corrosion, a phenomenon of great practical importance. Corrosion studies of Cu-Ni alloy exposed to NaCl solutions were carried out by Hulett and associates (1972b). The oxide layer on aluminum, including hot-rolled aluminum, has been studied by several investigators using the techniques of electron spectroscopy (Tripathi and Clark, 1973; Farrell and Naybour, 1973; Clark and Tripa thi, 1973). The study of catalysis and catalysts is, in general, closely related to surface research using electron spectroscopy. Rather early in the history of the applications of electron spectroscopy, a fairly long paper was published not only to survey the possibilities but to summarize the results obtained by the authors themselves (Deglass et al., 1970). They found that studies in the characterization, activation, and aging poisoning of catalysts are feasible using electron spectroscopy. A list of recent results with references can be found in the relevant section of the Amlyrical Chemistry reviews by Hercules (1972) and Hercules and Carver (1974). We shall next discuss only two examples as characteristic. When using a palladium-on-carbon catalyst in a factory to chemically reduce a nitrogen-containing organic compound, it was noted that, after some use, the palladium-on-carbon lost its catalytic ability. Sulfur or a halogen was suspected to be poisoning the catalyst. XPS examinations showed that a nitrogen-containing by-product coated the surface of the catalyst, and that was the reason for the loss of its activity (Du Pont information, Industrial Research, June 1974, p. 12). Similarly, an XPS study before and after use of a CuCr,O, heterogeneous catalyst shows clearly that the catalytic effect is caused by the positive chromium ion, although the negative chromium ion is also present before the intensity of peaks from the latter in the XPS spectrum is about the same before and after use (Varian brochure). Another area of research with the electron spectroscope is concerned with very urgent problems of increasing interest to society today. This field is

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that of environmental pollution studies. Not long ago a paper was devoted to a survey of possible areas of study (Hercules and Hercules, 1972), and some work has already been done in the field. For example, Hulett et al. (1972a) found that sulfur exists in sulfate, sulfite, and in even lower oxidation states on the surface of fly ash and smoke particles in which SO, is absorbed. Novakov and associates (1972) determined the lead, sulfur, and nitrogen concentrations, as well as the relative quantities of the latter two in different chemical states, in smog particles. To clarify the dynamics of atmosphericaerosol interactions the above measurements were carried out as a function of particle size and time of day. Recently, the chemical shifts for several zinc compounds (borate carbonate, chloride, fluoride, iodide, phosphate, titanate) in XPS spectra were measured, and the significance of these measurements for applicability to environmental samples was shown (Cothern et al., 1974). There are several other uses of electron spectroscopy that have practical importance, and we note some briefly. The doping of semiconductors important to industry, namely Ge, GaAs, and CdSe, was studied using XPS (Sharma et al., 1971). A definite shift of the Si 2p peak in the X-ray ejected electron spectrum was found for heavily doped Si of n- and p-type (Hedman et al., 1972). In an XPS study of wear (i.e., in a tribochemical application of it), it was proved that an MoS, film under sliding conditions will be oxidized and sulfur in an elemental as well as in a highly oxidized state will appear (Atkinson and Swift, reference 18 in Drummond et al., 1974).Electron spectroscopy can be used to measure the quantity of the slip agent (amide) present on a polymer surface (polyethylene) as was done in a lubrication study by W. Riggs (cited in Baitinger and Amy, 1974). The quality ofwool jiber depends on the treatment of its surface. The changes in chemical structure on the surface of wool yarn caused by corona and low temperature discharge were investigated by XPS techniques. The spectra showed a difference in the oxidation state of sulfur according to the treatment of the yarn (Millard, 1972a,b). We have noted a number of applications of practical importance using electron spectroscopy. One can say, however, that the potentialities for the use of electron spectroscopy are much greater than its applications to date. VI. CONCLUSIONS AND PERSPECTIVES

The application of electron spectroscopy in the analysis and research of the structure of matter is one of the many new powerful methods in this field, one with special advantages and disadvantages. We cannot here undertake a complete survey and intercomparison of these methods from ESCA to SIMS and from Mossbauer spectroscopy to ESCA. We should like only to show

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how electron spectroscopy (including Auger spectroscopy) fits in with other modern methods. As a technique for exploring the chemical structure of compounds, electron spectroscopy can be compared, e.g., with NMR and Mossbauer spectroscopy. The resolution with the latter methods is much higher than with electron spectroscopy. In the case of NMR the ratio of the maximum shift to the half-width is about 1O00, and in the case of the Mossbauer method it is even higher, while this ratio has a maximum of about 20 in electron spectroscopy. At the same time the limitations of NMR and Mossbauer are well known with respect to the elements to be examined and the form of the samples (liquids and crystals, respectively). We have seen, however, that all elements can be investigated using electron spectroscopy; and the sample can be gaseous, solid, or liquid. Electron spectroscopy is also a method for surface studies. Its sensitivity is less than that of SIMS, e.g.; but it is a nondestructive method, and it gives surface information about not only the elemental composition but also the chemical structure. In addition, electron spectroscopy is more appropriate to quantitative surface analysis because its sensitivity does not depend so strongly on atomic number as that using SIMS. There have been various methods used in the investigation of molecular orbitals; but, in general, all can determine the first ionization potential of the molecules. Using different forms of electron spectroscopy, the energy and features (e.g., d or 7~ types) of molecular orbitals can be traced down to the atomic levels. These experimental possibilities have given a new impetus to research in quantum chemistry. We have mentioned a number of the trends in the development of electron spectroscopy in this paper: monochromatization of X rays or the use of synchronous radiation with respect to instrumentation, more intensive utilization of angular correlation, of spin polarization techniques for measurable quantities, and of utilization of electron spectroscopy in the solution of different and new problems in various fields of applications. What I should like to emphasize, finally, is the trend toward complex applications of various modern methods including electron spectroscopy in the solution of diverse problems. It is a practical consequence of the character of contemporary methods that they supplement each other. It is now quite customary to have XPS, UPS, AEA, and LEED in the same equipment. Sometimes SIMS and ISS (ion scattering spectroscopy) as well as INS (ion neutralization spectroscopy) are also included. An instrument has been constructed with which one can study the same sample by using electron spectroscopy and XRF (X-ray fluorescence spectroscopy). Complex application of different methods, however, does not mean necessarily that they are included in the same equipment. It means related

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investigations of a problem using different techniques. There is a recent example in which the surface of glass has been examined by SIMS, by ISS, by AES, and by XPS in the same study (Rynd and Rastogi, 1975). A t the close of this survey we should like to point out once again that although electron spectroscopy is currently a very useful method in the family of the contemporary methods for the research of matter, its capabilities are by no means exhausted. ACKNOWLEDGMENTS The author wishes to thank the collaborators in the Group for Nuclear Atomic Physics in ATOMKI for their valuable discussions and comments. The author is also indebted to the following copyright owners for permission to reproduce certain diagrams o r tables: The Almqvist and Wiksell Periodical Co., Stockholm (Fig. 5 ) ; G E C Journ. Sci. and Technology, Wembly, England (Fig. 6); North-Holland Publ. Co.. Amsterdam (Figs. 7, 9, 15, 16 and 18); Elsevier Sci. Publ. Co., Amsterdam (Figs. 8, 13, 14, 19, 20 and 21); Institute of Physics, University of Uppsala. Uppsala, Sweden (Figs. 10 and 12 as well as Tables 111 and I V ) : The Royal Society, London (Fig. 1 1 ) ; The American Physical Society, New York (Figs. 3 and 17); The American Chemical Society, Washington (Fig. 22) and to the authors of the corresponding papers.

REFERENCES Abbati. I., Braicovich, L., and De Michelis, B. (1974). J. Phps. C 7. 3661. Adams, D. B. (1974). J. Electron Spectrosc. Relar. Phenom. 4, 72. Adams. D. B., and Clark, D. T. (1973). J. Electron Spectrosc. Relat. Phenom. 2, 201. ANrossman, A., and Fuggle, J. (1974). J. Elecfron Spectrosc. Relat. Phenom. 3, 449. Albridge, R . G. (1973). Proc. I n ( . ConJ Inn. Shell loniz. Phenom. Future Appl., 1972 p. 2305. Al-Joboury, M. 1.. and Turner, D. W. (1963). J. Chem. SOC.p. 5141. Allan, C. J., and Siegbahn, K. (1971). Electron Spectroscopy for Chemical Application, UUIP-754. Institute of Physics, Uppsala University, Uppsala. Allan, C. J., Gelius, U., Allison, D. A., Johansson, G., Siegbahn, H., and Siegbahn, K. (1972). .I. Elecrron Spectrosc. Relat. Phenom. I. 131. Allan. M., Hellbronner, E.,and Kloster-Jensen, E. (1975).J. Electron Spectrosc. Relar. Phenom. 6, 181. Allison, D. A., Johansson. G., Allan, C. J., Gelius, U., Siegbahn, H., Allison. J., and Siegbahn, K. (1973). J . Electron Spectrosc. Relar. Phenom. 1. 269. Almhof. J. (1973). J . Electron Speclrosc. Relat. Phenom. 2, 51. Atkinson, S. J.. Brundle, C. R., and Roberts, M. W. (1973).J . Elecrroti Specfrosc. Relar. Phenom. 2. 105. Bachrach, R . Z., Brown. F. C., and Hagstrom, S. B. M. (1975). J. Vac. Sci. Technol. 12, 309, Baer, Y.,Heden. P. F., Hedman, J., Klasson. M., Nordling, C., and Siegbahn, K. (1970). Phys. Scr. 1. 55. Baerends, E. J., Oudshoorn. C., and Oskam, A. (1975).J. Electron Spectrosc. Relat. Phenom. 6, 259. Bagus. P. S., and Wahlgren. U. I . (1974). Php.7. F m n . 9, Suppl.. 295 Baitinger, W. E.. and Amy, J. W. (1974). I n d . Res. June, 60.

106

D. BERkNYl

Baker, A. D., and Betteridge, D. (1972). “ Photoelectron Spectroscopy Chemical and Analytical Aspects.” Pergamon, Oxford. Bank, W., Barz, A,, Kocian, P., Noller, H. G., Polaschegg, H. D., Schillaties, H., Spohr, R., and Wischnewski, K. (1972). Vacuum 22, 497. Barber, M., Connor, J. A,, Hillier, I. H., and Meredith, W. N. E. (1972a). J . Electron Spectrosc. Relat. Phenom. 110. Barber, M., Connor, J. A,, Guest, M. F., Hall, M. B., Hilier, I. H., and Meredith, W. N. E. (1972b). Faraday Discuss. Chem. SOC.54, 219. Barrie, A. (1973). Chem. Phys. Lett. 19. 109. Bauer, E. (1972). Vacuum 22, 539. Bauer, R. S., and Spicer, W. E. (1972). Electron Spectrosc., Proc. Int. Con& 1971 p. 569. Benninghoven, A. (1973). Appl. Phys. 1, 3. Berenyi, D. (1974). A T O M K I Kozl. 16, 27. Bergmark, T., Karlsson, L., Jadrny, R., Mattsson, L., Albridge, R. G., and Siegbahn, K. (1974). J . Electron Spectrosc. Relat. Phenom. 4, 85. Berkowitz, J., Dehmer, J. L., Shimada, K., and Szwarc, M. (1973). J . Electron Spectrosc. Relat. Phenom. 2, 211. Berkowitz, J., Dehmer, J. L., Shimada, K., and Szwarc, M. (1974). J. Electron Spectrosc. Relat. Phenom. 3, 164. Berthou, H., and Jorgensen, C. K. (1974). Phys. Fenn. 9, Suppl., 321. Berry, R. S . (1969). Ann. Ren. Phys. Chem. 20, 357. Betteridge, D., and Baker, A. D. (1970). Anal. Chem. 42, 44A. Betteridge, D., and Williams, M. A. (1974). Anal. Chem. 46,No. 5, p. 125R. Betteridge, D., Carver, J. C., and Hercules, D. M. (1973).J . Electron Spectrosc. Relat. Phenom. 2, 327. Betteridge, D., Baker, A. D., Bye, P., Hasannudin, S. K., Kemp, N. R., and Thompson, M. (1974). J . Electron Spectrosc. Relat. Phenom. 4, 163. Betteridge, D., Williams, M. A., and Chandler, G. G. (1975). J . Electron Spectrosc. Relar. Phenom. 6, 327. Bieri, G., Brogli, F., Heilbronner, E., and Kloster-Jensen, E. (1972).J . Electron Spectrosc. Relat. Phenom. I, 67. Bishof, P., Gleiter, R., Kukla, M. J., and Paquette, L. A. (1974). J . Electron Spectrosc. Relat. Phenom. 4, 177. Bock, H., and Ramsey, B. G. (1973). Angew. Chem., Int. Ed. Engl. 12, 735. Bodor, N., Chen, B. H., and Worley, S. D. (1974). J . Electron Spectrosc. Relat. Phenom. 4, 65. Bremser, W. (1973). New Methods Chem. 36, 1. Brinen, J. S., and McClure, J. E. (1972). Anal. Lett. 5, 737. Brinen, J. S.. and McClure, J. E. (1974). J . Electron Spectrosc. Relar. Phenom. 4, 243. Brundle, C. R. (1974). J. Vuc. Sci. Technol. 11, 212. Brundle, C. R. (1975). Surface Sci. 48,99. Brundle, C. R., and Jones, G. R. (1973). J. Electron Spectrosc. Relat. Phenom. 1, 403. Brundle, C. R., and Roberts, M. W. (1972). Proc. R. SOC.London, Ser. A 331, 333. Brundle, C. R., and Roberts, M. W. (1973). Chem. Phys. Lett. 18, 380. Brundle, C. R., Roberts, M. W., Latham, D., and Yates, K. (1974). J. Electron Spectrosc. Relat. Phenom. 3, 241. Burger, K., and Fluck, E. (1974). Inorg. Nucl. Chem. Lett. 10, 171. Burhop, E. H. S., and Asaad, W. N. (1972). Adu. At. Mol. Phys. 8, 163. Burness, U. H., Dillard, J. G., and Taylor, L. T. (1974). Nucl. Chem. Leu. 10, 387. Busch, G., Campagna, M., and Siegmann, H. C. (1970). J. Appl. Phys. 41, 1044. Busch, G., Campagna, M., Pierce, D. T., and Siegmann, H. C. (1972). Phys. Rev. Letr. 28,611. Carlson, T . A. (1972). Electron Spectrosc, Proc. Int. ConJ, 1971 p. 53. Carlson, T. A,, and McGuire, G. E. (1972). J . Electron Spectrosc. Relat. Phenom. 1, 161.

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

107

Carlson, T. A., and Nestor. C. W . (1973). Phys. Rev. 8, 2887. Carlson, T. A, McGuue, G . E., Jonas, A. E., Cheng, K. L., Anderson, C. P., Lu, C. C., and Pullen, B. P. (1972). Electron Spectrosc., Proc. Int. Conf, 1971 p. 207. Carver. J. C., Carlson, T. A., Cain, L. C., and Schweitzer, G. K. (1972). Electron Spectrosc., Proc. Jnr. Conf., 1971 p. 803. Cavell, R. G . (1975). J . Electron Spectrosc. Relat. Phenom. 6, 281. Chadwick, D., and Katrib, A. (1974). J . Electron Spectrosc. Relat. Phenom. 3, 39. Chang, C. C. (1975). SirrJace Sci. 48, 9. Chau, F. T., and McDowell, C. A. (1975). J. Electron Sprctrosc. Relar. Phenom. 6, 357. Citrin, P. H., Shaw. R. W., Jr., and Thomas, T. D. (1972). Electron Spectrosc., Proc. Int. Conf., 1971 p. 105. Citrin, P. H., Einsenberger, P. M., Marra, W. C., Aberg, T., Utriainen, J., and Kallne, E. (1974). Phys. Rev. B 10, 1762. Clark, D. T., and Adams, D. B. (1973). J . Elecrron Spectrosc. Relat. Phenom. 1, 302. Clark, D. T., and Tripathi, K. C. (1973). Nature (London), Phys. Sci. 244, 77. Clark. D. T., Kilcast. D., Adams, D. B., and Scanlan, 1. (1972). J . Eiectron Spectrosc. Relat. Phenom. 1, 153. Clark, D. T., Kilcast, D., Adams, D. B., and Musgrave, W. K. R. (1975). J . Electron Spectrosc. Relat. Phenom. 6, 117. Coad, J. P., and Cunningham, J. G. (1974). J . Electron Spectrosc. Relat. Phenom. 3, 435. Cothern, C. R.,Langer, D. W., and Vesely, C. J. (1974). J . Electron Spectrosc. Relat. Phenom. 3, 399. Daniels, J., Festenberg, C. V., Raether, H., and Zeppenfeld, K. (1970). Springer Tracts M o d . Phys. 54, 78. Davis, D. W., and Shirley, D. A. (1974). J. Electron Spectrosc. Relat. Phenom. 3, 137. Deglass, W. N., Hughes, T. R., and Fadley, C. S. (1970). Catal. Rev. 4, 179. Delwiche, J., Natalis, P., Momigny, J., and Collin, J. E. (1973). J . Electron Spectrosc. Relat. Phenom. 1, 219. Dickinson, T., Povey, A. F., and Sherwood, P. M. (1975). J . Chem. SOC., Faraday Trans. 171, 298. Dill, D. (1972). Electron Spectrosc., Proc. Jnr. ConJ.., 1971 p. 277. Dixon, R. N., Duxbury, G., Horani, M., and Rostas, J. (1971). Mol. Phys. 22, 977. Dobbyn, R. C., McAlister, A. J., Cuthill, J. R., and Erickson, N. E. (1974). Phys. Lett. A 47,251 Drummond, 1. W., Errock, G . A,, and Watson, J. M. (1974). G E C J . Sci. & Technol. 41, 94. Eastman, D. E. (1972). Electron Spectrosc., Proc. Inr. Con$, 1971 p. 487. Eastman, D. E.. and Kuznietz, M. (1971). Phys. Rev. 26, 846. Ebel, M. F., and Ebel, H. (1974). J . Electron Spectrosc. Relat. Phenom. 3, 169. Egelhoff, W. F., and Perry. D. L. (1975). Phys. Rev. 34, 93. Escard, J., Contour, J. P., and Pontvianne, B. (1975). J . Electron Spectrosc. Relat. Phenom. 6, 17. Fabian, D. J., Padalia, B. D., and Lang, W. (1974). Phys. Fenn. 9, Suppl. 313. Fadley, C. S . (1972). Electron Spectrosc., Proc. Int. Conf, 1971 p. 781. Fadley, C. S., and Bergstrom, S. A. L. (1972). Electron Spectrosc., Proc. Int. Conf., 1971 p. 233. Fadley, C. S., Miner, C. E., and Hollander, J. M. (1969a). Appl. Phys. Lett. 15, 223. Fadley, C. S., Shirley, D. A,, Freeman, A. J.. Bagus, P. S., and Mallow, J. V. (1969b). Phys. Rev. Leu. 23, 1397. Fadley, C. S., Healey, R . N., Hollander, J. M.. and Miner, C. E. (1972). Electron Spectrosc., Proc. Int. C o n f , 1971 p. 121. Farrell, T., and Naybour, R. D. (1973). Nature (London), Phys. Sci. 244, 14. Freund, F., and Scharpen, L. H. (1974). J . Electron Spectrosc. Relal. Phenom. 3, 305. Friedman, R. M., Hudis, J., Perlman, M. L., and Watson, R. E. (1973). Phys. Rev. 8, 2433. Fuggle, J. C., Watson, L. M., Fabian, D. J., and Affrossman, S. (1975). J . Phys. F 5, 375. Gelius, U. (1972). Electron Spectrosc, Proc. Inr. C o n f . 1971 p. 311.

108

D. BERENYI

Gelius, U. (1974). Phys. Fenn. 9,Suppl., 290. Ginnard, C. R., Swingle, R. S., and Monroe, B. M. (1975). J. Electron Spectrosc. Relat. Phenom. 6, 77. Golob, L., Jonathan, N., Morris, A., Okuda, M., and Ross, K. J. (1973). J . Electron Spectrosc. Relat. Phenom. 1, 506. Gonbeau, D., Guimon, C., Deschamps, J., and Pfister-Guillouzo, G. (1975). J . Electron Spectrosc. Relat. Phenom. 6, 99. Grimblot, J., d’Huysser, A,, Bonnele, J. P., and Beaufils, J. P. (1975). J . Electron Spectrosc. Relat. Phenom. 6, 71. Grimm, F. A. (1972). Electron Spectrosc, Proc. Int. Conf., 1971 p. 199. Hagstrom, S. B. M. (1972). Electron Spectrosc., Proc. Int. Conf., 1971 p. 515. Hagstrom, S. B. M., and Fadley, C. S. (1974). I n “ X-Ray Spectroscopy” (L. V. Azaroff, ed.), p. 379. McGraw-Hill, New York. Hagstrom, S. B. M., Nordling, C., and Siegbahn, K. (1964). Z. Phys. 178, 439. Harris, L. A. (1968a). J. Appl. Phys. 39, 1428. Harris, L. A. (1968b). Anal. Chem. 24A,40. Harris, L. A. (1974). J. Vac. Sci. Technol. 11, 23. Hedman, J. (1974). At. Energy Rev. 763. Hedman, J., Heden, P. F., Nordling, C., and Siegbahn, K. (1969). Phys. Lett. A 29, 178. Hedman, J., Klasson, M., Nilsson, R., and Nordling, D. (1971). Phys. Scr. 4, 195. Hedman, J., Baer, Y., Berndtsson, A., Klasson, M., Leonhardt, G., Nilsson, G., and Nordling, C. (1972). J . Electron Spectrosc. Relat. Phenom. 1, 101. Helmer, J. C., and Weichert, N. H. (1968). Appl. Phys. Lett. 13,266. Hengehold, R. L., and Pedrotti, F. L. (1972a). Phys. Rev. 6, 2262. Hengehold, R. L., and Pedrotti, F. L. (1972b). Phys. Reo. 6, 3026. Hercules, D. M. (1972). Anal. Chem. 44,No. 5, 106. Hercules, D. M., and Carver, J. C. (1974). Anal. Chem. 46,No. 5, p. 133R. Hercules, S. H., and Hercules, D. M. (1972). Int. J . Enuiron. Anal. Chem. 1, 169. Hillier, I. H., and Kendrick, J. (1975). J. Electron Spectrosc. Relar. Phenom. 6, 325. Hollander, J. M., and Shirley, D. A. (1970). Annu. Rev. Nucl. Sci. 20, 435. Holm, R. (1972). GIT Fachz. Lab. 16, 12. Holm, R. (1973a). G I T Fachz. Lab. 17, 929. Holm, R. (1973b). Metalloberjlaeche-Angew. Elektrochem. 27, 199. Holm, R. (1974). Vak.-Tech. 23,208. Hopfgarten, F. (1973). J . Electron Spectrosc. Relat. Phenom. 2, 13. Houk, K. N., Davis, L. P., Newkome, G. R.. Duke, R. E., and Nauman, R. V. (1973). J . Am. Chem. SOC.95,8364. Huang, J. T. J., Elison, F. 0.. and Rabalais, J. W. (1974). J. Electron Spectrosc. Relat. Phenom. 3, 339. Hiifner, S., and Wertheim, G. K. (1974). Phys. k t t . A 47, 349. Hiifner, S., and Wertheim, G. K. (1975a). Phys. L e f t . A 51,299. Hiifner, S., and Wertheim, G. K. (1975b). Phys. Lett. A 51, 301. Hulett, L. D., Carlson, T. A., Fish, B. R., and Durham, J. L. (1972a). Proc. Symp. Air Quai., 1971 (cited in Carlson, 1972). Hulett, L. D., Bacarella, A. L., LiDonnici, L., and Griess, J. C. (1972b). J. Electron Specrrosc. Relat. Phenom. 1, 169. Ignatiev, A., and Rhodin, T. N. (1973). Am. f a b . 4, No. 11, 9. Itikawa, Y. (1973). J . Electron Spectrosc. Relat. Phenom. 2, 125. Johansson, G., Hedman, J., Berndtsson, A., Klasson, M., and Nilsson, R. (1973). J . Electron Spectrosc. Relat. Phenom. 2, 295. Jolly, W. L. (1972a). Electron Spectrosc., Proc. Int. Confi, 1971 p. 629.

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

109

Jolly, W. L. (1972b). Faraday Discuss. Chem. SOC.54, 13. Jonas, A. E., Schweitzer, K., and Grimm, F. A. (1972). J . Electron Spectrosc. R e l a f . Phenom. 1, 29. Jonathan, N.. Morris, A., Okuda, M., Ross, K. J., and Smith, D. J. (1972). Faraday Discuss. Chem. SOC.54, 48. Kellerer, B., Cederbaum, L. S., and Hohlneicher, G. (1974). J. Electron Spectrosc. Relat. Phenom. 3, 107. Keski-Rahkonen, 0 .(1975). J. Phys. C . 8, 541. Khodeyev, Y. S., Siegbahn, H., Hamrin, K., and Siegbahn, K. (1972). ESCA Applied to High Temperature Molecular Beams of Bismuth and Lead. UUIP-802. Institute of Physics, Uppsala University, Uppsala. Kieser, J. (1974). Phys. Fenn. 9, Suppl., 173. Kim, K. S., and Davis, R. E. (1973). 1. Electron Spectrosc. Relat. Phenom. 1, 251. Klasson, M., and Manne, R . (1972). Electron Spectrosc., Proc. Int. Con$, 1971 p. 471. Klasson, M., Hedman, J., Berndtsson, A., Nilsson, R., Nordling, C., and Melnik, P. (1972). Phys. Scr. 5, 94. Kosmus, W., Rode, B. M., and Nachbaur, E. (1973). J. Electron Spectrosc. Relat. Phenom. I, 409. Kowalczyk, S. P., McFeely, P. R.,Ley, L., Pollak, R. A., and Shirley, D. A. (1974). Phys. Rev. 9, 3573. Kramer, L. N., and Klein, M. P. (1972). Electron Specrrosc., Proc. Int. Con$, 1971 p. 733. Krause, M. 0. (1972). Adv. X - R a y Anal. 16, 74. Krause, M. 0.(1974). Phys. Fenn. 9, Suppl., 281. Krause, M. O., and Wuilleumier, F. (1972). Electron Spectrosc., Proc. Inr. Conf., 1971 p. 759. and Wuilleumier, F. (1973). Proc. In!. Con$ Inn. Shell loniz. Phenom. Future Krause, M. 0.. Appl., 1972 p. 2331. Larson, P. E. (1974). J. Electron Spectrosc. Relat. Phenom. 4, 213. Leckey, R. C. G. (1972). Elec. Eng. Trans. 38. Lee, D . D. (1972). Rev. Sci. Instrum. 43,1291. Lee, J. D. (1973). Rev. Sci. Instrum. 44,893. Levy, B., Milie, P., Ridard, J., and Vinh, J. (1974). J. Electron Spectrosc. Relat. Phenom. 4, 13. Ley, L., Kowalczyk. S. P., McFeely, F. R., and Shirley, D. A. (1974). Phys. Reo. B 10, 4881. Lindau, I., Helmer, J. C., and Uebbing, J. (1973). Rev. Sci. Instrum. 44,265. Lohr, L. L., Jr. (1972). Electron Spectrosc., Proc. I n t . Con$, 1971 p. 245. Liith, H., and Jussell G. J. (1974). Surface Sci. 45, 329. McFeely, F. R., Kowalczyk, S. P., Ley, L., and Shirley, D. A. (1973). Phys. Lett. A 45, 227. McGlynn, S. P., and Meeks, J. L. (1975). J. Electron Spectrosc. Relat. Phenom. 6, 269. Manson, S. T. (1973). J. Electron Spectrosc. Relat. Phenom. 1, 413. Matsuzawa, M . (1974). J. Electron Spectrosc. R e h . Phenom. 4, 1. Menzel, D. (1975). J. Vac. Sci. Techno/. 12, 313. Millard, M.(1972a). Electron Spectrosc., Proc. Int. Con$, 1971 p. 765. Millard, M. (1972b). Anal. Chem. 44,828. Millard, M., and Masri, S. M. (1974). Anal. Chem. 46,1820. Mirza, M. Y., and Hurt, W. B. (1974). J. Electron Spectrosc. Relat. Phenom. 3, 166. Moddeman, W. E., and Cothern, C. R. (1975). J. Electron Spectrosc. Relat. Phenom. 6, 253. Morishima, I., Yoshikawa, K., and Yonezawa, T. (1972). Chem. Phys. Lett. 16, 336. Natalis, P., Delwiche, J., and Collin, J. E. (1971). Chem. Phys. Lett. 9, 139. Nefedov, V. I., Salyn, Ya. V., Domashwvskaya, E. P., Ugai Ya. A., and Terekhov, V. A. (1975). J . Electron Spectro.rc. Relat. Phenom. 6, 231. Nemoshkalenko, V. V., Aleshin, V. G.,Kucherenko, Yu.N., and Sheludchenko, L. M. (1975). J. Electron Specrrosc. Relat. Phenom. 6, 145. Nishijima, A., Kudo, M., Nihei, Y.. and Kamada, H. (1974). Phys. Fenn. 9, Suppl., 324.

110

D.

BERBNYI

Nordling, C. (1972). Angew. Chem., Int. Ed. Engl. 11, 83. Nordling, C., Sokolowski E., and Siegbahn, K. (1958). Ark. Fys. 13. 483. Novakov, T., and Hollander, J. M. (1968). Phys. Rev. Lett. 21, 1133. Novakov, T., and Prins, R. (1972). Electron Spectrosc., Proc. In t . Con/., 1971 p. 821. Novakov, T., Mueller, P. K., Alcocer, A. E., and Otvos, J. W. (1972). J. Colloid Interface Sci. 39, 225. Okusawa, M., Ishii, T., and Sagawa, T. (1974). Phys. Fenn. 9, Suppl., 298. Page, P. J., Trimm, D. L., and Williams, P. M. (1974). J. Chem. Soc., Faraday Trans. I 70, 1769. Palmberg, P. W. (1975). J. Vac. Sci. Technol. 12, 379. Perry, W. B., and Jolly, W. L. (1974). J. Electron Spectrosc. Relat. Phenom. 4, 219, Poole, R. T., Leckey, R. C. G., Jenkin, J. G., and Liesegang, J. (1973a). Phys. Reu. B 8. 1408. Poole, R. T., Kemeny, P. C., Liesegang, J., Jenkin, J. G., and Leckey, R. C. G. (1973b). J . Phys. F 3, 47. Price, W. C. (1974). Ado. At. Mol. Phys. 10, 131. Price, W.C., Potts, A. W, and Streets, D. G. (1972). Electron Specrrosc., Proc. l n t . Con/., 1971 p. 187. Rabalais, J. W., and Colton, R. J. (1972). J. Electron Specrrosc. Relat. Phenom. 1, 83. Rabalais, J. W., Bergmark, T., Werme, L. 0.. Karlsson, L.. Hussain, M., and Siegbahn, K. (1972). Electron Spectrosc, Proc. f n t . Con/., 1971 p. 425. Ramsey, J. A. (1971). Vacuum 21, 115. Riach, G. E., and Goff, R. F. (1974). Ind. Res. June, 84. Risley, J. S. (1972). Reu. Sci. fnstrum. 43, 95. Roberts, R. F. (1974). J. Electron Spectrosc. Relar. Phenom. 4, 273. Rosencwaig, A., Wertheim, G. K., and Guggenheim, H. J. (1971). Phys. Reu. Lett. 27, 479. Rynd, J. P., and Rastogi, A. K. (1975). Surface Sci. 48, 22. Samson, J. A. R., and Petrosky, V. E. (1974). J. Electron Spectrosc. Relat. Phenom. 3, 461. Schafer, W., Schweig, A., Vermer, H., Bickelhaupt, F., and de Graaf, H. (1975). J . Ekctron Spectrosc. Relat. Phenom. 6, 91. Schon, G., and Lundin, S. T. (1972). J. Electron Spectrosc. Relat. Phenom. 1, 105. Schwartz, M.E., Switalski J. D., and Stronski, R. E. (1972). Electron Spectrosc., Proc. I n t . Con/., 1971 p. 605. Schweig, A., and Thiel, W. (1973). J. Electron Spectrosc. Relat. Phenom. 2, 199. Schweig, A., and Thiel, W. (1974). J. Electron Spectrosc. Relat. Phenom. 3, 27. Sharma, J., Staley, R., Rimstidt, J., Fair, H., and Gora, T. (1971). Chem. Phys. Lett. 9, 564. Shaw, R. W., and Thomas, T. D. (1972). Phys. Rev. Lett. 29, 699. Shepherd, F. R., and Williams, P. M. (1974). J . Phys. C 7, 4427. Shevchik, N. J.. Gardona, M.,and Tejeda, J. (1973). Phys. Rev. 8, 2835. Shirley, D. A. (1975). J . Vac. Sci. Technol. 12, 280. Siegbahn, H., and Siegbahn, K. (1973). J. Electron Spectrosc. Relat. Phenom. 2, 319. Siegbahn, K. (1972a). Electron Spectrosc, Proc. Inr. Con/., I971 p. 15. Siegbahn, K. (1972b). Electron Spectroscopy for Chemical Analysis. UUIP-793. Institute of Physics, Uppsala University, Uppsala. Siegbahn, K. (1974). Electron Spectroscopy-An Outlook. UUIP-880. Institute of Physics, Uppsala University, Uppsala. Siegbahn, K. (1975). Electron Spectroscopy and Molecular Structure. UUIP-909. Institute of Physics, Uppsala University, Uppsala, Sweden. Siegbahn, K., Nordling, C.. and Sokolowski, E. (1958). Proc. Rehouoth Con/. Nucl. Struct., 1957 p. 29. Siegbahn, K., Nordling, K., Fahlman, A., Nordberg, R., Hamrin, K., Hedman, J., Johansson, G., Bergmark, T., Karlsson, S. E., Lindgren, I., and Lindberg, B. (1967). Noua Acto Regiae Soc. Sci. Ups. [4] 20.

RECENT APPLICATIONS OF ELECTRON SPECTROSCOPY

111

Siegbahn, K., Nordling, C., Fahlman, A,, Nordberg, R., Hamrin. K., Hedman, J., Johansson, G., Bergmsrk, T., and Karlsson. S. E. (1968). Ann. Phys. (Leipzig) [7] 3, 281. Siegbahn, K., Nordling, C.. Johansson, G.. Hedman, J.. Heden, P. F., Hamrin, K., Gelius, U., Bergmark, T.. Werme. L. O., Manne, R., and Baer. Y. (1969). “ESCA Applied to Free Molecules.’’ North-Holland Publ., Amsterdam. Siegbahn, K., Gelius, U., Siegbahn, H., and Olson, E. (1970). Phys. Lett. A 32, 221. Siegbahn, K., Hammond, D., Fellner-Feldegg H., and Barbett, E. E. (1972). Science 176, 245. Siegmann, H. C. (1975). Phys. Rep. 17c. 39. Sokolowski, E.. Nordling, C., and Siegbahn, K. (1958). Phys. Reo. 110, 776. Spanget-Larsen, J. (1974). J . Electron Spectrosc. Relat. Phenom. 3, 369. Streets, D. G., and Williams, T. A. (1974). J. Electron Specrrosc. Relat. Phenom. 3, 71. Suffolk, R. J. (1974). J. Electron Spectrosc. Relat. Phenom. 3, 53. Swartz, W. E. (1973). Anal. Chem. 45, 788A. Swartz, W. E., and Alfonso, R. A. (1974). J. Electron Spectrosc. Relat. Phenom. 4, 351. Swartz, W. E., and Hercules, D. M. (1971). Anal. Chem. 43, 1774. Swartz. W. E., Watts, P. H., Watts, J. C., Brasch, J. W., and Lippincott, E. R. (1972). Anal. Chem. 44,2001. Swingle, R . S., 11. (1975). Anal. Chem. 47, 21. Tam, W . C., Yee, D., and Brion, C. E. (1974). J . Elecrron Spectrosc. Relar. Phenom. 4, 77. Todd. C. J. (1973). Vacuum 23, 195. Traum, M. M.. Rowe, J. E.. and Smith, N. E. (1975). J. Vac. Sci. Technol. 12, 298. Trautwein, A., and Keune. W. (1972). Metall. Tech. 26, 435. Tripathi, K. C . , and Clark, D. T. (1973). Nature (London), Phys. Sci. 241, 162. Turner, D. W. (1968). Proc. R . SOC.London, Ser. A 307, 15. Turner, D. W.. and Al-Joboury. M. 1. (1962). Chem. Phys. 37, 3007. Turner, D. W., Baker, C . . Baker, A. D.. and Brundle, C . R . (1970). “Molecular Photoelectron Spectroscopy.” Wiley (Interscience), London. van den Ham, D. M., van der Meer, D., and Feil, D. (1974). J . Electron Spectrosc. Relat. Phenom. 3, 479. van der Wiel, M. J., and Brion, C . E. (1973). J. Electron Spectrosc. Relat. Phenom. I, 309. van Hoorn, M. D. (1975). J . Electron Spectrosc. Relar. Phenom. 6, 65. Veal, B. W., and Lam, D. J. (1974a). Phys. Lert. A 49, 446. Veal. B. W.. and Lam, D. J. (1974b). Phys. Reo. B 10, 4902. Viinikka, E.-K.. and o h m , Y. (1974). Phys. Fenn. 9, Suppl., 304. Vilesov, F. I.. Kurbatov, 9. L., and Terenin. A. N. (1961). Dokl. A k a d . Nauk SSSR 138, 1329. Waclawski, 9. J., Vorburger, T. V., and Stein, R. J. (1975). J. Vac. Sci. Technol. 12, 301. Wagner, C. D. (1972a). Anal. Chem. 44. 1050. Wagner, C. D. (1972b). Electron Spectrosc, Proc. Int. Conf, 1971 p. 861. Wannberg, B., Gelius, U., and Siegbahn, K. (1974). J. Phys. E 7 , 149. Wertheirn, G. K., Conen, R. L., Rosencwaig, A., and Guggenheim, H. J. (1972). Elecrron Spectrosc., Proc. Inr. ConJ, 1971 p. 813. Wieczorek, J. S., Koenig, T., and Balk, T. (1975). J. Electron Spectrosc. Relar. Phenom. 6, 215. Wooten, F., Huen. T., and Winsor, H. V. (1972). Elecrron Spectrosc., Proc. Int. Conf., 1972 p. 283. Worley, S. D. (1971). Chem. Ret!. 71, 295. Worley, S. D.. Mateescu, G . D., McFarland, W. C., Fort, R. C., and Sheley, C. F. (1973).J . Am. Chem. SOC. 95, 7580. Yates, K., Barrie, A., and Street, F. J. (1973). J. Phys. E 6, 130. Yin, L. I., and Adler, I. (1974). Phys. Reo. A 9, 1070. Yu, K. Y..McMenamin. J. C . . and Spicer, W . E. (1975). J . Vac. Sci. Technol. 12, 286. Zatko. D. A., and Prather, J. W. (1973). J. Elecrron Specrrosc. Relat. Phenom. 2, 191.

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Laboratory Isotope Separators and Their Applications S. B. KARMOHAPATRO Saha Institute of Nuclear Physics Calcutta, India

........................... 113 ........................... 118 A. Hot Cathode Arc Discharge Sources ...................................... ..I19 B. Thermal Ion Sources .......... .............................................. ,127 C. Ion Source Technology for On-Line Isotope Separation ........................ ,128 D. Extraction and Formation of the Ion Beam ...................................... ,132 111. Acceleration of Ions ............. ..............,135 IV. Mass Analyzers .................. ...............137 1. Introduction

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11. Production and Formation of Intense Ion Beams

A. B. C. D. E.

V.

VI.

VII.

VIII.

Direction Focusing Magnetic First Order Single Direction Focusing Magnetic Analyzers Second Order Single Focusin Higher Order Focusing Magnetic Analyzers.. .................................... .140 Two-Directional Focusing with the Fringing Field of a Homogeneous Sector Magnet ............................................................................... 141 F. Two-Directional Focusing with an Inhomogeneous Magnetic Field . . . . . . . G. Magnetic Analyzers with Higher Dispersion.. ............................... H. Axial Beam Crossing with Magnetic Analyzers ................................... .146 Performance ............................................................................. .146 A. Dispersion and Resolution ................................. 147 B. Enrichment Factor and Contamination ........................................... ,148 C. Efficiency ............................................................................ ,152 Collection ................................................................................ 152 A. Factors Influencing Direct Deposition . . . . . . . . . . . . . . . . ......... 153 B. Sputtering Method of Preparation of Thin Samples . .........154 C. Electrostatic Retardation Method .................................................. 155 D. Collection of Short-Lived Isotopes with On-Line Separators .................... 155 Applications .............................................................................. 157 A. Application of Separated Ions ...................................................... 157 B. Application of Isotope Separators as Mass Spectrometers.. ..................... ,160 C. Isotope Separators as Low Energy Accelerators.. ................................ ,161 Conclusions .............................................................................. 168 References ............................................................................... .169

I. INTRODUCTION Since the introduction of the principle of mass spectroscopy the subject has been developed to a high degree of perfection and is used today in different forms in various fields of science. Besides their conventional use as 113

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analytical instruments related to measurements of the relative abundances of isotopes, sophisticated high resolution mass spectroscopes have been extensively employed for accurate measurement of nuclear masses. Electromagnetic isotope separators, developed as a result of the needs of high intensity applications of mass spectroscopy, have been important tools for quantitative enrichment of isotopes. Though the possibility of the latter method of isotope separation was suggested by Lindemann and Aston (1919) during the early days of mass spectroscopy, the method could not be applied in practice because of technical limitations. In those days, methods of producing intense ion beams and collecting ions after separation had not been developed to the extent needed for the mass spectroscopic method of isotope separation. With the emergence ofparticle accelerators, the need arose for the perfection of isotopic targets for nuclear reaction studies; and an early attempt by OIiphant et ai. (1934) succeeded in separating lithium isotopes with the help of Wien’s velocity filter. A thermal emission type source used in the experiment could produce current of a few microamperes of ’Li ions; but collection was not sufficient due to sputtering phenomena, which were not well understood at that time. This instrument was improved by Yates (1938) who collected boron isotopes with good efficiency. An electrostatic lens system for focusing the ion beam and a retarding potential before collection to avoid sputtering, used by Yates in this instrument, were new features. These techniques are even now considered very useful for satisfactory performance of a high intensity mass spectrometer when applied for producing enriched isotopes. For isotope separation, Smyth e l al. (1934) used a special magnetic analyzer system that produced samples of isotopes of the order of milligrams of lithium, potassium, and strontium. Walcher (1937, 1938) constructed an electromagnetic isotope separator that is considered to be the prototype for all modern laboratory isotope separators. It has been redesigned for improved performance a number of times and is still in use. In its present form it resembles a 90” sector focusing homogeneous magnetic mass spectrometer. It is of relatively large size due to a number of factors, including: the use of a high intensity ion source; a high accelerating voltage ( - 20 keV) for extraction of the space charge limited ion current; an electrostatic lens system for focusing the ion beam prior to its entry into the magnetic analyzer in place of a conventional slit system as used in mass spectrometry; a magnetic analyzer with a 6-cm gap to accommodate an intense ion beam; and finally due to the necessity of using a large radius of curvature Ro (100 cm) and a large field strength ( 5000 G) which are appropriate for analyzing fast ( 20-keV) ions as can be seen from the mass spectrometer equation for a singly charged ion

-

N

BoRo = 144(MV)”2

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where Bo is the magnetic field in gauss, R , is the radius of curvature of the trajectory of the ions in the magnetic analyzer in centimeters, M is the mass of the ions in amu, and V , in volts, is the voltage through which the ions are accelerated. The main differencebetween a laboratory isotope separator and a conventional mass spectrometer lies in the use of a high intensity ion source which results in the allied problems associated with focusing the intense beam onto a suitable collector and collection of the required isotope for enrichment. Walcher developed a few types of high intensity ion sources for both gases and solids and successfully collected a number of isotopes after separation. Separated stable isotopes and preparation of thin isotopic targets for nuclear studies with accelerators were urgently needed ; in this context the separation method of Walcher showed a possible direction for technological development. Following the method of Walcher, Koch and Bendt-Nielsen (1944) constructed the first Scandinavian laboratory isotope separator which was followed by a number of similar instruments constructed there. They belong to a class well known for the wide range of possible beam intensities from low values to a few hundred microamperes of mass-analyzed beam and also for the achievement of high resolving power. During World War 11, the large electromagnetic isotope separators built in Oak Ridge for separation of uranium isotopes proved over the course of time to be not so useful for this purpose as compared to the diffusion But their usefulness for separatechnique developed for separation of 235U. tion of stable isotopes was greatly realized. In spite of the large production potential of these devices, small-scale separation instruments of the type of the laboratory isotope separators developed earlier began to be preferred for their versatility. Large electromagnetic isotope separators at Oak Ridge known as Calutrons are 180" focusing machines of massive size with sufficient space for accommodating the ion source inside the magnetic analyzer. For laboratory isotope separators with much smaller magnets, it is convenient to use sector magnets of focusing angle 4 < 180" which reduce the weight of the magnet; it was convenient to place the ion source at a distance from the magnet. Scandinavian isotope separators satisfy these characteristics. However, the results of the experiences with Calutrons, when declassified and published, were of great use in developing further the techniques needed for laboratory isotope separators. Calutrons of alpha [122-cm (48-in.) radius] and beta [61-cm (24-in.) radius] types and in particular the electromagnetic isotope separator of beta type at Harwell are even now the largest sources of stable isotopes. Their colossal structure and the efforts needed in their operation rule out their installation in a laboratory, but it was necessary to develop a laboratory type mass separator for beam intensities of the orders obtained with the giant machines. In view of this Kistemaker and

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co-workers (Zilverschoon, 1954) at Amsterdam constructed an electromagnetic isotope separator with a maximum intensity of a few milliamperes, which was higher than that obtained with the Scandinavian type isotope separators. A special feature of this instrument was that it could be used as a low energy accelerator in the range of energies usually used for experiments in atomic physics. The Amsterdam machine consists of a 180" focusing inhomogeneous magnetic analyzer. Later, a few similar medium intensity laboratory isotope separators with homogeneous magnets of 60" focusing angle were developed by Bernas and co-workers. The medium intensity laboratory isotope separators are inferior in resolving power to the Scandinavian low intensity machines but produce higher ion currents without using electrostatic lenses as is done in Calutrons. It was shown (Freeman, 1963) that both machines may be developed to such an extent that there is little to choose in their performances as regards resolution and intensity. A large number of laboratory isotope separators of both types are now in operation in various laboratories of the world. Though they are not competitors of the production machines, they have been versatile in various uses such as ion implantation, studies on sputtering and channeling phenomena, charge exchange and excitation by ion impact, atomic diffusion and ranges in solids, and various other problems associated with the borderline between atomic and nuclear physics. In view of the problems with respect to other than stable isotope separation, it was expected that faster ions than those usually used with a laboratory isotope separator would be of great use. Usually power supplies ranging from 10 to 100 k V for accelerating the ions are used in laboratory isotope separators. One way to accelerate the ions to an energy faster than that available with the usual power supply of an isotope separator is to use multiply charged ions from the ion source. This again involves the development of a technique for production of an intense multiply charged ion beam. On the other hand, an enhanced voltage for accelerating the ions can also be used. The latter method was applied by Bogh et al. (1962) and Nielsen (1967) in constructing a laboratory isotope separator with 600-kV accelerating voltage at Aarhus. This pioneering work raised the isotope separators to the status of low energy ion accelerators. A few such isotope separators have been developed so far in different laboratories. Alternative methods for raising the target or the analyzer to a high voltage for postacceleration of the ion beam have been applied by Wilson (1967) and Guernet et al. (1970), respectively. A major application of laboratory isotope separators in separating radioactive isotopes has been possible due to their intrinsic higher efficiency compared to that of conventional mass spectrometers. Of course a mass spectrometer is useful for separation of short-lived isotopes since intensity is

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not a factor here. This was demonstrated quite early by Yamaguchi (1941) and Dempster (1944) who were pioneers in this respect. However, for more efficient separation of these isotopes, it was thought necessary to develop a small-scale laboratory isotope separator. Hayden and Lewis (1946) constructed such an instrument to investigate reactor-induced radioisotopes at Chicago. Reynolds et al. (1949) at Berkeley installed a similar laboratory isotope separator for analyzing cyclotron-induced activities. Handling of radioactive isotopes for transporting to the ion source as well as their magnetic analysis and collection posed special problems different from those with separators for stable isotopes. The problems are being solved with improved techniques. Andersson and Rudstam (1964) have shown the advantages of a laboratory isotope separator over a mass spectrometer in measuring nuclear cross sections. A number of instruments in different laboratories are in use for preparing thin radioactive targets for nuclear spectroscopy and for yield and cross-section measurements. However, an instrument off line from the source of radioactive isotopes, e.g., a reactor or an accelerator, is limited by the available techniques to the analysis of isotopes with half-lives of less than a few minutes. In recent years, the necessity of analyzing short-lived isotopes far away from the a-stability line required the analysis of half-lives as low as a fraction of a second. For this purpose, laboratory isotope separators on line with the accelerator or reactor were developed along with modern detectors. In an on-line separation system the processes of target irradiation, transport of the short-lived nuclei to the ion source, mass separation, collection,and spectroscopy form a continuous chain process, so that an investigation of nuclear systematics can be extended to the nuclei far off the stability line. The on-line separation technique was initiated by Kofoed-Hansen and Nielsen (1951) in the past for analyzing isotopes of half-lives up to a few seconds. Through improved techniques, with an on-line isotope separator in its present form it is possible to analyze isotopes with half-lives as low as 0.1 sec (Klapisch and Bernas, 1965). Though the techniques developed in mass spectrometry and large electromagnetic isotope separators like Calutrons have been rightly utilized in the improvement of laboratory isotope separators, techniques from other fields have also proved to be useful. High intensity ion sources are common to both the isotope separator and accelerator fields. Research on ion sources has benefitted both fields. Research on plasma physics has given valuable guidelines for perfection of the techniques for production of intense ions. Based on investigationsof the focusing of charged particles, necessary for the design of cyclic accelerators, a-ray spectrometers, nuclear spectrometers, and mass spectrometers, ideas have been introduced for development of new types of magnetic analyzers for laboratory isotopes separators. Research on

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the behavior of fast ion beams colliding with solids has been used to improve techniques for collection of an isotopic ion beam. Thus the development of laboratory scale electromagnetic isotope separation in its present form is the result of the successful interaction of various fields of physics. With regard to the associated technology, it is a sophisticated electronic instrument with versatile capabilities for use in research in atomic and nuclear physics. Apart from the classification of laboratory isotope separators according to their being low intensity or medium intensity machines, there are alternative ways of classifying them according to their uses, the type of magnetic analyzer used, resolving power, and the range of the accelerating voltage. But all of them have common fundamental component systems, such as ion beam production and formation, magnetic analysis of the beam, and collection of isotopes. The gradual development of the instruments since their historical introduction has been summarized by Smith (1957) and Koch et al. (1958); further progress has been reviewed by Alvager and Uhler (1968) and Brown (1968). Proceedings of a number of international conferences held at Harwell (Smith, 1956), Amsterdam (Kistemaker et d.,1958), Vienna (Higatsberger and Viehbock, 1960), Orsay (1962), Aarhus (Koch and Nielsen, 1965), Berkeley (Asilmore) (1967), Marburg (Wagner and Walcher, 1970), and Sweden (Skovde) (Andersson and Holmen, 1973) include a large number of papers on the gradual development of modern isotope separators for their uses in various lines of research. In the present paper, we shall describe the general features of laboratory isotope separators, their applications in separating stable and radioactive isotopes, and their indirect uses as low energy accelerators and high transmission mass spectrometers in atomic and nuclear physics research.

11. PRODUCTION AND FORMATION OF INTENSEIONBEAMS

We have already mentioned that the isotope separator differs appreciably from a conventional mass spectrometer in that ion sources that can produce intense ion beams are used. For a high intensity ion source, the extraction and formation of an ion beam and its transport prior to magnetic analysis pose special problems which have been solved in various ways. High intensity ion sources, as common components of accelerators, isotope separators, and thermonuclear machines, have been the subject of elaborate research by a large number of workers in different fields. Many specialized papers on this topic appear in the proceedings of the conferences on electromagnetic isotope separation referred to in the introduction and in those of the conferences and symposia on ion sources held in Saclay (1969), Wash-

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ington, D.C. (1970), Brookhaven (1971), Gatlinburg (1971), and Vienna (1972). Various aspects of ion sources useful for laboratory isotope separators have been summarized in greater detail in a number of reviews by many authors. We mention some of these which include a large number of references on the subject. An excellent review has been given by Freeman (1973)on the production and manipulation of ion beams in various types of ion sources used with separators for ion implantation. In a separate paper, Freeman and Sidenius (1973) have summarized technical details on the choice of construction materials, behavior of ion sources under the influence of chemical and metallurgical reactions. It includes in tabular form recommendations for source feed materials and for operational procedures for the elements covering the periodic table based on operational experience with a range of ion beam facilities. Though meant for general applications, the review given by Septier (1967) on the extraction conditions for various ion sources is useful for isotope separation techniques. Though the specific requirement for choosing an ion source for separators is that of high intensity, other qualities such as minimum energy spread, high efficiency for producing ions from neutral atoms, and good stability of the beam cannot be neglected for obtaining optimal resolution and intensity in the separator. We shall confine our discussions to a few types of high intensity ion sources that are suitable for separators. Generally, high intensity ion sources commonly used with isotope separators are generated in arc discharge sources forming a plasma, consisting of ions and electrons, of very 108-10’4 particles/cm3. Ions are extracted from the plasma high density, by an extractor electrode held at a negative potential with respect to that of the source through a small orifice in the container of the plasma. For extraction of an intense ion beam with small divergence and free of aberrations, the system has to be designed properly.

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A. Hot Cathode Arc Discharge Sources

If the electrons produced by heating a filament are accelerated to the anode box, with a potential difference of 10-50 V between them, at a gas pressure between lo-‘ and ton, an arc strikes, which is a stable plasma. At a pressure of 10-6-10-4 torr, the source is a conventional electron impact ion source as used in mass spectrometry, which produces low ion current. For an arc source, precision and selectivity of the ionization by single collision is absent; but due to the plasma sheath near the cathode the large electron current extracted from the filament produces intense ions in

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the arc. The problem of constriction of the arc discharge is solved by using an external magnetic field applied in the direction of the axis of the discharge chamber. Inherent instabilities of the arc plasma and the dependence of the extraction of ions on the boundary of the plasma near the exit slit are two major problems to be considered in designing an efficient arc ion source. 1. Conventional Magnetic Oscillation Sources

Magnetic oscillation arc sources with either axial or lateral extraction have been developed by many authors for use with laboratory isotope separators. Axial extraction sources normally have circular extraction apertures and were developed primarily for Scandinavian low intensity isotope separators with high resolution. Nielsen (1957)and Almen and Nielsen (1957)have made comprehensive studies of these sources; technical modifications in applications of the same sources for laboratory isotope separators will be found in reviews by Burgman and Anderson (1958), Uhler and Alvager (1958), Almen (1962), and Nielsen (1970). Lateral extraction sources were originally developed by the Calutron group and are now used in Calutrons for resolved high intensity ions in the 10-100-mA range. These sources are operated in the field (2000 G) of the main magnetic analyzer and suffer from “hash,” an effect due to plasma instability leading to a defocusing of the ion beam. Dawton (1958) constructed a scaled-down version of the Calutron source and used it with a sector field intermediate intensity isotope separator with an auxiliary variable magnetic field ( < 1000 G) for the source. They generally have extraction slits delivering a wedge-shaped beam, and with thinner filaments have been successfully applied by Bernas (1954) for intermediate intensity isotope separators. A modified version of this source is used by Chavet and Bernas (1967a,b),and detailed studies of the behavior of this type of source are due to Chavet (1965) and Chavet and Bernas (1967a,b).Though the ion optical behavior and extracted beam intensity are different in the two types of ion sources, either of them can be properly designed to be used with any laboratory isotope separator to attain resolution or intensity as desired in various applications. In its general form suitable for gases or vapors, a Nielsen axial extraction source consists of a tungsten spiral as the directly heated cathode, a graphite or molybdenum cylinder as anode (easily replaceable for different charge materials to avoid memory effect), and an insulated end plate with a circular aperture for extraction of ions. The source is placed inside a solenoidal magnetic field variable up to a few hundred gauss (Fig. 1). In the oscillating mode, the ion source is operated with the end plate at the cathode potential so that the electrons, contained mainly in the magnetic axis, oscillate along

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I

I

I I

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I

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2-

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FIG.1. Diagram of a Nielsen-type ion source ofdirect mode operation: (1) magnet coil, (2) container of the coil, ( 3 ) bellow, (4)vitreosil insulator tube containing the ion source, ( 5 ) brass flange with hermetically sealed electrical leads, (6) metal flange connecting to the insulating flange, (7) filament coil, (8) cylindrical graphite anode, (9) extraction electrode, (10) gas inlet.

it. In the direct mode of operation, the end plate is connected to the anode potential for electrons passing to it directly. Though the direct mode has the disadvantage of higher discharge current and heat, it is more suitable for poorly volatile charge materials. Otherwise an oscillating mode is universally used for gases and volatile charge materials. Almen (1962) has discussed the operational characteristics of several variations of this type of source used for different charge materials. In his studies, the simplest low temperature source for gases was made of a stainless steel anode within a ceramic or quartz tube. A modified version with a larger filament and radiation shields surrounding the anode has been used for materials to be handled at 500-600°C. For medium temperatures, i.e., 500-800"C, a graphite anode with pyrophyllite insulator can be used. For rare-earth chlorides requiring a temperature range of 800-1000"C, direct mode operation with a graphite anode surrounded by molybdenum heating elements, properly shielded together with a quartz insulator, gives very high efficiency. Sidenius and Skilbreid (1961) developed a method of

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chlorination used for the rare-earth elements. This method employs CCl, gas passing through the oxides heated to 800°C; the chloride thus formed evaporates into the discharge chamber. This method has proved successful for rare-earth metals, Be, B, Al, V, Ge, Sn, Ba, Hf, W, etc. Recently Love (1973) has observed, while applying the method of chlorination of B e 0 with CCI,, that introduction of BC1, enhances the Be+ ion output by more than three times. Further experimental results can ascertain the reason for such enhancement. For very high vapor pressure compounds like ZrCl,, MoCI,, TaC1, , and WCI,, a water-cooled container for the charge materials is used. In other cases the heat of the discharge chamber, to which a tube containing the charge material heated to vaporize by a furnace is protruded, is sufficient to keep the material in a state for ionization. Almen (1962) has described the results of his studies in tabular form in which the charge materials, ion sources with modifications, and furnace temperature required for all elements up to bismuth are given with additional remarks.

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n

FIG.2. Axial cross-sectional view of Bernas-type lateral extraction ion source: ( 1 ) filament-anode assembly, (2) extraction slit, (3) heat shields, (4) oven for charge material.

Figure 2 shows the schematic representation of a lateral extraction source of Bernas type in which the filament is within the discharge chamber 1 kG. The discharge chamber has a and the external magnetic field is lateral extraction slit for forming a wedge-shaped ion beam; it does not require an end plate as anticathode. If properly designed, the source is capable of delivering a high intensity ion beam of good resolution in an isotope separator. With furnaces for solid elements, the source can be easily operated for most elements. For refractory metals, to avoid the use of a high temperature furnace, Druaux and Bernas (1955) used a sputtering probe for ionization of these metals sputtered from it by impact of ions of a support

-

LABORATORY ISOTOPE SEPARATORS

123

gas introduced into the source for maintaining the discharge. Sputtering ion sources suitable for metals and volatile elements are described by Freeman (1369b) and Rautenbach (1960) for lateral and axial extraction of ions respectively. Many authors have successfully used sputtering ion sources for laboratory isotope separators and the results of the operational experiences show that proper care must be taken in designing the source so that the un-ionized sputtered atoms are not deposited on the insulators or d o not block the orifice of the chamber used for ion extraction. A wide range of metals, alloys, or compounds can be used as the cooled or uncooled sputtering probe in the source for obtaining ionization of a specific element. A comprehensive collection of data on sputtering yields and mechanisms will be found in the works of Kaminsky (1965), Carter and Colligon (1968), Almen and Bruce (1961), and Rol et a!. (1960). Both lateral and axial extraction sources can be used for stable and radioactive isotope separation with various modifications. An interesting modification is due to Uhler and Alvager (1958) who used two furnaces in one axial extraction source for separating small quantities of short-lived radioisotopes. One of the furnaces is charged with a stable material and is initially operated for observing an equilibrium condition of the source and the material. The other oven with radioactive material is operated next, and thus the effective separation time is greatly reduced without any loss of material. To avoid the loss of material or any contamination in the initial stage of separator operation, a two-furnace or multifurnace arrangement is very efficient, when small quantities of stable isotope separation are involved. Uhler and Alvager (1958), Sarrouy et al. (1965; Sarrouy and Kalpisch, 1961),and Sarrouy (1969) have described ion sources with two or more furnaces and the usefulness of their applications in isotope separation. 2. Freernun Source Freeman (1963, 1969b) developed a lateral extraction source that has a magnetron configuration in which the 4.5-mm tantalum rod filament lies 3 mm behind the extraction slit. Figure 3 shows the principle diagram of the

Anode Cathode Extractor FIG. 3. Principle diagram of a Freeman ion source

124

S. B. KARMOHAPATRO

ion source. The filament lies parallel to the extraction slit of the arc chamber. A magnetic field of 0-150 G provides the field parallel to the filament. In the conventional scaled-down version of the Calutron source, the magnetic field induced by the heated filament interacts with the field of the source magnet to produce distortion of the arc, deteriorating the performance of the source. Moreover, the complex behavior of the discharge maintained by the source magnet is responsible for “hash.” The Freeman source can even be operated without an auxiliary source magnet, when the magnetic field induced by the filament current, fixed in a space of the length of the chamber, can maintain the discharge with good ionizing efficiency. The low field of the auxiliary magnet adds a similar effect to enhance the efficiency of the source with a resultant double spiral electron path. Results show that a hash-free, high resolution ion beam at several milliampere intensity can be obtained for laboratory isotope separators with a Freeman ion source. The source is relatively free from instability of the beam and from contamination of the products due to ion-atom collisions in the extraction slit or the vacuum chamber. 3. Magnetron Source

Magnetron discharge as a source of intense singly and multiply charged ions with a crossed field was known early. Perovic (1957) and Cobic et al. (1963) reported a magnetron ion source for gases and solids and used it with a laboratory isotope separator. Figure 4 shows the schematic diagram of a magnetron ion source in which the radial electric and axial magnetic fields

FIG.4. Cross-sectional view of a magnetron ion source. F, filament. Poles of the magnet produce field in the direction of the filament. Axial extraction slit (not shown) is in the direction perpendicular to the plane of the diagram.

125

LABORATORY ISOTOPE SEPARATORS

produce intense ions due to the characteristic electron path in the source. It consists of a cylindrical anode with a filament heated by DC or AC voltage at its axis, stretched along the length of the anode. Though a magnetron source with lateral extraction slit has a configuration similar to that of the Freeman source, the principle of operation is different. Much higher magnetic fields than the Freeman source are used in it. The electron path in the magnetron source is subjected to a complex force resulting from the axial magnet, the tangential magnetic field induced by the current used for heating the filament, an axial electric field due to the potential difference between the filament terminals, and the radial electric field due to the anode voltage. The resultant electron path due to the combined action of the above forces makes spirals with the filament as axis. The spiral radius is determined by the strength of the source magnet and by the anode voltage. According to the condition given by Hull (1921), the anode current is cut off for

H , ( G )= 6.72VB’/2/r, (1) where Va is DC anode voltage in volts and ra is the anode radius in centimeters. Since the spiral steps are determined by the tangential magnetic field due to the filament and by the axial electric field due to the potential difference at the filament ends, the use of AC voltage for heating the filament will change the spiral steps in time, and evidently ionization will be enhanced for an increase in the number of revolutions and the total path length of electrons in the spiral. A magnetron source with lateral extraction slit can be conveniently used with a sputtering probe for charge materials with high melting points. 4. Hollow Cuthotie ion Source ( H C I S ) Sidenius (1965, 1969) developed a novel ion source using a hollow cathode principle to meet the specifications of an ideal ion source for a laboratory isotope separator. These specifications include such qualities as high efficiency for different elements in a large number of runs, high ion densitystable for maximum resolution, capability of operation at high temperature and small volume for the arc chamber for short-lived radioisotopes and for the possibility of heating the source to a high temperature with relatively low power consumption. The idea of using a hollow cathode follows from consideration of the goal of reducing the volume of discharge. The directly heated hollow cathode filament is made of a tantalum tube of 4-mm 0.d. and 3-mm i.d. turned down to a wall thickness of 0.05 mm over a length 8 mm and fitted into the outer tube for return of the filament current. Alternatively, a similar piece of graphite or tungsten wire tightly wound like a tube may be used for the same purpose.

-

126

S. B. KARMOHAPATRO

The filament,when heated, produces electrons that ionize the gas or vapor to form a plasma within the small volume of the cathode. A strong inhomogeneous magnetic field (1-3 kG) is used to confine the plasma to a narrow beam which passes through the exit hole of the anode to the extractor. In another version of a three-electrode system, a cathode for reflecting the electrons is used between the anode and the extractor to which the ions are drawn through an exit hole in the cathode (Fig. 5).

Fllrrnrnt

FIG. 5. Principle diagram of the threeelectrode hollow cathode ion source.

-

A microoven for vaporizing solids is placed within the source and can be heated to 2000°C. Sidenius (1970) reports the performance of a HCIS in an isotope separator to be excellent for gases and metals. But the high temperature of the source is not satisfactory for the chlorination method, and instead CIF, gives better performance when oxide compounds are used. A source with a 200-G electromagnet consumes 200 W total power. The use of a HCIS with a multiple-ion source system with provisions for automatic exchange of five ion sources for an on-line isotope separator is reported by Sidenius (1970) and Lindahl et af. (1970). Because of its small dimensions, a HCIS has proven to be an efficient source for short-lived isotopes due to the small holding time required in the course of separation and to its optimal efficiency with relatively low loss of material. 5. Plasmatron Sources Arc discharge ion sources are partially ionized plasmas, and the degree of ion density in the same sources is dependent upon the design of the source. With a view to attaining a denser plasma in an arc discharge, von Ardenne (1956) has described arc discharges of plasmatron type, which can be used for production of an intense ion beam of high density. The first in this line is a unoplasmatron source which does not require a magnetic field for operation and in which the arc is constricted by an intermediate electrode (IE) (Freeman et af., 1961). A duoplasmatron source due to von Ardenne (1956, 1961, 1962) surpasses all other sources with respect to arc density, which may be as high as

LABORATORY ISOTOPE SEPARATORS

IRON

ST.STEEL

127

ISOL

FIG. 6. Cross-sectional view of a duoplasmatron ion source (Septier, 1967): F, hot cathode; IE, intermediate electrode; A, anode plate; M, magnetic circuit; C, magnetic coil; E, extractor of stainless steel. Anode plate is isolated from IE by insulator.

-

1014 ions cm-3 near the extractor. Figure 6 shows a cross-sectional view of a duoplasmatron ion source that consists of a directly heated cathode and an anode made of soft iron. The inhomogeneous magnetic field between the intermediate electrode (IE) made of soft iron and the anode, along with the electric field of the IE concentrate the arc discharge to a high density of ions near the ion exit hole in the anode. Though duoplasmatron ion sources are not generally used for isotope separation, the high ion current produced by these sources with various modifications may be of use in a number of specific experiments. Winter and Wolf (1974) have described the operation of a duoplasmatron source under suitable conditions to obtain multiply charged ions. Masic et al. (1969), Illgen et al. (1972), and Shimzu el al. (1973) have reported the use of duoplasmatron sources for ionizing solid materials. A large number of papers have been published on the construction, operation, applications, and modified versions of duoplasmatron sources, references to which will be found in the works of Septier et al. (1966), Septier (1967), Lejeune (1971, 1974),Aubert (1972, Aubert et al., 1973), and Gautherin and Lejeune (1973).

B. Thermal Ion Sources

The use of a n ion source based on the principle of thermionic emission dates back to the time of the development of the isotope separator by Yates (1938) who, following the suggestions of Blewett and Jones (1936), made an

128

S. B. KARMOHAPATRO

ion source for lithium with a filament coated with synthetic lithiumaluminum silicate (Li,O,, 2 SiO,). Couchet (1954) has shown that for lithium, the said synthetic compound corresponding to natural B-eucryptite has maximum emissivity. Johnson (1962) gives the temperature range between 1200 and 1350°C for the emission of lithium ions reaching to a high current 1- 1.5 mA/cmZ near 1350°C. density Septier and Leal (1964) have shown that such an ion source can produce 10 mA for a comparatively long time. Other alkali Li' ion current of metals can be ionized in a similar way by using alkali aluminosilicates Al,O,-n S O , , M 2 0 (M = Li, Na, K, Rb, Cs) which emit ions and neutral 100-pA Cs' ion atoms when heated. Dawton (1956) was able to produce beams using a mixture of cesium oxide, alumina, and ferric oxide. The mechanism of thermal ionization formulated by Saha is applicable to thermal ion sources, and the phenomenon of surface ionization in which atoms are emitted from a hot metal surface has been investigated by Langmuir and Kingdon (1923a,b) and Langmuir and Taylor (1937). The ratio of ions to neutral atoms evaporated from a heated surface is given by a formula known as the Saha-Langmuir equation:

-

-

-

n'/no = Y exp[ -e($+ - w ) / K T ]

(2) where n + and no are the ions and neutrals respectively, q5i is the ionization potential of the substance on the hot surface of the filament of work function W , Y is a statistical constant characteristic for a particular element, e is the charge of an electron, and K is the gas constant. Alkali substances with low ionization potentials on the surfaces of metals like tungsten, platinum, rhenium, etc. are easily ionized, and practically the n+/no ratio for these elements may tend to unity. Some authors (Michel, 1953) have used multiple-beam sources with a large number of single filament sources to obtain high intensity ion beams. C . Ion Source Technology for On-Line Isotope Separation

Mass separation of short-lived nuclei produced by nuclear reactions requires rapid transfer of the reaction product to the ion source, and fast chemical separation is necessary for obtaining a suitable feed material for the source. So the method adopted for on-line isotope separation consists of a modification in the ion source technology by using an integrated targetion source system; or an alternative way is to transport the product by a carrier gas. Generally for on-line isotope separation, elements are prefered to compounds as targets since elements are easily vaporized and ionized and the damage due to nuclear radiation is less serious with elements as targets. For an element that cannot be evaporated and ionized, the use of a com-

LABORATORY ISOTOPE SEPARATORS

129

pound as a target is inevitable. G. K. W o l f e tal. (1973) have investigated the behavior of nuclear reaction products evaporating from chlorides and oxides of a few elements swept together with various gases into the ion source. Feldstein and Amiel (1973) have used helium carrier at molecular flow conditions for transport of noble gases produced by emanation from either UO, stearate or a mixture of U,O, and Ba stearate. A novel technique of fast transport through long distances by laminar flow has been achieved by Macfarlane et al. (1969) using a helium jet recoil transport (HeJRT) system, which is now in use with many ISOL (isotope separator on line) systems. Besides this, an integrated target ion source (ITIS) system is used with various modifications in ion sources, depending upon the nature of the elements originating in the reaction product. The complexity of the problem associated with ion source technology of ISOL systems requires more special attention than that necessary in conventional isotope separation. In the following sections we shall discuss the developments on HeJRT and ITIS systems used with ISOL instruments.

1. Helium Jet Recoil Transport System A helium jet recoil transport system consists of a chamber containing helium at about 1 or 2 atm, in which recoils from a nuclear reaction are slowed down to thermal velocity by collision with helium. Small impurities like water vapor or organic substances introduced in the He chamber form clusters of high molecular weight each consisting of 50-100 molecules for most common vapors. Evidently the radii of such stable clusters are -6-10 A, and their masses are as heavy as 107-108 amu. Wien et al. (1972) have shown that the formation of such clusters is enhanced by nuclear radiation. Also an ultraviolet radiation of wavelength shorter than 2000 A may be used for increasing the cluster formation process with an impurity in helium. The recoil products bound by the clusters are transported by laminar flow of helium through capillaries through distances of a few meters. Dantet et al. (1973) have shown that velocities up to sonic speed are achieved in the capillaries enabling this long distance transport within even 0.1 sec. At the end of the capillary the helium gas is separated from the clusters by passing it through a conical jet of dispersion angle less than 4" through an orifice at the top to an evacuated space, where helium leaving the nozzle of the capillary expands very fast and is pumped out. The clusters, which have large momenta, tend to crowd near the axis and enter the vacuum chamber connected to the capillary through the conical jet. Recoil products are now collected in this chamber or are directly introduced to an ion source. Figure 7 shows the schematic diagram of a HeJRT device used with ISOL systems.

-

130

S. B. KARMOHAPATRO

TO ION 501JRCE

FIG.7. HeJRT system containing cluster breeder and skimming device: ( 1 ) recoil product mm Hg, (3) collector chamber at chamber at p % 1 atm, (2) nozzle chamber at p = p x 10-2-10-4 mm Hg, (4) lead to the roots blower, (5) lead to the diffusion pump.

Many authors have investigated various aspects of the helium jet device. Snider et al. (1973) have observed that the transport efficiency of a HeJRT system is seriously affected at a gas pressure lying below 1 torr, when there is a transition region between laminar and molecular flow. Wilhelm et al. (1973) have shown that instead of pure helium, an additive like water, carbon tetrachloride, trichloroethylene, or ethanol as an impurity mixed with helium irradiated with uv radiation enhances transport efficiency. Aysto et al. (1974) used acetylene, a mixture of 46% propane and 54% butane, trichloro-trifluor propane, carbon tetrachloride, and vacuum pump oil vapor as additives. They observed that the vacuum pump oil vapor as an additive shows the best efficiency since it has a vapor pressure lower than the pressure of the collection chamber or the ion source. Air as an additive has been used by Treytl and Valli (1967), which enhances the collection efficiency by an order of magnitude over pure helium. That oxygen and acetylene are poorly efficient additives is verified by Aysto et al. (1974). Wilhelm et al. (1973) have tried aerosols like stearic acid, diffusion pump oil, etc. as cluster materials with good success. Aysto et al. (1974) have described technical details of the mixing chamber for vacuum pump oil and for additives that are liquid at room temperature and pressure. Kosanke et al. (1974) have described a HeJRT device running with its low pressure and at atmospheric pressure instead of using an evacuated collection chamber, making fast aqueous chemical procedures simpler. The exact mechanism of a HeJRT system is not clear. However, the device has been an important tool for rapid handling of short-lived nuclei of a large number of elements. Wien et al. (1972), Dantet et al. (1973),Jungclass et al. (1971), Schmidt-Ott et al. (1973), Torgerson et al. (1968), and other authors have described and employed HeJRT systems for on-line separation of fission and nuclear reaction products with success. A recoil atom mass

LABORATORY ISOTOPE SEPARATORS

131

analyzer (RAMA) described by Nitschke (1970) with a HCIS shows the elegance of the helium jet technique for on-line separation of very short-lived isotopes. 2. Integrated Target-Ion Source System Use of an integrated target-ion source (ITIS) device is a very fast technique for analyzing many nuclides. A number of methods have been used by various authors in applying this device to ISOL systems for various targets of varying size and chemical properties. We shall discuss the physical basis of the technique associated with a few of these methods. Borg (1970) has reported a simple device for analyzing fission fragments, in which 235U is deposited on the inner surface of the graphite anode of a conventional plasma type ion source. The fission recoil fragments from 23sUare stopped in a cylinder of graphite cloth covering the uranium layer and diffuse through it. They are vaporized by the heat of the source and are ionized. Anderson et al. (1973) have described an improved version of higher efficiency and used it for processing fission products. The device is equally applicable to many other target materials. Amarel et a/. (1966) and Klapisch et al. (1967) introduced a method of using the ionizer of a surface ionization source, which, chosen suitably, will diffuse the reaction product from its opposite side for on-line separation by mass spectrometry. Since diffusion and ionization are enhanced by increasing the temperature of the ionizer, the reaction products may be separated at high efficiencies in this way. The source is, however, selective for elements of low ionization potentials. Venezia and Amiel(l970) have extended the range of the use of a surface ionization source by employing the ionizer as the target for elements of high electron affinity. This is based on the negative surface ionization phenomena governed by a formula similar to the Saha-Langmuir equation (2):

n - / n o = Y exp[e(A - 4 ) / K T ]

(3)

where n - is the number of negative ions of electron affinity A. Wollnik et al. (1973) have used the method for fission products Rb and Cs. Amiel et al. (1973) have described a versatile integrated target-surface ionization source for both positive and negative ions. The method of using the target as ionizer for surface ionization source is restricted to a small number of elements. However, for conventional plasma type ion sources, a suitable target for a particular element is chosen for the ITIS device. Patzelt (1970) has reported targets of partially dehydrated hydroxides of Zr, Ce, and Th at room temperature for emanation of K, Xe, and Rn respectively with prompt ionization, short delay time, and good

132

S. B. KARMOHAPATRO

yield. An alternative target is the use of a molten metal or alloy, at high temperature under vacuum. Bachmann (1970) Hagebn (1970), and Ravn et al. (1973) have reported developments on ITIS devices and the complex of related problems. Hansen et al. (1973) have developed a system in which T h o , powder at very high temperature can diffuse elements, from Au to Rn, to the ion source. A CeO, target has also been used in a similar way for the elements from Sb to Xe. A modified multiple integrated target ion source (MITIS) system has been described by the authors for using these hot oxide targets. D . Extraction and Formation of the Ion Beam

Generally, ions from the plasma of a source held at positive accelerating voltage are extracted through an extraction electrode that is at a negative potential with respect to the source. Accelerating voltages in the laboratory isotope separation range between 10 and 100 keV for handling ion current densities from a few microamperes to 50 mA/cm2 emitted from the ion source. The emission orifice may be a hole or slit on whose dimension the intensity of the extracted ion current is partly dependent. The ion source and the extractor should be so designed that an intense ion beam of little divergence is produced in a manner such that it does not strike the focusing electrodes following the extractor. Almen and Nielsen (1957) have given the relation between the space charge limited ion current of cross section A , extracted from an ion source with extraction voltage U as

-

'n

= x(A,

U3I2/d2@)

(4)

where d is the distance of the extraction electrode in centimeters, m+ is the mass of the ions, and x is a constant depending upon the geometry of the emission side of the source. The plasma in the discharge consists of an equipotential region, and it diffuses through the emission orifice of cross section A , to the extraction electrode at a negative potential. A plasma sheath of length d will be formed between the plasma and the extractor. Kamke and Rose (1956) give the following expression for d : d

= (2.36

x 10-4)(U3/2/J+@)'i2

(cm)

where J + is the current density. From Eq. ( 5 ) it is observed that for an increase of U , the voltage between the plasma and the extractor, or for a decrease in J , , d increases. This shows that the parameters of the source determining J , can affect to a great extent the sheath length or the formation of its surface. The geometry of the discharge as well as its relative position with respect to the emission orifice controlling J, will influence extraction conditions.

LABORATORY ISOTOPE SEPARATORS

133

Bernas et al. (1954), Walcher (1958), Rautenbach (1961), Almen and Nielsen (1957), Septier (1967), and Chavet ( 1965, 1970) have investigated various aspects of the beam extraction mechanism in detail. Results of these investigations reveal that the space charge limited ion current from a source tends to diverge during acceleration due to space charge effects. This divergence can theoretically be controlled by extraction electrodes shaped as calculated by Pierce (1949) for electron guns. For a surface ionization type ion source of fixed geometry, the system may be adequate, but for a plasma ion source with the sheath formed between the extractor and the emission orifice with the boundary varying as a function of different operating parameters, the shaping of the extraction electrodes is not so helpful for minimizing divergence of the beam. Chavet (1970) showed that the curvature of the plasma meniscus yo plays a vital role in affecting the divergence of the ion beam. The divergence is given by 2a

=

Y1yo

(6)

where 9, is the width or diameter of the emission slit or hole, respectively. yo = I/r is positive for a convex, or negative for a concave plasma meniscus, where r is the radius of curvature of the meniscus. Chavet (1965, 1970) derived a relation for a fixed accelerating voltage; 110=4(dz-;) 5 d

(7)

from the data given by Langmuir and Blodgett (1923, 1924) for a flat beam emitting through a rectangular slit; and for a round beam through a circular hole,

where d o is the hypothetical distance between the emission ordice and a plane electrode fulfilling the condition n+ = (5.45 x 10-~)11~’*/d;JiF

(9)

and do is in centimeters. Equation (7) can be approximated by a linear relation

so that

2u = 2 . 3 9 ,

[a] d

do

134

S. B. KARMOHAPATRO

A convenient method for minimizing 2a, based on the above relations, is to vary the distance d with the accelerating potential kept fixed. Figure 8 shows the flat beam extraction system used in the Orsay separator in which the extraction voltage is almost the same as the accelerating voltage, the extraction electrode being held at a negative bias of a few hundred volts as electron barrier potential. Chavet (1965) has shown that for the system shown in Fig. 8, the divergence of the ion beam is reduced to a large extent by varying d for a fixed extractor voltage and a fixed extracted current density.

A B G FIG.8. Extraction system of the Orsay separator: A, extraction-acceleration field; 9, electron barrier field; G, ground potential electrode.

An alternative method of reducing the divergence is to vary U and keep the value of d fixed. This is used in the Calutron extraction system. In the extraction system shown in Fig. 9, used in some Scandinavian type on-line

C

D

G

FIG.9. Extraction system of the Scandinavian type on-line isotope separator: A, acceleration voltage; 9, extraction voltage lower than A ; C, lens potential; D, electron barrier field; G, ground potential.

isotope separators, a combination of both methods of varying U and d is in use. The extraction voltage U in these systems is lower than the accelerating voltage V, which follows the extraction field; a lens system is used between the extraction electrode and the electron barrier field as shown in the figure. For reducing divergence in the conventional Scandinavian isotope separators, an electrostatic lens system, shown in Fig. 10, is employed, which fol-

LABORATORY ISOTOPE SEPARATORS

A

G

6

135

G

FIG. 10. Extraction system of the conventional Scandinavian type isotope separators: A, extraction-acceleration field; G, movable electrode at ground potential; B, lens system; G, ground potential.

lows a single extraction-acceleration fixed field for the round beam between the emission hole and the extraction electrode at a fixed distance d. Brown (1968) has described the Scandinavian type Chalk River isotope separator in which the source is held at the accelerating voltage V and ions are extracted by a potential drop of 10-15 keV. The typical lens system of a Scandinavian type isotope separator consists of a cylindrical focusing electrode with adequate potential and horizontal plates at suitable voltage to compress the beam in the vertical direction to attain vertical focusing of ions. Chavet (1965) has shown that for the curvature of meniscus yo = 0, the use of a curved electrode (Fig. 11) for extraction

FIG. 11. Axial focusing by a curved electrode: E, entrance slit; P,, P,, poles of the deflecting magnetic analyzer; I, image.

slit gives a focal distance of 2.5 times the radius of curvature of the electrode in the axial plane, so that by proper design of the electrode the beam may be made to converge axially at the image location, resulting in a reduced image length. 111. ACCELERATION OF IONS

Laboratory isotope separators in their conventional design are low energy accelerators that accelerate and analyze masses of ions in the energy range 10-100 keV. Apart from their applications for collection of enriched isotopes, they are useful for research in atomic and solid state physics. Generally, a stabilized high voltage unit consisting of a transformer-rectifier

136

S. B. KARMOHAPATRO

system is used to raise the potential of the ion source to attain the desired energy of ions which are then extracted at a negative potential with respect to the source. The extraction and the lens electrodes between the source and the magnetic analyzer are placed within the main vacuum chamber at ground potential, from which they are properly insulated. The ion source at the positive accelerating voltage is adequately insulated from the chamber. The need for extending the range of the accelerating voltage to a higher value than that used in conventional isotope separators is seriously felt for ion implantation and other experiments. Nielsen (1967) and his group were pioneers in constructing an isotope separator with an electrostatic generator and a constant field acceleration tube to energize heavy ions to a voltage of 600 keV. Goode (1971) has described a similar machine employing a 500-keV Cockcroft-Walton accelerator. Brown (1973) has described an isotope separator with a 2-MeV “Pelletron” accelerator and a HCIS system. The main disadvantage with these types of high voltage isotope separators is the restricted accessibility to the ion source at the high voltage terminal and the need to use a bigger magnetic analyzer for analyzing high energy heavy ions. However, a few such machines described above or with Van de Graaff generators are in use for studies of atomic collisions in gases and solids and for ion implantation. Guernet et al. (1970) have described a 160-keV high voltage isotope separator, which employs an alternative method for post acceleration of ions where the analyzer is at high voltage. In this method, the mass analysis is performed at a low energy and the extracted ion current is independent of the final acceleration voltage. The disadvantage of the method lies in the vacuum chamber being at high potential and consequently requiring it to be isolated electrically from the analyzer or alternatively the analyzer and its power supplies to be insulated from the ground potential. However, the ready accessibility of the target chamber allows the same experimental facility as a conventional isotope separator. Wilson (1967) reported a different version of a high voltage isotope separator in which a conventional machine is used with the target chamber at high voltage, which is varied for achieving the desired energy of ions. This method does not permit ready accessibility of the target chamber in many experiments, but is convenient for studies on ion implantation. Freeman (1967) and Freeman and Gard (1970) have described high voltage isotope separators based on this principle. A simpler method for multiplying the energy of the ions uses multiply charged ions in a conventional isotope separator. Most of the ion sources described above produce a significant number of multiply charged ions, and these may be used for this purpose; or an ion source capable of producing

-

LABORATORY ISOTOPE SEPARATORS

137

multiply charged ions (Bennet, 1971a,b) may be chosen with proper consideration given to the quality of the beam required for a particular experiment.

IV. MASSANALYZERS A mechanism for analyzing the masses of ions for quantitative separation is a major component of a laboratory isotope separator. The mass analyzers used in mass spectrometry are more or less suitable for isotope separation. However, direction focusing magnetic analyzers are more often used. There are a few types of analyzers, such as Wien velocity filters or electric quadrupole analyzers, that are used for isotope separation. Wahlin (1964, 1965) has described a laboratory isotope separator based on the principle of the Wien velocity filter used earlier by Oliphant et al. (1934) for the same purpose. The electric quadrupole mass analyzer is based on the principle of focusing by the time of flight method and has been described by Paul and Raether (1955). The high intensity version of the method has been used by von Busch et al. (1961; von Busch and Paul, 1961) for isotope separation and by Bahrami et al. (1970) for ion implantation. Apart from a few unusual instruments referred to above, there exists a large number of isotope separators developed in various laboratories with mass analyzers based on the principle of direction focusing by magnetic fields. In the following, we shall discuss various aspects of direction focusing of ions by magnetic analyzers for mass separation in electromagnetic isotope separators. A. Direction Focusing Magnetic Analyzers

The principle of direction focusing magnetic analyzers applied to mass spectrometers is common to isotope separation. Ions of mass M in au, taking M = 16 for oxygen, and of charge state N accelerated to V volts are deflected in a circular path of radius Ro obeying the mass spectrometer equation BoRo = 144(M1//N)”2

(12)

which is applicable to a deflecting axially symmetric magnetic field within a certain limit of inhomogeneity in the radial direction. Besides the property of deflecting the ions for dispersion according to their masses as expressed in the mass spectrometer equation, a magnetic field of suitable conditions of inhomogeneity and boundary acts as a lens for refocusing the divergent monoenergetic ion beam for the same mass, which is useful for mass analysis without much loss in intensity.

138

S. B. KARMOHAPATRO

For a general magnetic field with 0 c n < 1, where the field index n = - ( R , /Bo)(aB,/dR), Bo being the magnetic field at the mean radius of curvature R , , the equations of motion of an ion can be solved to obtain conditions determining the parameters of direction focusing to first order for a magnetic analyzer in both the radial and axial plane. Following the treatment by Sternheimer (1952), these conditions are

where

- (1 - n)'/21,

cot1' = 1 + 1, tan and for n = 0.5

cot

El

cot[n'/24

12

= n1/2

'2

-n ' 4 , = 1 + I , tan

El

+ tl]

+ tan e2 cot[n'/24 + t,]

(14)

where E , and E~ are the angles made by the central ion ray with the normal to the boundary at the point of incidence and emergence, and I , and I, are respectively the distances of the source and the collector from the field boundary in terms of R, (Fig. 12). 4 is the angle of deflection in the analyzing magnet.

b

Source

e

FIG. 12. Schematic diagram of a sector-shaped magnetic analyzer for focusing of ions.

B. First Order Single Direction Focusing Magnetic Analyzers For a symmetric homogeneous magnetic analyzer, the central ion beam being perpendicular to the entrance and exit boundary of the magnet

l2

= 1, = 1

and

el = e2 = 0

Eqs. (13) and (14) then reduce to 1 = cot

which gives for

4 = 180°, 1 = 0.

4 + csc 4

(15)

LABORATORY ISOTOPE SEPARATORS

139

This is a particular configuration for a magnetic analyzer with first order direction focusing in the radial direction used with Calutron isotope separators. Since Calutrons are very large in size, it is not inconvenient to place the ion source within the magnet with 1 = 0. It is usual to use a sector magnet of (b = 90" with 1 = 1 for the Scandinavian isotope separators and a magnet with (b = 60" with I = 0.7 for Bernastype intermediate isotope separators. However, for any angle (b < 180", focal points can be determined, so that the ion source and the detector may be placed fairly far outside the magnetic field. The lateral magnification G of the image due to the source slit is generally unity with these sector-shaped magnetic fields, and the mass dispersion is given by D = (R0/2M)(I G ) (16) where D is the mass dispersion assigned to the plane of the image perpendicular to the central ray. Mass dispersion D is defined as the distance between the images of ions with M = 1 and may be expressed as d f A M ,where d is the distance between M and M + AM in the collector side. The resolving power, RP = R o / b , , , where b , , , is the width of the image at half maximum of the peak. Though the separating power S P expressed as the resolving power gives little information regarding the completeness of separation in the region for D % ht,*, it defines well the focusing conditions. One of the factors contributing to the width b , , , is the aberration ha, due to the magnetic focusing, which is R o a 2 for first order single focusing magnetic analyzers as has been calculated by Stephens (1934), where the half divergence angle a is very small.

+

C . Second Order Single Focusing Magnetic Analyzers

From the above, it is evident that the separating power of a laboratory isotope separator can be increased by reducing ha,, which requires the elimination of the second order term cx2 in the width of the image. Kerwin (1949) traced the ideal boundary of the pole pieces of a sector magnetic field for perfect focusing of ions (Fig. 13). If we approximate the

IY

I

Source

FIG. 13. Ideal boundary of the sector magnets for perfect focusing of ions (after Kerwin).

140

A

S. B. KARMOHAPATRO

FIG.14. I, straight line approximation at the point of inflection of the ideal boundary curve M.

ideal boundary by drawing a straight line at the point of inflection of the curve (Fig. 14), the second order term is eliminated. For such inflection type magnetic analyzers, Persson (1951) gives the value = &a3

(17) Arcipiani et al. (1959) described a second order focusing inflection type laboratory isotope separator with R , = 40 cm and 4 = 90". Alternatively, the ideal field boundary may be approximated by a normal circle, a special case of which is shown in Fig. 15; and for such analyzers dab

dab = R o d

(18)

For a magnetic analyzer with 4 = 90°, the radii of curvature of the normal circle type symmetric magnetic analyzer are equal to R o , the radius of curvature of the ion trajectory.

FIG. 15. Approximation of the ideal boundary curve by a normal circle (NC).

Dunjic (1965) has discussed the use of a second order focusing circular homogeneous magnet for a multi-ion source isotope separator following the proposal by Bainbridge (1952). D . Higher Order Focusing Magnetic Analyzers

Beiduk and Konopinski (1948) have shown that in a radially varying inhomogeneous magnetic field of type B, = Eo[l - a(6R/Ro)2] the monoenergetic ions produce aberration dab= Roa4

(19)

141

LABORATORY ISOTOPE SEPARATORS

for the median plane, where B, = 0 and the focusing angle is 180". In this type of field, a weak axial focusing exists. Achievement of aberration of fourth order makes it useful for higher resolution in spite of its peculiarly shaped pole faces. Zilverschoon (1954) has given detailed consideration of such a field in connection with the development of an electromagnetic isotope separator. E. Two-Directional Focusing with the Fringing Field of a Homogeneous Sector Magnet

We have so far considered single direction focusing magnetic analyzers for focusing of ions in the radial direction. For homogeneous sector magnets with angles < BOO, the source and the detector are outside the magnetic field, so that the fringing field of the boundary of the magnet disturbs the focusing conditions, for which corrections have to be made. Cotte (1938) and Lavatelli (1946) have considered the possibilities of utilizing such fringing fields to focus the ion beam in the axial direction. When ions enter the region of the fringing field with the central beam at an angle with the normal to the field boundary, they will face a B, component of the field normal to the beam trajectory generated by the gradient in B, produced by the curved fringing field. The B, component under suitable conditions will focus the ion beam in the axial plane. Camac (1951) and Cross (1951) have analyzed the conditions on parameters for vertical or axial focusing, and these conditions may be deduced from (13) and (14) with n = 0 as tan

E

1 2-2

- - [tan($

- A)]

+ 4 - 1cot q

and 1

r;

~

1

= tan(q5 - A) - ____ 4 - cot r j

where tan A

=

1 tan c1 + - ~ , tan 11

rj =

tan

1

--

4

and I'; is the distance of focal point, of both radial and vertical focusing in units of R , . Anderson et al. (1961, 1964)and Rudstam et al. (1964) have given design parameters and characteristics of a 55" homogeneous fringing field focusing type isotope separator at CERN in which parallel beams enter the magnet

S. B. KARMOHAPATRO

142

and the central beam is deflected about 55" to achieve two-directional focusing over a large mass range. Rudstam (1965) has described a similar magnet for the on-line separator a t CERN. Tarantin et a/. (1965) have described the isotope separator of the Laboratory of Nuclear Reactions at the Joint Institute of Nuclear Research, Dubna, in which two-directional focusing with a homogeneous magnet is obtained with second order radial focusing by circular boundaries. The values chosen in the separator are

1, = 1.86, = O",

e2 = 45",

l'; = 1.16,

Ro = 70 cm

R , = 58.1 cm,

R , = -50.5 cm

R , and R2 are the radii of curvature of the entrance and exit boundaries of the magnetic analyzer, respectively. Freeman (1970) has described a twodirectional focusing isotope separator of variable radius with rotatable pole tips attaining high experimental flexibility and the performance of a much larger conventional instrument, based on the fringe-field focusing principle.

F. Two-Directional Focusing with an Inhomogeneous Magnetic Field

A radially varying magnetic field of the type B, = Bo(Ro/R)"

(22)

has a radial component B, and is capable of focusing ions in the axial direction. In the course of a single revolution, ions will be focused at an angle $z in both the radial and axial directions with a magnetic field with n = 0.5. Svartholm and Siegbahn (1946) considered a two-directional focusing #&rayspectrometer of the type described above. An important effect that is associated with these types of magnetic analyzers is the increase in mass dispersion along with higher transmission as a result of axial focusing. The mass dispersion with these inhomogeneous magnets is given by Svartholm and Siegbahn (1946) as l / ( l - n ) times greater than the single-direction focusing magnets. Thus for n = 0.5, the dispersion with a $z inhomogeneous magnet is twice that of a homogeneous magnet. Evidently the resolving power is doubled in comparison to a similar homogeneous magnet. Karmohapatro (1959, 1960) has described a $ n focusing magnetic analyzer for a high intensity mass spectrometer (Fig. 16) for atomic collision experiments and enrichment of thin isotopic targets. The magnet has conical pole faces with an angle of the surface to the symmetry plane equal to 2" and

143

LABORATORY ISOTOPE SEPARATORS

VOLTAGE SUPPLY

I

1

FILAMENT SUPPLY

D.p,L H.L.

F. P .

FIG. 16. Schematic

agram ofa JZX magnetic analyzer for isotope separation.

the gap width is slightly greater than 50 mm, so that the field may be represented by

+

in the region of R , = 381 mm resulting in ha, = $R,(a2 $’), where c( and $ are the radial and axial half divergence angles at incidence respectively. The author has discussed the advantages of this type of magnetic analyzer for separation of isotopes over other systems. Banic et al. (1962) have described an experimental two-directional focusing separator with n = 0.5 and its applicability in a Calutron separation program using a f i n focusing angle and R, = 5 1 cm (20 in.), which gives nearly 1.6 times more dispersion than a 61-cm (24-in.) radius Calutron. Trahin (1965) has described a II focusing isotope separator for producing isotopes in commercial quantities and has discussed the aberrations with one or two ion sources within the magnet. Judd (1950) and Rosenblum (1950) have worked out the possibility of two-directional focusing of charged particles with 4 < ~ I so that I the ion source and the collector can be placed outside the magnet.

3

144

S. B. KARMOHAPATRO

From Eqs. (13) and (14) when E , = E~ = 0 we obtain two-directional focusing at a distance of I'; from the exit boundary as

I"- [Il/( 1 - n)l/'] cot(1 - n)'/'+] + ( 1 - n)2 I , - ( I - n)'/' cot[(l - n)1/2Q,]

For n = 0.5, this equation satisfies first order focusing for any value of 1, and Q,, Q, = $ n being one limiting case for 1, = 1, = 0. Viehbock (1961) described the design parameters of an inhomogeneous magnetic analyzer with R, = 100 cm, and n = 0.5 which has Q, = 169.42'for axial focusing along with second order radial focusing. The magnet has conical pole faces as described by Svartholm (1950). Bernas et al. (1961a,b) described a tandem isotope separator with two stages of magnetic analyzers, one of which is an inhomogeneous two-directional focusing magnet of angle 180" corresponding to a deflection of 191.4"because of the fringing field. The magnet has a field index n = 0.5 produced by a conical pole face yielding a first order field BAR) = Bz(Ro)2Ro/(R+ Ro)

(25)

given by Svartholm (1950). Hanser et al. (1962) and Fabricus et al. (1965) have described a separator for radioactive isotopes with a magnetic analyzer of n =0.5, 4 = 191", and R, = 102.5 cm. Foucher et al. (1973) have described the ORSAY on-line separator which has n = 0.5, Q, = 75", Ro = 80 cm. The machine provides an arrangement for varying E ~ for , any deviation in the angle between the ion path and the focal plane due to the deviation of n. Wilson Whitehead and White (1972) report the construction of a three-stage isotope separator with each magnet of 4 = 180", n = 0.5, and Ro = 24 in. A few isotope separators based on this principle have been developed in different laboratories in recent times. Mention may be made of " Sidonie," constructed by the French group in Orsay, which has a magnet with n = 0.5, Ro = 85 cm, Q, = 138" as reported by Camplan et al. (1970) and Alexander et al. (1970). A similar machine described by Camplan et al. (1973) consists of an inhomogeneous magnet with asymmetric arrangement, so that for I'; > I,, dispersion is increased along with the lateral magnification G. It is shown that G contributing to the image width is widely compensated by the increased dispersion to obtain samples of increased purity. Abrahamsen et al. (1973) have described a miniature isotope separator for 1-80 amu with a magnet with n = 0.5, R, = 15 cm, which has a resolving power less than

LABORATORY ISOTOPE SEPARATORS

145

200 and beam current density 1.5 mA/cmZ and is suitable for atomic collision experiments and high energy chemistry research. Wagner and Walcher (1961) have suggested the use of a spiral ridge magnet for two-directional focusing of ions based on the principle of acceleration by azimuthally varying field cyclotrons. They have shown that for accelerated ions, spirally shaped straight grooves milled at an angle 1' on the poles of a 180" homogeneous magnet with the width of pole faces small compared to R , will produce betatron oscillations of frequencies vz

=.f(ftan2 y ' ) ' I 2 ,

v, = 1

(26)

where f is the so-called flutter amplitude of the ridge. For y' = 72" and f = 0.3, a two-directional n focusing magnet can be realized with ion optics similar to the homogeneous fields. However, no such isotope separator is known to exist. G . Magnetic Analyzers with Higher Dispersion

Artsimovich et al. (1957) have described an isotope separator with

4 = 1.25 of field shape

) ~ the dispersion with a This type of field produces dispersion ( 4 / ~times homogeneous magnet. It is important to note that the shape of the third order focusing magnet described in Section IV,D can be derived from Eq. (27) with 4 = n. The field also represents the radially varying field described by Eq. (23) limited to the second order term with the coefficient d replaced by The dispersion attained with a 1.25n magnet is 1.56 times larger than with the homogeneous magnet, and it is by no means superior to a radially varying magnetic analyzer when dispersion and transmission are considered. Alekseevsky et al. (1955) have considered radially varying magnets with n = 0.8 or 0.9, which will give mass dispersion 5 or 10 times higher than that for homogeneous magnets, respectively, when axial focusing is not desired. Dagenhart and Whitehead (1970) have described the ion optical design of the Oak Ridge 180" isotope separator with n = 0.8 and R , = 61 cm (24 in.) to attain the high dispersion effect.

A.

146

S. B. KARMOHAPATRO

H . Axial Beam Crossing with Magnetic Analyzers

In Section II,D we have already discussed a method of compressing the axial beam by horizontal electrodes as used in the Scandinavian isotope separators. Curved electrode geometry for the intermediate separators for reducing the image length in the 2 direction given by Chavet (1965) has also been described. The use of two-directional focusing magnetic analyzers described in Sections IV,E and IV,F serves a similar purpose for increasing the transmission to a great extent. For reducing axial beam divergence, a few homogeneous magnets with special ion optical designs have been introduced. One of them is due to von Ardenne (1960, 1962) and has an entrance boundary of a suitable curvature for attaining crossing of the axial beam at the center of the ion path between the pole pieces, so that a magnetic analyzer with reduced gap width could be employed. Timpl (1965) has studied the dependence of beam current and resolving power with this type of magnet as a function of several operating variables. The second method is due to Chavet (1970) in which the magnet is designed to produce axial beam crossing at the entrance boundary. The isotope separator MEIRA reported by Chavet (1973) based on this design has R, = 196.1 cm and R , = 349 cm. The analyzer is symmetric with 4 = 64", = 0, and c 2 = 29"40'. Preliminary results are reported. Cramer and Schmidt (1966) have discussed the use of a single quadrupole magnet instead of an electrostatic lens system for beam orientation in correcting the beam shape. Chavet (1970) has suggested its use for compressing the axial beam from the emission slit without any disturbance to a space charge neutralization. V. PERFORMANCE The performance requirements of a laboratory isotope separator are determined by the application of the instrument to a particular problem. For separation of stable isotopes the beam intensity is more important than the efficiency defined by the charge material actually collected, and the latter is a predominant factor for separating radioactive isotopes for which the charge material and the time of separation are limited. Besides these two characteristics, other performance characteristics of an instrument are dispersion, resolution, the shape of the ion beam, energy, beam stability, and so on. In the following sections, we discuss some factors affecting the performance of an instrument in general.

LABORATORY ISOTOPE SEPARATORS

147

A . Dispersion and Resolution

Dispersion and resolution are the primary parameters that determine the performance of a separator. Dispersion D measured perpendicular to the central ion beam in a symmetric homogeneous magnetic analyzer for M = 1 is given by D = R,/M

(28)

where R, is the mean radius of curvature of the magnetic analyzer. Dispersion is a property of the geometry of the magnet; for inhomogeneous magnetic analyzers, it becomes 1/( 1 - n ) times the value of D given by (28). Thus inhomogeneous magnets with n = 0.5 and 0.8 as described earlier will give dispersion higher by 2 and 5 times respectively than that of a symmetric homogeneous magnetic analyzer of the same radius of curvature. An alternative way to increase dispersion is to use a multistage analyzing system, i.e., a system consisting of a few magnetic analyzers in series. Bernas et al. (1961a,b) have described a 180" inhomogeneous magnetic analyzer in series with a 60" intermediate separator (Section IV,F). Wilson, Whitehead, and White (1972) have given design parameters of a laboratory separator for producing high purity stable isotopes in milligram quantities with a system of three inhomogeneous magnetic analyzers of field index n = 0.5 in series; each of the analyzers has a focusing angle 4 = x, R, = 61 cm (24 in.) (Section IV,F). Resolving power or separating power is defined as the ratio of the dispersion and the image width b , / , at half of the peak height in terms ofmass M: RP

= DM/b,,,

(29)

In (29) the parameter 61,2 is dependent upon the geometry of the instrument as well as the operating conditions. 'For ions of constant mass,

S is the virtual source width, equal to the source aperture. It may sometimes be larger or smaller than the source aperture depending upon the method of beam extraction, resulting in the formation of a virtual object of larger or smaller width respectively which arises from the broadening of the beam due to the aberration inherent in a magnetic analyzer; d V / V and dB/B, are the instabilities of the accelerating voltage and the magnetic field respectively due to the power supplies. A E is the energy divergence of the ions. Though power supplies with stability of 1 : lo4 or a higher degree are normally used,

148

S. B. KARMOHAPATRO

the instabilities arising in the ion source shift the position of the beam. To reduce such instability a beam position stabilizer is used in some separators. It consists of two vertical pins of variable gap placed before the collector; before collection the beam passes between them. The unstable currents to the two pins are amplified by a differential amplifier and are fed back to produce a correcting voltage to a pair of vertical plates placed in the lens system on the ion source side (Section 11,D).For on-line isotope separation, such vertical pins can accumulate radioactivity, and it is difficult to shield the y-ray emission from them. Halbig et al. (1974) describe a split tape stabilization system in which the moving tape collector (Section VI,D) consisting of aluminized plastic tape has grooves through the aluminum layer for the length of the tape down the center. The tape has two electrically insulated conducting parts, which act as stabilizer pins. To measure the resolving power or to identify an ion beam and examine its quality, the simplest method is to place a probe on the collector side and scan the mass spectrum on a recorder by varying the magnetic field. Alternatively a visual checking of the beam quality may be done by hitting a glass surface placed in the collector plate coated with a fluorescent material like ZnS or KBr, the latter being superior to other materials (Hall, 1965).A more precise arrangement is the use of a massmeter described by Dropesky et al. (1967) in which masses are scanned on a linear scale in an x-y recorder with the probe current for the y axis; the x signal is fed from a circuit squaring the field term satisfying the mass spectrometer equation. Oscilloscope display of the mass spectrum is also possible by modulating the accelerating voltage with a voltage signal; the beam can be swept across a fixed probe (Zilverschoon, 1954). Alternatively a mechanical x-y beam scanner described by Nielsen and Skilbreid (1957) or a vibrator arrangement described by Alvager and Uhler (1957) on the collector side can be used for display of mass spectrum on an oscilloscope.

B. Enrichment Factor and Contamination For D 9 b,,,, the separating power does not provide information about the degree of separation and describes only the performance of the instrument as to its focusing properties. This is because the profile of a line corresponding to an isotope with mass M has a tail T (Fig. 17) arising from aberration and other processes, such as gas scattering. The tail of the profile is responsible for contaminating an isotope of mass number M , with a neighboring isotope of mass number M 2 .The contamination factor measuring the fraction of M, contaminating the required isotope M , is defined by

LABORATORY ISOTOPE SEPARATORS

149

A

FIG.17. Profile of the spectrum of an isotope of mass M with the half-width b,,, and a tail T

where (IMz) M , is the beam intensity of M , at the position of M I and I,, is the total beam intensity of M , . The enrichment factor e(f) is the reciprocal of the contamination factor and specifies the operational characteristics for obtaining a purer isotope with an instrument. Cossignol ( 1962) investigated the various factors affecting contamination. The results show that the two main sources of contamination are (1) scattering and (2) hash. For high pressure in the separation chamber, scattering predominates over the hash disturbances resulting in a preferential contamination by lighter isotopes, and the reverse happens for a chamber with a very low pressure when the enhanced hash phenomena result in a preferential contamination of heavier isotopes. Nielsen (1961) and Freeman (1961) have described the theoretical aspects of the contamination factor and their applications to controlling the beam shape and for obtaining purer isotopes. Bernas et al. (1961a) have studied isotopic contamination with a two-stage magnetic isotope separator. The asymmetry of contamination in the profile of the mass spectrum suggests that its origin is due to a process of interaction between ions and the residual gas. In general, the controllable contamination factor is partly dependent upon operational characteristics such as initial energy divergence of the beam, aberrations of the magnetic and electrostatic lens, hash, and space charge effects. Moreover, the beam defining slit may also be responsible for a defocusing effect (Freeman and Bell, 1963). The above effects are equally responsible for reducing the resolving power of the instrument. The major contribution to the tails of the intensity distribution is due to the interaction of the ion beam with the residual gas. Since the mean free path of a heavy ion in the chamber of a separator at a pressure lo-’ mm Hg is comparable to the total path length L in the separator, gas scattering is a dominant factor which influences the shape of the ion beam and increases contamination, which is residual in character. Both elastic and inelastic scattering of ions may occur.

-

150

S. B. KARMOHAPATRO

Elastic scattering between ions and residual gases is responsible for the slight deviation of the normal ion trajectories resulting in the formation of tails on both sides of the beam profile. Menat (1964) and Menat and Freider (1965) treated the effect of elastic scattering in enhancing contamination in various types of extraction and analyzer systems of isotope separators. The results of their calculations show that the contamination factor is minimal for separators with two-directional focusing magnetic analyzers of focusing angle 4 2 100". The use of a second magnetic analyzer in series reduces the contamination to a large extent (Bernas et al., 1961a). The effect of elastic scattering can be well reduced by operating the separator at a reduced pressure or by suitably designing the instrument for reducing the total path length L of the ions, so that the figure of merit D M / L is at a maximum. Alvager and Uhler (1968) have compared the various types of single-stage separators as regards their characteristic behavior of dispersion, total path length, and the contamination factor; and they indicate the superiority of a sector inhomogeneous field of n = 0.5 over the other systems for obtaining isotopes of high purity. 1. Charge Transfer

A number of types of inelastic collision leading to charge transfer between ions and residual gases may be responsible to some extent for enhancing the contamination factor in an isotope separator. The following reactions may take place: symmetric charge transfer: asymmetric charge transfer:

+X+ + Y+ xz++ Y + X + + Y + Xf + X +X X + + Y -+ X

(32) (33)

(34)

where X and Yare the atoms or molecules. The resonant symmetric charge transfer process usually has a high cross section for ions of low energy; and this sort of reaction takes place near the ion source when a slower ion collides with its neutral parent atom. The ion so produced is accelerated through the extraction system and has an energy defect causing contamination in the lower mass side of the mass spectrum. The effect has been described by Camplan et al. (1961) and Uhler (1963). The effect of this type of reaction contributing to the contamination factor can be reduced by increasing the efficiency of the source and by reducing the length of the acceleration path. The use of a compound instead of an element as the charge material reduces the effect to some extent. An asymmetric charge transfer of type (33) between the ion and the residual gas, mainly air, is responsible for contamination at the heavier mass

LABORATORY ISOTOPE SEPARATORS

151

side of the spectrum and self-contamination. The process occurs at any place in the vacuum chamber and is rather large for near resonant reactions where the ion and the reactant atoms have nearly equal ionization potentials. Uhler (1963) studied the effect in detail and found that nearly 90% of the tailing in a krypton beam at a pressure 2x mm Hg was due to the reaction K r f + N -, Kr + N + for which the difference in ionization potentials is 0.6 eV. The effect can be minimized by operating the separator at a reduced pressure or by using an electrostatic deflection field in front of the collector to separate the ions from the neutrals. Reaction type (34) occurs near the ion source and is thus similar to a symmetric reaction. But this type of reaction increases the tail of the mass spectrum on both the heavy and low mass sides. Since the populations of the doubly charged ions are produced less by an order of magnitude than that of the singly charged ones, the effect is also smaller by that amount. Uhler and Rossi (1963) have shown this to be true if the two reactions have similar cross sections.

-

-

2. Aston Bands Aston (1920) observed a number of secondary ions as impurities in the mass spectrometers and explained their origin in the collisions between the primary ions and residual gas atoms. Cobic et al. (1963) reported these Aston bands to be the cause of an added contamination in the isotope separators. Due to the formation of impurity ions, the isotope separator may contain a number of broad bands corresponding to ions that have a different charge to mass ratio depending upon collisions at different parts of the vacuum analyzer. Consider a charge transferring collision like X z + + Y -, X + + Y + . This reaction gives rise to a background from an apparent mass number 4X to 2X, the latter is observed for the collision happening before the analyzing magnet and for collisions within the magnet it will be less than 2x. Care must be taken in handling multiply charged ions usually used for extending the energy range in an isotope separator. As an example, when using X4+ ions, the Aston peaks of X4+ions with lower energy originate due to the reaction X + + Y -,X + + Y + e. Such Aston peaks lower the resolution and can be suppressed by reducing the flow of vapor or gas and the pressure of the vacuum tank. Rautenbach and Lubringe (1965), Fabricus et al. (1965), and Boge et al. (1970) have given some experimental results on contamination due to atomic collisions in the separators. Chavet (1965) and Freeman (1965) have reviewed various factors in the design and performance of isotope separators causing isotopic contamination and methods for its reduction. +

152

S. B. KARMOHAPATRO

C . Eficiency

The total efficiency of a separator for collecting isotopes is determined by the product of the efficiencies of ion source, transmission, and collection. It is defined as the fraction of the charge material actually collected. Generally the collection efficiency is dependent upon the nature of the collector; for ion energies above 5 keV, a high collection efficiency can be obtained by choosing suitable parameters for the lens system and the magnetic analyzer and by operating the instrument at a lower pressure. Thus the efficiency of the ion source is the major factor contributing to the efficiency of separation. Ion source efficiency is defined as the fraction of the input charge material actually ionized in the ion source; it depends upon the type of the source for an optimal flow of gas or vapor from the charge material. Though ion sources like duoplasmatrons are reported to reach very high efficiency for hydrogen, generally ion sources used in isotope separation have efficiency values of 10-60% for singly charged ions depending upon the type of the source and the nature of the element to be ionized. The overall efficiency achieved for isotope separators generally lies in the region of 25 %, though high efficiencies are sometimes reported. For elements like Re, Nielsen and Nielsen (1958) report separation efficiency as low as 0.01%.

VI. COLLECTION Generally the methods used for collection of isotopes consist of either direct deposition of the ion current in a suitable backing or collection of ions in pockets. The latter method is applied in Calutrons and in some intermediate isotope separators for collecting milligram quantities of isotopes by sputtering. The incoming ions penetrate the target and are sputtered by further collisions to be collected in the pockets. The method is followed by chemical treatment for isolation of a particular element from the target material. Dawton and Smith (1958) discussed the method in full detail, and reference could also be made to the proceedings of the EMIS conference given in the introduction. Since collection of medium and small quantities of isotopes is involved with laboratory isotope separators, we confine our discussion to the method of direct deposition. The transport of ions to the collector target can be calculated from Faraday’s law, and the weight of the directly deposited ions in micrograms can be given for a singly charged ion with beam current i in microamperes flowing for t hours by m = 0.0373tiM

(35)

LABORATORY ISOTOPE SEPARATORS

153

where M is the mass number of the separated isotope. For transport of radioactive isotopes, the amount of activity is more important than the weight. The activity So in microcuries can readily be calculated from the relation So = (1.7 x lO8)tiAe-"

(36)

where i is the ion current in microamperes, t the time of flow in hours, and 1 the decay constant of the separated radioactive isotope per hour. Relations (35) and (36) are calculated on the assumption that 100% of the ions are transported to the collector. However, all the transported ions are not directly collected on the target. In the process of transfer of the fast ions to the solid state, consideration of the sticking factor of the ions, details of the sputtering mechanism, and questions of ion ranges and diffusion are required to understand the process of direct deposition of ions with a uniform distribution and of unlimited quantity. Relation (36) shows that So is maximal for t = 111. So separation time required for collection of radioactive isotopes is determined by the decay constant. This condition is well satisfied, in general, for a separator with very low intensity beam or even a mass spectrometer. But separators are preferred to mass spectrometers for separation of radioactive isotopes because of their high efficiencies. A. Factors Injluencing Direct Deposition

The direct deposition method of isotope collection was applied to a large number of isotopes by Brostr~met al. (1947), Elbek et al. (1957, 1959), Hansen et al. (1961), and some other authors with considerable success. But there are several problems encountered with the direct deposition method. The major trouble arises due to the sputtering of the target material. Since the range of the ions in the energy range common to isotope separators penetrating the target is very low, the ions are deposited in the surface layers. The sputtering of the surface material along with the deposited isotope by further ion bombardment hampers the collection of isotopes to a large extent. Thus, after the deposition of a certain quantity of an isotope, the target is saturated and no further deposition becomes possible. Saturation quantities are largely dependent upon the sputtering process since a target reaches the saturation value for a particular projectile ion when the loss of the collected ions equals the number of incoming ions. The relation between the saturation quantities % and 3 for two types of ions 1 and 2 on a particular target is given by Almen and Bruce (1961): y

I

P 2

= S2PI

154

S. B. KARMOHAPATRO

where S1 and S, are the sputtering yields a t saturation of the target by types 1 and 2 respectively. Though the sputtering yield slowly varies with the energy of ions in the range of the separators, the increase of the range with energy being linear, the saturation quantities are proportional to the energy of ions. Almen and Bruce (1961) have investigated sputtering phenomena and saturation of various targets and ion combinations. They observed that for a combination of average sputtering yield, targets of thickness 5 pgm/cmz can be made without difficulties for many substances. Besides saturation phenomena, there are other factors that influence the collection of isotopes, such as the sticking factor and diffusion. The sticking factor is the fraction of ion retained by the target. Brown and Davies (1963) have measured the retentivity of rare gas ions in various targets. Generally, for ion energies greater than 5 keV, the sticking factor is unity and is lowered for light ions on heavy targets even at a higher energy. Diffusion of ions during or after bombardment is another cause of inefficient collection by direct deposition. So, in selecting a target for deposition of ions, proper care must be taken so that normal or damage diffusion is a minimum. AI, Ti, Ta are very suitable backing materials for direct deposition. Santry and Sitter (1970) have observed from studies of range and retentivity of various targets for 40-keV antimony ions that titanium suffers a weight loss of 10 pgm/cm2 and can retain 83% of the ions after bombardment of an ion dose of 5 x 10l6 ions/cm2, whereas gold shows no retentivity with a weight loss of nearly 514 pgm/cm2 bombarded by the same dose.

-

-

B. Sputtering Method of Preparation of Thin Samples For stable isotope separation, ions of kiloelectron volt energy may be considered for producing thin samples, but for beta spectroscopy work monolayer thick samples are sometimes required. Moreover, for preparing thin samples of stable isotopes of uniform distribution, it is necessary to enrich the target at a low energy since the backing foil may be destroyed under high energy bombardment and the high energy beams penetrate deeper into the foil, distorting the uniformity in distribution of the isotope. A method of collection of ions at lower energy consists of bombarding a reflector like graphite to saturation and sputtering the reflector by further ion bombardment to emit the isotopic material along with the material of the reflector. Since sputtered atoms have energies of the order of a few electron volts, the targets thus prepared have thicknesses up to a few milligrams per square centimeter. The drawbacks include the mixing of the reflector material; also, a small fraction of the target has uniform distribution of the isotope. The method has been used by Formann and Viehbock (1966) to prepare thin

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isotopic targets. An alternative method is the electrostatic retardation technique in which separated ions are retarded to low energy before collect ion. C . Electrostatic Retardation Method

In earlier days of electromagnetic isotope separation Yates (1938) introduced the retardation technique between the slit and the collector for increasing the collection efficiency. In the simplest form of the method, the target is raised to the positive retarding potential with a grounded diaphragm in front. An electrode with a few hundred volts negative potential is placed between the grounded diaphragm and the target, to suppress the secondary electrons. In this device, since no consideration is given to the ion optical behavior of the beam in the small decelerating region, the beam suffers defocusing and space charge elimination is not possible due to the target being at positive potential, resulting in extraction of all the secondary electrons of the plasma immediately. In order to control the shape and size of the isotope distribution on the target, an immersion lens may be designed depending upon the desired parameters of the source. Dionisio and de Lima (1968) and Uhler (1963) have given an ion optical treatment of such lens systems. Elbek et al. (1959) earlier introduced a system of retardation devices obtained by a trial and error method, which has proven to be ion optically suitable for efficient collect ion. Bergstrom et al. (1963) investigated various aspects of target preparation by retardation techniques with proper care for deflecting the paraxial incoming ions to eliminate the neutrals. Uhler (1963) has given results on target preparation for circular and slit geometry of the beam. Gorodetzky er al. (1965) have described the retardation technique with space charge elimination for a beam intensity 600 pA at 0.5 keV. The retardation lens system can be treated ion optically now by computer analysis and can be applied to obtain a target of desired geometry. Tortschanoff and Viehbock (1970) reviewed the preparation of thin sources by retardation techniques with reference to the existing systems and the trends for designing ion optical lens systems by computer routines.

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D . Collection of Short-Lived isotopes with On-Line Separators

Short-lived isotopes separated by on-line isotope separators are generally collected on a conducting plastic tape which is kept moving continually or at discrete steps with the help of a motor (Fig. 18). The moving

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tape collector (MTC) permits one to study the parent isotope with least interference from its isobaric daughter product. The daughter product can also be studied after a suitable delay time following collection. The tape advances within a fraction of a second to bring the collected isotope to the positions of the alpha, beta, and gamma counters for measurements. The collection and measurement periods are automatically controlled. The advantages of a moving tape are demonstrated in the work of Borg et al. (1965). They observed two y-ray peaks at 94.3 and 109.2 keV on a fixed collector contributed by two isobars 91Kr ( T l i 2= 10 sec) and its daughter 91Rb ( T l i 2= 14 min). Using the moving tape collector with a speed of SEPARATED BEAM

1

n

FIG. 18. A moving tape collector system for on-line isotope separators (Naumann er al., 1970).

2 mm/sec, it was possible to identify the 109.2-keV pray peak of "Kr by instantaneous detection, and after sufficient build-up time 94.3-keV y shows as belonging to the daughter 'lRb. Moving tape systems have been described by Naumann et al. (1970) (Fig. 18) and Kjelberg and Rudstam (1970), and at present these are used for all on-line separators with more or fewer modifications. Talbert et al. (1973) have described the use of two moving tape collectors with the TRISTAN facility, one of which is adapted to be used with a ,,hn P-spectrometer. In one of the modes of the latter, tape moving with a discontinuous motion, the /? particles of parent isotope are analyzed after collection; in the daughter enhancement mode, the collected activity is delayed to allow the building of the daughter product and the tape is then moved to the source position. Halbig et al. (1974) have described in detail three basic modes of operation of MTC, namely parent enhancement, daughter enhancement, and equilibrium, as well as the use of the /?-spectrometer through MTC.

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VII. APPLICATIONS A . Application of Separated Isotopes 1. Stable Isotopes

Dawton and Smith (1958) and Smith (1957) have discussed different uses of separated isotopes and established the various advantages of enriched stable isotopes in various fields of science, including nuclear physics, solid state physics, etc. The isotopes produced by the large production machines are still considered to be the main source of stable isotopes. But laboratory isotope separators have the advantage that thin targets can be prepared to high purity by direct deposition, requiring no further processing. When such limitations on the shape and quantity of the isotopic target are not required, isotopes can still be obtained to a higher degree of purity with a laboratory isotope separator than what can be produced in a large machine. High resolution machines are considered to produce high quality stable isotopes suitable as starting materials for artificially producing radioisotopes for studying their decay scheme and characteristic radiation and for assigning their masses. Such stable isotopes are conveniently used for nuclear reaction studies including (p, y ) resonances, K capture, Coulomb excitation, and fission reactions. Laboratory isotope separators are especially convenient for producing isotopic targets of gases by direct deposition. Koch (1958) has reviewed the uses of isotopes produced by laboratory separators in nuclear physics made by different groups. Koch and Rasmussen (1949) have used a laboratory isotope separator to produce Kr and Ne isotopes to study their optical hyperfine structure and.isotope shifts. Until now a large number of laboratory isotope separators have been installed in various laboratories producing almost all stable isotopes of high purity. They complement the large production machines in some cases, even superseding them in the event of the requirement of a purer or directly deposited isotope. Elbek (1965) has described the uses of better quality enriched targets for profitable nuclear reaction studies. The experimental results of his group show that with enriched Yb isotopes of mass number 168 and 170, studies of the localization of the low-lying single particle levels by (d,p) reactions due to increased neutron production are interfered with by contamination. A laboratory isotope separator could prepare a pure target of 50 pgm/cm2 thickness from a 20% enriched charge material of 16*Ybfor accurate determination of the (d,p) spectrum without any impurity peak. With an isotopic

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purity of 99% or better, laboratory isotope separators can efficiently produce rare isotopes or odd mass isotopes. For studies of level systematics, these machines can produce samples of all isotopes of an element with identical properties by direct deposition. 2. Radioactive Isotopes by Ofl-Line Separation The most attractive application of laboratory isotope separators lies in separating radioactive isotopes produced by an accelerator or a reactor. Such directly deposited targets are more suitable for nuclear spectroscopy work and mass number determination. High resolution P-spectrometry of radioisotopes requires, especially for studies of low energy conversion lines, thin, carrier free sources on thin backing foils. Laboratory isotope separators can efficiently produce such sources. Bergstrom (1952) and Thulin (1955) have reported results on measurements of separated radioactive isotopes of noble gases. Further results on the noble gas isotopes were published by Brown et al. (1961), Graham et ai. (1962), and Geiger et al. (1962) with the use of a $n P-spectrometer and an isotope separator. Uhler (1961) has shown the importance of a very thin radioactive target obtained for studying the conversion lines of 205Biand 206Bi in the Auger region. His studies on the decay scheme of the fissionproduced isotopes of low activity show the advantage of radioactive isotope separation for elimination of the isotopes with stronger activity. Anderson (1961) has outlined the use ofa separator for nuclear spectroscopic studies of neutron deficient nuclides. Nielsen and Skilbreid (1961) have shown some typical examples of radioactive separation without which nuclear spectroscopy is not possible. These include 242Pu,232Th,RaB, RaC, 59Gd, ' 'Tb, and lS2Tb.For y-spectroscopy work a thin source is not required in general, but separation of radioactive isotopes helps to provide mass identification and removal of unwanted activities. A small impurity, which is quite common with the rare-earth elements, can produce large y-ray peaks with a sensitive Ge(Li) detector. These detectors are efficient for detection of weaker lines. The weaker lines of the wanted spectrum under study have to be studied in the presence of the unwanted prominent weak lines due to impurities. Brown (1968) has shown that ' "Gd obtained from neutron irradiation of separated 158Gdsupplied by Oak Ridge and chemically purified show 830- and 950-keV y lines, due to impurities that are eliminated, when lS9Gditself is processed with a separator, Further, the impurity was proved to be of chemical origin by examining the adjacent Gd isotopes. Alvager (1961) has described the importance of decreasing prompt and chance coincidences by using a separated target when coincidencesbetween X rays and a photoline are measured for finding new isomeric states. Alvager

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and Ryde (1960) have studied weak transitions such as double quantum emission from lmXeafter separation. For a-spectroscopy, the requirement of a thin sample is essential as for the sources for /?-spectrometry. The effect of a thick CI source is to shift the a spectrum to a lower energy, whereas similar /? sources result in reduced resolution. Domeij et al. (1963) have studied this effect in detail. Anderson (1961) has shown the advantages of separation of a-emitting isotopes, which have weaker branching ratios. 3. Short-Lived Isotopes by On-Line Separation Separation of radioactive isotopes by a separator requires a finite time of the order of a few minutes between production and measurement of radioactivity. So, off-line isotope separators are suitable for handling isotopes with half-lives of the same order. To obtain more information on the short-lived isotopes far away from the /?-stability region and also to measure prompt yields in fission and spallation, it is necessary to connect the separator with the accelerator or reactor in a manner such that the reaction takes place within the ion source or to carry the reaction produced promptly to the ion source of the separator. It is also required to place the detection device to run with the separator. Such on-line isotope separators are suited for study of isotopes with half-lives in the millisecond range. Moreover, the on-line separation technique is of decisive importance for measuring the properties of nuclides of half-lives extending to a n hour or so. Though earlier KofoedHansen and Nielsen (1951) used such a device for studying 91Krof half-life 10 sec produced by fission, the technique could not be well developed until 1965, when Borg et al. (1965) repeated and extended the same experiment and Sidenius et al. (1965) used on-line separation of isotopes from spontaneous fission of 252Cf.Full-scale systems of on-line separators have been installed in many laboratories at present, and a survey of the existing instruments is given by Talbert (1970). The Isolde Collaborations ( 1969) have measured half-lives ranging between 3.6 sec and 18 min of on-line separated isotopes of a large number of elements produced by spallation. Besides measurements of half-lives, nuclear spectroscopy with on-line separators has provided much new information related to these short-lived isotopes in connection with the structure of their excited levels of the transitional and the deformed regions with 50 < Z , N < 82 and with 50 < N < 82 and 28 < Z < 50. On-line isotope separators are ideal for these studies since they can separate the complex mixture of the reaction products and can produce thin and pure samples for nuclear spectroscopy. Hansen (1973) has surveyed recent trends in studies of the short-lived nuclei including determination of nuclear masses, their spins and moments, their properties of a and proton

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activity, P-delayed particle emission, and P-7 radioactivity. Naumann (1969) has reviewed on-line separator studies of short-lived neutron rich isotopes produced by fission and neutron deficient isotopes produced by spallation or heavy ion reactions by various groups. So far as the short-lived activities of these isotopes are concerned, on-line separation with flight time of microseconds is a unique method for investigating their properties. To date, off-line separated radioactive nuclei have been used in atomic beam magnetic resonance experiments for determining their spins and moments. Ekstrom et al. (1973) have described plans for using on-line separated radioactive ions focused at the oven position of the atomic beam apparatus for determination of their spins and moments. On-line separation of a particular isotope from a mixture of a number of nuclei is meaningful before the atomic beam experiment is carried on.

B. Application of Isotope Separators as Mass Spectrometers Since isotope separators operate on the same principle as mass spectrometers, and their efficiencies and mass analyzed beam intensities are higher than the latter instruments, they can conveniently be used for certain experiments as mass spectrometers. For studying the behavior of different types of sources of singly and multiply charged ions, in connection with determination of the parameters of an ion source to increase the multiply charged fraction of heavy ions, isotope separators can conveniently be used. Such studies are of much importance for heavy ion reactions with nuclear accelerators and for the isotope separation technique itself to increase the energy range with multiply charged ions. Isotope separators can analyze inert gas molecular ions like LiKr+ produced in the source, as observed by Freeman and McIlroy (1964). Synthesis and study of similar molecules can throw light on their stabilities and modes of formation. Baumann et al. (1971) have observed some doubly charged negative heavy ions with a Penning ion source. Isotope separators may be more useful for investigating their properties in detail and also for studying negative ion sources in general. Identification of the mass of a radionuclide can be done in any experiment made with a separator. Due to preferential ionization of a particular element due to its difference in volatility or chemical behavior, the appearance of isobaric masses is limited to some extent. The isotope separator method of mass identification has greater sensitivity than is possible with mass spectrometers. Rook et al. (1974) have used a mass separator and neutron activation to identify trace elements like cadmium. Here mass separation is considered to be simpler than any chemical seperation. Eastwood et al. (1964)describe the use of a mass separator to

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estimate the number of atoms of long-lived "Kr for measuring its half-life (T,,, = 2.13 x lo5 yr). Anderson and Rudstam (1964) have discussed the advantages of a mass separator over conventional mass spectrometer techniques with respect to efficiency and accuracy. The isotope separator is a convenient tool for measuring the relative yields or cross sections of production of isotopes of an element produced by a nuclear reaction by determining the activity of the isotopes, though there are limitations arising due to variations in efficiency of the separator for different elements. Anderson (1954), Rudstam (1957), and Aagaard et al. (1957) studied the relative yields from spallation and high energy fission with the Uppsala separator. Andersson (1961) has mentioned the effect of contamination, e.g., hydride formation, in some cases and the advantages of isotope separators for measurement of isomeric cross-section ratios. Bernas (1961) has described relative cross-section measurements for the production of radioactive isotopes of Hg from Au by 155-MeV protons. Brown (1968) has evaluated methods already familiar in mass spectrometric studies for extending the usefulness of relative yield measurement experiments with isotope separators. These methods relate to determination of the absolute yield by comparison with an isotope of known independent yield or by isotope dilution techniques with radioactive tracers. C . Isotope Separators as Low Energy Accelerators

A laboratory isotope separator with typical acceleration voltage or with enhanced acceleration of ions (Section 111) can conveniently be used as a source of fast ions for a wide variety of experiments: scattering of ions in gases and solids, beam foil spectroscopy, inner shell ionization, radiation damage studies, and so on. In technology separators are applied in ion implantation in solids and as ion injectors for other accelerators. Studies of ranges of ions in solids, sputtering, sticking factor, and saturation of solids under ion bombardment by isotope separators are useful as regards the collection of ions (Section VI) for exploring collision processes in solid state physics. Channeling of ions in single crystals associated with the above studies has gained importance in recent years, and the isotope separator is an important tool for studying such phenomena. In the following, we shall briefly discuss applications of separators as accelerators showing their versatility in science and technology. 1. Ionization and Excitation in Atomic Collisions

Charge transfer, ionization, electron loss, or excitation are the fundamental processes for fast ions moving through gases. To determine the cross sections of the above processes simpler equipment would be sufficient. But the use ofa separator as the source of fast ions

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is an attempt to obtain optimal experimental conditions. The group of Kistemaker did pioneering work in applying fast ions from an isotope separator to study the above processes. The work of this group initiated with the studies by de Heer (1956) consisting of the investigation of the charge transfer and ionization processes for a number of ion-atom combinations using the isotope separator as a source of the accelerated ion beam. It was followed by the work of Sluyters (1959a)who studied charge transfer, ionization, and electron loss between Ar' and atoms of noble gases. Van Eck et al. (1962; Van Eck and Kistemaker, 1960) studied the same process for Li+ in H, and He. The cross sections of the same as a function of ion velocity have been measured by the above authors more accurately than with earlier methods. In some case, reasonable agreement with theory has been obtained. Sluyters (1959b) has studied the excitation mechanism of Ar' from an isotope separator in collision with noble gases. Emission and excitation cross sections as a function of ion energy are determined from the spectral lines obtained by a vacuum spectrograph. Van Eck et al. (1962)have studied the excitation of Li, HeI, and HP spectral lines excited by collision of Li+ in He or H, . In an experiment, Andersen et al. (1974) have used the fast ion beam of an isotope separator to investigate inelastic channels: X+

+ Ne<

+ AE (18.38 - 18.96 eV) (excitation)

(37)

+ Ne+ + AE (18 5 AE 5 19.2 eV) (charge transfer)

(38)

Ne*(2P53p)+ X f X'

where X' is an ion of one of N, 0, Na, and Mg atoms. Due to the interference between excitation (37) and charge transfer (38) collision channels, regularly spaced oscillations that are in counterphase have been observed, satisfying the law of conservation of the total probability of the levels of the quasi-molecule of the ion and the atom. For experiments similar to those described above or for studying any atomic or molecular reaction as a function of ion velocity, the isotope separator will remain an important source for accelerated ions. 2. X-Ray Production in Heavy Ion-Atom Collisions Violent collisions of heavy ions of energy higher than that used for charge transfer or excitation of outer shell electrons, as described in Section VII,C,l, produce X rays more efficiently than proton impact and create inner shell vacancies. The phenomena are qualitatively explained by the crossing of molecular orbitals in the quasi-molecule formed during the collision. Saris and Onderdelinden (1970)used an isotope separator as the source of an accelerated ion beam to study collisions of Ar', Ne', and H' on Ar and Ne' on N. Saris (1971)measured the cross sections for Ar L-shell X-ray

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emission in collisions of He+, C', O+, Al', Ti', Fe+, and Cu' on Ar. Fortner (1974) presented L-X-ray spectral measurements for P+, S', Cl+, Ar', and K + incident on Ar and CH, gas for projectile energies between 15 and 200 keV. The experiments described above are limited to the energy range used in isotope separators; even so, the results give important information : (1) X-ray production in heavy ion-atom collisions is more efficient than in proton-atom collisions. (2) X-ray emission cross sections as a function of atomic number show maxima when the binding energy of the inner shell under study is about equal to that of the inner shells of the collision partner. (3) Fluorescence yield is influenced by the mechanism of primary vacancy production. (4) From the energy dependence of the cross section near threshold, critical internuclear distances for inner shell excitation can be determined. (5) Electron promotion mechanisms based on the molecular orbital model given by Fano and Lichten (1965) and Lichten (1967) can explain the phenomena qualitatively.

For thick targets like solids, similar results are obtained with the exception of some solid state effects. For 80-keV Ar+ ions incident on graphite, Der et al. (1971) observed C K-X rays, the intensity of which is very small in the case of gaseous target CH, . This is explained by the molecular orbital theory, assuming the stripping of three or more M-shell electrons or one L-shell electron of the projectile. In ion-solid interactions, Saris et al. (1972) have observed molecular orbital X rays that are not identified as the characteristic X rays of the colliding partners. Similar results have been obtained by Macdonald and Brown (1972) by bombarding 25-200-keV Ar ions on targets of C, Al, and Si. Such noncharacteristic X rays are assumed to originate from the collision of Ar with a previously implanted Ar in the solid during the lifetime of the quasi-molecule formed by the colliding partners or some type of iontarget atom interactions not consistent with molecular orbital systematics. Cairns (1973) has applied a technique for X-ray production in solids to detect impurity elements at or near surfaces of solids. Detection of sulfur on the surfaces of copper, nickel, and stainless steel has been possible using 30-60-keV Ne' ions. Cairns er al. (1973) have assessed the potentialities of the method and the consequent complications involved in solid analysis by ion-induced X rays. The experiments discussed above involve the energy range available with an isotope separator, and its use will make convenient further studies in the same field. The reader is referred to the review by Garcia et al. (1973) for a detailed discussion of the subject.

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3. Beam Foil Spectroscopy Beam foil spectroscopy is a technique for studying a number of problems by passing a fast ion beam through a thin carbon foil and studying the emergent ions in various charge and excitation states. The problems that can be studied by this technique include: (i) mean lives of excited levels, (ii) autoionization processes, (iii) Stark, Zeeman effects and fine structure or hyperfine structure of atoms, (iv) radiative transitions and energy levels of atoms and ions, and so on. Martinson and Gaupp (1975) have reviewed the details of beam foil spectroscopy with the aid of various accelerators. Generally, low energy accelerators are used as sources of fast ion beams; the use of a laboratory isotope separator with a stable and intense ion beam for this purpose in some cases is convenient since at low ion energy, beam foil spectroscopy with higher sensitivity and resolution can produce valuable results. Secondly, the low velocity of the ions makes the device useful for measuring mean lives of 10-7-10-8 sec when the intensity ofthe spectral lines decreases at the measuring distance. For the ions and neutrals of high probability of populating states at low energies, isotope separators are convenient sources of fast ion beams. As an example, the work of Bickel et al. (1969) may be cited, in which an isotope separator has been used for beam foil spectroscopy of 'Li' ions; 26 spectral lines of Li I and Li I1 have been identified. They have measured the decay times of eight stronger lines to determine the mean lives of upper levels related to the decays. From the results, the feasibility of the instrument for similar studies at the limit of its accelerating voltage is well ascertained.

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4. Atomic Collisions in Solids and Channeling Phenomena When a fast ion beam of low energy is incident upon a solid target, a great variety of physical phenomena occur, namely, sputtering of solid, energy loss of ions, secondary electron emission, X-ray production, and back-scattering of ions. If the solid is homogeneous and isotropic, the results of the interaction of ions with the solid will not be influenced by the direction of the beam and the target. When the target is a monocrystal, the interactions will be strongly dependent upon the orientation of the beam and the target. The effect is due to the channeling of ions through crystals, and it is dependent upon the degree of anisotropy of the crystalline solid and the repulsive potential existing between the ion and the lattice atoms. Due to the repulsive potential, ions of suitable energy and direction of incidence will move through the channel acting like an open tube suffering minimal energy loss due to successive small-angle scattering. The effect of energy loss can be determined by the range of ions that have exhausted their energies and been captured by the cohesive forces of the solid.

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A common technique of measuring the range in a solid is that of bombarding it with radioactive ions from an isotope separator followed by anodic oxidation or chemical stripping. Domeij et al. (1963) have used the method of measuring the energy spectrum of a-emitting '"Rn implanted into a solid and correlating the degradation of the energy of a particles with the depth of penetration, so that the range can be estimated. Bergstrom et al. (1963) have applied a similar technique by implanting p-emitter '"Xe into a solid. Jespergird and Davies (1967) and Domeij er al. (1964) have measured the ranges of various ions in amorphous materials, the results of which are in good agreement with the theory of Lindhard et al. (1963). Davies et al. (1969) observed abnormally large ranges in crystalline solids due to channeled ions experiencing a very low energy loss in the crystal. For discussions and references to the extensive literature on the ranges of low energy ions from isotope separators, the reader is referred to the reviews by Dearnaley (1973) and Gemmell (1974). Since the discovery of sputtering of solids under ion bombardment, measurements of sputtering yield as a function of ion energy, angle of incidence of ions, and the nature of the ions has been made by many experimentalists. Isotope separators as sources of ion beams have played an important role in experiments on sputtering phenomena. Almen and Bruce (1961) and Rol et al. (1960b) have given comprehensive data on sputtering with the aid of isotope separators. Rol et al. (1960a), Almen and Bruce (1961), and Onderdelinden (1968a,b) have given results on sputtering in single crystals, showing the lowest yield in the more open channeling directions of the crystals. Since the first observation of the orientation dependence of the deposits of the sputtered atoms by Wehner (1955), extensive work has been done by many authors, showing preferential ejection near the simple close-packed direction of the crystal. These observations as evidence of the existence of focused collision sequences are supported by sputtering experiments (Onderdelinden, 1968a,b; Dey et al., 1970; Basu er al., 1971; Bhattacharya er ul., 1974). Rutherford scattering or back-scattering of ions from solids is used to detect small impurities on their surfaces, using the techniques of measurement of the energy loss observed in the back-scattered ions as a function of the mass of the target atom. For an element lighter than the target, the impurity peak cannot be discriminated from the random spectrum. Davies et al. (1967) have used a channeled back-scattered beam from a single crystal to surmount such drawbacks. Nelson and Thompson (1963) were pioneers in observing evidence of channeling of 50-keV H + , He+, Ne', and Xe+ ions back-scattered from a Cu single crystal. Both the back-scattering and the influence of channeling on it can be studied with the aid of an isotope separator. Bhattacharya and Karmohapatro (1973) have observed channeling phenomena in studying low energy scattered ions from Ag crystals.

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The influence of channeling of ions in light emission (Khan et al., 1967a) and in characteristic X-ray production (Brandt er al., 1965, 1968; Khan et al., 1967a,b) has also been observed. Davies et al. (1969) and Picraux er al. (1969) have compared the effect of channeling on back-scattering and X-ray production in crystals by direct measurement. Picraux e f al. (1969) have used characteristic X rays to study sublattices in polyatomic crystals. Domeij and Bjorkquist (1965; Domeij, 1966) implanted "'Rn in a W crystal and measured the blocking effect in the emission of ci particles from implanted Rn to determine its location in the lattice. Matzke and Davies (1967) implanted 13'Xe in KCl and studied the back-scattering of 3-MeV protons to locate xenon in the lattice. The results have been useful for locating impurities in a crystal lattice. The isotope separator has been an important tool for such low energy ion implantation responsible for the discovery of blocking effects in crystals. Collisions of ions in solids and crystals and channeling phenomena have been extensively studied by numerous authors, a few examples of which have been cited in the present discussion. Accelerators are universal tools for these studies, and an isotope separator within its range of energy has been important for specific applications in this field. For further references and critical discussions readers are referred to Gemmell (1974), Morgan (1973), and Dearneley (1973).

5. Ion Implantation We have mentioned (Section VII,C,4) the uses of isotope separators for implanting a radioactive isotope in crystals for range measurement, channeling, and blocking experiments. The major application of ion implantation by isotope separators has gained importance in semiconductor electronics. Shockley ( 1954) discussed the doping of semiconductors by ion implantation for producing narrow base transistors, the effect of high energy ioninduced damage in crystals, and the necessity of annealing them. Doping a semiconductor with impurity atoms alters its electrical properties on the surface extending to a depth of a few layers. Methods of introducing impurities in semiconductors are thermal diffusion, epitaxial techniques, alloying processes, and implantation of energetic ions by an accelerator. The latter method, using an isotope separator as an accelerator, has advantages over other methods with respect to controlling the depth of penetration, concentration, purity, and spatial distribution of the doping atoms. Unlike the thermal diffusion process, ion implantation takes place at a low temperature and ions of very low diffusion velocity or low solubility can be doped to a certain depth in the semiconductor. The main disadvantages of ion implantation are the slowing down of ions in collision processes and the damage of

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the crystals produced by fast ions. For ion implantation to be useful, proper care must be taken as regards the energy dependence of range, penetration distribution of the doped ions, as well as the structural defects of the crystal under ion bombardment. Generally, the energy required for ion implantation ranges from 10 to 500 keV, which can be attained by an isotope separator with enhanced acceleration described in Section 111. For most applications, ion energies around 100 keV with 10-7-10-5-A current are sufficient. Manchester rt a/. (1965) described the use of an isotope separator to evaluate the controllability of the uniformity and depth in implantation of boron and phosphorous ions in silicon. Results of the experiments show the applicability of such machines with energy less than 100 keV in silicon planar technology in producing electrical parameters equivalent to those produced by thermal diffusion. Structural defects of the crystals at that energy range and low ion intensity can be easily annealed by mild and short duration thermal treatment. Various aspects of ion implantation were discussed at the Aarhus, Koch and Nielsen (1965) conference. Since then a large number of papers has been published on the applications of ion implantation techniques for fabricating devices such as resistors of small dimensions with high resistivity and uniformity, and diodes for various uses. The major application of ion implantation in fabricating metal oxide semiconductor transistors (MOST) is the most important one since the method is accepted as unique for production of these devices for integrated circuits on an industrial scale. In addition, ion implantation well defines the implanted and doped zones to obtain greater compactness and density of the elements and lower energy consumption than the diffused devices. As regards industrial scale production, high reproducibility of the characteristics by ion implantation is responsible for a higher production yield of MOST devices. Doping of semiconductors with defects such as dislocations, grain boundaries, etc. by ion implantation is more advantageous than diffusion processes, which distort the electrical profile of the junction in imperfect semiconductors. Ion implantation is a prospective method for producing bipolar transistors and for doping insulators or compound semiconductors. Techniques of ion implantation in semiconductors and their applications have been reviewed by Mayer et al. (1970) and Stephen (1972). Proceedings of the conferences in Thousand Oaks (1970) and Bavaria (1971) include an account of progress in this field. Ion implantation in materials other than semiconductors is finding applications in many fields. To enhance superconductivity of the superconductors, to activate inorganic phosphors, to control the properties of magnetic bubble garnets in thin films of the Fe-Co-Ni alloy system, or to modify

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the refractive index of materials, ion implantation has proven to be a useful technique. For measurement of hyperfine interaction, radioactive nuclei are embedded in the solid and the interactions of the gamma decay of the embedded nuclei involving the product of the magnetic and the electric field gradients and the multipole of the embedded ion are observed by one of the methods such as nuclear orientation techniques, perturbed angular correlations, and Mossbauer effect. For embedding radioactive nuclei, the ion implantation technique gives more concentration of ions than the method of injection by recoil of a nuclear reaction product. Freeman (1969a) has given operational characteristics of an isotope separator for ion implantation in hyperfine interaction studies. Thompson (1970) has reviewed the applications of ion implantation in various fields other than semiconductors and its future prospects (see also Namba and Masuda, 1975). A detailed discussion by Stephen (1973) on the applications of ion implantation contains a large number of references on the subject. Readers are referred to the review by Freeman (1973) on ion accelerators and separators for ion implantation and to a discussion on the physicill state of ionimplanted solids by Nelson (1973) for further details. VIII. CONCLUSIONS Laboratory isotope separators originated from the principles of mass spectrometry and have reached a level of sophistication sufficient to separate stable isotopes for use in many experiments and to separate radioactive isotopes of very short duration. There is continuous development of different components such as ion sources, focusing elements, collection methods oriented to needs for high efficiency, less contamination, and high resolution. A separator in one form or other can perform the separation of isotopes and can be used as a low energy accelerator for other applications or as a high intensity mass spectrometer. It is possible to interchange the components of a separator to use it as a high or low intensity machine and to use it as a separator or an accelerator. Its applications cover a broad area in physics including atomic physics, solid state physics, and nuclear physics. On-line isotope separators and separators for ion implantation are developing rapidly for fruitful research in nuclear physics and solid state electronics; the impact of the latter in the industrial field has shown promising results. Further applications of isotope separators in radiation damage studies, radiation chemistry and surface chemistry studies, and as injectors for accelerators (not discussed in the present article) illustrate the versatility of the instruments. In future, tandem isotope separators, besides producing higher dispersion, will be used for measuring energies and angular distributions in collision experiments.

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ACKNOWLEDGMENT The author is grateful t o Professor A. K. Saha for valuable suggestions and to Professor D. N. Kundu for his keen interest in this work.

REFERENCES Aagaard. P., Andersson, G., Burgman, J. O., and Pappas, A. C. (1957). J . Inorg. Nucl. Chem. 5, 105. Abraharnsen, P., Jorgensen, H. E., and Skilbried, 0. (1973). Proc. I n t . Electromagn. Isotope Sep. Con$, 8th. 1973 p. 205. Alekseevsky, N. E., Prudkovsky, G. P., Kosurov, G. E., and Filimnov. S. 1 . (1955). Proc. Acad. Sci. U S S R 100, No. 2, 229. Alexandre, K., Camplan, J., Ligonniere, M., Meunier, R., Sarrouy, J. L., Smith, H. J., and Vassent, B. (1970). Nucl. Instrum. & Methods 84, 45. Almen, 0. (1962). PhD. Thesis, University of Goteborg. Almen, O., and Bruce, G. (1961). Nucl. Instrum. & Methods 11. 279 and 257. Almen, O., and Nielsen, K. 0. (1957). Nucl. Instrum. 1, 302. Alvager, T. (1961). Proc. Int. Symp. Electromagn. Sep. Radioact. Isotop., 1960 p. 279. Alvager, T., and Ryde, H. (1960). Ark. Fys. 17, 535. Alvager, T., and Uhler. J. (1957). Ark. Fys. 13, 145. Alvager, T., and Uhler, J. (1968). Prog. Nuci. Tech. Instrum. 3, 191. Amarel, I., Bernas, R., Chaumont, J., Foucher, R., Jastrzeleski*J., Johnson, A., Klapish, R., and Teillac, T. (1966). Ark. F y s . 36,77. Amiel, S., Nu-El, Y., Shmid, M., Venezia, A,, and Wismontsky, 1. (1973). Proc. Int. Conf. Electromagn. Isotope Sep., 8th 1973 p. 412. Andersen, T., Kirkegaard Nielsen, A., and Olsen, K. J. (1974). Phys. Reii. A 10, 2174. Andersson C., Grapengiesser, B., and Rudstam, G. (1973). Proc. Int. Electromagn. Isotope Ser. Cot$. 8th, 1973 p. 463. Andersson, G . (1954). Philos. Mag. [7] 45, 621. Andersson, G., Hedin. B., and Rudstam, G. (1961). Proc. Int. Symp. Electromagn. Sep. Radioact. Isotop., 1960 p. 103. Andersson, G. (1961). Proc. I n t . Symp. Electrornagn. Sep. Radioact. Isotop., 1960 p. 292. Andersson, G., and Holmen, G., eds. (1973). " Low Energy Ion Accelerators and Mass Separators," Proc. Int. EMIS Conf. Chalmers University of Technology, Goteborg, Sweden. Andersson. G., and Rudstam, G. (1964). Nucl. Instrum. & Methods 29, 93. Andersson, G., Hedin. B., and Rudstam, G . (1964). Nucl. Instrum. & Methods 28, 245. Arcipiani, B., Barsanti, G., Fumelli, M.. Musumeci, L., and Talini, N. (1959). Nuouo Cimento [lo] 12, 611. Artsimovich, L. A,, Shehepkin, G . Ya., Zhukov, V. V., Makov, B. N., Maksimov, S. P., Malov, A. F.. Nikelichev, A. A., and Panin, B. V. (1957). Sol. J . At. Energy 3(12), 1961. Aston, F. W. (1920). Proc. Cambridge Philos. SOC.19, 317. Aubert, J. (1972). Thesis, 3rd Cycle, Orsay. Aubert, J., Gautherin. G., and Lejeune, C. (1973). Proc. I n ? . Electromagn. Isotope Sep. Con5 8th. 1973 p. 166. Aysto, J., Peritti, P., and Kalevi, V. (1974). Nucl. Instrum. & Methods 115, 65. Bachmann, K. (1970). Proc. Int. Conf Electromagn. Isotope Sep. Tech. Appl., 1970 p. 126. Bahrami, H..Chernow, F., Denison, D., and Eldridge, G. (1970). Proc. Eur. ConJ Ion Implant. 1970 p. 26.

S. B. KARMOHAPATRO

170

Bainbridge, K. T. (1952). 1n“Experimental Nuclear Physics” (E. Segre, ed.), p. 559. Wiley, New York. Banic, G. M., Bell, W. A., Jr., and Love, L. 0.(1962). Proc. Int. Conf:Phys. Electromagn. Isorope Sep. Method, 1962 (unpublished). Basu, D., Dey, S. D., and Karmohapatro, S. B. (1971). Nucl. Instrum. & Methods 93, 403. Baumann, H.,Heinicke, E., Kaiser, H.J., and Bethge, K. (1971). Nucl. Instrum. & Methods 95, 389.

Bavaria (1971). “Proceedings of the Second International Conference on Ion Implant. Semiconductors.” Springer-Verlag, Berlin and New York. Beiduk, F. M., and Konopinski, E. J. (1948). Rev. Sci. Instrum. 19, 94. Bennett, J. R. J. (1971a). Proc. Int. Conf.Multiply Charged Heavy Ion Sources Accelerating Syst., 1971.

Bennett, J. R. J. (1971b). lEEE Trans Nucl. Sci. 18, 55. Bergstrom, I. (1952). Ark. Fys. 5, 191. Bergstrom, I., Brown, F., Davies, J. A., Geiger, J. S., Graham, R. L., and Kelly, R. (1963). Nucl. Instrum. & Methods 21, 249. Brostrnm, K. J., Huus, Y.,and Koch, J. (1947). Nature (London) 160, 498. Berkeley (Asilmore). (1967). “International Conference on Electromagnetic Isotope Separators” (unpublished). Bernas, R. (1954). Thesis, University of Paris. Bernas. R. (1961). Proc. In?. Symp. Electromagn. Sep. Radioact. Isotope 1960 p. 300. Bernas, R., Kaluszyner, L., and Druaux, J. (1954). J. Phys. Radium [8] 15, 273. Bernas, R.,Camplan, J., Van Ment, M., and Sarrouy, J. L. (1961a). Proc. Int. Symp. Electromagn. Sep. Radioact. Isotope., 1960 p. 68. Bernas, R., Sarrouy, J. L., and Camplan, J. (1961b). Proc. Int. Symp. Electromagn. Sep. Radioact. Isotop., 1960 p. 121. Bhattacharya, R. S., and Karmohapatro, S. B. (1973). Nucl. Instrum. & Methods 109, 191. Bhattacharya, R. S., Basu, D., and Karmohapatro, S. B. (1974). Radiat. Eff 21, 275. Bickel, W. S., Martinson, I., Lundin, L., Buchta, R., Bromander, J., and Bergstrom, I. (1969). J. Opt. SOC.Am. 59, 830. Blewett, J. P., and Jones, E. J. (1936). Phys. Rev. 50, 464. Boge, M., Bouriant, M., Chantereau, E., Dousson, S., and Bouchez, R. (1970). Proc. I n t . Con$ E M l S Tech. Appl. 1970 p. 381. Bogh, E., Dahl, P.. and Nielsen, K. L. (1962). Proc. I n t . Conf: Phys. Electromagn. Sep. Method, 1962 (unpublished). Borg, S . (1970). Proc. Inr. Conf: Electromagn. Isotope Sep. Tech. Appl., 1970 p. 51. Borg, S., Fagerquist, U., Holm, G., and Kropff, F. (1965). Nucl. Instrum. & Methods 38, 296. Brandt, W., Dobrin, R., Jack, H.,Laubert, R., and Roth, S. (1968). Can. J . Phys. 46, 537. Brandt, W., Khan, J. M., Potter, D. L., Worley, R. D., and Smith, H. P. (1965). Phys. Rev. Lett. 14, 42.

Brookhaven. (1971). “Proceedings of the Symposium on Ion Sources and Formation of Ion Beams.” US. Brookhaven Natl. Lab., Upton, New York. Brown, F. (1968). Nucl. Chem. 2, 298. Brown, F. (1973). Proc. Int. Electrornagn. Isotope Sep. Con$, 8th. 1973 p. 180. Brown, F., and Davies, J. A. (1963). Can. J. Phys. 71, 844. Brown, F., Graham, R. L., Ewan, G. T., and Uhler, J. (1961). Can. J . Phys. 39, 779. Burgman, J. O., and Anderson, G. (1958). Nucl. Instrum. 3, 33. Cairns, J. (1973). Surf: Sci. 34, 634. Cairns, J., and Marwick, A. D. (1973). Thin Solid Films 19, 91. Camac, M. (1951). Rev. Sci. Instrum. 22, 197. Camplan, J., Menat, M., and Bernas, R. (1961). J. Phys. Radium 91A, 22.

LABORATORY ISOTOPE SEPARATORS

171

Camplan, J., Meunier, R., and Sarrouy, J. L. (1970). Nucl. Instrum. & Methods 84, 37. Camplan, J., Meunier, R., and Fatu, C. (1973). Proc. Int. Electromagn. Isotope Sep. C o n f , 8th, 1973 p. 186. Carter, G., and Colligon, J. S. (1968). “Ion Bombardment of Solids.” Heinemann, London. Chavet, I. (1965). Thesis, University of Paris. Chavet, I. (1970). Proc. Int. Conf: Electromagn. lsotope Sep. Tech. Appl. 1970 p. 303. Chavet, I . (1973). Proc. I n t . Electromagn. Isotope Sep. ConJ. 8th, 1973 p. 192. Chavet, I., and Bernas, R. (1967a). Nucl. Instrum. & Methods 47, 77. Chavet, I., and Bernas, R. (1967b). Nucl. Instrum. & Methods 51, 77. cobic, B., ToSic, D., and Perovic, B. (1963). Nucl. Insfrum. & Methods 14, 358. Cossignol, C. (1962). In “Electromagnetic Separation of Radioactive Isotopes” (M. J. Higatsberger and F. P. Viehbock, eds.), p. 33. Springer-Verlag, Berlin and New York. Cotte, M . (1938). Ann. Phys. (Leipzig) [S] 10, 333. Couchet, G. (1954). Ann. Phys. (Paris) [13] 9, 731. Cramer, J. G., and Schmidt, F. H. (1966). Nucl. Instrum. & Meth. 45, 325. Cross, W. G. (1951). Rev. Sci. Instrum. 22, 197. Dagenhart, W. K., and Whitehead, T. W., Jr. (1970). Nucl. Instrum. & Methods 85, 215. Dantet, H., Gujrathi, S., Weischahn, W. J., DAuria, J. M., and Pate, B. (1973). Nurl. Instrum. & Methods 107, 43. Davies, J. A., Denhartog, J., Eriksson, L.,and Mayer, J. W. (1967). Can. J . Phys. 45, 4053. Davies, J. A,, Eriksson, L., Johansson, N. G. E., and Mitchell, I . V. (1969). Phys. Rev. 181, 548. Dawton, R. H. V. M. (1956). In “Electromagnetically Enriched Isotopes and Mass Spectrometry” (M. L. Smith, ed.), p. 37. Butterworth, London. Dawton, R. H. V. M. (1958). In “Electromagnetic Isotope Separators” (J. Koch, ed.), p. 156. North-Holland Publ., Amsterdam. Dawton, R. H. V. M., and Smith, M. L. (1958). In “Electromagnetic Isotope Separators” (J. Koch, ed.), p. 101. North-Holland Publ., Amsterdam. Dearnaley, G. (1973). In “Ion Implantation” (S. Amelinckx, G. Gevers, and J. Nihoul, eds.), p. 9. North-Holland Publ., Amsterdam. de Heer. F. J. (1956). Ph.D. Thesis, Amsterdam. Dempster, A. J. (1944). Metall. Rep. CP-1507. 25. Der, R. C., Fortner, R . J., Kavanagh, T. M., and Garcia, J. D. (1971). Phys. Rev. Lett. 27, 1631. Dey, S. D., Basu, D., and Karmohapatro, S. B. (1970). Nucl. Insfrum. & Methods 77, 242. Dionisio, J . S., and de Lima, D. X. ( (1968). Nucl. Instrum. & Methods 61, 260. Domeij. B. (1966). Ark. Fys. 32, 179. Domeij, B., and Bjorkquist, K. (1965). Phys. Lett. 14, 127. Domeij, B., Bergstrom, I., Davies. J. A., and Uhler, J. (1963). Ark. Fys. 24, 399. Domeij, B., Brown, F., Davies, J. A,, and McCargo, M. (1964). Can. J . Phys. 42, 1624. Dropesky, B. J., Deal, J. B., Buchen, J. F., and Kelly, J. M. (1967). Nucl. Instrum. & Methods48, 329. Druaux, J., and Bernas, R. (1955). In “Electromagnetically Enriched Isotopes and Mass Spectrometry” (M. L. Smith, ed.), p. 30. Butterworth, London. Dunjic. B. (1965). Nucl. Instrum. & Methods 38, 109. Eastwood, T. A,, Brown, H., and Crocker, 1. H. (1964). Nucl. Phys. 58, 328. Ekstrom. C.. Lindgren, I., and Olsmats, M. (1973). Proc. Int. Electromagn. Isotope Sep. Conf:, 8th 1973 p. 405. Elbek, B. (1965). Nucl. Instrum. & Methods 38, 314. Elbek, B., Nielsen, K. O., and Oleson. M. C. (1957). Phys. Rev. 108, 406. Elbek, B., Oleson. M . C., and Skilbreid, 0. (1959). Nucl. Phys. 10, 294. Fabricus, H.. Freitag, K., and Goring, S.(1965). Proc. Inr. Conf: Electromagn. Isotope Sep. Appl., 1965 p. 65.

172

S. B. KARMOHAPATRO

Fano, U., and Lichten, W. (1965).Phys. Rev. Lett. 14, 627. Feldstein, H., and Amiel, S. (1973). Proc. Int. Electromagn. Isotope Sep. Conf, 8 t h 1973 p. 420. Formann, E., and Viehbock, F. P. (1966). Nucl. Instrum. & Methods 42, 3 1 1 , Fortner. R. J. (1974). Phys. Rev. A 10, 2218. Foucher, R., Paris, P., and Sarrouy, J. L. (1973).Proc. Int. Electromagn. Isotope Sep. Conf, 8th, 1973 p. 341. Freeman, J. H. (1961). Proc. Int. Symp. Electromagn. Sep. Radioact. Isotop., 1960 p. 40. Freeman, J. H. (1963). Nucl. Instrum. & Methods 22, 306. Freeman, J. H. (1965).Nucl. Instrum. & Methods 38, 49. Freeman, J. H. (1967). In “Application o f Ion Beams to Semiconductor Technology” (P. Glotin, ed.), p. 75. C.E.N., Grenoble, France. Freeman, J. H. (1969a). Proc. R. SOC., London, Ser. A 311, 123. Freeman, J. H. (1969b). Proc. lnr. Conj3 1.N.S.T.N.. 1969 p. 369. Freeman, J. H. (1970).Proc. Int. Con$ Electromagn. Isotope Sep. Tech. Appl., 1970 p. 373. Freeman, J. H. (1973). I n “Ion Implantation” ( S . Amelinckx, G. Gevers, and J. Nihoul, eds.), p. 255. North-Holland Publ., Amsterdam. Freeman, J. H., and Bell W. A. (1963). Nucl. Instrum. & Methods 22, 317. Freeman, J. H., and Card, G. A. (1970). U.K., At. Energy Res. Establ., Rep. R 6330. Freeman, J. H., and McIlroy, R. W. (1964).Nature (London) 201, 69. Freeman, J. H., and Sidenius, G. (1973). Nucl. Instrum. & Methods 107, 477. Freeman, N. J., Young, W. A. P., Hardy, R. W. D., and Wildon, H. W. (1961). Proc. Int. Symp. Electromagn. Sep. Radioact. Isotopes, 1960 p. 83. Gatlinburg. (1971). “Proceedings of the International Conference on Multiply Charged Heavy Ion Sources and Accelerating Systems.” Gatlinburg, Tennessee. Gautherin, G., and Lejeune, C. (1973). Proc. Int. Electromagn. Isotope Sep. Conf, 8th, 1973 p. 17. Geiger, J. S., Graham, R. L., and Brown, F. (1962). Can. J . Phys. 40, 1258. Gemmel, D. S . (1974). Reo. Mod. Phys. 46, 129. Goode, P. D. (1971). Nucl. Instrum. & Methods 92, 447. Gorodetzky, S., Denimal, J., Ricaud, C., Robin, B., and Ambruster, R. (1965). Nucl. Instrum. & Methods 38, 79. Garcia, J. D., Fortner, R. J., and Kavanagh, T. M. (1973). Rev. Mod. Phys. 45, 1 1 1 . Graham, R. L., Bergstrom, I., and Brown, F. (1962). Can. J. Phys. 40, 1258. Guernet, G., Esteve, P., and Holtin, P. (1970). “Note technique,” LETI/ME, 659, CEA, C.E.N.G. Report. Grenoble, France. Hageb~,E. (1970). Proc. Int. EMIS Conf Tech. Appl., 1970 p. 146. Halbig, J. K., Wohn, F. K., and Talbert, W. L., Jr. (1974). Rev. Sci. Instrum. 45, 789. Hall, D. H. (1965). Rev. Sci. Instrum. 36, 1512. Hansen, P. G. (1973). Proc. Inr. Electromagn. Isotope Sep. Con$, 8th, 1973 p. 102. Hansen, F., Lindahl, A., Nielsen, 0.B., and Sidenius, G. (1973).Proc. Int. Electromagn. Isotope Sep. Conf. 8th I973 p. 426. Hansen, O., Oleson, M. C., Skilbreid, 0..and Elbek, 9. (1961). Nucl. Phys. 25, 634. Hanser, A., Goring, S., and Langmann, H. J. (1962). Proc. Int. Con5 Phys. Electromagn. Isotope Sep. Method, 1962 (unpublished). Hayden, R. J., and Lewis, L. G. (1946). Phys. Rev. 70, 1 1 1 . Higatsberger, M. J., a n d Viehbock, F. P. (eds.) (1961). Proc. Int. Symp. Electromagn. Sep. Radioactive Isotop., 1960. Hull, A. W. (1921). Phys. Rev. 18, 31. Illgen, J., Kirchner, R., and Schulte, J., in der Baumen (1972). IEEE Trans. Nucl. Sci. 19, 35. Isolde Collaboration. (1969). Phys. Lett. B 28, 415. Jespergird, P., and Davies, J. A. (1967). Can. J . Phys. 45, 2983. Johnson, F. M. (1962). R C A Rev. 23, 427.

LABORATORY ISOTOPE SEPARATORS

173

Judd, D. L. (1950). Reu. Sci. Instrum. 21, 213. Jungclass, H., Macfarlane, R. D., and Fares, Y. (1971). Radiochim. Acta 16, 141. Kaminsky, M. (1965). “Atomic and Ionic Impact Phenomena on Metal Surfaces.” SpringerVerlag, Berlin and New York. Kamke, D., and Rose, H. J. (1956).Z . Phys. 145, 83. Karmohapatro. S. B. (1959). Indian J . Phys. 33, 139. Karmohapatro, S. B. (1960).Indian J . Phys. 34, 407. Kerwin, L. (1949). Reu. Sci. Insrrum. 20, 36. Khan, J. M., Potter, D. L., Worley, R. D., Salem, S. I., and Smith, H. P. (1967a). Phys. Rev. Lett. 19, 950. Khan, J. M., Potter. D. L., Worley, R. D., and Smith, H. P. (1967b). Phys. Rev. 163, 81. Kistemaker, J., Bigeleisen, J., and Nier, A. 0.C., eds. (1958).“Proceedings of the International Symposium o n Isotope Separation, Amsterdam, 1957.” North-Holland Publ., Amsterdam. Kjelberg, A., and Rudstam, G., eds. (1970). “Isolde Collaboration,” CERN Rep., pp. 70-73. Klapisch, R., and Bernas, R. J. (1965). Nucl. Instrum. & Methods 38, 291. Klapisch, R., Chaumont, J., Phillipe, C., Amarel, I., Fergau, R., Salome, M., and Bernas, R. (1967). Nucl. Instrum. & Methods 53, 216. Koch, J . (1958).I n “Electromagnetic Isotope Separators” (J. Koch, ed.), Chapter IV. NorthHolland Publ., Amsterdam. Koch, J., and Bendt-Nielsen, B. (1944). I<. Dan. Vidensk. Selsk., M a t . Phys. M e d d . 21, No. 8. Koch. J., and Rasmussen, E. (1949). Phys. Rev. 76, 1417. Koch, J., and Nielsen, K. O., eds. (1965). Proc. Int. Con$ Electromagn. Isotope Sep., 1965. Koch. J.. Dawton, R. H. V. M., Smith, M. L., and Walcher, W. (1958).“Electromagn. Isotope Separators” (J. Koch, ed.). North-Holland Publ., Amsterdam. Kofoed-Hansen, O., and Nielsen, K. 0. (1951). K . Dan. Vidensk. Selsk., M a t . Fys. Medd. 26, No. 7. Kosanke, K. L., McHarris, W. C., Warner, R. A,, and Kelly, W. H. (1974). Nucl. Instrum. & Methods 115, 151. Langmuir, I., and Blodgett, K..B. (1923). Phys. Rev. 22, 347. Langmuir, I., and Blodgett, K. B. (1924). Phys. Rev. 24, 49. Langmuir, I., and Kingdon, K. H. (1923a).Science 57, 58. Langmuir, I., and Kingdon, K. H. (1923b).Phys. Rev. 21, 380. Langmuir, I., and Taylor, J . B. (1937). Phys. Rev. 51, 753. Lavatelli, L. S. (1946).A.E.C. Document MDDC-350. Lejeune, C. (1971). Doctorate Thesis No. 810, Orsay. Lejeune, C. (1974). Nucl. Instrum. & Methods 116, 417 and 429. Lichten, W. (1967). Phys. Rev. 164. 131. Lindahl, A,, Nielsen, 0. B., and Sidenius, G. (1970). Proc. Int. Workshop Meet. Tech. Prob. On-Line Sep., 1970. Lindhard, J., Scharff, M., and Schintt, H. E. (1963).K . Dan. Vidensk, Selsk., Mat.-Fys. M e d d . 33, No. 14. Lindemann, F. A., and Aston, F. W. (1919). Philos. Mag. [6] 37, 523. Love, L. 0. (1973). Proc. Int. Electromagn. Isotope Sep. Con$, 8th, 1973, p. 6. Macdonald, J. R., and Brown, M. D. (1972). Phys. Rev. Lett. 29, 4. Macfarlane, R. D., Cough, R. A., Oakey, N. S., and Torgerson, D. F. (1969). Nucl. Instrum. & Methods 73, 285. Manchester, K. E., Silbey, C. B., and Alton, G. (1965). Nucl. Instrum. & Methods 38, 169. Martinson, I., and Gaupp, A. (1975). Phys. Rep. 1 5 , 113. Masic, R., Warnecke, R. J., and Sautter, J. M. (1969). Nucl. Instrum. & Methods 71, 339. Matzke, H . J., and Davies, J. A. (1967).J . Appl. Phys. 38, 805.

174

S. B. KARMOHAPATRO

Mayer, J. W., Eriksson, L., and Davies, J. A. (1970). “Ion Implantation in Semiconductors: Silicon and Germanium.” Academic Press, New York. Menat, M. (1964). Can. J . Phys. 42, 164. Menat, M., and Freider, G. (1965). Can. J . Phys. 43, 1525. Michel, M. C . (1953). UCRL-2267. Morgan, D. V., ed. (1973). “Channeling-Theory, Observation and Applications.” Wiley, New York. Namba, S., and Masuda, K. (1975). Adv. Electron. Electron Phys. 37, 263. Naumann, R. A. (1969). In “Radioactivity in Nuclear Spectroscopy” (J. H. Hamilton and J. C. Manthuruthil, eds.), Vol. 1, p. 449. Gordon & Breach, New York. Naumann, R. A., Hiibel, H., Loeser, F., and Spejewaski, E. (1970). Proc. Int. Conf. E M I S Tech. Appl., 1970 p. 49. Nelson, R. S. (1973). I n “Ion Implantation” (S. Amelinckx, R. Gevers, and J. Nihoul, eds.), p. 154. North-Holland Publ., Amsterdam. Nelson, R. S., and Thompson, M. W. (1963). Philos. Mag. [8] 8, 1977; erratum, Philos. M a g . [8] 9, 1069 (1964). Nielsen, K. 0. (1957). Nucl. Instrum. 1, 289. Nielsen, K. 0. (1961). *‘ Electromagnetic Separation of Radioactive Isotopes,” p. 35. SpringerVerlag, Berlin and New York. Nielsen, K. 0.(1967). I n “Application of Ion Beams to Semiconductor Technology” (P. Glotin, ed.), p. 27. C.E.N., Grenoble, France. Nielsen, K. 0. (1970). Recent Deo. Mass Spectrosc, Proc. Int. Conf. Mass. Spectrosc, 1969 p. 506. Nielsen, K. O., and Nielsen, 0. B. (1958). Nucl. Phys. 5, 319. Nielsen, K. O., and Skilbreid, 0. (1957). Nucl. Instrum. & Methods 1, 159. Nielsen, K. O., and Skilbreid, 0. (1961). Proc. Int. Symp. Elecrromagn. Sep. Radioact. Isorop., 1960 p. 307. Nitschke, J. M. (1970). Proc. Int. Conf. on the Properties of Nuclei f a r from the Region of Beta Stability, Leysin, Switzerland, 1972, Vol. 7, p. 15. Oliphant, M. L., Shire, E. S., and Crowther, B. M. (1934). Proc. R . SOC.,London, Ser. A 146,922. Onderdelinden, D. (1968a). Can. J. Phys. 46, 729. Onderdelinden, D. ( 1968b). Ph.D. Thesis, University of Leiden. Orsay. (1962). “Conference on the Physics of the Electromagnetic Separation Method” (unpublished). Patzelt, P. (1970). Proc. I n t . Con$ Electromagn. Isotop. Sep. Tech. Appls., 1970 p. 158. Paul, W., and Raether, M. (1955). 2. Phys. 140, 262. Perovic, B. (1957). Proc. Int. Con/: lonir. Phenom. Gases, 3rd. 1957 p. 813. Persson, R. (1951). Ark. Fys. 3, 455. Picraux, S . T., Davies, J. A., Eriksson, L., Johansson. N. G . E., and Mayer, J. W. (1969). Phys. Rev. 1%0, 873. Pierce, J. R. (1949). “Theory and Design of Electron Beams.” Van Nostrand-Reinhold, Princeton, New Jersey. Rautenbach, W. L. (1960). Nucl. Instrum. & Methods 9,199. Rautenbach, W. L. (1961). Nucl. Instrum. & Methods 12, 196. Rautenbach, W. L., and Lubringe, K. (1965). Nucl. Instrum. & Methods 38, 60. Ravn, H. L., Sundell, S., and Westgaard, L. (1973). Proc. I n t . Elecrromagn. Isotope Sep. Conf., 8th, 1973 p. 432. Reynolds, F. L., Karraker, D. G., and Templeton, D. H. (1949). Phys. Reo. 75, 313. Rol, P. K., Fluit, J. M., Viehbock, F. P., and de Jong, M. (1960a). Proc. Int. Con/: loniz. Phenom. Gases, 4th, 1959 p. 257. Rol, P. K., Fluit, J. M., and Kistemaker, J. (1960b). Physica (Utrecht) 26, 1009.

LABORATORY ISOTOPE SEPARATORS

175

Rook, H. L.. la Fleur. P. D., and Suddueth. J. E. (1974). Nucl. Insrrum. & Merhods 116, 579. Rosenblum, E. S. (1950). Re[’. Sci. Insfrum. 21, 586. Rudstam, G. (1957). Ph.D. Thesis. University of Uppsala. Rudstam, G . (1965). Nucl. Insfrum. & Merhods 38. 282. Rudstam, G., Sundell, S., and Anderson, G. (1964). Nucl. Instrum. & Methods 28, 255. Saclay (1969). “Proceedings of the First International Conference o n Ion Sources.” I.N.S.T.N., Saclay, France. Santry, D. C., and Sitter, C. W. (1970). Proc. Inr. Conf: Electromagn. Isotope Sep. Tech. Appl., 1970 p. 505. Saris, F. W. (1971). Physica (Utrecht) 52, 290. Saris, F. W., and Onderdelinden, D. (1970). Physica (Utrechr) 49, 441. Saris, F. W.,Vander Weg, W. F., Tawara, H., and Laubert, R. (1972). PhJJS.Reu. Leu. 28, 717. Sarrouy, J. L. (1969). “Proceedings of the First International Conference on Ion Sources,” p- 341. 1.N.S.T.N.-Saclay, France. Sarrouy, I . L., and Klapisch, R. (1961). Proc. lnr. Symp. Elecrromagn. Sep. Radioact. Isoiop., 1960 p. 184. Sarrouy, J. L., Camplan, J., Dionisio, J. S.. Fournet-Fayas, I., Levy. G., and Obert, J. (1965). Nucl. Instrum. & Methods 38. 29. Schmidt-Ott. W.-D.. Wolf, D., and Mlekodaj, R. L. (1973). Proc. Int. Elecrromagn. Isorop. Sep. Conf:, 8th, 1973 p. 445. Septier, A. (1967). ”Focussing of Charged Particles,’’ Vol. 2. Academic Press, New York. Septier. A., Leal, H., and Gautherin, G. (1964). Nucl. Insrrum. & Methods 29, 257. Septier, A., Prangere. F., and Ismail, H. (1966). Nucl. Insrrum. & Merhods 38,41. Shimzu. K.. Kawaktsu. K., and Kanaya. K. (1973). Nucl. Instrum. & Methods 111, 525. Shockley. W. (1954). US. Patent 2,787,564. Sidenius, G . (1965). Nucl. Instrum. & Merhods 38, 19. Sidenius, G. (1969).“ Proceedings of the First International Conference on Ion Sources,” p. 401. I.N.S.T.N., Saclay, France. Sidenius. G . (1970). Proc. Inr. Cot$ Electromagn. Isotop. Sep., 1970 p. 423. Sidenius. G., and Skilbreid. 0. (1961). Proc. Int. Symp. Sep. Radioact. Isorop., 1960 p. 243. Sidenius, G.. Gammon. R. M., Naumann. R. A,, and Thomas, T. D. (1965). Nucl. Instrum. & Methods 38, 299. Sluyters, T. J. M . (1959a). Physica (Utrechr) 25, 1376. Sluyters, T. J. M. (1959b). Physica (Urrechr) 25. 1389. Smith, M . L.. ed. (1956). “Electromagnetically Enriched Isotopes and Mass Spectrometry.” Butterworths, London. Smith, M. L. (1957). Prog. Nucl. Phys. 6 . Smyth, W . R.. Rambaugh, L. H.,and West. S. S. (1934). Phys. Rev. 45. 724. Snider, D. F., Wagner, H., Jungclas, H., Wollnik, H., Mazumder, A. K., and Wilhelm, H. G . (1973). Proc. l n r . Electromagn. Isorope Sep. Conf., 8rh. 1973 p. 490. Stephen, J. (1972). RuJio Elecrron. Eng. 42, 265. Stephen, J . (1973). In ”Ion Implantation” (S. Amelinckx. R. Gevers. and J. Nihoul. eds.), p. 476. North-Holland Publ., Amsterdam. Stephens. W. E. (1934). P h j s . Rev. 45, 513. Sternheimer. R. M. (1952). R e [ . Sci. Insrrum. 23, 629. Svartholm. N. (1950). Ark. Fys. 2, No. 14, 115. Svartholm. N.. and Siegbahn. K. (1946). Ark. Mar., Astron. Fys. 24, Talbert. W. L., Jr. (1970). Proc. Inr. Conf: E M I S Tech. Appl., I970 p. 14. Talbert. W . L., Jr.. McConnell, J. R., Halbig. J . K.. and Sleege, G. A. (1973). Proc. Inr. Electromagn. Isorope Sep. Con$ 8rh, 1973 p. 397.

176

S. B. KARMOHAPATRO

Tarantin, N. J., Demyanov, A. V., Dyachikhin, Yu. A., and Kabachenko, A. P. (1965). Nucl. lnstrum. & Methods 38, 103. Thompson, M. W. (1970). Proc. Eur. Conf. l o n Implant., 1970 p. 109. Thousand Oaks. (1970). “Proceedings of the First International Conference on Ion Implantation in Semiconductors.” Gordon & Breach, New York. Thulin, S. (1955). Ark. Fys. 9, 107. Timpl, F. (1965). Nucl. lnstrum. & Methods 38, 113. Torgerson, D. F., Couch, R. A,, and Macfarlane, R. D. (1968). Phys. Rev. 174, 1494. Tortschanoff, T., and Viehbock, F. P. (1970). Proc. l n t . Con$ Electrornagn. lsotop. Sep. Tech. Appl., I970 p. 482. Trahin, M. (1965). Nucl. lnstrum. & Methods 38, 121. Treytl, W., and Valli, K. (1967). Nucl. Phys. A 97, 405. Uhler, J. (1961). Proc. l n t . Con5 Electrornagn. Sep. Radioact. lsotop., 1960 p. 258. Uhler, J. (1963). Ark. Fys. 24, 349. Uhler, J., and Alvager, T. (1958). Ark. Fys. 14, 473. Uhler, J., and Ross6 J. (1963). Ark. Fys. 24, 369. Van Eck, J., and Kistemaker, J. (1960). Physica (Utrecht) 26, 629. Van Eck, J. de Heer, F. J., and Kistemaker, J. (1962). Proc. Int. Conf. loniz. Phenom. Gases, Sth, 1961 p. 54. Venezia, A., and Amiel, S. (1970). Proc. l n t . Conf. E M I S Tech. Appl., 1970 p. 467. Viehbock, F. P. (1961). Proc. l n t . Symp. Electromagn. Sep. Radioact. lsotop., 1960 p. 91. Vienna (1972). “Proceedings of the Second International Conference on Ion Sources” (F. Viehbock, H. Winter, and M. Bruck, eds.), IClS 2. von Ardenne, M. (1956). “Tabellen fur Elektronen, Ionenphysik und ubermikroskopie.” VEB Dtsch. Verlag Wiss., Berlin. von Ardenne, M. (1960). Kernenergie 3, 1177. von Ardenne, M. (1961). Exp. Tech. Phys. 9, 227. von Ardenne, M. (1962). Kernenergie 5, 305. von Busch, F., and Paul, W. (1961). Z. Phy?. 164, 581 and 588. von Busch, F., Paul, W., Reinhard, H. P., and Zahn, U. V. (1961).Proc. lnt. Symp. Electromagn. Sep. Radioact. lsotop., 1960 p. 113. Wagner, H., and Watcher, W. (1961). Proc. l n t . Symp.Electromagn. Sep. Radioact. lsotop., 1960 p. 109. Wagner, H., and Walcher, W., eds. (1970). “Proceedings of the International Conference on Electromagnetic Isotope Separators and the Techniques of Their Applications,” Rep. K 70-28. ZAED. Wahlin, L. (1964). Nucl. Instrum. & Methods 27, 55. Wahlin, L. (1965). Nucl. lnstrum. & Methods 38, 133. Watcher, W. (1937). Phys. Z. 38, 961. Watcher, W. (1938). 2.Phys. 108, 376. Walcher, W. (1958). In “Electromagnetic Isotope Separators” (J. Koch, ed.), p. 230. NorthHolland Publ., Amsterdam. Washington (1970). “Heavy Ion Sources,” Publ. WASH-1 159. US. At. Energy Comm., Washington, D.C. Wehner, G. K. (1955). Phys. Rev. 102, 690. Wien, K., Fares, Y., and Macfarlane, R. D. (1972). Nucl. lnstrum. & Methods 103, 181. Wilhelm, H. G.,Jungclas, H., Wollnik, H., Snider, D. F., Brandt, R., and Robig, G. (1973). Proc. l n t . Electromagn. Isotope Sep. Conf., 8th. 1973 p. 481. Wilson, R. G. (1967). In “Applications of Ion Beams to Semiconductor Technology” (P. Glotin, ed.), p. 105. C.E.N., Grenoble, France. Wilson Whitehead, T., and White, F. A. (1972). Nucl. lnstrum. Methods 103, 437.

LABORATORY ISOTOPE SEPARATORS

177

Winter. H., and Wolf, B. H. (1974). P/asma Phys. 16, 791. Wolf. G. K., Fritsch, T., and Dreyer, J. (1973). Proc. I n t . Electromagn. Isotope Sep. Conf., 8th. 1973 p. 456. Wollnik, H., Brandt, R., Ewald, H., Jungclas, H., Kornahl, G., Snider, D., Wagner, H., Walcher, W., and Wilhelm, H. G. (1973).Proc. Int. Electromagn. Isotope Sep. Conf, 8th, 1973, p. 477. Yamaguchi, S. (1941). Proc. Phys.-Math. SOC.Jpn., 23, 264. Yates, E. L. (1938). Proc. R . Soc. London, Ser. A 168, 148. Zilverschoon, C. J. (1954). Thesis, Amsterdam.

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Light-Emitting Devices, Part I: Methods HERBERT F. MATARE ISSEC International Solid State Electronics Consultants, Los Angeles, CaliJbrnia

1. Introduction ................................................................................. 179 2. Radiative and Nonradiative Recombination ......... ..... 185 189 3. Radiative Recombination and the Injection Process ...................................... 195 References for Sections 1-3 ................................................................. 4. Materials for Light Emitters ................................................................ 195 A. 111-V Compounds ....................................................................... 198 B. Direct and Indirect Semiconductors ................................................... 207 C. Ternary Compounds .................................................................... 214 References for Section 4 ................................ 5 . The Heterojunction .................................... A. Injection Efficiency and Confinement .................................................. 222 B. Lattice Match ............................................................................ 229 C. Defects and Radiative Efficiency ....................................................... 234 D. Efficiency of Radiative Recombination and Balance between Stimulated and Spontaneous Emiss' References for Section Appendix . . . . . . . . . . . . . . References for Append 6 . Methods of Junction Formation ........................................................... 246 A. Diffusion ................................................................................. 247 B. Chemical Vapor Deposition .............................................. ... 254 C. Liquid Phase Epitaxy ..................................................... ... 262 D. Molecular Beam Epitaxy ........................................... ... 275 278 References for Section 6 .....................................................................

1. INTRODUCTION Progress in the field of electrooptics and especially in the area of materials for electrooptical use has been so decisive recently that it can be foreseen now that a new industry is being created. Especially, the technology of communications will be enriched as new and needed frequency domains open up. Due to the extension of the useful carrier bands into the near-and farinfrared parts of the spectrum, a wealth of new technical possibilities appears. Where before the microwave generator was the most important active device for wide-band communications, a new dimension in power generation is available at a frequency range well beyond the millimeter-wave I79

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HERBERT F.

MATARB

region. One generally assumes that the limit of 0.1-mm wavelength defines the transition from microwaves to the infrared. This corresponds to a frequency of 3000 GHz or 3 x 10’’ Hz. Expressed in micrometers in the infrared scale, we begin here at a lower limit of 100 pm and dispose of a band up to 6 x lOI4 Hz or a wavelength I, z 0.5 pm (see Fig. 1.1). Energywise, the microwave band extends from

FIG.1 . 1 . Frequency (wavelength) chart showing the microwave band and the optical band with associated energy (eV) values.

’

10- eV (1000-cm wavelength) to 3000 GHz. The iTfrared band covers the range from 100 pm to the visible band below 1-pm wavelength or the frequency range from 3 x lo’* to around 1OI5 Hz. We infer from this enormous frequency extension that we can add a considerable amount of useful bandwidth to the usual scale of useful microwave power. It has to be added here that the useful microwave range does not cover the full range to the lo-’ eV point since the millimeterwavelength range lacks power generators and practical detectors. There is little technical coverage between the points from below 1-cm to 100-pm wavelengths. The development of cheap and efficient sources for the infrared range down to the visible spectrum is a welcome addition to the spectrum for information transfer and communications in general. As the sources and

18 1

LIGHT-EMITTING DEVICES

detectors for this range developed, so did our knowledge of transmission through the atmosphere and through optical fibers. The complex absorption in the atmosphere shows several useful "windows " for certain frequency ranges, especially in the 800-900-nm band and beyond 1 pn; and OHgroup absorption in fibers has been measured and decreased, as have scattering losses in modern fibers (Fig. 1.2). Figure 1.3 shows the typical loss for an industrial fiber (Nippon Electric Company, Selfoe-Cable SLC 100) in the spectral area of interest. The minimum absorption is at 850 nm or 0.85 pm. 100 %

80

60

40

20

n..i

I

2

I

3

s

lOOpm

10

Bands FIG. 1.2. Spectral characteristics of optical waves between 0.1 and 100-pm wavelength. Transmission in percent per km path length along earth's surface at percipitable water vapor. From N . S. Kopeika and J. Bordogna, "Background Noise in Optical Communications Systems." Proc. I E E E , October 1970. p. 1573. -Absorption

150-

dB/km

05

0'6

017

$8

d9

1'0

1'1

1'2

t,'3

FIG I .3. Losses in dBJkm dBikm cof "Selfoc" fiber over optical wavelength in pmI (relative absorption and scattering losses).

182

HERBERT F. MATARE

While we have to take into account losses near 30 dB/km for these cables, the theoretical lower limit is at 4 dB/km (1.2).In Fig. 1.2, the limiting measured absorptive and scattering attenuations are also plotted as a function of wavelength. The sum of these values represents the total fiber loss. For doped fused silica core with fused silica cladding, the absorption curve coincides well with the calculated losses. Between 800 and 850 nm, the total attenuation is therefore so low that infrared data transmission over kilometers of fiber can be achieved without a repeater. As the technology of glass purification and pulling of fibers improves, prices will be reduced and an essential part of infrared transmission hardware will become available. Recent progress (Corning) has resulted in the availability of fiber wave guides of 10-km length and only 5.4-dB/km damping at around 800-nm wavelength with a pulse broadening of 1 nsec/km! On the other hand, progress in high speed detectors for the infrared range, based on 111-V and ternary compound heterojunctions, is so decisive (1 .2) that only the light-emitter properties seem to limit present infrared communication techniques. The present survey of the field of light-emitting diodes (LEDs) is intended to assess the technological status in this important area as of the date of this writing. While the first work on light emission dates back to before World War I (1.3) the really important contributions were made on the basis of 111-V compounds described first in 1952 by Welker (2.4). These compounds were sufficiently developed in the 1960s (1.5) and most of the decisive progress has occurred since then. Since some major 111-V compounds (GaAs, GaSb, InAs, InP, InSb) and also some ternary compounds, like GaAsxP,-x (x 2 0.60) and Ga,Al,_,As (x 2 0.55)), are direct semiconductors, their usefulness in applications based on radiative recombination became clear after their properties had been studied. The classical semiconductors (germanium and silicon) are indirect semiconductors and, therefore, unsuitable as LED materials. This difference is a fundamental one and involves the behavior of the electronic wave functions or Bloch waves in the lattice. As the junction operates to inject electrons from the n-type side into the p side of the crystal, how their energy is spent is critical, either generating phonons (lattice vibrations) or generating photons in direct recombination with defect electrons. These two alternatives are decided upon by the shape of the energy versus wave vector relation in momentum space. If the injected electrons (respectively, holes) can reach each other across the k = 0 axis in the energy versus momentum diagram, no wave vector adjustment is necessary and no phonon energy is involved. We shall see later that similar conditions can prevail in 111-V “ indirect” semiconductors when isoelectronic trap levels are involved. The property of direct transition, however, is very important for high efficiency radiative

183

LIGHT-EMITTING DEVICES

Ge

Ga As

FIG. 1.4. Simplified band structure of silicon, germanium, and gallium arsenide with minimum gap E , .

recombination and is intimately connected with the type of lattice and the resulting band form along the Brillouin zones (see Fig. 1.4). Here we have plotted the simplified energy diagrams for germanium, silicon, and gallium ar senide. The maximum of the valence band (with the split-off light mass band) is located at k = (000) at the center, and it is from this point that the energy gap is measured. In silicon and germanium, EG is lowest for conductionband minima outside k = (OOO), while for GaAs the k = (O00) wave vector point is also the nearest point to the conduction band minimum. The most efficient light emission occurs in those cases where the stoichiometry allows for direct transitions without phonon adjustment via isoelectronic trap levels (Fig. 1.5a).This is the case for most infrared emitters in the range 850-1 100 nm. Figure 1.5b shows the indirect case, and Fig. 1 . 5 the ~ quasi-direct (isoelectronic impurity) case. The importance of these emitters for infrared communications will be dealt with later in this chapter. While this development is going on, the display sector has already created a wide market (worldwide about $300 million in 1974) for the basic red, yellow, and green LEDs. There is a general trend to replace all mechanical indicators by LED displays. This refers not only to the digital read-out in electrical instruments

184

HERBERT F. MATARE

‘t

k

k0

(0)

FIG. 1.5. (a) Schematic of direct-gap semiconductor. (b) Schematic of indirect-gap semiconductor with intermediate transition process from E,(k,) to E,(k,). (c) Transition via isoelectronic trap level.

and watches, but also to those cases where one wants to preserve a geometrical input to the viewer. This means that arrow-type or handle indicators are replaced by a series of illuminated lines or points, and the level or reading is indicated by the last illuminated point or line in a row, thereby improving in a remarkable fashion the clarity and directness of the read-out.

LIGHT-EMITTING DEVICES

185

2. RADIATIVEAND NONRADIATIVE RECOMBINATION

There are a number of processes in p-n junctions resulting in radiative recombination or photon emission. Prominently known since the early days of detailed work on silicon and germanium junctions is radiation by internal field emission. Visible recombination light and deexcitation radiation are emitted by silicon p-n junctions operated in the avalanche (reverse) mode where high fields prevail within the widened depletion layer. Since the emitted photon energy hv is not related to the band gap in this case, the higher energy is due to the field emission process itself. This became clear when a detailed observation of light-emitting areas showed correlation with edges and dislocations, since these areas produce localized high field regions (2.1, 2.2). Since this radiative recombination occurs in indirect semiconductors, and at higher than band gap energies, it is obvious that the k = 0 valleys (see Fig. 1.4) are not directly involved. A much more efficient way to produce radiation is via the minimum gap at the r symmetry point in the Brillouin zone (2.3) because less energy is needed to build up the carrier density required. A condition is also that no phonon energy ( k vector variation or thermalization) is involved in the recombination process since the occupied states allow direct transitions according to the k selection rule. After the importance of this fact had been recognized (2.4, the development of a semiconductor laser was only a question of time. There are, however, a number of possibilities for electrons and holes to recombine in direct semiconductors. Figure 2.1 gives a schematic view of four fundamental transitions. There are (1) band to band transitions, (2) donor to valence band recombinations, (3) conduction band to acceptor level recombinations, and (4) donor to acceptor transitions.

FIG.2.1. Possible recombination paths: ( I ) band to band, ( 2 ) donor to valence band, (3) conduction band to acceptor, (4) donor to acceptor.

186

HERBERT F.

MATARB

In case (I), a minority carrier recombines either directly with a free majority carrier or after an intermediate exciton state has formed. While all these processes are radiative at various optical frequencies v, depending on the energy levels involved, the efficiencies may vary according to the types of levels, their capture cross section, density, and relaxation time. If carrier density variations An and Ap are induced, the photon emission rate will increase in proportion to the product

(n + W

( P + AP)

where n, p are the initial carrier densities of electrons and holes, respectively. The total recombination rate per volume is

=lorv dv u)

R,

= Ro(n

+ A4(P + A P h P

(2.2)

Ro is the equilibrium recombination rate and is given by (see Ref. 2.5) Ro =

k? dv S, 8n exp(hv/kT)' ' Kth

O0

where k, = n,v/c is the wave vector for the radiation; n, is the refractive index; v the frequency; c, the speed of light; k, the Boltzmann constant; T, the absolute temperature; and K t h , the absorption coefficient for interband transitions in the absence of carrier injection. Kth is a function of the absorption-edge parameter A : K,h =

A(hv - E G ) ' l 2

(2.4)

where A

= 2 ~ e ~ ( 2 r n (pmo , ) ~ /12/3rnGn~o~h3~ ~

(2.5)

Here e is the electron charge; m,, the reduced mass; I P , , , ~ ) ~ , the square of the momentum matrix element; rn,, the free electron mass; n, the refractive index; E , , the dielectric constant; h, Planck's constant; and EG is the band separation in electron energy. A varies only slowly with the photon energy since (P,,,,,(~is only a slow function of k in the vicinity of the band gap. pmocan be represented by the band edge value for the minimum band separation k, :

p,,(ko)

5

= ih ur(r, ko)Vu,(r, ko) dr

(2.6)

LIGHT-EMITTING DEVICES

187

where u, and u, are the amplitude functions in the Bloch wave functions with the corresponding lattice periodicity (u* is the conjugate complex of u). This integral (2.6) vanishes unless the transitions are vertical in k space:

k,

= k,

( = k)

(2.7)

A general expression for the probability that a transition has occurred after a time t is derived from first order perturbation theory (2.5) and yields an expression similar to the Fraunhofer diffraction function :

In the presence of radiation w = 2nv, the transition probability P,,(t) from a filled state 0 to an empty state m is proportional to the intensity I(v) of the radiation and to 1 pmo1.’ The diffraction term shows the usual sharp amplitude rise for w = om,. However, function (2.8) has to be integrated over all pairs of energy states in the crystal having energy separations close to homo.The transition probability then becomes

Here p(o,,)h A(om0gives the number of energy level pairs with separations h ~ , , to h(wmo Ammo). For indirect transitions (see Fig. 1.5b), a change in wave vector

+

q p

=km -

k,

(2.11)

is involved. The whole transition process can be envisioned via an intermediate state E , ( k , ) in the conduction band, and the expression for Pm,(t) has a form similar to (2.8) where the product of two perturbation matrix elements H i , I2 and H m g1’ replaces 1 H,, 1’ and the frequency difference shows the additional phonon frequency +up:

I

I

Hi0 is an optical matrix element; Hmi is the matrix element for electronphonon interaction; wi, is the transition frequency of the intermediate to the ground state; and hwiais the width of the forbidden gap at the valence band maximum (Fig. 1.5b).

188

HERBERT F. MATARE

The sign for a,, (phonon frequency) refers to two possibilities:

- wp , phonon absorption;

+up, phonon emission

Again (2.12) has to be integrated over all contributing conduction-band states. We can see that in principle radiative transitions are possible in indirect semiconductors when correct phonon-assisted transition processes are involved, allowing the sequence of events shown in Fig. 1.5b. A transition between the initial state Eo(ko) and the final conduction band state Em(km) takes place via the intermediate states Ei(k,) and EI(kk). There are wellknown cases where such conditions can be met due to a particular impurity complex in indirect semiconductors like Gap. In this semiconductor, which has primary importance for visible light emitters (LEDs), there are known several complexes that induce impurity level emission via excitonic states that have high quantum efficiency. Some cases are depicted in Fig. 2.2. We can see here that independently of the fact that G a P is an indirect semiconductor, efficient radiative recombination can occur. This is due to the fact that in this case radiative lifetime for decay via excitonic transitions is shorter than the process of reexcitation into the conduction band. The famous oxide complexes form such exciton levels in 111-V lattices. In Gap, e.g., oxygen is a deep donor (0.89 eV) and zinc a shallow acceptor (0.06 eV); but when these atoms can be present in adjacent lattice sites (in p-type material), they form a neutral complex that traps electrons (0.3-eV level), Such efficient traps with short relaxation times change completely the properties of the semiconductor. Electrons trapped here recombine with lowlying acceptor levels without encountering nonradiative centers (2.5).

FIG. 2.2. Energy level diagram for recombination and emission processes involving impurities in Gap. E , = binding energy of a hole to a negatively charged Zn-0 complex. hv,, is the energy of a bound exciton (radiative).

LIGHT-EMITTING DEVICES

189

Similarly, one can replace one of the constituents of the basic lattice of a 111-V compound by an element from the same column of the periodic table, e.g., phosphorous by nitrogen or bismuth in G a P or arsenic by cobalt in AIAs; or in a 11-VI compound like ZnTe, one can replace Te by oxygen. In these cases, neutral localized states form, which act as traps for free charge carriers (2.5). Such trap levels in compounds are very active in radiative recombination since excitons bound to these traps decay with intense light emission (such atomic arrangements do not seem feasible in elemental semiconductors). Especially interesting is G a P with a wide forbidden gap (2.8 eV) since such “ isoelectronic impurities induce radiative recombination within the visible spectrum (green-yellow emission) with a relatively high quantum efficiency. The reason for this is the particular form of the wave function of the trapped electron, which seems to equal conduction band Bloch wave functions for the central region of the Brillouin zone. Electron-hole recombination can thus take place vertically in k space as is the case in a direct semiconductor (see Fig. 1.5~).We can see that in the case of such indirect semiconductors, e.g., Gap, isoelectronic traps create a situation that might be called quasi-direct. In the case of nitrogen in Gap, a further advantage is the high solubility of N in the G a P lattice (up to 1019 cm-3) while the Zn-0 complex density in G a P is limited to values in the 5 x 10’7-1018-cm-3 range. In the latter case, Auger processes set in for higher Z n - 0 densities, transferring the energy to free electrons instead of generating phonons. There are numerous Auger recombination processes involving impurity levels and bands. In n-type material, the free electron is ejected into higher conduction band levels or into vacuum. Other recombination mechanisms that are nonradiative are defect-induced recombinations at dislocations, point defects in low angle lineage boundaries, and grain boundaries. In all these cases, equivalent phonon energy can be generated by multiphonon processes, making it unnecessary to find just one phonon of the correct energy, an unlikely, even impossible event if the electron energy is in excess of the Debye energy (kO). ”

3. RADIATIVERECOMBINATION AND

THE

INJECTION PROCESS

Excitation of electrons from the valence band to the conduction band is the basis for radiative recombination. In general, scattering out of this higher energy state occurs due to collisions with phonons, impurities, defects, and other imperfections in the lattice. If the electron returns directly to the valence band state, “recombination radiation occurs. This process generally requires a longer lifetime for the excited carrier than nonradiative ”

190

HERBERT F. MATARB

collision processes. Therefore, the amount of carriers available for radiative processes is much smaller than for nonradiative recombination since natural crystals are highly imperfect. External excitation with radiative sources thus needs strong radiation sources to provide sufficient photons per second to observe emitted radiation, the radiative lifetime of carriers ranging from microseconds to at most milliseconds. A more efficient way to generate a sufficient shift of electrons into the conduction band is by means of p - n junctions. The high electric field strength in depletion layers will impart sufficient energy to create free carriers. The maximum energy of these “hot” carriers is the pair production threshold, above which they rapidly lose energy by producing new electron-hole pairs. Such energetic carriers will fall back to the band edge under emission of radiation. Generally, a broad emission band results. This is the case for reverse biased junctions, and such light emission has been observed in indirect semiconductors as germanium and silicon. Internal field emission generally occurs preferentially at dislocations where midband levels are available and high field regions increase the carrier energy (2.1). Other less efficient processes are tunneling and impact ionization combined with multiplication phenomena. In the case of direct or quasidirect semiconductors, a p-n junction biased in the forward direction is a means of injecting carriers of sufficient energy to force radiative recombination across the gap or in cooperation with suitable donor-acceptor states. The relatively small diffusion voltage in the forward direction is sufficient to impart the necessary energy when a number of conditions are fulfilled. Among these are high population density, low defect density (or sufficiently long radiative lifetime), and abrupt p-n change. Since the junction is forward biased with a voltage V , (Fig. 3.1) and e t N EG, the population is inverted and electrons are thrown across the depletion region into the p region where they recombine under emission of radiation: hv = EG . The number of quanta emitted is proportional to the product of the number of electrons in the conduction bandf, and the number of holes in the valence band 1 -f, .f, andf, are the known Fermi distribution functions

’

1 = 1 + exp(E, - F,)/kT’

=1

+

1 exp(E, - F,)/kT

(3.1)

where El is the upper energy state; F , , the quasi-Fermi level; E 2 ,the lower energy state; and F,, the quasi-Fermi level. For the absorption process, the product isf,(l -f,). The total number of quanta absorbed and emitted is also a function of the transition probability Wand the density p of photons at the energy hv. Thus, Qabs Qernit

= AWJ(1 =

-Mv)

AWf,(l - ~ , ) P ( v )

(3.2) (3.3)

LIGHT-EMITTING DEVICES

191

-

space charge

(b 1 FIG. 3.1. Energy diagram of a p-n junction in highly doped material. (a) Without bias; (b) with forward bias. Shaded area: filled states. F. Fermi level; F ; ; Fi = quasi-Fermi levels; E , conduction band level; E, , valence band level; V, , external voltage.

.

In the case of a laser, stimulated emission occurs for Qemit> Qabs orS, >f,. From this condition and Eqs. (3.1t(3.3), one derives the condition F, - F, > El - E,

hv

(3.4) a condition first derived by Bernard and Duraffourg (3.1, 3.2). There are numerous possibilities of carrier injection in p - n junctions depending on doping ranges, carrier densities, field strength, band gap, etc.

192

HERBERT F. MATARE

As we have discussed before, isoelectronic traps can induce strong light emission in an indirect 111-V semiconductor like Gap. Such transitions between a band and an impurity level can involve shallow or deep levels. We have seen (Fig. 2.1) that donor-acceptor transitions also take place. Such recombination processes may have longer time constants than is usual for the lifetime of injected excess electrons (10- sec). An example is silicon as an amphoteric impurity in GaAs. We shall come back to this important case later. Other possibilities are band-to-band tunneling, which is significant in degenerately doped junctions. Here the junction is very narrow so that the wave functions of the conduction band electrons overlap those of the holes in the valence band. Another possibility is photon emission plus tunneling. In this case, the emitted frequency is smaller than eK ( V , is the external bias voltage) for the sum of the separation of the quasi-Fermi levels from their respective band edges. For further detail on these injection processes, see Pankove (3.3). As we have already considered, for efficient injection, doping ranges have to be high. In almost all cases at least one side of the junction is degenerate or close to degenerate. The Bernard-Duraffourg condition (3.4) states that the difference of the quasi-Fermi levels should exceed hv. For band-to-band transitions, this means that at least one of the bands is degenerate. The doping gradient is also important with respect to the configuration of the active region of the p - n junction. Diffusion generally results in a linearly graded junction, while alloyed and epitaxially deposited junctions are mostly abrupt. In the first case, the gradient is in the range of 10” ~ m - ~ / l Ocm - ~N 1021 cmW4with the depletion layer cm wide, while for abrupt junctions much higher values result especially for near or fully degenerate doping: 10’’ ~ m - ~ / l O -cm ’ = cm-4 (depletion layer lo-’ cm wide). The active region is the area where the injected carriers recombine. In most cases, it is the less doped side of the junction where the minority carrier from the other side reaches the end of the depletion layer and finds a sufficient number of majority carriers for radiative recombination. We have marked the transition region in Fig. 3.1. Its width can be estimated from the carrier diffusion length

L, = (DnT,)”2 (3.5) D, is the diffusion constant and T, the carrier lifetime (steady state). z, is inversely proportional to the average hole concentration p: z,

B is the recombination constant.

2:

1pp

(3.6)

LIGHT-EMITTING DEVICES

193

The average hole concentration p now is proportional to the impurity gradient G at the junction and the diffusion length L,:

p = GL,,

(3.7)

With (3.5)-(3.7), one obtains L, ‘v (D,/GB)’’3 (3.8) ~ m - while ~ , For the abrupt junction in GaAs, we calculated G = B rr lo-’’ cm3/sec (Ref. 3.2) and D, = 15 cm2/sec (electrons). Therefore, L,, is, according to (3.8), cm = 1 pm L,, N There are two possibilities for carrier transport and recombination across a junction. In one case the carriers are swept to the other junction side across the transition region and recombine there. In the other case, recombination occurs within the space charge region itself. The latter case is the preferred mode of operation for high efficiency because the depletion region is swept free of mobile carriers and offers less chance for absorption and nonradiative recombination. Once the injected electrons have drifted through this region and have reached the p-type bulk zone, their recombination is more likely nonradiative because of the higher number of centers (defects) offered there. It was, therefore, a fundamental step made by Kroemer (3.4) when he proposed the application of heterojunctions that would cause potential barriers and confine the injected carriers to the space charge region. (We shall discuss this in detail in Section 5.) This scheme has become a cornerstone for LEDs as well as lasers with respect to increased quantum efficiency and lowered laser threshold. The heavily doped semiconductor has a property that is important for efficient injection of carriers, referred to as “band tailing.” What is meant by this is shown in Fig. 3.2. Here the energy distribution of the densities of states p ( E ) is plotted. The normally parabolic form of the density function (energy versus state density) is drawn out from both conduction and valence bands and has tails narrowing the gap. The cause for this effect lies in the randomization of the band edges due to localized impurity states. In normally degenerate semiconductors, one expects an overlap of the density function over the Fermi level or states available at energies below EF (3.5)forming impurity bands. At higher impurity densities, band perturbations, however, result in conduction band states at relatively low potentials and valence band states at high potential regions. Pankove (3.3)found that these tails extend exponentially into the forbidden gap P ( J 9 ‘v exp(EIE0)

where E , increases with the doping level and is typically lo-’ eV.

194

HERBERT F. MATARB

I

I

I I

I

___)

d(El

diEi

n - region

p - region

dW

EC

--

5e

E"

n

(b) FIG.3.2 (a) Distribution of energy states u(E) in the vicinity of a p-n junction in thermal equilibrium. (b) Radiative recombination and injection involving conduction band tail (1) horizontal, (2) diagonal tunneling.

LIGHT-EMITTING DEVICES

195

The application of a forward bias to the junction shown in Fig. 3.2 will result in injection of electrons from the n region by “horizontal tunneling” (1) into the conduction band tail from where radiative recombination occurs (Fig. 3.2b). Diagonal recombination (2) in Fig. 3.2b will also occur when the tail levels are progressively filled as the current increases (shift to higher photon energies).

REFERENCESFOR SECTIONS 1-3 1 . 1 . D. B. Keck. R. D. Maurer, and P. C. Schultz, Appl. Phys. Lett. 22, 307-309 (1973). 1.2. See, e.g., R. C. Eden, 1.06 Micrometer Photodetector,” North American Rockwell Science Center Report (NASA-Contract No. NASS-23 134). North Am. Rockwell Sci. Cent., Thousand Oaks, Calif., 1972. 1.3. C. H. Gooch, “Injection Electroluminescent Devices.” Wiley, New York, 1973. 1.4. H. Welker, Z. Narurforsch., Teil A 7, 744 (1952); 8, 248 (1953); German Patent 970,420 (1958); H. Welker and H. Weiss, Solid State Phys. 3, 1-78 (1956). 1.5. 0. Madelung, “Physics of 111-V Compounds.” Wiley, New York, 1964. 2.1. A. G. Chynoweth, Prog. Semicond. 4, 95-123 (1960). 2.2. H. F. Matare, Defect Electronics in Semiconductors,” p. 234. Wiley (Interscience). New York, 1971. 2.3. See, e.g.. W. A. Harrison: “Solid State Theory,”p. 90etc. McGraw-Hill, New York, 1970. 2.4. For a historic survey, see, e.g., M. Gershenzon, Srmicond. Semimetals 2, 289-369, (1966). 2.5. See, e.g., T. S. Moss, G. L. Burrell, and B. Ellis, “Semiconductor Opto-Electronics.” Butterworth, London, 1973. 3.1. See, e.g., M. G. A. Bernard and B. Duraffourg, Phja. Status. Solidi 1, 699 (1961). 3.2. See, e.g., M. J. A d a m and P. T. Landsberg, In “Gallium Arsenide Lasers” (C. H. Gooch, ed.), pp. 5-79. Wiley (Interscience), New York, 1969. 3.3. J. I. Pankove, In “Solid State Physical Electronics Series” (N. Holonyak, Jr., ed.), Elec. Eng. Ser., pp. 174ff. Prentice-Hall, Englewood Cliffs, New Jersey, 1971. 3.4. H. Kroemer, Proc. IEEE 51, 1782-1783 (1963). 3.5. See, e.g., E. Spenke: “Electronic Semiconductors,” 2nd ed., pp. 416fT. Springer-Verlag, Berlin and New York, 1965. I‘

I‘

4. MATERIALS FOR LIGHTEMITTERS The basis for all modern light emitters is compound semiconductors of the A”’BV type and their ternary varieties: AILIBVCV or A:~B:”.C~ which were first discovered and described by Welker ( 1 . 4 ) ; see the table in Fig. 4.1 with atomic spacings indicated. The original idea of designing new semiconductors was prompted by the recognition of the limitations of basic materials constants of the then prevailing elemental semiconductors, germanium and silicon. Above all, the mobility of charge carriers is limited to values of up to 3600 cm2/V sec in germanium and 1400 cm2/v sec in silicon, while certain 111-V compounds

196

HERBERT F. MATARE Period

Peril

I 'i

I

I

I

I

Wurtzite Structure

FIG.4.1. Interatomic spacings for basic 111-V compounds.

like InSb produce electron mobilities that are easily 10 times higher. This is due to several factors peculiar to the 111-V lattice with tight tetrahedral binding (small thermal vibrations) and the low effective mass of conduction electrons. However, the pioneering work on these compounds by Welker and co-workers coincided with the rapid development of silicon device technology and microminiaturization where earlier limitations in transit time were overcome and very high frequency transistors opened up microwave microcircuitry. It also turned out that it is not only the lattice mobility value of electrons, but the hole-electron mobility product that is decisive for the high frequency response of bipolar devices. And in this respect, most compound semiconductors are inferior to germanium with the exception of InSb (2.2, see p. 45) which, however, is not usable at room temperature on account of its small band gap. In Table 4.1, a number of compound semiconductors are listed together with the elemental semiconductors and their respective band gaps, electron and hole mobilities, lattice constants, as well as the electron-hole mobility products. This product is then considered in relation to germanium (last column). We see that with the exception of InSb, all compounds are inferior to germanium. Materials close to the value p e p , , for germanium are Sn, GaAs, InP, InAs, and HgTe. But we know today that even with silicon, which has only one-tenth the value for the mobility product of germanium, very high frequency devices are feasible by a constructional decrease in transit time and/or correct choice of device type. Therefore, also compounds

197

LIGHT-EMITTING DEVICES

TABLE 4.1

Crystal IV

c Si Ge Sn

E , (eV) at 300°K

Electron mobility (cm2/v sec)

Hole mobility (cm2/v sec)

Lattice constant

(A)

Mobility product peph ( x lo6)

relative to Ge

5.4 1.15 0.65 0.08

1800 1900 3800 2500

1200 480 1800 2400

3.567 5.42 5.646 6.47

2.15 0.9 7.0 6.0

0.3 0.13 1.o 0.85

60

8

4.35

4.8 x 10-4

0.7 x 10-4

1200 400 80 8 500

5.63 6.13 5.44 5.65 6.095 5.869 6.058 6.48

2.4 x lo-' 6 x lo-' 1.36 x 10-3 3.4 2.6 3.2 7.2 70

3.4 x 8.5 x 10-3 0.2 x 0.48 0.37 0.46 1.02 10

pe p h

IV-IV S i c

3.0

111-V AlAs AlSb GaP GaSb InP InAs InSb

2.3 1.52 2.25 1.35 0.69 1.27 0.35 0.17

4600 3 x 104 7 x 104

200 150 17 400 650 700 240 loo0

IV-VI PbS PbSe PbTe

0.37 0.26 0.25

800 1500 1620

I 000 1500 7 50

7.5 6.14 6.45

0.8 2.25 1.22

0.14 0.32 0.17

V-VI Bi,Te,

0.15

1250

515

10.48

0.65

0.09

11-VI ZnSe CdTe HgTe

2.6 1.5 0.2

100 650 22 103

16 45 160

1.6 x 10-4 2.9 x 3.5

0.23 x 1 0 - ~ 0.41 x 0.5

GaAs

4Ooo

5.667 6.48 6.429

like GaAs, InP, AISb, CdTe are useful for speclfic devices; GaAs, e.g., for FETs for U H F (field effect transistors) and CdTe for thin film surface barrier transistors. However, the importance of the compound semiconductors lies more in their particular band structure which, in the case of GaAs, displays differential negative mobility characteristics with applied bulk field, leading to microwave oscillations (Gunn effect) and, in the case ofjunctions, to the possibility of efficient radiative recombination as used in LEDs and laser diodes. In Table 4.1 we find a few important IV-VI, V-VI, and 11-VI compounds. Their device potential is small compared to the 111-V compounds. This is due to some material properties (instability) and also in view of the relatively restricted values of electronic constants, like mobility and their varied lattice constants (lattice mismatch). Due to their extreme miscibility, the Ill-V compounds allow for an extraordinary flexibility with respect to band gap and lattice constant matching-two important reasons for their

198

HERBERT F.

MATARB

use, as we shall see. It is in fact these metallurgical properties coupled with high mobility that have given the 111-V compounds their importance. It was Welker's contribution to have been the first to stress clearly the importance of these compounds. A . Ill-V Compounds The fundamental properties of the binary and ternary compounds of the elements of groups 111and V of the periodic chart are basic for light emitters. Especially the phosphides, arsenides, and antimonides of gallium and indium, which crystallize in the zinc-blende structure, are of greatest interest electronically. The general properties have been described in many review papers (1.4, 1.5, 2.4, 2.5). Therefore, we can limit our discussion to the main points touching on their optoelectronic behavior. In view of the close stoichiometric fit of mixed compounds, it is interesting to compare the interatomic spacings. In the table in Fig. 4.1 we have plotted the interatomic spacings for the most important 111-V compounds. Their values are close (within three decimals) for compounds formed from the same row of the periodic chart. The W a v e l e n g t h X (pm)

P h o t o n e n e r g y (eV)

FIG. 4.2. Absorption coefficient a (cm-') versus wavelength (photon energy in eV) for (1.3-1.6 eV). n-type monocrystalline GaAs, n = 3 x 10I6

199

LIGHT-EMITTING DEVICES

elemental semiconductors Si, Ge, and Sn can be fitted according to their period in the periodic chart. It is of great importance for the miscibility of these compounds that these spacings are so close; see, e.g., AlAs and GaAs or AlSb and GaSb. This has a direct bearing on the perfection of mixed compound crystals with tailored band gap and heterojunctions, which we discuss subsequently. As described before, one outstanding property of the direct-gap materials with zinc-blende structure is the low effective electron mass, which is roughly given by the relation rn*/mo 0.05EG EG is the energy gap in electron volts. The resulting high mobility in combination with the possibility of direct transitions gives these compounds their importance for optoelectronic applications. The close lattice match (Fig. 4.1) is indicative of predominantly homopolar bonds; however, the relatively high difference in high frequency (optical) and low frequency (electrical) dielectric constants of these compounds shows that there is a significant degree of ionic bonding (Welker). Since gallium arsenide is the most important 111-V compound, we include in a number of figures some of the well-known characteristics ( 4 . 1 ) of this crystal type (Figs. 4.2-4.12). This basic 111-V compound has found so

-

Photon energy ( e V ) 40,1.6

,

1.2

0;62

0;41

0;31 0;25

0.

t

30

11 I 0 8 10

2

3

4

5

6

7 8 9

Wavelength X (Fm)

FIG.4.3. Same as Fig. 4.2, for

11 =

4.9 x lo” c m - 3 (1.6-0.12 eV)

100 I

50 40 I

30

2 0 2 0 10

5

4

1

I

I

1

f

1

3 I

-

C

0

u

\

-oz5 1020) ._

u .-

c 0

--"

b

/

0

I

a I

10'

I

1

1

I

l

l

I

FIG.4.4. Hall coefficient as a function of temperature for n-type, single crystal GaAs lot6 c m - 3 doping, respectively. (a,b,c) lo1’,5 10" 8-

6-

4 -

2-

10'

-5

-

8 -

6 -

I

,

I

I

l

l

1

I

I

l

l

I

1

Temperature ( O K 1 FIG.4.5. Electron mobility as a function of temperature for single crystal n-type GaAs. (1,2) n , = 3 x 1 0 ' ~cm-'; (3) n n = 5 x 1 0 ' ~ cm-'; (4) n , = 4 . 5 x l o i 7 cm-'; ( 5 ) n, = 5 x 10" (polycrystalline). 200

7000

I

T

6000 ..-. u

u) 01

’ E

2

-

I

’ ’

I

-

-

-

5000-

-

-

1000-

>r

-

n

-

0

E 0 c

L

3000-

-

-

U

aJ

i;

-

2000-

10’~ ,

I

I 0’7

10’8 I

1

I

1

1

I

I .

10‘6 1

I

I

I

I015

. .

FIG.4.6. Electron mobility as a function of carrier concentration in n-type single crystal GaAs at 300°K. Wavelength X ( p m )

78

OK

300 OK

Photon energy ( e V )

FIG. 4.7. Photoconductive response of n-type GaAs at 3 0 0 and 78°K. p300.y = 2 x 10’ R cm. 20 1

Wavelength h ( p m )

2.1

701

6o 50

t

1.6

1.2

1 .o

0.9

I

I

I

I

C

DIODE EMISSION

FI

tin-

I I[

i

\

20 10

n A W,"

n7 w.,

n W."n

I n V.'o

I

in I."

I

...

1 1

1 1,.* 7

.."

1.7

.--

1.d

1-5 .--

..-

1.6

Photon energy (eV)

FIG. 4.8. Photoluminescence and electroluminescence (diode emission) as functions of wavelength for single crystal n-type GaAs, tellurium-doped.77"K, n = 1.6 x 10'' cm- '. Photon energy ( e V )

0.95

0.88

0.83

I

100

-

-.In c

C 3 0)

> .L

O 0

Y L

.-5 5 0 In c 0)

c

c .C

0 .c 0 .-

U

0

a

Wovelength A ( p m )

FIG.4.9. Photon luminescence as a function of wavelength for p-n junctions in GaAs at three temperatures: 300, 200, 77°K.

20 3

LIGHT-EMITTING DEVICES Wavelength X ( p i n )

0.36

0.33

0.30

0.27

0.26

t

2 Wave number A x 10“ n

2

FIG. 4.10. Reflectivity as a function of wavelength for n-type GaAs at 300°K. 10’’-10’8c m - ’ .

many applications in microwaves and light emitters that it is one of the best known compounds today. In advanced LED processes, it serves mainly as substrate material; but its electronic behavior and impurity level schemes have been the subject of numerous publications. We can refer only to a small number of these. The growth of 111-V crystals has become a new art since the volatile compounds have to be grown under their vapor pressure (4.2, 4.3). The role and energy levels of all important impurities have been assessed (4.4). Lithium as an impurity has been studied thoroughly (4.5-4.8). Especially important is silicon as an amphoteric impurity in GaAs (4.9). The importance of the band tails, due to heavy doping, in light emission has been the subject of intense studies. F. Stern has contributed essentially to the clarification of the role of band tails to the linear dependence of gain on excitation levels in light emitters (4.10). The explanation of the decrease in luminous efficiency for high doping levels beyond the 6 x lO”-cm-’ range has been ascribed at an early date to Auger recombination. Zschauer (4.11) has reviewed this situation recently (see literature cited in this paper).

204

HERBERT F. MATARE

Photon energy ( e V )

FIG. 4.11. Refractive index as a function of wavelength for n-type GaAs at three temperatures. n c 6 x 10l6cm-3.

For the other most important III-V compounds, we shall briefly indicate their usefulness in light emitters. Gallium Phosphide Due to its high band gap (indirect: Ex 1: 2.2 eV; direct: ET N 2.8 eV), this compound is of major importance for visible light emission. In spite of the fact that its lowest conduction band point is not at k = 0, it plays a decisive role in the technology of LEDs. This is, as mentioned, due to the possibility of converting this material to a quasi-direct semiconductor and due to its miscibility in defined stoichiometric proportions with GaAs. We discuss GaAsP in Section 4,C on ternary compounds. The indirect gap of GaP at k = 0 is 2.29 eV at 300°K. The valence band consists of light and heavy mass bands, degenerate at k = 0 and a spin-orbit

LIGHT-EMITTING DEVICES

6o 40

205

3

Temparoture

(OK)

FIG.4.12. Thermal conductivity as a function of temperature for n-type GaAs. Dashed line n = lo'* ~ 1 3 1 solid ~ ~ ; line n = 7 x lOI5 C I I - ~ .

split-off band 0.127 eV below. There is thus a great similarity between silicon and gallium phosphide. But due to the possibility of a replacement of phosphorous by nitrogen and gallium by zinc and zinc oxide complexes, broad exciton levels can be formed. Isoelectronic impurity states present a number of energy levels in the forbidden gap, which can be filled by optical pumping or junction injection processes and are responsible for the high radiative recombination efficiency. Gallium phosphide, therefore, can emit a wide frequency band from red, orange, yellow, to green depending on the type of dopant. The normal donor-acceptor (Zn-0) pair recombination shows a strong maximum at 1.8eV (0.7 pm)and is the basis for the widely used red-emitting LEDs. A particular feature of these lamps is their low current requirement (10 mA as compared to 5 and 10 times this value in GaAsP lamps for similar efficiencies).Saturation ofemission sets in due to depopulation of the impurity centers, while in GaAs a continuously higher injection level will result in higher radiative output, a very important feature for infrared communications with GaAs.

206

HERBERT F. MATARE

For green and yellow emission in Gap, all traces of oxygen should be eliminated to maximize the Zn or Si acceptor to S or Te donor transitions (ev): Zn-S: Zn-Te:

2.2; 2.215;

Si-S: 2.213 Si-Te:

2.226

or according to A = 1.24/EGN 0.550 pm = 5500 8, = 550 nm. Best results have been achieved in nitrogen-doped LPE (liquid phase epitaxial) grown junctions with appropriate donor-acceptor spacing (4.12 ) and minority carrier lifetime (4.1 3, 4.1 4). Constant progress regarding efficiency of these lamps makes this field at present a highly competitive industrial area of activity (see below). Indium Antimonide

Due to its small band gap, this material has found major applications in infrared detectors. As a direct semiconductor, this material is of use for LEDs and lasers, when cooling is applied (10°K) and in addition carrier bunching is induced by an applied magnetic field (operation of Landau levels) (2.5).The tunability of such laser structures by means of an external magnetic field is a technical advantage and will probably play a role in future devices, in spite of the low temperature requirement. The high magnetoresistance values of InSb due to the high electron mobility have been the subject of intense studies and applications in electromagnetic devices (4.15 ) . Other materials with similarly high magnetoresistance values ( p e , cm2/v sec) are: InAs: pe z 30,000

HgSe: pe z 18,000

p e = 15,000

HgTe: p e x 22,000

Cd,As,: Indium Arsenide

Also in this case low temperature operation is imperative for laser action (EG = 0.35 eV). Pumping action by junction injection and external irradiation has been achieved, and emission peaks at 0.413 and 0.398 eV have been measured. The position in the far infrared (3 pm) makes this material interesting for infrared technologies, limited however by the requirement of cooling below 10°K.Above 10"K, the threshold for light emission and laser action increases rapidly (30 times as great at liquid nitrogen temperatures) (4.1 6).

LIGHT-EMITTING DEVICES

207

Indium Phosphide

This compound with a direct gap of 1.34 eV (300°K) and a relatively high electron mobility (4600) is the closest lattice fit to G a P and the ideal compound for a ternary combination. Other important compounds are given in Fig. 4.1 and in Table 4.1. To complete this survey, we have to mention gallium nitride, GaN. This material has been synthesized recently by chemical vapor deposition. With a direct band gap of 3.5 eV (300"K),this crystal is suitable for excitation and recombination radiation in the blue spectral range and has many advantages compared to Sic. Recent results (4.17, 4.18) have led to a low efficiency blue emission when crystals were contacted, and Schottky barrier formation has been achieved. The highly insulative character of these crystals has made production of a p-n junction impossible so far. However, Nick1 and co-workers have achieved Zn doping to compensate the donor concentration in insulating GaN layers on sapphire. The interface between the intrinsic (compensated) and the grown n-type layers shows a blue emission peak at 2.86 eV. Similar results have been obtained by Pankove and co-workers (4.19). Further work has been done recently to refine GaN film properties, and very nearly perfect crystal structure has been achieved (4.20). Recent work on ion implantation of zinc into GaN has led to a more controlled distribution and peak emission at 2.8 eV (4.21). Instead of using the normal Zn doping in the compensated GaN layer, Mg-doped GaN on undoped n-type (nitrogen vacancies) GaN leads to an emission peak near 3 eV in the violet region. The light seems to originate in dislocation boundaries between GaN-grown islands (4.22). For the role of defects, see Matare (4.23) p. 234. €3. Direct and Indirect Semiconductors

As we have seen, the question of whether a semiconductor is direct and allows for vertical transitions in the energy band scheme is important for applications in light emitters. For laser action, it is an essential condition; however, in the case of G a P we have noticed the possibility of using isoelectronic trap levels which permit a wave vector shift within the exciton band and vertical transitions at k # 0. As a rule, the III-V combinations around group IV of the periodic chart form semiconductors with decreasing binding energies as one moves down the periods and thus through decreasing melting points, decreasing band gaps, and generally increasing electron mobilities. This is apparent from the periodic chart and the derivation of Welker's scheme or rule. In Fig. 4.13, we

208

HERBERT F. MATARE [

I

II

IV

V

VI

I

1

1

1

FIG.4.13.Center portion of periodic chart of the elements with the important homopolar group IV semiconductors in the center and indication of the most useful combinations of these leading to Welker's rule.

have plotted the center portion of the periodic chart with the compound combinations. According to Welker's rule, the scheme shown in Table 4.11 can be derived. Vertically downward binding energies and melting points decrease while mobilities increase. Horizontally, the group IV semiconducTABLE 4.11 C (diamond) Sic Si Sio.5Geo.5 Ge

Sn

' Direct.

BN BP

AIN BAS AIP GaN BSb AlAs GaP InN AlSb GaAs" InP GaSb" InAs" InSb"

209

LIGHT-EMITTING DEVICES

tors at the left have lower binding energies, forbidden gaps, and mobilities than the isoelectronic compounds on the right. If one also connects the group I1 and group VI compounds in the chart in Fig. 4.13, other interesting materials orginate as listed in Table 4.111. We know that also the ternary compounds PbSnSe and PbSnTe are of great importance for infrared technology. TABLE 4.111

Compound

ZnS ZnSe ZnTe CdS CdSe CdTe PbS PbSe PbTe HgS HgSe HgTe

E , (eV)

p, (cm2/Vs)

3.58 2.67 2.26 2.42 1.7 1.44 0.37 0.26 0.25 2.5 0.3 0.2

I20 530 530 340 600 700 800 1500 1620 ? 18500 22000

d

(8)

3.814 5.667 6.103 4.137 4.298 6.477 7.5 6.14 6.45 5.852 6.08 6.429

direct direct direct direct direct direct indirect direct direct indirect direct direct

With CdSe and PbSe diode lasers for 7-pm (respectively, 22-pm) emission wavelength have been made (4.24). With Pb,-,Sn,Se, lasing at wavelengths depending on x between 0.04 eV (31 pm) and 0.16 eV (7.7 pm) has been achieved (x = 0 for 0.16 eV; x = 0.1 for 0.04 eV) (4.25, 4.26). While in PbSe and PbTe the mobility product is high and the crystals show relatively good stability on account of their melting points (1062 and 904"C, respectively), their low band gap makes low temperature operation imperative. The same is true for HgSe and HgTe. Their lattice constants match reasonably well: PbSe: 6.14A 4.4 % difference PbTe: 6.45 8, HgSe: 6.08 8, 5.4% difference HgTe: 6.4298, The difference is high, however, compared to certain 111-V compound pairs. In the production of heterojunctions and thin film deposition or heteroepitaxy, a lattice match can be achieved by a gradual admixture of one component. Notwithstanding, a close match is desirable to decrease the dislocation density. Even a lattice match as close as 10leads to 10' dislocations per cm2 (4.27).

2 10

HERBERT F. MATARB

The condition of close lattice match in combination with a tailored band gap by stoichiometry within the range of direct transitions makes the ternary compounds interesting for devices. Typical cases are GaJl, - 4 s and Gap,&, - x . If one plots the 100%pure direct and indirect gaps at k = (OOO) and k = (100) at both ends of the ordinate and EG (eV) on the abscissa for GaAs, Gap, and AlAs and connects these points by straight lines, interpolation yields the approximate band gap for these compounds. Figure 4.14 shows the case of GaAs-Gap, and Fig. 4.15 GaAs-AlAs. The measured data deviate somewhat from the straight line connection. In most cases of light emitters, maximum efficiency is achieved for points near the direct corner (GaAs in Figs. 4.14and 4.15) (4.28)up to the intersec-

< t 1.2

Ga As

0.2

0.6

0.4

X-

0.8 Gap

FIG. 4.14. Emission near the band edge in G a P & _ , crystals as a function of stoichiometry x. T = 77’K; k = (100). indirect; k = (OOO), direct branch. Measured values range from: cathodoluminescence, junction electroluminescence, forward and reverse bias.

21 1

LIGHT-EMITTING DEVICES

tion of the two lines. In the case of Ga, - ,AI,As, direct transitions can extend beyond the x = 0.45 point. In the case of G a P A s l _ , , the x = 1 point (pure Gap) with isoelectronic traps will result in enhanced efficiency due to quasidirect transitions. Another important direct gap compound for band tailoring is InP. In combination with G a P and AIP, two useful compounds are formed. If one plots again the direct compound at the left corner in the E G ( x ) diagram (Figs. 4.16 and 4.17) (4.29,4.30), one can easily find the compositional range of maximum efficiency for light emitters of the desired spectral range. The linear approximation for the band gap energy E , (respectively, E x ) :

-measured data range

AI,Ga,+,As

1.2

1 0

Ga A s

FIG

I 0.2

I 0.6

d.4

X-

I

0.8

3 Al AS

4 15 Emission (eV) for AI,Ga, xAs at 300 K as f ( u ) Indirect band gap estimated (4 28)

212

HERBERT F. MATARE

2.8

i

eV

Ga, In,+ P ,measured

data range

1

In P

I X-

Ga P

FIG.4.16. Emission (eV) for GaJn,_,P asf(x) (4.29, 4.30).

can be improved by adding a quadratic term with a relatively constant function is “bowing” parameter of 0.3. The complete 4 3 = J%O)

+ P ( 1 ) - J5,O)lX + 0.3(x2 - x)/[0.5(E,o, + 4 1 ) ) Y 2

(4.2)

This representation is valid for all direct materials and the direct ternary compounds: GaAsP, AlGaAs, GaInP, and AlInP; while the case of quasidirect transitions in indirect material like G a P is more complex, as we have seen, due to the A and NN lines. Excitonic levels originating from isoelectronic defect centers have small radiative recombination lifetimes. Electrons trapped at these centers have momentum values throughout the first Brillouin zone. A contribution of the Bloch states from the r minimum of the conduction band to the wave

213

LIGHT-EMITTING DEVICES

2.8-

2.6

)

2.4-

Al,

In

P

0

FIG.4.17. Emission (eV) for AI,In, -xPand measured peak position near crossover point ( M ) .

function of the bound states increases the probability of a no-phonon radiative decay of a bound exciton. The cases of GaAs,-,P, with N and Zn doping (x I 0.46)have been studied in detail (4.31). The effect of the dopants on emission frequency and efficiency depends on x to some extent. At 77"K, e g , a shift in peak output energy of 28 meV was measured when Zn was added as a dopant. GaAs,-,P, : N peaks at a higher gap value (or shorter wavelength) than GaAs, - ,P, :N:Zn. This shift decreases with increasing x. In the indirect portion of the E ( x ) diagram (x > '0.46) emission intensity is still high due to NN-pair-to-valence band and NN-pair-to-acceptor transitions. Similar results have been achieved with tellurium and Te:Zn doping.

214

HERBERT F.

MATARB

However, the quantum efficiencies for LEDs are highest for direct semiconductors, especially in cases of small difference in lattice constants of adjacent epitaxial layers. This point will be discussed later. C . Ternary Compounds

It is one of the most important features of the 111-V compounds that they can be combined to ternary and quaternary compounds of varied stoichiometry while maintaining ordered crystal structure and high mobility. This property has resulted in a new dimension for crystal formation and tailor-making of electronic properties, heretofore unthinkable with elemental semiconductors. In particular, faultless growth of heterojunctions and multiple layers of compounds with different band gaps was achieved with ternary compounds with close match of their lattice constants. Similar close matching is possible with the above mentioned 11-VI compounds and IV-VI compounds. However, compound systems fulfilling the conditions for LEDs, such as close lattice match, use of one component as the solvent, high mobility, and compound stability, are more frequent for 111-V compounds. Also the availability of appropriate bulk substrates is realized with all major 111-V base compounds: GaAs, InP, InSb, Gap, GaSb. The fact that the lattice parameters vary quite linearly with composition facilitates epitaxial procedures. The gradual increase of one compound in the composition allows minimization of lattice defects. Another important condition is the adaptation of the thermal expansion coefficient at the interface to avoid lattice strain. TABLE 4.IV Compound

,

GaAs - .P, Gal .AI& Ga,In, - .P Al&, - xP

L1 Emission range (pm) 0.63-0.9 0.59-0.89 0.56-0.92 0.540.92

The most important ternary compounds for LEDs are shown in Table 4.1V. The range comprises all colors from the infrared, red, orange, to yellow. For green emission, the band gap of G a P (2.25 eV) is more suitable in combination with nitrogen-induced isoelectronic trap leveIs. In effect, the extension into the Er range (the direct gap is in the 2.8-eV range) allows excitation clearly into the green spectral range (2.3-2.4 eV). The same is true in principle for AIP with a direct value E x = 3.7 eV. The compounds GaJn, - *P and AlJn, - xP have their highest luminous efficacy in the range

LIGHT-EMITTING DEVICES

215

x = 0.3 or near the E x = E , crossover point between 2.0 and 2.2 eV, i.e., in

the orange spectrum. Archer has pointed out that quantum efficiency of these compounds should surpass GaAsP combinations (4.29) by as much as a factor of 20. Modern device design has partially overcome the metallurgical and geometrical difficulties, e.g., by reflection of radiation at the contact area. We shall come back to these problems when discussing the device aspects. Ternary compounds allow moreover a precise determination of the optical frequency emitted by stoichiometric variation, an important property. A ternary compound that has undergone intense study is Ga(As, - xPx). Already at an early date, Spitzer and Mead established the complete direct and indirect threshold energies along the E , and E x lines in the stoichiometry diagram (Fig. 4.14) from photovoltaic response measurements (4.32). Later, GaAs, -xPx(x = 0.35) was subjected to optical excitation (pumping), and the photoluminescence behavior of nitrogen-doped crystals with distinct laser peaks was shown to be due to isoelectronic traps (4.33).The solution growth of In,-,Ga,P mixed crystals was achieved at this time (4.34). Also quaternary compounds have been prepared successfully for double heterojunction lasers (4.35). The most important compound G a A l , -,As has found ample consideration and many papers refer to this compound on account of the fact that it allows a very nearly perfect growth of junction structures of varied stoichiometry and heterojunction layer structures suitable for LEDs and lasers. This compound is the basis for all modern infrared-emitting devices of high quantum efficiency. We refer here to the recent summary articles by Panish and Hayashi (4.36) on heterostructure lasers and by Kressel and Nelson (4.37)on liquid phase epitaxy and to the literature cited in these articles. The important method of LPE (liquid phase epitaxy) will be discussed in Section 6. For all types of epitaxial structures, one is interested in a most perfect crystal growth, i.e., a small number of dislocations. This again is not only a function of the respective lattice parameters of substrate and film, but also of the difference in thermal expansion, for strain-induced dislocations can also act as nonradiative recombination centers. Most important for the choice of ternary compounds is their common solubility in one of the components. Hall (4.38) was the first to establish systematically the solubilities of InSb, InAs, and InP in indium and of GaSb, GaAs, and G a P in gallium (Fig. 4.18). In reading this diagram, the value of x defines the temperature at which of P will saturate InP at saturation occurs; e.g., an atom fraction x of 650°C. Knowledge of these values enables the correct setting of temperature for epitaxial growth from a saturation solution. As we construct ternary

HERBERT F.

MATARB

i

1 200

I 400

I

600

I

800

I

1000

I do0

I 1400

FIG.4.18. Solubility of InSb, InAs, and InP in indium and of GaSb, GaAs, and GaP in gallium. After Hall (4.38).

compounds, preferably from 111-V groups with a common component and close lattice relations, the groupings shown in Table 4.V are suggested for heteroepitaxial junctions. We see from Table 4.V that only a few cases of pairs of compounds are close with respect to lattice constants and expansion coefficients. In their order of compatibility, the compound systems in Table 4.VI are most apt to form perfect ternary systems. In a simplified model for the linear dislocation density caused by these differences in lattice constants,

TABLE 4.V

MAINCOMPOUNDS E , , 1,and d Energy gap (eV) (300°K)

Lattice constants

Expansion coefficient at 300°K ( x W6/”C)

Melting temperature

(4

Energy gap type

*AIAs GaAs

2.15 1.43

5.661 5.653

Indirect Direct

5.2 5.8

2013 1511

*GaAs GaP

1.43 2.26

5.653 5.451

Direct Indirect

5.8 5.3

1511 1738

GaSb GaP

0.73 2.26

6.095 5.45 1

Direct Indirect

6.9 5.3

98 5 1738

GaSb GaAs

0.73 1.43

6.095 5.653

Direct Direct

6.9 5.8

98 5 1511

InAs GaAs

0.35 1.43

6.058 5.653

Direct Direct

4.5(5.3) 5.8

1210 1511

*InAs InP

0.36 1.34

6.057 5.870

Direct Direct

4.5(5.3) 4.5

1210 1343

*AIP GaP

2.4 2.26

5.451 5.451

Indirect Indirect

? 5.3

2823 1738

*A& GaSb

1.65 0.73

6.135 6.095

Indirect Direct

3.7 6.9

1323 98 5

AlSb AIP

I .65 2.4

6.135 5.451

Indirect Indirect

3.7 ?

1323 2823

AlSb AlAs

1.65 2.15

6.135 5.661

Indirect Indirect

3.7 5.2

1323 2013

*AIP AlAs

2.4 2.16

5.451 5.661

Indirect Indirect

?

5.2

2823 201 3

InAs AlAs

0.35 2.16

6.057 5.661

Direct Indirect

4.5(5.3) 5.2

1210 2013

*InAs InSb

0.35 0.17

6.057 6.479

Direct Direct

4.5(5.3) 4.9

1210 803

*InSb GaSb

0.17 0.73

6.479 6.095

Direct Direct

4.9 6.9

80 3 98 5

*InSb AlSb

0.17 1.65

6.479 6.135

Direct Indirect

4.9 3.7

80 3 1323

InSb InP

0.17 I .34

6.479 5.869

Direct Direct

4.9 4.5

803 1343

*InP GaP

1.34 2.26

5.869 5.45 1

Direct Indirect

4.5

5.3

1343 1738

*InP AIP

1.34 2.4

5.869 5.457

Direct Indirect

4.5 ?

1343 2823

Compound system’

The systems marked with an asterisk are closely matching.

(OK)

218

HERBERT F. MATARE

TABLE 4.VI COMPOUND SYSTEMS WITH CLOSE

LATTICEMATCH

Compound system

Percent difference in lattice constants

AIP-GaP AM-GaAs GaSb- AlSb InAs-InP GaAs-GaP AIP-AIAs InSb-A1Sb InSb-GaSb InAs-InSb InAs-GaAs InP- A1P InP-GaP

< 0.01 0.14 0.65 3.2 3.6 3.8 4.7 6.1 6.6 6.9 7.1 7.2

one would have a mean dislocation density d,

6 = ao/6 6 = Aa/ao is the displacement index, usually given as a percentage. The normally given dislocation density is 1 A [ ~ m -= ~] 10l6(J in

a2

For the case of

A) = a2 10l6 a0

AIP-Gap, e.g, 6 = ~] A [ ~ m -= ~

(5.45 1)2

and

1016 = 3.3 x 106

The linear dislocation density would be A [cm-

6 '1 = 1 lo8 = lo8 = 1.8 x -

d

lo3

a0

In cases of a lesser adaptation of the two lattices, e.g., in the case of 7.2% (InP-Gap), the linear density is

A [cm-l]=--

0.072 lo8 = 1.2 x lo6 5.86

and the area density is A [ ~ m - =5.862 ~ ]0'0722 1016 - 1.5

1012

LIGHT-EMITTING DEVICES

219

In reality such cases of higher misfit with several percent for the displacement index are grown together in a graded fashion. In the last case, e.g., the compound In,Ga, - xPis epitaxially deposited on G a P and x is varied from a small value to the actually desired value x = 0.6. This is especially easy in VPE (vapor phase epitaxy) by a control of the gas flow. In LPE, there have also been found methods for deposition in steps. The ideally controlled method is, of course, molecular beam deposition which, however, has not yet developed for use as an industrial tool (see Section 6). The ternary compound has opened a wide field of activities .inasmuch as now all parts of the optical spectrum can be covered with light emitters and suitable detectors. In Fig. 4.19, we indicate the most important compounds and their position in the wavelength scale together with the ranges of selenium detectors, silicon detectors, and the human eye.

BLUE

GREEN

YELLOW

RED

-1.R

FIG. 4.19. Emissive range of some ternary compounds in the optical spectrum within sensitivity range of selenium and silicon detector and the human eye.

In Fig. 4.20, we have plotted a number of ternary compounds with their range of stoichiometry covering a defined frequency spectrum and some of the lasing compounds. The possible range covered by one ternary compound is limited by the metallurgical possibilities for growing a perfect layer of the desired stoichiometry x. In most cases, one has to start from an available compound bulk crystal in wafer form as substrate. Not all interesting compounds can be grown

I

Ga As, PI-,

Ternary Compounds

1

1

Pb SnTe

Ga,ln,.,As

FIG.4.20. Emissive range of some ternary compounds of variable composition (x) and some typical semiconductor laser emissions in the wavelength scale (experimental range).

1

In,

Ga,..Sb

1

In, Al ,-, As Al As

In As

I

AlSb

7cls

Nl y , n z

111 I 0.~3

0'4

d5 d6

In, Al,-, Sb

iinSbx P,-,)

Te

1

inSb

Sn, Pb,_,Te

II d! 1

FIG.4.21. Possible coverage of the optical spectrum by stoichiometric variation of ternary compounds.

LIGHT-EMITTING DEVICES

22 1

easily and fewer can be purchased at good quality. The crystals easily available are InSb (direct), GaAs (direct), InP (direct), G a P (indirect), GaSb (direct). Most of these are the basis for the ternary compounds listed in Fig. 4.20. A further problem is the fact that overgrowth in a graded form (x = 0 to 1 ) may not be easily achieved while placing a junction into the layer. Dopant-induced defects and transition too far into the indirect branch of the E ( x ) curve may degrade quantum efficiency. The full range of optical activity for a number of known ternary compounds is shown in Fig. 4.21. We see that the spectral region from the violet to the red and into the infrared is covered if certain 11-VI compounds are included. The enrichment of electrooptics due to this enormous flexibility in material choice and properties is obvious, and it is only a matter of the effort spent in materials research to achieve complete coverage of this spectrum by monochromatic, high efficiency semiconductor light emitters.

REFERENCES FOR SECTION 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8. 4.9. 4.10. 4.11. 4.12. 4.13. 4.14. 4.15. 4.16. 4.1 7 . 4.18. 4.19. 4.20. 4.21. 4.22. 4.23. 4.24.

M. Neuberger. “Gallium Arsenide.” Data Sheets, Electronic Properties Information Center Report DS-144. Air Force Mater. Lab., Dayton, Ohio. A. G. Fischer, J . Electrochem. Soc. 117, 41C-47C (1970). J. J. Nickl and W. Just, J . Crpst. Growth 11, 11-20 (1971). C. S . Fuller and K. B. Wolfstirn, J. Appl. Phys. 31, 2287-2289 (1963). C. S. Fuller and K. B. Wolfstirn, J . Appl. Phps. 33. 745-746 (1962). C . S. Fuller and K. B. Wolfstirn, J . Appl. Phgs. 34, 1 9 1 4 1920 (1963). C. S. Fuller and H. W. Allison. J . Appl. Phys. 35, 1227-1231 (1964). C. S. Fuller and K. B. Wolfstirn. J . Appl. P h p . 33, 2507-2514 (1962). J . K . Kung and W. G. Spitzer, J. Appl. Phys. 45, 4477-4486 (1974). F. Stern. P h y s . Rev. 148, 186-194 (1966). K . H. Zschauer. Solid Stare Commun. 7. 1709-1712 (1969). G. B. Stringfellow, H. T. Hall, Jr., and R. A. Burmeister. J . Appl. Phys. 46,3006-3011 (1975). B. Hamilton. A. R. Peaker. S. Bramwell, W. Harding,and D. R. Wright, Appl. P h p . Letr. 26, 702-704 (1975). K . Mettler and K. Richter, Siemens Forsch. Enrwicklungsher. 2, No. 4 (1973). H. Weiss, Semicond. Semimetals 1, 315-376 (1966). I . Melngailis and R. H. Rediker. Appl. PhI.s. Lprr. 2. 202 (1963); J . A p p l . Phys. 37, 899 ( 1966). H. P. Maruska and J . J. Tietjen. Appl. P h w Letr. 15, 327-329 (1969). J. J. Nickl. W. Just. and R Bertinger, Marer. Res. Bull. 9, 1413-1420 (1974). J. I . Pankove. H. P. Marnska. and J. E. Berkeyheiser, Appl. Phys. krr..17, 197 (1970). W. Hosp, Sirniens Forsch. Entwicklunyshw. 1, No. 3 (1972). P. I . Pankove and J. A. Hutchby, Appl. Phys. Lerr. 24, 281-283 (1974). H. P. Maruska and D. A. Stevenson, Solid Srate Elecrron. 17, 1171-1179 (1974). H. F. Matare: ..Defect Electronics in Semiconductors,” p. 234. Wiley (Interscience), New York. 1971. J. M. Besson. A . R. Calawa. and W. Paul, in “Solid State Research,” Lincoln Lab. Rep., No. 3, p. 1 . 1965; C. E. Hurwitz; ihrd. No. 3 , p. 6 (1967).

222

HERBERT F. MATARE

4.25. A. R. Calawa and I. Melngailis, in “Solid State Research,” Lincoln Lab. Rep., No. 3, pp. 1-6. 1967; J. F. Butler and T. C. Harman, ibid. No. 3, pp. 13-15 (1968); J. P. Donnelly, A. R. Calawa, T. C. Harman, A. G. Foyt, and W. T. Lindley, ibid. No. 3, pp. 5-7 (1971); I. Melngailis and T. C. Harman, ibid. No. 1, p. 7 (1972). 4.26. T. C. Harman, J. Melngailis, A. R. Calawa, and J. 0. Dimmock, in “Solid State Research,” Lincoln Lab. Rep., No. 3, p. 1. 1969; A. Mooradian, A. J. Strauss, and J. A. Rossi, ibid. No. 1, p. 13 (1972). 4.27. H. F. Matare, “Defect Electronics in Semiconductors,” p. 474, etc., Fig. 14.9. Wiley (Interscience), New York, 1971. 4.28. H. C. Casey, Jr., and M. B. Panish, J . Appl. Phys. 40,4910 (1969). 4.29. R. J. Archer, J . Electron. Muter. 1 (1972). 4.30. G. B. Stringfellow, P. F. Lindquist, and K. A. Burmeister, J. Electron. Muter. 4, 437-457, (1972).

4.31. J. C. Campbell, N. Holonyak, Jr., M. H. Lee, M. J. Ludowise, M. G. Craford, D. Finn, and W. 0. Groves, J . Appl. Phys. 45, No. 2, 795-799 (1974); M. H. Lee, N. Holonyak, Jr., J. C. Campbell, W. 0. Groves, and M. G. Craford, ibid. No.4, pp. 1775-1778. 4.32. W. G. Spitzer and C. A. Mead, Phys. Rev. 133, A872-A875 (1964). 4.33. N. Holonyak, Jr., D. R. Scifres, R. D. Burnham, M. C. Crawford, W. 0. Groves, and A. H. Herzog, Appl. Phys. Lett. 19, 254-256 (1971); N. Holonyak, Jr., D. R. Scifres, M. G. Crawford, W. 0. Groves, and D. L. Keune, ibid. pp. 256-258. 4.34. H.Itoh, K. Hara, A. Tanaka, and T. Sukegawa, Appl. Phys. Lett. 19, 348-349 (1971). 4.35. R . D. Burnham, N. Holonyak, Jr., H. W. Korb, H. M. Macksey, D. R. Scifres, J. B. Woodhouse, and Z . 1. Alferov, Appl. Phys. Lett. 19, 25-28 (1971). 4.36. M. B. Panish and I. Hayashi Appl. Solid State Sci. 4, 235-328 (1974). 4.37. H. Kressel and H. Nelson, Phys. Thin Films 7 , 115-256 (1973). 4.38. R. N. Hall, J. Electrochem. SOC. 110, 385-388 (1963).

5. THEHETEROJUNCTION A . Injection EfJiciency and Confinement

Real progress was achieved in the use of semiconductors for LEDs and lasers after it was recognized that p-n junctions biased in the forward direction are efficient carrier injectors and that especially the use of heterojunctions enhances injection and light emission in many ways and makes room temperature lasing possible. First, we recall Kroemer’s reasoning when introducing the heterojunction as an efficient carrier injector (5.1). In terms of electron- and defect-electron current densities j , andJ, at a p-n junction, the injection efficiency is

for holes. The relative injection deficiency is defined as

223

LIGHT-EMITTING DEVICES

and j , and j , are given by the usual equations for electron and hole flux through p-n junctions :

One can easily show that for normal injection currents Di is given by

with D,and D, being the diffusion constants for electrons in the p region and for holes in the n region; Lnand L,, the corresponding diffusion lengths; np, the net donor density in the p side; and p n , the net acceptor density in the n side (5.2). The p region can be associated, e.g., with the emitter, and the n region with the base of a diode or transistor @ . I ) , if one expresses the equilibrium minority carrier densities by the intrinsic densities in emitter nie and base nib: noe = n i e / p e ;

pob = &/Nb

(5.5)

where P , is the net acceptor density in the emitter and N , is the net donor density in the base. One can introduce the state densities according to a Fermi distribution: n: = N , N , exp[ - (ECe- E,J/kTl

(5.6)

on both sides [see also Ref. (5.3)]. With the usual effective state densities ( p side) N , = 2(2nm,*kT/h2)3’2, N , = 2 ( 2 ~ m , * k T / h ~ ) ~ ” (5.7) and similarly for N , and N , for the n side, the expressions (5.6) become ni?,= ( ~ , * L m 3 exp[-(ECe ~” - Evc)/kT] nfb = (m,*,m,8)3i2exp[ - ( E c b - E,,)/kT]

(5.8)

the ratio is

In Fig. 5.1, we have plotted the heterojunction with a gap step AE = E,, - E C z . A slight bending of the valence band at the space charge interface is due to the difference in electron affinity and is disregarded in the calculation. Thus we assume Ece - E,, = AE,

Eve - E,, = 0

(5.10)

P

E(eW

N

\

E,-L---

'-.

--.-.-.-.-

t

t

1

,

Ec

.-

EF

E,

f

E"

Therefore the injection deficiency is

The factor characterizing the heterojunction is therefore exp( -AE/kT) when the effective masses are considered equal for holes and electrons on either junction side. Assuming, e.g., a heterojunction between Ga0,,A1,,,,As and GaAs with the respective band gaps of 1.85 eV and 1.43 eV, AE = 0.42 eV. Since kT (300°K) = 0.025 eV, we have for exp( -AiE/kT) = exp( - 16.8) = 5.2 x lo-,

+

Correspondingly, y = l/(Di 1) is very close to one with practically no injection loss. Strong injection has been dealt with for the case of power devices, and the corresponding carrier density variations have been taken into account by Spenke (5.2, pp. 159ff.) Such density variations (see Fig. 5.2) have a strong effect on the space charge extension and carrier mobilities. In this case, y is no longer a materials dominated magnitude, but is governed by the carrier density p ( x f l )in the low impurity density side (n-type side in Fig. 5.2): (5.12) which is close to one for the case p ( x n ) 4 p p . Injection loss occurs strongly, however, when the injection density p(x,) of holes into the n-type side (lower doping) approaches the hole density p p at the higher doped p-type side of the junction. In LEDs high injection densities must be assumed. Doping levels of 10'' or higher in the n-type side (Fig. 5.2) have to be balanced by even

225

LIGHT-EMITTING DEVICES

C 1019

10

1

-

loP-

/

/

xp X" space charge

-X

FIG 5.2 Carrier concentration along heterojunction interface with strong electron injection.

higher concentrations at the injector side. Here the heterojunction helps considerably in keeping Di [Eq. (5.1l)] at a low value due to the exponential factor exp( - AE/kT). In detail, the heterojunction has several important functions in LEDs and semiconductor lasers and has made possible tremendous development of these light sources. Even such useful sources as the helium-neon laser are now largely replaced by the new room temperature semiconductor lasers (RCA). The reason for this is simply the compactness, ruggedness, and efficiency, low power requirements, and solid state compatibility of these light sources; in addition there is the wide spectral range and the possible exact placement onto a defined optical frequency (see Fig. 4.21). There are numerous pairs of semiconductors that match each other not only with respect to lattice constant, but also with respect to thermal expansion coefficient. They are useful in this context and add new possibilities to the list of ternary compounds already given. In Table 5.1, we have plotted the most useful combinations with their main constants and their electron affinities x in electron volts. These values are added because there is some influence of x on the interface potential barrier, which generally is small, however, and overshadowed by other parameters such as surface states and differences in the effective masses of holes and electrons. In fact, the higher hole effective mass shrinks the gap more in p-type material than in n-type material, typically for lo-' eV. The barrier for injected electrons into the p side of a p-n junction is reduced, favoring

226

HERBERT F.

MA TAR^

TABLE 5.1 IMPORTANT PAIRSOF 11-VI

AND

111-V SEMICONDUCTORS WITH CLOSE LATTICE MATCH

Lattice constants

Expansion coefficient at 300°K ( x 10-60C-')

D, direct I, indirect

Type

E , (eV)

(A)

D I

GaAs Ge

1.43 0.7

5.654 5.658

5.8 5.7

Se, Te (n) Al, Ga, In (P)

4.07 4.13

D 1

ZnSe Ge

2.67 0.7

5.667 5.658

7.0 5.7

Al, Ga, In (n) Al, Ga, In (P)

4.09 4.13

D D

ZnSe GaAs

2.67 I .43

5.667 5.654

7.O 5.8

Al, Ga, In (n) Zn, Cd (P)

4.09 4.07

I I

GaP Si

2.25 1.11

5.451 5.431

5.3 2.33

Se, Te (n) Al, Ga, In (P)

4.3 4.0 1

D D

GaSb InAs

0.68 0.36

6.095 6.058

6.9 445.3)

Te, Se (n) Zn, Cd (P)

4.06 4.9

D D

ZnTe GaSb

2.26 0.68

6.103 6.095

8.2 6.9

cu (PI

3.5 4.06

D D

ZnTe InAs

2.26 0.36

6.103 6.058

8.2 4.5(5.3)

c u (PI

3.5 4.9

D I

ZnTe AlSb

2.26 1.6

6.103 6.136

8.2 3.7

cu (PI

Se, Te (n)

3.5 4.59

D D

CdTe InSb

1.44 0.17

6.477 6.479

4.9

L i Sb (P) Se, Te (n)

4.28 4.59

D D

ZnTe CdSe

2.26 1.7

6.103 6.05

-

cu (PI

3.5 4.95

-

Typical dopants

Se, Te (n)

Se, Te (n)

Cl, Br, I

(eV)

injection and recombination on the p side ( 5 in Fig. 5.1). This will be considered in more detail in the following. We have seen that numerous semiconductor pairs form valid heterojunctions. Such structures are of great value not only because one side has a higher band gap than the other, but also due to differences in optical absorption and due to a confinement for both charge carriers at the transition region. Kroemer (5.4) first recognized this fact and has discussed the decisive double heterojunction properties. According to this scheme, the following properties are essential: (1) High injection efficiency (high gap to low gap, see Fig. 5.3). (2) Charge carrier confinement within the transition zone between the

227

LIGHT-EMITTING DEVICES

N E W

degenerate electron population

5h

-

._.

%AIlps

4

c Ga As

--I-

GaxAll_xAs

FIG. 5.3. Double heterojunction for Ga,AI, _.As/GaAs/Ga,AI, -,As laser and confinement area for injected carriers. After Kroemer (5.4).

two space charge regions. Injected holes and electrons must overcome potential barriers on both sides preventing bulk recombination and forcing radiative recombination within the transition region. (3) Optical confinement, important in lasing structures. Due to planar zones of different indexes of refraction, a low-loss guidance of the optical power is achieved. All this is under the assumption of most perfect crystallographic order along the junction interfaces (matching lattice constants and expansion coefficients; see Table 5.1). In the case of ternary compounds with one common compound, these conditions can be met by graded stoichiometry as described before. Figure 5.3 shows the original idea of Kroemer. Here a small zone of a narrow gap semiconductor (GaAs, Kroemer proposed Ge) is contacted on either side by a wide gap zone (GaAlAs, Kroemer proposed GaAs because the technique of ternary compounds had not been developed sufficiently in 1963). We see that injected electrons as well as holes from both the n side and the p side of the junction find a barrier within the intermediate P - layer preventing drift or diffusion of these carriers into the bulk. This idealized scheme satisfies conditions (1)-(3) given above and is the basis for all modern laser structures and also partially for LED structures of high efficiency. In modern faser structures conditions (2) and (3) are satisfied by the

228

HERBERT F. MATARE

so-called SH and DH (single and double heterostructure) lasers. In a DH laser, the “cavity” layer, i.e., the innermost generally p-type layer of small width (in the submicron range) is sandwiched between two larger gap ternary compound layers followed by two similar layers of even larger band gap. While this structure offers metallurgical advantages-gradual change to the ternary compound and better lattice match-it also separates carrier confinement and light confinement and allows for a wider spread of the radiative energy along the outer junction planes and thus better energy distribution (see Fig. 5.4). These structures have led to a considerable lowering of the laser threshold to even below lo00 A/cmZ and to room temperature operation. We discuss these devices in Part I1 when summarizing the outstanding work by M. B. Panish and I. Hayashi (Bell Laboratories) and H. Kressel, H. Nelson, H. F. Lockwood (RCA), and others (5.5, 5.6). EleV)

a(Refractive Index )

(Optical

I

A

FIG 5.4. Double heterojunction with addi:ional heterolayer to form a broad optical confinement layer (see text).

229

LIGHT-EMITTING DEVICES

B. Lattice Match We now consider in more detail the heterojunction and its operation since it is essential to the functioning of efficient LEDs. The way in which two different band gap materials unite at the interface is subject to many variations depending on lattice constant match, thermal expansion coefficients, impurity types, diffusion constants, electron affinities, dielectric constants, etc. When two materials of different band gap unite, a few basic facts have to be considered : (a) The Fermi levels equalize (voltage-free case). (b) This causes a kink in the formerly even vacuum levels and also a discontinuity in the conduction band. (c) The valence band curves up for the amount of the difference in gap energy minus the work function difference responsible for the curving up in the conduction band. (d) Interface states and dislocations generally present at a density of 106-107cm-2 mask the slight difference in x and introduce additional band deformation due to the presence of interband states. We have plotted these changes in Fig. 5.5 without regard to defectinduced interband levels. The two semiconductors-one n-type, the other p-type-are brought into contact, a process that brings their respective Fermi levels in line (EF, = EF2)and causes a bending of the projected vacuum levels VDnand VDpfor the difference E,, - EF2: E F ~- EF2

=

VDn

+ VDp

= EZ

+ (xZ

- xl) -

(61

-k 6Z)

(5.13)

P

AEC Here x1 and x2 are the electron affinities of both semiconductors; 6, and 6, are the energy differences between the Fermi levels and the respective bands: 6 , = E,, - E F , ~

62 = E F~ E,,

(5.14)

If one assumes transition regions depleted over the distances x, and x,, then X,

N , = X, N ,

(5.15)

Poisson's equation applied to VDnand VDpleads to d2 V,Jdx; = - 4np/El

and

(5.16)

230

HERBERT F. MATARE

FIG.5.5. (a) Heterojunction interface formation with x , and x 2 (vacuum)levels, 6 , and 6, are energy differentials between Fermi levels and respective bands, and (b) band deformation after junction growth. V,, Yo, are diffusion voltages (electrons, holes); AE, and AE,are barriers (see text).

with el being the dielectric constant and p the charge density. Since -4np = ND, we have

vDn= NDX f / 2 E

(5.18)

Therefore, the ratio of the diffusion voltages on both sides is

(5.19) which on account of (5.15) is

23 1

LIGHT-EMITTING DEVICES

As an example, we consider the heterojunction between an n-type GaP and a p-type silicon crystal. The lattice constants are reasonably close: Gap, 5.451 ; Si, 5.431. The electron affinities are: Gap, 4.3 eV; Si, 4.01 eV. GaP may be doped with Se or Te. Silicon may be p-type due to Al, Ga, In. We shall not consider that the expansion coefficients are quite far apart: Si, 2.33; Gap, 5.3 ( x "C-' at 300"K), but will consider the band curvings (Fig. 5.6).

n-type

Ga P

I

J---rp-type

Si

Vacuum Lmel

-7-

FIG.5.6. Heterojunction interface for GaP/Si with values indicated for vacuum levels and barriers.

We have noticed that the differences AE, and AE, are given respectively by the difference of the vacuum levels and by the difference in energy gap:

AEc = x 2 - x1 AE,, = E l - E2 - AEc

(5.21) (5.22)

Since

AE, -I- AE,

= El -

E2

(5.23)

In Fig. 5.5, the energy spike AE, is due to the matching of different electron affinities. If the larger gap material is p-type (e.g., GaAs in Fig. 5 4 , the spike may appear in the valence band rather than in the conduction

232

HERBERT F.

MA TAR^

band (5.7) but as we noted already, interface states and traps due to mismatch and difference in thermal expansion will dominate the heterojunction and overshadow the relatively small effect of the difference in x. The importance of a good lattice match becomes evident when we calculate the number of dislocations due to a difference in the lattice parameter. The lattice disregistry is measured by the “displacement index 6: ”

6 = Aa/ao

(5.24)

where Aa = lab - a, 1, 6 = lab - a, I/a, is generally given in percent (5.3). The linear dislocation distance is then given by

a = aSruwP

(5.25)

and in the simple case, the lattice translation vector a,,”,,,as a Burgers vector can be replaced by a. . This leads to a linear dislocation density

l/d

Y

Aala;

(5.26)

The area density is then 2

A [ern-'] = (1)

d

* Aa2/a: =

2

10l6 for a, in angstroms

(5.27)

a0

The figure for A [crn-’] is relatively high for a direct interface between two different semiconductors. For the case of GaAs

5.654

GaP

5.451 Aa

= 0.203,

6 = 3.59%

one finds

A [cm-’] z 4 x 10” But for a gradual increase in the phosphorous amount from the GaAs surface, the x in GaAs, -xP,can begin with x = 0 at the interface, a distinct possibility in CVD (chemical vapor deposition). In this case (5.28) (5.29) Measured values for Ac/Ax in percent of phosphorous per micrometer of epitaxial layer are in good agreement with measured dislocation densities in the epitaxial layer (between lo7 and lo9 cm-’) for Ac/Ax in the O.l-lO%

233

LIGHT-EMITTING DEVICES

range. For the density of dislocations at the top of a crystal decreases also due to lateral dislocation outgrowth and the formation of inclined dislocations (see Refs. 5.6 and 5.8). This is evident from figures for A, calculated for cases of measured dislocation densities. For example, Mader and Blakeslee (5.9) have shown that misfit dislocations dissociate into partials and form networks, but that their main density measured in the (220), (TTI),and (311) directions are essentially the same. For GaAs,.,P,,,/GaAs, one calculates Aa = 0.0676 A or a linear dislocation density of

=2 x

105/cm

while these authors measured only 0.5 x 105/cm. We see that interface dislocations are an essential feature of epitaxial growth and have to be dealt with. Their appearance and density depends partially on the type of epitaxy used and it seems that LPE has some advantages compared to other methods due to a definable melt-back and elimination of surface states.

t

FIG. 5.7. Interface dislocations and resulting band changes: (a) bicrystal; (b) heterojunction with dislocations. T,, T2 = tunnel currents, F , = recomb. flux.

234

HERBERT F. MATARB

It is known from earlier work on dislocations in homopolar crystals that edge and also screw-type dislocations introduce trap levels in the forbidden gap. Depending on the type of crystal and the growth method used, many varied energy levels can be created (5.3).The scheme of a grain boundary in a monocrystal can serve as a model for a heterojunction with interface dislocations. Figure 5.7a shows such a grain boundary in the energy (el‘) versus X diagram with and without (dashed curve) external bias (V,). The interface dislocations cause a curving up of the bands as these states are pulled out from the valence band. The bias causes an increase in the barrier height from 4 to 4‘ or el‘, to e V o . In the case of a heterojunction with interface dislocations, the influence of the interband states is a loss of injected carriers. These can tunnel through the interface spike AEv or AEc (TI)or into the interband traps (T’), and a considerable portion can be lost to the radiative recombination process by a recombination flux F , through the interband levels (Fig. 5.7b).

C . Defects and Radiative Efficiency

Defects can play a role in radiative recombination as interband levels or “pump levels.” This is especially the case in homopolar semiconductors. In cases where grain boundaries form, a planar space charge can give rise to high local fields as in p-n junctions and can form an area of efficient radiative recombination (5.3). In III-V semiconductors, complications arise due to the possibility of two types of dislocations, those with a core bond from an A side (a dislocation) and those with a core B atom (/3 dislocation). In GaAs, e.g., one has (Ga) dislocations on the A( 111) or the gallium side and /3 (As) dislocations on the B(TT1) side. Since the a dislocation in GaAs has a negative core charge in n-type materials (due to electron attraction to the free bond), a positive space charge forms. The same tendency prevails for /3 (As) dislocations in p-type material. The effects of a and /3 dislocations on resistivity, mobility, radiative and nonradiative recombination, etc. have been studied by several authors [one can find a summary in Matare (5.3),pp. 168 and 2381. Recently, Heinke and Queisser (5.10) have plotted isointensity curves around dislocations proving the light decrease due to nonradiative recombination in photoluminescence experiments. In the case of junctions, there is a definite dependence on the orientation of the junction with respect to the dislocation. If we designate by aI a

235

LIGHT-EMITTING DEVICES

dislocation vertically oriented to the junction plane and by a,, the parallel case, the following rule can be established for GaAs LEDs (5.1f,5.3): radiative recombination nonradiative recombination Bl + enhanced nonradiative recombination PI, + enhanced radiative recombination. tll + enhanced

all + enhanced

Other authors have defined ctL as the a dislocation vertical to the current flow direction and correspondingly a , ,, PL,and Pll with respect to the current flow. The influenceof these dislocations on basic transport parameters is certainly much dependent on the dopant environment (accepted carriers and their energy level). Some of the known effects on carrier transport are presented in Table 5.11. As a general rule, defects do not enhance radiative recombination except for the cases of a linear or planar arrangement with resultant space charge layer. In GaAs,,,P,,,, the usual material for red LEDs, a multilevel defect with a large hole capture cross section has been identified by measurements of photocapacitive transients (5.12). The behavior of this defect, 0.9 eV from the conduction band edge, suggests a combination of dislocation and oxygen levels. Oxygen is ever present in Czochralski-grown GaP crystals due to contact with the boron oxide cover layer. The proposed model is shown in Fig. 5.8. (5.12). Several emission processes via complex levels are shown. eth and e; are thermal emissions into the conduction and valence bands, respectively.

Po,4)= 1.95 eV

FIG. 5.8. Multiple defect trap level in GaAs, 6Po,4with capture and emission processes. q;, thermal electron emission; e : , thermal hole emission; C,, and C,, are hole capture coefficients: e i l and e;> are optical electron emission processes. After Forbes and Fogle (5.12).

TABLE 5.11 INFLUENCE OF DISLOCATION TYPEON ELECTRONIC PARAMETERS OF Ge, GaAs,

Crystal

Dislocation type _

Ge GaAs

_

_

Conductivity

Mobility

U

P

0.2-0.5 eV acceptor

u i i> u0

p , ,= p o

01

Pl

(Ga)-a

acceptor in n-type (donor in p-type GaAs) acceptor in n-type

u,,> uo ul < uII

Onta

(SbH

Diffusion length D

InSb

Remarks

~

edge in n-type

( A S H

lnSb

E, - ED (ev.) and type of dislocation level

AND

0.12 eV acceptor in n-type donor in p-type N < 1016 cmn-type: donor N > 1 0 1 7 cm-3 n-type: acceptor 0.22 eV

D , , > Do D l
(5.3),p. 266-275. 382, 454

fill > p o pl < p,,

D , , > Do

Values measured by Esquivel el a/. (5.26)

>>Po < PI1

D,, > Do


u,,b 6 0 < 011

PI1

ul

PI

ol, < uo

p,,< p o


Pl


D, < D,, D, < Dll

D , , < Do

DL
estimated relations for GaAs and InSb

(5.3). p. 382; (5.27)

LIGHT-EMITTING DEVICES

237

Optical electron emissions (several steps, e; and ei2) have also been found; however, most of the recombinations are nonradiative in these junctions. The growth of GaAs,-,P, crystals either by vapor phase or liquid growth has made great progress recently, and lattice distortion and defect densitities have been assessed carefully (5.13). Generally, growth occurs better on the B surface of an A,,,B, compound in the (111) direction. However, in the case of LPE, this rule may be less apparent when melt attack and wetting problems are involved (see Section 6 ) . Much has been published concerning the influence of defects and crystal growth striations on LEDs and laser diodes. Ziegler and Henkel (5.14) report that crystal growth striations (impurity density variations) are of minor importance for the efficiency of lasing junctions. A variety of vacancydefect levels due to annealing of GaAs were identified by Chang et al. (5.15). Mismatch dislocations in heterojunctions have been identified early in VPE of Ge on GaAs (5.16) and also in diffused GaAs LEDs (5.17). Direct correlation between dislocation density on one side and minority carrier diffusion length (respectively, external quantum efficiency of edge emission) on the other side was found in LPE-grown GaAs LEDs (Te and Ge doped). The different dislocation densities were induced by surface roughening (5.28). Bulk degradation is a known effect in semiconductor lasers and LEDs and has been investigated since the early days of commercial light emitters. It is in fact still today the main cause for the slow introduction ofjunction lasers into industrial use. The space charge region, as the most vulnerable part of the light emitter, is the area where defects interfere with radiative efficiency. Nonradiative centers are, e.g., impurity clusters, metallic impurities (Cu), stoichiometric faults, cracks, voids, defect centers, dislocations. All these centers introduce excess current in the space charge region and decrease the quantum efficiency. Operation of a junction under hard conditions causes: (1) field-induced migration of impurities, especially those with a high diffusion coefficient like copper; (2) rearrangement of lattice structure in the vicinity of the junction, especially when the junction has unequal current density due to hot spots; (3) relief of built-in stress due to thermal activation with attendant formation of defects, dislocations, or even cracks.

In the case of a laser, the optical confinement and high energy density within a small area of the junction plane are of particular influence and generally limit lifetime to a few thousand hours. Bulk degradation of GaP LEDs has been connected with the action of copper. This cause is particularly probable in diffused devices due to expo-

238

HERBERT F. MATARE

sure to an impurity atmosphere. Gallium as a sink for copper as used in LPE seems to eliminate this cause (5.19). Mettler and Pawlik show correlation between dislocation density and degradation. They also suppose that Cu migration plays a role in the activity of these defects as well as strain-induced nonradiative centers (5.20). The abbreviation DLD, used for dark line defects, shows how numerous the appearances are of the famous dark spots or nonradiative recombination centers. Petroff and Hartman (5.21) have shown by transmission electron microscopy investigation of these defects that they originate from dislocations crossing the epitaxial layer and extending from there by a climb mechanism during device operation, i.e., during local heating. It is apparent that dislocations originally present near or at the space charge region are subject to rather strong field gradients and will form hot spots due to their own space charge (5.3). Upon further increase in driving power and output, these localized high field regions will heat up and act as polygonization centers causing climb and glide, which enlarge to a dislocation network. Such structures are visible in transmission electron microscopy (see Refs. 5.21 and 5.22). Their appearance depends on the orientation with respect to the junction plane, and what are called DLD (dark line defects) or DSD (dark spot defects) seem to originate from the same physical source (5.23). Elimination of stress by the appropriate atomic mix in the compound seems to be a distinct possibility. Phosphorous in particular can compensate the mismatch between GaAlAs and GaAs when the quaternary compound Gal-J&l-yPy is used as an epilayer on GaAs. Rozgonyi and Panish have studied this compound as an LPE layer on GaAs with the result that coincidence of the Bragg reflection was established for y = 0.015 or an atom fraction of 2.5 x 10- of P in the growth liquid (5.25). At this point, the average stress in the epitaxial layer is minimized as measured by plotting the change in the position of the Bragg angle as a function of the sample position and using X-ray topography for a measurement of substrate-layer lattice parameter difference.

D . EfJiciency of Radiative Recombination and Balance between Stimulated and Spontaneous Emission We consider a simplified case ofa space charge layer of width wand area A in a p-n junction as shown in Fig. 5.9. The recombination flux is W

Irecomb

= -qA

1

'0

u dx

(5.30)

LIGHT-EMITTING DEVICES

239

t-4 FIG. 5.9. p-n junction and barrier layer of width W (simplified model for recombination efficiency calculation).

q is the electron charge. U is the recombination rate defined by trap levels within w only, but not by bulk recombination. We can derive U from the Shockley-Read recombination mechanism: pn - nz (5.31) u = CTU,hN, n p 2n, Here u,,, is the thermal velocity; CT the conductivity; N , , the trap density; n, p are the electron and hole densities; and n, is the intrinsic density. N, can be any one of the frequency-defining impurity-defect combinations or, e.g., in the case of a amphoteric-doped LED (GaAs-(:Si)p on GaAs-(:Si)n) the element Si. For a particular VF (forward voltage) the recombination rate U has a maximum value in the barrier layer where p + n is a minimum. The electron-hole concentration has a minimum where p = n. This is the case where the Fermi level for the intrinsic semiconductor is halfway between the quasi-Fermi levels EFn and E F p .In this case

+ +

p = n = a, exp(q 1 VF )/2kT) Therefore, we get for the recombination rate a maximum value

(5.32)

(5.33)

and the recombination current is according to (5.30) (5.37)

240

HERBERT F. M A T A ~

This has to be compared to the normal bulk diffusion currents of the carriers :

(5.38) (5.39) The diffusion currents for electrons (5.38) and for holes (5.39) are governed by the respective diffusion constants D, and D, and NA and ND, acceptor and donor density as well as the diffusion lengths L, and L, of the carriers. Using (5.38) and assuming that

one has with V, 9 kT/e for the recombination ratio, i.e., the ratio of current due to space charge recombination and the diffusion current:

One can now introduce the effective lifetime of the carriers: and the diffusion length

L, = ( 0 ~ 5 , ) ’ ’ ~

(5.42)

D, = L,ZOU,,N A

(5.43)

and write With (5.40) and (5.35), this leads to a simplified form for R,,,:

The trap density N , now replaces the acceptor density because we made U,,, dependent on N , . Without the condition VF 9 kT/q, we can write for the recombination/diffusion current ratio:

(5.45) (5.46)

24 1

LIGHT-EMITTING DEVICES

A Rrec 5 0 t relative1

4 0-

30-

2.0-

1.0-

I

0.5

I

1.0

I

1.5

i.0

2'5

I

I

3.0

This is a rapidly decreasing function with V, increasing as expected because the diffusion current dominates for higher bias voltages (Fig. 5.10). To derive optimizing factors from this expression, we have to bear in mind that we started with simplifying conditions with respect to the recombination current. It can be derived from (5.44) that for injection into a n-doped region, e.g., the trap or recombination center density N , should obviously be large. Due to the integration over w (in 5.30) also the space charge width should be large. We know that this conflicts with the general fact that in laser structures, e.g., the actual recombination layer should be small to lower the laser threshold since confinement is best operative in "small " layers. These are relative terms since it is technologically difficult to achieve submicron flat layers. The conclusion from (5.44) with respect to the diffusion length is also relative since one usually measures high quantum efficiency for large values of L. But within the confines of our model, a minimization of L means that carriers should not leave the space charge region (confinement). We know that this is physically done by the heterojunction potential barriers and thus more efficiently than by a minimization of L. The junction width is generally given by

(5.47)

242

HERBERT F.

MA TAR^

where E,, is the dielectric constant of vacuum; E, the dielectric constant in barrier; and &,, the electrostatic barrier potential. This can be maximized in 111-V junctions especially GaAs where 4, is high and a graded doping can extend the space charge. With respect to the bias dependence, of R,,,, we note that V, should be small, a condition derived here from the fact that carrier transport beyond the space charge layer should be minimized. In practical devices, this condition is also combined with the fact that the loss current (i2R losses in spreading resistance) should be minimized. As to the injection efficiency or the magnitude of I,,, according to our formula (5.37), we have to add that the extreme doping levels used in LEDs generate band tailing as discussed earlier. This decreases the necessary current density for a specific efficiency or gain. According to Stern (5.25), an increased doping decreases the necessary current density for a chosen gain factor. For instance for g = 100 cm-’ at T = 300”K,one finds that the current densities shown in Table 5.111 are needed for the respective doping ranges. An increase by two orders of magnitude in dopant concentration decreases the necessary current density by one order of magnitude. TABLE 5.111 Current density (A/cm2)

Doping range (cm-3)

3 x 103 1 x 104 2.5 104 3 x 104

3 x 1019 1 1019 3 x lo’* 3 x 1017

REFERENCES FOR SECTION 5 5.1. H. Kroemer, Proc. IRE 45, 1535 (1956). 5.2. E. Spenke, “Electronic Semiconductors,” 2nd ed., pp. 155 and 164. Springer-Verlag,

Berlin and New York, 1965. 5.3. H. F. Matare, ’‘ Defect Electronics in Semiconductors,” pp. 455ff. Wiley (Interscience), 5.4. 5.5. 5.6. 5.7. 5.8. 5.9. 5.10. 5.11. 5.12.

New York, 1971. H. Kroemer, Proc. IEEE, 51, 1782-1783 (1963). M. B. Panish and 1. Hayashi, Appl. Solid State Sci. 4, 235-238. Academic Press (1974). H. Kressel and H. Nelson, Phys. Thin Films 7, 115-256 (1973). A. G. Milnes and D. L. Feucht, “ Heterojunctions and Metal Semiconductor Junctions.” Academic Press, New York, 1972. H. J. Queisser, J . Crysr. Growth 17, 169-172 (1972). S. Mader and A. E. Blakeslee, Appl. Phys. Lett. 25, 365-367 (1974). W. Heinke and H. J. Queisser, Phys. Reu. Lett. 33, 1082 (1974). V. B. Osvenskii, G . P. Proshko, and M. G. Milvidskii, Sou. Phys.-Semicond. 1, 755-760 (1967). L. Forbes and R. M. Fogle, Appl. Phys. Lett. 25, 152-156 (1974).

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243

5.13. A. R . Badzian, K. Wisniewska, B. Widaj, B. Krukowska-Fulde, and T. Niemyski, J . 5.14. 5.15. 5.16. 5.1 7. j.18. 5.19.

5.20. 5.21. 5.22. 5.23. 5.24. 5.25. 5.26. 5.27.

Crysr. Growih 5, 222-224 (1969). G. Ziegler and H. Henkel, 2. Angew. Phys. 19, 4 0 - 4 0 4 (1965). L. L. Chang, L. Esaki, and R. Tsu, A p p l . Phys. Lett. 19, 143-145 (1971). G . 0. Kratise and E. C. Teague, Appl. Phys. Lett. 10, 251-253 (1967). K. H. Zschauer, Solid State Commun. 7, 335-337 (1969). M . Ettenberg, J . Appl. Phys. 45, 901-906 (1974). A A. Bergh, 1 E E E Trans. Electron Deuices ed-18, 166-170 (1971). K. Mettler and D. Pawlik. Siemens Forsch.-Entwicklungsber. 1, 274-278 (1972). P. Petroff and R . L. Hartrnan. Appl. Phys. Lett. 23, 469-471 (1973). P. W. Hutchinson, P. S. Dobson, S. OHara, and D. H. Newman, Appl. Phys. Lett. 26, 250-252 (1975). S. Kishino. H. Nakashirna, R. Ito, and 0. Nakada, Appl. Phys. Lett. 27,207-209 (1975). G. A. Rozgonyi and M. B. Panish. Appl. Phys. Lett. 23, 533-535 (1973). F. Stern, Phys. Rev. 148, 186-194 (1966). L. Esquivel, S. Sen, and W. N. Lin, J. A p p l . Phys. 47, 2588-2603 (1976). R . L. Bell and A. F. W. Willoughby, J . Mater. Sci. 1, 66 (1966).

APPENDIX TO

SECTION

5

The general relation for the gain in a system with spontaneous and induced transitions is derived from the classical case of blackbody radiation. In considering the interaction between photons and electrons in the semiconductor body, one is interested in the probability ratio for emission and absorption of photons. The classical model for the emissivity of radiation applies, where the number of cells gi per phase space V/h3 (I/ is the volume considered and h is Planck’s constant (see Ref. I ) , is

A = 2(4np2 d p ) V/h The factor 2 is added due to two possibilities of polarization. p = E/c = hvjc is the momentum vector. Similarly, the number of electromagnetic modes in a solid of refractive index n, in a frequency band dv is (2) N,, dv

=

2(4zq2 d q )

(2)

where q = n, v/c = nr/A.

(3)

Since the group velocity ug of a wave package is V,

= dv/dq

(4)

we have N,, = 8?rnfv2/c2v,

(5)

HERBERT F. MATARB

244

Emission and absorption may be considered to be governed by the same coefficient ko when a spectral linewidth Av, is specified. The probability of a photon being absorbed in unit time is then

KO u g For n, photons, the rate is

KO Vg n, If two energy levels are considered, the lower occupied, the upper one empty, e.g., and the system has a positive probability of emission Pe/Pa = (nv

+ 1)/nv

(8) n, is the number of photons per mode, P, is the probability of emission, and Pa is the probability of absorption, then the rate of spontaneous absorption is given by (7), while for the opposite case (lower level empty, upper level occupied) the rate is K Ou s . We now express the spontaneous emission rate in the modes [Eq. (2)] that interact with electrons in the respective levels and define (3)f, as the occupational probability for an electron in the upper level and fh as the occupational probability for a hole in the lower level ; 3

r, Av, =f, fh K Oug N, Avs (photons/sec/unit volume) ( n = 1, Pauli exclusion principle). For stimulated emission the rate is f e fhKOUgnvNv

(9)

Avs

and the rate for photon absorption is (l -fe)(l -fh)KOugn,Nv Avs The difference between (10) and (11) is the absorption coefficient:

(11)

A = KO[1 - f e - f h l ( 12) In terms of the chosen level scheme, a gain coefficient related to the spontaneous emission rate can be defined:

g=-A

=

K o [ f ,+fh - l ] / r , Av,

(13)

and the gain is G = gr,

With (9) we have

and with ( 5 )

(14)

LIGHT-EMITTING DEVICES

245

To apply this general equation to a semiconductor, one associates the upper level with E , (conduction band) and the lower level with the valence band E , . fe and f h are associated with the respective Fermi-Dirac statistics:

+ expC(Ec - Ce)/kTI}f h = {l + exp[(ih - E v ) / k T ] ) - ' fe

= (1

(17)

In general, these statistics apply also when shallow levels are involved, as long as the carrier collision times are short compared to their recombination times. Introducing (17) into (16) leads to a useful expression for the gain:

hv - AC 1 - exp ____ kT where hv = E , - Ev is the photon energy and A t = Ce - C h is the quasiFermi level separation. Equation (18) is applicable to all states that can be associated with these quasi-Fermi levels and that contribute to the emission of photons of frequency v. The gain is positive according to (16) when

Generally the arguments leading to the condition for negative absorption are simplified by stating that the two rates of stimulated emission Re,s t and absorption Ra.st are = Afe(l

-fh)

= Afh(1

-fe)

(21) (A is a constant accounting for density of states in both bands, the transition probability, and the photon flux)$, andf, are again the occupation probabilities of the energy states at or near EE and E , . E , - E , = hv or the photon energy of radiation. For an amplification of the photon flux, Re,,, must exceed R , , , ; thus Ra,

st

or As Bernard and Duraffourg ( 4 ) stated originally, radiative recombination will exceed absorption when

246

HERBERT F. MATARE

REFERENCESFOR APPENDIX I. See, e.g., L. Brillouin, “Quantum Statistics.” Springer-Verlag, Berlin and New York, 1931. 2. P. A. M. Dirac, “The Principles ofQuantum Mechanics.” Oxford Univ. Press (Clarendon), London and New York, 1958. 3. T. S. Moss, G. J. Burrell, and B. Ellis, ‘*SemiconductorOpto-Electronics.” Butterworths, London, 1973. 4. M. G. A. Bernard and G. Duraffourg, Phys. Status M i d i 1, 699 (1961).

6. METHODS OF JUNCTION FORMATION

While the 111-V compounds allow for a multiplicity of compositional variations and thus band gap variation in connection with p - n junction formation as we have seen, they are not so easily amenable to diffusion as a method of junction formation as are the elemental semiconductors. There are several reasons for this. Above all, diffusion processes are relatively long time methods since the resulting impurity profile depends on the time t and deeper penetration requires longer time. This also means that the semiconductor is subjected to relatively high temperatures during longer periods. Even such stable lattices as those of the homopolar crystals, like germanium and silicon, suffer from extended diffusion processes, and much research and development has been done to minimize “ process-induced” defects in these materials. The movement of vacancies from the surface to the interior of the crystal is a decisive agent for atomic redistribution by a substitutional mechanism. An important magnitude is the self-diffusion coefficient. This coefficient is high for 111-V compounds as compared to silicon and especially germanium. The diffusion coefficient can be expressed by the simple formula

D = Do exp( -Q/kT)

(6.1) where Do is a constant and Q is the activation energy for diffusion. D is known to be a factor of 2-3 higher for As in GaAs than for Si in Si a t 1200°C. D for Ga in GaAs is an order of magnitude higher than D for Si in Si. For As in GaAs, at higher temperature D is even larger. Seeger and Swanson (6.2) point out that a vacancy as an acceptor has a higher solubility in a donor environment (e.g., in l O I 9 cm-3 doped n-type germanium) than in p-type material. In the case of 111-V compounds like GaAs, the diffusion of external impurities will be accompanied by an even stronger self-diffusion and probably vacancy migration, which have a destructive effect on the crystal stoichiometry since the energy for the formation of a Frenkel defect is only -9 eV in GaAs as compared to 12-14 eV in Si and Ge respectively. We shall see that all this establishes limitations as regards the choice of

LIGHT-EMITTING DEVICES

247

diffusants in 111-V compounds, and that long time and deep diffusions, as they are known in silicon technology, are generally excluded in 111-V device technology. Therefore, the methods of epitaxial deposition either by vapor phase or liquid phase epitaxy or finally molecular beam deposition are much more suited to this type of material. It is interesting that this particular technology cannot be applied to the homopolar crystals because no miscible compounds exist that could form graded epitaxial layers. This is one of the reasons why the technology of silicon on sapphire (or spinel) has encountered so many difficulties. We can, therefore, deal with diffusion in a cursory manner since it is of incidental importance only for 111-V compound devices, especially LEDs. O n the other hand, the other methods of junction production and crystal formation have to be discussed thoroughly in this case. A . DSffusion

In spite of its complex lattice structure, GaAs has relatively good stability when subject to diffusion processes. In comparison to Ge and Si, the self-diffusion coefficient is much higher, and stoichiometric and other defect production much more probable; but when maintaining a sufficient partial pressure of the components, especially arsenic, the diffusion can give relatively useful results. In this case, the concentration profile follows the usual error function: C(X, t ) = C , erfc(x/2JDt) 2 x/2(Dt)'/z erfc = 1 - erf = 1 exp( - t2)d t ~

711'2

(6.2)

s,

D is the diffusion coefficient and t the time. In case ofa fixed surface concentration C, as produced by a gallium melt on the surface,

Von Munch has pointed out already (6.2) that the diffusion coefficients for GaAs vary in wide limits. This is due to the complex mechanism of diffusion in compounds. The substitutional diffusion takes place via the sublattice for gallium atoms in the galluim sublattice and for arsenic within the arsenic sublattice. The activation energies for these processes are quite different (As substitution, % 4 eV; gallium substitution, = 2.5 eV), and it is certain that the diffusion process entails within the basic lattice stoichiometry variations that

248

HERBERT F. MATARE

depend on the partial pressure of the components on the surface of the crystal subject to diffusion. It is therefore customary to maintain especially the arsenic partial pressure at a high value during diffusion of impurities. In the case of LEDs, only very shallow diffused layers are used, and in this case diffusion time and/or temperature are so low that damage to the crystal is less important, even if no external partial pressure is maintained. Some techniques of closed and semiclosed tube diffusion with zinc are based on this fact. The normal outdiffusion is accompanied by vacancy transport, and the equilibrium with the outer partial pressure defines the number of vacancies created, which are the vehicle for substitutional diffusion. If, e.g., the As partial pressure is high enough to maintain predominantly the As, species, the constant k of the mass action law for the vacancies, [ha][

‘As]

=

(6.4)

is found as the product of k l and k , in the individual vacancy formations of the partial lattices:

Obviously, the increase in As pressure is accompanied by an increase in substitutional diffusion in gallium sites and a decrease in substitutional diffusion in arsenic sites. In the case of amphoteric impurities like silicon and germanium in GaAs, the situation is more complex, and depends on temperature and As concentration. (In the presence of gallium, as in liquid epitaxy, e.g., silicon and germanium move preferably into As sites, a feature discussed in more detail in Section 6,C.) More complex cases are those where both substitutional and interstitial diffusion take place, as is the case with zinc, an important diffusant in the production of LEDs. Penetration values as given by the relation (6.2)with a time dependence of x: x

‘v

( D p

(6.7)

are valid only for unchanged doping of the base material. This explains the rather wide range of measured diffusion constants in GaAs. The preferred elements used as donor diffusants are sulfur, selenium, and tellurium. Elements used as acceptors are magnesium, zinc, cadmium, and manganese. Silicon and germanium as amphoteric impurities in GaAs can be incorporated both ways, and special methods have been developed to direct this incorporation in one way or the other.

249

LIGHT-EMITTING DEVICES

The group VI elements of the periodic chart, S, Se, and Te, can easily form independent compounds with gallium; and if used in higher concentration, will decompose the GaAs at the exposed surface ( Co > 10" cm- '). To control the partial pressure of these elements, one uses compounds rather than the elemental form: for sulfur: Al,S,, As2S2,As$,, Ga,S,, GeS,, GeS,, In,S,; for selenium: Al,Se,, As,Se,, As,Se, , MoSe,; for tellurium: Al,Te, .

)I

I

l

I

l

I

I

d.6

0!7

I

1

I I

0.8

I I

0.9

I I

I

1.0

1.1

I

107~0~ FIG. 6.1. Diffusion coefficient D (cm'jsec) versus temperature for the major dopants in gallium arsenide.

The diffusion constants for some important diffusants are plotted in Fig. 6.1. Tin (group IV) is preferably used as a donor element and is not subject t6 the conditions of the other elements since it does not attack the GaAs surface.

250

HERBERT F.

MATARB

1

XCUI

FIG.6.2. Concentration profile for tin diffusion in gallium arsenide. From von Miinch (6.2).

Figure 6.2 shows a typical concentration profile for tin diffusion (concentration versus penetration x) which follows the complementary error function. Measured values are very close to this curve (6.2). For the other elements, elaborate methods of surface protection with oxides ( S O , A1,0,) have been worked out in connection with the production of transistor structures. Since these techniques are of minor importance for optoelectronic devices (LEDs), we do not consider them here [for details, see, e.g., von Munch (6.2)j. The case of acceptors as diffusants leaves a smaller choice since group I1 contains the elements beryllium (Be), magnesium (Mg), cadmium (Cd), and mercury (Hg), and zinc (Zn). Of these only cadmium and zinc have technical importance so far.

LIGHT-EMITTING DEVICES

25 1

In the case of zinc, the most important impurity for p-type diffusion, we also have a dependence on the type of diffusion, i.e., the alternative possibility of substitutional or interstitial mechanisms of transport. If the zinc ), diffusion (gallium concentration is small (I 3 x 10l8 ~ r n - ~substitutional replacement) is predominant and the diffusion profile is given by Eq. (6.2). In this case, the zinc atom (substitutional) Zn, combines with a gallium vacancy YGa(6.3): Zn,

+

VGa

4 Zn,,

+ e+

For higher zinc concentrations, deviations from (6.2) are seen, as shown in Fig. 6.3 for Ga + 1% Zn source (see also Table 6.1). Here interstitial

FIG. 6.3. Concentration profiles for zinc diffusion from three differently concentrated Ga + Zn sources. For Ga + 1 y; Zn. deviation from a complementary error function. From von Munch (6.2).

252

HERBERT F. MATARE

TABLE 6.1 ZINCCONCENTRATION A N D DIFFUSION COEFFICIENT I N GaAs"

Source G a + 1%Zn Ga + 0.1 % Zn Ga + 0.01% Zn

Zn concentration C , (cm- ') at GaAs surface no sio, layer

SiO, layer, 6500 A thick

1.3 x 1019 2.7 x 10" 1.3 x 10''

3 x 10lR 6 x 10'' 1.6 1017

Diffusion coefficient D in GaAs (cm'isec) no SiO, layer G a + 1%Zn Ga + 0.1 % Zn Ga + 0.01% Zn

7 x 10-11

2.9 x lo-'' 2.3 x lo-''

SiO, layer 6500 A thick 2.2 x 7.1 x 1 0 - 1 3 3.5 x lo-"

From von Munch (6.2).

diffusion contributes to the penetration, and the total concentration is the sum of two concentrations Ci + C, (Ci being interstitial concentration and C, substitutional concentration) or the effective diffusion constant is

D, is the diffusion constant for substitutional transport; Di the diffusion constant for interstitial transport; and the fractions of the concentrations are the probabilities for substi tu tional (respectively, interstitial) movement. It can be shown that for high Zn concentrations, the diffusion coefficient is proportional to the square of the zinc concentration. If one maintains also a high arsenic partial pressure, D is rather independent of the concentration, and the diffusion is effective at rather low temperatures. [See Fig. 6.1 for ZnAs, (6.3a).] Elaborate methods have been worked out to diffuse Zn and Sn from doped SiO, (Table 6.1) and to apply the planar technology to GaAs. Such methods are useful in connection with display devices where alfanumerical patterns are deposited on larger crystals. Experience gained by the work in transistor technology has been usefully applied to the production of LEDs. This is also evident from the work by Strack et al. (6.4). These authors also pointed out that iron-sulfur diffused junctions result in higher device yield. At an early date, they also concluded that liquid phase epitaxy is superior to other methods in the production of n-p layers in GaAs. A technique for applying doped SiO, layers to GaAs via Si(C,H,O), and the corresponding compounds for the dopants, like Sn(C,H,), and

253

LIGHT-EMITTING DEVICES

Zn(C,H,), , has been developed for transistors, but has the drawback of oxygen diffusion, a danger for light-emitting junctions. Pure SiO, masking on the other hand is less damaging when sufficient Zn or Sn surface concentration is applied to the etched out regions since the diffusion process can be shortened. Most of the modern light-emitting devices based on GaAsP are therefore produced in closed or semiclosed diffusion runs (Figs. 6.4 and 6.5). The oxide thickness is decisive for dopant penetration, especially for Zn diffusion; lateral diffusion along the SO,-GaAs interface can partially defeat the masking. Si,N, layers are better masks, but need higher deposition temperatures of 750-1000°C. Results of surface concentration and diffusion coefficients in GaAs for different sources have been given by von Munch (6.2) (Table 6.1).The use of aluminum oxide Al,O, as a masking agent has also been studied, and it was found that Al,O, will protect the 111-V compound from decomposition in an open tube diffusion, while it is transparent to zinc as a diffusant. The A1,0, layer was applied using an organic aluminum compound such as triisobutyl aluminum and triethoxy aluminum. The best zinc diffusion source is ZnAs, . This open or semiopen tube diffusion can be carried out at 700°C and yields a relatively high zinc surface concentration under the A1,0, layer (6.3~). 0

0

C

0

0

0

n u

Ga+Zn

000 0

0

0

0

w 0

T2 >T FIG.6.4. Closed tube diffusion: W, wafer; C , heater coil; Q, quartz tube.

C 0

0

0

0

0

0

0

0

0

0

0

0

0

0

T >T2 FIG. 6.5. Semiclosed tube diffusion: W, wafers; C, heater coil; S, spacer.

254

HERBERT F. MATARE

B. Chemical Vapor Deposition Vapor deposition in general was applied early to 111-V compounds (6.4). The difficulty of bringing down on a substrate at equal rates two or more components of different vapor pressure has led to a number of approaches, among these “flash evaporation.” It was recognized also that there is a critical temperature for the substrate. If the temperature is too low (e.g., < 400°C for GaAs), no oriented growth occurs. If the temperature is too high (e.g., > 530°C for GaAs), one observes reevaporation of the more volatile component (group V element). The temperature of the substrate has rather precise limits because (1) supersaturation is necessary, but should be low enough for multiple bonds to be stable (not single bonds); (2) the substrate lattice temperature should supply enough vibrational energy to the substrate atoms to form free surface bonds for epitaxial growth. Walton has defined an epitaxia1 temperature from the activation energy of the process of cluster formation (6.5):

T=

u + QD k In(Ra2/v)

One defines R as the rate of incidence of atoms on substrate; T as the temperature; a, the distance between absorption sites; v, the frequency of vibration; U , the binding energy of an atom to a cluster; Q D , the binding energy of a single atom to the surface; and k, is the Boltzmann constant. This results from the statement that the rate of incidence on the surface element (a2)is given as Ra2 = v exp( U

+ QD)/kT

(6.10)

In vacuum deposition utmost care has to be taken to deposit on a clean surface. Sputtering and/or ion cleaning have been used in conjunction with deposition and annealing. The complications arising from these steps and the relatively complex equipment needed have led to a more frequent application of chemical vapor deposition where a catalytic action of the substrate surface increases the efficiency of monocrystallization. In this latter case, the surface can actually be cleaned by a reactive carrier gas, and nucleation is enhanced by the formation of vapor-IiquidJiquidsolid drops. The frequency of nuclei formation is even enhanced in catalytic nucleation of this type (6.6).Because of the success of CVD in industrial epitaxy, wide applications of this method have covered all interesting semiconductors. Even in the case of silicon, heteroepitaxy on foreign substrates (spinel, sapphire) has shown results in terms of material parameters: mobility values

LIGHT-EMITTING DEVICES

255

close to the bulk value. Here, however, anisotropy across the layers and high defect density are unavoidable in two-dimensional deposition (lattice mismatch), while in 111-V compound growth, graded deposition allows for a gradual addition of one or more components and therefore a change to another semiconductor without the damaging high density of defects. The combination of CVD and LPE has also been used for the main semiconductors. In this case, one speaks of VLS growth (vapor-liquid-solid) and the layer grows out from a “solvent” or “agent” on the substrate that is in contact with a vapor saturated with the deposited component or compound (6.7). The case of CVD has led to wide application, especially on the basis of halide vapor transport. Monocrystalline layers of good perfection are grown on related substrate materials, a particular advantage in the case of 111-V compounds. Substrate materials are available in monocrystalline form for all basic compounds, as mentioned. The most widely used combination is GaAsP on GaAs. The matching problems reported in early work (6.846) have been overcome to a considerable degree by graded growth. Dislocation structures and pits, however, are always a criterion for layer quality. But it seems that the high degree of doping in LEDs (in the high 10’’-cm- range) neutralizes to some degree the electronic properties of dislocation networks. Epitaxial layer perfection and matching to the GaAs substrate gradually improved as more became known about growth conditions. Important progress was made due to the relatively simple recipe for first growing a GaAs layer on a GaAs substrate. As we have discussed, the epitaxial layer perfection improves with thickness over and above the substrate perfection by lateral outgrowth of dislocations. Upon growth of a few microns of an epitaxial layer, the gas flow is gradually changed and PH, is added to the continuous ASH, and H, flux. Gallium is generally present in the reaction tube in liquid form or is arranged within the mixing chamber at a point of higher temperature. In this case, HCI is also injected into the reaction tube, generating gallium subchlorides. In some cases, gallium is maintained at a temperature sufficient to keep the Ga partial pressure at the substrate high enough for a reaction. Another advantage of this system is the controlled addition of impurities for doping purposes. For n-type doping, selenium can be added as gaseous hydrogen selenide, and zinc as p-type dopant can be added in the form of vapor. A typical epitaxial reactor arrangement is shown in Fig. 6.6. Relative high mobilities have been measured, and Fig. 6.7 shows p e as a function of doping density (electron carrier density). Similar open flow systems and good mobilities of CVD GaAs have been reported by other authors (6.9, 6.10).

256

HERBERT F. MATARE

SUBSTRATE

I I

HCI + H2 EXHAUST FIG.

6.6. Typical set-up for chemical vapor deposition (CVD) and reactants used.

Many variations of substrate-epilayer resistivity relations have been made. For some devices, a high resistivity substrate is used as an insulator support for doped layers; in other cases one wants to deposit films of low carrier concentration on highly doped substrates. Orientation dependence of the layer quality was studied (6.11)and it was found that films grown on the (TIT)and (100) planes were of higher perfection than films grown on the (1 11) and (I 10) planes. It had been reported earlier that the B side of compound crystals has better properties for perfect growth. Much work has been done to take advantage of the highly insulative character of compensated GaAs (chromium doped) and to form selective areas of epitaxial deposition, so-called islands, a process known from SO, insulated silicon islands. This method was first considered an important variant for discrete devices or groups of devices that could be interconnected as in LSI (large scale integration) (6.12).In the case of LED displays, oxide (Si,N,) masking and selective diffusion turned out to yield sufficient separation since these devices work at high currents and low voltages. The formation of defects as twins and edge dislocations (6.13a,b) the dependence of the deposition rate on substrate temperature (6.14 ) , and impurity behavior in the grown layers have been studied (6.15). Other aspects of the problem are surface structure and its dependence on the substrate holder. It was shown that quartz is attacked by gallium, especially in sratu nascendi,and that graphite is a preferred substrate holder in this case (6.16, 6.21).

LIGHT-EMITTING DEVICES j.+[crn2/V

257

sec]

f 10000-

80006000-

40004 2000-

FIG. 6.7. Electron mobility measured as a function of carrier density in chemical vapor deposited GaAs.

Rapid progress was made in carrying out successful epitaxial deposition of other important compounds. In,,,Ga,,,P was deposited from separate gas streams of GaCl and InCI, while HCl and P H 3 were used as additional reactants with the dopant. Photoluminescence with a maximum in the spectral range of 6300 8, and compatibility with GaAs as a substrate make this compound technically interesting (6.17). The use of organometallic compounds for CVD, first introduced by Manasevit and further refined (6.18, see literature in this paper) by Ito et al., has been proposed to decrease the substrate temperature requirements and eliminate the HCI flow system, which has complications with respect to the early reactions in the presence of gallium in quartz (see Ref. 6.16). Good quality GaAs was grown on GaAs substrates, but InAs was also grown on GaAs in this way (6.19). While all these epitaxial layers can be grown by LPE and there is indication of superior layer perfection, some material combinations are not feasible otherwise than by CVD. So far, aluminum nitride has been grown only by CVD. Sapphire is used as the substrate material. One way that was successfully applied is the simultaneous deposition of trimethyl aluminum (CH,)3Al and ammonia in the presence of H, at 1200°C (6.20). In general, CVP is still the most important method for modern LED production, especially of the green, yellow, and red indicator species (6.21), while LPE is of greatest importance for infrared LEDs and laser diodes (see the next section).

258

HERBERT F.

MA TAR^

Finally, we have to mention a very important area of heteroepitaxy: the CVD of GaAs on germanium and of GaP on silicon. The lattice match in either case is very good: GaAs : 5.654 A

Ge :

5.658 8, Aa = 0.004 8,

a=-=:

Aa a.

0004 = 7 x lo-’% 5.658

which corresponds to initially lo7 dislocations/cm’

a=-=;

Aa a.

GaP:

5.451 A

Si:

5.431 A

002 2 0.00369 N 3.7 x 10- % 5.431

or about 5 x lo* dislocations/cm’ at the interface. This is a much closer fit than, e.g., silicon on sapphire or spinel (4.23). It has been a research program for some time to deposit a suitable III-V compound on one of the well-known homopolar semiconductors because one could then use well-established crystal growth facilities for these crystals. Since their size and form is well in hand, one needs only the gradual deposition of GaAs/GaAsP or other compound combinations based on GaAs (or G a P in the case of silicon) to a thickness generally below the 30-pm mark for the active layer. The normal method is based on a carefully cleaned germanium surface subjected to hydrogen chloride transport epitaxy in which either the compound GaAs is directly introduced into the reaction chamber, or hydrides and chlorides or metalloorganic compounds of the components are introduced (6.22, 6.23). In the case of GaAs on Ge, one has to avoid as much as possible autodoping by the Ge substrate because germanium as an amphoteric impurity causes trapping of charge carriers and a decrease in radiative recombination if it interferes within the junction area where abrupt p-n changes are required. It has been found that HCI as carrier gas etches the substrate and carries Ge into the epitaxy. Therefore, recently progress has been made by a process without extrinsic HCI using the compounds ASH,, P H 3 , AsCI,, and gallium.

259

LIGHT-EMITTING DEVICES

+

+

Some results were reported with TMG (trimethyl gallium) PH3 ASH,; however, the purification of TMG from spurious impurities like Si, S, Fe, Cu, Zn, 0, and C seems to be a problem. Devices made by this process had a low radiative efficiency. For a device of the kind, shown in Fig. 6.8,

Ga Asoh Poa ( 5 x 10’: n )

Px (graded 1

Ga As,, X=0-+04

i

SUBSTRATE (GaAs) Ge(n)

FIG.6.8. LED profile as deposited in a gradual CVD process on a substrate.

several processes are applicable: (1)

~

HCI (g) + Ga (1)

_

_

_

atrn 85O’C-l0-’atm

,‘x>

_ 9, \ \

,,’

\,‘\\

, \,

80°C

-

- - _ _

0,’‘’.\

\

1 5 x 10- atm I

- - _ _

0°C - 8 x

‘x) AICI, 20°C I

v’ ,‘ , I

,

(3) W C H , ) ,

+ lo-’ atm

x‘

>:,

(2) GaCI,

’.’..

As 420°C

\

i 10-

’ atm

\

\

0’

atm

b

ASH, 1o-l atm

As mentioned, processes (2) and (3) are the preferable ones, as is obvious from the absence of free HCI during the reaction: (1) 4 GaCl

+ As, Ga(CH3), + ASH,

( 2 ) 6 GaCl (3)

+ As, + 2 H,

750-c

4 GaAs + 4 HCl

750.c

4 GaAs + 2 GaCI,

---+

700°C

GaAs + 3 CH,

HERBERT F. MATARE

TABLE 6.11 ~

No.

Year

Remarks

1962

GaAs o n G e

1963 1964

GaAs o n G e GaAs o n G e

1964

GaAs o n G e

1964

GaAs on G e

1965

GaAs on G e

1966

Ford

J . Electrochem. Soc. 113, 724 J . Appl. Phys. 31, 4687

1966

GaAs,_,P, on Ge GaAs o n G e

Honeywell I.B.M.

J . Appl. Phys. 31, 4295 Electrochem. Technol. 6, 78

1966 1968

GaAs on G e GaAs on G e

Hewlett Packard Hi tach i Hitachi

Trans. A I M E 245, 565 J . Appl. Phys. 43,1792-1798 Solid-State Electron. 16, 913 J . Appl. Phys. 35, 580 J. Phys. SOC. Jpn. 16, 2591

1969

GaAsP o n Ge G a P on G e GaAsP o n Ge GaAs on G e GaAs on G e

Authors

Company Bell

4

R. Moest and B. Shupp J. Amick M. Weinstein, R. Bell, and A. Menna T. Gabor

5

T. Gabor

Westinghouse

6

H. Holloway, K. Wollmann, and A. Joseph J. Tientjen, and J. Amick L. Bobb, H. Holloway, K. Maxwell, and E. Zimmerman R. Schulze R. Kontrimas and A. Blakeslee R. Burmeister and A. Regher H. Kasano H. Kasano

Ford

1 2 3

7 8

9 10 11 12 13 14 15

16 17

18

19 20

21

E. Muller T. Okada, T. Kano, and Y. Sasaki P. RaiChoudhury T. Saitoh and S. Minagawa H. Manasevit G. Crawford and W. Groves R. Burmeister, G. Pighini, and P. Greene T. von Munch

RCA TYCO

Westinghouse

RCA

?

Nipon Electric Westinghouse

Reference J . Electrochem. SOC. 109, 1061 R C A Reo. 24, 555 J . Electrochem. Soc. 111, 674

J . Electrochem. SOC. 111, 825 J . Electrochem. SOC.111, 817 Philos. Mag. [8] 111, 263

1972 1973 1964 1961

1969

Monsanto

J . Electrochem. SOC. 116, 1745 J . Electrochem. SOC. 120, 656 J . Cryst. Growth 13/14, 306 Proc. IEEE 61, 862

Hewlett Packard

Trans. A I M E 245, 587

1969

Technische Universitit Hannover

J. Cryst. Growth 9, 144

1971

Hitachi Autonetics

1973 1972 1973

T M G CaAs on AsGa T M G GaAsP on AsCa TMGGaAs o n Ge C V D GaAsP on GaAs C V D GaAsP on GaAs C V D review

(continued)

26 1

LIGHT-EMITTING DEVICES

TABLE 6.11 (continued)

No.

Authors

22

General Electric Hitachi

Solid -State Technol. 13, 45-50 J . Electrochem. Soc. 119, 617

Bell Bell

26

A. Bradshaw and J. Knappett S. Kishino, M. Osirima, and K. Kurata J. DiLorenzo J. DiLorenzo and G. Moore M. Weiner

21

J . Burd

Monsanto

28

T. Okada

Nipon Electric

29

H. Kressel and M. Ettenberg

RCA

J . Cryst. Growth 17, 189 J . ELctrochem. Soc. 118, 1823 J . Electrochem. SOC.119, 496 Trans. A1 M E 245, 57 1 J p n . J . Appl. Phys. 2, 206 Appl. Phys. k r t . 23, 51 1 J . Electrochem SOC. 120, 1419

23

24 25

30

s. 110,

T. Shinohara, and Y. Seki

Company

Bell

Nipon Electric

Reference

Year

Remarks

1970

CVD general

1972

E P D in C V D GaAsP

1972 1971

1969

C V D parameters G e and Si contamination Si contamination C V D GaAsP

1963

AsGa on G e

1973

Ge doping

1973

T M G o n GaAs

1972

As a summary of the work being carried out in this important area, we add a condensed list of references in chronological order (see Table 6.11). It is clear that a process based on a substrate made from an elemental semiconductor would have strong industrial implications. Above all, perfect monocrystals of a wide variety of dopants and forms are available and much of the costs and hazards of producing compound bulk semiconductor substrates could be avoided. CVD based on the GaP/Si combination has not been pursued to the extent quoted for GaAs/Ge, but it is to be expected that here also more activity will develop in time. We have to mention finally that the protection of the backside of the substrate is carried out by a layer of SiO, or/and Si,N,. In a prolonged epitaxy, however, hydrogen and HCI may reduce this layer. It is also a hazard to subject the wafer to a high zinc concentration for diffusion since these layers are partially transparent for Zn, especially in the case of thin layers (pinholes !). But since successful epitaxial island growth of GaAs by SiO, masking was achieved, one can expect good results also in the case of heteroepitaxy of GaAs on Ge. In the absence of HCI, the GaAs deposited on SiO, will react as follows (6.24): SiOz (s) + GaAs (s) = SiO (g) + Ga,O (g)

+ 3 As,

(g)

262

HERBERT F. MATARE

A mixed Si02-Si3N, layer is a rather solid protection for the substrate, and it seems that such techniques will yield viable new processes of device production in the near future.

C . Liquid Phase Epitaxy (Liquid phase epitaxy) (LPE) is the most promising method for highly perfect epitaxial growth. It has found wide applications wherever radiative recombination has to be optimized. While the original scheme was considered a simple or cheap method of junction fabrication-no complex gas flow system was needed-ontinued development and applications to the semiconductor double heterostructure lasers of low threshold values have led to more and more sophisticated methods and equipment. We shall outline the various methods used, the trends in this area of technology, and look at the results. From here, some conclusions regarding the future development are possible. The amount of literature in this field is so enormous that a careful choice had to be made to maintain an illustrative and relevant flow of information. The choice made should in no way establish a value judgment. The original scheme for successful LPE on GaAs goes back to Nelson (6.25), and many variants of this method have been discussed in the literature [see summary report on LPE by Kressel and Nelson (6.25a)I. The method using a tipping furnace in its simplest form (Fig. 6.9, without S) has

~

FIG.6.9. Tilting-furnace LPE set-up after Nelson: F, furnace; T, quartz-tube; G, graphite crucible; W, wafer; M, melt; C, clamp; S, support (sapphire or other).

LIGHT-EMITTING DEVICES

263

many shortcomings. Control of the melt saturation is difficult, and melt attack of or deposition on the substrate depends on a number of uncontrollable factors, such as temperature gradient across the package and temperature difference within the furnace. The method was practically copied when applied to other materials. Nelson applied it to GaAs and gallium melts saturated with GaAs, while others used this technique, e.g., for G a P on silicon (6.26) and GaN (6.27) originating from a reaction of nitrogen gas with molten gallium. Some variants of the method were also patented in respect to the substrate temperature gradient. Lockwood (6.28) recognized that the melt-substrate package requires a vertical temperature gradient for better growth, i.e., cooling from the substrate. His patent adds the substrate support S (Fig. 6.9), which can be a transparent sapphire window, to the Nelson scheme. In this fashion, a temperature gradient across the meltsubstrate sandwich is introduced. This is essential for melt homogenization since the melt is depleted near the substrate and requires a temperature gradient for homogenization (see later in this section under the discussion of limited melt). Already at an early date, new methods were tested to carry out LPE with better control than is possible in the rather unsophisticated tilt furnace, which, however, showed clearly that LPE has advantages. Strack et al. (6.4) reported on solution growth of GaAs and used already the so-called “dipping technique” (Fig. 6.10) to grow GaAs islands onto insulator gallium arsenide substrates. This method was developed subsequently, and Lorenz (6.29) has applied this technique already to the growth of GaAlAs/GaAs devices with variable band gap. The fact that melt depletion can be avoided by a temperature gradient in the melt (see Ref. 6.28) was patented by Stone (6.30), while other methods concentrated on the production of thin epitaxial layers (6.31). All these methods were not suitable however for growth of complex or multiple layers as needed for the production of heterojunction lasers. The horizontal sliding boat or multiple bin graphite boat method for sequential deposition was apparently first developed and applied in 1968 to GaAlAs/GaAs lasers by a group at Bell Laboratories [see Ref. 6.32 and literature cited]. Panish and Sumski in an early paper on this method (6.33)point out that for the GaAlAs/GaAs system the horizontal tipping apparatus or moving crucible type system is better due to the cleaning action of the slider and elimination of scum. Later it was recognized that this method has other important advantages due to the melt limitation and multilayer growth possibility. Figure 6.11 shows the first type of such equipment (6.33, 6.32) with the melt below the substrate. This arrangement allows a deposition of thin

264

HERBERT F. MATARE

-FURNACE -CRUCIBLE -MELT

FIG.6.10. Typical arrangement of a dipping apparatus for LPE. Quartz tube Q can be used to add dopant. From Kressel and Nelson (6.25).

layers only and can be reversed so that the melt travels over the substrate. In this form, it is in use in many places for the production of multiple layers as in the case of double heterostructure lasers (Fig. 6.12). Many researchers implemented this method with respect to melt saturation (6.34) and monocrystallinity (6.35) avoiding edge-dendrite growth by covering the substrate leading edge during epitaxy. In order to understand the complex systems parameters in LPE, phase equilibria have to be known, especially for the important systems GaAs/Ga and GaP/Ga. As Thurmond noted (6.36), the liquid phases of these systems are not “regular” solutions (regular is defined as referring to those solutions

265

LIGHT-EMITTING DEVICES i

H2

G

TH

Q FIG. 6.11. Tipping LPE apparatus [after Panish and Hayashi (6.32)]:Q, quartz tube; G , graphite boat; L. liner; S, slider (graphite); R, ram (fused silica); W, wafer; TH, thermocouple. From Panish and Hayashi (6.32).

for which the heat of mixing is proportional to the atom fractions of the two components and the excess entropy of mixing is ideal). The most important system is the GaAIAs/GaAs combination. Figure 6.13 shows the gallium corner of the liquidus isotherms in the Al-Ga-As phase diagram, and Fig. 6.14 gives the solidus isotherms for this system (6.32). As Panish and Hayashi note, GaAs cannot exist in equilibrium with a Ga-Al-As melt. But, when GaAs in contact with a Ga-A1 melt is heated, GaAs dissolves until the liquid reaches a ternary composition on the liquidus isotherm in the phase diagram (Fig, 6.13). Here the point reached is defined by the initial GaAl ratio. Each point on the liquidus isotherm (at a particular temperature) defines the liquidus in equilibrium with a specific ternary solid solution of Gal - .AI,As. It is apparently this compound a t the GaAs-melt interface that protects the GaAs substrate from further dissolution. Liquid phase epitaxy has now been applied to a great variety of compounds and has been the decisive step in most cases where enhanced radiative recombination is the issue. This is the reason for the enormous activity to bring this technology into the realm of real mass production. Before dealing with these efforts, which have created a wealth of published detail (6.37), we shall review briefly the efforts to apply LPE to other than GaAIAs/GaAs compound combinations. G a P as a basic material for visible LEDs was discussed earlier. Zn, 0-doped type LPE layers can be grown in a Nelson-type tilting furnace arrangement and yield LPE layer thicknesses in the 30-60-pm range with varied doping levels in the 10’6-1017-cm-3 range (6.38). InAs, -,Sb, was also grown by LPE on InAs monocrystals (6.39). The method used in this case is the so-called “steady state liquid phase epitaxy technique” (Fig. 6.15). Here the melt functions as the transport medium

266

HERBERT F.

MATARB L

F

SOURCE

SEED

F

T

L

T

STOP

(b) FIG.6.12. (a) Complete cycle tipping equipment for multiple layer growth [after Panish and Hayashi (6.32)]:F, furnace; L, gradient liner; T, fused silica tube; GS, graphite slider bar; GB, graphite barrel; TH, thermocouple. (b) Melts 1-5 and temperature cycle for melt sequence.

from a source of InAs to the substrate. The difficulty with these methods lies in the surface morphology of the LPE layers, as we shall discuss in the section on limited melts. The important compound In, - $a,P was also synthesized and p-n junctions fabricated by the tilting method (6.40). Substrates used were Si, InP, Gap, and GaAs. But the author states that the quality of the grown layer was poor on all substrates except [ 1113 oriented GaAs. This is not astonishing in view of the facts that lattice mismatch is considerable and that a large melt without the necessary temperature gradient was used.

267

LIGHT-EMITTING DEVICES

ATOM % Al

FIG. 6.13. Liquidus isotherms of the Al-Ga-As system near the Ga corner of the phase diagram. From Panish and Hayashi (6.32).

I

09 a8 lo: 0.70.6-

0504-

0.3-

020.1

1

Ib -3

I

A

X

a

FIG.6.14. Solidus isotherms in the Al-Ga-As system.

,

1-1

268

HERBERT F. MATARE FURNACE

t

X

FIG.6.15. Schematic diagram ofa “steady state LPE” apparatus used to grow InAs, xSb, on InAs substrates. G, graphite crucible; TH, thermocouple; M, melt (In + As + Sb); InAs( 1). source; InAs(2). substrate.

Interesting combinations of layers were grown by LPE using the already mentioned good lattice match between Si and G a P and Ge and GaAs (6.41). This case is of importance since it could lead to use of high quality homopolar substrate crystals for 111-V layer growth. In,Ga, -,As has been grown by LPE on GaAs substrates in a horizontal sliding crucible system (6.42). The measured band gap in electron volts at 300°K as a function of stoichiometry (x) is given in Fig. 6.16. It has also been shown that phosphorous added to the melt will substantially improve the perfection of the epitaxial layers in this case. The final Ga,In, - .As (x = 0.8) layer is of better quality if grown onto a quaternary layer of Ga,In, - .AsyPl - y . The latter is continuously graded from the GaAs substrate to the final Ga,In, - ,As layer (6.43).It had escaped these authors that such methods had been used earlier and that stress compensation had been measured as the decisive reason for the improvement of the layer morphology (6.44). There are other more exotic methods for LPE, e.g., the “centrifugal tipping” method where substrates are wetted by a melt in rotation (6.45). The advantage is a regular distribution of a thin melt over the substrate surface. We shall discuss this in connection with the importance of thin melts. Most important is progress with respect to the morphology of the epitaxial layer since flat and faultless surfaces are an important condition for the industrial use of LPE. In this respect, recent progress with respect to the

269

LIGHT-EMITTING DEVICES

I

0

1

I

0.05

0.1

I

0 15

1

0.2

I

0.25

- X

InAs MOLE FRACTION

FIG.6.16. Band gap of In,Ga, _,As at 300°K as a function of x. Line equation given fits most measured data from spectra of photoluminescence or photoreflectance. Spread between lines-measured data.

application of Peltier heating/cooling to LPE has to be mentioned. The need to maintain a sufficient temperature gradient across the melt-substrate interface is at the root of some of the problems of surface morphology. Here a current flow across the sandwich is the ideal means ofcreating a temperature gradient difficult to realize by external means. Current densities in the range 10-40 A/cmZ are sufficient for epitaxial layers of up to 60-pm thickness with external oven temperatures much below normal values (as low as 600°C) (6.46).In combination with Peltier heating of one side and cooling of the other side of the melt, a driving force is set up, tending to move the melt in the direction of the heat gradient (away from the cool interface). There may be some influence of ionic field migration on the melt (6.47). The Limited Melt in LPE As mentioned before, surface morphology of epitaxial layers is of primary importance for the industrial use of these layers. In LEDs, certain geometries are needed, as in alphanumerical devices; also contact evaporation works best on flat surfaces. Dicing and bonding need flat surface textures for successful contacts, and lasers necessitate flat active regions. It is a

270

HERBERT F. MATARE

well-known fact that LPE surfaces suffer from growth inhomogeneities. Terrace structures, wavy surfaces, and microcrystallites are found with orientation variations in the form of polygonization and hillock formation (6.48). The probability of a flat surface structure is much enhanced by the use of small or “limited” melts. This is due to the influence of “constitutional supercooling.” This expression describes the effect of inhomogeneous freezing of the melt when depletion of one component leads to local solidification and nonstoichiometric growth. Some authors maintain that surface morphology depends only on substrate surface perfection and orientation within 5’ of (100).A more detailed analysis of the available data, however, shows that only very thin layers at or below the 1-pm mark are flat, and that thicker layers suffer from terrace growth steps even for perfect substrate surface orientation (6.49). This has to be expected since thicker layers have to be grown from thicker melts, and in these cases the melt is depleted near the substrate. Therefore, a compensating temperature gradient has to be set up across the melt. Thicker epitaxial layers are necessary in many industrial applications since the epitaxial surface must eventually allow for an alloying step, a diffusion step, a lapping operation, passivation, etching, etc. Also, the perfection of the upper layer improves during growth, as we have seen, and therefore thick epitaxial layers are desirable. In addition, control of substrate orientation within 5 arc minutes is not a cheap process, and it seems that this is not even necessary when “limited melts” are used. By “limited” we understand the minimum melt size necessary for build-up of a particular layer thickness, e.g., 25 pm. Minden (6.50) was the first researcher to point out clearly that one cannot expect faultless growth in a system where constitutional supercooling can occur. He refers to prior work by Tiller (6.51)who showed that interface growth is dependent to a remarkable degree on constitutional supercooling and that instabilities result, as Ga inclusions, voids, terraces, etc., unless the temperature gradient across the melt is sufficient. Minden also points out that a limited melt would greatly reduce the need for a strong temperature gradient across the melt. He bases his considerations on the fact that constitutional supercooling can be avoided when the following condition for the concentration gradient holds: aC d C d T -<-ax - d T dx

(6.1I )

where d C / d T is the slope of the liquidus line at the interface temperature. The temperature gradient therefore has to be sufficiently high to balance the flux of solute atoms at the solid-liquid interface. The diffusion flux is given by J = - D SC/dx (6.12)

27 1

LIGHT-EMITTING DEVICES

with D, e.g., the diffusion coefficient of As in gallium. Equation (6.11) expressed in terms of the necessary temperature gradient is

-dT> - -d C d T dx - a x d C

(6.13)

Now considering the diffusion equation

D a2c/ax2 = aclat

(6.14)

with D being the diffusion constant of, e.g., As in Ga(l), the boundary conditions for a large size melt (unbounded) are (1)

C=Co,

(2)

lim x-E,

(3)

t=O

ac -=0

for all t > 0

ax

1dC

c at

--

=

1 ~

forallx

= const

at x

= 0,

(6.15)

all t

In the bounded case (limited melt), one has: (l)C=Co,

t=0,

allx

(2) a C / a x = O ;

x

all t

(3)

lac

--= at

c

=W;

1 ~., x = O

>o

(6.16)

atallt>O

Here t = toTo/T

(6.17)

At temperature 17; the initial concentration is C, for all x. The factor /To is the furnace cooling rate. A small cooling rate means To/K or t large. In discussing the solution of the diffusion equation (6.18) Minden shows that condition (6.13)combined with the solution (6.18) leads to two very different functions for aT/ax in the unbounded and bounded cases. In the first case, the required temperature gradient iricreases continually. In the bounded (limited) case, the temperature gradient asymptotically approaches a fixed value. It appears that this result is a sufficient explanation for the fact that large size melts cause gallium inclusions and surface imperfections, characteristic for constitutional supercooling occurring near the surface of the forming epilayer.

272

HERBERT F.

MA TAR^

In practice, the use of limited melts had been introduced some time ago. In horizontal crucible or sliding crucible methods, only a thin layer remains on the substrate, thick enough, however, to form layers of several microns thickness (6.52, 6.53). A design to decant the melts and to form so-called "aliquot melts" was first indicated by researchers from Bell Laboratories (6.54). The system allows decanting of a defined portion of the melt and carrying it over to the substrate surface. In this way, a saturated melt of larger size can be retained at a point of higher temperature, and a portion of it is brought in contact with the substrate at another lower temperature. These authors also calculated the minimum temperature gradient dT,/dx required across the melt of thickness w at the end of the deposition to avoid constitutional supercooling, using Minden's analysis and solution for the bounded case. The result of their analysis confirms the importance of the limited melt. The necessary temperature gradient across the melt of thickness d

(dT/dx)d = dT increases so strongly that a melt of more than 1-mm thickness requires a gradient that is very difficult to realize (45°Cacross 1 mm corresponds to 100-125"C/in.). Tables 6.111 and 6.IV show these values calculated for G a P and GaAs (GaAIAs melt). One can see that dT across the melt or (dT/ax)d increases considerably with melt thickness. An increased cooling rate also increases the necessary temperature gradient. However, limited melts allow higher cooling rates. This occurs to the point where the cooling rate equals the cooling range. If (dT/dx)d > AT, constitutional supercooling occurs. (See 2-mm melt a t TABLE 6.111" GaP Melt thickness d(mm)

T range AT("C)

0.5

108

Melt depletion (%)

Cooling rate (T/min)

Gradient ("C/mm)

67 33 17 8 3

3.6 1.3 0.6 0.3 0.1

6 4.5 4.1 2.9 1.1

Across melt dT ("C) ~~

1.o

2.0 4.0 12.0

40 18 8 3

~

~

0.3 4.5 8.2 11.6 13.6 ~~

Total cooling time, 30 min; LPE layer thickness, 20 pm; initial q ,1040°C; range AT = T - T( (T( is the substrate T a t end of deposition).

273

LIGHT-EMITTING DEVICES

TABLE 6.IV GaAs (GaAIAs)

(dT/dx)d at cooling rates ('C/min) d (mm)

0.5

1 .o

2.0

Cooling range AT ("C)

0.5 1.0 2.0 4.0

0.41 1.66 6.7 29.4

0.83 3.3 1 13.9 65.3

1.66 6.66 29.4 166.0

150 50 22 10

.

-v-

_r

dT

29.4"C/min cooling rate. Cooling rates and temperature ranges for deposition (AT)are adjusted for a 20-pm thick layer growth in 30 min.) An important conclusion from these data is that the limited melt allows for higher cooling rates if the temperature gradient across the melt is sufficiently high. Gradients of l-S"C/mm or 10-50"C/cm can be realized easily in normal, horizontal or vertical melt ovens. This fact combined with reasonable cooling rates makes the limited melt an important development in epitaxy. There is constant progress in LM-LPE (limited melt liquid phase epitaxy) and methods have been devised to overcome the difficult control mechanisms of melt transport decanting and doping with simultaneous temperature control. For the case of GaAlAs/GaAs, a special process of LED junction formation is based on a melt, saturated with the necessary ternary compound (Al) and a dopant (e.g., Zn) which also affects a melt-back of the substrate (GaAs) which is doped with an amphoteric impurity (e.g., Si) which upon dissolution in the gallium-rich melt converts to a p-type impurity (6.55). In this way, a one-step process can be designed such that the complete, but limited melt is brought onto the wafer, and the package is simply subjected to an external temperature profile. In this case, a vertical oven with its normal temperature gradient will allow programmed movement to optimize the layer growth during which a p - n junction is created. Siemens researchers (6.56) have designed a similar method for Gap. Here the melt is also limited and saturated in a melt-back procedure from where outgrowth occurs. The dopants are, however, introduced by the gas flow system, in this case zinc and nitrogen (as NH,). There are of course some problems in those cases of combined action of impurities, e.g., the transition temperature for silicon (change from n- to

274

HERBERT F. MATARE

S M

I I

A

D(IPANTS

H

I

I"

-H.

Y

G

S

D

LIGHT-EMITTING DEVICES

215

p-type impurity) decreases with increasing amount of aluminum in the lattice (6.57). There are other problems related to the solubility of silicon in GaAs with increasing A1 amount. While the solubility of Si decreases, the solubility of zinc increases with increasing amount of Al. These problems have been dealt with in more detail recently (6.58, 6.59). Finally, we put together in Fig. 6.17 a survey of the most important techniques used in LM-LPE. It remains to be seen which technique will ultimately be used for mass production of LEDs. In the case of multiple structures (lasers), the sliding crucible apparatus is the best solution at this time.

D. Molecular Beam Epitaxv A newer method which is related to vacuum evaporation, but allows more control, is molecular beam deposition. The development of this method, especially for 111-V compounds, was primarily carried out by A. Y. Cho, Bell Laboratories. Molecular beam epitaxy (MBE) is distinct from evaporation inasmuch as focused beams of each of the elements, forming the compound in question, are directed toward the target (substrate) where they combine. In order to keep these molecular beams controllable, one works in ultrahigh vacuum. It seems at first glance that this method is ideally suited for the deposition of such multiple layers of varied compounds as needed in DH lasers. We shall discuss certain limitations at the end of this section. The first application of this method was the synthesis of GaAs with a defined amount of dopant. The large difference in vapor pressure between the constituents does not interfere in the deposition since the sticking coefficients are both near unity because a surplus of one species increases the sticking coefficient for the other species. Figure 6.18 shows the typical set-up as described by Ctio (6.60). In a series of precise experiments concerning the FIG.6.17. Limited melt epitaxy techniques. LM-LPE: ( a ) Semicylindrical rotation (180’). Wetting of wafer W by melt M during rotation (Donahue and Minden, 1970). (b) Centrifugal tipping. W. wafer; M, melt; G, graphite crucible; S, shaft (rotating); C, coil; Q, quartz-tube (6.45). (c) Single-slice graphite slider. Decanted or aliquot melt A. G , , G , . G , , graphite boats or sliders (6.53).(d) Aliquot melt A from main melt M over wafer S (seed crystal). After epitaxy A is pushed into dump well D (6.54). (e) High capacity LM-LPE apparatus: S, substrates; M, melt; C. coil; R, rotation handle. (Lorimor. Saul, Dawson and Paola, 1973). (f) Apparatus for isothermal solution mixing growth: C, cylinder with flat for wafer W; G, graphite crucible; M, melt; R, rotational axis; D, opening for melt (dopant) introduction. (g) Limited, stationary melt in temperature gradient A T and temperature program T = . f ( t ) ; G , graphite plate (eventually with holes for doping): W, wafer (635.6.56). ( h ) Peltier induced LPE: G, graphite crucible; M, melt; Q, quartz tube; S. stainless steel rod + contact; BN. boron nitride insulating layer (6.46).

276

HERBERT F. MATARE r - EAI OUADRUPOLE

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formation of monocrystals, the molecular sticking coefficient, the photoluminescence response, the doping profiles obtained by impurity depositions from additional beams, and layer mobility and resistivity measurements, Cho established the viability of this method especially for the growth of thin and complex epitaxial layer structures. One difficulty is the fact that for GaAs those dopants with reasonable solubility have high vapor pressure and low sticking coefficients; e.g., zinc is difficult to evaporate. Also gold and tin show difficulties when evaporated onto p-type GaAs, for example for Schottky barriers (6.61). The method is ideally suited, however, to growing periodic structures (superlattices) like GaAs/GaAs, -,P, with alternate layers by pulsing PH, in the reaction tube (6.62), and dielectric wave guides for integrated optics (6.63). Other applications are concerned with microwave devices, IMPATT diodes, varactors, and the like (6.64, 6.65, 6.66, 6.67). With respect to the main application considered here, it is important to know that also sequences of ternary compound layers have been deposited successfully and that GaAs-Al,Ga, -,As double heterostructure lasers have been prepared (6.68).However, threshold values were high as compared to LPE-generated structures. An annealing step improves the characteristic considerably and

LIGHT-EMITTING DEVICES

277

also decreases the forward resistance, which is generally high in MBEdeposited junction devices. The problem encountered here centers around the crystallographic order and the number of nonradiative recombination centers. Stoichiometric variations are probable at 625°C for the substrate temperature. The necessary annealing step at 750-850°C shows that crystallographic order is improved by the higher temperature treatment, which corresponds to the growth regime of heterojunctions in LPE. There is general difficulty in doping correctly and at low substrate temperature in order to keep the sticking coefficient high, which interferes with monocrystallization that would be optimal at substrate temperatures well above the temperatures used. Doping profiles for Sn, Si, and Ge in GaAs also depend on substrate temperature and show larger deviations from the increases (6.69). profile of the dopant intensity in the beam, when Kubslrale Therefore, an additional feature was introduced into MBE recently, i.e., ionization of the molecular beam. Zinc was successfully deposited in ionized form. In this way, the sticking coefficient was enhanced and an important molecular species normally used as dopant was added to the groups of dopants applicable to MBE (6.70). It remains to be seen whether further experiments will implement this type of approach, which could also be applied to the compounds themselves and allow an increase in substrate temperature. It seems important to induce short-range crystallographic order in these films grown with rather abruptly changing interfaces. It has been found that stress is not the reason for the lesser efficiency of MBE-deposited laser structures. Dingle and Wiegmann (6.71) have shown that there are other reasons for the need for an annealing step. They also point out that the dramatic reduction in the lasing threshold upon annealing may have to do with nonradiative recombination centers that disappear at higher temperature ( > 800°C). We believe that this difference between MBE and LPE touches on an old, well-known difference between thin films and bulk crystals. As early as 1950, semiconductor researchers wondered why vacuum evaporated germanium crystals were not comparable to bulk material in the form of Czochralski-grown boules. And even today, we find researchers who wonder why evaporated thin film crystals with good mobility values cannot compete with bulk crystals. We have to state that mobility alone is not the decisive value. Mobility, homogeneity, and lifetime are already more demanding from a crystallographic order point of view (6.58). Even CVD (chemical vapor deposition) with the advantage of catalytic growth cannot compete with bulk crystal growth

278

HERBERT F. MATARE

from the liquid state as far as perfection is concerned (6.6). (Twodimensional, planar deposition). It is still a research project to find ways and means for depositing thin layers of a structural perfection comparable with bulk crystals. REFERENCES FOR SECTION 6 6.1. A. Seeger and M. L. Swanson, in “Lattice Defects in Semiconductors” (R. R. Hasiguti ed.), pp, 93-130. Penn. State Univ. Press, University Park, 1968. 6.2. W. von Munch, Tech. Phys. Einzeldarst. 16, 588 (1969). 6.3. M. B. Panish and H. C. Casey, 1.Phys. Chem. Solids 28. 1673-1684 (1967). 6.3a. E. Adachi and K. Saito (Hitachi). U.S. Patent 3,794,533 (1974). 6.4. H. A. Strack et a/., “Gallium Arsenide Transistor Studies,” Final Tech. Rep. AFAL-TR-66-361. Air Force Avionics Lab., Texas Instruments, Inc., December 1966. 6.5. D. Walton, Philos. Mag. [8] 7, 1671-1679 (1962). 6.6. H. F. Matare, Sci. Electr. 15, 95 and 129 (1969). 6.7. R. L.. Barns and W. C. Ellis, J. Appl. Phys. 36,2296-2301 (1965). 6.8a. C. M. W o k , C. J. Nuese, and N. Holonyak, Jr., J. Appl. Phys. 36, 3790-3801 (1965). 6.8b. J. J. Tietjen and J. A. Amick, J. Electrochem. Soc. 113, 724-728 (1966). 6.9. D. Efier, J . Electrochem. Soc. 112, 1020-1025 (1965). 6.10. Z. I . Alferov, V. I. Korolkov, M. K. Trukan, and S. P. Chashchin, Sou. Phys.-Solid Stare 7, 1915-1918 (1966). 6.11. R. R. Moest, J . Electrochem. Soc. 113, 141-146 (1966). 6.12. D. W. Shaw, J. Electrochem. Soc. 113,904-908 (1966). 6.13a. H. Holloway and L. C. Bobb, J. Appl. Phys. 38, 2893-2896 (1967). 6.136. S. Kishino and. S. Iida, J. Electrochem. Soc. 119, 1113-1 118 (1972). 6.14. D. W. Shaw, J. Electrochem. Soc. 115, 40-408 (1968). 6.15. R. C. Taylor, J. Electrochem. Soc. 118, 364-368 (1971). 6.16. K. Richter, J . Cryst. Growth 17, 207-211 (1972). 6.1 7. H. Kressel, C. J. Nuese, and I. Ladany, J . Appl. Phys. 44, 3266-3272 (1973). 6.18. S. Ito, T. Shinohara, and Y.Seki, J. Electrochem. Soc. 120, 1419-1423 (1973). 6.19. B. J. Baliga and S. K. Ghandi, J. Elecrrochem. Soc. 121, 1642-1650 (1974). 6.20. J. K. Lin, K. M. Lakin, and K. L. Wang, J . Appl. Phys. 46, 3703-3706 (1975). 6.21. G. 9. Stringfellow and D. Kerps, Solid State Electron. 18, 1019-1028 (1975). 6.22. H. Holloway, K. Wollmann, and A. S. Joseph, Philos. Mag. [8] 11, 263-276 (1965). 6.23. L. C. Bobb, H. Holloway, K. H. Maxwell, and E. Zimmerman, J. Appl. Phys. 37, 4687-4693 (1966).

6.24. 6.25. 6.25a. 6.26. 6.27. 6.28. 6.29. 6.30. 6.31. 6.32. 6.33.

P. Rai-Choudhury and D. K. Schroder, J. Electrochem. Soc. 118, 107-110 (1971). H. Nelson, R C A Rev. 24, 603 (1963). H. Kressel and H.Nelson, Phys. Thin Films 7, 155-256 (1973). G. A. Antypas et al., U.S. Patent 3,810,794 (1974). F. Z. Hawrylo et a/., U.S. Patent 3,811,963 (1974). H. F. Lockwood, US. Patent 3,785,884 (1974). M. R. Lorenz, US. Patent 3,677,836 (1972). L. E. Stone, U.S. Patent 3,785,885 (1974). R. Deitch, US. Patent 3,791.887 (1974). M.B. Panish and I. Hayashi, Appl. Solid State Sci. 4, 235-238 (1974). M. B. Panish and S. Sumski, 1. Phys. Chem. Solids 30, 129-137 (1969).

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6.34. M. Ettenberg et al., US. Patent 3,767.481 (1973). 6.35. D. P. Marinelli et al., US. Patent 3,825,449 (1974). 6.36. C. D. Thurmond, J. Phys. Chem. Solids 26, 785-802 (1965). 6.37. G . M. Blom, S. L. Blank, and J. M. Woodall, eds., “Liquid Phase Epitaxy.” NorthHolland Publ., Amsterdam, 1974. 6.38. R. H. Saul and W. H. Hackett, Jr., J. Electrochem. Soc. 117, 921-924 (1970). 6.39. G. B. Stringfellow and P. E. Greene, J. Electrochem. Soc. 118, 805-809 (1971). 6.40. B. W. Hakki, J. Electrochem. SOC. 118, 1469-1473 (1971). 6.41. F. E. Rosztoczy and W. W. Stein, J . Electrochem. Soc. 119, 1119-1 121 (1972). 6.42. R. E. Nahory, M. A. Pollack, and J. C. DeWinter, J . Appl. Phys. 46, 775-782 (1975). 6.43. H. Nagoi and Y. Noguchi, Appl. Phys. Leu. 26, 108-110 (1975). 6.44. G. A. Rozgonyi and M. B. Panish, Appl. Phys. Lett. 23, 533-535 (1973). 6.45. S. Y. Lien and J. L. Bestel, J . Electrochem. SOC.120, 1571-1573 (1973). 6.46. J. J. Daniele, Appl. Phys. Lett. 27, 373-375 (1975). 6.47. D. T. J. Hurle, J. B. Mullin, and E. R. Pike, Philos. Mag. [8] 9, 473 (1964). 6.48. E.g., B. L. Mattes and R. K. Route, J. Cryst. Growth 27, 133-141 (1974). 6.49. E. Bauser, M. Frik, K. S . Loechner, L. Schmidt, and R. Ulrich, J. Cryst. Growth 27, 148-153 (1974). 6.50. H. T. Minden, J. Cryst. Growth 6, 228-236 (1970). 6.51. W. A. Tiller and C. Kang, J . Cryst. Growth 2, 345-355 (1969). 6.52. B. I. Miller, E. Pinkes, 1. Hayashi, and R. J. Capik, J. Appl. Phys. 43, 2817-2826 (1972). 6.53. J. M. Blum and K. S. Shih, Proc. IEEE 59, 1498-1502 (1971). 6.54. A. A. Bergh, R. H. Saul, and C. R. Paola, J. Electrochem. Soc. 120, 1558-1563 (1973). 6.55. H. F. Matare, Solid State Technol. 15, 41-45 (1972). 6.56. C. Wyrich, G. H. Winstel, K. Mettler, and M. Plihal, Inst. Phys. Conf: Ser. 24, 145-154 (1975). 6.57. W. G. Rado, W. J. Johnson, and R. L. Crawley, J. Appl. Phys. 43, 2763-2765 (1972). 6.58. H. F. Matare, Crit. Rev. Solid State Sci. 5, 499-545 (1975). 6.59. D. R. Campbell and K. K. Shih, Appl. Phys. Lett. 19, 330-333 (1971). 6.60. A. Y. Cho, J . Vac. Sci. Technot. 8, S31-S38 (1971). 6.61. A. Y. Cho and 1. Hayashi, J. Appl. Phys. 42, 4422-4425 (1971). 6.62. A. Y. Cho, Appl. Phys. Lett. 19, 467-468 (1971). 6.63. A. Y. Cho and F. K. Reinhart, Appl. Phys. Lett. 21, 355-356 (1972). 6.64. A. Y. Cho and H. C. Casey, Jr., J. Appl. Phys. 45, 1258-1263 (1974). 6.65. A. Y. Cho and F. K. Reinhart, J. Appl. Phys. 45, 1812-1817 (1974). 6.66. A. Y. Cho. C. N. Dunn, R. L. Kuvas, and W. E. Schroeder. Appl. Phys. Lett. 25,224226 (1974). 6.67. A. Y. Cho and W. C. Ballarny, J. Appl. Phys. 46, 783-785 (1975). 6.68. A. Y. Cho and H. C . Casey, Jr., Appl. Phys. Lett. 25, 288-290 (1974). 6.69. A. Y. Cho, J. Appl. Phys. 46, 1733-1735 (1975). 6.70. M. Naganuma and K. Takahashi, Appl. Phys. Lett. 27, 342-344 (1975). 6.71. R. Dingle and W. Wiegmann, J. Appl. Phys. 46, 4312-4315 (1975).

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AUTHOR INDEX Numbers in parentheses are reference numbers and indicate that an author’s work is referred to although his name is not cited in the text. Numbers in italics show the page on which the complete reference is listed. Andersen, T., 162, 169 Anderson, C. P., 78, 107 Andersson, C., 118, 131, 169 Andersson, G., 117, 141, 158, 159, 161, 169, I75 Antypas, G. A., 263(6.26), 278 Appleman, E. H., 16, 35 Archer, R. J., 211(4.29), 212(4.29), 215(4.29), 222 Arcipiani, B., 140, 169 Arndt, R. A,, 50, 51, 54 Artsimovich, L. A,, 145, 169 Asaad, W. N., 60, 106 Ast, T., 24. 35 Aston, F. W., 114, 151, 169, 173 Atkinson, S. J., 94, 103, 105 Aubert, J., 127, 169 Ausloos, P. V., 15, 16, 21, 37, 39 Aysto, J., 130, 169

A

Aagaard, P., 161, 169 Abbati, I., 89, 105 Aberg, T., 63, 73, 107 Aberth, W. H., 16, 35, 36 Abrahamsen, P., 144, 169 Adachi E., 252(6.3a), 253(6.3a), 278 Adams, D. B., 83, 85, 96, 99, 105, 107 Adams, M . J., 191(3.2), 195 Adler, I., 72, 111 AtTrossman, A., 92, 94, 105, 107 Akerman, R. J., 25, 35 Albridge, R. G., 74, 80, 96, 105, 106 Alcocer, A. E., 103, 1 10 Aldrich, H . S., 26, 36 Alekseevsky, N. E., 145, 169 Aleshin, V. G., 89, 109 Alexandre, K., 144, 169 Alferov, Z . I., 215(4.35), 222, 255(6.10), 278 Alfonso, R. A,, 98, 1 I 1 Al-Joboury, M.I., 57, 105, 1 1 1 Allan, C. J., 72, 76, 77, 80, 95, 96, 99, 105 Allison. D. A., 76, 77, 95, 99, 105 Allison, H. W., 221 Allison, J., 95, 105 Allison, J. F., 42, 48, 49, 50, 51, 54 Almen, o.,120, 121. 122, 123, 132. 133, 154, 165, 169 Almhof, J., 83, 105 Alton, G., 167, 173 Alvager, T., 118, 120, 123, 148, 150, 158, 169, I76 Amarel, I., 131, 169, 173 Ambruster, R., 155. 172 Amick, J. A., 255(6.8b), 278 Amiel, S., 129, 131, 169, 172, 176 Amy, J . W., 25, 35, 103, 105 Anbar, M., 16, 35, 36 Andersen, C . A., 8, 9, 35

B Bacarella, A. L., 102, 103, 108 Bachmann, K., 132, 169 Bachrach, R. Z., 71, 105 Badzian, A. R., 237(5.13), 243 Baer, Y.,75, 76, 87, 103, 105, 108, 111 Baerends, E . J., 83, 96, 105 Bagus, P. S., 75, 96, 105, 107 Bahadur, K., 16, 38 Bahrami, H.,137, 169 Bainbridge, K. T., 140, 170 Baitinger, W. E., 24, 35 Bakale, D. K., 10, 35 Baker, A. D., 59, 63, 80, 106, 111 Baker, C., 80, 101, I 1 I Baldeschwieder, J. D., 20, 35 Balducci, G., 25, 35 Baliga, B. J., 257(6.19), 278

28 1

282

AUTHOR INDEX

Ballamy, W. C., 276(6.67), 279 Balle, T., 85, 111 Banic, G. M.,143, 170 Bank, W., 62, 106 Barber. M., 98, 106 Barbett, E. E., 66, 111 Barns, R. L., 255(6.7), 278 Barrie, A., 65, 93, I06, I I I Barron, R., 16, 36 Barsanti G., 140, 169 Barz, A., 62, 106 Basu, D., 165, 170, 171 Batinger, W . E., 103, I05 Bauer, E., 60, 106 Bauer, R. S., 65, 90,106 Baumann, H., 160, I70 Bauser, E., 270(6.49), 279 Beachey, J. E., 25, 38 Beaufils, J. P., 94, 108 Beck, D., 34, 35 Beckey, H. D., 16, 17, 35, 36 Beggs, D. P., 20, 35 Begley, R. F., 32, 35 Beiduk, F. M.,140, 170 Bell, R. L., 236(5.27), 243 Bell, W. A,, 149, 172 Bell, W. A,, Jr., 143, 170 Bendt-Nielsen, B., 115, 173 Bennett, J. R. J., 137, 170 Bennett, S. L., 25, 26, 35, 37 Benninghoven, 4.. 8, 9, 35, 91. 106 Berenyi, D., 60,106 Bergh, A. A., 238(5.19), 243, 272(6.54), 275f6.54). 279 Bergmark, T., 10, 39, 59, 69, 74. 75, 76, 84, 96, 101, 106, 110, 1 1 1 Bergstrom, I., 155, 158, 164, 165, 170, 171, 172 Bergstrom, S . A. L., 78, 107 Berkeyheiser, J. E., 207(4.19), 221 Berkowitc J., 16, 26, 35, 36, 99, 106 Bernard, M. G. A,, l91(3.1), 195, 245, 246 Bernas. R., 117. 120, 122, 131, 133, 144, 149, 150, 161, 169, 170, 171. 173 Berndtsson, A., 60, 67, 103, 108 Berry. R. S., 78, 106 Berthou, H., 92, 106 Bertinger, R.. 207(4.18). 221 Beske, H. E.. 8, 35

Besson, J. M., 209(4.24), 221 Bestel, J. L., 268(6.45), 275(6.45), 279 Bethge, K., 160, 170 Betteridge, D., 59, 63, 69, 78, 101, 106 Beynon, J. H., 20, 24, 25, 35 Bhattacharya, R. S., 165, 170 Bickel, W. S., 163, 170 Bickelhaupt, F., 85, 110 Bieri, G., 85, 106 Bigeleisen, J., 118, I73 Binghan, F . W., 6, 35 Bishof, P., 84, 106 Bjorkquist, K., 166, 171 Blair, F. J., 15, 36 Blakeslee, A. E., 233, 242 Blank, S. L., 237(6.37), 279 Blewett, J. P., 127, 170 Blickensderfer, R. P., 18, 36 Blodgett, K. B., 133, 173 Blom, G. M.,265(6.37), 279 Blum, J. M., 272(6.53), 275(6.53), 279 Bobb, L. C., 256(6.13a), 258(6.23), 278 Bock, H., 80, 106 Bodor, N., 83, 106 Boers, A. L., 6, 39 Bogt, M.,151, 170 Bogh, E., 116, I70 Bonneele, J. P.,94, 108 Borg, S., 131, 156, 158, 170 Bouchez, R., 151, 170 Bourgelas, F. N., 25, 39 Bouriant, M., 151, 170 Bowers, M. T., 23, 35 Bradley, J. N., 34, 35 Bradley, R. C., 7, 35 Braicovich, L., 89, 105 Brarnwell, S.. 206(4.13), 221 Brandt, R., 130, 131, 177 Brandt, W.. 166, 170, 176 Brasch, J . W., 68, I 1 1 Brernser, W., 59, 72, 95, 106 Brillouin, L.. 243( 1). 246 Brinen, J . S., 100, 106 Brion, C. E., 71, 99, 1 1 1 Brogli, F., 85, 106 Bromander. J.. 164, 170 Brostram, K. J., 170 Brown, F., 118, 135, 136, 154, 155, 158, 161, 165, 170, I72 Brown, F. C., 71, 105

AUTHOR INDEX

Brown, H., 160, 171 Brown, H. L., 16, 36 Brown, M. D., 163, 173 Bruce, G., 123, 154, 165, 169 Brundle, C. R., 63, 65, 67, 80, 84, 91, 94,

283

Cederbaum, L. S., 109 Cermak, V., 20, 38 Chaban, E. E., 15, 38 Chadwick, D., 85, 107 Chandler, G. G., 101, 106 105, 106, 111 Chang, C. C., 12, 36, 60,107 Buchen, J. F., 148, 171 Chang, L. L., 230(5.15), 237, 243 Buchta, R.,164, 170 Chantereau, E., 151, 170 Burger, K., 99, 106 Chapin, D. M., 42, 54 Burgman, J. 0.. 161, 169 Chashchin, S. P., 255(6.10), 278 Burhop, E. H. S., 13. 19, 38, 60, 106 Chau, F. T., 99, 107 Burlingame, A. L., 2, 3, 36 Chaumont, J., 131, 169, 173 Burmeister, R. A., 206(4.12), 211(4.30), Chavet, I., 120, 133, 134, 135, 146, 151, 171 212(4.30), 221, 222 Chen, B. H., 83, 106 Burness, V. H., 95, 106 Cheng, K. L., 77, 107 Burnham, R. D., 215(4.33, 4.39, 222 Cheng, T. M. H., 20, 36 Burns, R. P., 25, 36 Chernow, F., 137, 169 Burrell, G. L., 186(2.5), 187(2.5), 188(2.5), Cho, A. V., 275, 276(6.61, 6.62, 6.63, 6.64, 189(2.5), 206(2.5), 195, 198(2.5), 244(3), 2 6.65, 6.66, 6.67), 277(6.68, 6.69), 279 Busch, G., 71, 106 Chong, S. L., 20, 36 Butler, J. F., 209(4.25), 222 Christman, S. B., 15, 38 Bye, P., 63, 106 Chupka, W. A., 15, 16, 24, 25, 35, 36 Byer, R. L., 32, 35 Chynoweth, A. G., 185(2.1), 190(2.1), 195 Citrin, P. H., 63, 73, 107 Clark, D. T., 85, 96, 99, 102, 105, 107 Coad, J. P., 93, 107 C Cbbic, B., 124, 151, 171 Coburn, J. W., 7, 36 Cain, L. C., 86, 87, 99. 107 Cocke, D. L., 25, 26, 36 Cairns. J., 163, 170 Cohen, M. J., 18, 36 Cairns, R. B., 15, 36 Colby, B. N., 10, 35 Calawa, A. R., 209(4.24), 210(4.25, 4.26), 221, Cole, C., 24, 37 222 Colligon, J. S., 5, 7, 36, 123, 171 Caldwell, C. W., Jr., 6, 39 Collin, J. E., 74, 84, 107, 109 Camac, M., 141, 170 Colton, R. J., 99. 110 Campagna, M., 71, 106 Cooks, R. G., 20, 24, 25, 35 Campbell, D. R., 275(6.59), 279 Cooper, B. C., 7, 39 Campbell, J. C., 222 Conen, R. L., 75, 111 Camplan, J., 123, 144. 150, 169, 170, 171, 175 Connaday, S. S., 48, 54 Capik, R. J., 272(6.52), 279 Connor, J. A., 98, 106 Carlson, G. A,, 25, 39 Contour, J. P., 94, 107 Carlson, T. A., 59, 72, 76, 78. 86, 87, 94, 99, Cossignol, C., 149, 171 106, 107, 108 Cothern, C . R., 96, 103, 107, 109 Carter, G., 5, 7, 36, 123, 171 Cotte, M., 141, 171 Carver, J. C., 59, 69, 86, 87, 99, 102, 106, Couchet, G., 128, 171 107, 108 Cough, R. A,, 129, 173 Casey, H. C., Jr., 210(4.28), 211(4.28), 222, Covington, M. A., 27, 30, 37 276(6.64), 277(6.68), 278, 279 Covrick, P. J., 2, 36 Castaing, R., 9, 36 Cox, R. E., 2, 36 Cavell, R. G., 96, 97, 107 Craford, M. G., 222

284

AUTHOR INDEX

Cramer, J. G., 146, 171 Crawford, M. C., 215(4.33), 222 Crawley, R. L., 275(6.57), 279 Crocker, I. H, 160, 171 Cross, W. G., 141, 171 Crowther, B. M., 114, 137, 174 Cunningham, J. G., 94, 107 Cusacks, L. C., 26, 36 Cuthill, J. R., 89, 107

D Dagenhart, W. G., 145, 171 Dahl, P., 116, 170 Dalgarno, A., 20, 38 Dalton, J., 15, 36 Damico, J. W., 16, 36 Danby, C. J., 16, 36 Daniele, J. J., 269(6.46), 275(6.46), 279 Daniels, J., 60,107 Dantet, H., 129, 130, 171 DAuria, J. M., 129, 130, 171 Davies, J. A., 154, 155, 165, 166, 167, 170, 171, 172, 173, 174 Davis, D. W., 107 Davis, L. P., 93, Y6, 98, 108 Davis, R. E., 109 Dawton, R. H. V. M., 118, 120, 128, 157, 171 Deal, J. B., 148, 171 Dearnaley, G., 165, 166, 171 DeCorpo, J. J., 32, 34, 36, 39 Deglass, W. N., 94, 102, 107 De Graaf, H., 85, 110 de Heer, F. J., 162, 171 Dehmer, J. L., 26, 36, 99, 106 Deitch, R., 263(6.31), 278 de Jong, M., 123, 165, 174 de Lima, D. X., 155, 171 Delwiche, J., 74, 84, 107, 109 DeMaria, G., 25, 35, 36 De Michelis, B., 89, 105 Dempster, A. J., 117, I71 Demyanov, A. V., 142, 176 Denhartog, J., 165, 171 Denimal, J., 155, 172 Denison, D., 137, 169 Depaus, R.,25, 39 Der, R. C., 163, 171 Deschamps, J., 85, 108 Dewinter, J. C., 268(6.42), 279

Dey, S. D., 165, 170 d'Huysser, A., 94, 108 Dibeler, V. H., 15, 16, 36 Dickinson, T., 94, 107 Dill, D., 78, 107 Dillard, J. G., 14, 20, 36, 38, 95, 106 Dimmock, J. O., 209(4.26), 222 Dingle, R., 277, 279 Dionisio, J. S.,123, 155, 171, 175 Dirac, P. A. M., 243(2), 246 Dixon. R. N., 85, 107 Djerassi, C., 13, 38 Dobbyn, R. C., 89, 107 Dobrin, R., 170 Dobson, P. S.,238(5.22), 243 Dole, M., 18, 36 Domashwvskaya, E. P., 80, 99, 109 Domeij, B., 165, 166, 171 Donnelly, J. P., 209(4.25), 222 Dougherty, R. C., 15, 36 Dousson, S., 151, 170 Dreyer, J., 129, 177 Dropesky, B. J., 148, 171 Drowart, J., 25, 36, 37 Druaux, J., 122, 133, 170, 171 Drummond, I . W., 60, 73, 103,107 Duke, R. E., 98, 108 Dunbar, R. C., 23, 37 Dunjic, B., 140, 171 Dunn, C. N., 276(6.66), 279 Dunn, G. H., 24, 39 Duraffourg, B., 191(3.1), 195, 245, 246 Durham, J. L., 108 Duxbury, G., 85, 107 Dyachikhin, Yu. A,, 142, 176

E Eastman, D. E., 78, 87, 88, 107 Eastwood, T. A., 160, 171 Ebel, H., 69, 107 Ebel, M. F., 69, 107 EFfer, D., 255(6.9), 278 Egelhoff, W. F., 78, 107 Eisenberger, P. M., 63, 73, 107 Ekstrom, C., 160, I71 Eland, J. H. D., 16, 36 Elbek, B., 155, 157, 171, 172 Eldridge, G., 137, 169 Elison, F. 0..83, 108

AUTHOR INDEX

28 5

Ellis, B., 186(2.5), 187(2.5), 188(2.5), 189(2.5), Fluck, E., 99, 106 206(2.5), 195, 198(2.5) Fluit, J. M., 123, 165, 174 Ellis, W. C., 244(3), 246, 255(6.7), 278 Fogdall, L. B., 48, 54 Eltenton, G. C., 33, 36 Fogel, Y.M., 8, 37 Entner, P., 26, 39 Fogle, R. M., 235(5.12), 242 Erickson, N. E., 89, 207 Foltz, R. L., 15, 37 Eriksson, L., 165, 166, 167, 171, 174 Foner, S. N., 32, 34, 37 Errock, G. A,, 60, 73, 103, 107 Fontijn, A., 14, 37 Esaki, L., 230(5.15), 237(5.15), 243 Forbes, L., 235(5.12), 242 Escard, J., 94, 107 Formann, E., 154, 172 Esquivel, L., 236, 243 Fort, R. C., 83, 1 I 1 Esteve, P.,116, 136, 172 Fortner, R. L., 163, 171, 172 Ettenberg, M., Jr., 237(5.18), 243, 264(6.34), Foucher, R., 131, 144, 169, 172 279 Fournet-Fayas, I., 123, 175 Evans, C. A., Jr., 9, 10, 11, 35, 36 Foyt, A. G., 209(4.25), 222 Ewald, H., 131, 176 Franklin, J. L., 20, 26, 36, 37, 38 Ewan, G. T., 158, 170 Freeman, A. J., 75, 107 Freeman, J. H., 116, 119, 123, 136, 142, 149, 151, 160, 168, 172 F Freeman, N. J., 126, 172 Freider, G., 150, 174 Fabian, D. J., 88, 92, 107 Freitag, K., 144, 151, 271 Fabricus, H., 144, 151, 271 French, J . B., 21, 37 Fadley, C. S., 59, 62, 75, 76, 78, 94, 102, Freund, F., 90,107 107, 108 Friedman, L., 20, 23, 37, 38 Fagerquist, U., 156, 159, 166, 170 Friedman, R. M., 89, 107 Fahlman, A., 10, 39, 59, 69, 96, 101. 110, 111 Frik, M., 270(6.49), 279 Fair, H., 103, 120 Fristrom, R. M., 30, 31, 32, 37 Falconer, W. D., 27, 28, 39 Fritsch, T., 129, 177 Fales, H. M., 15, 16, 17, 36 Fuggle, J. C., 92, 94, 105, 107 Fano, U., 163, 172 Fuller, C. S., 41, 54, 203(4.4, 4.5, 4.6, 4.7, 4.8). 222 Farber, M., 26, 39 Fares, V., 129, 130, 173, 176 Fumelli M., 140, 169 Farrell, T., 102, 107 Futrell, J . H., 23, 39 Fatu, C., 271 Fehsenfeld, F. C., 22, 36 Feil, D., 85, I 1 1 G Feldstein, H., 129, 172 Fellner-Feldegg, H., 66, 12 I Gammon, R. M., 159, 175 Fergau, R., 131, 173 Garcia, J. D., 163, 171, 172 Ferguson, E. E., 20, 36, 38 Card, G. A., 136, I72 Festenberg, C. V., 60, 107 Gardona, M., 89, 1 I0 Feucht, D. L., 232(5.7), 242 Gaupp, A., 164, 173 Field, F. H.. 14, 20, 35, 36, 38 Gautherin, G., 127, 169, 272, 175 Filimnov, S. I., 145, 169 Gegel, H. L., 26, 37 Finn, D., 222 Geiger, J. S., 155, 158, 165, 170, 172 Fintiman, A. F., Jr.. 15, 37 Gelius, U., 62, 66, 75, 76, 77, 78, 83, 95, 99, Fischer, A. G., 203(4.2), 221 105, 107, 208, 121 Fish, B. R., 108 Gemmel, D. S., 165, 166, 172 Fisher, I . P., 33, 36 Gerhard, W., 7, 38 Fite, W. L., 34, 36 Gershenzon, M., 185(2.4), 195, 198(2.4)

AUTHOR INDEX

Hansen, F., 132, I72 Hansen, 0.. I72 Hansen, P. G., 159, I72 Hanser, A,, 144, 172 Haque, C. A,, 11, 37 Hara, K., 215(4.34), 222 Harding, W., 206(4.13), 221 Hardy, R. W. D., 126, I72 Harland, P. W., 26, 37, 38 Harman, T. C., 209(4.25, 4.26), 222 Harris, L. A., 11, 37, 60,108 Harrison, H., 15, 36 Harrison, W. A,, 185(2.3), 195 Hartman, R. L., 238(5.21), 243 Harvey, A. B., 32, 35 Hasannudin, S. K., 63, 106 Hasted, J. B., 28, 29, 37 Hawrylo, F. Z., 263(6.27), 278 Hayashi, I., 215, 222, 228, 242, 263(6.32), 265(6.32), 266(6.32), 272(6.52), 276(6.61), 278, 279 Hayden. R. J., 117, I72 Haynes, R. M.,21, 37 Haynos, J., 50, 51, 54 Healey, R. N., 62, 107 Heden, P. F., 75, 76, 87, 105, 108, 111 Hedin, B., 141, 158, 169 Hedman, J., 59, 67, 69, 70, 75, 16, 87. 96, 101, 103, 105, 108, 109, 110, 111 Hedman, K., 10, 39 Heilbronner, E., 85, 105, 106 Heinicke, E., 160. 170 Heinke, W., 234, 242 Helmer, J. C., 63, 108, 109 Henderson, E., 33, 36 H Hengehold, R. L., 60,108 Henkel, H., 237, 243 Hackett, W. H., Jr., 365(6.38),279 Hercules, D. M.,59, 69, 100, 102, 103, 106, Hackworth. J. V.,26, 37 108, I I I Hagebe, E.,132, 172 Hercules, S. H., 103, 108 Hagstrom, S. B. M.,59, 71, 87, 102, 105, 108 Herman, Z., 22, 37 Herzog, A. H., 215(4.33), 222 Hakki B. W., 266(6.40), 279 Herzog, R. F. K., 8, 9, 37 Halbig, J. K., 148, 156, 172, 175 Hierl, P. M.,23, 24, 37 Hall, D. H., 148, 172 Hies], P. M.,23, 37 Hall, H. T., Jr., 206(4.12), 221 Higatsberger, M.J., 118, I72 Hall, M.B., 98, 106 Hildebrand, D. L., 25, 37 Hall, R. N., 215, 216, 222 Hillier, I. H., 76, 83, 98, 106, 108 Hamilton, B.,206(4.13), 221 Hinthorne, J. R.,8, 9, 35 Hammond, D., 66, I I I Hoch, M.,26, 37 Hamrin, K., 59, 69, 75, 76, 87, 96, 101, 109, Hohlneicher, G., 109 110,111

Ghandi, S. K., 257(6.19), 278 Gilles, P. G., 25, 35 Gilmartin, D. E., 25, 39 Gingerich, K. A,, 25, 26, 36, 37 Ginnard, C. R.,99, 108 Gleiter, R.,84, 106 Goring, S., 144, 151, 171, 172 GOIT, R. F., 6, 37, 91, 110 Goldhammer, L. J., 49, 54 Golob, L., 85, 108 Comer, R., 16, 37 Gonbeau, D., 85, 108 Gooch, C. H., 182(1.3), 195 Goode, P. D., 136, 172 Cora, T., 103, 110 Gorden, R.,Jr., 15, 16, 37, 39 Gorodetzky, S., 155, 172 Couch, R. A., 130, 176 Graham, R. L., 155, 158, 165, 170, 172 Grapengiesser, B., 131, 169 Greene, F. T., 25, 38 Greene, P. E.,265(6.39), 279 Griess, J. C.. 102, 103, 108 Grimblot, J., 94, 108 Grimley, R. T., 25, 37 Grimm, F. A., 74, 78, 108, I O Y Groves, W. O., 215(4.33), 222 Guernet, G., 116, 136, I72 Guest, M.F., 98, 106 Guggenheim, H. J., 76, 110,111 Guido, M.,25, 35 Guimon, C., 85, 108 Gujrathi, S., 129, 130, 171 Gunies, J., 18, 36

287

AUTHOR INDEX

Hollander, J. M., 62, 76, 107, 108, 110 Holloway, D. M., 11, 37 Holloway, H., 256(6.13a), 258(6.22, 6.23). 278 Holm, G., 156, 159, 166, 170 Holm, R., 59, 69, 80, 91, 95, 96, 97, 108 Holmen. G., 118, 169 Holonyak, N., Jr., 215(4.33, 4.35), 222, 255(6.8a), 278 Holtin, P., 116, 136, 172 Honig. R. E., 7, 8, 11, 37 Hopfgarten, F., 83. 108 Horani, M., 85, 107 Hosp. W., 207(4.20), 221 Houk, K. N., 98, 108 Huang, J. T. J , 83, 108 Hiibel, H., 156, 174 Hudis, J., 89, 107 Hudson, B. S., 35 Hudson, R. L., 32, 34, 37 Hiifner, S., 73. 74, 88, 108 Huen. T., 78, 11 1 Hughes, T. R.. 94, 102, 107 Hulett, L. D., 102, 103, 108 Hull, A. W., 125, 172 Hurle, D. T. J., 269(6.37), 279 Hurt, W. B., 85, 109 Hussain, M., 84, 110 Hutchby, J. A., 207(4.21), 221 Hutchinson, P. W., 238(5.22), 243 Huus, V., 170

I Ignatiev, A,, 60, 108 Iida, S., 256(6.13b), 278 Iles, P., 48, 54 Illgen. J., 127, 172 Inghram, M. G.. 16, 25, 36 Ishii, T., 76. 110 Ismail, H., 127. 176 Itikawa, Y.,80,108 Ito, R.,243 Ito, S., 257. 278 Itoh, H., 215(4.34), -722

J Jack, H., 170 Jadrny, R., 74, 106

Jastrzeleski, J., 131. 169 Jenkin, J. G., 88, 110 Jespergird, P., 165, 172 Johansson, G.. 10, 39, 59, 67, 69, 75, 76, 77, 95, 96. 99, 101, 105, 108, 110, I f 1 Johansson, N. G . E., 165, 166, 171, 174 Johnson, A,, 131, 169 Johnson, F. M., 128, 172 Johnson, J. E., 34, 36 Johnson, W. J., 275(6.57), 279 Jolly, W. L., 84, 95, 108, 109, 110 Jonas, A. E., 74, 78, 107, 109 Jonathan, N., 85, 108, 109 Jones, E. J., 127, 176 Jones, G. R., 84, 106 Jones, R. W., 26, 37 Jorgensen, C. K., 92, 106 Jorgensen, H. E., 144, 169 Joseph, A. S., 258(6.22), 278 Judd, D. L., 143, 173 Jungclas, H., 129, 130, 131, 173, 175, 176, 177 Jurela, Z., 8, 37 Jussell, G. J., 60, 109 Just, W., 203(4.3), 207(4.17), 221

K Kabachenko, A. P., 142, 176 Kaiser, H. J., 160, 170 Kallne, E., 63, 73, 107 Kalevi, V., 130, 169 Kaluszyner, L., 133, 170 Kamada, H., 93, 109 Kaminsky, M., 5, 37, 123, 173 Kamke, D., 173 Kanaya, K., 127, 175 Kang, C.. 270(6.51), 279 Karesek, F. W., 18, 36 Karlsson, L., 74, 84, 96, 106, 110 Karlsson, S. E., 10, 39, 59, 69, 101, 110, 111 Karmohapatro, S. B., 142, 165, 170, 171, 173 Karraker, D. G., 117, 1 74 Katrib, A,, 85, 107 Kavanagh, T. M., 163, 171, 172 Kawaktsu, K., 127, 175 Kay, E., 7 , 36 Kebarle, P., 21, 37 Keck, D. B., 182(1.1), 195 Kellerer, B., 109 Kelly, J. M., 148, 171

AUTHOR INDEX

Kelly, R., 155, 170 Kelly, W. H., 130, 173 Kemeny, P. C., 88, 110 Kemp, N. R., 63, 106 Kendrick, J., 76, 83, 108 Kerps, D., 256(6.21), 257(6.21), 278 Kerstetter, J. D., 22, 37 Kerwin, L., 139, 173 Keski-Rahkonen, O., 101, 109 Keune, D. L., 215(4.33), 222 Keune, W., 87, 111 Khan, J. M.,166, 170, 173 Khodeyev, Y. S.,87, 109 Kieser, J., 89, 109 Kilcast, D., 85, 99, 107 Kim, K. S.,93, 109 Kingdon, K. H., 128, 173 Kirchner, R., 127, 172 Kirkegaard Nieisen, A,, 162, 169 Kiser, R. W., 30, 37 Kishino, S., 243, 253(6.13b), 278 Kistemaker, J., 115, 118, 123, 162, 165, 173, 174, I76 Kistiakowsky, G. B., 34, 35, 37 Kjelberg, A., 156, 173 Klapisch, R., 117, 123, 131, 169, 173. 175 Klasson, M.,67, 70, 81, 83, 87, 103, 105, 108, I09 Klein, F. S.,20, 38 Klein, M.P., 67, 94, 109 Kloster-Jensen, E., 85, 105, 106 Koch, J., 115, 118, 157, 167, 170, 173 Kocian, P., 62, 106 Koenig, T., 85, I 1 I Kofoed-Hansen, O., 117. 159, 173 Kohl, F. J., 26, 39 Komiya, S.. 11, 37, 38 Konopinski, E. J.. 140, 170 Korb, H.W., 215(4.35), 222 Kordis, J., 25, 26, 36 Kornahl, G., 131, 177 Korolkov, V. I., 255(6.10), 278 Kosanke, K. L.. 130, 173 Kosmus, W., 83, 109 Kosurov. G. E.,145, 169 Kowalczyk, S. P., 74, 88, 89. 109 Kramer, J . M.,23, 37 Kramer, L. N., 67. 94, 109 Krause, G . O., 237(5.16), 243 Krause, M. 0.. 99, 101, 109

Kressel, H., 215, 222, 223(5.6), 228, 233(5.6), 242, 257(6.17), 262, 278 Kroerner, H., 191(3.4), 192(3.4), 193, 195, 222(5.1), 223(5.1), 226, 242 KropR, F., 156, 159, 166,170 Krukowska-Fulde, B.. 237 (5.13), 243 Kucherenko, Yu. N., 89, 109 Kudo, M.,93, 109 Kukla, M.I.. 84, 106 Kung, J. K., 203(4.9), 221 Kurbatov, B. L., 57, I I 1 Kuvas, R. L., 276(6.66), 279 Kuznietz, M.,78, 87, 107 Kydd, P. A., 34, 37

L Ladany, I., 257(6.17), 278 la Fleur, P. D., 160, 175 Lakin, K. M.,257(6.20), 278 Lam, D. J., 88, 89,111 Lampe, F. W., 20, 34, 36, 37 Lander, J. J., 11, 37 Landsberg, P. T., 191(3.2), 195 Lank W., 88, 107 Langer, D. W., 103, 107 Langmann, H. J., 144, I72 Langmuir, I., 128, 133, 173 Larkins, J. T., 16, 34, 38 Larson, P. E., 92, 109 Latham, D., 63, 67, 106 Laubert, R., 163, 170, 175 Lavatelli, L. S.,141, 173 Leal, H., 128, 175 Leckey, R. C. G., 87, 88, 109, 110 Lee, A., 23, 37 Lee, D. D., 109 Lee, J. D., 63, 109 Lee, M.H., 222 Lejeune, C., 127, 169, 172, 173 Leonhardt, G., 103, 108 Leventhal, J. J., 23, 37 Levy, B., 83. 109 Levy, G., 123. 175 Lewis, L. G., 117, 172 Lewis, R. K., 9, 12, 38 Ley, L., 74. 88, 89, 109 Lichten, W.. 163, 172, 173 LiDonnici. L., 102, 103, 108

AUTHOR INDEX

Liebl, H. J., 8, 9, 37 Lien. S. Y., 268(6.45), 275(6.45), 279 Liesegang, J., 88, I10 Lifschitz, C., 20, 38 Ligonniere, M., 144, 169 Lin, J. K., 257(6.20), 278 Lin, S. S., 25, 35, 38 Lin, W. N., 236(5.26), 243 Lincoln, K. A., 27, 30, 38 Lindahl, A,, 126, 132, 172, 173 Lindau, I., 63, 109 Lindberg, B., 10, 39, 59, 69, 96, 101, I I0 Lindemann, F. A., 114, I73 Lindgren, I., 10, 39, 59, 69, 96, 101, 110, 160, 171 Lindhard, J, 165, 173 Lindley, W. T., 209(4.25), 222 Lindmayer, J., 42, 54 Lindquist, P. F., 211(4.30), 212(4.30), 222 Lippincott, E. R., 68, 111 Lockwood, H. F., 263(6.28), 278 Loechner, K. S., 270(6.49), 279 Loeser, F., 156, 174 Lohr, L. L., Jr., 80, 109 Lorenz, M. R., 263, 278 Losing, F. P., 33, 38 Love, L. O., 122, 143, 170, 173 Lu, C. C., 78, 107 Lubringe, K., 151, 174 Ludowise, M. J., 222 Lundin, L., 164. I70 Lundin, S. T., 93, 110 Luth, H., 60, 109 Lynch, A. W., 25. 38

M McAlister, A. J., 89, 107 McCargo, M., 165, 171 McClure, J. E., 100, 106 McConnell, J. R., 156, 175 McDaniel, E. W., 20, 38 MacDonald, J. R., 163, 173 McDowell, C. A., 99, 107 McDowell, M. V., 32, 35, 36, 39 McFadden, W. H., 3, 38 McFarland, W. C., 83, 1 I I Macfarlane, R. D., 129, 130, 173, 176 McFeely, P. R.,74, 88, 89, 109

289

McGee, H.A., 4, 38 McGlynn, S. P., 84, 99, 109 McGuire, G. E., 78, 94, 106, 107 McHarris, W. C., 130, 173 McIlroy, R. W., 160, 172 McKeown, M., 18, 21, 38 Macksey, H. M., 215(4.35), 222 McMenamin, J. C., 91, I 1 1 Madelung, O., 182(1.5), 195, 198(1.5), Mader, S., 233, 242 Makov, 3. N., 145, 169 Maksirnov, S. P., 145, 169 Mallow, J. V., 75, 107 Malone, T. J., 4, 38 Malov, A. F., 145, 169 Manchester, K.E., 167, I73 Mangrave, J. L., 37 Mann, J. B., 27, 38 Manne, R.,75, 76, 81, 83, 109, 111 Manson, S. T., 78, 109 Margrave, J. L., 25, 26, 38 Marinelli D. P., 264(6.35), 279 Marr, G. V., 15, 38 Marra, W. C., 63, 73, 107 Martin, J., 4, 34, 38 Martin, T. W., 38 Martinson, I., 164, 170, 173 Maruska, H . P., 207 (4.17, 4.19, 4.22), 221 Marwick, A. D., 163, 170 Masic, R., 127, I73 Masri, S. M., 94, 109 Massey, H. S. W., 13, 18, 38 Masuda, K., 168, I74 Matare, H. F., 185(2.2), 207(4.23), 209(4.27), 195, 196(2.2), 207(4.23), 209(4.27), 221, 222 223(5.3), 232(5.3), 234(5.3), 235(5.3), 236, 238(5.3), 242, 254(6.61), 273(6.55), 275(6.58, 6.55), 277(6.58, 6.6), 278, 279 Mateescu, G. D., 83, 111 Matsuzawa, M., 85,109 Mattes, B. L., 270(6.48), 279 Mattsson, L., 74, 106 Matzke, H. J., 166, 1 73 Maurer, R. D., 182(1.1), 195 Maxwell, K. H., 258(6.23), 278 Mayer, J. W., 165, 167, 171, 174 Mazumder, A. K., 130, 175 Mead, C. A., 215(4.32), 222 Meeks, J. L., 84, 99, 109 Melngailis, I., 209(4.25, 4.26), 221, 222

290

AUTHOR INDEX

Melnik, R.,70, 109 Melton, C. E., 14, 34, 38 Menat, M., 150, 170, 174 Menzel, D., 94, I09 Meredith, W. N. E., 98,106 Mettler, K., 206(4.14), 221, 238(5.20), 273(6.56), 275(6.56), 279 Meulenberg, A., 50, 51, 54 Meunier, R., 144, 169, 171 Meyer, R. T., 25, 38 Michel, M. C., 128, 174 Milie, P., 83, 109 Millard, M., 94, 103, 109 Miller, 8. I., 272(6.52}, 279 Milne, C. W. A., 16, 36 Milne, T. A., 25, 38 Milnes, A. C., 232(5.7), 242 Milvidskii, M. G., 235(5.11), 242 Minden, H. T., 270, 279 Miner, C. E., 62, 107 Mirza, M. V., 85, 109 Mitchell, I . V., 165, 166, I71 Mitscher, L. A., 15, 37 Mlekodaj, R. L., 130, 175 Moddeman, W. E., 96, 109 Moest, R. R.,256(6.1I), 278 Momigny, J., 74, 84, 107 Monroe, B. M., 99, 108 Mooradian, A,, 209(4.26), 222 Morabito, J. M.,9, 11, 38 Morgan, D. V.. 166, 174 Morishima, I., 85, 109 Morris, A,, 85, 108, 109 Moss, T . S., 186(2.5), 187(2.5), 188(2.5), 189(2.5), 195, 198(2.5), 206(2.5), 222, 2443). 246 Muccino, R. P., 13, 38 Miiller, E. W.,16, 38 Mueller, P. K., 103, 110 Mullin, J. B., 269(6.47), 279 Munson, M. S. B., 14, 38 Murad, E., 25, 37 Musgrave, W. K. R., 85, 99, 107 Musurneci, L., 140, 169

Nahory, R. E., 268(6.42), 279 Nakada, 0.. 243 Nakashima, H., 243 Namba, S., 167, 174 Narusawa, T., 11, 37, 38 Natalis, P., 74, 84. 107, 109 Nauman, R. V., 98, 108 Naumann, R. A., 156, 159, 160, 174, 175 Naybour, R. D., 102, 107 Neckel, A., 26, 39 Nefedov, V. I., 80. 99, 109 Nelson, H., 215, 222, 223(5.3), 228, 233(5.6), 242, 262, 278 Nelson, R. S., 165, 168, 174 Nemoshkalenko, V . V.. 89,109 Nestor, C. W., 76. 107 Neuberger, M., 221 Newkome, G . R., 98, 208 Newman. D. H., 238(5.22), 243 Nichaus, A., 38 Nick], J . J., 203(4.3), 207(4.18), 221 Niehaus, A., 34, 37 Nielsen, A., 162, 169 Nielsen, K. L., 116, I70 Nielsen, K. O., 116, 117, 118, 120, 132, 133, 136,148,149,158,159,167,169,l71,173,174

Nielsen, 0. B., 126, 132, 172, 173, 174 Niemyski. T., 237(5.13), 243 Nier, A. 0.C., 118, 173 Nihei, Y.,93, 109 Nikelichev, A. A,, 145, 169 Nilsson, G., 103, 108 Nilsson, R.,67, 70, 87, 108, 109 Nir-El, Y., 131, 169 Nishijima, A,, 93, 109 Nitschke, J. M., 131, 174 Noller, H. G., 62, 106 Noguchi, Y., 268(6.43), 279 Nordberg, R., 10, 39, 59, 69, 96, 101, 110 Nordling, C., 10, 39, 57, 70, 75, 76, 87, 102, 103, 105, 108, 109, 110, 111 Nordling, D., 87, 102, 108 Nordling, K., 59, 69, 96, 110 Novakov, T., 76, 77, 103, 110 Nuese, C. J., 255(6.8a),257(6.17), 278

N 0

Nachbaur, E., 83, 109 Naganuma, M., 277(6.70), 279 Nagoi, H.,268(6.43), 279

Oakey, N. S., 129, 173 Obert, J., 123, 175

29 1

AUTHOR INDEX Oechsner, H., 7, 38 o h m , Y.,96, 111 OHara, S., 238(5.22), 243 Okuda, M., 85, 108, 109 Okusawa, M., 76, 110 Oleson, M. C., 162, 171, 172 Oliphant, M. L., 114, 137, 174 Olsmats, M., 160, 171 Olson, E., 78, 11 1 Onderdelinden, D.. 162, 165, 174, 175 Orsay, 118, 174 Osberghaus, O., 34, 35 Oskam, A,, 83, 96, 105 Osvenskii, V. B., 235(5.1 I), 242 Otvos, J. W., 103, 110 Oudshoorn, C., 83, 96, 105

P Padalia, B. D., 88, 107 Page, P. J., 94, 110 Paleev, V. I., 17, 39 Palniberg, P. W., 63, 110 Panin. B. V., 145, 169 Panish, M. B., 210(4.28), 215, 222, 228, 242, 243, 251(6.3), 263(6.32, 6.33). 265(6.32), 266(6.32), 268(6.44), 278, 279 Pankove, J. I., 191(3.3), 192, 193, 207(4.21), 195, 221

Paola, C. R., 272(6.54), 279 Pappas, A. C., 161. 169 Paquette, L. A,, 84, 106 Paris. P., 144, 172 Parr, G. R., 15, 38 Pate, B., 129, 130, 171 Patzelt, P., 131, 174 Paul, W., 137, 174, 176, 209(4.24), 221 Pawlik, D., 238(5.20), 243 Peaker, A. R., 206(4.13), 221 Pearson, G. L., 41, 54 Pedrotti, F. L., 60, 108 Peritti, P., 130, 169 Perlman, M. L., 89, 107 Perovic, B., 124, 151, 171, 174 Perry, D. L., 78. 107 Perry, W. B., 84. 110 Persson, R., 140. 174 Pertel, R., 31, 32, 38 Petroff, P., 238(5.21), 243 Petrosky, V. E., 85. 110

Petty, F., 26, 38 Pfister-Guillouzo, G., 85, 108 Phillipe, C., 131, 173 Piacente, V., 25, 35 Picraux, S. T., 166, 174 Pierce, D. T., 71, 106 Pierce, J. R., 133, 174 Pike, E. R., 269(6.47), 279 Pinkes, E., 272(6.52), 279 Plihal, M., 273(6.56), 275(6.56), 279 Polaschegg, H. D., 62, 106 Pollack, M. A., 268(6.42), 279 Pollak, R. A,, 89, 109 Pontvianne, B., 94, 107 Poole, R. T., 88, 110 Potter, D. L., 166, 170, 173 Potts, A. W., 78, 79, 110 Povey, A. F., 94. 107 Prangere, F., 127, 175 Prather, J. W., 99, I11 Press, P.. 8, 39 Preston, F. J., 34, 38 Price, W. C., 60, 78, 79, 110 Prins, R., 76, 77, 110 Proshko, G . P., 235(5.11), 242 Prudkovsky, G. P., 145, 169 Pullen. B. P., 78, 107

Q Queisser, H. J., 233(5.8), 234, 242

R Rabalais, J. W., 83, 84, 99, 108, 110 Rado, W. G., 275(6.57), 279 Raether, H., 60,107 Raether, M., 137, 174 Rai-Choudhury, P., 261(6.24), 278 Rambaugh, L. H.,114, 175 Ramsey, B. G., 80, 106 Ramsey, J. A., 60,110 Rapp, R. A., 25,38 Rasmussen, E., 157, 173 Rastogi, A. K., 91, 105, 110 Rauh, E. G., 25, 35 Rautenbach, W. L., 123, 133, 151, 174 Ravn, H. L., 132, 174 Rebbert, R. E., 15, 37

292

AUTHOR INDEX

Rediker, R. H., 221 Reed, R. I., 19, 38 Reeher, J. R., 34, 38 Reid, N. M, 21, 37 Reinhard, H. P., 176 Reinhart, F. K., 276(6.63, 6.65), 279 Revesz, A. G., 48, 54 Reynolds, F. L., 117, 174 Reynolds, J. H., 48, 54 Rhodin, T. N, 60, 108 Rhyde, H., 169 Rhyne, T. C., 20, 38 Riach, G. E., 91, 110 Ricaud, C., 155, 172 Richardson, J. H., 23, 38 Richter, K., 206(4.14), 221, 256(6.16), 257(6.16), 278 Ridard, J., 83, 109 Ridley, T., 25, 35 Rimstidt, J., 103, 110 Rittner, E. S, 42, 54 Risley, J. S., 64, 110 Roberts, J. D., 36 Roberts, M . W., 63, 65, 67, 91, 94, 105, 106 Roberts, R. F., 93, 110 Robertson, A. J . B, 16, 38 Robig, G., 130, 176 Robin, B., 155, 172 Rode, B. M., 83, 109 Rol, P. K., 123, 165, 174 Rook, H. L., 160, 175 Rose, H. J., 173 Rose, T. L., 37 Rosenblum, E. S., 143, 173 Rosencwaig, A., 76, 110, 1 1 I Rosenstock, H. M., 16, 38 Ross, K. J., 85, 108, 109 Rossi J., 151, 176 Rossi J. A., 209(4.26), 222 Rostas, J., 85, 107 Rosztoczy, F. E., 268(6.41), 279 Roth, S., 170 Route, R. K., 270(6.48), 279 Rowe, J. E., 15, 22, 38, 91, 111 Rozgonyi G. A., 243, 268(6.44), 279 Rudstam, G., 117, 131, 141, 142, 158, 161, 169, 173, 175 Ruedl, E., 7, 35 Rummel, R. E., 34, 38 Russell, M. E., 24, 36

Ryde, H., 159, 169 Rynd, J. P., 91, 105, 110

S Saalfeld, F. E., 32, 34, 35, 36, 38, 39 Sagawa, T., 76, 110 Saito, K., 252(6.3a), 253(6.3a), 278 Salomk, M., 131, 173 Salyn, Ya. V.,80, 99, 109 Samson, J. A. R., 110 Santry, D.C., 175 Saris, F. W., 162, 163, 175 Sarrouy, J. L., 123, 144, 150, 169, 170, 171, 172, 175 Satake, T., 11, 37, 38 Saul, R. H., 265(6.38), 272(6.54), 275(6.54), 279 Saunders, R. A., 34, 38 Sautter, J. M., 127, 173 Scanlan, I., 107 Schafer, W., 85, 110 Schaen, R. I., 15, 36 Scharff, M., 165, 173 Scharpen, L. H., 90,107 Schillaties, H., 62, 106 Schiertt, H. E.,165, 173 Schmidt, F. H., 146,171 Schmidt, L., 270(6.49), 279 Schmidt-Ott, W.-D., 130,175 Schnitzer, R., 20, 38 Schon, G., 93, 110 Schroeder, W. E., 276(6.66), 279 Schroder, D.K., 261(6.24), 278 Schulte, J., 127, 172 Schultz, P. C., 182(1.1), 195 Schwartz, M. E., 95, 110 Schweig, A,, 80, 85, 110 Schweitzer, G. K., 86, 87, 99, 107 Schweitzer, K., 74, 109 Scifres, D. R., 215(4.33, 4.35), 222 Searles, S. K., 21, 39 Seeger, A., 246, 278 Seki, Y.,257(6.18), 278 Semeluk, G. P., 33, 38 Sen, S., 236(5.26), 243 Septier, A., 119, 127, 128, 133, 175 Shapiro, R. H., 13, 39 Sharma, J., 103, ZZO Shaw, D. W., 256(6.12, 6.14). 278

29 3

AUTHOR INDEX Shaw, R. W., 72, 110 Shehepkin, G. Ya., 145, 169 Sheley, C. F., 83, 111 Sheludchenko, L. M.,89, 109 Shepard, F. R.,88, 110 Sherwood, P. M.,94, 107 Shevchik, N. J., 89, 110 Shimada, K., 99, 106 Shimzu, K., 127, 175 Shin, K.S., 272(6.53), 275(6.59, 6.53), 279 Shinohara, T., 257(6.18), 278 Shire, E. S., 114, 137, 274 Shirley, D. A., 11, 39, 74, 75, 76, 88, 89, 91, 96. 107,108, 109, 110 Shmid, M.,131, 169 Shockley, W., 167, 175 Showalter, H. D . H., 15, 37 Sidenius, G., 119, 121, 125, 126, 132, 159, 172, 173, 175 Sieck, L. W., 16, 21, 39 Siegbahn, H., 63, 76, 77, 78, 87, 95, 99, 105, 109, 110, 111 Siegbahn, K., 10, 39, 57, 59, 62, 63, 66, 67, 68. 69, 71, 72, 74, 75, 76, 77. 78, 79, 80. 81, 84, 87, 95, 96, 99, 100, 102, 105, 106, 108, 109, 110, I l l , 142, 175 Siege], M.W., 18, 21, 38 Siegmann, H. C., 71, 106, 1 1 1 Silbey, C. B., 167, 173 Silfvast, W. T.. 39 Sinnott, G. A,, 20, 39 Sitter, C . W., 175 Skilbried. 0..121, 144, 148, 158, 169, 171, 172, 174, I75 Sleege, G. A,, 156, 175 Sloan, R. H., 8, 39 Slodzian, G. S.. 9, 36 Sluyters, T. J. M.,162, 175 Smith, D. L., 23, 39. 85, 109 Smith, H. J., 144, 166, 169 Smith, H. P., 170 Smith, M.L., 118. 157, 171, 175 Smith, N . E., 91, 111 Smolinsky, G., 29, 39 Smyth, W. R., 114. 175 Snider, D. F.. 130, 131, 175, 176, 177 Sodeck, C.,26, 39 Sokolowski, E., 57, 110, 111 Spanget-Larsen, J .. 1 11 Spejewaski, E, 156, 174 Spenke, E., 1930.5). 195, 223(5.2), 224, 242

Spicer. W. E., 65, 90,91, 106 Spitzer, W. G., 203(4.9), 215(4.32), 221, 222 Spohr, R., 62, 106 Srivastava, R. D., 26, 39 Stafford, F. E., 26, 27, 37, 38, 39 Staley, R., 103, 110 Statler, R. L., 49, 54 Steams, C. A., 25, 39 Steele, W. C., 25, 39 Steiger, R. P., 26, 38 Stein, R. J., 91, I I 1 Stein, W. W., Jr., 268(6.41), 279 Stephen, J., 167, 168, 175 Stephens, W. E., 139, 175 Stern, F., 203(4.10), 238(5.25), 242, 243 Sternheimer, R. M.,175 Stevenson, D. A,, 207(4.22), 221 Stevie, F. A,, 27, 28, 39 Stone, L. E., 263, 278 Strack, H. A., 252, 254(6.4), 278 Stratten, L. W., 23, 37 Strauss, A. J., 209(4.26), 222 Street, F. J., 65, 111 Streets, D. G., 78, 79, 84, 110, I l l Stringfellow, G . B., 206(4.12), 211(4.30), 212(4.30), 221, 222, 256(6.21), 257(6.21), 265(6.39), 278, 279 Stronski, R. E., 95, 110 Su, T., 23, 35 Suddueth, J . E., 160, 175 Suffolk, R. J., 84, 111 Sukegawa, T., 215(4.34), 222 Sumski, S., 263(6.33), 278 Sundell, S., 132, 141, 174, 175 Suumeijer, E. D. T. M., 6, 39 Svartholm, N., 142, 144, 176 Svec, H. J., 34, 38 Swanson, M. L., 246, 278 Swartz, W. E., 59, 68, 72. 95, 98, 100, I l l Swingle, R. S., 11, 94, 99, 108. I l l Switalski, J. D., 95, 110 Szwarc, M.,99, 106

T Taglauer, E.. 7, 36 Takashashi, K., 277(6.70), 279 Talbert, W. L., Jr., 148, 156, 159, 172, 175

294

AUTHOR INDEX

Talini N., 140, 169 Tam, W. C., 99, 111 Tanaka, A., 215(4.34), 222 Tannenbaum, H. P., 36 Tarantin, N . J., 142,' I76 Tassa, R.,20, 38 Tawara, H., 163, 175 Taylor, J. B., 128, 173 Taylor, L. T., 95, 106 Taylor, R. C., 256(6.15), 278 Taylor, W., 15, 38 Teague, E.C., 237(5.16), 243 Teer, D., 18, 36 Teillac, T., 131, 169 Tejeda, J., 89, 110 Templeton, D. H., 117, 174 Terekhov, V. A., 80, 99, 109 Terenin, A. N., 57, I 1 1 Thiel, W., 80, 110 Thomas, T. D., 72, 110, 159, 175 Thompson, M., 63, 106 Thompson, M. W., 165, 168, 176 Thulin, S., 158, 176 Thurmond, C. D., 264, 279 Tietjen, J. J., 207(4.17), 221, 255(6.8b), 278 Tiller, W. A,, 270, 279 Timpl, F., 146, 176 Todd, C. J., 91, 111 Tomer, K. B., 13, 39 Torgerson, D. F., 129, 130, 173, 176 Tortschanoff, T., 155, 176 ToSik, D., 124, 151, 171 Trahin, M., 143, 176 Traum, M. M., 91, I I 1 Trautwein, A., 87, 111 Treble, F. C., 49, 54 Trevisan, G., 25, 39 Treytl, W., 130, 176 Trimm, D. L., 94, 110 Tripathi K. C., 102, 107 Trukan, M. K., 255(6.10), 278 Tsu, R., 230(5.15), 237(5.15), 243 Tsuchiya, M., 34, 38 Turk, J., 13, 39 Turner, D. W., 57, 80, 105, 111

U Uebbing, J., 63, 109 Ugai, Ya. A., 80, 99, 109

Uhler, J., 118, 120, 123, 148, 150, 151, 155, 158, 165, 169, 170, 171, 176 Ulrich, R.,270(6.49), 279 Utriainen, J., 63, 73, 107

V

Valli, K., 130, 176 van den Ham, D. M., 85, 1 1 1 van der Meer, D., 85, 111 Vander Weg, W. F., 163, 175 van der Wiel, M. J., 71, 111 Van Eck, J., 162, I76 van Hoorn, M. D., 85, 111 Van Ment, M., 144, 150, 170 Vasile, M . J., 27, 28, 29, 39 Vassent, B., 144, 169 Veal, B. W., 88, 89, 111 Venezia, A., 131, 169, I76 Verrner, H., 85, 110 Vesely, C. J., 103, 107 Viehbock, F. P., 118, 121, 144, 154, 155, 165. 172, 174, 176 Viehboek, R. P., 8, 37 Viinkka, E.-K., 96, 1 I 1 Vilesov, F. I., 57, 111 Vinh, J., 83, 109 von Ardenne, M., 126, 146, 176 von Busch, F., 137, 176 von Munch, W., 247, 248(6.2), 250, 253, 278 Vorburger, T. V., 91, 111 Vy, 0. M., 26, 39

W

Wachi F. M., 25, 39 Waclawski B. J., 91, 111 Wagner, C. D., 92, 100, 11I Wagner, H., 118, 130, 131, 145,175, 176, 177 Wahlbeck, P. G., 25, 39 Wahlgren, U. I., 96, 105 Wahlin, L., 137, 176 Wahrhaftig, A. L., 13, 39 Walcher, W., 115, 118, 131, 133, 145, 176, 177 Walker, J . A,, 16, 38 Walls, F. L., 24, 39 Walton, D., 254(6.5), 278 Wang, J. L. -F., 26, 38 Wang, K. L., 257(6.20), 278

295

AUTHOR INDEX

Wannberg, B.. 62, I I 1 Warnecke, R. J., 127, 173 Warner, R. A., 130, I73 Watson, J. M., 60, 73. 103. 107 Watson, L. M., 92. 107 Watson, R. E.. 89, 107 Watts, J. C., 68. 1 1 1 Watts. P. H., 68, 111 Wehner, G. K.. 165, 176 Weichert, N. H., 63, 108 Weischahn, W. J., 129, 130, 171 Weiss, H., 206(4.15), 221 Welker, H., 182, 194(1.4), 195 Werme, L. O., 75, 76, 84, 110, I l l Wermer, H. W.. 9, 39 Wertheim, G. K., 73. 74, 76, 88, 108, 110, 111

West, S. S., 114, I75 Westenberg, A . A., 31, 32, .37 Westgaard, L., 132, 174 Wheatley. G. H., 6. 39 White, F. A,, 144, 176 Whitehead, T. W., Jr., 145, I71 Whitmore. D. H., 26, 37 Widaj, B., 237(5.13), 243 Wieczorek. J. S., 85. 111 Wiegmann, W., 277, 279 Wien. K.. 129, 130. 176 Wildon, H. W., 116. 126, 127. 172 Wilhelrn. H. G., 130, 175, 176, 177 Williams, M. A,, 78, 101, 106 Williams, P. M., 89. 94, 110 Williams, T. A., 84, 1 1 1 Willoughby. A. F. W.. 236(5.27), 243 Wilson, R. G., 136, 176 Wilson Whitehead, T., 144, 176 Winkler, H. I., 16, 36 Winsor, H. V., 78. 1 I I Winstel, G. H., 273(6.56), 275(6.56), 279 Winter, H., 127. 177 Wischnewski K., 62. I06 Wismontsky. 1.. 131. 169 Wisniewska, K.. 237(5.13), 243 Wohn, F. K., 148, 156, 172 Wolf, B. H., 127, 177 Wolf, D.. 130, 175 Wolf, G. K., 129, 177

Wolf, M.. 48. 54 Wolfe, C. M., 255(6.8a), 278 Wolfgang, R., 22, 37 Wolfstirn, K. B., 221 Wollmann, K., 258(6.22), 278 Wollnik. H., 130, 131, 175, 176, 177 Wong, J . L.F., 26, 37 Woodall. J . M., 265(6.37), 279 Woodgate. S. S., 20, 35 Woodhouse. J. B., 21514.35). 222 Woodyard. J. R.. 7, 39 Wooten, F.. 78, 111 Worley, R. D.. 166, 170 Worley. S. D., 80, 83, 106, I l l Wright, D. R., 206(4.13), 221 Wu, H. Y., 25, 39 Wuilleumier, F., 99. 101, 109 Wyatt, J. R., 23. 26, 32, 37, 39 Wyrich, C.. 273(6.56), 275(6.56), 279

Y Yamaguchi, S., 117, 177 Yates. E. L.. 114, 127. 154, 1 7 7 Yates, K., 63. 65, 67, 106, 1 1 1 Yee, D.. 99. I 1 I Yin, L. I., 72, I l l Yonezawa, T., 85, 109 Yoshikawa, K., 85, I09 Young, W. A. P., 126. 172 Yu, K. Y., 91, 1 1 1

2

Zahn, U . V., 176 Zandberg, E. Ya., 17, 39 Zatko, D. A., 98, 111 Zavitsanos, P. D., 25, Zeppenfeld, K., 60,107 Zhukov. V. V., 145, 169 Ziegler, G., 237. 243 Ziherschoon, C . J., 116, 141, 148, 177 Zimrnerman, E., 258(6.23), 278 Zschauer, K. H., 203(4.11), 237(5.17), 221, 243

This Page Intentionally Left Blank

SUBJECT INDEX Channeltron, in electron spectroscopy, 67 Charge transfer, in isotope separators,

A

Accelerators, isotope separators as, 161 AES, see Auger electron spectroscopy Analytical Chemistry, 56, 59 Angular distribution. in electron spectroscopy measurements, 77-78 Aston bands, in isotope separators, 151 Atomic collisions ionization and excitation in, 161- 162 isotope separator and, 164-165 Auger electron spectroscopy, 5, 11, 59, 91,

150-151

Chemical compounds, electron spectroscopy in study of, 94-99 Chemical ionization, 14-15 Chemical shift, in core electron spectroscopy, 97

Chemical vapor deposition in junction formation, 2 5 4 2 6 2 typical set-up for, 256 CI, see Chemical ionization CNR cell, see COMSAT nonreflective cell Combustion sampling, in mass spectroscopy,

104- 105

X-ray photoelectron spectroscopy and, 91-92

30-33

Avalanche mode, in radiative recombination.

COMSAT Laboratories, 42 COMSAT nonreflective cell, 42, 50-53 current-voltage characteristics of, 51-53 Core electron spectroscopy, chemical shift in,

185

B

97

Band tailoring in doped semiconductors, 193 indium phosphide in, 21 1 Beam foil spectroscopy, 164 Bernas-type lateral extraction ion source,

CVD, see Chemical vapor deposition Cyclopentanediones, UPS spectra for, 98

D

122-123

Biological systems, electron spectroscopy in study of, 95-99 Blackbody radiation, for light-emitting devices, 243-244 Bloch waves, 182 Brillouin zone, 183, 185

Diffusion, in junction formation, 247-253 Direct-gap materials or semiconductors, 182-184

low effective electron mass of, 199 Direction focusing magnetic analyzer, 137- 139

Direct semiconductors, 207-214 Duoplasmatron ion source, for isotope separators, 127

C

Calutrons, 115-1 17 for high-intensity ions, 120 ion beam extraction system for, 134 Catalysts, electron spectroscopy study of, 102 Chalk River isotope separator, Scandinavian type, 134-135 Channeling phenomena, isotope separators and, 164-165

E Electromagnetic isotope separators, see Isotope separators Electron impact ion source, 12-14 disadvantages of, 13 Electron mobility

297

298

SUBJECT INDEX

in chemical vapor deposition, 257 in light emitting materials, 200 Electron spectrometer detector in, 63 in electron spectroscopy in chemical analysis, 60 magnetic, 62-64 magnetic shielding for, 62 Electron spectroscopy Auger, see Auger electron spectroscopy basic concepts of, 57-58 classification of field in, 58-60 control and data handling in, 67 experimental results in, 83-84 line splitting and satellite structure in, 7477 main fields of application of, 80-101 origin and development of. 56-57 photoionization cross section in, 81 present scope of, 55-56 recent applications of, 55-105 shift for inner shells in, 8 1 in solid state structure, 87-90 in solution of practical problems, 101-103 in structure of chemical compounds and biological systems, 9 4 9 9 surface studies with, 90-94, 104 system of lines in, 81 X-ray spectroscopy and, 99-101 Electron spectroscopy in chemical analysis, 5, 10-1 I, 58-59 block scheme for, 61 calibration in, 67-69 energy gap in, 66 energy resolution of, 66 escape depth in. 69 main components of, 60 pumping system of, 61-62 samples and irradiation in, 64-67 sampling depth in, 69-70 ultraviolet light irradiation in, 65 Electron spectroscopy measurements angular distribution in, 77-78 line shift, linewidth, and line shape in, 71-73 observables in, 70-80 photoionization cross section in, 78-80 Energy loss spectroscopy (ELS), 59 Environment pollution studies, electron spectroscopy in, 102-103

ESCA, see Electron spectroscopy for chemical analysis

F Fermi distribution functions, in radiative recombination, 190 FI, see Field ionization Field desorption ionization, in mass spectroscopy, 17 Field effect transistors, 197 Field ionization, in mass spectroscopy, 16- 17 Flame studies, in mass spectroscopy, 34 Flow systems, in ion-molecule studies, 2 1-22 Freeman ion source, for isotope separators, 123- 124

G Gallium arsenide band structure of, 183 electron mobility in, 200 in junction formation, 246-248 as light emitting material, 196-197 maximum eficiency in, 210 photoluminescence of, 202 reflectivity in, 203 refractive index for, 204 zinc and, 25 1-252 Gallium phosphide, as light emitting material, 204206 Germanium, energy gap in, 183 Gunn effect, 197

H HCIS, see Hollow cathode ion source Helium jet recoil transport system, for isotope separators, 129-131 Heterojunction defects and radiative efficiency in, 234-238 dislocation distance in, 232 dislocation vs. minority carrier diffusion length in, 237 double, 228 Fermi levels in, 229 functions of, 225-226

SUBJECT INDEX

injection efficiency and confinement for, 222-228 interface dislocations in, 232-233 interface formation in, 230-231 lattice match in, 229-234 as light-emitting device, 222-242 radiative recombination efficiency for, 238-242 stimulated vs. spontaneous emission in, 238-242 High temperature studies, mass spectrometry in, 25-28 Hole-electron pairs, in silicon, 41 see also p-n junction Hollow cathode ion source, for isotope separators, 125 Homogeneous fringing field, of isotope separator, 141 -142 Hot cathode arc discharge sources, 119-127 1

ICR, see Ion cyclotron resonance technique IKES, see Ion kinetic energy spectroscopy IMPATT diodes, 276 Indirect-gap semiconductor, 182-184 Indirect semiconductor, 207-214 Indium antimonide, in infrared detectors, 206 Indium arsenide. light emission and laser action in, 206 Indium phosphide band tailoring and, 21 1 electron mobility and direct gap for, 207 Induced electron emission, 59 Industrial Research, 102 Inhomogeneous magnetic field, of isotope separator, 142-144 Injection efficiency, for heterojunction, 222-224 INMS, see Ionized neutral mass spectrometry INS, see Ion neutralization spectroscopy Integrated target ion source system, 129 Ion acceleration, in isotope separators, 135- I37 Ion-atom collisions, X-ray production in, 162- 163 Ion beams extraction and formation of an isotope separator, 132-135

299

hot cathode arc discharge sources in, 119-127 in isotope separator research, 117-118 production and formation of, 118-135 Ion cyclotron resonance technique, 23 Ion-detection microprobe, 9 Ion implantation, 166-168 in isotope separators, 119 Ionization chemical, 14 field, 16-17 field desorption, 17 Pennmg, 18 photoionization, 15- 16 surface, 17-18 Ionization efficiency curves, 18-19 Ionization processes electron impact, 12-14 mass spectroscopy and, 12-19 Ionized neutral mass spectrometer, first practical, 7 Ionized neutral mass spectrometry, 5-8 Ion kinetic energy spectroscopy, 24 Ion-molecule reactions, 19-25 flow systems in, 21-22 low pressure, single collision techniques in, 22-23 quadruple trap in, 24 reactant internal energy and, 24 static systems in, 2C-21 Ion neutralization spectroscopy, 104 Ion scattering spectrometry, 5-7, 104 surface analysis by, 6 Ion sources, thermal, see Thermal ion sources Ion source technology, for on-line isotope separation, 128-132 Isolde Collaborations, 159 Isoelectronic defect centers, excitonic levels and, 212 Isoelectronic trap levels. 182-183 Isoelectronic traps, light emission and, 192 lsokinetic probes, in combustion sampling, 31 Isotopes collection of in isotope separator, 152-156 direct deposition of, 153- 154 on-line separation of, 159-160 radioactive, 158-159 short-lived, 155- 156, 159- 160

SUBJECT INDEX

300

stable, 157-158 Isotope separators, 113-168 accelerating voltage for, 135-136 applications of, 116-117, 157-168 Aston bands in, 151 and atomic collisions in solids, 164-166 in beam foil spectroscopy, 164 Bernas-type lateral extraction ion source for, 122-123 Calutrons as, 115-117, 120, 134 channeling phenomena and, 164- 166 charge transfer in, 150-151 collection of isotopes in, 152-156 direct deposition of isotopes in, 153-154 dispersion and resolution of, 147-148 efficiency of, 152 electrostatic retardation method for, 155 enrichment factor and contamination in, 148- 150

Freeman ion source for, 123-124 helium jet recoil transport system for, 129- 13 1

with higher dispersion, 145 hollow cathode ion source for, 125-126 homogeneous fringing field, 141-142 inhomogeneous magnetic field of, 142-144 integrated target-ion source system for,

sputtering method for, 154-155 for stable isotopes, 157-158 thermal ion sources for, 127-128 transformer-rectifier system for, 135-136 two-directional focusing with inhomogeneous magnetic field, 142- 144 and X-ray production in heavy ion-atom collisions, 162- 163 ISS, see Ion scattering spectroscopy ITIS, see Integrated target ion source system

J

Jahn-Teller splitting, 74, 81-82 Joint Institute of Nuclear Research, 142 Journal of Electron Spectroscopy and Related Phenomena, 56 Junction formation in light-emitting devices, 246-278 liquid phase epitaxy in, 255-257, 262-275 molecular beam epitaxy in, 275-278

K Knudsen cell studies. 25-28

131-132

ion acceleration in, 135-137 ion beam extraction and formation in,

L

132- 135

ion beam production and formation in, 118- 135

ion implantation and, 166-168 and ionization and excitation atomic collisions, 161-162 as low energy accelerators, 161 magnetic oscillation sources for, 120- 123 magnetron source for, 124-125 mass analyzer as component of, 137-146 as mass spectrometer, 116-117, 160-161 mass spectroscopy and, 114 on-line, 128-132, 155-156 performance of, 146-152 plasmatron sources for, 126-127 power supply for, 116 radioactive isotopes by off-line separation in, 158-159 Scandinavian type, 115, 120, 134-135 short-lived isotopes in, 155-156, 159-160

Laboratory isotope separators, see Isotope separators Lattice match, in heterojunction, 229-234 Lattice mismatch, 197 LED, see Light-emitting diodes LEED, see Low energy electron diffraction Light-emitting devices, 179-278 see also Light-emitting diodes; Lightemitting materials blackbody radiation and, 243-244 chemical vapor deposition in, 254-262 close lattice match in, 214-218 conditions for, 214 electron mobility in, 201 heterojunction in, 222-242 junction formation in, 241-278 liquid phase epitaxy in, 262-275 materials for, 195-221 molecular beam epitaxy in, 275-278

SUBJECT INDEX

radiative and nonradiative recombination and, 185-190 ternary compounds in, 214-221 Light-emitting diodes chemical vapor deposition in, 254262, 277-278 junction formation in, 248-250 semiconductors and, 182-183 Light-emitting displays, market value of, 183 Light-emitting materials photoluminescence in, 202 reflectivity in, 203 refractive index of, 204 thermal conductivity of, 205 111-V compounds as, 198-207 in liquid phase epitaxy, 270-275 Line splitting, in electron spectroscopy, 74-77 Liquid phase epitaxy, 277 in junction formation, 255-257, 262-275 limited melt in, 269-275 multiple layer growth and, 266 steady state, 268 typical arrangement for, 264 Low energy electron diffraction, 91-92, 104 Lubrication studies, electron spectroscopy in, 103

M Magnetic analyzer axial beam crossing with, 146 direction focusing, 137-138 first-order single direction focusing, 138-139 with higher dispersion, 145 higher-order focusing, 140- 141 second-order single focusing, 139-140 Magnetic oscillation arc sources, for isotope separators, 120-123 Magnetic splitting, 74-76 Magnetron discharge, as ion source in isotope separators, 124 Mass analyzer see also Magnetic analyzer as component of isotope separator, 137 direction focusing magnetic, 137-139 two-dimensional focusing with fringing field, 141-142

30 1

Mass spectrometer conversion of molecules to ions in, 19-20 and electron spectroscopy for chemical analysis, 10 focusing magnetic, 142- 143 isotope separators as, 1-161 spark source, 3 chemical ionization in, 14-15 high temperature studies and, 25-28 isotope separators and, 117 Knudsen cell studies and, 25-28 Mass spectroscope basic components of, 2-3 direct-insertion probe for, 4 gas chromatograph inlet in, 3 ion beam mass sorting in, 4 ion detection in, 5 ion source for, 4 viscous-leak inlet in, 3 Mass spectroscopy, 1-35 see also Electron spectroscopy; Mass spectrometer; Mass spectrometry chemical ionization, 14-15 combustion sampling in, 30-33 field desorption ionization in, 17 field ionization in, 16-17 instrumental design and techniques in, 2-5 ionization processes and, 12- 19 ionized neutral, 5-8 ion-molecule studies in, 19-25 ion scattering, 5-7, 104 isotope separators and, 114 in nuclear mass measurement, 114 Penning ionization in, 18 photoionization, 15-16 plasma sampling in, 28-30 radical sampling in, 33-35 sampling of reactive species in, 28-35 secondary ion, 8-10, 91, 104-105 surface ionization in, 17-18 surface studies in, 5-12 MBE, see Molecular beam epitaxy Metal oxide semiconductor transistors, ion implantation in, 167 Microwave band, vs. optical band, 179-180 Molecular beam epitaxy, in junction formation, 275-278 Mossbauer spectroscopy, 103-104 MOST, see Metal oxide semiconductor transistors

302

SUBJECT INDEX

Multichannel plate detector, in electron microscopy, 67

N Nielsen axial extraction source, 120- 121 Nuclear magnetic resonance, 104

0 Oak Ridge (Tenn.) isotope separators, 115 On-line isotope separator, 128-132 see also Isotope separators Optical band energy values, 179-180 Optical fibers, 181-182

PI, see Photoionization Plasmas, sampling of in mass spectroscopy, 28-30 Plasmatron sources, for isotope separators, 126-127 p - n junction electron shift in, 190 energy diagram for, 191 energy states distribution in, 194 horizontal tunneling in, 195 radiative recombination in, 185 Polyatomic compounds, molecular structure of, 84

Q Quadrupole mass spectrometer, in high temperature studies, 27 Quadrupole trap, 24

P Paramagnetic gas molecules, magnetic splitting in, 75 PAX, see Photoelectron spectrometry for the analysis of X-rays Perfluorodiazines, electron spectroscopy for, 84 PESIS, see Photoelectron spectroscopy of inner shells PESOS, see Photoelectron spectroscopy of outer shells Phonons, in LED materials, 182 Photoelectron lines, shift of, 72 Photoelectrons in silicon solar cell back surface, 48 X-ray spectroscopy by, 101 Photoelectron spectroscopy, 59 for analysis of X-rays, 101 in high temperature studies, 26 of inner shells, 59 of outer shells, 59 solid state structure in, 87-90 Photoionization, in mass spectroscopy, 15-16 Photoionization cross section, in electron spectroscopy measurements, 79-8 1 Photoluminescence, of light-emitting materials, 202 Photon emission, in p - n junctions, 185 Photons, in LED materials, 182

R Radiation emissivity, classical model of, 243 Radiative recombination basis of, 189 defects in, 234 efficiency of for heterojunctions, 238-242 injection process and, 189-195 in p - n semiconductors, 185-189 pump levels in, 234 Radical sampling, in mass spectroscopy, 33-35 RAMA, see Recoil atom mass analyzer Reactive series, sampling of in mass spectroscopy, 28-35 Recoil atom mass analyzer, 130-131 Recombination, radiative and nonradiative, 185-189 Recombination paths, in p - n junctions, 185 Recombination radiation, 189-190 REEL (reflection-electron energy loss spectroscopy), 59

S Saha-Langmuir equation, 17, 131 Satellite structures, in electron spectroscopy, 76-77

303

SUBJECT INDEX

Scandinavian type isotope separator, 115, 120, 134-135 Secondary ion emission, 8 Secondary ion mass spectrometry, 8-10, 91, 104-105

Semiconductors, 103 A"'BV type, 195-196 band tailing in, 193 direct and indirect, 207-214 direct-gap, 182- 184 indirect, 182-183, 207-214 indirect-gap, 182-184 isoelectronic traps and, 182-183. 192 radiative recombination in, 185 SI, see Surface ionization Silicon, energy gap in, 183 Silicon solar cells antireflection coatings vs. wavelength in, 47 COMSAT nonreflective type, 50-53 optical matching in, 48 for space use, 41-53 violet cell and, 42-50 SIMS, see Secondary ion mass spectrometry Soft X-ray emission spectrometry, 89 Solar cells ohmic.back surface contact in, 48 silicon, see Silicon solar cells Solid state structure, electron spectroscopy in, 87-90 Spectroscopy, electron, see Electron spectroscopy Spectrometer, mass, see Mass spectrometer Spectrometry, ion scattering, see Ion scattering spectrometry Spin-orbit splitting, 74 Splitting, multiple or exchange, 74, 76 Surface ionization, in mass spectroscopy, 17-18 Surface studies with electron spectroscopy, 90-94 X-ray photoelectron spectroscopy and, 92-93 SXS, see Soft X-ray emission spectroscopy T Ternary compounds emissive range of, 220 in light-emitting devices, 214-221

Thermal ion sources, for isotope separators, 127- 128 Transmission-electron energy loss spectroscopy, 59 Trimethyl gallium, in junction formation, 259

U Ultraviolet photoelectron spectroscopy, 59, 91, 104 and ionization potentials of molecules, 83 energy in bands of, 71 linewidths in, 72 valence band studies in, 87-88 UMPA (universal microprobe analyzer), 9 UPS, see Ultraviolet photoelectron spectroscopy Uranium-235, separation of, 115 Uranium isotopes, electromagnetic separators for, 115

V Vacuum ultraviolet photoelectron spectroscopy, 59 Valence bands, in electron spectroscopy, 87 Vapor-liquid-solid growth, in junction formation, 255 Violet cell, 42-50 see also Silicon solar cells vs. conventional cell, 48-49 current-voltage characteristics of, 48-49 efficiency advantage of, 49 light penetration in, 43 open-circuit voltage of, 44 VUV-PES, see Vacuum ultraviolet photoelectron spectroscopy

X XPS, see X-ray photoelectron spectroscopy X-ray fluorescence spectroscopy, 104 X-ray photoelectron spectroscopy, 59, 91, 99, 102-105 Auger electron spectroscopy and, 91-92 in determination of ionization potentials, 83-85

304

SUBJECT INDEX

line shift in, 72 in surface studies. 92-93 X-rays, monochromatization of, 104 XRF, see X-ray fluorescence spectroscopy

2

Zinc, in light-emitting diodes, 250-252

A

6

8 7 C

8

0 9 E F G H 1 J

O 1 2 3 4 5