Trace Analysis of Semiconductor Materials PDF
Preview Trace Analysis of Semiconductor Materials
INTERNATIONAL SERIES OF MONOGRAPHS ON ANALYTICAL CHEMISTRY Vol. 1. WEISZ—Microanalysis by the Ring Oven Technique Vol. 2. CROUTHAMEL (Ed.)—Applied Gamma-Ray Spectrometry Vol. 3. VICKERY—The Analytical Chemistry of the Rare Earths Vol. 4. HEADRIDGE—Photometric Titrations Vol. 5. BUSEV— The Analytical Chemistry of Indium Vol. 6. EL WELL and GIDLEY—Atomic Absorption Spectrophotometry Vol. 7. ERDEY—Gravimetric Analysis Vol. 8. CRITCHFIELD—Organic Functional Group Analysis Vol. 9. MOSES—Analytical Chemistry of the Actiniae Elements Vol. 10. RYABCHIKOV and GOL'BRAIKH—The Analytical Chemistry of Thorium TRACE ANALYSIS OF SEMICONDUCTOR MATERIALS EDITED BY J. PAUL CALI Chief, Analytical Section United States Air Force Cambridge Research Laboratories Bedford, Massachusetts PERGAMON PRESS OXFORD · LONDON · NEW YORK · PARIS 1964 PERGAMON PRESS LTD. Headington Hill Hall, Oxford 4 and 5 Fitzroy Square, London W.l PERGAMON PRESS INC. 122 East 55th Street, New York 22, N.Y. GAUTHIER-VILLARS ED. 55 Quai des Grands-Augustins, Paris 6e PERGAMON PRESS G.m.b.H. Kaiserstrasse 75, Frankfurt am Main Distributed in the Western Hemisphere by THE MACMILLAN COMPANY · NEW YORK pursuant to a special arrangement with Pergamon Press Incorporated Copyright © 1964 PERGAMON PRESS INC. Library of Congress Catalogue Card Number 63-18922 Set in Monotype Times 10 on 12 pt. and printed in Northern Ireland by The Universities Press, Belfast PREFACE ALTHOUGH the experienced analytical chemist needs no warning, it may be well to point out to less experienced readers that this volume is not in any sense a "cook book". The procedures given in the various chapters are usually directly adaptable only to the specific system under consideration. In ultra-trace analysis, with which this book is concerned, a change in the matrix usually necessitates some modification of the procedure. Moreover, different instrumentation from that used may require procedural changes. In some cases, the order of the separation of the various elements introduces variables in the analysis not considered in the quoted procedure. Of what value then, it may be asked, is this volume, if a given procedure may have to be varied to meet different conditions imposed by the require ments of a specific analysis ? Primarily this volume should be considered as a guide book. A good guide book shows the traveller those paths which lead most directly to the desired goal. Where the experienced traveller has trod, there the novice may step with some assurance. Such a book should also point out the best means of getting to the destination; in an analogous manner, this volume discusses specific instrumentation required for the various techniques covered. Guide books also give warnings of danger spots and recommend detours around them. Thus considered, this volume should serve as a useful adjunct in solving many of the extremely difficult problems posed by our present day quest for purer and still purer materials. A rapid glance through the book will set many readers to wondering whether the editor really knows what a semiconductor material is. We have used the term semiconductor material in the title in a rather unrestricted sense to mean semiconductor materials/?^ se, e.g., silicon, precursor materials e.g., silicon tetrachloride, and indeed, any material or substance, e.g., quartz, which enters into semiconductor preparation or technology in any important sense whatsoever. There is no point in trying to remove highly pure silicon crystals from a quartz crucible loaded with a phosphorus impurity. If such a calamity has happened (as the author knows occurred in the early days), then the chemist must have the techniques for analyzing the crucible as well as the ingot. When one considers the tremendous diversity of inter- metallic compounds now being developed for experimental and industrial uses, it is not surprising to find references to more than half the elements in the periodic table. Although only six distinct techniques are discussed in this monograph, it must not be construed that other techniques are not available for trace vii viii PREFACE analysis. It is, however, the editor's opinion and experience that these six aforementioned are the most widely used and applicable to more varied problems than any of the other available possibilities. Considerations of time and space also imposed restrictions on a volume of wider scope. Each of the contributors has drawn freely from the work of many scientists in his field, as the wealth of references will attest. More important, however, is the wide range of practical experience which each contributor has brought to his particular chapter. All the authors are actually engaged in an intimate way with the difficult problems presented in the field of ultra-trace analysis. In general, the work reported herein covers the various fields through 1961. The time gap between conception and writing and final publication is, although seemingly large, about par for the course. Some of the delay may be attributed to some changes in the editor's professional life which unavoid ably interfered with the rapid translation from manuscript to print. The text is nevertheless timely because each author agreed beforehand to try to write in such a manner as best to illustrate the basic techniques peculiar to his speciality. In a rapidly changing technology, details change, but general principles and good practice remain valid over a longer period of time. Each chapter covers the following: (1) an introduction where some general statements and principles are presented, (2) the theoretical basis for the technique involved—not in great detail because this has been adequately covered elsewhere in the literature, (3) instrumentation available; practical considerations in using the techniques and finally, (4) applications to trace analysis with special reference to semiconductor materials. The references following each chapter should serve as an excellent starting point for readers who wish to go further into the field. Although there was originally planned a chapter on physical measurements, e.g., resistivity, Hall effect measurements, and similar techniques, it was decided that such a chapter would be inconsistent with the intent and general tenor of this International Series, addressed as it is to Analytical Chemistry. While the techniques mentioned above are used widely in measuring many of the physical and electrical parameters of semiconductors, they are not widely applicable to trace analysis, especially where more than a few impurities are present, as is usually the case. The editor thanks everyone who has in one way or another contributed to this volume. A faulty memory does not, however, excuse him from thanking those who have made major contributions and acknowledging the help of those who have given generously of their time. First and foremost thanks are given to the authors of the various chapters without whose contributions this book would not have been possible. Their understanding and patience through many delays was especially gratifying. The U.S. Air Force Cambridge Research Laboratories Library staff aided the editor materially in obtaining references for Chapter I. Their always PREFACE ix cheerful help is gratefully acknowledged. The mathematical rearrangement and exposition of Rubinson's equation, also in Chapter I, is by Mr. Edward Burke, also of U.S.A.F. Cambridge Research Laboratories. Without the encouragement and time granted from regular duties by Mr. C. D. Turner and Dr. B. Rubin, Chiefs of the Solid State Chemistry Branch and Purification and Properties Section, respectively, Chapter I could not have been written nor the book edited. This liberal U.S.A.F. policy is gratefully recognized and commended. It has been a pleasure to deal with Pergamon Press through its editors, especially the New York staff. Professors L. Gordon and R. Belcher, the International Series Editors, contributed many useful and valuable suggestions both as to format and content. Finally, the editor warmly thanks his wife, Dale, for the many hours of assistance both in the proof reading and in editing help in those spots where sticky grammatical construction was found. Thanks are also due to my four daughters, but especially Andrea, for many hours of cheerful and unsolicited assistance. INTRODUCTION BERNARD RUBIN U.S. Air Force Cambridge Research Laboratories, Bedford, Massachusetts THE analysis of trace impurities in solids implies the quantitative and quali tative examination of very small amounts of chemical imperfections in solid- state matrices. In general, the level of the impurities that falls within the realm of trace-analysis is the part per million range and less. Interest in this range of impurity level was stimulated initially by semiconductor science and technology, and more recently, by the increasing demand for more reliable industrial materials. Thus, the need for ultrapure materials has given an additional impetus to the growth of the science of trace analysis. There is no intent to disregard the important role of physical imperfections on the properties of solids or the interactions between chemical and physical imperfections and their effects. It is perhaps because chemists have played such a major role in contributing to the text of this book that chemical imperfections have been emphasized. Furthermore, many more sensitive methods are available for detecting chemical rather than physical impurities, and the role of the former impurity is perhaps better understood. Historically, the need for ultrapure materials arose during World War II when extraordinarily pure silicon and germanium were required as semiconductors. A brief description of this class of materials will indicate the reason for the need of controlled ultrapurity and the necessity of monitoring this purity. Semiconductors may be considered as systems of electrons and nuclei whose properties are defined by a particular energy distribution of the electrons. According to the band structure theory, electronic energy levels are grouped together in bands in which the energy separation between levels is infinitesimal. The band-gap energy is defined as the difference in energy between the valence band and the conduction band. According to this band structure theory, the distinction between a semiconductor and a metal is that semiconductors have non-zero band-gap energies, while for metals, the energy gap is zero. By this definition an insulator is a large band-gap semi conductor. At temperatures above the absolute zero, some electrons will be thermally excited from the valence band to the conduction band. The absence of an electron in an energy level is defined as a hole. As a result of thermal excitation, therefore, holes are produced in the valence 1 2 TRACE ANALYSIS OF SEMICONDUCTOR MATERIALS band. When the number of holes equals the number of electrons, a semi conductor is called intrinsic. How is all this related to purity or pure materials! An atom or molecule added to the system, which is different from the atoms or molecules present in the pure system, is called an impurity, and impurities can introduce energy levels which modify the properties of the materials. It should be emphasized at this point that minute quantities of impurities can affect these changes i.e. quantities as small as 1012 atoms per cubic centimeter of matrix. The levels introduced by the impurity generally are different in energy from the levels of the pure system. Thus, the addition of an impurity usually introduces levels in the band-gap. If some of the new levels are occupied and lie just below the conduction band, the electrons can be thermally excited to the conduction band, thereby increasing the number of electrons. In this case, the impurity is called a donor. If some of the new levels lie just above the valence band and are unoccupied, valence band electrons can be thermally excited to these levels. This excitation increases the number of holes, and the impurity is called an acceptor. The addition of impurities is called doping. Because conduction is a function of the number of holes and electrons, it is apparent that by the controlled addition of impurities, the conductivity of a semiconductor can be increased. This implies that, for a desired con ductivity, controlled introduction of impurities is necessary, and furthermore that a pure matrix is required to begin with in order to dope to a prescribed level. There is another way in which impurities can affect the properties of a semiconductor. If unoccupied levels are present near the center of the band- gap, excess conduction band electrons can drop to these levels and then to the valence band. This two-step process is much faster than the direct transition and these levels are called recombination centers, trapping centers or traps. It should be pointed out that the latter arise from impurities other than the donor or acceptor type and gives rise to a loss in conductivity. Thus, by adding small amounts of impurities to intrinsic semiconductor materials, some very interesting, controllable, and useful electrical character istics result. If atoms of an impurity are substituted for atoms of a pure material in the crystal structure, the valence electrons of the impurity atoms determine the electrical characteristics of the sample over a wide temperature range. The example cited (semiconductor solids) is only one where impurities play an important role, and where knowledge of the level of the impurities is necessary for proper control of the electronic properties. Recently, it was discovered that by increasing the impurity level from the range of 1012 atoms per cm3 to 1020 atoms per cm3 on both sides of a p-n junction, new current- voltage characteristics were obtained for this diode. Where previously the slope of the current-voltage curve was positive, a negative resistance region INTRODUCTION 3 was obtained to yield what is termed the tunnel diode. New devices became apparent from this more heavily-doped material and the theory of semi conductors expanded. From this it became even more apparent that con trolled chemical imperfection can lead to new and important electronic devices. In other related fields, controlled amounts of impurities affect the properties of a matrix material. In magnetics it is the impurity that is responsible for the action of masers and lasers. It is the controlled amount of chromium ion in aluminium oxide that is responsible for the light amplification in the ruby system. Furthermore, the host material must be devoid of any other con taminants. Otherwise, line broadening in the magnetic spectrum of the dopant occurs and the electromagnetic properties become altered. In the field of electron tubes, the work function of thermionic emitters is altered by the presence or absence of small amounts of impurities. In the field of plasmas, high-purity gases are required if controllable and reproduc ible parameters are to be obtained. However, it is not necessary to confine this discussion of impurities and their effects on properties to the domain of electronics. The mechanical and structural properties of materials are also modified by contaminants. Tungsten, whose use was essentially relegated to filaments in electric-light bulbs and whose handling was always a problem because of brittleness, has been purified and no longer possesses this undesirable characteristic. It thus offers many new applications as a key material in rocketry. Beryllium oxide, used as a coating material or as a moderator in nuclear reactors, is difficult to fabricate unless it is made in very high purity. Aluminium manufacturers and airframe companies are extremely inter ested in producing high purity aluminium, because impurities give rise to metal fatigue and other undesirable properties that may result in structural failure. Carbon and graphite are currently made at a level called spectrographically pure because small amounts of boron with its high cross-section for neutron capture can attenuate the flux of a nuclear reactor. In the realm of organic materials it has been shown recently that electrical conductivity in conjugated systems is in some way related to the impurity content. The rates of chemical reactions are affected by small amounts of impurities present in the reactants. It has been demonstrated that small amounts of iron impurity increased the oxidation rate of graphite 540 times. The impact of the varied role of impurities has already been felt. Several industrial houses are currently producing a line of ultra-pure chemicals related to the semiconductor business. Not only are ultra-pure matrices being produced, but diffusants, alloys, dots, and dopants are available. Chemicals related to the synthesis of these materials are produced as a 4 TRACE ANALYSIS OF SEMICONDUCTOR MATERIALS special line. Solvents, such as acetone and alcohol, as well as acids for etching, may now be purchased. New terms—extra-pure, ultra-pure, high quality, semiconductor-grade, electronic grade, and transistor-grade, have only recently been introduced into the scientist's vocabulary. New laboratories and manufacturing facilities have special clean-rooms constructed as part of the facility to minimize contamination of items produced. Special attention is paid to materials of construction, ambient atmospheres, and personnel, so that ultra-pure conditions are always maintained. The attainment of chemical perfection of some of the elements has made them an integral part of our industrial scheme. Whereas twenty years ago elements such as germanium silicon, gallium, arsenic, phosphorus, and boron were chemical oddities, and in many cases discarded, today they are produced in large quantities to meet the demands of the electronics industry. They command high prices per unit weight. The rare-earths are currently enjoying an unprecedented demand, because their purities are now in the range suitable for electronic application. It is difficult to predict how many other elements will fall into this category once sufficiently high purity has been attained to reveal their real and useful properties. It is clear that the chemical perfection of materials yields new and inter esting properties, and precise knowledge of the imperfection levels is man datory to control them. It is regrettable that the demands imposed today by technology exceed the level of knowledge of materials. It would be far better for materials producers to exceed the demand for materials whose purity is above the levels required. To monitor the necessary purity levels, techniques for detecting impurities quantitatively and qualitatively must also improve. The chemist has many tools available, amongst them emission spectrography, neutron activation analysis, mass spectrography, chromatography, polarography, colorimetry, X-ray spectrography and fluorimetry, infrared absorption analysis, and elec trical conductivity. The physicist has introduced such techniques as nuclear magnetic resonance absorption, electron paramagnetic resonance absorption, and Hall effect measurements. No one of these is the perfect method. For example, electron spin resonance must deal with atoms with unpaired electrons. Nuclear magnetic resonance provides information about the magnetic moment of atomic nuclei in the presence of internal fields and is best applied to elements or compounds. However, NMR has not been developed to the point where it is applicable to impurity detection. Electrical conductivity and Hall effect measurements do not determine the chemical identity and concentration gradients of impurities, and compensation effects of donors and acceptors interfere with the understanding of the material. Although infrared absorption has been used extensively for the analysis of solutions, and more recently has been applied to the determination of band
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