Table Of ContentINTERNATIONAL 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.
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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
