Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties, and Applications

254 Pages·1975·4.621 MB·English

by  J. L. Shay, J. H. Wernick and B. R. Pamplin (Auth.)

OTHER TITLES IN THE SERIES IN THE SCIENCE OF THE SOLID STATE
VOL. 1. GREENAWAY AND HARBEKE—Optical Properties and Band Structures of Semiconductors.
VOL. 2. RAY—II-VI Compounds.
VOL. 3. NAG—Theory of Electrical Transport in Semiconductors.
VOL. 4. JARZEBSKI—Oxide Semiconductors.
VOL. 5. SHARMA and PUROHIT—Semiconductor Heterojunctions.
VOL. 6. PAMPLIN—Crystal growth.
TERNARY CHALCOPYRITE
SEMICONDUCTORS: GROWTH,
ELECTRONIC PROPERTIES,
AND APPLICATIONS
J. L SHAY
BELL TELEPHONE LABORATORIES, INCORPORATED
HOLMDEL, NEW JERSEY 07733
AND
J. H. WERNICK
BELL TELEPHONE LABORATORIES, INCORPORATED
MURRAY HILL, NEW JERSEY 07974
P E R G A M ON P R E SS
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Copyright © 1975 Bell Telephone Laboratories Incorporated
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publishers
First edition 1975
Library of Congress Cataloging in Publication Data
Shay, Joseph Leo, 1942-
Ternary chalcopyrite semiconductors growth,
electronic properties, and applications.
(International series of monographs in the science
of the solid state, v. 7)
Includes bibliographies.
1. Semiconductors. 2. Chalcopyrite crystals.
3. Crystals-Growth. 4. Energy-band theory of solids.
I. Wernick, Jack Harry, 1923-joint author.
Π. Title.
QC611.8.C45S52 1975 537.6'22 74-5763
ISBN 0-08-017883-9
Printed in Great Britain by A.Wheaton & Co., Exeter
ACKNOWLEDGMENTS
This book probably would never have been written without the efforts of several indi-
viduals over the last few years. P. A. Wolff, A. G. Chynoweth and C. Κ. N. Patel have
encouraged and supported our own research, and provided helpful suggestions and ad-
vice. Our colleagues, K. Bachmann, Ε. Buehler, G. Boyd, H. Kasper and Β. Tell, have
contributed enormously to our understanding of ternary chalcopyrite semiconductors.
The cooperation of our secretaries, Miss Susan Feyerabend and Miss M. P. Reitter,
in typing the several drafts of the manuscript is gratefully appreciated.
EDITORS PREFACE
THE diamond structure is one of the simplest and most symmetrical arrangements of
atoms known in crystallography. Only the two important semiconducting elements, Si
and Ge, habitually crystallize in this structure and their commercial importance can be
traced back to this fact. The four electron per site rule, which can be deduced from the
Grimm-Sommerfeld rule of chemical bonding, can account for their semiconducting
and doping properties and the strength of the bonding down from diamond through Si
and Ge to grey Sn can account for the change in energy gap and thermal conductivity.
The rule can also be used to predict semiconductivity in a vast family of compounds
related to diamond which is variously called the tetrahedral or adamantine or simply
diamond-like family. The latter name does not strictly include the structures derived
from wurtzite which I feel are in this family, because every anion is bonded tetrahedrally
to four cations and every cation to four anions.
As one proceeds from the group IV elements to the III-V and II-VI compounds new
properties become available for study and exploitation. A wider choice of energy gaps
and band structure enables devices depending on larger and sometimes direct energy
gaps (like light-emitting diodes and heterojunction lasers) or on high mobilities (like
Hall effect and magnetoresistance devices) to be made which cannot be made with the
elements. Some completely new devices like those based on the Gunn Effect also become
possible. Moving on to ternary compounds the choice becomes wider still and the drop
in symmetry due to superlattice formation opens the way for non-linear devices and inter-
esting optical properties.
This book deals with the two most interesting ternary cousins of the tetrahedral family.
The I-III-VI and the II-IV-V compounds, when they form, usually have the chal-
2 2
copyrite structure and fifty such compounds are now known. It is interesting to note that
no antichalcopyrite compounds (e. g. of the families III -IV-VI II -V-VII) have been
2 2
found to exist in spite of many attempts to make them. These materials are now perhaps
in the same state that the III-V compounds reached ten years ago. The preliminary work
has been done on many of them and the door is open for applications, but a great deal
of scientific and technological development particularly in crystal growth, characteri-
zation, detailed band structure studies and device exploitation remains to be done. I
welcome this book which provides a timely and thorough summary of the work done
and pointers to the future.
Chalcopyrite itself was known many years before semiconduction in the elements was
discovered. Its crystal structure was the subject of an early controversy in crystallography.
Burdick and Ellis (1917) proposed a unit cell based on a tetragonally distorted diamond
unit cell. Fifteen years later Pauling and Brockway using single crystals confirmed the
now generally accepted body centred tetragonal structure with a unit cell twice the size.
The Burdick and Ellis structure is a possible superlattice structure for a tetrahedral
A-B-C compound and may yet be found to exist if people will look for it. Chalcopyrite
2
is antiferromagnetic and is one of only a very small number of magnetic tetrahedral
semiconductors.
la OXS ix
Early work on these compounds was performed out of purely scientific interest, this
was spurred on in the early sixties by the hope of thermoelectric applications. These
materials have very low lattice thermal conductivities, which for many devices is a
disadvantage. Some, like CdSnAs, have remarkably high mobilities. It was hoped that
2
one member of the family would be found, chemically stable and cheap to produce,
which might have a sufficiently high figure of merit to enable it to be developed for the
one-in-every-home or one-in-every-cav mass market for refrigerators or battery chargers.
However it soon became clear that this dream was not be be realized and some steam
went out of the drive for new compounds.
Careful scientific work continued and application-minded scientists watched the devel-
opment of work in the III-V and II-VI compounds on light-emitting junctions and
heterojunctions, Gunn effect devices and acoustooptical effects, and on magnetic semi-
conductors and amorphous materials. As this book reports, all these topics can be studied
in the ternary compounds. But pessimists will argue that such studies are admirable
topics to amuse academic workers and PhD students but unlikely to prove of commercial
interest because of the technological difficulties inherent in three element semiconduc-
tors. They said the same about GaAs-AlAs alloys ten years ago !
The authors have drawn out the present state of possible applications of these com-
pounds for optical communications systems as infrared- or visible light-emitting materials
and for use with other lasers as pumps or up convertors. There may be a break-through
in the far infrared by down conversion or spin flip action or in the blue or ultraviolet
region using some of the newly discovered nitrides. Again the peculiar non-linear pro-
perties of these compounds may have an application round the corner in some electrical
device—who knows what hot electron effects may be found in tetragonal materials with
pseudo-direct gaps or how phonon interactions may be affected.
The First International Conference on Ternary Chalcopyrite Semiconductors was
held at Bath in November 1973. Over seventy participants from eight countries reported
on and discussed these compounds and their properties. I hope this book will be available
for participants at the Second Conference in Strasbourg in 1975.1 am writing these notes
a few days after returning from a six-week lecture and study tour of Japanese semicon-
ductor laboratories under the auspices of the Royal Society. I can report great interest
in these materials both in Japan and at the international conference and school on crystal
growth held there this spring. I am sure that this admirable book by two eminent workers
in the field, who make such a splendid matched pair in their diverse yet overlapping
interests, and who work at the leading semiconductor research laboratory in the world,
will be read with close interest by a wide variety of scientific workers and used as a
Bible by workers in the fields of ternary compounds and non-linear optics. I am sure it
will stimulate much further work.
B.R.P.
CHAPTER 1
I N T R O D U C T I ON
ALTHOUGH many binary and ternary naturally occurring compounds were known to
exhibit behavior now associated with semiconductors long before the invention of the
transistor, it was this development that stimulatedsystematic searches for new semiconduc-
tors in the early 1950s. The first materials investigated in this manner were the III-V
compounds which exhibit the diamond structure. Blum et al. (1950) and Goryunova
and Obukhov (1950) reported InSb to be a semiconductor, but it appears that Welker
(1952) was the first to appreciate the importance of the III-V compounds as a class of
new semiconductors. High carrier mobilities and the ability to dope n- and /?-type were
properties investigators strived for.
A natural extension in the search for new semiconductors was to examine ternary
compounds exhibiting diamond-like or tetrahedral coordination, and Goodman and
Douglas (1954) discussed the possibility of semiconductivity in the I-III-VI compounds
2
synthesized a year earlier by Hahn et al (1953). Goodman (1957) further showed that
by ordered substitution of Groups II and IV atoms for the Group III atoms in the
III-V compounds, new ordered semiconducting II-IV-V compounds could be pre-
2
pared.
Although only two or three of the II-IV-V compounds exhibited electron mobilities
2
of a few thousand, the II-IV-V and I-III-VI compounds, and in particular, solid
2 2
solutions of them with their III-V and II-VI analogs, were investigated from the thermo-
electric standpoint. For utility as practical thermoelectric materials, high-quality single
crystals generally are not required.
The discovery of the laser and the interest in light-emitting materials in the late
1950s and early 1960s stimulated renewed effort in these materials, particularly by the
Russian workers. Studies aimed towards understanding the electronic structure and
elucidating the nonlinear optical properties of these materials, with the hope of produc-
ing a technological material, require high quality single crystals. Many of these materials
are difficult to grow in the form of large, high quality crystals, and it has only been
within the past few years that researchers have been successful in this effort.
Ternary chalcopyrite crystals are currently of technological interest since they show
promise for application in the areas of visible and infrared light-emitting diodes, infrared
detectors, optical parametric oscillators, upconverters, and far infrared generation. It
has been discovered that several ternary compounds can be obtained both p- and «-type.
In addition it has been found that two of them, CuGaS and CuAlS, can be made
2 2
/7-type and have direct bandgaps in the visible and ultraviolet respectively. They are
unique in this respect, and have generated activity in the area of heterojunctions with
large bandgap II-VI compounds, which can only be obtained «-type. Infrared light-
emitting diodes have been prepared by epitaxial growth of CdSnP on InP.
2
By virtue of the noncubic crystal structure, these compounds are optically biré-
fringent, and therefore nonlinear optical interactions can be phase-matched. Further-
la* 1
TERNARY CHALCOPYRITE SEMICONDUCTORS
more, these compounds are covalently bonded to a large extent, so that they have large
nonlinear coefficients, a measure of their usefulness as nonlinear optical materials.
These compounds are also of interest from a fundamental point of view since the
chalcopyrite structure is the simplest, noncubic ternary analog of the well-understood
binary zincblende structure. It has been found that the energy band structure of ternary
crystals differs from that of binary compounds in several nontrivial ways by virtue of
the noncubic structure. Degeneracies are lifted, and forbidden electronic transitions
become allowed. Whereas the valence bands of most zincblende crystals are composed
of s- and /?-like orbitals, the noble metal ^-levels in I-III-VI compounds hybridize
2
with the otherwise s- and /?-like orbitals, and lead to several anomalous features of the
energy band structure.
In Chapter 2 we review the crystal growth of II-IV-V and I-III-VI single crystals.
2 2
In Chapters 3 and 4 we review studies of the electronic structure of II-IV-V and
2
I-III-VI compounds respectively. And in Chapters 5-8 we review the luminescence,
2
nonlinear optical, electrical properties, and miscellaneous physical properties of chalco-
pyrite crystals.
References
BLUM, A. N., MOKROVSKI, N. P., and REGEL, A. R. (1950) Seventh All-union Conference on the Pro-
perties of Semiconductors, Kiev.
GOODMAN, C. H. L. (1957) Nature 179, 828.
GOODMAN, C. H. L., and DOUGLAS, R. W. (1954) Physica 20, 1107.
GORYUNOVA, Ν. Α., and OBUKHOV, A. P. (1950) Seventh All-union Conference on the Properties of Semi-
conductors, Kiev.
HAHN, Η., FRANK, G., KINGLER, W., MEYER, Α., and STORGER, G. (1953) Z. anorg. Chem. 271, 153.
WELKER, Η. (1952) Z. Naturforsch. 11, 744.
2
CHAPTER 2
THE CHALCOPYRITE STRUCTURE A ND
CRYSTAL G R O W TH
2.1 The Chalcopyrite Structure
The I-III-VI and II-IV-V (ABC ) phases possess the tetragonal El structure-type
2 2 2 x
space group ÏAld, with four formula units per cell.f The known phases exhibiting this
structure are tabulated in Tables 2.1 and 2.2. Each A- and B-atom is tetrahedrally co-
ordinated to four C-atoms, while each C-atom is tetrahedrally coordinated to two A-
and two B-atoms in an ordered manner (Fig. 2.1). The atomic distribution is as
follows:
equivalent position j 000 ;
4A in (a) positions, 000; 0
4B in (b) positions, 00 — ; 0
8C in (d) positions, χ ; χ ; — χ — ; — χ —
w F 48 48 4 8 3 8
If the cations A and Β were distributed at random, the cubic zincblende structure
would result. Thus the chalcopyrite structure is a superlattice of the zincblende structure
with the c\a ratio approximately equal to 2. The tetrahedral coordination implies that
the bonding is primarily covalent with sp3 hybrid bonds prevalent, although there is
some ionic character present because the atoms are different.
The I-III-VI and II-IV-V compounds can be regarded as the ternary analogs of
2 2
the II-VI and III-V binary compounds respectively. They can be derived from the
binary phases by ordered substitution of other group atoms so as to maintain an elec-
tron-to-atom ratio of 4. The derivation of these phases from the pure covalent crystals,
Si and Ge, is schematically illustrated in Fig. 2.2. The ordered arrangement of the
metal atoms (or cations) leads to the formation of the tetragonal superlattice.
The quantity 2—c\a is a measure of the tetragonal distortion which may occur as
a result of ordering. The x-parameter which precisely locates the C-atom position,
depends on the difference between the A-C and B-C interactions. Since 2—cja is
greater than zero for most II-IV-V compounds, χ greater than 0.25 (the value for the
2
cubic zincblende structure) represents a motion of the V-atom toward the IV-atom and
away from the column II atom.
t The prototype phase is the mineral chalcopyrite, CuFeS.
2
J Add the coordinates of the equivalent positions to the coordinates of the a, b, and d sites to obtain
the coordinates of all of the atoms.
3
TERNARY CHALCOPYRITE SEMICONDUCTORS
TABLE 2.1. Lattice Parameters for I-HI-VI2 Compounds
β (A) c(A) c/a References
CuAlS 5.323 10.44 1.96 a
2
5.312 10.42 1.96 b
5.31 10.41 1.96 e
5.336 10.444 1.958 1
CuAlSe 5.617 10.92 1.94 a
2
5.60 10.90 1.95 b
5.60 10.86 1.94 e
5.6035 10.977 1.959 m
CuAlTe 5.967 11.80 1.97 a
2
5.96 11.74 1.97 e
CuGaS2 5.34741 10.47429 1.9588 j
5.360 10.49 1.96 a
5.35 10.48 1.96 c
5.34 10.47 1.96 e
5.351 10.484 1.959 f,g
5.328 10.462 1.964 f,h
5.349 10.480 1.959 i
CuGaSe 5.618 11.01 1.96 a
2
5.602 11.00 1.964 d
5.60 10.98 1.96 e
5.6159 11.0182 1.962 m
CuGaTe 6.006 11.93 1.98 a
2
5.99 11.92 1.99 e
6.025 11.948 1.983 k
CuInS 5.528 11.08 2.00 a
2
5.51 11.02 2.00 e
5.524 11.13 2.01 f
5.52279 11.13295 2.0158 j
CuInSe 5.785 11.57 2.00 a
2
5.77 11.54 2.00 e
CuInTe 6.179 12.36 2.00 a
2
6.16 12.32 2.00 e
CuTlS 5.58 11.16 2.00 e
2
CuTlSe 5.83 11.60 1.99 e
2
AgAlS 5.707 10.28 1.80 a
2
5.69 10.24 1.80 e
AgAlSe 5.968 10.77 1.80 a
2
5.95 10.71 1.89 e
AgAlTe 6.309 11.85 1.88 a
2
6.29 11.83 1.88 e
AgGaS 5.755 10.28 1.79 a
2
5.74 10.27 1.79 e
5.75 10.305 1.79 f
5.75722 10.3036 1.7897 j
AgGaSe 5.985 10.90 1.82 a
2
5.97 10.87 1.82 e
5.9920 10.8863 1.817 m
AgGaTe 6.301 11.96 1.90 a
2
6.29 11.95 1.90 e
4