Semiconducting III-V Compounds

247 Pages·1961·13.89 MB·English

by  cyril hilsum

OTHER TITLES IN THE SERIES ON SEMICONDUCTORS
Vol. 2 Photo and Thermoelectric Effects in Semiconductors.
JAN TAUC.
Vol. 3 Semiconductor Statistics.
J. S. BLAKEMORE.
Vol. 4 Thermal Conduction in Semiconductors.
J. R. DRABBLE and H. J. GOLDSMD.
SEMICONDUCTING
III-V COMPOUNDS
by
G. HILSUM
and
A. C. ROSE-INNES
Services Electronics Research Laboratory
Baldocky Hertfordshire England
y
PERGAMON PRESS
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1961
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Copyright
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1961
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Library of Congress Card Number 60-53574
Set in Baskerville irfispt. and
printed in Gnat Britain by Spotnswoodt, Ballantyru & Co., Ltd., London and Colchester
To our wives
BETTT and BARBARA
who spent so many hours
helping with
this book
PREFACE
IT is now ten years since the semiconducting properties of the III-V
compounds were first reported. Since then several hundred papers and
articles have been written about them, and a stage has now been reached
where a broad study of these materials is possible. We have attempted
such a study with two objects in mind: firstly, to describe the properties
of III-V compounds as a family of semiconducting crystals and, secondly,
to relate these compounds to the better known and better understood
monatomic semiconductors, silicon and germanium. The rapid advance
that has been made in semiconductor physics and devices is a notable
example of how pure research and technology can stimulate each other,
and for this reason we have thought it important to include a chapter on
applications of III-V compounds.
In this book we have assumed that the reader is acquainted with the
fundamental ideas about semiconductors and that there is no need to
explain basic concepts, such as "A-space", "hole" or "donor impurity".
We have also assumed that the reader has some knowledge of the pro­
perties of germanium and silicon, and we often compare the properties
of III-V compounds with those of these "prototype" covalent semi­
conductors.
Even in so specialized a topic as semiconducting III-V compounds it is
impossible to refer to all the published work, and it would not be desirable
to include references simply for fear of leaving somebody out. We have
chosen references either as sources of information given in the text or as
suitable further reading and we are sorry if any injustice has arisen from
this choice.
It is a pleasure to acknowledge the many helpful discussions we have
had with the members of the Solid State Division at the Services Elec­
tronics Research Laboratory, and we especially thank Mr. J. W. Allen,
Dr. F. A. Cunnell, Dr. E. H. Rhoderick, Dr. O. Simpson and Mr. R. I.
Walker for reading and commenting on the manuscript. We are grateful
to the many authors who have given us permission to quote from their
unpublished work.
C.H.
A. C. R.-L
Services Electronics Research Laboratory
Baldock, Herts
August 1960
Chapter 1
INTRODUCTION
ATOMS from group Illb of the periodic table combine with atoms from
group Vb to form crystalline semiconducting compounds. We call these
semiconductors "III-V compounds". They are not alloys but definite
chemical compounds with a 1:1 atomic ratio between the III and V
atoms, which occupy alternate sites in the crystal lattice. For example, if
we melt a mixture of indium and antimony, the compound InSb is
formed, and, on solidification, any excess of either constituent is thrown
out as a second phase.
Ill atoms have one valence electron less, and V atoms one electron
more than those atoms from group IV of the periodic table which form
the semiconducting crystals, diamond, germanium, silicon and grey tin.
III-V compounds have, therefore, the same average number of electrons
per atom as the group IV semiconductors, and it is found that the com­
pounds indeed have a crystal structure and electronic properties which
are in many ways similar to those of the group IV semiconductors.
Nevertheless, III-V compounds possess characteristic properties which
distinguish them from the group IV semiconductors. These differences
arise chiefly from the fact that the compound crystals have a lower
symmetry than the group IV crystals and that, whereas the group IV
semiconductors consist of covalently bound neutral atoms, the III-V
compounds contain positive and negative ions at the lattice sites.
The first of the III-V compounds to be reported was InP, prepared
over fifty years ago by Thiel and Koelsch (Tl). Huggins (HI) pointed
out in 1926 that binary compounds formed between group Illb and
group Vb elements should crystallize in a form similar to the diamond
structure taken up by the group IV semiconductors, and this was con­
firmed by the experiments of Goldschmidt (GÌ), who in 1929 investigated
the crystal structures of InSb, GaSb, GaAs, GaP, AlSb, AlAs, AIP and
AIN. The fact that one of the compounds, InSb, is a semiconductor akin
to germanium and grey tin, was reported in 1950 by Blum, Mokrovski
and Regel' (Bl) and by Goryunova and Obukhov (G2). Welker (Wl),
i 1
2 INTRODUCTION
however, appears to have been the first to appreciate the importance
of the III-V compounds as a new family of semiconductors, when in
1952 he described the semiconducting properties of several of these com­
pounds and drew attention to some of their special properties, such
as the very high electron mobility and small energy gap of indium
antimonide.
Binary IIIa-Vb compounds have been prepared (II) between the Ilia
rare earths and the Vb elements, for example, CeAs, PrN, but these
compounds have the sodium chloride crystal structure and are probably
essentially ionic crystals. No compounds or alloys between group Ilia
-and Va elements have been reported and it seems likely that they do not
exist Lanthanum and niobium are insoluble in each other, both in the
liquid and solid state (SI), and lanthanum and tantalum also appear to
be mutually insoluble in both states.
Though III-V crystals have a structure similar to that of diamond,
there are two kinds of atom in the unit cell, and so they form a more
general system than the group IV semiconductors. There are a large
number of possible combinations and these compounds cover a wide
range of semiconducting properties, useful both for a study of semi-
conduction and for the manufacture of devices. In spite of the fact that
the compounds have two different atoms in the unit cell, they sometimes
show simpler behaviour than the monatomic group IV semiconductors.
Indium antimonide and indium arsenide, for example, approximate
rather closely to the simplest model of a semiconductor in which a
spherical conduction band and a spherical valence band both he at the
centre of the zone. On the other hand, III-V compounds are usually more
difficult to prepare in a pure form than the group IV semiconductors,
and, for many of the compounds, the measured properties are those
governed by impurities rather than those of the pure material, even for
the purest samples yet prepared.
In Table 1.1 we list some of the important properties of the III-V
compounds. Indium antimonide has probably been the most intensively
studied because it has remarkable properties resulting from its small
energy gap and very high electron mobility. Furthermore, it is relatively
easy to grow pure single crystals of this material. In several of the com­
pounds, the mobility of the electrons is much greater than that of the
holes. As a result, the familiar semiconductor terms, "n-type" and "/>-
type", must be used with some caution. For instance, indium antimo­
nide shows properties characteristic of negatively charged carriers, such
as a negative Hall coefficient, even when there are a hundred times more
INTRODUCTION 3
Table l.I. Some properties of III-V compounds and group IV semiconductors
Hall mobility
at room
Energy gap
Melting temperature*
Crystal at room
point (cm*sec-W-1) Remarks
structure temperature
(°K)
(eV)
Electrons Holes
BN hexagonal,
also ZB ~10-0
A1N Würz >2400
GaN Würz 3-3
InN Würz
BP ZB 2000-3000 5-9 100
AIP ZB 30 unstable
in air
GaP ZB 1450-1500 2-25 110 75
InP ZB 1062 1-29 4600 150
BAs ZB
AlAs ZB >1600 2-16 unstable
in air
GaAi ZB 1237 1-4 8500 420
InAs ZB 942 0*36 33,000 460
BSb ZB?
AlSb ZB 1050 1-62 200 420 unstable
in moist air
GaSb ZB 712 0-67 4000 1400
InSb ZB 525 0-17 78,000 750
InBi tetrag 103 metallic
TIBi CsQ? 230 metallic
C diamond -5-2
Si diamond 1420 1-08 1450 500
Ge diamond 958 0-66 4500 3500
a-Sn diamond (150) 0-08
Würz a wurzite; ZB » zinc-blende; tetrag » tetragonal.
* Best experimental value reported.
4 INTRODUCTION
holes than electrons. We shall use the term " n-type " to describe material
in which there are more conduction electrons than holes, and "/>-type"
to describe material with more holes than electrons, irrespective of the
sign of the measured electrical properties.
Many physical processes which are of importance in deciding the
behaviour of III-V compounds are of little importance, or do not occur,
in monatomic semiconductors such as germanium and sihcon. In this
book we have tried to concentrate on these processes which are peculiar
to III-V compounds, and not to dwell on those aspects which are com­
mon to semiconductors in general.
REFERENCES
Bl. BLUM, A. N., MOKROVSKI, N. P. and REGEL', A. R., Seventh All'Union Conference on the
Properties of Semiconductors, Kiev (1950).
Gl. GOLDSCHMIDT, V. M., Trans. Faraday Soc. 25, 253 (1929).
G2. GORYUNOVA, N. A. and OBUKHOV, A. P., Seventh All-Onion Conference on the Properties of
Semiconductors, Kiev (1950).
HI. HUOGINS, M. L., Phys. Rev. 27, 286 (1926).
II. IANDELLI, A. and BOTTI, E., Atti accad. nazi. Lincei 25, 129, 498, 638 (1937).
SI. SAVITSDI, E. M., TEREKHOVA, V. F. and BUROV, I. V.,Zhur.Neorg. KhimA, 1462 (1959).
Tl. THIEL, A. and KOELSCH, H„ Z. anorg. Chem. 65-66, 288 (1910).
Wl. WELKER, H., Z. Naturforsch, 11, 744 (1952).
Chapter 2
CRYSTAL STRUCTURE AND BINDING
2.1 CRYSTAL STRUCTURE
NEARLY all the III-V compounds crystallize in an arrangement where
each atom is at the centre of a regular tetrahedron, at the four corners
of which he atoms of the other kind. These tetrahedra can be arranged
into two forms of crystal structure: " zinc-blende'' which is cubic, and
" wurzite" which is hexagonal.
The cubic zinc-blende (or "sphalerite'') structure is the same as the
diamond form except that the two different kinds of atom occupy
alternate positions in the lattice (Plate I). The III and V atoms each lie
on a face-centred-cubic sub-lattice, the two sub-lattices being displaced
relative to each other by one quarter of the body diagonal of the cube.
The wurzite structure is similar to zinc-blende except that alternate
(111) layers are rotated through 180° about the [111] axis, giving the
structure hexagonal symmetry.
The twelve compounds that boron, aluminium, gallium and indium
make with phosphorus, arsenic and antimony all have the zinc-blende
structure. The nitrides of aluminium, gallium and indium have the
wurzite form. Boron nitride normally has a hexagonal structure similar
to graphite, though tiny crystals of the zinc-blende form ("Borazon")
have been prepared (§ 5.3). The bismuthides appear to be metallic
compounds without tetrahedral structure. The forms and constants of
the crystal structures are collected in Table 2.1.
We can assign a characteristic radius to each III and V atom, such
that the separation between the nuclei of neighbouring atoms in the
tetrahedrally bonded compounds is the sum of the two atomic radii.
This "tetrahedral radius" is virtually independent of the compound in
which the atom finds itself. In Fig. 2.1 are given the values of the tetra­
hedral radii for III and V atoms. As an example, the sum o fthe radii
of indium and antimony is 2-80 Â, which is the separation o fthe nuclei
of these atoms in indium antimonide.
5