Table Of ContentOTHER TITLES IN THE SERIES IN SEMICONDUCTORS
Vol. 1 Semiconducting III-V Compounds
C. HILSUM and A. C. ROSE-INNES
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. GOLDSMID
Vol. 5 Electroluminescence
HEINZ K. HENISCH
Vol. 6 Imperfections and Active Centres in Semiconductors
R. G. RHODES
Vol. 7 Electrical Properties of Semiconductor Surfaces
DANIEL R. FRANKL
Vol. 8 Structure and Application of Galvanomagnetic Devices
H.WEISS
Vol. 9 Silicon Semiconductor Data
HELMUT F.WOLF
I N T E R M E T A L L IC
S E M I C O N D U C T I NG
F I L MS
H.H. W I E D ER
NWC Corona Laboratories,
Corona, California
P E R G A M ON PRESS
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Copyright © 1970
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without the prior permission of Pergamon Press Inc.
First edition 1970
Library of Congress Catalog Card No. 76-80293
PRINTED IN GERMANY
08013367 3
For my wife,
DORA C. W
F O R E W O RD
MANY advances in the technology of semiconductor devices were made
possible by fundamental research concerned with a quantitative description
of the physical and chemical phenomena in semiconducting materials.
Device technology, in turn, provided some of the impetus and motivation
as well as the framework for the formulation of many research problems.
Current extensive basic and applied investigations on semiconducting
AiiiBv compound films fit such a feedback cycle. They represent a logical
extension of continuing studies made of the structure and properties of bulk
semiconducting crystalline compounds formed from elements in groups Ilia
and Va of the periodic table. They are also concerned with the search for
better, cheaper, and more reliable solid state devices and components than
those in present use.
The wide range of the energy bandgaps of the AmBy compounds and
their high electron mobilities are of considerable fundamental as well as
practical significance. Most of them, including those which are the iso-
electronic analogs of silicon and germanium, are direct gap semiconductors;
electron transitions across the gap proceed with the conservation of electron
momentum. Others, such as GaP or AlSb, have an indirect gap; electron
transitions across the bandgap proceed with the emission or absorption of
phonons. The low effective mass and high electron mobility of direct gap
materials is in contrast with the high eflfective mass and low electron mobility
of the indirect gap compounds.
Materials with a high electron mobility are needed for galvanomagnetic
devices such as Hall generators and magnetoresistors. For the construction
of high-sensitivity photoconductive and photovoltaic devices, the need is for
materials in which the electrons have a low effective mass, a high electron-
to-hole mobility ratio, a low intrinsic carrier concentration, and long charge
carrier lifetimes. Most of these requirements can be met with the AmBy
compounds InSb and InAs.
The electric field-induced transfer of electrons from the low eflfective mass,
high electron mobility, (000) conduction band minima of GaAs, InP or
GaAS;cPi-xj to subsidiary, high eflfective mass, low mobility minima along
<100> directions is the basis of the Gunn eflfect. This two-terminal negative
resistance phenomenon can be used for the generation and amplification of
coherent high-frequency and microwave signals.
A variety of Ai„By compounds, GaP, GaAs, InP, InAs, GaSb, and InSb,
and their solid solutions have been used in the construction of high-efficiency
ix
χ FOREWORD
injection electroluminescent p-n junction diodes. Laser action has been
obtained in direct bandgap compounds and visible light can be generated by
GaP or GaAs^^Pi-x junctions.
The wide bandgap, = 1.4 eV, and the high electron mobility, //„ = 8.5
X 10^ cm^/Vs, of GaAs are of particular significance. They afford an exten
sion in the operational range of diodes, transistors, and other components
to higher temperatures and higher frequencies than similar devices made of
silicon. A considerable effort is being expended at this time on the devel
opment of GaAs as a substitute for the integrated circuit technology based
on silicon.
At present, polycrystalline AmBy films are used primarily for galvano
magnetic device applications. Single-crystal homo- and heteroepitaxial films,
grown onto insulating or conducting substrates, are needed for both active
and passive components and circuits. The purity of such films can be higher
and their electron mobility can be greater than those of melt-grown bulk
crystals. Nevertheless, serious impediments remain. They must be overcome
if ΑιπΒγ films are to be used more extensively than at present. The chemical
thermodynamics of film nucleation and growth are, to a large extent, still
undetermined. The structure, composition, and physical properties of films
need to be investigated in greater detail, particularly where differences appear
between bulk and film properties. Although continuing research is steadily
adding to the available knowledge on AmBy compounds, many of the
problems relating to their surfaces and surface properties remain unsolved.
These problems are acute in films because of their large surface-to-volume
ratio. The physical and chemical characterization of AmBy films is aggravated
by the small film mass and volume, by interfacial imperfections and cross-
diffusion of impurities at the film-substrate interface.
The main purpose of this monograph is to introduce the reader to the
physics and technology of AmBy compound films, to guide him to the
available literature on their preparation and characterization and to provide
him with an assessment of their present-day and contemplated device
applications. The book is intended for graduate students, just starting out
on experimental investigations on semiconducting films, and for research
scientists, whose background includes basic and applied solid state and
semiconductor physics, and who are interested in the status of intermetallic
AniBy films up to December 1967.
I should like to acknowledge my debt to the many authors, their editors
and publishers, who generously allowed the use of figures and illustrations
adapted from the original manuscripts.
I am particularly grateful to Prof. H. K. Henisch, whose suggestion and
encouragement prompted this book, and to my colleagues, A.Nedoluha,
A. R. Clawson, N. Davis, D. Stierwalt, and R. F. Potter, for their perceptive
criticism and valuable comments.
H. H.W.
CHAPTER 1
PREPARATION OF III-V COMPOUND LAYERS
CONTENTS
1.1 Survey of Techniques 2
1.2 Vacuum Deposition 4
1.2.1 Evaporation of the Compound 5
1.2.2 Flash Evaporation 9
1.2.3 Coevaporation of the Elements 16
1.2.4 The Three-temperature Method 19
1.2.5 Recrystallization from the Liquid Phase 27
1.3 Chemical Vapor Phase Growth 30
1.3.1 Closed-tube Transport Reactions 36
1.3.2 Open-tube Transport Reactions 47
1.3.3 Close-spaced Transport Reactions 57
1.4 Liquid Phase Epitaxial Growth 62
1.5 Cathodic Sputtering 64
1.6 Electron Beam Crystallization and Zone Refining 68
1.7 Casting of Thin Films from the Bulk 73
1 ISF
2 INTERMETALLIC SEMICONDUCTING FILMS
1.1 SURVEY OF TECHNIQUES
To a large extent, the choice of any one of the many available methods
for growing thin films of the III-V compounds is a function of the required
crystalline order, perfection and the impurity concentration in the films.
Vacuum evaporation and condensation of the compound or its synthesis
in a vacuum chamber by the coevaporation of its elemental constituents
represents the simplest and most direct procedure for preparing poly-
crystalline films with impurity concentrations of the order of or greater than
10^^ cm-^
Flash evaporation of a III-V compound in the form of a fine-mesh powder
in vacuum is a relatively simple process. If the powder is dispensed uniformly
and continuously onto a heater maintained above the evaporation tempera
ture of the least volatile constituent of the compound, then the film deposited
on a suitably heated substrate will be monophase with the precise stoichiome-
try of the original compound. Such films may be grown epitaxially on single-
crystal substrates; however, they are multiply twinned as a rule, and contain
many growth defects.
A widely used method which insures stoichiometry of a vacuum-deposited
film is the 'Three-temperature Method". It requires control and regulation
of three interdependent temperatures: that of two individual crucibles, each
containing one of the constituent elements of a binary III-V compound,
and that of the heated substrate. The latter must be maintained above the
condensation temperature of the most volatile elemental component and
below the melting point of the compound.
Vacuum evaporation does not afford control of the thermodynamic
parameters of the crystallization process to the same degree as crystallization
from the melt. Consequently, vacuum-deposited films consist of many small
crystallites usually less than 1 μm in diameter. Larger crystallites with
dimensions of the order of millimeters may be grown by a two-step process.
Composite elemental films of a compound are first deposited either
simultaneously or sequentially on hot or cold substrates. They are sub
sequently heated to a temperature slightly above their melting point to form
a contiguous liquid film. Careful control of the fractional content of the
constituents insures the stoichiometry of the crystallized solid film produced
by solidification of the liquid. The thermal gradients determine the solid-
liquid interface and the nucleation and growth of crystallites. The dis
advantage of this method is that the evaporated films must be heated to their
melting point. During melting and solidification, incomplete mixing may
cause localized inhomogeneities and incomplete alloying of the recrystallized
films.
Chenucal vapor phase synthesis is more advantageous if a preselected
type and concentration of impurities must be introduced into a film and if
the growth of a single-crystal film is mandatory.
PREPARATION OF III-V COMPOUND LAYERS 3
Polycrystalline and epitaxial single-crystal III-V films may be grown by
chemical vapor phase transport reactions at temperatures considerably
below the melting points of the compounds. Either a closed-tube, i.e.,
constant volume, or an open-tube, i.e., constant pressure, process is used
to react a halogen or similar reactive gas, diluted in a neutral transfer gas
such as hydrogen, with a polycrystalline III-V compound in the form of a
wafer or in powder form. The reaction products are transported along a
thermal gradient to a substrate where they recombine and are deposited as
a compound film. The quality, perfection, and orientation of the substrate
determine, to a considerable extent, the crystallographic perfection of homo-
or heteroepitaxially deposited layers.
Epitaxial single-crystal III-V compound films may also be grown by an
open-tube gaseous phase transport reaction in which the spacing between a
polycrystalline source wafer and a single-crystal substrate is reduced to less
than 100 μm and the thermal gradient between them is slight. Even though
the temperature gradient is small, the short transfer path produces a rapid
growth of the layers deposited on the substrate. Epitaxial single-crystal films
may be deposited in this manner using either a halide gas or water vapor as
the reactive gas component mixed with a neutral gas transfer agent.
Liquid phase epitaxial growth is a valuable technique used particularly
for the fabrication of injection electroluminescent diodes and other semi
conductor devices. The basic process consists of dissolving an Αιι,Βγ
compound into a suitable metal solvent at a temperature well below the
melting point of the compound. The liquid solution is brought in contact
with a single-crystal substrate and cooled slightly. An epitaxial layer is
formed during the cooling cycle and the solution is then decanted. Very
plane p-n junctions can be grown in this manner by suitable doping of the
melt.
It is remarkable, but not very surprising, that other methods, less frequently
used and in a more rudimentary stage of development, have been concerned
almost exclusively with the synthesis of InSb films. This is due primarily to
the low melting temperature and the comparatively small energy of formation
of this compound.
Cathodic sputtering of polycrystalline III-V compound films in a low-
pressure argon glow discharge has been used to a limited extent to deposit
polycrystalline films of III-V compounds. Films produced by sputtering
consist as a rule of small crystallites. The substrates must be heated, or post-
deposition annealing is required in order to promote the growth of larger
crystallites.
Electron beam microzone melting may be used for the recrystallization of
a previously vacuum-deposited film which consists of the elemental con
stituents of a compound. By sweeping the beam in the form of a raster, a
molten zone can be induced in the film. Synthesis of the compound takes
place by solidification of the melt. Zone refining of a film or homogenization
4 INTERMETALLIC SEMICONDUCTING FILMS
of impurities introduced during deposition may be accomplished by the
interaction between a high-intensity, well-focused electron beam and AmBy
films adequately protected from decomposition while they are in the liquid
state.
Self-supporting polycrystalline layers can be grown by the rapid solidi
fication of a molten drop of the compound between two plane-parallel
quartz plates maintained in an inert gas environment. This method has been
used for the preparation of fragments of InSb with a surface area of 1 cm^,
a thickness of 10 μm, and electrical properties comparable in many respects
to those of bulk InSb.
1.2 VACUUM DEPOSITION
The evaporation of intermetallic compounds in vacuum and condensation
of the vapor onto chemically inert crystalline or amorphous substrates
represents one of the simplest methods for the preparation of thin films.
Such a procedure is indeed feasible for alloys which have nearly identical
vapor pressures at their evaporation and condensation temperatures and for
compounds which have a very high heat of formation.
Intermetallic compounds cannot be deposited by vacuum evaporation
procedures without due consideration of their dissociation, which takes place
even below their melting points, and of the concomitant fractional distillation
of their components. The initial layer produced by vacuum deposition of a
III-V compound is that of the more volatile group V constituent. As the
evaporation continues, the vapor composition changes and becomes richer
in the less volatile group III constituent. The outer layer of the film is
composed entirely of the group III element. As a rule, such films contain a
small amouiit of the recombined compound formed by interdiffusion of a
fraction of the condensate and large quantities of the unreacted elemental
components. A subsequent thermal reaction is required in order to recombine
the condensate into a homogenous compound.
The synthesis of compound films may be effected in a single-step process
by the coevaporation in vacuum of their constituent elements. The difference
in the partial vapor pressures of group III and group V elements complicates
the problem of maintaining the proper stoichiometric ratio required in order
to obtain monophase films. Vacuum deposition thus affords either of two
choices in procedure: a single-step process such as flash evaporation of the
compound and its synthesis on a hot substrate, or the evaporation of the
constituents from separate crucibles at a rate chosen to meet the condition
of stoichiometry of the condensed compound layer. Alternatively, the inter
dependent control of the temperatures of two or more crucibles and that of
the substrate may be used in order to obtain stoichiometric binary or ternary
AiiiBy compound films.
