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OCR for page 113
Semiconcluctors
INTRODUCTION
There are only a handful of scientific or technological discoveries
that have revolutionized society. Within the past few decades, none
has held as central a role as the computer and communication
technologies. Spectacular progress in these is directly connected to
materials research in semiconductors and other materials used in
electronic devices (Figure 5. 1~. If the rate of progress that has
characterized this technology is to continue into the next decade, our
scientific understanding of such subjects as semiconductor surfaces,
interfaces, and defects and of deliberately structured materials-either
geometrically or spatially will be indispensable. instead of reaching a
plateau after its initial explosive growth following the discovery of the
transistor based on semiconductor physics and materials, materials
research related to semiconductor technology is expected to receive
another impetus to further growth from the advent of very-large-scale
integration (VLS11.
Alongside these exciting technological developments semiconductor
physics has continued to be a surprisingly rich and fertile field of
scientific inquiry. Current experimental and theoretical developments
offer tantalizing suggestions that we may be able to understand some of
the properties of these materials at a microscopic level. However,
recently discovered new phenomena such as the quantum Hall eject or
113
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114 A DECADE OF CONDENSED-MATTER PHYSICS
FIGURE 5. ~ An experimental I-megabit dynamic random-access silicon memory chip.
The width of the smallest features is just 1 micrometer. Access time is 150 nanoseconds.
(Courtesy of IBM Thomas J. Watson Research Center.)
deeper insights into the transport properties of disordered solids are
constant reminders that not everything can be anticipated or claimed to
be understood. The ability to prepare materials deliberately with
atomic arrangements not found in nature is another reminder that
science and technology are often symbiotic. The tools for preparing
these materials were developed for computer technology and are now
used to prepare and characterize materials that may, in the future,
provide even more powerful and cost-effective computer components.
In this chapter, it is not our intent to discuss technology in any detail
or the essential role of semiconductor materials in it, but rather we
hope to convey a brief perspective of the status in semiconductor
science. However, as it is a field that is characterized by a relatively
unique interplay between science and technology, it is useful in this
introductory section to indicate the relationship of science, to be
described in the following sections, to technological development.
Semiconductor science includes the growth and characterization of
materials as well as the study of physical phenomena. Spurred by
technology, the study of growth and characterization of materials will
remain a competitive area. New and improved materials for applica
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SEMICONDUCTORS 1 15
tions ranging from infrared detectors, solar cells, and solid-state lasers
to transistors and VLSI circuits will continue to be in great demand.
The properties of surfaces of semiconductors is an active area of
solid-state physics. New theoretical calculations and experimental
techniques show promise of resolving many of the outstanding issues
related to the structure of semiconductor surfaces. The interactions of
ions, atoms, and molecules with semiconductor surfaces play a signif-
icant role in the processing of semiconductor materials, ranging from
epitaxial deposition to reactive ion-beam etching. In related studies,
most semiconductor device materials require contacts of one kind or
another. Semiconductor-solid interfaces are therefore an important
area of investigation.
The role of point, line, and planar defects in the electronic properties
and yield of semiconductor devices has long been recognized, and
much has been understood about such defects. However, there are still
outstanding issues that need resolution, and it is expected that these
will continue to interest scientists and technologists for the foreseeable
future.
One of the more exciting areas of solid-state physics in the past few
years has been the role of disorder and dimensionality on transport in
solids. Lithographically produced structures, two-dimensional inver-
sion layers, and high-mobility semiconductors have been widely used
in the investigation of phenomena related to quantum transport local-
ization, Coulomb interaction effects, and the integer and fractional
quantum Hall effects. Heterostructures continue to be investigated for
their optical and electrical properties. Recent technological develop-
ments in the production of high-speed transistors in GaAs-based
epitaxial multilayers has brought renewed and increasing emphasis on
this class of materials. Over the coming decade we expect these
materials to be explored intensively for electronic applications requir-
ing high speed and high-density integration.
The search for new semiconductor materials or ingenious methods
for fabricating known semiconductors with the potential for novel
device geometries is expected to be an active area of interest. The
desire to obtain an understanding of amorphous semiconductors re-
mains strong. Technological applications such as copying, solar cells,
and optical storage will continue to drive this field.
SURFACES AND INTERFACES
Scientific interest in surfaces and interfaces of semiconductors is
worldwide. The use of ultrahigh-vacuum technology over the last
decade has provided data on clean surfaces, surfaces with controlled
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116 A DECADE OF CONDENSED-MATTER PHYSICS
exposure to impurities, and interfaces. The availability of synchrotron
radiation has made possible the generation of data on the electronic
properties of semiconductors with unprecedented resolution. The
recently developed scanning tunneling microscope has provided, for
the first time, a direct image of the arrangement of atoms on one of the
more complicated surfaces of silicon t(111) 7 x 7] (see Figure 7.1 in
Chapter 7~. Theoretical approaches have provided reliable estimates of
the energies of semiconductor surfaces as functions of atomic posi-
tions. The latter enable one to rule out a variety of possible surface
models by comparing their relative energies.
The last decade can be characterized as one in which a great variety
of experimental and theoretical techniques were developed. It can also
be characterized as one in which it was realized that understanding
surfaces and interfaces is difficult but important problems to solve.
A combination of the results of a variety of experimental techniques
using x rays, electrons, ions, and atoms has provided evidence that our
understanding of the atomic arrangement at surfaces is only now
beginning. In fact, it is generally agreed that only one semiconductor
surface that of GaAs (1101- is currently known reliably and accu-
rately. A particularly encouraging development in the theory of clean
surfaces has been the ability to calculate the total energy of crystals as
a function of atomic geometry. Such calculations have already pro-
vided evidence (confirmed experimentally) that the notion of the
buckling of surfaces a long-held view in this field is valid more for
ionic semiconductors such as GaAs than for the covalently bonded Si.
More surprisingly, it has been proposed, and current experiments
support this view, that a surface of Si rearranges its atomic positions to
form bonding more characteristic of carbon (pi bonding) than of Si.
Parallel to the study of clean surfaces, the effects of impurities, both
physisorbed and chemisorbed, have been investigated on a number of
different semiconductors and their surfaces. These impurities include
H. O. C1, and F. as well as metals such as Al or Pd. on Si. The spatial
location and electronic effects of impurities have been investigated
both by structural studies and, for example, by vibrational high-
resolution electron energy loss and infrared absorption spectroscopies.
Semiconductor-metal, semiconductor-oxide, and semiconductor-
semiconductor interfaces have been intensely investigated over the last
decade. With the availability of high-resolution probes and controlled
environments under which such interfaces can be prepared, the
number of theoretical models that can explain the properties of such
interfaces has been sharply reduced. In the case of semiconductor-
metal contacts, a particularly important result has been the demonstra
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SEMICONDUCTORS 1 17
tion that chemical reaction between semiconductor and metal is a
dominating factor in interracial properties. This reactivity studied at a
monolayer level by atomic and electronic structural techniques has
posed some fundamental questions about the formation of Schottky
barriers.
DEFECTS IN SEMICONDUCTORS
Defects in semiconductors are essential for the operation of semi-
conductor devices as well as detrimental. They have therefore been
studied for a number of decades. The properties of shallow impurities
were well understood during the 1960s primarily through optical
absorption experiments and effective-mass theory. In contrast,
progress in understanding deep impurities and other point defects
(deep centers) has been slow, largely because of technical difficulties.
Most experimental techniques using bulk samples ran into problems
associated with the presence of shallow impurities at concentrations
greater than the defects of interest. Theoretical techniques using
primarily the cluster approximation, which simulates the infinite crys-
tal by a small number of atoms, often yielded poor results.
Major advances in both experimental and theoretical techniques for
the study of deep centers occurred in the last decade. During the early
and mid 1970s, a new family of experimental techniques was developed
using junctions (pen junctions and metal-semiconductor junctions, for
example) instead of bulk samples. The advantage here is that one can
use electric fields to sweep mobile carriers out of the junction region,
thus effectively simulating a material without shallow impurities. A
variant of these techniques, known as deep-level transient spectros-
copy, is particularly powerful because individual deep levels appear as
peaks on a continuous spectrum. In the past 5 years or so, we have
seen the evolution of a large number of hybrid techniques. For
example, optical detection of magnetic resonance combines electron
spin resonance (ESR) with luminescence and is thus capable of
simultaneously probing the local symmetry and the chemical identity of
atoms (which is an ESR feature) and electronic energy levels (which is
a luminescence feature).
Also during the past 5 years or so, a new theoretical technique has
been developed, based on the mathematical tool called Green's func-
tions, which enables one to avoid the cluster approximation and treat
an isolated defect in an infinite crystal with accuracy comparable with
that achieved in the study of the perfect host crystal. This has provided
a detailed picture of the electronic structure of many classes of deep
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118 A DECADE OF CONDENSED-MATTER PHYSICS
centers. For each deep center, the number, relative energy positions,
and wave-function character of localized states can now be explained
in terms of simple physical models. Trends for classes of impurities (or
for the same impurity in different hosts) are better understood.
In the area of extended defects, the most noteworthy developments
have been the achievement of very-high-resolution micrographs and of
theoretical simulation techniques, which lead to more reliable identi-
fication of the nature of the defects. Major advances have been made
in understanding the role of some extended defects in electronic
devices. Most notable is the appreciation of the role of dislocations in
Bettering defects from the active region of devices and the similar role
played by oxygen precipitates.
REDUCED DIMENSIONALITY IN SEMICONDUCTORS
Advances in semiconductor technology, especially in the silicon
metal-oxide-semiconductor technology used to make the devices that
are central to computer memories and other applications, have also
made possible the observation and study of two-dimensional electron
systems in which the electron density can easily be varied over two
orders of magnitude (from about 10" to about 10'3 cm-21. These
systems are two dimensional in the sense that the motion of the
electrons in the direction perpendicular to the semiconductor-insulator
interface is constrained to a region of about 10 rim by the interface
barrier and externally applied electric fields. The first clear demonstra-
tion of the two-dimensional character of these electrons was made in
1966.
Many kinds of structure have now been shown to exhibit the reduced
dimensionality first seen in metal-oxide-silicon devices. They include
heterojunctions (structures in which two different materials adjoin,
usually epitaxially), quantum wells formed by two heterojunctions, and
superlattices formed by periodic arrays of quantum wells or by periodic
variations of impurity concentrations. If the confining potentials quan-
tize the energy levels to give level spacings comparable with or greater
than the thermal energy kBT and the energy level broadening, then the
motion of the electrons will have a two-dimensional character. The
metal-insulator-semiconductor structure has the advantage that the
carrier density is easily varied by changing the voltage across the
device. Although work on superlattices has concentrated on the
GaAs-(Ga,Al)As system, because of the favorable growth conditions
and simple band structures that they present, there is also considerable
work on so-called type II superlattices, in which occupied energy
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SEMICONDUCTORS 1 19
levels in one material lie at the same energy as empty levels in the
other.
Intensive studies of electron transport, hot-electron effects, strong
and weak localization in one and two dimensions, impurity-band
erects, piezoresistance, many-body erects, charge-density-wave ef-
fects, and magnetotransport effects including the quantum Hall effect
have been carried out on these systems. The ability to vary the electron
density has been of great help in allowing meaningful comparisons
between theory and experiment to be made. Structures involving
GaAs-(Ga,Al)As heterojunctions have the advantage of high mobility,
especially when donor impurities are spatially separated from elec-
trons. This can lead to electron mean-free paths of the order of I lam at
low temperatures, more than an order of magnitude larger than the best
values attained in silicon. The smaller electron effective mass in GaAs
leads to larger energy splittings both for the quantum levels induced by
confining fields and for the Landau levels induced by magnetic fields,
which means that lower magnetic fields and higher temperatures can be
used than for comparable phenomena in silicon.
The reduced dimensionality of the systems being discussed here has
made possible the experimental observation of a number of important
physical erects. For example, weak localization effects and the re-
markable quantized Hall conductance phenomena, both discussed in
Chapter 1, have been observed in these systems. The two dimension-
ality of these systems also leads to a situation where the electron-
electron interactions make a major contribution to the electronic
energy levels, as has been verified in far-infrared spectra of silicon
inversion layers.
OPTICAL PROPERTIES OF COMPOUND SEMICONDUCTORS
As interesting (and important) as the optical properties of elemental
semiconductors are, compound semiconductors, mainly the Ill-V
materials (A~BV), add much more scope to this area of work. The
llI-Vs cover a wide energy-gap range (0.172-2.24 eV), are direct gap
(not just indirect gap) in much of the range, possess high electron
mobilities, can be made into alloys, and, above all, can be made into
heterojunctions and are powerful light emitters. Thus, they have the
potential to be made into light-emitting diodes (LEDs) and lasers, not
to mention photodetectors (and various high-speed transistors as well).
In fact, optoelectronics is totally dependent on these materials, i.e., on
their optical properties. The binary crystals GaAs (the prototype),
GaP, and InP have become important bulk substrate materials, and
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120 A DECADE OF CONDENSED-MATTER PHYSICS
their optical properties, in their own right, are heavily studied. Also,
their bulk optical properties serve as a reference for an entirely new
and large area of work that is unique: lII-Vs permit the construction of
heterojunctions, and this in turn makes possible the construction of
quantum-well heterostructures (QWHs) and superlattices (SLs), and
thus deliberately designed quasi-two-dimensional structures. This
achievement (i.e., quasi-two-dimensional heterostructures) of mainly
the past decade puts III-V materials, and their technology, in a special
category that promises to be of a revolutionary nature. Also this
development is of immediate and long-range use in devices.
The two-dimensional nature of a QWH or SL breaks the crystal bulk
symmetry and, for example, removes the heavy-hole, light-hole degen-
eracy of, let us say, GaAs of thickness smaller than 500 A. In addition,
the confined-particle states, electron and hole, partition the conduction
and valence bands and permit excitor absorption (and recombination)
to be observed in an abnormally large range, including (300 K, 0-10
kbar) up into the region (energy) of higher band minima (L and X). All
the usual optical properties are modified by the quasi-two-dimen-
sionality of QWHs or SLs. This, of course, is becoming an intensive
area of study for a variety of III-V heterostructures, which prefera-
bly are lattice-matched (e.g., AlxGa~-xAs-GaAs)' but even in some
cases can be strained-layer heterostructures (e.g., GaAs-ln~Ga~_yAs
or GaAs~_xPx-GaAs). The undoped QWH or SL is of interest at
low and at high carrier levels and serves, moreover, as a reference
and comparison for similar heterostructures with impurities intro-
duced into the wells or barriers, as is necessary for device applica-
tions.
In the area of optoelectronics, III-V QWHs and SLs promise to have
a profound effect. Already major improvements have been effected in
semiconductor laser performance. In the form of QWHs, monolithic,
single-diode structures have achieved laser power levels from 100 mW
to over 2 W. In these heterostructures the large asymmetry in
electron-hole behavior permits a major redesign of valence photo-
detectors and makes possible other unique hot-electron devices. Of
further interest, impurity-induced disordering can be used to convert,
selectively, quantum-layer regions to bulk-layer regions, or lower gap
to higher gap, thus making possible interesting device geometries (and
microgeometries) and consequently integrated optical and electronic
structures. There is little doubt that the optical properties of III-V
materials will be a major area of study for 10 and more years and, in
general, will be the basis for many further developments in
optoelectronics (more sophisticated lasers, LEDs, detectors, real
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SEMICONDUCTORS 12 1
space negative resistance devices, higher-speed transistors, and inte-
grated versions of all of these processing simultaneously charge and
photons). Clearly, progress in this area of work will depend on the skill
and progress in III-V crystal growth and development. It is not
unreasonable to assume also that QWHs and SLs will be constructed
in Il-VI and other semiconductor crystal systems with interesting
optical properties.
AMORPHOUS SEMICONDUCTORS
Amorphous-semiconductor physics is concerned with the structural,
vibrational, and electronic properties of noncrystalline semiconduc-
tors. By material, the field divides into two principal subfields: (1)
tetrahedrally bonded group IV elements, mostly Si or Ge, and alloys
with each other or with H and (2) the chalcogenides S. Se, or Te,
alloyed with each other or with group IV or V elements. By phenom-
ena, the field has numerous subtopics that parallel much of semicon-
ductor physics as a whole.
Within the past decade, by far the most important discovery has
been n- and p-type doping of hydrogenated group IV amorphous
semiconductors, abbreviated a-C:H, a-Si:H, and a-Ge:H for hydro-
genated amorphous carbon, silicon, and germanium, respectively.
Related technological applications have rapidly followed, led by world-
wi~le efforts in nhotc~voltaics but also including demonstrated applica-
tions in xerography, vidicons, and thin-film transistors. Most of the
attention for both the physics and technology is focused on a-Si:H
because in this system more than in others there is the promise of
studying the intrinsic disorder of a prototypical amorphous semicon-
ductor. Because of overconstrained bonding conditions, true glasses
cannot be expected with fourfold coordination. Experimentally this is
seen in the form of incomplete or dangling bonds and other local
structural inhomogeneities, which lead to gap states that obscure the
basic semiconducting properties of interest, for example, activated
conductivity, doping, and distinct band gaps. For Si, hydrogenation
heals the dangling and other weak bonds and thereby permits the study
of most of the previously observed effects. In the last decade there has
been a burst of activity worldwide to capitalize on the scientific and
technological promise of doped, hydrogenated group IV amorphous
semiconductors.
The bonding between atoms in amorphous chalcogenides is different
from that present in the group IV semiconductors. Hence, their atomic
arrangement and the defects associated with this arrangement are also
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122 A DECADE OF CONDENSED-MATTER PHYSICS
quite different. It is generally believed that electrons are paired at
dangling bonds (defects) in the chalcogenides in contrast to, say,
amorphous Si where the dangling bond is associated with a single
charge. The chalcogens show strong photostructural changes at photon
energies comparable to band gaps rather than bonding energies. For
example, volume changes of several tens of percent are observed. The
microscopic origins of these changes are not known. Chalcogenide
materials are being explored for photoresist and optical storage appli-
cations.
While real qualitative advances have been made in the past decade,
quantitative and predictive understanding of the basic phenomena
associated with disorder are still lacking in both classes of amorphous
semiconductors. There is considerable scientific challenge in the
problems of knowing the principal sources of disorder (bond angles,
intrinsic defects, and role of impurities, to name a few) and in
discovering which semiconductor phenomena are unique to the disor-
dered, amorphous state rather than remnants of analogous effects in
the ordered, crystalline state. Key to the physics is the sorting out of
the innumerable chemical and materials-science preparation and char-
acterization aspects.
FUTURE PROSPECTS
It is our belief that the rate of progress in understanding phenomena
and materials and in manipulating materials to obtain deliberately
arranged geometrical and spatial structures will accelerate in the next
decade. We expect semiconductor research to be an active area of
interest not just because of technological forces but also because of our
increased experimental and theoretical capabilities.
Semiconductor Surfaces and Interfaces
A variety of experimental and theoretical techniques will be applied
to investigate the nature of atomic rearrangement or reconstruction on
semiconductor surfaces. There will be an increasing trend toward
understanding the nature of gas-surface interactions for both scientific
and technological reasons. Reactions such as etching or deposition of
materials with directed external radiation such as that introduced by
ions or lasers are likely to be of increasing importance in semiconduc-
tor technology.
Basic research on semiconductor interfaces will grow in the coming
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SEMICONDUCTORS 123
years for the following two reasons: (1) The essential role played by
semiconductor interfaces in microelectronic devices will become even
more significant as the trend toward miniaturization continues; (2)
experimental techniques and computational methods are available now
for interracial studies that bridge theory and experiment. Key areas of
interest and progress can be identified by the following interfaces.
SEMICONDUCTOR-SEMICONDUCTOR INTERFACES
Of special interest is the low-temperature growth of pen (or n+-n,
p+-p) junctions. Since the diffusion distance (which increases with
temperature) is an intrinsic limitation on device dimension, low-
temperature processing is likely to become crucial for achieving
submicrometer structures in VLSI devices. The low-temperature
epitaxial growth of high-quality Si on Si with a controlled doping is a
key issue. This will require understanding of the atomic process of
growth of pure Si in ultrahigh vacuum, the addition of dopant, and
interracial defect formation and control during growth.
Ion implantation and laser annealing have been combined to obtain
a dopant concentration in Si much higher than its equilibrium solubil-
ity. Ultrafast interracial growth by energetic beam annealing will be a
subject of continuing interest. The motion of a liquid-solid interface at
high speed, its nonequilibrium nature, the heat dissipation, and the
atomic mechanism involved will be subjects of study.
Interfaces in man-made superlattice structures will be another sub-
ject of study. Epitaxial growth of materials will be of particular
interest, as will the growth of defect-free and stoichiometric GaAs
layers for VLSI devices.
SEMICON DOCTOR-INSULATOR INTERFACES
Currently the most important semiconductor-insulator interface is
the Si-SiO2 interface. This is almost entirely due to the use of this
combination of materials in curved electronic devices. However, other
insulators on Si or GaAs will be actively explored as new device
concepts are explored. More studies are required in order to under-
stand the atomic structure, composition, and property correlation of
the interface, for example, its charge-trapping states. Defect formation
on Si during high-temperature oxidation is a subject of current study;
it may lead to a better understanding of the intrinsic defects in Si and
also of the structure and kinetic processes that determine the behavior
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124 A DECADE OF CONDENSED-MATTER PHYSICS
of the interface. Laser annealing of Si on SiO2 and thermal growth of
large grains of Si on SiO2 are subjects of technical importance.
SEMICONDUCTOR-METAL INTERFACES
The research on metal-St interfaces has experienced rapid growth.
This growth has been fostered by technological demands. We expect it
to continue for several years. Also, we expect an increasing emphasis
on the study of metal-compound semiconductor interfaces. The link
between ultrahigh-vacuum studies and those carried out in ambient
environments may bridge basic research and technological applications
of these interfaces. A desire to seek fundamental understanding of the
origin of Schottky barrier formation will motivate continuing research
on this topic.
Defects in Semiconductors
Ion implantation is currently used to introduce controlled amounts of
shallow impurities in semiconductor devices. The samples are then
annealed in order to redistribute the impurities to electrically active
sites. This process depends on the type of annealing used. Shallow-
impurity diffusion is also affected by oxidation, the growth of a silicide,
and other processing steps. The understanding of migration mecha-
nisms both with and without thermal equilibrium is a challenging and
important problem for both science and technology.
The problem of identifying deep centers is essential for a complete
scientific understanding of defect processes and valuable for technol-
ogy in enabling appropriate processing steps in the fabrication of
devices to be chosen. The study of defect reactions under external
stimulation (electron injection, temperature, and laser irradiation, for
example) is only beginning, and many effects are not understood.
Extended defects such as dislocations are detrimental in device per-
formance. The electrical properties, for example, of the core structure
of dislocations and the role of impurities in making them conducting are
still not understood. Overall, the study of extended defects is closely
related to materials processing for devices. The main problems are the
understanding of the conditions under which these defects grow, their
identification, migration kinetics, and role in reactions. Surface and
interface defects are becoming more important as technology evolves
toward the use of shallower junctions. Understanding of the pinning of
the chemical potential at Schottky barriers is an outstanding problem
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SEMICONDUCTORS 1 25
whose resolution may involve defects. The nature of defects at the
Si-SiO' interface still poses a difficult problem. Process-induced sur-
face defects are important for technology, but current understanding is
limited. The theory of surface and interface defects is primitive.
Systems of Reduced Dimensionality
Where might one expect further activity in systems of reduced
dimensionality?
QUANTIZED HALL EFFECT
Work on the fractional and integer quantum Hall effect will continue,
and its observation in a wider range of materials can be expected. If the
precision being attained now is confirmed by additional work, the quan-
tized Hall resistance may become a resistance standard or a secondary
standard.
GROWTH TECHNIQUES AND LITHOGRAPHY
Continuing improvement in semiconductor growth techniques and in
lithography can be expected to lead to use of a wider range of materials
and to new device structures. In particular, it is possible to reduce
dimensions to the order of 10 rim by lithography and to the order of 5
rim by shadowing techniques. This means that it is possible to
construct conventional devices small enough so that electrons have a
low probability of being scattered during their motion from one contact
to the other and should behave ballistically.
SMALL STRUCTURES
The increased ability to fabricate small structures now makes it
possible to reduce the effective carrier dimensionality even further.
Narrow lines on surfaces can be expected to lead to corresponding
confinement of the carriers inside the semiconductor. For conductivity
processes to appear one dimensional, it is only necessary that the
relevant mean-free-path parameter be large compared with the channel
width. Such one-dimensional behavior has already been observed in a
variety of samples. One-dimensional behavior in a quantum sense
requires that the carriers be confined in a distance comparable to the
electron wavelength at the Fermi surface, which is just out of reach at
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126 A DECADE OF CONDENSED-MATTER PHYSICS
present but can be expected to be achieved in the coming decade.
Studies of magnetic-Hux quantization in small structures of normal
metals are also being pursued.
HETEROSTRUCTU RES
Superlattices of type II are materials in which there is energy overlap
between filled states in one material and empty states in another;
InAs-GaSb is the prototype. They are likely to be studied more
extensively and in a wider range of materials. In these systems,
electrons and holes lie in adjacent layers. This makes possible new
experiments involving excitonic effects and collective effects. Strained-
layer superlattices, in which the conditions on lattice-constant match-
ing across an interface are relaxed, should also extend the range of
materials and structures that can be studied. Given the ability to make
small surface structures, it is possible to construct surface superlattices
in which the carrier density and the strength of the potential can be
varied.
THE TWO-DIMENSIONAL WIGNER CRYSTAL
The elusive two-dimensional Wigner crystal, the electron crystal
expected in a degenerate low-density electron system at low tempera-
tures, may finally be observed in inversion layers at semiconductor
surfaces in the coming decade, as its classical analogue was observed
in electrons on liquid helium in the past decade. Exciting new possi-
bilities arise if electrons are placed on a thin film of liquid helium on a
substrate in which a periodic- or random-potential is imposed.
Representative terms from entire chapter:
semiconductor surfaces
