Table Of ContentContributors
Editor: Maurice .H Francombe, Department of Physics and Astronomy, Georgia
State University, University Plaza, Atlanta, GA 30303-3083
Ge _~iS x Epitaxial Layer Growth and Application to Integrated Circuits: David
.W Greve, Department of Electrical and Computer Engineering, Carnegie
Mellon University, Pittsburgh, AP 15213
Platinum Silicide Internal Emission Infrared Imaging Arrays: Freeman .D
Shepherd, Rome Laboratory RL/ER, Hanscom AFB, MA 01731-2909
Thin Film Epitaxial Layers on Silicon for the Detection of Infrared Signals: Paul
.W Pellegrini, Rome Laboratory RL/ER, Hanscom AFB, MA 01731-2909
Jorge .R Jimenez, Rome Laboratory RL/ER, Hanscom AFB, MA 01731-2909
III-V Quantum-Well Structures for High-Speed Electronics: Elliott .R Brown,
Defense Advances Research Projects Agency, 3701 .N Fairfax Dr., Room 850,
Arlington, AV 22203-1714, and Lincoln Laboratory, Massachusetts Institute
of Technology, 244 Wood Street, Room -E 124, Lexington, MA 02173-9108
.K .A Mclntosh, Lincoln Laboratory, Massachusetts Institute of Technology, 442
Wood Street, Room E-124, Lexington, MA 02173-9108
Quantum-Well Devices for Infrared Emission: .A .G .U Perera, Department of
Physics and Astronomy, Georgia State University, University Plaza, Atlanta,
GA 30303-3083
.W-.J Choe, Department of Physics, Kyung eeH University, Suwon 449-701, Korea
.M .H Francombe, Department of Physics and Astronomy, Georgia State
University, University Plaza, Atlanta, GA 30303-3083
Preface
In volume 12 of this Thin Films book series we presented, under the title
Homojunction and Quantum-Well Detectors, five review chapters covering recent
and ongoing developments in the fields of infrared detectors and transistors, and
high-efficiency solar cells. Significant progress also has occurred during the past
few years in other closely related device technologies, which were not covered by
volume 21. This research, which is surveyed in volume 23 by a number of well-
known experts, is based on a variety of layered heterostructure devices formed in
Group IV (Si and Si-Ge) and Group III-V semiconductors. The examples treated
include Si/(Si-Ge) heterojunctions for high-speed integrated circuits, Schottky-
barrier arrays in Si and Si-Ge alloys for infrared imaging, III-V quantum-well
detector structures operated in the heterodyne mode for high-data-rate com-
munications, and III-V heterostructures and quantum-wells for infrared emission.
The first chapter by D. .W Greve presents a detailed review of recent numerous
published results in the field of Si-Ge epitaxial growth and applications to high-
performance integrated circuits. Included is a critical comparison of the main
vapor growth techniques, based on MBE and CVD, in relation to the special
requirements of processing heterojunction bipolar (HBT) and field-effect (FET)
transistor structures, in which the critical Si-Ge alloy layer constitutes the base or
channel region. Emphasis to date has been on MBE and UHV/CVD growth
methods, and these have been utilized for the processing of extremely high-
mobility alloy layers, in device structures operable at frequencies higher than 001
GHz. Optimization of device parameters, through control of crystal quality,
doping, and band-offset conditions at interfaces, depends sensitively on careful
surface cleaning and maintenance of low-temperature processing throughout. The
improved transistors are targeted for a wide range of uses, for example, in mixed-
mode or analog power applications, portable communications, and (in MOSFET
devices) for CMOS circuits.
The third and third articles, which are closely related, are authored by
Hanscom Air Force Base infrared scientists, who over the past several years have
pioneered the successful development of high-performance Schottky-barrier
detector arrays for military and commercial applications.
The second article by .F (Freeman) D. Shepherd offers an authoritative
overview of the development and present status of platinum silicide (PtSi)
staring-mode infrared sensors, discusses the physics of the internal emission
xii PREFACE
detection process, and extends the present device model to include the effects of
photo-generated carrier tunneling through the barrier. Factors influencing
uniformity, quantum efficiency and minimum resolvable scene temperature are
discussed, and approaches are considered for modifying the barrier height and
profile. The collection of signal current via tunneling can thereby be enhanced,
while selectively suppressing dark current through filtering. Finally, changes in
PtSi processing and detector cell structure are recommended, which are expected
to improve camera sensitivity, extend spectral response to longer wavelengths,
and in some cases increase detector operating temperatures.
The third chapter by E (Paul) .W Pellegrini and J. (Jorge) R. Jimenez reviews
recent research (primarily at Hanscom and Jet Propulsion Laboratories, JPL) on
the use of thin epitaxial layers on silicon for the detection of infrared signals.
The article begins with a description of atmospheric transmission characteristics
and of the spectral "windows" suitable for infrared imaging. The sources of
optical radiation available for passive sensing, and the spectral variation of their
relative intensity, also are demonstrated. Next follows a very relevant outline of
the properties and current relative status of available infrared detector
technologies. The main body of the article addresses newer studies on Schottky
and heterojunction internal photoemission (HIP) sensors (in particular the
feasibility of extending spectral response into the long wavelength infrared
(LWIR) range and beyond). A key aspect of this work is the novel utilization of
MBE and UHV/CVD epitaxy techniques to achieve lower-gap Si-Ge alloy
layers, and delta-doped Si layers, permitting the bandgap engineering of detector
structures with superior LWIR response behavior. The review ends with an
interesting and useful section discussing growth and fabrication of various Si-
based detector structures.
Volume 12 of this book series placed strong emphasis on quantum-well
infrared photodetectors (QWIPs) developed primarily for infrared imaging in the
LWIR spectral range. The fourth article of the present volume, by E. (Elliott) R.
Brown and K. A. Mclntosh, discusses the development and optimization of
QWIPs, based on the GaAs/AIGaAs system, designed specifically for high-
speed opto-electronics. In particular, these approaches exploit the superior high-
frequency (up to approximately 50 GHz) detection capabilities of QWIPs
(compared with HgCdTe detectors) in relation, for example, to the special needs
of high-resolution molecular spectroscopy and high-bit-rate optical communi-
cations. It is shown that by operating the QWIP detector in the heterodyne mode,
beating a local-oscillator (LO) source signal such as a ~OC laser against the
signal to be measured, the beat-frequency heterodyne signal seen by the QWIP
is significantly enhanced, resulting in great improvement in the noise-equivalent-
power (NEP) of the detector. This mode of operation can be used effectively to
achieve markedly improved sensitivity and resolution in molecular spectroscopy,
PREFACE xiii
and for CO 2 laser systems operating at difference-frequencies in the 50 to 100
GHz range for high-bit-rate space communications.
The fifth and final chapter, by A. G. U. (Unil) Perera, J.-W. Choe and M. H.
Francombe, deals with a rapidly emerging area that is complementary to the
articles surveying IR detector developments, i.e. semiconductor sources for
infrared generation. Earlier activity in this field has focussed mainly on the
development of double-heterostructure (D-H) diode emitter configurations,
processed in wider bandgap III-V alloys, designed for the spectral range 0.82
to 1.55 mxI used in fiber optic link technology. However, the wavelength range
of interest has expanded rapidly to embrace the mid-infrared (2-5 ~m)
and LWIR (8-12 Ixm), suitable for applications such as laser radar, optical
communications, remote sensing, pollution monitoring, molecular spectroscopy,
medical care, and infrared countermeasures. The development of useful emitter
devices for these longer wavelengths has posed significant challenges in the
areas of band-structure physics, epitaxial growth of complex III-V compound
alloys, and evolution and testing of novel device configurations. Using narrower-
gap alloys, together with strained layers for the active region, the performance of
D-H devices, both as light-emitting diodes (LEDs) and as lasers, has been
improved and extended into the mid-IR range. However, the most exciting and
promising breakthroughs, especially for applications at longer wavelengths, have
occurred in MQW devices, in which emission of IR radiation is stimulated by
relaxation of excited carriers between subband energy levels in the quantum
wells. Normally, this is a low-efficiency process. Studies at Lucent Technologies
(previously AT&T Bell Labs), however~ have demonstrated that by sophisticated
bandgap engineering the efficiency, power capability, and operating temperature
of such devices can be significantly improved. Recent development of cascade
laser devices incorporating novel and effective carrier injection features is
reviewed, together with operating characteristics over spectral ranges extending
to the LWIR.
Maurice H. Francombe
THIN FILMS, VOLUME 32
GexSil. x Epitaxial Layer Growth and
Application to Integrated Circuits
D. W. GREVE
Department q'Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh,
Pennsylwmia 15213 (Tel. (412) 268-3707) dg r 7 @ andrew.cmu.edu
I. Introduction ............................................... 2
II. Ge x ~_~iS Heterojunctions--General Considerations ....................... 3
III. Growth by Molecular Beam Epitaxy ................................ 6
A. MBE Systems ........................................... 7
B. Surface Cleaning and Preparation ................................ 9
C. Germanium Incorporation and Abruptness .......................... 10
D. p-type Doping ........................................... 12
E. n-type Doping ........................................... 14
F. Incorporation of Adatoms and Growth Temperature Limits ................ 51
G. Gas Sources ............................................. 12
IV. Growth by Chemical Vapor Deposition ............................... 21
A. Low-temperature Growth ..................................... 22
B. Growth Systems .......................................... 23
C. Surface Reactions ......................................... 27
I). Kinetics of Layer Growth--Hydride Reactants ....................... 37
E. Kinetics of Layer Growth-- Dichlorosilane ......................... 41
.F Transition Abruptness ...................................... 43
G. Minimum Growth Temperature ................................. 44
H. Surface Preparation ........................................ 44
I. Overview/Summary ........................................ 50
.V Application to Heterojunction Bipolar Transistors ........................ 5 I
A. Operation of the HBT ...................................... 5 !
B. Early Reports of HBTs ...................................... 54
C. MBE-Grown HBT Process ................................... 56
I). UHV/CVD-Grown HBT Process ............................... 59
E. Profile Design for the UHV/CVD HBT ............................ 64
.F H BT Future Prospects ...................................... 68
VI. The Ge Si~_~ Channel MOSFET ................................... 68
VII. Conclusions and Future Prospects .................................. 73
Acknowledgments ........................................... 74
References ................................................ 74
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2 D.W. GREVE
I. Introduction
Germanium-silicon alloys have attracted considerable attention over the past
several years, and it is now recognized that the GexSi~ x/Si material system is the
most practical route to heterojunction devices on silicon substrates. Early work
by Kasper et al. (1) demonstrated epitaxial growth of GexSi~ X and yielded some
working devices; but the observed critical thickness before formation of misfit
dislocations was disappointingly small. Bean et al. (2) subsequently showed that
metastable films could be grown to considerably greater thicknesses by reducing
the growth temperature. Several more devices were demonstrated in this period"
the band offsets were measured and the dependence of layer morphology on
growth conditions was established. Then, in 1987 Smith and Welbourn (3) and
Mertens et al. (4) pointed out the possibility of a heterojunction bipolar transis-
tor (HBT) using a _~iSxeG X base and a first demonstration using molecular beam
epitaxy (MBE) growth was reported (5,6). Succeeding years saw a rapid expan-
sion of research, with the development of chemical vapor deposition (CVD)
growth techniques of several types. HBTs were reported by a number of groups
using both MBE (7-10) and CVD growth techniques (11,12).
There has been a strong interrelationship between the capabilities of growth
techniques and device design in the Ge iS/x_~iS material system. In part, this is
because the goal from the outset was to integrate heterojunction devices into sili-
con integrated-circuit processes with relatively minor changes, which imposed
significant constraints on the epitaxial growth process. In addition, the nature of
the germanium-silicon system itsell' imposes other constraints. The lattice con-
stant of germanium is 4% greater than the silicon lattice constant; thus disloca-
tion-free layers will be strained and possibly metastablc. Further, the cncrgctics
are such that germanium tends to grow in Stranski-Krastanov mode on silicon
substrates. From the beginning, then, growth techniques operating far from ther-
modynamic equilibrium have been favored.
This paper will explore the interrelationship between growth technology and
device design as well as describe a selection of important growth techniques and
explain the physical and chemical principles behind them. It will also point out
the limitations and advantages of particular growth techniques. At present, the
device with the greatest degree of maturity is undoubtedly the hetcrojunction
bipolar transistor; I will outline the initial stages of its development and the pre-
sent state of the art and briefly describe a number of other devices. Because of
the emphasis on devices that can be integrated into silicon processes, I will limit
the discussion entirely to devices grown on (100) silicon wafers. I will exclude
devices that must be fabricated on relaxed buffer layers, as I believe that such
devices have much less potential for integration into silicon-integrated circuit
processes. This is because the thickness required for relaxed buffer layers is at
_~iSxeG x EPITAXIAL LAYER GROWTH AND APPLICATION TO INTEGRATED CIRCUITS 3
present too great to maintain surface planarity, which is highly critical in state-
of-the-art processing. In addition, relaxed buffer layers, even when optimized,
still have a high density of threading dislocations.
There is now considerable literature on various aspects of GexSil_ (cid:141) epitaxial
growth and devices, and a number of review articles treating various portions of the
literature. A nonexhaustive list of other reviews includes a recent review by Bean
(13), a book by Jain (14), and a series of articles in a volume edited by Kasper (15).
My particular objective in this review is to discuss the relations between the funda-
mental processes occuring during growth and the devices that can be fabricated.
II. GexSi~_ x Heterojunctions~General Considerations
Consider the growth of a thin germanium-silicon epitaxial layer on a silicon
substrate. Assuming that the layer grows uniformly (planar growth), the germa-
nium-silicon layer will be compressively strained in the plane of the wafer so
that it conforms with the silicon lattice constant (this is known as commensurate
growth). As the thickness of the layer is increased, the strain energy increases
and eventually it is energetically more favorable if the strain is relieved by misfit
dislocations (incommensurate growth). The boundary between these two
regimes is the equilibrium critical thickness, which decreases with increasing
germanium fraction. Figure 1 shows the equilibrium critical thickness (16) plot-
ted as a function of germanium fraction (solid line).
In the definition of equilibrium critical thickness, there is no consideration of
how the misfit dislocations are created. In fact, dislocations must nucleate and prop-
agate through the crystal and there is a considerable energy barrier that must be sur-
mounted for this to occur. A layer that is grown at low temperatures can be grown
far in excess of equilibrium critical thickness before a significant density of disloca-
tions is observed. The thickness at which dislocations become detectable is known
as the metastable critical thickness, which is a function of the growth temperature,
the quality of the epitaxial layer growth, and the resolution of the technique used to
detect dislocations. Figure 1 also shows measured critical thicknesses for a range of
growth temperatures. A decrease in growth temperature from 900 to 500~ results
in nearly an order of magnitude increase in the measured critical thickness.
Layers thinner than the metastable critical thickness can be grown with essen-
tially no dislocations. However, subsequent anneals above the growth temperature
provide the additional thermal energy required to surmount the kinetic barriers to
dislocation formation and propagation. So in practice, subsequent thermal process-
ing is severely constrained for layers greater than the equilibrium critical thick-
ness. Anneals to activate ion implants are typically performed near 900~ only
layers below the equilibrium critical thickness are likely to survive such a process.
4 D.W. GREVE
FIG. !. Critical thickness for growth of Ge Sil_ ~ on Si(100). The solid line shows the equilibrium
critical thickness as calculated by Matthews and Blakeslec (16). The broken lines show the
observed critical thicknesses for various growth temperatures. For growth temperatures below
900"C mctastablc lilrns can bc grown well in excess of the equilibrium critical thickness.
(Figure from D. C. Houghton, J. Appl. Phys. 70, 2136-2151 (I 991), used by permission.)
The operation of heterojunction devices depends on the formation of band off-
sets in either the conduction band, the valence band, or both. Strained X GexSi~_
on (100) silicon forms a type I heterojunction, that is, the bands of the narrow-
gap x GexSi~_ lie within bands of the large bandgap silicon (Fig. 2). A good fit to
E~;(x)
the measured is (13)
;.~E ~ 1.15 - 0.96x + 0.43x 2 -0.17x 3 [eV]. ) 1 (
_liSxeG x EPITAXIAL LAYER GROWTH AND APPLICATION TO INTEGRATED CIRCUITS 5
Si GexSi x-1
I
EC
.FA V
t
Ev
FIG. 2. Band diagram of a heterojunction lormed when strained (or commensurate) Ge Sil_, is
grown on a (100) silicon substratc.
Although there is some disagreement in the literature concerning the size of the
conduction band offset (17), it is generally considered that about 80% of the
bandgap difference appears in the valence band, so to a good approximation (13)
AE v = 0.84x eV. (2)
The measured bandgaps of strained (18) and unstrained _~iSxeG x (19) are plotted
in Fig. 3. The bandgap of a strained epitaxial layer can thus be modulated by
varying the germanium fraction. The resulting valence band offset can easily be
many times the thermal energy kT at room temperature at values of x where sub-
stantial thicknesses can be grown commensurately. For example, at x = 0.20, the
metastable critical thickness is about 2000 * and the band offset is 0.168
eV ~ 6kT.
There are some devices which must be grown on relaxed buffer layers to ob-
tain an appreciable band offset in the conduction band. I will not discuss these
devices here. I will also not discuss in any more detail the kinetics of dislocation
nucleation and propagation. There is considerable literature on relaxation of
strained epitaxial layers in general and Ge ~_~iS layers in particular. A detailed
review and more references can be found in the book by Jain (14). There is also
a published model useful for prediction of the degree of relaxation resulting
from particular postgrowth thermal treatments (20).
.D .W GREVE
t.2
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STRAIN-SPLIT V.B. --' -~ t
- (CALC)
- 2.0
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Si Ge FRACTION , x Ge
FIG. .3 Bandgap of unstraincd Ge ~_~iS and straincd Gc ~iS ~ grown on (i()0) silicon. Strain causcs
the degeneratc light and heavy hole valence bands to split. The strained energy gap corrcsponds
to the lower curve (labeled 3/2, _+ 3/2). (Figure l'mm D. .V Lang, R. Peoplc, J. C. Bean, and A. M.
Sergent, Appl. Phys. Lett. 47, 1333-1335 (1985), used by permission.)
III. Growth by Molecular Beam Epitaxy
Most of the early work on GexSi~_x/Si heteroepitaxy was performed using
MBE, and to this day MBE has some capabilities difficult to match with alterna-
tive CVD growth techniques. In this section, we review the development and
general characteristics of MBE, with particular emphasis on factors which im-
pact the types of devices which can be fabricated.
Growth of epitaxial silicon layers under conditions roughly comparable to
modern MBE (550~ UHV) was reported as early as 1964 (21). However, prac-
tical MBE of III-V materials--especially GaAs--was developed before MBE
