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Chemical beam epitaxy of Hg1 − xCdxTe and related binaries
Prog. Crystal Growth and Charact. Vol. 29, pp. 161-216, 1994 Copyright 0 1995 Elsevier Science Ltd Printed in Great Briiain. All rights reserved 0960-6974/94 $26.00
0960-9974(94)0001
S-9
’
CHEMICAL BEAM EPITAXY OF Hg,,,Cd,Te RELATED BINARIES
AND
C. J. Summers, B. K. Wagner and R. G. Benz II Quantum Microstructures Laboratory, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A.
coNTEwrs
m
DescriDtion
................ .. .. .. ............................................................................... ........... ....... ..... 162
1.
Introduction
2.
Rationale for Chemical Beam Epitaxy ................................................. .. .............. ........ .. ......... ...... 163 2.1
Nucleation Kinetics .......................................................................... .. ............ ...... .. ............ 164
2.2
Growth Technology Issues ................................................................... .. .......... ....... .......... 164
System Description .. ...... ... ............................ .... .. ....... ... .... ... .. .. .... .. .. ..................... ........... .... .. ........ 167 3.1
Gas Handling System ................ ............................................... ................... ......... ......... ..... 168
3.2
Gas Injection Design ........................................................................ ............... ........ ........... 169
3.3
Mercury Pressure Controlled Vapor Source .................... ........ ............ ........... ........... ....... 179
3.4
Metalorganic Precursor Cracking Characteristics .......................... ........... ........... ............. 171
3.5
In-situ Characterization
3.6
Surface Nucleation Kinetics ....... .. .. ................................ ........... ................. ...... ............ ..... l*O
of System. ...................................................... ............................ 176
Properties of HgCdTe Grown By CBE ........................................... 4.1
Characterization
4.2
CdTe/HgCdTe
......................... ....... ... 183
Studies of HgCdTe Alloys .............................. .._............ ......... ..... .... *84 Superlattice Growth ................................................. .............. ...... .......... ... ls9 161
C. J. Summers et al.
Doping by Chemical Beam Epitaxy ............................................................................................. 190 5.1
Iodine Doping of CdTe .....................................................................................................
5.2
Iodine Doping of HgCdTe ................................................................................................ 198
5.3
P-Type Doping of CdTe ...................................................................................................
192
199
6.
Chemical Beam Epitaxy of Related Materials _..,.........................................................................
203
7.
Manufacturability
206
7.1 8.
Issues.. .............................................................................................................
Characterization
SUmnXXy
of CBE Grown HgCdTe .......................................................................
207
..... .. ........................................................................ .... ... ............... ............ ............ 210
1. INTRODUCTION
Since its inception by Faurie and Million in 1981’ the molecular beam epitaxy (MBE) of HgCdTe has not only advanced the knowledge
of the growth processes
of this material system, but has also, by the
growth of special structures, led to an increased understanding
of the physical properties of this system and
opened up new concepts for advanced detector designs. The basic issues of epitaxiaf growth are to produce defect free bulk material and interfaces, and to achieve stable and reproducible
n- and p-type doping characteristics.
structures is critically dependent
In fact, the realization
on obtaining material structures with these properties
perform in-situ n- and p-type doping.
of new device
and on the ability to
A review of the material properties of HgCdTe shows that the most
important issues in the MBE growth of HgCdTe and related alloys are the inability to control ahoy composition and stoichiometry
precisely
and to reproducibly
dope these alloys n- and p-type.
These areas presently
represent a basic technological block to the development of high performance, low cost infrared sensors. To address these problems a new growth technique for HgCdTe, chemical beam epitaxy (CBE), has been developed demonstrated
by Summers
the ability to produce high quality heterointerfaces
avalanche photodiodes, modification
First applied to III-V systems by Tsang,” et a1.2,3,4,5,6~7,8.9.10,‘1
and optical and electrical devices such as
distributed Bragg reflector lasers and heterostructure
of MBE where the solid evaporation
CBE has
bipolar transistors.‘3
CBE is a
sources are replaced by hydride or organometallic
gas
sources. These gas sources allow more flexibility in the choice of molecular species to be applied to the growth surface, depending on which is found to best optimize the nucleation and growth processes.
For example, it has
been shown that the use of dimer arsenic (elemental arsenic normally evaporates as a tetramer) in the MBE growth of GaAs yields smoother
surface morphologies,
better optical and electrical
carbon incorporation than growth utilizing tetramer arsenic.‘4.‘5
properties,
and lower
Chemical Beam Epiiaxy
We review here the initial development
this system, alloys have been grown with electron concentrations cm*V’i’.
163
and application of CBE to the growth of HgCdTe. Using down to 3 x lOI4 cmm3and mobilities up to lo6
New doping schemes, such as the iodine n-type doping of these alloys, which has the advantage of
being insensitive
to the stoichiometry
of the growth conditions,
using amine has also been investigated
have also been investigated.
and higher incorporation
efficiencies
elemental arsenic, but complete electrical activation has been difficult to realize. growth and manufacturability
obtained
P-type doping
than when using
Recent work on superlattice
issues for HgCdTe are also discussed.
2. RATIONALE
FOR CHEMICAL
BEAM EPITAXY
The MBE of Hg-based compounds has been active for just over a decade and has already equaled more established
growth techniques in several areasI
However, there is a fundamental
limit to conventional
MBE because it relies on the evaporation of elements and, thus, little control is possible over the atomic species applied to the growth surface.
The evaporation of the group VI constituent elements and the group V p-type
dopants produce dimer and tetramer molecules, respectively, which are difficult to effectively incorporate at the growth surface. Therefore,
conventional
MBE is unable to produce a monomer
Te species.
Variation of the
chemical growth species is an important aid in controlling surface growth processes, and can have a significant influence
on material properties.
Although
high temperature
cracking furnaces have been used to crack
tetramer molecules into dimers in BI-V MBE, the thermal cracking of dimer Te into monomer Te requires very high temperatures
(3000 K).”
These high temperatures cause deleterious outgassing and significant radiative
heating of the substrate surface from the high temperature cracker and, thus, are not feasible. deposition
The laser-assisted
technique has been used by Cheung to generate monomer Te, but it is cumbersome
significant modification
and requires
of the growth system.”
Difficulties have also been reported in doping during MBE growth.
Although lithium and silver
p-type dopants and gallium and indium n-type dopants evaporate to produce monomers and have successfully produced doped Hgt.,Cd,Te layers by MBE, the diffusion rates of these dopants on the cation sublattice have prevented precise dopant profiles from being obtained.‘9*20*2’ Practical limitations
of HgCdTe MBE also arise because the high vapor pressures of the host
elements require low furnace temperatures which make temperature control difficult and setpoint changes slow due to the small heat loss. The problem is compounded temperatures,
for Cd and Te since they are solids at their operating
which causes poor thermal contact between the charge and furnace crucible and aggravates
accurate flux control over long time periods. obtain precise alloy compositions
Since HgCdTe growth requires tight control over flux ratios to
and good stoichiometry,
both of which strongly affect fundamental electrical
and optical properties,
these problems
operating temperatures
also increase the difficulty in removing impurities such as oxygen and halogens by
source degassing.
are more significant
than for other MBE grown material.
The low
164
C. J. Summers et al.
2.1
Nucleation Kinetics From this brief analysis, the major limitations of the application of MBE to HgCdTe growth are
the lack of control over the chemical growth species and the difficulties reproducibility.
associated with flux regulation and
CBE,‘* therefore, should offer significant advantages over current growth technologies
VI compounds
as there is considerable
HgCdTe growth.
evidence that modification
for II-
of nucleation kinetics can be beneficial in
Cheung has reduced the kinetic hindrance to the growth of CdTe by MBE by growing with
monomer tellurium produced by high temperature laser evaporationz3
Wu et al. have obtained arsenic p-type
doping of HgCdTe only through the use of monomer Te produced by thermal cracking of dimer Te.*’ In principle, either conventional
the control possible with CBE is significantly
or laser-MBE
because it provides
reactions and nucleation mechanisms and dopant monomer,
incorporation
a direct method to manipulate
which control chemical composition,
(substitutional
or interstitial).
more easily incorporated cracking techniques
By cracking
hydride
compounds
sources,
The actual choice depends on
Monomer sources could also result in dopants being
(< 200°C) growth of Hgi.,Cd,Te
of the
are very important
gas sources were used for the Te growth species since typical organometallic
such as diethyltelluride
and diisopropyltelluride
designed cracking gas injector should decompose formation.
or metalorganic
into the host lattice. Thus, the selection of source gases and the development
required for the low temperature
Metalorganic
the surface chemical
crystalline quality, defect chemistry,
dimer or tetramer species can be supplied to the growth surface.
which species is found to optimize the nucleation process.
issues.
greater than can be obtained with
The incorporation
Te
have a single Te in each molecule, and a properly
the Te compound yielding monomer Te without Te dimer
of dopant species is also expected to be enhanced by the use of monomer dopant
species. However, very high temperatures are needed to crack elemental As and Sb, for example, from tetramer to dimer and monomer forms.
Thus, the use of amine has been investigated.
for n-type doping on the Te-sublattice,
also are dimers in the vapor form.
Cl, I and Br, which can be used An investigation
of these elements
shows that iodine best satisfies all of the physical and chemical properties required of an n-type dopant in the HgCdTe alloy system, and is very compatible doping in the Te-sublattice
with low temperature
epitaxial growth.
preserves the advantages of low temperature cation-rich
growth which has been
shown to produce planar surfaces and also is expected to promote dopant incorporation Low temperature conditions,
growth also enhances the dopant sticking coefficient
should suppress the formation of compensating
For example, iodine
on the Te-sublattice.
and along with group II rich growth
Vcd, Tei, and [Vcd-Donor] acceptor defects.
As
iodine substitutes on the more stable Te-sublattice, its diffusion coefficient is over an order of magnitude lower than that of indium, which incorporates minimal
diffusion
on the group II sublattice.25*26 Advanced
for abrupt dopant profiles.
Because
of these advantages,
device structures require
a study was made of the
effectiveness
of iodine doping in CdTe and HgCdTe by Rajavel and Summers, and Benz et al.27’28
2.2
Growth Technolow Issues The use of gas sources whose flow rates can be rapidly, accurately, and reproducibly
with precision
flow controllers
makes CBE an attractive alternative to conventional
MBE.
controlled
Accurate flow
165
Chemical Beam Epitaxy
control is particularly important for the growth of homogeneous Hgl.,Cd,Te.
layers of the long-wavelength
Calculations by Summers et al. predicted a ten-fold improvement
compositions
in cutoff wavelength
of
variation
for CBE compared to MBE grown layers? As discussed determines
the compositional
in detail by Wagner et al.29 the stability of the host element accuracy. To quantitatively
fluxes directly
estimate the full potential of CBE to control alloy
uniformity in the growth direction Wagner et al. have calculated the variation in cut-off wavelength,
a,
as a
function of the cut-off wavelength, k,, and growth rate, and compared it to MBE using the procedure discussed by Farrow and Reno.30’3’ To a first approximation, incident Cd to Tea or Te flux ratio. stability, x-value compositional
in the MBE of HgCdTe the x-value is determined
Thus, the variation in x-value, Ax, is directly dependent
and the absolute temperature
accuracy is also dependent
of the furnaces.
The latter dependence
by the
on the flux
implies
that the
on the growth rate, which for MBE of HgCdTe is principally
controlled by the Te flux. 1.5
a
CUTOFF Figure
1.
AT
77
K
@m)
Variation in 77 K cut-off wavelength as a function of cut-off wavelength for HgCdTe grown by MBE and CBE at various growth rates.
As previously elemental
WAVELENGTH
shown, temperature
Cd and T~Q sources
control of better than f 0.2”C is required
in order to approach
the required
uniformity
in x-value
for both the and cut-off
wavelength.30S3’ This degree of control is extremely difficult to achieve over the long (- 4 - 10 hr) run times needed to grow uniform 10 pm thick layers for long wavelength
(> 10 pm) applications
and is difficult to
maintain over long production cycles. For example, a temperature
stability of 0.1 - 0.2”C in the thermal Hg
166
C. J. Summers et a/.
source developed by Cook et al. required very extensive considerations
and careful design.”
Figure 1 shows
the variation in & versus h, calculated for typical MBE source temperature variations of AT = 5 0.2”C. shown, &
As
increases rapidly with increasing h, and decreases with increasing growth rate. For detectors grown
by MBE with 5, 10 and 15 Frn cut-off wavelengths the value of A& decreases from 0.12 to 0.09, from 0.34 to 0.28 and from 0.7 to 0.58 l.trn, respectively,
as the growth rate increases from 0.5 to 5 @hr.
faster growth rates can improve the compositional
Thus, for MBE
uniformity in the growth direction.
Calculations were also performed for the expected variation in Lw, for CBE growth for a stability specification
of pressure based flow controllers of approximately 0.05% of full scale. It should be noted that the
flow rate has a small temperature dependence which has less than a 0.01% effect on the variation of k,. Figure 1 also shows the dependence
of Ah, on h, for CBE growth, and demonstrates
accurate than that calculated for the best possible MBE growth conditions. rate dependence
that it is from 2 to 10 times more Additionally, CBE has little growth
because it is possible to recalibrate the flow controllers for a maximum flow corresponding
each predetermined
to
growth rate. The improved stability for CBE is particularly important for long wavelength
materials (8 - 20 pm) in which a 3-fold improvement
in homogeneity is predicted.
This increases to a factor of
10 for - 0.5 /.tm/br growth rates and, thus, is particularly useful for the accurate growth of graded heterojunction structures which are very important for high-performance Of equal importance
imaging systems.
for advanced device structures is the precision of extrinsic doping levels.
Therefore, a comparison was also made of the stability in doping obtainable by MBE and CBE. Assuming that the dopant incorporation
rate is insensitive to variations in the Group II fluxes and is directly proportional
to
dopant flux, the obtainable doping precision is affected by the stability of the dopant flux and the growth rate which, in the case of MBE HgCdTe, is controlled by the Te flux. Figure 2 shows the percentage variation in doping as a function of doping level for MBE using thermal dopant sources (1 pm/hr growth rate and AT = f 0.2”C) and for CBE using a dopant pressure-controlled calculations
vapor source (PCVS) discussed
indicate that CBE is capable of a factor of ten improvement
dopant-PCVS doping levels.
was determined
experimentally
over MBE.
at a particular set of flow conditions
in Section 3. The The stability of the
and extrapolated
The three sets of CBE curves correspond to thme different orifice conductances
used by the dopant-PCVS
to other
that must be
to cover the full range of doping levels.
In addition, for CBE long term stability is improved because there are no source depletion effects and the growth chamber does not need to be opened and baked when recharging the sources.
Thus, this growth
technique is extremely well suited to the reproducible growth of HgCdTe alloys and novel device structures. CBE also allows pm-mixing
of the constituent gases, thereby minimizing the number of gas injectors required
and achieving enhanced compositional This technique, therefore, by other HgCdTe growth technologies graded structures, precise stoichiometric
uniformity.33 has the potential to solve many of the problems currently experienced in obtaining abrupt heterojunctions,
smooth surfaces, compositionally
adjustment, and complex n- and p-type extrinsic doping profiles, all of
which are essential for the fabrication of advanced focal plane array structures.
167
Chemical Beam Epitaxy
2.0 MBE MCT:As
1.5 =: MBE MCTdn
[IO n’ = -6 0.5 MOMBE MCT smaller orifice \
0.0
Figure 2.
\
MOMBE MCT larger orifice
10
Variation of doping precision as a function of doping concentration for gas source doping for MBE and CBE growth of HgCdTe.
3. SYSTEM DEXXIPTION
A schematic of the system developed for CBE HgCdTe growth is shown in Figure 3. The CBE equipment was retrofitted to a Varian GEN II machine and thus differs significantly from the MBE system by the addition of gas handling systems for both host (Hg, Cd, Te) and doping gases, specially designed
LIQUID NIKCGEN
2”
~CRYOfwMP -NRBOMOlICUIAR
Figure 3.
Schematic of chemical beam epitaxy system developed for HgCdTe growth.
gas
166
C. J. Summers et al.
b)
Figure 4.
Schematic of gas handling systems developed for CBE a) host sources, b) dopant sources
injectors for cracking the source gases, a Hg pressute- controlled vapor source for control of the Hg flux, and a special pumping system to handle the large gas loads generated during CBE. coefficient of the Hg, very high pressures are present during MBE growth.
Because of the low sticking
Therefore, the CBE approach must
both minimize excess gas loads and m aximize pumping speed. Special gas injectors and pumps were therefore developed to minimize the gas load and to provide high pumping speeds. These details are briefly summarized in the next three sections.
Gas Handling Svstem
3.1
The gas handling systems specially designed and built for this system am shown in Figure 4. To reduce the gas load and achieve precise flow control, flow controllers based on pressure control were used rather than the conventional minimum
inlet pressures
metalorganics,
Thermal mass flow controllers
typically require
Because of the low vapor pressures
of most
it is necessary to entrain them in a carrier gas by bubbling to achieve sufficient pressure for use
with conventional temperature
thermal mass flow controllers. of 50 torr for proper operation.
mass flow controllers.
As a consequence the metalorganic flow is determined by the bubbler
whose stability limits tire precision of the flow controller.
The use of a carrier gas also further
increases the gas load and makes molecular flow conditions more difficult to maintain in the growth chamber. fn contrast, pressure-based
flow controllers require inlet pressures around 1 - 2 torr and hence can
be operated without any carrier gas. For the host materials the MKS Instruments Model 115OB, which operates
169
Chemical Beam Epitaxy
on the principle of choked viscous flow across an orifice, was chosen. The important properties of choked flow are that the flow rate is directly proportional downstream
to the pressure upstream of the orifice, is independent
pressure, and has a square. root dependence
1150 uses a highly accurate capacitance
on the absolute gas temperature.
of the
The MKS Model
manometer
which provides feedback control to a solenoid valve to
maintain a constant pressure upstream of an orifice.
These flow controllers can be provided with a full scale
range of 0.8 - 10 seem to cover growth rates between 1 - 10 pm/hr.
Following the flow controllers the gases are
fed into the gas injector through a combination isolation and pump-out valve (Figure 4a). A revolutionary implemented
development
in pressure
controlled
to address the challenge of doping by CBE.
provides control over approximately
vapor
delivery
systems
was
also
This was necessary because the MKS 1150 only
two orders of magnitude whereas the fluxes required for doping must be
varied over four orders of magnitude at a flux level that is four to six orders of magnitude less than that of the constituent
gases.
In addition, the dopant flow controller must be flexible in its maximum flow rate because
quantities such as dopant incorporation
coefficient and cracking efficiency were unknown at the time of system
design. Therefore, as shown in Figure 4b, a flow controller was built based on feedback control of the pressure upstream of a variable orifice, similar to the MKS 1150. The large range of flows was accommodated accuracy MKS capacitance
manometer.
The capacitance
manometer
feedback controlled
by a high
the pressure via a
stepper motor driven Granville Phillips leak valve. In place of a fixed orifice, the dopant source used a manual leak valve as a variable orifice, which allowed the maximum flow rate to be easily varied.” controller the dopant gas was fed into the gas injectors through a combination
From the flow
isolation/pump-out
valve.
dopant gas handling system thus operated on the same principle as the Te and Cd flow controllers
The
but had a
significantly larger dynamic range. Because of the large volume of toxic gases a specially designed pumping system was developed to safely handle organometallics, turbomolecular evacuates
their by-products,
The pumping system was based on a
pump (Balzers MBE series) because it is specially suited to pumping
gases from the chamber
turbomolecular
and Hg vapor.3
rather than entraining
them as in a cryopump
large gas loads and or ion pump.
The
pump was followed by a foreline isolation valve and a 50 cfm Alcatel Corrosion Series backing
pump. The exhaust gases then passed into an Emcore toxic gas scrubber filled with a sulfur impregnated activated charcoal capable of absorbing up to 40% of its weight in Hg. A Varian cryopump was also used to remove Hg vapor and metalorganic during CdTe growtb hydrocarbons.
by-products.
This combination
of pumps gave a background
pressure
of lo4 to 10m5torr, with the majority of the residual gas being ethyl and propyl
The ion pump was isolated during CBE growth to prevent failure from continuous pumping of
hydrocarbons. 3.2
Gas Iniector De&n To achieve the full advantages of CBE special gas injectors were designed and constructed
efficiently
decompose
the source gases, so that variations in the surface chemistry
generation of monomer and dimer species.
to
could be studied by the
170
C. J. Summers et al
Atmosphere
Vacuum
a) Schematic of (a) CBE gas injector and (b) cracker diffuser.
Figllre 5.
Special decomposition
consideration
temperature
and source cryoshroud.
was given to the gas injector
design
in order to obtain
and to minimize outgassing, thermal heating, and contamination
the lowest
of the substrate
A convoluted flow path was designed to maximize wall collisions to achieve efficient
decomposition
while maintaining
a large enough conductance
recombination
in the gas phase.
Construction
to avoid the high pressures
was of high purity, high temperature
reactive with either the source gas or any of the decomposition to ensure stable and reproducible cracking characteristics
by-products.
that could cause
materials that were not
Catalytic elements were not used
with time.
A schematic of a gas injector is shown in Figure 5a. Each consists of a 0.75 in diameter delivery tube heated by a standard MBE furnace heater. The Cd source tube was made from tantalum and the Te source tube from pyrolytic boron nitride (pBN) because of the reaction between Ta and Te or Hz at high temperatures. The diffuser design is also shown in Figure 5b. The gas flows through the holes in the diffuser, into the diffuser cone and then onto the substrate.
The small holes at right angles to the injector axis were
designed to ensure many wall collisions for efficient decomposition. of the holes was equivalent to the cross-sectional
To minimize flow resistance the total area
area of the central 3/8 in diameter tube. This central shaft has
an 8” taper which was calculated to give a rt 2% flux uniformity over a 2 in substrate. machined
from solid hot-pressed
binder
The diffusers were
free boron nitride (BN) of purity comparable
to pBN.
The
performance of the gas injectors is discussed in Section 3.4, with special emphasis on the cracking of both host and dopant species.
3.3
Mercury Pressure Controlled Vaoor Source In conventional
MBE Hg sources the flux is controlled by very tight temperature regulation of a
liquid Hg reservoir.32V35 However, due to their low operating temperature mass(-
1 kg) they have poor time response and control characteristics.
(100 - lfiO”C), and large thermal
A vapor source was therefore developed
by Wagner et al. to allow gas-like control of the Hg flux in order to be compatible with the CBE process.36 The
171
Chemical Beam Epitaxy
Hg-PCVS shown in Figure 6 operates on the same principle as the pressure based flow controllers in the CBE system. Mercury vapor was supplied by heating the reservoir to 145°C while the rest of the system was kept above 160°C to prevent Hg condensation.
The Hg pressure upstream of the control orifice was held at the set
point by feedback control of the control valve via the capacitance
manometer
signal.
The Hg flux
flows
through the isolation valve and heated nozzle and onto the substrate. Because the pressure is controlled directly by the control valve, the temperature
control requirements
of this system are much less stringent than the
thermally controlled Hg evaporation sources where there is an exponential dependence
of the Hg pressure (i.e.
flux) on temperature.
In fact, it has been shown that the Hg-PCVS is able to maintain a constant flux for Hg
reservoir changes off
10°C. This design can supply fluxes of up to 2 x 10e3torr and also allows for fast time
response
(10 s) and excellent
flux stability and setpoint repeatability
stability enables the Hg flux to be optimized to minimize stoichiometric
Figure 6.
(0.1%).
The fast response
and flux
doping effects, and defect generation,37
Schematic of Hg-Pressure Controlled Vapor Source.
and also to be changed as required to maintain optimum growth conditions due to changes in surface kink and step densities during the growth of HgCdTe.3*
3.4
Metahmmk
Preausor
Diethylcadmium isopropyltellurium
Cracking Studies
(DeCd),
dimethylcadmium
(DmCd),
diethylzinc
(DeZn)
(DipTe) precursors were used based on the fact that for these metalorganics
and
di-
the associated
radical was single bonded to a Cd, Zn or Te atom and, thus, on pyrolysis should produce monomer species. Also, previously reported work on these and similar precursors was used to define the selection. growth of GaAs, for example, layers grown using triethylgallium grown with trimethylgallium.39
showed less carbon incorporation
In the CBE that those
This was attributed to the stronger chemical bond between gallium and the
carbon complex in trimethylgallium
as opposed to triethylgallium.
The carbon-metal
bond strength of DeCd
172
C. J. Summers et al.
(26.5 kcal/mole)
is less than that of DmCd (33.5 kcal/mole) and therefore easier to pyrolyze.
Additionally,
because the ethyl complex is larger than the methyl complex, it was expected that it would produce less carbon incorporation.
However, later studies showed that DeCd was unstable and, thus, this and other higher order Cd-
alkyl complexes
which are expected to crack at lower temperatures
with lower carbon incorporation
are not
expected to be suitable for CBE.40 Similar arguments were used for the selection of DipTe which was the largest hydrocarbon-Te
complex available in high purity. Secondary ion mass spectrometry
of CdTe layers grown using these gas precursors showed that the carbon contamination
(SIMS) evaluation
was no greater than the
SIMS background level of 3 x 1016cm‘3. Quadtupole mass spectrometry sources used for CBE.
(QMS) was used to study the pyrolysis of the organometallic
gas
Ideally, the thermal pyrolysis should yield only monomer and organic by-products
both low and high injector pressures.
To optimize
the monomer
yield for Te, gas phase collisions
at
were
minimized by operating the gas injector under molecular flow conditions where the molecular mean free path is longer than the injector tube diameter.
For a 1 pm/hr CdTe growth rate, which required a DipTe flow of 0.4
seem, the maximum injector pressure was estimated to be 50 mtorr from the specifications flow controller. maximum
For these conditions,
the DipTe mean free path was calculated
hole diameter in the cracking cell and diffuser of the injector.
of the MKS 1150
to be comparable
The low probability
to the
of Te dimer
formation was confirmed in a study of the pyrolysis of DipTe for injector temperatures between 300 - 1000°C. Figure 7 shows the ion current as a function of injector temperature for DipTe, isopropyltelhnium
(i-FTe), Te,
Te2, and propyl radicals for a DipTe flow rate of 0.24 seem which was routinely used for the growth of CdTe epitaxial layers.
IO0
10200
300
400
500
600
700
600
900
1000
Group VI Injector Temperature (33) Fignre 7.
Dependence of QMS ion current for DipTe and decomposition by-products on injector temperature.
173
Chemical Beam Epitaxy
The onset of DipTe pyrolysis occured at an injector temperature of 400°C and was complete by 750°C.
The DipTe ion current decreased by approximately
four orders of magnitude and for higher injector
temperatures became less than the QMS detection limit; an indication of complete decomposition. Te, and propyl radicals were detected below 400°C due to fragmentation of Tez was detected.
I-propyl Te,
in the QMS, and also a small quantity
As the injector temperature was raised above the pyrolysis temperature, both the monomer
Te and propyl radical signals increased by over an order of magnitude and saturated above 650°C confirming that monomer Te was produced by the thermal pyrolysis of DipTe.
The activation energies for DipTe and i-
PTe were 56 kcal/mole and 26 kcal/mole, respectively. The different temperature dependence for i-FTe indicated that DipTe has a two step reaction path for monomer
formation.
The activation energy for DipTe bond cleavage was estimated
by subtracting
decrease in the Te-C bond energy (- 10 kcal/mole) due to free radical electron charge delocalization4’ 66 k&/mole
activation energy for the removal of the first methyl group from DmTe.
the
from the
This estimate of 56
kcal/mole is in excellent agreement with experiment for the energy of removal of the first propyl radical from DipTe. In addition, McAllister estimated that the ratio of the activation energies for the removal of the first and second propyl groups from DipTe for the homolytic bond cleavage mechanism was 2.42 The measured ratio of 2.15 is, therefore,
also in agreement
with theory.
These measurements
confirmed
the decomposition
mechanism for DipTe to yield monomer Te. The cracking efficiency was obtained by correcting the measured ion currents for the different elemental masses and ionization ratios and then ratioing the Te and Te2 flux densities.
The TeJTez flux density
ratio was found to be 24, indicating that more than 96% of the tellurium flux was cracked into monomer Te. Evidence that n&t
of the Tez was created by polymerization
pyrolysis efficiency
of DipTe is even higher.
cracking diffuser designs. DipTe decomposes
in the QMS, strongly suggests that the thermal
These results confirm the effectiveness
of the gas injector and
The propyl radical/DipTe flux density ratio of 2.22 also confirms the expectation that
into two propyl radicals.
From similar studies the pyrolysis of DmCd was observed at 1100°C and at temperatures between 500 - 800°C for DeCd. decomposition
These conditions
produced
into cadmium and organic byproducts.
was discontinued
complete
DmCd decomposition
and partial DeCd
DeCd was used in the initial experiments,
because of room temperature decomposition
in the bubbler.
but its use
This effect resulted in a mixture
of DeCd and lighter organic gases in the bubbler, which made proper operation of the pressure-based controller impossible
and, therefore, prevented determination
of the optimum cracking conditions
flow
for DeCd.
No such problems were. observed for DmCd. The potential of developing a complete chemical beam epitaxy process by using a Hg precursor, divinylmercury
@vHg), as a mercury source has recently been investigated by Zinck and Rajave1.43 However,
in a preliminary study of the growth kinetics it was found that although DvHg was adsorbed and dissociated on a Te surface at temperatures
as low as -1 WC, the temperature dependence of adsorption was so strong that its
probability became less than unity at typical HgCcITe growth temperatures
(150 - 200°C).
Thus, growth was
C. J. Summers et al.
174
only possible by thermally precracking
dissociation
the precursor.
Exposure to atomic hydrogen was found to enhance the
of DvHg and could be an alternative to thermal precracking.
Using ptecracked
DvHg, single
crystal (001) HgTe growth at 150°C was reported. Recently, Rajavel et al. have investigated iodine precursors
to determine
the best compounds
source molecular beam epitaxy.” their use as conventional
dopants
requires
of
for the n-type doping of CdTe and HgCdTe during gas
This was necessary because the high vapor pressure of the halogens prevents
elemental sources in MBE, and thus requires the use of compound
ZnC12, or alkyl-halide compounds source
the selection criteria and pyrolysis characteristics
as gas dopant precursors.
information
on their chemical
However, assessment properties,
pyrolysis
sources such as
of the effectiveness and doping
of gas
characteristics,
particularly for effective doping at low growth temperatures. The main experimental
issue is that low vapor pressure precursors increase the response time of a
PCVS, because it takes longer to pump out the injector vacuum feed line to attain a steady state flux. Consequently,
for effective pyrolysis and the stable operation of pressure controlled vapor sources, it was found
necessary to utilize source materials with a low bond energy and a high vapor pressure.
Although the bond
strength of 12(36 kcal/mole) is sufficiently low, the vapor pressure (0.3 torr at 20°C) is too low for a PCVS and too high for MBE. HI, a natural first choice
was not used as the expected
by-product,
hydrogen,
could be
incorporated into the growth surface. Also, the high bond energy of HI (71 kcal/mole), requires temperatures in excess of 1200°C for pyrolysis.
Ethyliodide, and to a greater extent allyliodide, have weaker bonds and hence
dissociate at lower temperatures.
The vapor pressures (- 50 torr at room temperature) of these compounds were
also experimentally
determined
to be high enough for the stable operation of a PCVS flow controller at room
temperature.
Butyliodide,
iodononane
and iododecane
have lower bond energies, but their vapor pressures
progressively
decrease with the length of the hydrocarbon chain.
For the experimental
geometry of the leak
valve delivery system described earlier it tequired several minutes to obtain a stable flux. The practical need to have a vapor pressure 150 torr (at room temperature) reduced the effectiveness as iodononane ethyliodide criteria.
and iododecane
as dopant sources for gas source applications.
of long chain compounds
such
From these empirical studies
and allyliodide were selected as suitable iodine precursors as their properties best fitted the above
To identify the products created by the injector, Rzjavel et al. performed mass scans for cracker cell
temperatures between 150 and 750°C. For ethyliodide, only the QMS signals for CaHs+, CpHsI+ and r’ showed a strong dependence
on injector temperature and between 400 and 650°C the ion current of the parent molecule
decreased
by over two orders of magnitude,
amount.
This indicated
Normalization
that ethyliodide
while the 12 ion current increased by approximately
was pyrolized
into dimer iodine and hydrocarbon
the same
by-products.
of the data showed that the onset of pyrolysis of ethyliodide occurred at 400°C and was 99.6%
complete at 650°C to produce dimer iodine molecules, as shown in Figure 8. A similar study for allyliodide showed that the onset of pyrolysis occurred at - 3OO“C and at 600°C 99.6% of the allyliodide molecules were decomposed
into dimer iodine and hydrocarbon by-products.
175
Chemical Beam Epitaxy
For a growth rate of 0.5 lunihr, the BEP of the host elements is approximately Assuming
100% incorporation,
a BEP of 1 x 10’) torr produces a dopant concentration
of - 10” cm”, and has
to be decreased by four orders of magnitude to obtain doping levels down to 1 x 10” cm”. of the dependence
of pyrolysis on the precursor flow rate, was therefore investigated
1 x 10e6 torr.
An understanding
for the range of values
currently possible. For this study the BEP of ethyliodide was varied over three orders of magnitude from 8 x 10m9to 2 x 10m5torr for an injector temperatme of 700°C. At a BEP of 8 x 10m9totr, the pyrolysis efficiency was 90% and increased rapidly with increasing dependence
flow rate to > 99.1% for a BEP of 2 x 10m5torr.
of the pyrolysis efficiency on flow rate, the dopant incorporation
Due to the non-linear
in the host lattice is also expected
to vary non-linearly with the precursor flow rate in the precracked mode. From this study the pyrolysis of ethyliodide and allyliodide was found to produce iodine dimers and the cracking efficiency
was found to be dependent
formation is not understood
but two possibilities
on the precursor flow rate.
are suggested.
The mason for dimer
As each parent molecule contains only one
iodine atom, 12 can be produced when two parent molecules combine to form a complex during pyrolysis, or when iodine monomers intermolecular pressure.
created after the dissociation
of the patent molecule recombine
to form dimers.
As
or interatomic collisions are required for the creation of 12,both processes are very dependent on
However, further studies were difficult because dimer iodine was dissociated to monomer iodine by
electron impact in the QMS. Thus, it was not possible to follow the complete chain of chemical reactions and the mechanism of the pyrolysis process as the dependence of monomer yield on precursor flow rate and cracker cell temperature could only be studied at low QMS ionizer electron energies which were not available. Thus, iodine has desirable attributes for the n-type doping of HgCdTe alloys. allyliodide
were found to have the most suitable properties
and to produce
Ethyliodide
iodine dimers
100 lo-'
90
80% g104
70
E
SG?
5
vi
10”
402
0
E 1o-‘O
“!j *O a 10
1cl-”
I
100
ZOO
’
t’
I
300
400
’
I
500
’
I
600
’
I
700
Injector Temperature (“C) -8.
g
QMS ion current for ethyliodide decomposition.
v
I
So0
0
’
Wa
and
for pyrolysis
176
C. J.
temperatures
Summers et al.
above 650°C and 600°C. respectively.
As described later, the ethyliodide
doping of CdTe and
HgCdTe has produced highly conductive layers with high mobilities and electron concentrations
as high as 5 x
1018cmm3. For the p-type doping studies amine was pyrolyzed in a heated pyrolitic boron nitride injector up to 1300”C.45 The QMS indicated that AsHs pyrolysis above 1200°C
capable of operating at temperatures
generated Asz. As4 and Hz. with the major species being dimer As at a ratio of 10: 1 As2 to As4. To control the flow of the high-pressure
amine gas, the pressure controlled
vapor source system described
previously
was
modified to supply very small flows of AsH3 from a high pressure source. Using this technique, a minimum BEP of 1 x 1O-1otorr was repeatedly achieved with the amine source.
In situ Characterization
3.5
of System
To facilitate the growth of multiple quantum well and superlattice
structures and to integrate
atomic layer epitaxy and migration enhanced epitaxy techniques with the conventional capability, the system was fully automated. chamber
and loadlock,
substrate
This involved controlling
heaters, temperature
measurement
temperatures,
opening and closing of cell shutters, and acquisition
spectrometer
and video RHEED
throughput and manufacturability
system.
Automation
chemical beam epitaxy
the following and control,
components:
growth
flow rates, furnace cell
of data from the QMS, Auger electron
of the growth process
also greatly increases
the
as described in Section 7.
The RHBED system was upgraded by the addition of a video camera, and a computer equipped with a digitizing board and software for RHEED analysis. The electron gun was also mounted on a bellows to allow the incident electron beam to be scanned from 0.05” to 3.5” by a combination of mechanical motion and electrostatic
deflection.
The diffraction
patterns
were analyzed by a RHEED
Data Acquisition
System
developed by Epitaxial Growth System and consisting of a DigiSector board with video monitor to display the diffraction
pattern and two software programs for analyzing intensity oscillations
Fourier transforms
and beam profiles.
Fast
of the intensity oscillation data were used to obtain accurate and real-time growth rates.
This system was also used to record the entire RHEED pattern. The RHEED system was found to be very effective in evaluating the performance
of the system.
The pyrolysis of DipTe and DmCd was studied by RHEED intensity oscillation measurements growth rate as a function of the injector temperature.
The combination
of the CdTe
of CBE and EWEED analysis has also
been applied to evaluate the preparation of substrates with optimum, low index atomically smooth The
system characterization
surfaces.
runs were used to precisely establish the necessary operating parameters
MBE system for the growth runs and the surface kinetics experiments.
For these calibration
for the
experiments
RHEED intensity oscillations on the CdTe (001) and (11 l)Te surfaces were used. An example of an intensity oscillation is shown in Figure 9, using monomer Te and Cd at 290°C under excess Cd conditions. the growth rate, the Fourier transformation spectrum corresponds
was taken, where the maximum
to the growth rate as shown in the inset in Figure 9.
amplitude
To calculate
of the frequency
177
Chemical Beam Epitaxy
3000
2000
1000
0
10
40
30
20
::c60
::::
:::::
:::::
:::::
o!:::
Time (seconds) Flgun? 9.
Representative RHEED intensity oscillation for CBE growth of CdTe. The inset shows the Fourier transform of the intensity oscillations.
Wagner et al. have also recently shown bow RHEED studies can be used to compare the ability of the MBE and CJ3E techniques
to obtain accurate and reproducible
alloy compositions.29
comparison of these growth methods, CdTe was first grown by MBE from a stoicbiometric CBE. In these studies it was observed tbat intensity oscillations measurements using a CdTe furnace nonuniformities
had an intensity
across the substrate.46
envelope
that exhibited
In a dynamic
source and then by
performed during ME3E gmwtb
fnzquency beating,
Figure 10 shows the intensity oscillations
as a result of flux
during solid source CdTe
7000 3 .z 3
8000
Je
6000
21 .%I
4000
5 E
3000 2000 0
lo
20
30 lime
F&ure
10.
40
60
60
(seconds)
Beats in the intensity oscillation envelope during solid source MBE gmwth. The data at later times is magnified by 2.7 X as shown in the inset.
178
C. J. Summers et al.
growth from a single CdTe furnace.
This behavior was not observed during CBE growth and was attributed to
the superior growth uniformity available with this technique. Figure 11 shows the frequency spectrum obtained for CdTe growth in which a stable monomer Te gas source was used to compare the fluctuations in growth rate resulting from a gas source of Cd and a solid source of Cd. As shown the Fourier transform spectra obtained using a solid Cd source is broad and tails to higher frequency values.
This was attributed to the cooling of the Cd furnace after the Cd shutter was opened
and the decrease of the Cd flux and CdTe growth rate to a steady state value.
For gas source growth the
frequency peak was narrower and showed no tailing, which proved that flux transients are not operative during CBE. Accurate growth rate measurements DmCd flow controllers. Te-limited conditions incorporation
were also found to be necessary to calibrate precisely the DipTe and
Figure 12 shows the CdTe (001) growth rate at 280°C under Cd-saturated,
as a function of the DipTe flow controller setpoint.
monomer
These conditions ensured complete
of monomer Te, so that the growth rate accurately represented the DipTe flow rate. The DipTe
flow controller exhibited a nonlinear flow vs. setpoint relationship which was fitted by a 3rd order polynomial. Similar behavior was reported for the DmCd flow controller. RHEED
intensity
oscillations
were
also used to study organometallic
decomposition
measuring the CdTe growth rate under saturated conditions as a function of gas injector temperatures. possible
because the surface catalytic decomposition
temperatures
so that the growth rate corresponded
injector into Te or Cd, respectively.
This was
at low growth
to the amount of DipTe or DmCd decomposed
by the
temperature dependence
which was defined as the point of maximum
growth rate.
and An
f::::::.:,::::::::::::r
O
0.6
-..-..
Gas Source Cd and Te
-
b&E Cd Source + Gas Source Te
1.0 Frequency
Figurell.
was negligible
These experiments yielded the decomposition
the optimum injector operating temperature,
4000
of the organometallic
by
1.6
I2.0
(Hz)
RHFiED oscillations observed for CdTe (001) growth with monomer Te and gas source Cd or solid source Cd illustrating solid source flux transients.
179
Chemical Beam Epitaxy
0
0
0.2
0.1
DipTe
Figure 12.
0.6
0.4
0.3
(seem)
Setpoint
CdTe (001) growth rate versus DipTe flow controller setpoint in the Cd-saturated growth W$ne.
example of these studies is shown in Figum 13 for the (001) CdTe growth rate as a function of the group VI injector temperature
at a set DipTe flux.
The substrate temperature
was 280°C and sufftcient
Cd flux was
supplied to saturate the growth rate. As shown, with increasing temperature the growth rate increased rapidly with an activation energy of 23 kcal/mole, in excellent agreement with the QMS cracking studies for i-PTe decomposition.
Therefore, these results also supported the two step bond cleavage decomposition
described earlier.
The growth rate decreased
6OQOC
i%i+ Gi ki > m 5
l
l
0
mechanism
for injector temperatures
above 650°C indicating this was an
7otyc
6OQ”C
l
6OQ%
0
.
0
.
0.6 --
s .E. Q, ;;; u 5 0.2 --
Activation
Energy
- 23 kcallmole
2
0.0006
Reciprocal
Figure 13.
0.0010 Temperature
0.0012
of Group VI Injector
0.1
(l/K)
CdTe growth rate versus the reciprocal of the DipTe injector temperature for Cd-saturated growth.
160
C. J. Summers et al.
optimum operating point. The change in growth rate was attributed to Te deposition in the injector. A similar study was performed
for DmCd, where the optimum operating temperature
for the
DmCd injector was determined to be 1120°C. For higher temperatures the growth rate decreased slowly from increased Cd deposition
in the injector, as verified by inspection
after removal from the CBE system.
The
growth rate increased rapidly up to 1120°C and exhibited two activation energies, 33 kcal/mole below 880°C and 14 kcal/mole mechanism
above 880°C.
This behavior was attributed to a two step bond cleavage decomposition
analogous to the DipTe decomposition
results in which the lower temperature activation energy of
33 kcal/mole corresponds
to removal of the first methyl radical and the high temperature activation energy of
14 kcal/mole corresponds
to removal of the second methyl radical.
Surface Nucleation Kinetics
3.6
An understanding necessary for the production
of the fundamental
mechanisms
involved
of material for advanced infrared applications.
in HgCdTe epitaxial With knowledge
growth is
of the growth
processes, the variation in material properties with growth conditions can be predicted and hence optimized. addition, advanced device concepts, such as superlattices, which rely on high quality heterojunction will benefit stoichiometry. allows
from increased
knowledge
of the growth
kinetics
as applied
Studies of surface growth kinetics therefore strongly complement
to interface
In
interfaces,
abruptness
and
CBE because of the choice it
over the chemical growth species, along with its ability for rapid, accurate and reproducible
control of
the growth fluxes. Studies of the surface growth kinetics that control the growth of CdTe and HgTe have been made by measuring
the growth rate as a function of the substrate temperature
and flux ratio for the (OOl),
(11 l)B, and (21 l)B orientations. Figure 14 shows the dependence of the (001) CdTe growth rate on Te flux for a fmed Cd flux for substrate temperatures
between 200 - 320°C. Also included in the figure are the growth conditions,
defined by
the ratio of tire growth fluxes, and the growth regimes, which am defined by the derivative of the growth rate with respect to monomer
Te flux.
As shown, initially the growth rate increases
monomer Te flux but at large flux values becomes independent the Cd overpressure
linearly with increasing
of the Te flux. Investigations
showed that the growth rate was independent
of the Cd flux at low Te fluxes and, thus,
these growth conditions defined a Te-limited growth regime, where the temperature dependence rate in this regime is representative
of the effect of
of the growth
of the Te species surface kinetics.
However, at high Te fluxes the growth rate varied linearly with the Cd flux. These conditions, therefore,
detine a Cd-limited growth regime, where the temperatum dependent growth rate is representative
the Cd surface kinetics.
of
The Cd -limited growth rate decreased with increasing temperature for all temperatures
studied, in contrast to Te-limited growth, indicating incomplete Cd incorporation
over
this
temperature range.
The growth rate data displayed Arrhenius type behavior, with a Cd activation energy of 5 1 meV. In the Te-limited regime the growth rate was independent of temperature below 300°C indicating complete
incorporation
of all incident
Te atoms, and so investigations
were made at higher
substrate
181
Chemical Beam Epitaxy
Cd -
Te -
0.6
0.6 ML/s
1.0
Monomer
Te
1.6
Flux
(ML/s)
Figure 14.
(001) CdTe growth rate for a constant Cd flux as a function of monomer Te flux for substrate temperatures between 200 - 320°C.
temperatures.
Figure 15 shows the Te-limited
from 275 - 400°C for two different Te fluxes.
growth rate as a function of reciprocal substrate temperature The growth rate decreases between 305°C and 360°C with an
activation energy of 150 meV, and at substrate temperatures resulting
from insufficient
greater than 36O”C, shows a very rapid decrease
Cd flux to insure saturated growth conditions
and prevent
sublimation.
The
activation energy in this regime of 1.9 eV is in agreement with the CdTe sublimation energy measured by Arias
409% r.
37vc
369%
32Q“C
30(X .l.MMUS
0.43 Mua
0.0015 Reciprocal
Figure 15.
0.0016 Substrate
0.0017 Temperature
(i/K)
(001) CdTe growth rate as a function of reciprocal substrate temperature under Te-limited, saturated growth conditions for two diierent monomer Te fluxes.
182
C. J. Summers et al.
A similar study was also performed
et aL4’
monomer
Te.
for dimer Te, with the results being qualitatively
Below 300°C the growth rate was independent
molecules were incorporated. of 85 meV, approximately
of temperature
similar to
and all incident
dimer Te
Between 300°C and 350°C the growth rate decreased with an activation energy half the monomer Te activation energy.
For substrate temperatures
greater than
35O”C, the growth rate decreased rapidly due to CdTe sublimation with an activation energy of 1.9 eV. Studies for the (11 l)B surface were qualitatively identical to the (001) surface but the Cd-limited growth rate decreased with temperature with an activation energy of 123 meV, as compared to 51 meV for the (001) surface.
The difference in Cd precursor energies implies that Cd exists in a more tightly bonded site on
the (11 l)Te surface, compared to the (001) surface, which agrees with observations for Hg incorporation
Initial studies for the CBE growth of HgTe on the slightly
HgCdTe growth on the (11 l)Te surface.4a misoriented
(11 l)B surface showed strong intensity oscillations
llO”C, indicating oscillations growm4’
a smooth
for substrate temperatures
atomic growth front during (11 l)B HgTe growth.
were not observed,
presumably
during
However,
because the Te adatom mobility was sufficient
between
165 -
above 17O”C, for step flow
Figure 16 shows the HgTe growth rate at 150°C as a function of the Hg flux for a fixed monomer Te
flux. The growth rate initially increases with increasing Hg flux, but plateaus for Hg fluxes > 2.6 x lo4 torr. However, the Te-limited growth rate at 150°C was found to be less than the incident Te flux indicating that not
0.30 -
0 _________________
,A+--
0
4 #’
0.29 ,’
5:
Li
z 2
0 0.28 L1?
SubstrateTempentw=15OC
S
:
i Atomic Te Flux = 0.363 ML/s
0.27 -
d 0.28
i
1
1.2OE-4
Figure
16.
1
I
1
1 ME-4
I
I
I
1
I
1
1
1
2.OOE4 2.4OE4 Hg Flux (toIT)
1
1
1
1
2.8OE4
1
1
1
1
3.2OE-4
HgTe growth rate on the (11 l)B surface at 150°C as a function of Hg flux for a set monomer Te flux.
183
Chemical Beam Epitaxy
all of the Te was incorporated.
Similar growth rates were measured for CdTe and HgTe under Te-limited,
saturated conditions indicating negligible scattering of the Te beam by the Hg flt.~x.“~~~ RHEED oscillation studies were also performed on the (2 1 l)B CdTe surface which is a vicinal surface. composed of (11 l)B terraces 3 atomic spacings wide in the ~11 l> direction, with step edges parallel to the direction. termination
A Cd-stabilized
for the Cd-stabilized
surface was observed during CdTe layer growth, which implies a bulk
surface.
However, during HgCdTe growth and Te annealing of the CdTe
surface at 300°C the diffraction pattern was characteristic (21 l)B HgCdTe growth occurs on a Te-stabilized stabilized surface.
of a singular, step free surface.
This implies that
surface, as opposed to (001) growth which occurs on the Cd-
Intensity oscillations were observed on the (21 l)B surface but no analysis of the nucleation
kinetics was reporh~L’~ The small desorption
energies
obtained
from these studies
suggests
that both atomic
molecular growth species exist in weakly bound precursor surface states before being incorporated
and
into the
lattice. This is similar to GaAs where molecular arsenic exists in a mobile, low binding energy precursor state. However, this is an unexpected result for atomic Cd and Te and suggests a possible kinetic hindrance to growth not observed during III-V MBE. The growth rate for MBE CdTe47*50shows a similar temperature dependence which confirms that the small activation energies am not unique to CBE growth, or a result of the interaction of organic species with the surface, but am intrinsic to CdTe surface processes. The different
precursor
energies
for monomer
and dimer Te show that diier
incident on the surface continue to exist on the surface as dimers, rather than dissociatively monomers.
These results also offer a possible
illumination,
which from measurements
that measured
for monomer
Te.
explanation
for the desorption
Te molecules adsorbing
as
of Te under He-Ne laser
by Benson et al.,5’ has an activation energy of 150 meV, the same as
In those experiments,
the He-Ne laser was resonant with the 1.9 eV Te
desorption energy, which implies that laser excitation breaks the surface Te dimer bonds and promotes the Te into a monomer precursor state, from which they then desorb. Despite the difference species were completely incorporated
between the dimer and monomer
Te precursor binding energies,
at saturated conditions under 300°C. In contrast, the incorporation
was not complete, despite the similar binding energies for Cd (51 mev) and dimer Te (85 mev).
both of Cd
This suggests
a kinetic limitation for Cd incorporation and shows that the Cd and Te nucleation kinetics are not simply related to the precursor state binding energy. It is possible that Cd incorporation is controlled by adatom incorporation at surface steps and not by nucleation on the terraces as has been observed during GaAs growth.49
4. PROPERTIES
OF HzCdTe
Before the growth of any Hg containing deposited on the GaAs or CdTe substrates. described previously
GROWN BY CBE
layers, l-6 pm thick CdTe buffer layers were first
HgTe and Hgi,Cd,Te
growth was performed using the Hg source
and DipTe and DmCd sources. Typically, growth rates of 0.3 - 2.7 p.m/hr at substrate
184
C. J. Summers et al.
temperatures
of 120 - 185’C were used. After HgCdTe growth, a 100 - 200 8, thick CdTe layer was deposited
as a passivating cap. This terminated the growth at the Hg layer growth temperature without degradation of the surface due to the preferential
evaporation
of Hg.
growth was reported while for the (lll)B HgCdTe.“”
For the (001) orientation
and (211)B orientation
CdTe, HgTe, and Hgi.,Cd,Te
growth was studied only for CdTe and
The nucleation conditions varied depending upon the epitaxial orientation and whether GaAs or
CdTe and ZnCdTe substrates were used. To ensure reproducible growth and doping, the substrate temperature was calibrated by the tellurium condensation
4.1
Characterization Extensive
layers in conjunction
technique.52*5’
Studies of HeCdTe AIIovs
studies have been reported of the structural, electrical, and optical properties of these with the growth studies.
layers by optical microscopy
Studies of the surface properties
and scanning electron microscopy
of epitaxial (001) HgCdTe
(SEM) showed the presence of small hillocks
on the surfaces, with the best sample having hillock densities of ld cme2 and the worst lo6 cme2. Although no optimization
was performed to lower the hillock density, the best values are similar to values reported for MBE
grown layems
Because of the improved surface morphology of (21 l)B HgCdTe first reported by Koestner et
a.l.?5 later work emphasized also used to estimate Hga.&la.aiTe growth
this orientation.
the composition
of the layers using reference
obtained from Cominco.
rates of CdTe,
SEM and energy dispersive spectroscopy samples
(EDS) analyses were
of CdTe, HgTe and bulk
This data indicated that samples with 0 < x < 0.4 were grown.
HgTe and HgCdTe
were determined
by the RHEED
technique
The latter measurements
or were
measurement
of the thicknesses
accomplished
by selectively etching the epilayers, as described by Leech et al., and then using profilometry
measure
the etch steps.56
transmission
measurements Figure
of the epilayers grown on GaAs substrates.
oscillation
The
Interference
fringes from long wavelength
Fourier Transform
to
Infrared (FTIR)
also were used to determine the thicknesses.
17 shows the room temperahne
FTIR transmission
spectra reported
for Hgi,Cd,Te
samples grown on (001) GaAs substrates. These thin (2 - 5 pm) layers exhibit relatively sharp cut-offs with 50% transmission
bandgaps corresponding
moves to longer wavelengths
to x-values of 0.19, 0.21, 0.24, and 0.37. Note that as the cut-off
(lower x-values),
the slope of the cut-off feature becomes
less abrupt both
because of the decreasing absorption coefficient and the increased distribution of thermally excited conduction electrons
in smaller bandgap alloys.
absorption edge to shorter wavelengths this technique transmission
without extensive
In fact the Moss-Burstein
effect in n-type material shifts the apparent
and thus makes it difficult to obtain precise x-value determinations
analysis.
Ideally, x-value determinations
require temperature
by
dependent
data. For this mason Wagner et. al used the strong dependence
of the electron
concentration
on
temperature to obtain the x-value.5 The rapid decrease in electron concentration between 300 K and 100 K is a direct measure of the change in the number of intrinsic electrons thermally excited across the bandgap. by fitting the variable temperature Hall data with a model incorporating
Fermi-Dirac
Thus,
statistics with the “small
Chemical
Beam Epitaxy
185
gap” Kane energy band structure,s7 an accurate value of the alloy composition
was determined.
This model
also incorporated two donor levels and an acceptor level to tit the data in the extrinsic temperature region. 100
3 1 \
80
3 C
5
rj c
60
s
‘5
.B E
40
t S 20
0 2
4
6
0
10
12
14
16
18
Wavelength (urn) F&llre 17.
Room tempemtum FlYR transmission spectra of four (001) Hg&d,Te grown on GaAs substrates.
Excellent
tits to the temperature
dependence
of the electron
layers of various compositions
concentration
temperature range were reported for samples with x-values of 0.161,0.185,0.183,0.226 confirmed the high uniformity and quality of CEE HgCdTe. net donor concentrations, cmm3,respectively.
over the entire
(Figure 18) and 0.33
All of these layers exhibit very low background
with the x = 0.183 and 0.185 layers having Nn - NA = 3.6 x lOI4 cmm3and 3.4 x lOI
These low background carrier concentrations
those currently obtained with conventional
are approximately
MBE.s8S59 Possible explanations
a factor of 2 - 3 lower than
for the low carrier concentrations
are that the use of monomer Te reduces the kinetic hindrances to growth and, thus, the creation of crystalline defects;
there may be different
conventional
types or levels of impurities
in the metalorganic
sources as compared
to
solid MBE source material; and the higher flux control and stability of the CEE sources and the
Hg-PCVS minimize any stoichiometric
doping effects during the growth of the layers.
In Figure 19 the dependence of the electron mobility on temperature is shown for two HgTe layers grown by CEE at 165°C on a (001) CdZnTe and a (001) GaAs substrate.
As shown, very comparable mobility
values were found for both samples, with the HgTe/CdZnTe layer showing slightly higher mobilities at the higher temperatums (150 - 300 K). The mobilities increase rapidly horn 2.5 x 104 cm%%-’ at 300 K to about 1.1 x 16 cm*V’s-’ at 20 K due to the reduction in LG-phonon dependence
scattering.
The mobility values and the temperature
are in relatively good agreement with the theory as outlined by Meyer et aL60 High quality
166
C.
HgTdChAs
aho
J.
Summers
et al.
has been grown at a low temperature of 120°C with a background No - NA concentration of 1 x
1Or6cm-3 and a mobility of 74,000 cm’V’s_’ at 20 K. These values compare favorably with the highest MBE HgTe mobility obtained and indicate that high quality HgTe can be grown by CBE at very low substrate temTcmtures.6’ An example of Hgt.,Cd,Te discussed previously.
mobility data is shown in Figure 20 for the HgD.&&Te
As shown, the mobility increases by an order of magnitude due to decreasing Lo-phonon
100.0 150.0 TEMPERATURE Figure
18.
layer
200.0
250.0
(deg. K)
Electron concentration versus temperature for two Hg&&Te layers (x = 0.183 and 0.226); dashed and solid lines represent theoretical fits of intrinsic and extrinsic concentrations, respectively.
scattering as the temperature is reduced from 300 - 100 K. At lower temperamms the mobility plateaus at a value of 3.2 x 16 cm*V’s-‘.
Another example of a high mobility sample is shown in Figure 21. This layer (x = 0.14)
exhibited a mom temperature mobility of 3 x IO4 cm’V’s_’ which increases to 8.7 x 16 cm*V’s-’ at 20 K. This mobility is one of the highest reported for a Hgt,Cd,Te As a further demonstration
layer grown either by MBE or CBE.
of the ability of the CBE technique to produce layers with high quality
surfaces, Figure 22 shows the first RHEED intensity oscillations
obtained for a Hgr.,Cd,Te
layer grown on
GaAs. This data indicated a growth rate of approximately 0.52 Mus. Recently, the optical studies of CBE grown HgCdTe have been extended to investigation infrared photoluminescence
and photoreflection
et al. were made using YAGNd spectrometer
properties.
The photoluminescence
of their
studies reported by Tomm
laser excitation with an Rower density of 16 Wcm-* and a low f-number
for sample temperatures from 4.2 to 300 K.“*
Chemical Beam Epitaxy 15E+MJ!5
I -Theo
HQTe 7/CdZnTe mnm Hgle//GaAs
AAAAA
O.OE+000
Figure 19.
I
l&l
sb
150
2dO
2!%
3&l
Electron mobility versus temperahue for two (001) HgTe layers (the solid line represents theory from ref. 60). IO0
II-
_ Hg<,Cd .Te x = ,161
4. 2s h
106:
f
I-
% I
’ 2IO'
F@ure 20.
’
dl
150
2bo
250
3 IO
Electron mobility versus temperature for a (001) Hgo.&,16Te layer grown on &As.
The 300 K photoluminescence Figure 23. The luminescence
data obtained for an undoped Hg.T&d.uTTe
sample is shown in
intensity is observed to be quite. asymmetric with a full width at half maximum of
- 85 meV and the peak luminescence rkzases
id0
being observed to occur at 0.205 eV. At lower energies the luminescence
rapidly while for highe.r energies the distribution is observed to extend to energies as high as 0.420
eV. This featunz is attributed to the 300 K Fermi Dirac electron distribution in the conduction
band due to a
C. J. Summers et
al.
_ 10". “E 9
8 a % 0 lOi5 0.0
, 50.0
.
I u I m I 100.0 150.0 200.0 (deg.
TEMPERATURE
Figure 21.
Temperature dependence
ofcarrierconcentration
m I 250.0
I
K)
and mobility for a (001) H&.&Q.,4Te layer grown on
H&Cd,,Te/GaAs
r = 0.49
0
RHEED
340 K electron gas.
(001)
MU.s
10 Time
Figure 22.
10’ 300.0
(&onds)
intensity oscillations obtained from a (001) H~,J.&&~T~/G~As CBE layer grown at 165°C. The peak PL at 300 K agrees well with the IR transmission
data recorded for these
samples. The low (15 K) temperature PL is shown in Figure 24. The peak PL has shifted down in energy to 0.125 eV (-10 w)
and has a considerably
narrower full width at half maximum of 11 meV.
The peak is
189
Chemical Beam Epitaxy
more symmetrical although some evidence of the Fermi distribution is still observed at higher energies.
Similar
data has also been obtained for CdTeIHgCdTe superlattices.
4.2
CdTe/I-kCdTe Su~rlattice Conventional
Growth
type II superlattices consisting of CdTe barriers and small bandgap Hgr .,Cd,Te (x =
0.25 - 0.3) semiconductor
quantum wells have also been grown by CBE using the procedures
HgCdTe and CdTe growth.
developed
for
Before growth on the (21 l)B CdTe substrates were annealed at 300°C for several
minutes with and without a Te flux incident on the surface. The substrate temperature was dropped to 180°C for the initiation of the superlattice growth. After the confiiation
of high quality growth, as determined
by
RHEED and diffuse laser scattering, the growth temperature was gradually lowered to 170°C to minimize layer
H&dTe (x=0.237) T=3OOK
MOO-
0.
I 180
[ 220
I,
I,
I,
260
300
I, 340
I/ 380
420
460
Energy (meV)
Figure 23. interdiffusion.
Photoluminescence spectrum obtained at ux) K for a Hgl,CdXTe undoped layer with x = 0.237. These layers were deposited at a growth rate of approximately
0.75 pm/hr and were deposited
directly onto the substrate with no buffer layer growth. During growth, a photon flux of 40 - 60 mWcm_* was incident upon the growth surface. Two types of superlattice
structures
were grown; 50 A CdTeIlOO 8, HgCdTe
CdTeLZOOA HgCdTe, with and without iodine doping. varied to shift the absorption superlattice layer.
edge.
The composition
Room temperature
and 100 8,
of the HgCdTe in the. wells was also
FTIR transmission
This data indicated that the layers had cutoff wavelengths
spectra were taken for each ranging from 2.4 to 5.0 pm for
C. J.Summers etal.
190
0 100
110
120
130
140
Energy(meV) Figure24.
Photoluminescence spectrum obtained at 15 K from an undoped Hg,..Cd.Te layer (x = 0.237).
these structures and HgCdTe composition interference
values.
The transmission
fringes indicative of high quality material.
absorption edge to longer wavelengths
by approximately
spectra showed sharp cutoffs and several
As expected, 0.2 pm.
the larger period structure shifted the
The cutoff wavelength
increasing Hg content in the wells. Figure 2.5 shows room temperature FIIR transmission h100
also increased with spectra from three 50
A (barrier/well) superlattices in which the barrier layers were grown under identical conditions but with a
different DmCd flux for the HgCdTe wells. The increase in cutoff wavelength for these layers corresponds decreased Cd content in the wells. Figure 26 shows FITS transmission identical conditions
spectra for two samples grown under
except for one having twice the barrier and well thicknesses.
longer cutoff
wavelength
expected
for such a change
measurements
of the doped superlattices
in the dimensions
This sample exhibits the
of the structure.
indicate room temperature electron concentrations
high-1Or6 cmm3to the mid-10’7 cme3. Infrared photoluminescence
to a
Electrical
ranging from the
spectra were also recorded for these samples
at room and low temperatures6*
5. DOPING BY CHEMICAL BEAM EPITAXY
In addition to the successful growth of CdTe and HgCdTe alloys by CBE, investigations undertaken
to obtain n- and p-type doped layers to explore the advantage it offers for developing
The lack of controllable extrinsic doping technology for HgCdTe.27728,45363
were
a mature
in-situ extrinsic doping is a major
191
Chemical Beam Epitaxy
8o1
20
01’1’1’ 4500
4000
I’ 3000
3500
I’ 2500
I’ 2000
I’ 1500
I’I 1000
500
Wavenumbers Figure 25.
Room temperature FITR spectra from three CdTeIHgCdTe superlattices with different compositions HgCdTe in the wells.
of
80
80
Q
2 3
5 ..-t2 E
40
i 20
0
Flpre26.
Room temperature FTlR spectra from two CdTe/HgCdTe and well thicknesses.
impediment
to numerous device applications
could result in a radical impiuvement
superlattices with one having twice the barrier
for these alloys and is essential for new device schemes which
in detection.
The wider range of dopants made possible by CBE makes it
prudent to review the selection criteria for dopants in HgCdTe so as to clearly define their desired properties. The three main criteria for dopant selection which have to be mutually compatible axe: 1. their electronic behavior in the host lattice,
C. J. Summers et a/.
192
2. their chemical characteristics,
stability and metallurgical
properties
such as volatility, segregation
and diffusion
and
3. their nucleation kinetics which determines
the segregation coefficient,
substitutional
or interstitial
behavior, and electrical activity. The first criteria is met by selecting elements in the periodic table from columns adjacent to the host elements
whose chemical
and bonding
characteristics
perturbation of the lattice while doping the semiconductor.
allows substitution
for a host ion with little
Possible p-type dopants for the HgCdTe system are
elements from group IB (Cu, Ag, Au) on a cation (Hg or Cd) lattice site, or the substitution of elements from group VA (N, P, As, Sb, Bi) on an anion (Te) lattice site. Conversely, elements from group III4 (B, Al, Ga, In) substituting on the cation lattice site, and elements from group VIA (F, Cl, Br, I) substituting on an anion lattice are possible n-type dopants. Since the bonding electronegativity,
forces in HgCdTe are more ionic than covalent
ionic radii, and atomic volume similar to the host ions are required.
in nature, dopants with For the Hg/Cd site Cu,
Ag, and In are closely matched to the properties of the host atoms, whereas for the Te lattice, P, As, I and Br are the most suitable dopants.
However, the metallurgical properties of these dopants show that atoms such as In,
Cu and Ag, which incorporate
on the HgKd
lattice diffuse very fast.64 Thus, the most stable dopants are
expected to substitute on the Te lattice, i.e., P, As or N for p-type doping and I or Br for n-type doping. The incorporation
of dopant species is also expected to be enhanced by the use of monomer
dopant species. However, these elements evaporate in molecular forms. Cl, I and Br, which can be used for ntype doping on the Te-lattice, also are dimers in the vapor form. Because of these limitations, metalorganic gas dopant sources were investigated, structures, the CBE technique doping where extensive
and have shown that in addition to the growth of alloys and superlattice
is also a very effective doping technique.
characterizations
This is particularly true for iodine
of CdTe and HgCdTe doped with iodine using an ethyliodide
gas
source have recently been reported as discussed be10w.“~~~~~~ Iodine h~ine
5.1
of CdTe
Because of the complexity of the HgCdTe alloy system the first CBE doping investigations performed
in CdTe.
In addition to the fact that it appears to best satisfy all of the physical and chemical
properties required of an n-type dopant in the HgCdTe alloy system, iodine also is very compatible temperature epitaxial growth. cation-rich
were
with low
Iodine doping on the Te-sublattice preserves the advantages of low temperature
growth which has been shown to produce planar surfaces and also is expected to simultaneously
promote
dopant incorporation
sticking
coefficient
on the Te sublattice.
and minimizes
the formation
Low temperature of native defects.
growth also enhances Additionally,
the dopant
group II rich growth
conditions should further suppress the formation of Va, Tei and [Vcd-donor] acceptor defects that compensate donors.
Also, because iodine substitutes on the more stable Te sublattice, its diffusion coefficient
order of magnitude
lower than that of indium, which substitutes for Cd.25*26 Low diffusion
realize abrupt dopant profiles in advanced device structures.
is over an
is important to
193
Chemical Beam Epitaxy The
high
doping was implemented source described
vapor
form. Therefore,
using an alkyliodine gas source that was delivered by the pressure controlled
in Section 3.1.”
substrate in the unpyrolyzed conductance
of iodine prohibits its use in the elemental
ptxsure
form.
Ethyliodide
was used as the iodine precursor,
The dopant-PCVS
iodine vapor
and was incident on the
was operated under choked flow conditions,
and the
of the orifice was related to the upstream pressure to accurately determine the dopant flow rate.
This enabled the measurement
of flow rates as small as 10m7seem, corresponding
to a BEP of lo-” torr.
However, because CdTe growth between 200 - 300°C using a binary source occurs under Te-rich conditions
due to the nonunity
sticking coefficient
dependent on the Cd overpressure?o
of Cd the surface stoichiometry
and growth rate are
The Cd-rich growth conditions that are necessary to optimize doping were
obtained by measuring the dependence
of the CdTe growth rate on excess Cd flux.
As shown in Figure 27,
with increasing Cd flux, the growth rate increases to a maximum of 0.27 prn/hr, for fluxes greater than 6 x 10m8 torr corresponding
to a Cme
flux ratio of 1.3. This indicates that for a growth temperature of 230°C. and PC&
= 3.7 x l@’ torr, the surface becomes cation-rich when the Cd BEP was greater than 6 x lOmatorr. This data was corroborated reconstructed
by RHEED studies which showed that the growth surface exhibited
pattern for low Cd overpressures,
and the co-presence
stabilized surface for Cd BEP’s of 1.5 x 10e7 torr or higher.” dependence
of the electrical propeaies
was investigated
a Te-stabilized
of a ~(2x2) reconstruction
Using these cation-rich
(2x1)
typical of a Cd-
growth conditions,
for ethyliodide dopant flow rates between
the
lo-’ - lo-*
seem.
I
0.350
(001)
CdTe: I/G&a
T. = 230°C
c g E
0.300
-
-
5
0.225 -
0.200
1
0.0
5.0x10-"
PCd Figure 27.
I
1.0x10‘
I
1.5x10-'
2.oxlO-7
ttorr)
Dependence of growth rate on excess Cd flax for MBE grown CdTe at 230°C. Flux values for which
(2x1) Te-stabilized and mixed (2x1) Te and ~(2x2) Cd-stabii are indicated.
surface reconstructions were observed
C. J. Summers et al.
194
As shown in Figure 28, the room temperature electron concentration root dependence
was found to have a square-
on the dopant flow rate for flows between 10.’ and lo-* seem. For this range of flow rates, the
electron concentration
increased from 8 x lOi to 3 x 10” cmm3and showed no evidence of saturation at high
flows indicating that doping higher than 3 x 1OL8cm-3 can be achieved in CdTe:I. This nonlinear dependence was attributed by Rajavel and Summers to a decrease in the dopant incorporation flow rate.*’ For films with room-temperature cm”, the corresponding
electron concentration
rate with increasing dopant
of 4.4 x lo”, 7.5 x lo”, and 3.1 x 10”
mobilities were 560, 570, and 460 cm2Vk’,
respectively,
and are among the highest
values recorded in heavily doped CdTe layers. Doping down to electron concentrations
in the lOI - lOI cme3 has recently been achieved and
given the highest mobilities yet reported for MBE grown CdTe. For a sample with a room temperature electron concentration
of 2.3 x 10” cmm3 and a 60 K concentration
to 9022 cm%&-‘.
The lowest concentration
cme3 at low temperature
of 2.8 x lOI cmm3,the mobility increased from 600
achieved was 4.4 x 1014 cme3 at room temperature and 5 x lOI
with corresponding
mobilities
of 300 and 5300 cm2V~‘s~‘.66 Further optimization
studies in this area are expected to result in even higher mobilities for this concentration range. SIMS measurements
corroborated
the electrical data and showed that the electrical activity and
iodine depth profile were well behaved in CdTe. This indicates that there was little compensation presence of native acceptor defects or the formation of
Cd12 precipitates
that are electrically
due to the
inactive.
4 _
(001) CdTe:l
2 - Growth
6.
rate = O.!Q.m/hr
I ,,,,I,,
I I I11111
466’
VP2
2
lOA
466’
10”
I I
I I I11111 2
466’
2
4
1o-2
Ethyliodide flow rate (seem) F&we
28.
Dependence of electron concentration for iodine doped CdTe as a function of ethyliodide flow rate.
As
Chemical Beam Epitaxy
shown in Figure 29 the iodine concentration
is constant
195
throughout
the layer and drops sharply at the
CdTeYCdTe
buffer layer interface; an indication
temperature.
(The accumulation of iodine at the substrate/layer interface was caused by dopant flux calibration
prior to growth.) concentration
Because of the mass interference
of the low diffusivity
of iodine in CdTe at this growth
in the SIMS measurements
was limited to a factor of five, and results in a conservative
the accuracy of the iodine
estimate of the iodine electrical
activity of between 50% and 100%.
1.oo DEPTH Figure 29.
2.00
3.00
(microns)
SIMS profile of iodine doped CdTe.
Photoluminescence
studies of iodine doped CdTe show that the PL emission intensity was up to
100 times greater than comparable undoped materials which made it possible to identify a very rich spectra.67 As shown in Figure 30, the low temperature spectra shows two very strong features with linewidths c 1 meV at 1.5927 and 1.5892 eV which were assigned to an iodine donor substituting acceptor exciton.
for Te and an unknown bound
A small shoulder was also observed on the low energy side of the acceptor bound exciton at
1.5860 eV and assigned to two-electron transitions. This feature occurs because some of the free exciton recombination
energy is transferred to the
donor, to excite an electron from the 1s ground state to the 2s first excited state, resulting in a lower energy recombination
feature. The presence of this feature is not only an indication of very high crystalline quality but
also allows a very accurate determination
of the donor ionization energy of - 15.0 -c 0.2 meV for iodine.
The
196
C. J. Summers et al.
temperature dependence energy.
of the 1.593 eV feature was also measured to further verify the iodine donor ionization
Figure 3 1 shows the quenching of this feature as the temperature is increased.
The high temperature
results indicated an iodine ionization energy of - 14 meV. A slight kink in the data near 45 K was explained by assuming an activation energy of 10 meV which corresponds to the 1s - 2s donor transition energy.
Thus, this
data confirms iodine’s shallow hydrogen like donor behavior with Eo - 15 meV. Figure 30 also shows a feature at 1.568 eV assigned to an LO phonon replica of the (A’,x) feature.
Because acceptor are deeper than donors
they are more strongly tied to the phonons, this is an indirect confirmation of this feature at 1.556 eV was not determined recombination donor-acceptor
but was ruled out as an acceptor or a donor-acceptor
because it showed no phonon replicas. pair recombination
involving
of the 1.589 eV feature. The origin pair
The lower energy feature at 1.54 eV was assigned to a
the iodine donors
and unintentional
doping by Na and Li
acceptors which have activation energies of -58 meV. At even lower energies, a series of features were observed between 1.49 1 and 1.35 eV. The sharp feature at 1.49 1 eV was only observed below 45 K and has recently been assigned to radiative recombination emitted when conduction
electrons recombine
through a neutral donor-acceptor
pair complex resulting from
nearest neighbor cation (Na& and anion &) defects. The energy level associated with this neutral (Na&-fre+) pair is 0.115 eV below the conduction band. This feature was accompanied by associated LO-phonon replicas
CdTe: I (T, = 210°C) 500 mWIcm* T = 4.9K
1.5860 TET
DAP
I
1
1.556
1
(A”J)-lLO
1.560 Energy Figure36.
1.576 @VI
Photoluminescencespectrum of iodine doped CdTe at 10 K.
1
Chemical Beam Epitaxy
197
loo.0
j
10.0
s .i? 2
B c
-
1.0
7
0.1
I
0.M I
0.05
0.03
0.0
CdB:I (-r,=170%) Ed9a emissionat 1.593 eV
l/Temperature
0
.Ofi
(K-l)
I
I
I
0.10
0.15
0.20
0 !5
1/Temperature (K“) Figure 31.
Quenching of 1.593 eV feature with increasing temperature.
occurring at energies
20.4
- 21.3
meV below the main feature and also by an interesting local mode of 36.5
meV which was attributed to the presence of Nacd in the defect center.68 The feature at 1.470 eV, followed by up to seven LO-phonon replicas spaced by 21.3 meV, has been assigned to a donor-to-acceptor the (Vc&J
pair recombination
transition from the ground state of the iodine donor to
acceptor state complex formed by adjacent Vcd and Ire sites. This assignment was made because
the PL emission intensity increased proportionally with increasing dopant concentration. in emission intensity was also observed for a sample grown with no Cd overpressure. to a larger concentration
A significant increase
This result was attributed
of acceptor defects which provide an increased path for recombination
indirectly confirms the effectiveness
of excess Cd in decreasing V,
and, therefore,
associated point defects and complexes.
This analysis results in an ionization energy of 0.125 eV for the (V&*-IT,+) acceptor complex. Confirmation
of the transition energies
observed
in the PL spectra has also been obtained by
monitoring the intensity of the 1.470 eV line as a function of temperature.
The luminescence
intensity can be
fitted as a function of inverse temperature by three activation energies; 15 meV at low temperatures, intermediate
temperatures,
and 125 meV above 140°C.
Estimates of the Huong-Rhys
67 meV at
factor from the LO-
phonon replica spectra range between 1.5 and 2 and are significantly less than the phonon coupling strengths for chlorine and bromine, reported as 2.2 and 2.9, respectively.
This reduction in phonon coupling is attributed to a
reduced lattice distortion since the Te and iodine have such similar size.
Thus, the lower level of (Vcd-1~~)
defects and reduced phonon coupling both support the argument of iodide being an effective donor in CdTe and related alloys.
198
5.2
C. J. Summers et al. Iodine Dopine of HgCdTe For this study ethyliodide doped HgCdTe layers were grown by CBE at 180°C on 10 pm thick
CdTe buffer layers deposited on (21 l)B oriented GaAs substrates. seem and 0.16 seem, respectively.
The Hg-PCVS
The DipTe and DmCd flow rates were 0.8
control pressure was 0.725 torr, for a beam equivalent
pressure of 1.4 x 10e4torr. These conditions gave an x-value of 0.241 and produced a growth rate of 2.3 pm/hr. The ethyliodide previously.27
was incident on the growth surface without cracking as for the CdTe doping discussed
The conductance
of the orifice was calibrated with the ethyliodide
from the product of the capacitance
manometer
pressure and the characteristic
flow rate being determined decay time of the orifice.
In
addition, the beam equivalent pressure was measured directly for larger flow rates. Figure 32 shows the electron concentration iodine-doped
as a function of sample temperature
for a range of
HgCdTe layers and undoped CBE HgCdTe layers. The low temperature electron concentration
varied over three orders of magnitude and indicated that iodine is an effective n-type dopant for HgCdTe. maximum electron concentration MBE grown material. concentration
was 5 x 1018cmm3and is the highest reported n-type carrier concentration
The lowest electron
concentration
sample was undoped
of 3.4 x 10” cme3. Figure 33 shows the temperature dependence
The in
and had a 20 K carrier
of the carrier concentration
and
mobility for a sample doped to 1.8 x lOI cme3. The peak mobility was 50,000 cm’V’s_’ which is the highest reported mobility at this carrier concentration measurements
for MBE HgCdTe material.
It should be noted that all of these
were made on as-grown material, without any low temperature, Hg-saturated anneals to remove
Hg vacancies as is usually given to MBE layers before measurements.
Therefore, the electron mobilities are
expected to improve after such treatments.
Figure 32.
Dependence of electron concentration on temperaturefor iodinedoped Hgi.,Cd,Te samples with x = 0.24.
Chemical Beam Epitaxy
199
Dependence of electron concentration and mobility on temperature for iodine doped Hgr.,Cd,Te (x = 0.24) with n = 1.8 x 10” cm-3.
Figure 33.
Figure 34 shows the electron concentration at 20 K as a function of the ethyliodide flow rate. The carrier concentration
increased as the square root of the ethyhodide flow and only deviated from this behavior at
high doping levels.
These results am in agreement with the previously discussed dependence
of n-type doping
on ethyliodide flux for CdTe.
5.3
P-Tvw Dovim of CdTe Due to the difftculty in p-type doping HgCdTe, a number of different
investigated
techniques
to activate As-doped HgCdTe layers. For example, As has been incorporated
layers of a CdTe/HgTe multilayer and then interdiffused
have been
only into the CdTe
into the adjacent HgTe layers, and laser illumination
has been used to promote high As doping in CdTe at low growth temperatures.65
Arias et aL6’ and Wu et aL6’
have reported that Cd-rich off-stoichiometric
growth flux conditions are required for the effective activation of
As and In, respectively.
of iodine,
stoichiometry.63
The activation
however,
is apparently
insensitive
to the surface
Also, epitaxial growth temperatures have to be minimized to obtain high dopant incorporation.
Thus, laser illumination
has been used with reduced growth temperatures
to maintain adequate CdTe adatom
surface mobilities. Because low temperature growth has been remarkably effective for incorporating
arsenic atoms
into CdTe,” Maruyama et al.” undertook studies to reduce the growth temperature by the addition of an excess Cd flux to compensate
for the preferential desorption of Cd atoms.
or accurate Cd-rich off-stoichiometric layers at high temperatures
Thus, stoichiometric
surface conditions,65
growth conditions can be used to achieve both high quality CdTe buffer
and high concentration Asdoped
At the higher temperatures
layers at low temperatures.
the CdTe growth rate was observed to increase with increasing Cd
flux and attained a constant value for CdRe flux ratios exceeding
1.8. This saturation of the growth rate at
C. J. Summers
200
et a/.
/'
10% E
./'
.
_. /'
1016 o.ooo1
I 0.01
* - 8a.
. .I 1
. - ..
. : :: 100
EthyliodideFluwrate, Fi (TOIT%‘) Figure 34.
Electron concentration (measured at 20 K) as a function of ethyliodide flow rate. Fitted line corresponds to square root dependence of carrier concentration on flow rate.
high excess Cd fluxes implies that the Cd desotption stoichiometric
was offset by the increased Cd flux, thus yielding a
surface for the CdTe layers as confirmed by RHEED. After buffer layer growth at 300°C with a Cme
was reduced to below 200°C and the growth conditions technique, as depicted in Figure 35. For temperatures
flux ratio of 1.8 the CdTe growth temperature
studied by the same RHEED intensity oscillation
less than 200°C no change was seen in the growth rate
using excess Cd. However, the RHEED patterns were shown to strongly depend on the growth conditions and Cd/Te flux ratio.
Streaky RHEED patterns were taken to indicate two dimensional
morphology
and good crystallinity,
accompanied
by twin-induced
experiments
growth, smooth surface
while spotty patterns were indicative of three dimensional
growth, and
diffraction features, which signaled the onset of crystallinity degradation.
These
were used to define a window of growth flux ratios at a given substrate temperature that yielded
high material quality. From this study optimum growth conditions were obtained for Cd!Te flux ratios between 1.02 and 1.18 at 180°C and between decreasing
temperature,
demonstrating
1.06 and 1.12 at 170°C. As shown, this window decreases in size with the need for precise flux and temperature
control at lower growth
temperatures. The CdTe layers were grown under different growth conditions and their properties evaluated by photoluminescence,
SIMS, and Hall effect measurements.
Amine was pyrolyzed to generate a large flux of
dimer arsenic, as comparedto the tetmmerAs species obtained in the conventional AS sublimation process.
Chemical Beam Epkaxy
Buffer:J,
14. =l .O
Growth Temperature (“C) 1.7
r
1.6 1.6 F 1.4 3
1.3 z ') 1.2 1.1 1.0 0.9 150
I
160
I
I
I
I
170
180
190
200
Growth Temperature (“C) b) Figure 35.
Ctystahinity of CdTe, estimated from RBEED patterns, as a function of the growth temperature and Cite flux ratios with (a) J&Jr. = 1.0 and (b) JcdJT, = 1.8.
Figure 36 shows a 10 K PL spectrum obtained for an As doped CdTe layer.
The four peaks
between 8200 and 8600 8, confirmed arsenic incorporation on the Te sublattice, where. the donor-acceptor (DAP) peak at 8200 8, corresponded
pair
to As atoms occupying Te sites and the remaining peaks are longitudinal-
optical (LO) phonon replicas of this peak.
From this study dimer arsenic was shown to incorporate
tellurium sublattice with a higher efficiency than was obtained with tetramer.
on the
Undoped CdTe layers grown
under similar conditions did not show any peaks around 8200 A. SIMS profile measumments
showed that the arsenic concentration was as large as 2.5 x lOI7 cm-‘,
for a large arsenic flux of 1.1 x 10e7torr BEP at 180°C.
The abrupt reduction of the arsenic signal at the
interface indicated that the diffusion of arsenic was very low at the CdTe growth temperature. of the As concentration
The dependance
on the cracked amine partial pressure is shown in Figure 37. The maximum arsenic
C.
202
1
I
A
Summers
et al.
I
I
I
CdTe:As
Ad(
j
J.
T=lOK PATHS= 4.8x10w7torr Jcd/J Te = 1.68 T~200”C
/
7t
10
8000
8400
Wavelength Figure 36.
8800 ( 8)
Photoluminescence specttum of a CdTe:As layer obtained at 10 K.
concentration
was 3.0 x lOi cmm3 for an arsine BEP of 1.3 x 10.’ torr for growth at 170°C.
concentration
incorporated
The As
in CdTe grown at 170°C was equivalent to that grown at 18O”C, despite the lower
amine pressure at 170°C. Thus, the incorporation efficiency of As was remarkably improved for 170°C growth. Doping using solid arsenic for the same growth conditions and for equivalent As tetramer flux, produced an As concentration
at the SIMS detection limit of 2 x 10” cme3. This indicates that the incorporation
efficiency of
dimer As is two orders of magnitude higher than that using As tetramers. However, carrier concentrations were considerably
room temperature
Hall effect measurements
and mobilities (3.5 x lOi5 cm-3, 81 cm%%-’
showed typical p-type values for the
and 1.7 x 1015cm-‘, 52 cm*V’s-‘) which
lower than measured by SIMS. The high resistivity was attributed to autodoping from the
CdTe substrates as the SIMS measurements
also showed high concentrations
of bromine, which acts as a donor
in CdTe. It is also possible that untracked or partially cracked amine was incorporated onto Te sites and caused self-compensation. As tetramers
These results indicate that As climers obtained by pyrolyzing amine are a better source than
for high-efficiency
arsenic doping, but that more work remains to be done to achieve high
electrical activity. P-type doping of Cd&Zn,Te
using atomic nitrogen supplied by a dc glow plasma source has
recently been reported7’ with hole concentrations doping
efftciency
concentration
decreased
significantly
of up to l$’
cm”
as the Cd mole fraction
of only 8 x 1015 cme3 was achieved for homogeneously
being obtained in ZnTe. was increased doped CdTe.
However, the
and a maximum
hole
Pulse doping (alternating
doped and undoped layers) was used to achieve a factor of ten increase in doping, to lOI7 cmm3for CdTe.
Chemical Beam Epitaxy
I
-
-
I
203
I
I
l Tg=180”C o Tg=170”C
j
0
I
lo-'0 10-9 10-8 1o-7 10-6 AsH3 pressure (torr) Figure 37.
Dependence of As concentrationon amine beam equivalent pressure.
6. CRE GROWTH OF RELATED MATERIALS: CdTe AND ZnU-Ig.Cd)Te
In this section only Te-based compounds grown by CBE are reviewed.
Excellent review articles
on the CBE growth of the wider bandgap Se and S based materials have been published by Yoshikawa and Konagai.r2*73 Rajavel and Zinck have recently extended the CBE technique to the growth of CdTe, CdZnTe and ZnTe.r4*7s*76 These growths were performed
in a Vacuum Generators
system in which the DmCd and
DeZn precursors were cracked in tantalum based cracker cells. In this study, diethyltelluride as the Te precursor
and was cracked in a quartz cracker to prevent unwanted
precursors were completely DeTe was decomposed
dissociated
into monomer
into the elemental species and hydrocarbon
(DeTe) was used
corrosion.
Both group II
by-products,
and dimer tellurium, partially alkylated tellurium
whereas the
compounds,
and
hydrocarbon by-products. prior to growth, surface kinetic studies were performed on (001) GaAs substrates.
The growth
kinetics of ZnTe and CdTe were studied by measuring the variation in growth rate as a function of substrate temperature and II/VI ratio using RI-TEED intensity oscillations.
Figure 38 shows the variation in growth rate
measured for a fixed Te flux as a function of the DeZn pressure and substrate temperature.
As shown, the
curves have the expected behavior with the growth rate increasing rapidly with DeZn flux at low flux values and then plateauing for higher values. The saturated maximum in the growth rate defines a Te limited growth rate regime,
which above 33O“C decreases
with increasing
incorporation
coefficient of the Te species under Zn-rich growth conditions are unity below 330°C but become
increasingly less than unity at higher temperatures.
temperature.
This result indicates
that the
A similar study also was performed for CdTe and showed
that the Te sticking coefficient became less than unity above 280°C. These results are similar to those reported
204
C. J. Summers
et
al.
by Benz et al. except that the critical temperature is significantly lower, presumably because of the presence of dimer Te and other Te species which have a lower activation energy than monomer Te.”
The difference
in
critical temperature between CdTe and ZnTe is attributed to the higher bond strength between Zn and Te.77 The physical properties measurements
of these 2 - 3 pm thick films as determined
from X-ray and SIMS
indicated the rocking curve linewidths were 200 - 220 arc-set for ZnTe and 480 - 580 arc-set for
I-
T,=315”C x Ts = 330 . T, = 350 . T, = 365 n Ts = 385 q
0.8
1.0
1.2
1.4
1.6 L
Figure 38.
1.8
2.0
2.2
2.4
(torr)
Variation in growtb rate as a function of substrate temperature and DeZn/DeTe flux ratio.
CdTe with lower values of 210 - 230 arc set being obtained for thicker (6 - 7 Frn) films. The PL spectra of the ZnTe samples were dominated by strong near band edge emissions as is shown in Figure 39. Free excitonic recombination
was observed at 2.382 eV and lower energy features which repeat at 26 meV intervals below the
free excitonic peak at 2.356 and 2.330 eV, were attributed to phonon replicas of the free exciton. Strong bound excitonic
recombinations
associated
with gallium donors and arsenic and zinc However, in the absence of
vacancy (Va) acceptors were observed at 2.379,2.374
and 2.368 eV, respectively.
PL excitation
The strongest bound excitonic feature was the arsenic
data, these assignments
are tentative.
bound excitonic transition which had a FWHM of 3 meV. vacancy complexes.
No evidence was found for the presence of zinc
Films grown under different B/W ratios showed similar features, however, the relative
intensities of the excitonic features varied.
These results demonstrate that the structural and optical properties
of CBE grown ZnTe films equal those prepared by other epitaxial growth techniques.
Chemical Beam Epitaxy
205
T
-
ZnTelGaAs T=5K
As”,X
vz,“,x
I
Ga”.X
I
J1,;_ X
I
I
I
I
I
I
1
2.30
2.32
2.34
2.36
2.38
2.40
Energy (eV) Figure39.
Photoluminescence spectra of ZnTe at 5 K.
SIMS measurements
on these samples indicated the presence of Ga and As at a 5 x 10” cme3
level, presumably due to contamination contamination
from the substrate.
High levels of Si, Al, and Br due to source material
were also observed and C and 0 identified at SIMS background levels of - 1Or7cmm3. The CdTe samples were grown using a ZnTe buffer layer to ensure (001) orientation.
crystalline quality sample was observed to exhibit a rich PL structure. CdTe film is shown in Figure 40. recombination
The highest
The PL spectrum of a representative
A strong free exciton peak was observed
at 1.596 eV and associated
from its fust excited state at 1.603 eV and its upper polariton branch at 1.598 eV. These free
excitonic features attest to the high quality of the CBE grown (001) CdTe/GaAs films. Also, a sharp (FWHM = 1 meV) and intense bound excitonic feature was observed at 1.590 eV and was attributed to excitons bound to arsenic acceptors.
Similar results were obtained for CdTe growth using DeCd.
Cdt,Zn,Te
(x - 0.04) films also were grown at 320°C on 0.2 pm thick ZnTe buffer layers
deposited on (001) GaAs substrates.
Films 8 - 9 pm thick exhibited x-ray FWHMs of 210 - 250 arc-s and 3 pm
thick films FWHMs of 400 - 450 arc-sec. bound excitonic recombination
The near band-edge region of the PL spectrum was dominated by
features associated with acceptors and donors at 1.615 and 1.619 eV, respectively of the free exciton was also observed at 1.622 eV.
and the
Donor acceptor pair recombination
was
observed at 1.5686 eV, and two phonon replicas with a spacing of 0.021 eV were observed at lower energies. A weak defect related band appeared at 1.494 eV. The surface morphology
of the (Cd,Zn)Te films prepared
206
C. J. Summers et al.
under near-stoichiometric
growth conditions
were smooth and nearly featureless as determined
by Nomarski
contrast microscopy. Studies of higher x-value alloys also show that the crystalline quality of the CBE material was very good. For example, a Cdi.,Zn,Te
(x = 0.17) grown on a (001) GaAs substrate exhibited a X-Ray FWHM
of 350 arc-sec. Also, PL spectra at 5 K exhibited strong donor bound excitonic transitions.
A”,X 1
I.580
(001) CdTe/GaAs T, = 280°C T=5K
I
I
I
1.585
1.590
1.595
I 1mO
I 1.605
1.610
Energy (eV)
Figure 40.
Photoluminescence
spectrum of a CdTe layer grown by CBE.
HgZnTe growth by CBE using elemental Zn also has been reported by Benz et al3 and Summers et al6
Figure 41 shows the room temperature
FUR spectra obtained for two Hgi.,Zn,Te
layers grown at
185°C. The cutoff wavelengths correspond to zinc mole fractions of 0.59 and 0.33.
7. MANUFACTURING
ISSUES
Of particular interest is the application of CBE as a flexible manufacturing systems.
technique for infrared
Given the material issues previously discussed for HgCdTe, it is important that a growth technique be
repeatable, precise, flexible and scalable to become a viable manufacturing tool. Wagner et al. have assessed the performance of CBE as a flexible manufacturing measurements
of the optical, electrical and compositional
measurements
of system performance
potential advantages of CBE.
techniques from
properties of CBE grown HgCdTe layers2’
RHEED
were also reported along with theoretical analysis which indicate the
Chemical Beam Epitaxy
1
3
5
7
9
11
13
Wavelength
207
15
17
19
21
(pm)
Room temperature FTIR spectra obtained for hvo HgZrrTe samples grown by CBE
Fipre41.
As previously mentioned,
a major advantage of CBE over conventional
MBE is the increased
stability possible with pressure based flow controllers as opposed to conventional thermal evaporation sources. This is especially important for the HgCdTe material system where the host elements and the group V and VII dopants have high vapor pressures. a small variation in temperature especially
important
For these solid sources, very stringent temperature control is required since
causes a relatively large change in the growth fluxes.
at the low growth temperatures
High flux control is
used for HgCdTe growth as any excess flux of the
constituent elements will not desorb from the growth surface and, therefore, will be incorporated
as defects.
The ease of computer control, ability to change sources externally and potential for selective area epitaxy and in situ processing
also make CBE an effective
manufacturing
demonstrated
the selective area epitaxy of CdTe by CBE?*
7.1
Characterization
achieved
In fact, Benson et al. have already
of CBE Grown HeCdTe
To experimentally undertaken
technique.
assess the potential of the CBE system, a series of HgCdTe growths was
over several months and under constant growth conditions over alloy composition.
measurements
These properties
were determined
to assess the control that could be by infrared transmission
and SIMS
to measure the alloy composition variations from run-to-run, laterally across the wafer and along
the growth direction. To transmission
determine
the
alloy
composition
and run-to-run
spectra were measured on each sample from 2 - 15 um.
variation,
room
temperature
FfIR
Figure 42 shows selected transmission
C. J. Summers et al
208
Hg,,CdxTe R = 0.242 80
80
4
FTIR transmission curves for three CBE gown
Figure 42.
10
8
Wavedgth
(urn)
HgCdTe samples whose cutoff wavelengths correspond
to the average and maximum variations of a series of fourteen layers.
curves obtained
for samples
whose compositions
obtained from a series of fourteen growths. corresponding
to the composition
correspond
to the average and the maximum
variations
The vertical lines in Figure 42 represent the wavelength (5.93 pm)
of the mean x-value and the mean plus or minus one standard deviation in x-
value. The data indicate a standard deviation of 0.17 pm. The cut-on wavelength was obtained from each of these curves by assuming that it corresponds The x-value was then calculated
to 50% of the average transmission
using Hansen and S&nit’s
expression
value at long wavelengths.
relating x-value, temperature
and
energy gap.” Figure 43 shows the range of x-values obtained from the above analysis for a series of 14 consecutively
grown samples.
As shown, the samples exhibit a mean composition
deviation of f 0.0043, a variation of less than 1.8%. It should be emphasized over a period of two weeks and were accomplished previous experiments.
of x = 0.241 and a standard
that these runs were performed
with no recalibration of the system and no feedback from
Also, growths performed months later with the same growth conditions yielded layers
with x-values within Ax of the mean x-value of 0.241. More recently, after implementing allowing
longer
compositional
substrate
of 14 growth
runs have produced
of the lateral variations in the HgCdTe composition
was carried out using
temperature
stabilization
times,
a series
computer control and
variations off 0.0016 for x = 0.229 material. An initial assessment
SIMS measurements.
In these measurements
the sampled area was 0.5 mm2 and several sets of data were taken
Chemical Beam Epitaxy
209
_I~1
Std. CM.: 0.004 Std. DeWMean:1.85%
I
I
0.2301: : : : : : :: :: : : : :: :: :: :: :: : : : : : : : : :: : : : : : : c 30 50 40 60
Run Number Figure 43.
Range of
compositions obtained from a series
of fourteen consecutively grown HgCdTe samples
over a 5 mm area. The studies indicated a variation of - 3% in the average lateral compositional
uniformity for
the x = 0.241 layers. FTIR scans taken on the x = 0.229 material indicated a lateral compositional
variation of
0.0005 over a 1.5 x 1.5 cm* sample. The issue of uniformity along the growth direction was also investigated. measuted
for several samples with the x-value being determined
shows the results obtained for three samples.
SIMS profiles were
using the Bubolac technique.s”
Figure 44
As shown, two of these layers exhibit sharp HgCdTeKdTe
interfaces while the non-ideal interface of the third sample was attributed to not allowing sufficient time for the sample to equilibrate at the growth temperature.
The average x-value of these layers was x = 0.241 and agrees
very well with the results obtained for the x-values using FIIR. shows a systematic increase in the x-value from the CdTekIgCdTe
Closer examination of the variation in x-value interface to the surface, and that this slope is
the major contribution to the variation observed in layer composition. Figure 45a shows the x-value as a function of depth for one of the layers shown in Figure 44. Note that at the CdTeJHgCdTe interface the x-value decreases from x = 1 to x - 0.24. There is also an increase in x-value evident as one approaches the surface of the layer. It is believed that this change is due to either the emissivity change of the substrate surface when HgCdTe growth is initiated or to measurement
inaccuracies in
the SIMS technology. Analysis of the variation in composition (neglecting the interface regions).
was first carried out across the HgCdTe layer thickness
The data obtained for these layers yield x = 0.240 f 0.0022, x = 0.235 f
0.0040 and x = 0.24 f 0.0056. Next, in order to minimize the long term variation in composition and measurement
due to growth
errors the data was fit with a low order (quadratic) polynomial (Figure 45b). The tit was then
subtracted from the original compositional
profile leaving a profile of the short term compositional
variation
210
C. J. Summers
et
al.
-r
1.2 (211)B Hg,.$d,Te/CdTe/GaAs 1.0 +z 2 9
0.8
& s ‘Z .8 E
0.6
0.4
0.2
0
4
Depth (,~rn)~ Figure 44.
SIMS composition profile of three HgCdTe samples.
(Figure 45~).
Analyzing
these profiles yielded variations
in the x-value of f 0.0005, 0.0008 and 0.001,
respectively, for the layers discussed above. This analysis shows that the short term changes in x-value that can be attributed principally to the control over the host gas delivery system (Cd + Te) and that potentially the control of the x-value can be better than f 0.00075.
This is very close to the predicted value from the simple model calculations
described in
Section 1. As a measure of the doping stability achievable with gas source doping technology, SIMS profiles of I-doped CdTe layers also were characterized.
From several layers results were obtained which indicate that
doping variations of less than 1% could be achieved on a routine basis.
8. SUMMARY
A detailed review has been presented of the science and technology issues of the CBE of HgCdTe and related compounds. The HgCdTe CBE system described features a specially designed pumping and purging system, a Hg-PCVS,
custom flow controller
systems for both the host elements and doping species, and special gas
cracker cells for efficient pyrolysis of the precursor gases. used to accurately calibrate system performance
A special RHEED system was also developed and
and to investigate
and optimize
substrate preparation
and
surface growth kinetics. Structural, optical, and electrical measurements
show that high quality HgCdTe alloys with x-
values between 0.0 and 0.4 can be achieved by the CBE technique at growth temperatures ranging from 120 to
Chemical
Beam Epitaxy
211
I 0.25
-
0.20
2 0
025
4.0
0.0
.
5
0.24
8 Q
0.23
..=
E
O
0
8.0
0002 .
0.000
4mo2
I I
I
I
I
I
I”
I ;
C) I
0.0
I 2.0
I
I 4.0
I
6.0
Depth (urn) Figure
185°C.
4%.
SIMS compositional profile of HgCdTe sample: (a) x-value as a function of depth for a HgCdTe layer grown at 180°C. (b) quadratic polynomial fit to data of (a). (c) short term compositional variation profile obtained by subtracting (b) from (a).
HgCdTe layers have been grown with background electron concentrations
low temperature mobilities of up to 9 x 10’ cm*Ns.
These HgCdTe properties equal or exceed those reported
for MBE grown material. The material characterization impurity contamination
as low as 3 x 1014cme3 and
results presented indicate that carbon or organometallic
did not significantly affect the electrical properties of CBE grown HgCdTe, making this
technique viable for infrared detector materials. N-type doping of CdTe and HgCdTe was accomplished
using a chemical beam of ethyliodide.
The films had electron concentrations
ranging from 4.4 x 1Or4 to 5 x lOI* cmm3;some of the highest values
achieved in MBE CdTe and HgCdTe.
Additionally, the highest electron mobilities were reported for the lightly
and heavily doped CdTe layers and the heavily doped HgCdTe layers. Iodine was shown to exhibit an electrical activity between 50% and 100% and to be a slow diffuser.
The near-edge 10 K luminescence
exhibited sharp bound exciton peaks and no long-wavelength
luminescence
spectra of CdTe:I
indicating that CBE iodine doping
C. J. Summers et al.
212
of CdTe is an effective
method to achieve high n-type conductivity
in CdTe and HgCdTe with the well
controlled doping profiles required for advanced device structures. A single study was conducted concentrations
on p-type doping of CdTe using amine. Although
high As
(> 5 x IO’s cme3) were observed in CdTe by SIMS, the electrical activation was low (10” - lOI
cmm3). Preliminary
N-plasma doping experiments
although low activation efficiency
indicated that N may be a suitable p-type dopant for CdTe
may be a problem and again no results were reported for HgCdTe.
This
problem with p-type doping is widespread throughout the HgCdTe community and is believed to result from impurities in poor substrate material. The inability to efficiently dope HgCdTe alloys p-type requires continued research. Additional cutoff wavelengths
superlattice
structures with
in the 2 - 5 pm region.
In addition, capabilities
research reported included the growth of HgCdTe-CdTe
recent studies of the CBE system performance
for the flexible manufacturing
obtained using this growth technique
have demonstrated
its excellent
of advanced HgCdTe material structures and devices.
demonstrate
the exciting potential of chemical
growth of advanced device concepts in HgCdTe, such as the development
The results
beam epitaxy for the
of avalanche photodiode
infrared
detectors for advanced imaging applications and infrared laser systems.
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J.P. Faurie, R. Sporken, S. Sivananthan, and M.D. Lange, J. Cryst. Growth 111,698 (1991).
38.
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39.
H. Heinecke, K. Werner, M. Weyers, H. Luth, and P. Balk, J. Cryst. Growth 81,270 (1987).
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40.
R.G. Benz, B.K. Wagner, and C.J. Summers, to be published.
41.
W.E. Hoke, P.J. Lemon&,
42.
T. McAllister, J. Crys. Growth 96,552 (1989).
43.
J.J. Zinck and D. Rajavel, Mat. Res. Sot. Symp. Proc. 282, 57 (I 993).
44.
D. Rajavel, A. Conte, C.J. Summers, J. Cryst. Growth 140, 327 (1994).
45.
K. Maruyama, R.G. Benz II, A. Conte-Matos, (1994).
46.
B.V. Shanabrook, D.S. Katzer, and R.J. Wagner, Appl. Phys. Lett. 59, 1317 (1991).
47.
J.M. Arias and G. Sullivan, J. Vat. Sci. Technol. A5,3143 (1987).
48.
S. Sivananthan, X. Chu, J. Reno, and J.P. Faurie, J. Appl. Phys. 60, 1359 (1986).
49.
B.A. Joyce, J. Zhang, T. Shitara, J.H. Neave, A. Taylor, S. Armstrong, M.E. Pembie, and C.T. Foxon, J. Crys. Growth 115,338 (1991).
50.
L. Ulmer, H. Mariette, N. Nagnea, and P. Gentile, J. Crys. Growth 111,711 (1991).
51.
J.D. Benson, D. Rajavel, B.K. Wagner, R. Benz II, and C.J. Summers, J. Crys. Growth 95,543 (1989).
52.
D. Rajavel, F. Mueller, J. D. Benson, B. K. Wagner, R. G. Benz, and C. J. Summers, J. Vat. Sci. Technol. A8, 1002 (1990).
53.
D. Rajavel, F. Mueller, J. D. Benson, B. K. Wagner, R. G. Benz, and C. J. Summers, J. Vat. Sci. Technol. B8, 192 (1990).
54.
R.J. Koestner, M.W. Goodwin, and H.F. Schaake, J. Vat. Sci. Technol. B9, 1731 (1991).
55.
R.J. Koestner and H.F. Schaake, J. Vat. Sci. Technol. A6,2834 (1988).
56.
P.W. Leech, P. J. Gwynn, and M. H. Kibel, App. Surf. Sci. 37,291 (1989).
57.
E.O. Kane, “The k.p Method” in Semiconductors Beer, Academic Press, NY 1966), p.75.
58.
M.D. Lange, S. Sivananthan, X. Chu, and J.P. Faurie, Appl. Phys. Lett. 52,978 (1988).
59.
Arias, S.H. Shin, J.T. Cheung, J.S. Chen, S. Sivananthan, J. Reno, and J.P. Faurie, J. Vat. Sci. Technol. A5,3133 (1987).
60.
J.R. Meyer, C.A. Hoffman, F.J. Bartoli, J.M. Perez, J.E. Fumeaux, R.J. Wagner, R.J. Koestner, and M.W. Goodwin, J. Vat. Sci. Technol. A6,2775 (1988).
61.
R.J. Koestner and H.F. Schaake, Mat. Res. Sot. Symp. Proc. 80,311 (1987).
62.
J.W. Tomm, T. Kelz, H. Kissel, A.R. Gamyeva, W. Hoer&l, T.K. Tran, B.K. Wagner, R.G. Benz II, R. Bicknell-Tassius, C.J. Summers, and T.H. Myers, submitted for publication in J. Elec. Mater.
63.
D. Rajavel, B.K. Wagner, R.G. Benz II, A. Conte, K. Maruyama, C.J. Summers, and J.D. Benson, J. Vat. Sci. Technol. BlO, 1432 (1992).
and R. Korenstein, J. Mater. Res. 3,329 (1988).
B.K. Wagner, C.J. Summers, J. Cryst. Growth 137,435
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Chemical Beam Epitaxy
215
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H.H. Woodbury,“Diffusion and Solubility Studies,” in Physics and Chemistry of II-VI Compounds, (Eds. M. Alven and J.S. Prener, John Wiley & Sons Inc., NY 1967), p.223.
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J.M. Arias, S.H. Shin, D.E. Cooper, M. Zandian, J.G. Pasko, E.R. Gertner, R.E. DeWames, and J. Singh, J. Vat. Sci. Technol. AS, 1025 (1990).
66.
N.C. Giles, J. Lee, T.H. Myers, Z. Yu, R.G. Benz, B.K. Wagner and C.J. Summers, to be published in J. Elec. Mater.
67.
N.C. Giles, J. Lee, D. Rajavel, and C.J. Summers, J. Appl. Phys. 73,454l
68.
J. Lee, N.C. Giles and C.J. Summers, submitted for publication in J. Elec. Mater.
69
Y.S. Wu, A. Waag, and R.N. Bicknell-Tassius,
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N.C. Giles, K.A. Bowers, R.L. Harper, Jr., S. Hwang and J.F. Schetzina, J. Crystal Growth 101,67, (1990).
71.
T. Baron, S. Taturenko, K. Saminadayar, N. Magnea, and J. Fontenille, Appl. Phys. Iett. 65, 1284 (1994).
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A. Yoshikawa, “Metal-organic Molecular Beam Epitaxy Growth and Properties of Widegap II-VI Compounds” in Widegap II-VI Compounds for Optoelectronic Applications, (Ed. H.E. Ruda, Chapman and Hall, London 1992), p. 98.
73.
M. Konagai, J. Cryst. Growth 120,261(1992).
74.
D. Rajavel and J.J. Zinck, Appl. Phys. Lett. 61, 1534 (1992).
75.
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76.
D. Rajavel and J.J. Zinck, J. Electron. Mat. 22,803 (1993).
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J.D. Benson and C.J. Summers, J. Appl. Phys. 66,5367 (1989).
78.
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79.
G.L. Hansen and J.L. Schmit, J. Appl. Phys. 54, 1639 (1983).
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216
C. J. Summers et al.
BIOGRAPHY Christopher J. Summers (M’82) was born in Oxford, England, on August 3, 1940. He received the B.S. and Ph.D. degrees in physics from Reading Univeristy, Reading, England, 1962 and 1966, respectively. After holding postdoctoral fellowship positions at Reading University and Bell Telephone Laboratories, he joined GTE Laboratories in 1970 as a member of the technical staff. In 1972 he moved to McDonnell Douglas Research Laboratories, where he became a Senior Research Scientist. He joined the Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, in 1981 and is currently a GTRI Fellow and Head of the Quantum Microstructures Branch and the Director of the Phosphor Technology Center of Excellence. His current research interests include optoelectronic properties of heterostructures and superlattices, the metalorganic and molecular beam epitaxy of II-VI and III-V semiconductors, and the growth and characterization of phosphor materials. Rudolph Cl. Benz, II was born in Detroit, Michigan, on February 22, 1962. He received the B.S. degree with honors in Engineering Science from the Pennsylvania State University in 1984, and the M.S. and Ph.D degree in physics from the Georgia Institute of Technology in 1988 and 1992, respectively. He is currently a research scientist at the Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta. His research interests include the growth and characterization of II-VI and III-V semiconductors by molecular beam epitaxy, the surface kinetics of epitaxial growth, and the application of GaAs charge transfer devices to solid state imager applications. Brent K. Wagner was born in Pottstown, PA, on June 22, 1962. He received his B.S.h. degree in Engineering Science at the Pennsylvania State University in 1984. He then received his M.S. degree in Physics and his Ph.D. degree in Electrical Engineering at the Georgia Institute of Tehnology in 1987 and 199 1, respectively. He is currently a research scientist at the Georgia Tech Institute, Georgia Insitute of Technology in Atlanta, Georgia. His research interests include the thin film growth and characterization of II-VI and III-V semiconductors and phosphor materials and the application of heterostructures to optoelectronic devices.
0960-9974(94)0001
S-9
’
CHEMICAL BEAM EPITAXY OF Hg,,,Cd,Te RELATED BINARIES
AND
C. J. Summers, B. K. Wagner and R. G. Benz II Quantum Microstructures Laboratory, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A.
coNTEwrs
m
DescriDtion
................ .. .. .. ............................................................................... ........... ....... ..... 162
1.
Introduction
2.
Rationale for Chemical Beam Epitaxy ................................................. .. .............. ........ .. ......... ...... 163 2.1
Nucleation Kinetics .......................................................................... .. ............ ...... .. ............ 164
2.2
Growth Technology Issues ................................................................... .. .......... ....... .......... 164
System Description .. ...... ... ............................ .... .. ....... ... .... ... .. .. .... .. .. ..................... ........... .... .. ........ 167 3.1
Gas Handling System ................ ............................................... ................... ......... ......... ..... 168
3.2
Gas Injection Design ........................................................................ ............... ........ ........... 169
3.3
Mercury Pressure Controlled Vapor Source .................... ........ ............ ........... ........... ....... 179
3.4
Metalorganic Precursor Cracking Characteristics .......................... ........... ........... ............. 171
3.5
In-situ Characterization
3.6
Surface Nucleation Kinetics ....... .. .. ................................ ........... ................. ...... ............ ..... l*O
of System. ...................................................... ............................ 176
Properties of HgCdTe Grown By CBE ........................................... 4.1
Characterization
4.2
CdTe/HgCdTe
......................... ....... ... 183
Studies of HgCdTe Alloys .............................. .._............ ......... ..... .... *84 Superlattice Growth ................................................. .............. ...... .......... ... ls9 161
C. J. Summers et al.
Doping by Chemical Beam Epitaxy ............................................................................................. 190 5.1
Iodine Doping of CdTe .....................................................................................................
5.2
Iodine Doping of HgCdTe ................................................................................................ 198
5.3
P-Type Doping of CdTe ...................................................................................................
192
199
6.
Chemical Beam Epitaxy of Related Materials _..,.........................................................................
203
7.
Manufacturability
206
7.1 8.
Issues.. .............................................................................................................
Characterization
SUmnXXy
of CBE Grown HgCdTe .......................................................................
207
..... .. ........................................................................ .... ... ............... ............ ............ 210
1. INTRODUCTION
Since its inception by Faurie and Million in 1981’ the molecular beam epitaxy (MBE) of HgCdTe has not only advanced the knowledge
of the growth processes
of this material system, but has also, by the
growth of special structures, led to an increased understanding
of the physical properties of this system and
opened up new concepts for advanced detector designs. The basic issues of epitaxiaf growth are to produce defect free bulk material and interfaces, and to achieve stable and reproducible
n- and p-type doping characteristics.
structures is critically dependent
In fact, the realization
on obtaining material structures with these properties
perform in-situ n- and p-type doping.
of new device
and on the ability to
A review of the material properties of HgCdTe shows that the most
important issues in the MBE growth of HgCdTe and related alloys are the inability to control ahoy composition and stoichiometry
precisely
and to reproducibly
dope these alloys n- and p-type.
These areas presently
represent a basic technological block to the development of high performance, low cost infrared sensors. To address these problems a new growth technique for HgCdTe, chemical beam epitaxy (CBE), has been developed demonstrated
by Summers
the ability to produce high quality heterointerfaces
avalanche photodiodes, modification
First applied to III-V systems by Tsang,” et a1.2,3,4,5,6~7,8.9.10,‘1
and optical and electrical devices such as
distributed Bragg reflector lasers and heterostructure
of MBE where the solid evaporation
CBE has
bipolar transistors.‘3
CBE is a
sources are replaced by hydride or organometallic
gas
sources. These gas sources allow more flexibility in the choice of molecular species to be applied to the growth surface, depending on which is found to best optimize the nucleation and growth processes.
For example, it has
been shown that the use of dimer arsenic (elemental arsenic normally evaporates as a tetramer) in the MBE growth of GaAs yields smoother
surface morphologies,
better optical and electrical
carbon incorporation than growth utilizing tetramer arsenic.‘4.‘5
properties,
and lower
Chemical Beam Epiiaxy
We review here the initial development
this system, alloys have been grown with electron concentrations cm*V’i’.
163
and application of CBE to the growth of HgCdTe. Using down to 3 x lOI4 cmm3and mobilities up to lo6
New doping schemes, such as the iodine n-type doping of these alloys, which has the advantage of
being insensitive
to the stoichiometry
of the growth conditions,
using amine has also been investigated
have also been investigated.
and higher incorporation
efficiencies
elemental arsenic, but complete electrical activation has been difficult to realize. growth and manufacturability
obtained
P-type doping
than when using
Recent work on superlattice
issues for HgCdTe are also discussed.
2. RATIONALE
FOR CHEMICAL
BEAM EPITAXY
The MBE of Hg-based compounds has been active for just over a decade and has already equaled more established
growth techniques in several areasI
However, there is a fundamental
limit to conventional
MBE because it relies on the evaporation of elements and, thus, little control is possible over the atomic species applied to the growth surface.
The evaporation of the group VI constituent elements and the group V p-type
dopants produce dimer and tetramer molecules, respectively, which are difficult to effectively incorporate at the growth surface. Therefore,
conventional
MBE is unable to produce a monomer
Te species.
Variation of the
chemical growth species is an important aid in controlling surface growth processes, and can have a significant influence
on material properties.
Although
high temperature
cracking furnaces have been used to crack
tetramer molecules into dimers in BI-V MBE, the thermal cracking of dimer Te into monomer Te requires very high temperatures
(3000 K).”
These high temperatures cause deleterious outgassing and significant radiative
heating of the substrate surface from the high temperature cracker and, thus, are not feasible. deposition
The laser-assisted
technique has been used by Cheung to generate monomer Te, but it is cumbersome
significant modification
and requires
of the growth system.”
Difficulties have also been reported in doping during MBE growth.
Although lithium and silver
p-type dopants and gallium and indium n-type dopants evaporate to produce monomers and have successfully produced doped Hgt.,Cd,Te layers by MBE, the diffusion rates of these dopants on the cation sublattice have prevented precise dopant profiles from being obtained.‘9*20*2’ Practical limitations
of HgCdTe MBE also arise because the high vapor pressures of the host
elements require low furnace temperatures which make temperature control difficult and setpoint changes slow due to the small heat loss. The problem is compounded temperatures,
for Cd and Te since they are solids at their operating
which causes poor thermal contact between the charge and furnace crucible and aggravates
accurate flux control over long time periods. obtain precise alloy compositions
Since HgCdTe growth requires tight control over flux ratios to
and good stoichiometry,
both of which strongly affect fundamental electrical
and optical properties,
these problems
operating temperatures
also increase the difficulty in removing impurities such as oxygen and halogens by
source degassing.
are more significant
than for other MBE grown material.
The low
164
C. J. Summers et al.
2.1
Nucleation Kinetics From this brief analysis, the major limitations of the application of MBE to HgCdTe growth are
the lack of control over the chemical growth species and the difficulties reproducibility.
associated with flux regulation and
CBE,‘* therefore, should offer significant advantages over current growth technologies
VI compounds
as there is considerable
HgCdTe growth.
evidence that modification
for II-
of nucleation kinetics can be beneficial in
Cheung has reduced the kinetic hindrance to the growth of CdTe by MBE by growing with
monomer tellurium produced by high temperature laser evaporationz3
Wu et al. have obtained arsenic p-type
doping of HgCdTe only through the use of monomer Te produced by thermal cracking of dimer Te.*’ In principle, either conventional
the control possible with CBE is significantly
or laser-MBE
because it provides
reactions and nucleation mechanisms and dopant monomer,
incorporation
a direct method to manipulate
which control chemical composition,
(substitutional
or interstitial).
more easily incorporated cracking techniques
By cracking
hydride
compounds
sources,
The actual choice depends on
Monomer sources could also result in dopants being
(< 200°C) growth of Hgi.,Cd,Te
of the
are very important
gas sources were used for the Te growth species since typical organometallic
such as diethyltelluride
and diisopropyltelluride
designed cracking gas injector should decompose formation.
or metalorganic
into the host lattice. Thus, the selection of source gases and the development
required for the low temperature
Metalorganic
the surface chemical
crystalline quality, defect chemistry,
dimer or tetramer species can be supplied to the growth surface.
which species is found to optimize the nucleation process.
issues.
greater than can be obtained with
The incorporation
Te
have a single Te in each molecule, and a properly
the Te compound yielding monomer Te without Te dimer
of dopant species is also expected to be enhanced by the use of monomer dopant
species. However, very high temperatures are needed to crack elemental As and Sb, for example, from tetramer to dimer and monomer forms.
Thus, the use of amine has been investigated.
for n-type doping on the Te-sublattice,
also are dimers in the vapor form.
Cl, I and Br, which can be used An investigation
of these elements
shows that iodine best satisfies all of the physical and chemical properties required of an n-type dopant in the HgCdTe alloy system, and is very compatible doping in the Te-sublattice
with low temperature
epitaxial growth.
preserves the advantages of low temperature cation-rich
growth which has been
shown to produce planar surfaces and also is expected to promote dopant incorporation Low temperature conditions,
growth also enhances the dopant sticking coefficient
should suppress the formation of compensating
For example, iodine
on the Te-sublattice.
and along with group II rich growth
Vcd, Tei, and [Vcd-Donor] acceptor defects.
As
iodine substitutes on the more stable Te-sublattice, its diffusion coefficient is over an order of magnitude lower than that of indium, which incorporates minimal
diffusion
on the group II sublattice.25*26 Advanced
for abrupt dopant profiles.
Because
of these advantages,
device structures require
a study was made of the
effectiveness
of iodine doping in CdTe and HgCdTe by Rajavel and Summers, and Benz et al.27’28
2.2
Growth Technolow Issues The use of gas sources whose flow rates can be rapidly, accurately, and reproducibly
with precision
flow controllers
makes CBE an attractive alternative to conventional
MBE.
controlled
Accurate flow
165
Chemical Beam Epitaxy
control is particularly important for the growth of homogeneous Hgl.,Cd,Te.
layers of the long-wavelength
Calculations by Summers et al. predicted a ten-fold improvement
compositions
in cutoff wavelength
of
variation
for CBE compared to MBE grown layers? As discussed determines
the compositional
in detail by Wagner et al.29 the stability of the host element accuracy. To quantitatively
fluxes directly
estimate the full potential of CBE to control alloy
uniformity in the growth direction Wagner et al. have calculated the variation in cut-off wavelength,
a,
as a
function of the cut-off wavelength, k,, and growth rate, and compared it to MBE using the procedure discussed by Farrow and Reno.30’3’ To a first approximation, incident Cd to Tea or Te flux ratio. stability, x-value compositional
in the MBE of HgCdTe the x-value is determined
Thus, the variation in x-value, Ax, is directly dependent
and the absolute temperature
accuracy is also dependent
of the furnaces.
The latter dependence
by the
on the flux
implies
that the
on the growth rate, which for MBE of HgCdTe is principally
controlled by the Te flux. 1.5
a
CUTOFF Figure
1.
AT
77
K
@m)
Variation in 77 K cut-off wavelength as a function of cut-off wavelength for HgCdTe grown by MBE and CBE at various growth rates.
As previously elemental
WAVELENGTH
shown, temperature
Cd and T~Q sources
control of better than f 0.2”C is required
in order to approach
the required
uniformity
in x-value
for both the and cut-off
wavelength.30S3’ This degree of control is extremely difficult to achieve over the long (- 4 - 10 hr) run times needed to grow uniform 10 pm thick layers for long wavelength
(> 10 pm) applications
and is difficult to
maintain over long production cycles. For example, a temperature
stability of 0.1 - 0.2”C in the thermal Hg
166
C. J. Summers et a/.
source developed by Cook et al. required very extensive considerations
and careful design.”
Figure 1 shows
the variation in & versus h, calculated for typical MBE source temperature variations of AT = 5 0.2”C. shown, &
As
increases rapidly with increasing h, and decreases with increasing growth rate. For detectors grown
by MBE with 5, 10 and 15 Frn cut-off wavelengths the value of A& decreases from 0.12 to 0.09, from 0.34 to 0.28 and from 0.7 to 0.58 l.trn, respectively,
as the growth rate increases from 0.5 to 5 @hr.
faster growth rates can improve the compositional
Thus, for MBE
uniformity in the growth direction.
Calculations were also performed for the expected variation in Lw, for CBE growth for a stability specification
of pressure based flow controllers of approximately 0.05% of full scale. It should be noted that the
flow rate has a small temperature dependence which has less than a 0.01% effect on the variation of k,. Figure 1 also shows the dependence
of Ah, on h, for CBE growth, and demonstrates
accurate than that calculated for the best possible MBE growth conditions. rate dependence
that it is from 2 to 10 times more Additionally, CBE has little growth
because it is possible to recalibrate the flow controllers for a maximum flow corresponding
each predetermined
to
growth rate. The improved stability for CBE is particularly important for long wavelength
materials (8 - 20 pm) in which a 3-fold improvement
in homogeneity is predicted.
This increases to a factor of
10 for - 0.5 /.tm/br growth rates and, thus, is particularly useful for the accurate growth of graded heterojunction structures which are very important for high-performance Of equal importance
imaging systems.
for advanced device structures is the precision of extrinsic doping levels.
Therefore, a comparison was also made of the stability in doping obtainable by MBE and CBE. Assuming that the dopant incorporation
rate is insensitive to variations in the Group II fluxes and is directly proportional
to
dopant flux, the obtainable doping precision is affected by the stability of the dopant flux and the growth rate which, in the case of MBE HgCdTe, is controlled by the Te flux. Figure 2 shows the percentage variation in doping as a function of doping level for MBE using thermal dopant sources (1 pm/hr growth rate and AT = f 0.2”C) and for CBE using a dopant pressure-controlled calculations
vapor source (PCVS) discussed
indicate that CBE is capable of a factor of ten improvement
dopant-PCVS doping levels.
was determined
experimentally
over MBE.
at a particular set of flow conditions
in Section 3. The The stability of the
and extrapolated
The three sets of CBE curves correspond to thme different orifice conductances
used by the dopant-PCVS
to other
that must be
to cover the full range of doping levels.
In addition, for CBE long term stability is improved because there are no source depletion effects and the growth chamber does not need to be opened and baked when recharging the sources.
Thus, this growth
technique is extremely well suited to the reproducible growth of HgCdTe alloys and novel device structures. CBE also allows pm-mixing
of the constituent gases, thereby minimizing the number of gas injectors required
and achieving enhanced compositional This technique, therefore, by other HgCdTe growth technologies graded structures, precise stoichiometric
uniformity.33 has the potential to solve many of the problems currently experienced in obtaining abrupt heterojunctions,
smooth surfaces, compositionally
adjustment, and complex n- and p-type extrinsic doping profiles, all of
which are essential for the fabrication of advanced focal plane array structures.
167
Chemical Beam Epitaxy
2.0 MBE MCT:As
1.5 =: MBE MCTdn
[IO n’ = -6 0.5 MOMBE MCT smaller orifice \
0.0
Figure 2.
\
MOMBE MCT larger orifice
10
Variation of doping precision as a function of doping concentration for gas source doping for MBE and CBE growth of HgCdTe.
3. SYSTEM DEXXIPTION
A schematic of the system developed for CBE HgCdTe growth is shown in Figure 3. The CBE equipment was retrofitted to a Varian GEN II machine and thus differs significantly from the MBE system by the addition of gas handling systems for both host (Hg, Cd, Te) and doping gases, specially designed
LIQUID NIKCGEN
2”
~CRYOfwMP -NRBOMOlICUIAR
Figure 3.
Schematic of chemical beam epitaxy system developed for HgCdTe growth.
gas
166
C. J. Summers et al.
b)
Figure 4.
Schematic of gas handling systems developed for CBE a) host sources, b) dopant sources
injectors for cracking the source gases, a Hg pressute- controlled vapor source for control of the Hg flux, and a special pumping system to handle the large gas loads generated during CBE. coefficient of the Hg, very high pressures are present during MBE growth.
Because of the low sticking
Therefore, the CBE approach must
both minimize excess gas loads and m aximize pumping speed. Special gas injectors and pumps were therefore developed to minimize the gas load and to provide high pumping speeds. These details are briefly summarized in the next three sections.
Gas Handling Svstem
3.1
The gas handling systems specially designed and built for this system am shown in Figure 4. To reduce the gas load and achieve precise flow control, flow controllers based on pressure control were used rather than the conventional minimum
inlet pressures
metalorganics,
Thermal mass flow controllers
typically require
Because of the low vapor pressures
of most
it is necessary to entrain them in a carrier gas by bubbling to achieve sufficient pressure for use
with conventional temperature
thermal mass flow controllers. of 50 torr for proper operation.
mass flow controllers.
As a consequence the metalorganic flow is determined by the bubbler
whose stability limits tire precision of the flow controller.
The use of a carrier gas also further
increases the gas load and makes molecular flow conditions more difficult to maintain in the growth chamber. fn contrast, pressure-based
flow controllers require inlet pressures around 1 - 2 torr and hence can
be operated without any carrier gas. For the host materials the MKS Instruments Model 115OB, which operates
169
Chemical Beam Epitaxy
on the principle of choked viscous flow across an orifice, was chosen. The important properties of choked flow are that the flow rate is directly proportional downstream
to the pressure upstream of the orifice, is independent
pressure, and has a square. root dependence
1150 uses a highly accurate capacitance
on the absolute gas temperature.
of the
The MKS Model
manometer
which provides feedback control to a solenoid valve to
maintain a constant pressure upstream of an orifice.
These flow controllers can be provided with a full scale
range of 0.8 - 10 seem to cover growth rates between 1 - 10 pm/hr.
Following the flow controllers the gases are
fed into the gas injector through a combination isolation and pump-out valve (Figure 4a). A revolutionary implemented
development
in pressure
controlled
to address the challenge of doping by CBE.
provides control over approximately
vapor
delivery
systems
was
also
This was necessary because the MKS 1150 only
two orders of magnitude whereas the fluxes required for doping must be
varied over four orders of magnitude at a flux level that is four to six orders of magnitude less than that of the constituent
gases.
In addition, the dopant flow controller must be flexible in its maximum flow rate because
quantities such as dopant incorporation
coefficient and cracking efficiency were unknown at the time of system
design. Therefore, as shown in Figure 4b, a flow controller was built based on feedback control of the pressure upstream of a variable orifice, similar to the MKS 1150. The large range of flows was accommodated accuracy MKS capacitance
manometer.
The capacitance
manometer
feedback controlled
by a high
the pressure via a
stepper motor driven Granville Phillips leak valve. In place of a fixed orifice, the dopant source used a manual leak valve as a variable orifice, which allowed the maximum flow rate to be easily varied.” controller the dopant gas was fed into the gas injectors through a combination
From the flow
isolation/pump-out
valve.
dopant gas handling system thus operated on the same principle as the Te and Cd flow controllers
The
but had a
significantly larger dynamic range. Because of the large volume of toxic gases a specially designed pumping system was developed to safely handle organometallics, turbomolecular evacuates
their by-products,
The pumping system was based on a
pump (Balzers MBE series) because it is specially suited to pumping
gases from the chamber
turbomolecular
and Hg vapor.3
rather than entraining
them as in a cryopump
large gas loads and or ion pump.
The
pump was followed by a foreline isolation valve and a 50 cfm Alcatel Corrosion Series backing
pump. The exhaust gases then passed into an Emcore toxic gas scrubber filled with a sulfur impregnated activated charcoal capable of absorbing up to 40% of its weight in Hg. A Varian cryopump was also used to remove Hg vapor and metalorganic during CdTe growtb hydrocarbons.
by-products.
This combination
of pumps gave a background
pressure
of lo4 to 10m5torr, with the majority of the residual gas being ethyl and propyl
The ion pump was isolated during CBE growth to prevent failure from continuous pumping of
hydrocarbons. 3.2
Gas Iniector De&n To achieve the full advantages of CBE special gas injectors were designed and constructed
efficiently
decompose
the source gases, so that variations in the surface chemistry
generation of monomer and dimer species.
to
could be studied by the
170
C. J. Summers et al
Atmosphere
Vacuum
a) Schematic of (a) CBE gas injector and (b) cracker diffuser.
Figllre 5.
Special decomposition
consideration
temperature
and source cryoshroud.
was given to the gas injector
design
in order to obtain
and to minimize outgassing, thermal heating, and contamination
the lowest
of the substrate
A convoluted flow path was designed to maximize wall collisions to achieve efficient
decomposition
while maintaining
a large enough conductance
recombination
in the gas phase.
Construction
to avoid the high pressures
was of high purity, high temperature
reactive with either the source gas or any of the decomposition to ensure stable and reproducible cracking characteristics
by-products.
that could cause
materials that were not
Catalytic elements were not used
with time.
A schematic of a gas injector is shown in Figure 5a. Each consists of a 0.75 in diameter delivery tube heated by a standard MBE furnace heater. The Cd source tube was made from tantalum and the Te source tube from pyrolytic boron nitride (pBN) because of the reaction between Ta and Te or Hz at high temperatures. The diffuser design is also shown in Figure 5b. The gas flows through the holes in the diffuser, into the diffuser cone and then onto the substrate.
The small holes at right angles to the injector axis were
designed to ensure many wall collisions for efficient decomposition. of the holes was equivalent to the cross-sectional
To minimize flow resistance the total area
area of the central 3/8 in diameter tube. This central shaft has
an 8” taper which was calculated to give a rt 2% flux uniformity over a 2 in substrate. machined
from solid hot-pressed
binder
The diffusers were
free boron nitride (BN) of purity comparable
to pBN.
The
performance of the gas injectors is discussed in Section 3.4, with special emphasis on the cracking of both host and dopant species.
3.3
Mercury Pressure Controlled Vaoor Source In conventional
MBE Hg sources the flux is controlled by very tight temperature regulation of a
liquid Hg reservoir.32V35 However, due to their low operating temperature mass(-
1 kg) they have poor time response and control characteristics.
(100 - lfiO”C), and large thermal
A vapor source was therefore developed
by Wagner et al. to allow gas-like control of the Hg flux in order to be compatible with the CBE process.36 The
171
Chemical Beam Epitaxy
Hg-PCVS shown in Figure 6 operates on the same principle as the pressure based flow controllers in the CBE system. Mercury vapor was supplied by heating the reservoir to 145°C while the rest of the system was kept above 160°C to prevent Hg condensation.
The Hg pressure upstream of the control orifice was held at the set
point by feedback control of the control valve via the capacitance
manometer
signal.
The Hg flux
flows
through the isolation valve and heated nozzle and onto the substrate. Because the pressure is controlled directly by the control valve, the temperature
control requirements
of this system are much less stringent than the
thermally controlled Hg evaporation sources where there is an exponential dependence
of the Hg pressure (i.e.
flux) on temperature.
In fact, it has been shown that the Hg-PCVS is able to maintain a constant flux for Hg
reservoir changes off
10°C. This design can supply fluxes of up to 2 x 10e3torr and also allows for fast time
response
(10 s) and excellent
flux stability and setpoint repeatability
stability enables the Hg flux to be optimized to minimize stoichiometric
Figure 6.
(0.1%).
The fast response
and flux
doping effects, and defect generation,37
Schematic of Hg-Pressure Controlled Vapor Source.
and also to be changed as required to maintain optimum growth conditions due to changes in surface kink and step densities during the growth of HgCdTe.3*
3.4
Metahmmk
Preausor
Diethylcadmium isopropyltellurium
Cracking Studies
(DeCd),
dimethylcadmium
(DmCd),
diethylzinc
(DeZn)
(DipTe) precursors were used based on the fact that for these metalorganics
and
di-
the associated
radical was single bonded to a Cd, Zn or Te atom and, thus, on pyrolysis should produce monomer species. Also, previously reported work on these and similar precursors was used to define the selection. growth of GaAs, for example, layers grown using triethylgallium grown with trimethylgallium.39
showed less carbon incorporation
In the CBE that those
This was attributed to the stronger chemical bond between gallium and the
carbon complex in trimethylgallium
as opposed to triethylgallium.
The carbon-metal
bond strength of DeCd
172
C. J. Summers et al.
(26.5 kcal/mole)
is less than that of DmCd (33.5 kcal/mole) and therefore easier to pyrolyze.
Additionally,
because the ethyl complex is larger than the methyl complex, it was expected that it would produce less carbon incorporation.
However, later studies showed that DeCd was unstable and, thus, this and other higher order Cd-
alkyl complexes
which are expected to crack at lower temperatures
with lower carbon incorporation
are not
expected to be suitable for CBE.40 Similar arguments were used for the selection of DipTe which was the largest hydrocarbon-Te
complex available in high purity. Secondary ion mass spectrometry
of CdTe layers grown using these gas precursors showed that the carbon contamination
(SIMS) evaluation
was no greater than the
SIMS background level of 3 x 1016cm‘3. Quadtupole mass spectrometry sources used for CBE.
(QMS) was used to study the pyrolysis of the organometallic
gas
Ideally, the thermal pyrolysis should yield only monomer and organic by-products
both low and high injector pressures.
To optimize
the monomer
yield for Te, gas phase collisions
at
were
minimized by operating the gas injector under molecular flow conditions where the molecular mean free path is longer than the injector tube diameter.
For a 1 pm/hr CdTe growth rate, which required a DipTe flow of 0.4
seem, the maximum injector pressure was estimated to be 50 mtorr from the specifications flow controller. maximum
For these conditions,
the DipTe mean free path was calculated
hole diameter in the cracking cell and diffuser of the injector.
of the MKS 1150
to be comparable
The low probability
to the
of Te dimer
formation was confirmed in a study of the pyrolysis of DipTe for injector temperatures between 300 - 1000°C. Figure 7 shows the ion current as a function of injector temperature for DipTe, isopropyltelhnium
(i-FTe), Te,
Te2, and propyl radicals for a DipTe flow rate of 0.24 seem which was routinely used for the growth of CdTe epitaxial layers.
IO0
10200
300
400
500
600
700
600
900
1000
Group VI Injector Temperature (33) Fignre 7.
Dependence of QMS ion current for DipTe and decomposition by-products on injector temperature.
173
Chemical Beam Epitaxy
The onset of DipTe pyrolysis occured at an injector temperature of 400°C and was complete by 750°C.
The DipTe ion current decreased by approximately
four orders of magnitude and for higher injector
temperatures became less than the QMS detection limit; an indication of complete decomposition. Te, and propyl radicals were detected below 400°C due to fragmentation of Tez was detected.
I-propyl Te,
in the QMS, and also a small quantity
As the injector temperature was raised above the pyrolysis temperature, both the monomer
Te and propyl radical signals increased by over an order of magnitude and saturated above 650°C confirming that monomer Te was produced by the thermal pyrolysis of DipTe.
The activation energies for DipTe and i-
PTe were 56 kcal/mole and 26 kcal/mole, respectively. The different temperature dependence for i-FTe indicated that DipTe has a two step reaction path for monomer
formation.
The activation energy for DipTe bond cleavage was estimated
by subtracting
decrease in the Te-C bond energy (- 10 kcal/mole) due to free radical electron charge delocalization4’ 66 k&/mole
activation energy for the removal of the first methyl group from DmTe.
the
from the
This estimate of 56
kcal/mole is in excellent agreement with experiment for the energy of removal of the first propyl radical from DipTe. In addition, McAllister estimated that the ratio of the activation energies for the removal of the first and second propyl groups from DipTe for the homolytic bond cleavage mechanism was 2.42 The measured ratio of 2.15 is, therefore,
also in agreement
with theory.
These measurements
confirmed
the decomposition
mechanism for DipTe to yield monomer Te. The cracking efficiency was obtained by correcting the measured ion currents for the different elemental masses and ionization ratios and then ratioing the Te and Te2 flux densities.
The TeJTez flux density
ratio was found to be 24, indicating that more than 96% of the tellurium flux was cracked into monomer Te. Evidence that n&t
of the Tez was created by polymerization
pyrolysis efficiency
of DipTe is even higher.
cracking diffuser designs. DipTe decomposes
in the QMS, strongly suggests that the thermal
These results confirm the effectiveness
of the gas injector and
The propyl radical/DipTe flux density ratio of 2.22 also confirms the expectation that
into two propyl radicals.
From similar studies the pyrolysis of DmCd was observed at 1100°C and at temperatures between 500 - 800°C for DeCd. decomposition
These conditions
produced
into cadmium and organic byproducts.
was discontinued
complete
DmCd decomposition
and partial DeCd
DeCd was used in the initial experiments,
because of room temperature decomposition
in the bubbler.
but its use
This effect resulted in a mixture
of DeCd and lighter organic gases in the bubbler, which made proper operation of the pressure-based controller impossible
and, therefore, prevented determination
of the optimum cracking conditions
flow
for DeCd.
No such problems were. observed for DmCd. The potential of developing a complete chemical beam epitaxy process by using a Hg precursor, divinylmercury
@vHg), as a mercury source has recently been investigated by Zinck and Rajave1.43 However,
in a preliminary study of the growth kinetics it was found that although DvHg was adsorbed and dissociated on a Te surface at temperatures
as low as -1 WC, the temperature dependence of adsorption was so strong that its
probability became less than unity at typical HgCcITe growth temperatures
(150 - 200°C).
Thus, growth was
C. J. Summers et al.
174
only possible by thermally precracking
dissociation
the precursor.
Exposure to atomic hydrogen was found to enhance the
of DvHg and could be an alternative to thermal precracking.
Using ptecracked
DvHg, single
crystal (001) HgTe growth at 150°C was reported. Recently, Rajavel et al. have investigated iodine precursors
to determine
the best compounds
source molecular beam epitaxy.” their use as conventional
dopants
requires
of
for the n-type doping of CdTe and HgCdTe during gas
This was necessary because the high vapor pressure of the halogens prevents
elemental sources in MBE, and thus requires the use of compound
ZnC12, or alkyl-halide compounds source
the selection criteria and pyrolysis characteristics
as gas dopant precursors.
information
on their chemical
However, assessment properties,
pyrolysis
sources such as
of the effectiveness and doping
of gas
characteristics,
particularly for effective doping at low growth temperatures. The main experimental
issue is that low vapor pressure precursors increase the response time of a
PCVS, because it takes longer to pump out the injector vacuum feed line to attain a steady state flux. Consequently,
for effective pyrolysis and the stable operation of pressure controlled vapor sources, it was found
necessary to utilize source materials with a low bond energy and a high vapor pressure.
Although the bond
strength of 12(36 kcal/mole) is sufficiently low, the vapor pressure (0.3 torr at 20°C) is too low for a PCVS and too high for MBE. HI, a natural first choice
was not used as the expected
by-product,
hydrogen,
could be
incorporated into the growth surface. Also, the high bond energy of HI (71 kcal/mole), requires temperatures in excess of 1200°C for pyrolysis.
Ethyliodide, and to a greater extent allyliodide, have weaker bonds and hence
dissociate at lower temperatures.
The vapor pressures (- 50 torr at room temperature) of these compounds were
also experimentally
determined
to be high enough for the stable operation of a PCVS flow controller at room
temperature.
Butyliodide,
iodononane
and iododecane
have lower bond energies, but their vapor pressures
progressively
decrease with the length of the hydrocarbon chain.
For the experimental
geometry of the leak
valve delivery system described earlier it tequired several minutes to obtain a stable flux. The practical need to have a vapor pressure 150 torr (at room temperature) reduced the effectiveness as iodononane ethyliodide criteria.
and iododecane
as dopant sources for gas source applications.
of long chain compounds
such
From these empirical studies
and allyliodide were selected as suitable iodine precursors as their properties best fitted the above
To identify the products created by the injector, Rzjavel et al. performed mass scans for cracker cell
temperatures between 150 and 750°C. For ethyliodide, only the QMS signals for CaHs+, CpHsI+ and r’ showed a strong dependence
on injector temperature and between 400 and 650°C the ion current of the parent molecule
decreased
by over two orders of magnitude,
amount.
This indicated
Normalization
that ethyliodide
while the 12 ion current increased by approximately
was pyrolized
into dimer iodine and hydrocarbon
the same
by-products.
of the data showed that the onset of pyrolysis of ethyliodide occurred at 400°C and was 99.6%
complete at 650°C to produce dimer iodine molecules, as shown in Figure 8. A similar study for allyliodide showed that the onset of pyrolysis occurred at - 3OO“C and at 600°C 99.6% of the allyliodide molecules were decomposed
into dimer iodine and hydrocarbon by-products.
175
Chemical Beam Epitaxy
For a growth rate of 0.5 lunihr, the BEP of the host elements is approximately Assuming
100% incorporation,
a BEP of 1 x 10’) torr produces a dopant concentration
of - 10” cm”, and has
to be decreased by four orders of magnitude to obtain doping levels down to 1 x 10” cm”. of the dependence
of pyrolysis on the precursor flow rate, was therefore investigated
1 x 10e6 torr.
An understanding
for the range of values
currently possible. For this study the BEP of ethyliodide was varied over three orders of magnitude from 8 x 10m9to 2 x 10m5torr for an injector temperatme of 700°C. At a BEP of 8 x 10m9totr, the pyrolysis efficiency was 90% and increased rapidly with increasing dependence
flow rate to > 99.1% for a BEP of 2 x 10m5torr.
of the pyrolysis efficiency on flow rate, the dopant incorporation
Due to the non-linear
in the host lattice is also expected
to vary non-linearly with the precursor flow rate in the precracked mode. From this study the pyrolysis of ethyliodide and allyliodide was found to produce iodine dimers and the cracking efficiency
was found to be dependent
formation is not understood
but two possibilities
on the precursor flow rate.
are suggested.
The mason for dimer
As each parent molecule contains only one
iodine atom, 12 can be produced when two parent molecules combine to form a complex during pyrolysis, or when iodine monomers intermolecular pressure.
created after the dissociation
of the patent molecule recombine
to form dimers.
As
or interatomic collisions are required for the creation of 12,both processes are very dependent on
However, further studies were difficult because dimer iodine was dissociated to monomer iodine by
electron impact in the QMS. Thus, it was not possible to follow the complete chain of chemical reactions and the mechanism of the pyrolysis process as the dependence of monomer yield on precursor flow rate and cracker cell temperature could only be studied at low QMS ionizer electron energies which were not available. Thus, iodine has desirable attributes for the n-type doping of HgCdTe alloys. allyliodide
were found to have the most suitable properties
and to produce
Ethyliodide
iodine dimers
100 lo-'
90
80% g104
70
E
SG?
5
vi
10”
402
0
E 1o-‘O
“!j *O a 10
1cl-”
I
100
ZOO
’
t’
I
300
400
’
I
500
’
I
600
’
I
700
Injector Temperature (“C) -8.
g
QMS ion current for ethyliodide decomposition.
v
I
So0
0
’
Wa
and
for pyrolysis
176
C. J.
temperatures
Summers et al.
above 650°C and 600°C. respectively.
As described later, the ethyliodide
doping of CdTe and
HgCdTe has produced highly conductive layers with high mobilities and electron concentrations
as high as 5 x
1018cmm3. For the p-type doping studies amine was pyrolyzed in a heated pyrolitic boron nitride injector up to 1300”C.45 The QMS indicated that AsHs pyrolysis above 1200°C
capable of operating at temperatures
generated Asz. As4 and Hz. with the major species being dimer As at a ratio of 10: 1 As2 to As4. To control the flow of the high-pressure
amine gas, the pressure controlled
vapor source system described
previously
was
modified to supply very small flows of AsH3 from a high pressure source. Using this technique, a minimum BEP of 1 x 1O-1otorr was repeatedly achieved with the amine source.
In situ Characterization
3.5
of System
To facilitate the growth of multiple quantum well and superlattice
structures and to integrate
atomic layer epitaxy and migration enhanced epitaxy techniques with the conventional capability, the system was fully automated. chamber
and loadlock,
substrate
This involved controlling
heaters, temperature
measurement
temperatures,
opening and closing of cell shutters, and acquisition
spectrometer
and video RHEED
throughput and manufacturability
system.
Automation
chemical beam epitaxy
the following and control,
components:
growth
flow rates, furnace cell
of data from the QMS, Auger electron
of the growth process
also greatly increases
the
as described in Section 7.
The RHBED system was upgraded by the addition of a video camera, and a computer equipped with a digitizing board and software for RHEED analysis. The electron gun was also mounted on a bellows to allow the incident electron beam to be scanned from 0.05” to 3.5” by a combination of mechanical motion and electrostatic
deflection.
The diffraction
patterns
were analyzed by a RHEED
Data Acquisition
System
developed by Epitaxial Growth System and consisting of a DigiSector board with video monitor to display the diffraction
pattern and two software programs for analyzing intensity oscillations
Fourier transforms
and beam profiles.
Fast
of the intensity oscillation data were used to obtain accurate and real-time growth rates.
This system was also used to record the entire RHEED pattern. The RHEED system was found to be very effective in evaluating the performance
of the system.
The pyrolysis of DipTe and DmCd was studied by RHEED intensity oscillation measurements growth rate as a function of the injector temperature.
The combination
of the CdTe
of CBE and EWEED analysis has also
been applied to evaluate the preparation of substrates with optimum, low index atomically smooth The
system characterization
surfaces.
runs were used to precisely establish the necessary operating parameters
MBE system for the growth runs and the surface kinetics experiments.
For these calibration
for the
experiments
RHEED intensity oscillations on the CdTe (001) and (11 l)Te surfaces were used. An example of an intensity oscillation is shown in Figure 9, using monomer Te and Cd at 290°C under excess Cd conditions. the growth rate, the Fourier transformation spectrum corresponds
was taken, where the maximum
to the growth rate as shown in the inset in Figure 9.
amplitude
To calculate
of the frequency
177
Chemical Beam Epitaxy
3000
2000
1000
0
10
40
30
20
::c60
::::
:::::
:::::
:::::
o!:::
Time (seconds) Flgun? 9.
Representative RHEED intensity oscillation for CBE growth of CdTe. The inset shows the Fourier transform of the intensity oscillations.
Wagner et al. have also recently shown bow RHEED studies can be used to compare the ability of the MBE and CJ3E techniques
to obtain accurate and reproducible
alloy compositions.29
comparison of these growth methods, CdTe was first grown by MBE from a stoicbiometric CBE. In these studies it was observed tbat intensity oscillations measurements using a CdTe furnace nonuniformities
had an intensity
across the substrate.46
envelope
that exhibited
In a dynamic
source and then by
performed during ME3E gmwtb
fnzquency beating,
Figure 10 shows the intensity oscillations
as a result of flux
during solid source CdTe
7000 3 .z 3
8000
Je
6000
21 .%I
4000
5 E
3000 2000 0
lo
20
30 lime
F&ure
10.
40
60
60
(seconds)
Beats in the intensity oscillation envelope during solid source MBE gmwth. The data at later times is magnified by 2.7 X as shown in the inset.
178
C. J. Summers et al.
growth from a single CdTe furnace.
This behavior was not observed during CBE growth and was attributed to
the superior growth uniformity available with this technique. Figure 11 shows the frequency spectrum obtained for CdTe growth in which a stable monomer Te gas source was used to compare the fluctuations in growth rate resulting from a gas source of Cd and a solid source of Cd. As shown the Fourier transform spectra obtained using a solid Cd source is broad and tails to higher frequency values.
This was attributed to the cooling of the Cd furnace after the Cd shutter was opened
and the decrease of the Cd flux and CdTe growth rate to a steady state value.
For gas source growth the
frequency peak was narrower and showed no tailing, which proved that flux transients are not operative during CBE. Accurate growth rate measurements DmCd flow controllers. Te-limited conditions incorporation
were also found to be necessary to calibrate precisely the DipTe and
Figure 12 shows the CdTe (001) growth rate at 280°C under Cd-saturated,
as a function of the DipTe flow controller setpoint.
monomer
These conditions ensured complete
of monomer Te, so that the growth rate accurately represented the DipTe flow rate. The DipTe
flow controller exhibited a nonlinear flow vs. setpoint relationship which was fitted by a 3rd order polynomial. Similar behavior was reported for the DmCd flow controller. RHEED
intensity
oscillations
were
also used to study organometallic
decomposition
measuring the CdTe growth rate under saturated conditions as a function of gas injector temperatures. possible
because the surface catalytic decomposition
temperatures
so that the growth rate corresponded
injector into Te or Cd, respectively.
This was
at low growth
to the amount of DipTe or DmCd decomposed
by the
temperature dependence
which was defined as the point of maximum
growth rate.
and An
f::::::.:,::::::::::::r
O
0.6
-..-..
Gas Source Cd and Te
-
b&E Cd Source + Gas Source Te
1.0 Frequency
Figurell.
was negligible
These experiments yielded the decomposition
the optimum injector operating temperature,
4000
of the organometallic
by
1.6
I2.0
(Hz)
RHFiED oscillations observed for CdTe (001) growth with monomer Te and gas source Cd or solid source Cd illustrating solid source flux transients.
179
Chemical Beam Epitaxy
0
0
0.2
0.1
DipTe
Figure 12.
0.6
0.4
0.3
(seem)
Setpoint
CdTe (001) growth rate versus DipTe flow controller setpoint in the Cd-saturated growth W$ne.
example of these studies is shown in Figum 13 for the (001) CdTe growth rate as a function of the group VI injector temperature
at a set DipTe flux.
The substrate temperature
was 280°C and sufftcient
Cd flux was
supplied to saturate the growth rate. As shown, with increasing temperature the growth rate increased rapidly with an activation energy of 23 kcal/mole, in excellent agreement with the QMS cracking studies for i-PTe decomposition.
Therefore, these results also supported the two step bond cleavage decomposition
described earlier.
The growth rate decreased
6OQOC
i%i+ Gi ki > m 5
l
l
0
mechanism
for injector temperatures
above 650°C indicating this was an
7otyc
6OQ”C
l
6OQ%
0
.
0
.
0.6 --
s .E. Q, ;;; u 5 0.2 --
Activation
Energy
- 23 kcallmole
2
0.0006
Reciprocal
Figure 13.
0.0010 Temperature
0.0012
of Group VI Injector
0.1
(l/K)
CdTe growth rate versus the reciprocal of the DipTe injector temperature for Cd-saturated growth.
160
C. J. Summers et al.
optimum operating point. The change in growth rate was attributed to Te deposition in the injector. A similar study was performed
for DmCd, where the optimum operating temperature
for the
DmCd injector was determined to be 1120°C. For higher temperatures the growth rate decreased slowly from increased Cd deposition
in the injector, as verified by inspection
after removal from the CBE system.
The
growth rate increased rapidly up to 1120°C and exhibited two activation energies, 33 kcal/mole below 880°C and 14 kcal/mole mechanism
above 880°C.
This behavior was attributed to a two step bond cleavage decomposition
analogous to the DipTe decomposition
results in which the lower temperature activation energy of
33 kcal/mole corresponds
to removal of the first methyl radical and the high temperature activation energy of
14 kcal/mole corresponds
to removal of the second methyl radical.
Surface Nucleation Kinetics
3.6
An understanding necessary for the production
of the fundamental
mechanisms
involved
of material for advanced infrared applications.
in HgCdTe epitaxial With knowledge
growth is
of the growth
processes, the variation in material properties with growth conditions can be predicted and hence optimized. addition, advanced device concepts, such as superlattices, which rely on high quality heterojunction will benefit stoichiometry. allows
from increased
knowledge
of the growth
kinetics
as applied
Studies of surface growth kinetics therefore strongly complement
to interface
In
interfaces,
abruptness
and
CBE because of the choice it
over the chemical growth species, along with its ability for rapid, accurate and reproducible
control of
the growth fluxes. Studies of the surface growth kinetics that control the growth of CdTe and HgTe have been made by measuring
the growth rate as a function of the substrate temperature
and flux ratio for the (OOl),
(11 l)B, and (21 l)B orientations. Figure 14 shows the dependence of the (001) CdTe growth rate on Te flux for a fmed Cd flux for substrate temperatures
between 200 - 320°C. Also included in the figure are the growth conditions,
defined by
the ratio of tire growth fluxes, and the growth regimes, which am defined by the derivative of the growth rate with respect to monomer
Te flux.
As shown, initially the growth rate increases
monomer Te flux but at large flux values becomes independent the Cd overpressure
linearly with increasing
of the Te flux. Investigations
showed that the growth rate was independent
of the Cd flux at low Te fluxes and, thus,
these growth conditions defined a Te-limited growth regime, where the temperature dependence rate in this regime is representative
of the effect of
of the growth
of the Te species surface kinetics.
However, at high Te fluxes the growth rate varied linearly with the Cd flux. These conditions, therefore,
detine a Cd-limited growth regime, where the temperatum dependent growth rate is representative
the Cd surface kinetics.
of
The Cd -limited growth rate decreased with increasing temperature for all temperatures
studied, in contrast to Te-limited growth, indicating incomplete Cd incorporation
over
this
temperature range.
The growth rate data displayed Arrhenius type behavior, with a Cd activation energy of 5 1 meV. In the Te-limited regime the growth rate was independent of temperature below 300°C indicating complete
incorporation
of all incident
Te atoms, and so investigations
were made at higher
substrate
181
Chemical Beam Epitaxy
Cd -
Te -
0.6
0.6 ML/s
1.0
Monomer
Te
1.6
Flux
(ML/s)
Figure 14.
(001) CdTe growth rate for a constant Cd flux as a function of monomer Te flux for substrate temperatures between 200 - 320°C.
temperatures.
Figure 15 shows the Te-limited
from 275 - 400°C for two different Te fluxes.
growth rate as a function of reciprocal substrate temperature The growth rate decreases between 305°C and 360°C with an
activation energy of 150 meV, and at substrate temperatures resulting
from insufficient
greater than 36O”C, shows a very rapid decrease
Cd flux to insure saturated growth conditions
and prevent
sublimation.
The
activation energy in this regime of 1.9 eV is in agreement with the CdTe sublimation energy measured by Arias
409% r.
37vc
369%
32Q“C
30(X .l.MMUS
0.43 Mua
0.0015 Reciprocal
Figure 15.
0.0016 Substrate
0.0017 Temperature
(i/K)
(001) CdTe growth rate as a function of reciprocal substrate temperature under Te-limited, saturated growth conditions for two diierent monomer Te fluxes.
182
C. J. Summers et al.
A similar study was also performed
et aL4’
monomer
Te.
for dimer Te, with the results being qualitatively
Below 300°C the growth rate was independent
molecules were incorporated. of 85 meV, approximately
of temperature
similar to
and all incident
dimer Te
Between 300°C and 350°C the growth rate decreased with an activation energy half the monomer Te activation energy.
For substrate temperatures
greater than
35O”C, the growth rate decreased rapidly due to CdTe sublimation with an activation energy of 1.9 eV. Studies for the (11 l)B surface were qualitatively identical to the (001) surface but the Cd-limited growth rate decreased with temperature with an activation energy of 123 meV, as compared to 51 meV for the (001) surface.
The difference in Cd precursor energies implies that Cd exists in a more tightly bonded site on
the (11 l)Te surface, compared to the (001) surface, which agrees with observations for Hg incorporation
Initial studies for the CBE growth of HgTe on the slightly
HgCdTe growth on the (11 l)Te surface.4a misoriented
(11 l)B surface showed strong intensity oscillations
llO”C, indicating oscillations growm4’
a smooth
for substrate temperatures
atomic growth front during (11 l)B HgTe growth.
were not observed,
presumably
during
However,
because the Te adatom mobility was sufficient
between
165 -
above 17O”C, for step flow
Figure 16 shows the HgTe growth rate at 150°C as a function of the Hg flux for a fixed monomer Te
flux. The growth rate initially increases with increasing Hg flux, but plateaus for Hg fluxes > 2.6 x lo4 torr. However, the Te-limited growth rate at 150°C was found to be less than the incident Te flux indicating that not
0.30 -
0 _________________
,A+--
0
4 #’
0.29 ,’
5:
Li
z 2
0 0.28 L1?
SubstrateTempentw=15OC
S
:
i Atomic Te Flux = 0.363 ML/s
0.27 -
d 0.28
i
1
1.2OE-4
Figure
16.
1
I
1
1 ME-4
I
I
I
1
I
1
1
1
2.OOE4 2.4OE4 Hg Flux (toIT)
1
1
1
1
2.8OE4
1
1
1
1
3.2OE-4
HgTe growth rate on the (11 l)B surface at 150°C as a function of Hg flux for a set monomer Te flux.
183
Chemical Beam Epitaxy
all of the Te was incorporated.
Similar growth rates were measured for CdTe and HgTe under Te-limited,
saturated conditions indicating negligible scattering of the Te beam by the Hg flt.~x.“~~~ RHEED oscillation studies were also performed on the (2 1 l)B CdTe surface which is a vicinal surface. composed of (11 l)B terraces 3 atomic spacings wide in the ~11 l> direction, with step edges parallel to the
A Cd-stabilized
for the Cd-stabilized
surface was observed during CdTe layer growth, which implies a bulk
surface.
However, during HgCdTe growth and Te annealing of the CdTe
surface at 300°C the diffraction pattern was characteristic (21 l)B HgCdTe growth occurs on a Te-stabilized stabilized surface.
of a singular, step free surface.
This implies that
surface, as opposed to (001) growth which occurs on the Cd-
Intensity oscillations were observed on the (21 l)B surface but no analysis of the nucleation
kinetics was reporh~L’~ The small desorption
energies
obtained
from these studies
suggests
that both atomic
molecular growth species exist in weakly bound precursor surface states before being incorporated
and
into the
lattice. This is similar to GaAs where molecular arsenic exists in a mobile, low binding energy precursor state. However, this is an unexpected result for atomic Cd and Te and suggests a possible kinetic hindrance to growth not observed during III-V MBE. The growth rate for MBE CdTe47*50shows a similar temperature dependence which confirms that the small activation energies am not unique to CBE growth, or a result of the interaction of organic species with the surface, but am intrinsic to CdTe surface processes. The different
precursor
energies
for monomer
and dimer Te show that diier
incident on the surface continue to exist on the surface as dimers, rather than dissociatively monomers.
These results also offer a possible
illumination,
which from measurements
that measured
for monomer
Te.
explanation
for the desorption
Te molecules adsorbing
as
of Te under He-Ne laser
by Benson et al.,5’ has an activation energy of 150 meV, the same as
In those experiments,
the He-Ne laser was resonant with the 1.9 eV Te
desorption energy, which implies that laser excitation breaks the surface Te dimer bonds and promotes the Te into a monomer precursor state, from which they then desorb. Despite the difference species were completely incorporated
between the dimer and monomer
Te precursor binding energies,
at saturated conditions under 300°C. In contrast, the incorporation
was not complete, despite the similar binding energies for Cd (51 mev) and dimer Te (85 mev).
both of Cd
This suggests
a kinetic limitation for Cd incorporation and shows that the Cd and Te nucleation kinetics are not simply related to the precursor state binding energy. It is possible that Cd incorporation is controlled by adatom incorporation at surface steps and not by nucleation on the terraces as has been observed during GaAs growth.49
4. PROPERTIES
OF HzCdTe
Before the growth of any Hg containing deposited on the GaAs or CdTe substrates. described previously
GROWN BY CBE
layers, l-6 pm thick CdTe buffer layers were first
HgTe and Hgi,Cd,Te
growth was performed using the Hg source
and DipTe and DmCd sources. Typically, growth rates of 0.3 - 2.7 p.m/hr at substrate
184
C. J. Summers et al.
temperatures
of 120 - 185’C were used. After HgCdTe growth, a 100 - 200 8, thick CdTe layer was deposited
as a passivating cap. This terminated the growth at the Hg layer growth temperature without degradation of the surface due to the preferential
evaporation
of Hg.
growth was reported while for the (lll)B HgCdTe.“”
For the (001) orientation
and (211)B orientation
CdTe, HgTe, and Hgi.,Cd,Te
growth was studied only for CdTe and
The nucleation conditions varied depending upon the epitaxial orientation and whether GaAs or
CdTe and ZnCdTe substrates were used. To ensure reproducible growth and doping, the substrate temperature was calibrated by the tellurium condensation
4.1
Characterization Extensive
layers in conjunction
technique.52*5’
Studies of HeCdTe AIIovs
studies have been reported of the structural, electrical, and optical properties of these with the growth studies.
layers by optical microscopy
Studies of the surface properties
and scanning electron microscopy
of epitaxial (001) HgCdTe
(SEM) showed the presence of small hillocks
on the surfaces, with the best sample having hillock densities of ld cme2 and the worst lo6 cme2. Although no optimization
was performed to lower the hillock density, the best values are similar to values reported for MBE
grown layems
Because of the improved surface morphology of (21 l)B HgCdTe first reported by Koestner et
a.l.?5 later work emphasized also used to estimate Hga.&la.aiTe growth
this orientation.
the composition
of the layers using reference
obtained from Cominco.
rates of CdTe,
SEM and energy dispersive spectroscopy samples
(EDS) analyses were
of CdTe, HgTe and bulk
This data indicated that samples with 0 < x < 0.4 were grown.
HgTe and HgCdTe
were determined
by the RHEED
technique
The latter measurements
or were
measurement
of the thicknesses
accomplished
by selectively etching the epilayers, as described by Leech et al., and then using profilometry
measure
the etch steps.56
transmission
measurements Figure
of the epilayers grown on GaAs substrates.
oscillation
The
Interference
fringes from long wavelength
Fourier Transform
to
Infrared (FTIR)
also were used to determine the thicknesses.
17 shows the room temperahne
FTIR transmission
spectra reported
for Hgi,Cd,Te
samples grown on (001) GaAs substrates. These thin (2 - 5 pm) layers exhibit relatively sharp cut-offs with 50% transmission
bandgaps corresponding
moves to longer wavelengths
to x-values of 0.19, 0.21, 0.24, and 0.37. Note that as the cut-off
(lower x-values),
the slope of the cut-off feature becomes
less abrupt both
because of the decreasing absorption coefficient and the increased distribution of thermally excited conduction electrons
in smaller bandgap alloys.
absorption edge to shorter wavelengths this technique transmission
without extensive
In fact the Moss-Burstein
effect in n-type material shifts the apparent
and thus makes it difficult to obtain precise x-value determinations
analysis.
Ideally, x-value determinations
require temperature
by
dependent
data. For this mason Wagner et. al used the strong dependence
of the electron
concentration
on
temperature to obtain the x-value.5 The rapid decrease in electron concentration between 300 K and 100 K is a direct measure of the change in the number of intrinsic electrons thermally excited across the bandgap. by fitting the variable temperature Hall data with a model incorporating
Fermi-Dirac
Thus,
statistics with the “small
Chemical
Beam Epitaxy
185
gap” Kane energy band structure,s7 an accurate value of the alloy composition
was determined.
This model
also incorporated two donor levels and an acceptor level to tit the data in the extrinsic temperature region. 100
3 1 \
80
3 C
5
rj c
60
s
‘5
.B E
40
t S 20
0 2
4
6
0
10
12
14
16
18
Wavelength (urn) F&llre 17.
Room tempemtum FlYR transmission spectra of four (001) Hg&d,Te grown on GaAs substrates.
Excellent
tits to the temperature
dependence
of the electron
layers of various compositions
concentration
temperature range were reported for samples with x-values of 0.161,0.185,0.183,0.226 confirmed the high uniformity and quality of CEE HgCdTe. net donor concentrations, cmm3,respectively.
over the entire
(Figure 18) and 0.33
All of these layers exhibit very low background
with the x = 0.183 and 0.185 layers having Nn - NA = 3.6 x lOI4 cmm3and 3.4 x lOI
These low background carrier concentrations
those currently obtained with conventional
are approximately
MBE.s8S59 Possible explanations
a factor of 2 - 3 lower than
for the low carrier concentrations
are that the use of monomer Te reduces the kinetic hindrances to growth and, thus, the creation of crystalline defects;
there may be different
conventional
types or levels of impurities
in the metalorganic
sources as compared
to
solid MBE source material; and the higher flux control and stability of the CEE sources and the
Hg-PCVS minimize any stoichiometric
doping effects during the growth of the layers.
In Figure 19 the dependence of the electron mobility on temperature is shown for two HgTe layers grown by CEE at 165°C on a (001) CdZnTe and a (001) GaAs substrate.
As shown, very comparable mobility
values were found for both samples, with the HgTe/CdZnTe layer showing slightly higher mobilities at the higher temperatums (150 - 300 K). The mobilities increase rapidly horn 2.5 x 104 cm%%-’ at 300 K to about 1.1 x 16 cm*V’s-’ at 20 K due to the reduction in LG-phonon dependence
scattering.
The mobility values and the temperature
are in relatively good agreement with the theory as outlined by Meyer et aL60 High quality
166
C.
HgTdChAs
aho
J.
Summers
et al.
has been grown at a low temperature of 120°C with a background No - NA concentration of 1 x
1Or6cm-3 and a mobility of 74,000 cm’V’s_’ at 20 K. These values compare favorably with the highest MBE HgTe mobility obtained and indicate that high quality HgTe can be grown by CBE at very low substrate temTcmtures.6’ An example of Hgt.,Cd,Te discussed previously.
mobility data is shown in Figure 20 for the HgD.&&Te
As shown, the mobility increases by an order of magnitude due to decreasing Lo-phonon
100.0 150.0 TEMPERATURE Figure
18.
layer
200.0
250.0
(deg. K)
Electron concentration versus temperature for two Hg&&Te layers (x = 0.183 and 0.226); dashed and solid lines represent theoretical fits of intrinsic and extrinsic concentrations, respectively.
scattering as the temperature is reduced from 300 - 100 K. At lower temperamms the mobility plateaus at a value of 3.2 x 16 cm*V’s-‘.
Another example of a high mobility sample is shown in Figure 21. This layer (x = 0.14)
exhibited a mom temperature mobility of 3 x IO4 cm’V’s_’ which increases to 8.7 x 16 cm*V’s-’ at 20 K. This mobility is one of the highest reported for a Hgt,Cd,Te As a further demonstration
layer grown either by MBE or CBE.
of the ability of the CBE technique to produce layers with high quality
surfaces, Figure 22 shows the first RHEED intensity oscillations
obtained for a Hgr.,Cd,Te
layer grown on
GaAs. This data indicated a growth rate of approximately 0.52 Mus. Recently, the optical studies of CBE grown HgCdTe have been extended to investigation infrared photoluminescence
and photoreflection
et al. were made using YAGNd spectrometer
properties.
The photoluminescence
of their
studies reported by Tomm
laser excitation with an Rower density of 16 Wcm-* and a low f-number
for sample temperatures from 4.2 to 300 K.“*
Chemical Beam Epitaxy 15E+MJ!5
I -Theo
HQTe 7/CdZnTe mnm Hgle//GaAs
AAAAA
O.OE+000
Figure 19.
I
l&l
sb
150
2dO
2!%
3&l
Electron mobility versus temperahue for two (001) HgTe layers (the solid line represents theory from ref. 60). IO0
II-
_ Hg<,Cd .Te x = ,161
4. 2s h
106:
f
I-
% I
’ 2IO'
F@ure 20.
’
dl
150
2bo
250
3 IO
Electron mobility versus temperature for a (001) Hgo.&,16Te layer grown on &As.
The 300 K photoluminescence Figure 23. The luminescence
data obtained for an undoped Hg.T&d.uTTe
sample is shown in
intensity is observed to be quite. asymmetric with a full width at half maximum of
- 85 meV and the peak luminescence rkzases
id0
being observed to occur at 0.205 eV. At lower energies the luminescence
rapidly while for highe.r energies the distribution is observed to extend to energies as high as 0.420
eV. This featunz is attributed to the 300 K Fermi Dirac electron distribution in the conduction
band due to a
C. J. Summers et
al.
_ 10". “E 9
8 a % 0 lOi5 0.0
, 50.0
.
I u I m I 100.0 150.0 200.0 (deg.
TEMPERATURE
Figure 21.
Temperature dependence
ofcarrierconcentration
m I 250.0
I
K)
and mobility for a (001) H&.&Q.,4Te layer grown on
H&Cd,,Te/GaAs
r = 0.49
0
RHEED
340 K electron gas.
(001)
MU.s
10 Time
Figure 22.
10’ 300.0
(&onds)
intensity oscillations obtained from a (001) H~,J.&&~T~/G~As CBE layer grown at 165°C. The peak PL at 300 K agrees well with the IR transmission
data recorded for these
samples. The low (15 K) temperature PL is shown in Figure 24. The peak PL has shifted down in energy to 0.125 eV (-10 w)
and has a considerably
narrower full width at half maximum of 11 meV.
The peak is
189
Chemical Beam Epitaxy
more symmetrical although some evidence of the Fermi distribution is still observed at higher energies.
Similar
data has also been obtained for CdTeIHgCdTe superlattices.
4.2
CdTe/I-kCdTe Su~rlattice Conventional
Growth
type II superlattices consisting of CdTe barriers and small bandgap Hgr .,Cd,Te (x =
0.25 - 0.3) semiconductor
quantum wells have also been grown by CBE using the procedures
HgCdTe and CdTe growth.
developed
for
Before growth on the (21 l)B CdTe substrates were annealed at 300°C for several
minutes with and without a Te flux incident on the surface. The substrate temperature was dropped to 180°C for the initiation of the superlattice growth. After the confiiation
of high quality growth, as determined
by
RHEED and diffuse laser scattering, the growth temperature was gradually lowered to 170°C to minimize layer
H&dTe (x=0.237) T=3OOK
MOO-
0.
I 180
[ 220
I,
I,
I,
260
300
I, 340
I/ 380
420
460
Energy (meV)
Figure 23. interdiffusion.
Photoluminescence spectrum obtained at ux) K for a Hgl,CdXTe undoped layer with x = 0.237. These layers were deposited at a growth rate of approximately
0.75 pm/hr and were deposited
directly onto the substrate with no buffer layer growth. During growth, a photon flux of 40 - 60 mWcm_* was incident upon the growth surface. Two types of superlattice
structures
were grown; 50 A CdTeIlOO 8, HgCdTe
CdTeLZOOA HgCdTe, with and without iodine doping. varied to shift the absorption superlattice layer.
edge.
The composition
Room temperature
and 100 8,
of the HgCdTe in the. wells was also
FTIR transmission
This data indicated that the layers had cutoff wavelengths
spectra were taken for each ranging from 2.4 to 5.0 pm for
C. J.Summers etal.
190
0 100
110
120
130
140
Energy(meV) Figure24.
Photoluminescence spectrum obtained at 15 K from an undoped Hg,..Cd.Te layer (x = 0.237).
these structures and HgCdTe composition interference
values.
The transmission
fringes indicative of high quality material.
absorption edge to longer wavelengths
by approximately
spectra showed sharp cutoffs and several
As expected, 0.2 pm.
the larger period structure shifted the
The cutoff wavelength
increasing Hg content in the wells. Figure 2.5 shows room temperature FIIR transmission h100
also increased with spectra from three 50
A (barrier/well) superlattices in which the barrier layers were grown under identical conditions but with a
different DmCd flux for the HgCdTe wells. The increase in cutoff wavelength for these layers corresponds decreased Cd content in the wells. Figure 26 shows FITS transmission identical conditions
spectra for two samples grown under
except for one having twice the barrier and well thicknesses.
longer cutoff
wavelength
expected
for such a change
measurements
of the doped superlattices
in the dimensions
This sample exhibits the
of the structure.
indicate room temperature electron concentrations
high-1Or6 cmm3to the mid-10’7 cme3. Infrared photoluminescence
to a
Electrical
ranging from the
spectra were also recorded for these samples
at room and low temperatures6*
5. DOPING BY CHEMICAL BEAM EPITAXY
In addition to the successful growth of CdTe and HgCdTe alloys by CBE, investigations undertaken
to obtain n- and p-type doped layers to explore the advantage it offers for developing
The lack of controllable extrinsic doping technology for HgCdTe.27728,45363
were
a mature
in-situ extrinsic doping is a major
191
Chemical Beam Epitaxy
8o1
20
01’1’1’ 4500
4000
I’ 3000
3500
I’ 2500
I’ 2000
I’ 1500
I’I 1000
500
Wavenumbers Figure 25.
Room temperature FITR spectra from three CdTeIHgCdTe superlattices with different compositions HgCdTe in the wells.
of
80
80
Q
2 3
5 ..-t2 E
40
i 20
0
Flpre26.
Room temperature FTlR spectra from two CdTe/HgCdTe and well thicknesses.
impediment
to numerous device applications
could result in a radical impiuvement
superlattices with one having twice the barrier
for these alloys and is essential for new device schemes which
in detection.
The wider range of dopants made possible by CBE makes it
prudent to review the selection criteria for dopants in HgCdTe so as to clearly define their desired properties. The three main criteria for dopant selection which have to be mutually compatible axe: 1. their electronic behavior in the host lattice,
C. J. Summers et a/.
192
2. their chemical characteristics,
stability and metallurgical
properties
such as volatility, segregation
and diffusion
and
3. their nucleation kinetics which determines
the segregation coefficient,
substitutional
or interstitial
behavior, and electrical activity. The first criteria is met by selecting elements in the periodic table from columns adjacent to the host elements
whose chemical
and bonding
characteristics
perturbation of the lattice while doping the semiconductor.
allows substitution
for a host ion with little
Possible p-type dopants for the HgCdTe system are
elements from group IB (Cu, Ag, Au) on a cation (Hg or Cd) lattice site, or the substitution of elements from group VA (N, P, As, Sb, Bi) on an anion (Te) lattice site. Conversely, elements from group III4 (B, Al, Ga, In) substituting on the cation lattice site, and elements from group VIA (F, Cl, Br, I) substituting on an anion lattice are possible n-type dopants. Since the bonding electronegativity,
forces in HgCdTe are more ionic than covalent
ionic radii, and atomic volume similar to the host ions are required.
in nature, dopants with For the Hg/Cd site Cu,
Ag, and In are closely matched to the properties of the host atoms, whereas for the Te lattice, P, As, I and Br are the most suitable dopants.
However, the metallurgical properties of these dopants show that atoms such as In,
Cu and Ag, which incorporate
on the HgKd
lattice diffuse very fast.64 Thus, the most stable dopants are
expected to substitute on the Te lattice, i.e., P, As or N for p-type doping and I or Br for n-type doping. The incorporation
of dopant species is also expected to be enhanced by the use of monomer
dopant species. However, these elements evaporate in molecular forms. Cl, I and Br, which can be used for ntype doping on the Te-lattice, also are dimers in the vapor form. Because of these limitations, metalorganic gas dopant sources were investigated, structures, the CBE technique doping where extensive
and have shown that in addition to the growth of alloys and superlattice
is also a very effective doping technique.
characterizations
This is particularly true for iodine
of CdTe and HgCdTe doped with iodine using an ethyliodide
gas
source have recently been reported as discussed be10w.“~~~~~~ Iodine h~ine
5.1
of CdTe
Because of the complexity of the HgCdTe alloy system the first CBE doping investigations performed
in CdTe.
In addition to the fact that it appears to best satisfy all of the physical and chemical
properties required of an n-type dopant in the HgCdTe alloy system, iodine also is very compatible temperature epitaxial growth. cation-rich
were
with low
Iodine doping on the Te-sublattice preserves the advantages of low temperature
growth which has been shown to produce planar surfaces and also is expected to simultaneously
promote
dopant incorporation
sticking
coefficient
on the Te sublattice.
and minimizes
the formation
Low temperature of native defects.
growth also enhances Additionally,
the dopant
group II rich growth
conditions should further suppress the formation of Va, Tei and [Vcd-donor] acceptor defects that compensate donors.
Also, because iodine substitutes on the more stable Te sublattice, its diffusion coefficient
order of magnitude
lower than that of indium, which substitutes for Cd.25*26 Low diffusion
realize abrupt dopant profiles in advanced device structures.
is over an
is important to
193
Chemical Beam Epitaxy The
high
doping was implemented source described
vapor
form. Therefore,
using an alkyliodine gas source that was delivered by the pressure controlled
in Section 3.1.”
substrate in the unpyrolyzed conductance
of iodine prohibits its use in the elemental
ptxsure
form.
Ethyliodide
was used as the iodine precursor,
The dopant-PCVS
iodine vapor
and was incident on the
was operated under choked flow conditions,
and the
of the orifice was related to the upstream pressure to accurately determine the dopant flow rate.
This enabled the measurement
of flow rates as small as 10m7seem, corresponding
to a BEP of lo-” torr.
However, because CdTe growth between 200 - 300°C using a binary source occurs under Te-rich conditions
due to the nonunity
sticking coefficient
dependent on the Cd overpressure?o
of Cd the surface stoichiometry
and growth rate are
The Cd-rich growth conditions that are necessary to optimize doping were
obtained by measuring the dependence
of the CdTe growth rate on excess Cd flux.
As shown in Figure 27,
with increasing Cd flux, the growth rate increases to a maximum of 0.27 prn/hr, for fluxes greater than 6 x 10m8 torr corresponding
to a Cme
flux ratio of 1.3. This indicates that for a growth temperature of 230°C. and PC&
= 3.7 x l@’ torr, the surface becomes cation-rich when the Cd BEP was greater than 6 x lOmatorr. This data was corroborated reconstructed
by RHEED studies which showed that the growth surface exhibited
pattern for low Cd overpressures,
and the co-presence
stabilized surface for Cd BEP’s of 1.5 x 10e7 torr or higher.” dependence
of the electrical propeaies
was investigated
a Te-stabilized
of a ~(2x2) reconstruction
Using these cation-rich
(2x1)
typical of a Cd-
growth conditions,
for ethyliodide dopant flow rates between
the
lo-’ - lo-*
seem.
I
0.350
(001)
CdTe: I/G&a
T. = 230°C
c g E
0.300
-
-
5
0.225 -
0.200
1
0.0
5.0x10-"
PCd Figure 27.
I
1.0x10‘
I
1.5x10-'
2.oxlO-7
ttorr)
Dependence of growth rate on excess Cd flax for MBE grown CdTe at 230°C. Flux values for which
(2x1) Te-stabilized and mixed (2x1) Te and ~(2x2) Cd-stabii are indicated.
surface reconstructions were observed
C. J. Summers et al.
194
As shown in Figure 28, the room temperature electron concentration root dependence
was found to have a square-
on the dopant flow rate for flows between 10.’ and lo-* seem. For this range of flow rates, the
electron concentration
increased from 8 x lOi to 3 x 10” cmm3and showed no evidence of saturation at high
flows indicating that doping higher than 3 x 1OL8cm-3 can be achieved in CdTe:I. This nonlinear dependence was attributed by Rajavel and Summers to a decrease in the dopant incorporation flow rate.*’ For films with room-temperature cm”, the corresponding
electron concentration
rate with increasing dopant
of 4.4 x lo”, 7.5 x lo”, and 3.1 x 10”
mobilities were 560, 570, and 460 cm2Vk’,
respectively,
and are among the highest
values recorded in heavily doped CdTe layers. Doping down to electron concentrations
in the lOI - lOI cme3 has recently been achieved and
given the highest mobilities yet reported for MBE grown CdTe. For a sample with a room temperature electron concentration
of 2.3 x 10” cmm3 and a 60 K concentration
to 9022 cm%&-‘.
The lowest concentration
cme3 at low temperature
of 2.8 x lOI cmm3,the mobility increased from 600
achieved was 4.4 x 1014 cme3 at room temperature and 5 x lOI
with corresponding
mobilities
of 300 and 5300 cm2V~‘s~‘.66 Further optimization
studies in this area are expected to result in even higher mobilities for this concentration range. SIMS measurements
corroborated
the electrical data and showed that the electrical activity and
iodine depth profile were well behaved in CdTe. This indicates that there was little compensation presence of native acceptor defects or the formation of
Cd12 precipitates
that are electrically
due to the
inactive.
4 _
(001) CdTe:l
2 - Growth
6.
rate = O.!Q.m/hr
I ,,,,I,,
I I I11111
466’
VP2
2
lOA
466’
10”
I I
I I I11111 2
466’
2
4
1o-2
Ethyliodide flow rate (seem) F&we
28.
Dependence of electron concentration for iodine doped CdTe as a function of ethyliodide flow rate.
As
Chemical Beam Epitaxy
shown in Figure 29 the iodine concentration
is constant
195
throughout
the layer and drops sharply at the
CdTeYCdTe
buffer layer interface; an indication
temperature.
(The accumulation of iodine at the substrate/layer interface was caused by dopant flux calibration
prior to growth.) concentration
Because of the mass interference
of the low diffusivity
of iodine in CdTe at this growth
in the SIMS measurements
was limited to a factor of five, and results in a conservative
the accuracy of the iodine
estimate of the iodine electrical
activity of between 50% and 100%.
1.oo DEPTH Figure 29.
2.00
3.00
(microns)
SIMS profile of iodine doped CdTe.
Photoluminescence
studies of iodine doped CdTe show that the PL emission intensity was up to
100 times greater than comparable undoped materials which made it possible to identify a very rich spectra.67 As shown in Figure 30, the low temperature spectra shows two very strong features with linewidths c 1 meV at 1.5927 and 1.5892 eV which were assigned to an iodine donor substituting acceptor exciton.
for Te and an unknown bound
A small shoulder was also observed on the low energy side of the acceptor bound exciton at
1.5860 eV and assigned to two-electron transitions. This feature occurs because some of the free exciton recombination
energy is transferred to the
donor, to excite an electron from the 1s ground state to the 2s first excited state, resulting in a lower energy recombination
feature. The presence of this feature is not only an indication of very high crystalline quality but
also allows a very accurate determination
of the donor ionization energy of - 15.0 -c 0.2 meV for iodine.
The
196
C. J. Summers et al.
temperature dependence energy.
of the 1.593 eV feature was also measured to further verify the iodine donor ionization
Figure 3 1 shows the quenching of this feature as the temperature is increased.
The high temperature
results indicated an iodine ionization energy of - 14 meV. A slight kink in the data near 45 K was explained by assuming an activation energy of 10 meV which corresponds to the 1s - 2s donor transition energy.
Thus, this
data confirms iodine’s shallow hydrogen like donor behavior with Eo - 15 meV. Figure 30 also shows a feature at 1.568 eV assigned to an LO phonon replica of the (A’,x) feature.
Because acceptor are deeper than donors
they are more strongly tied to the phonons, this is an indirect confirmation of this feature at 1.556 eV was not determined recombination donor-acceptor
but was ruled out as an acceptor or a donor-acceptor
because it showed no phonon replicas. pair recombination
involving
of the 1.589 eV feature. The origin pair
The lower energy feature at 1.54 eV was assigned to a
the iodine donors
and unintentional
doping by Na and Li
acceptors which have activation energies of -58 meV. At even lower energies, a series of features were observed between 1.49 1 and 1.35 eV. The sharp feature at 1.49 1 eV was only observed below 45 K and has recently been assigned to radiative recombination emitted when conduction
electrons recombine
through a neutral donor-acceptor
pair complex resulting from
nearest neighbor cation (Na& and anion &) defects. The energy level associated with this neutral (Na&-fre+) pair is 0.115 eV below the conduction band. This feature was accompanied by associated LO-phonon replicas
CdTe: I (T, = 210°C) 500 mWIcm* T = 4.9K
1.5860 TET
DAP
I
1
1.556
1
(A”J)-lLO
1.560 Energy Figure36.
1.576 @VI
Photoluminescencespectrum of iodine doped CdTe at 10 K.
1
Chemical Beam Epitaxy
197
loo.0
j
10.0
s .i? 2
B c
-
1.0
7
0.1
I
0.M I
0.05
0.03
0.0
CdB:I (-r,=170%) Ed9a emissionat 1.593 eV
l/Temperature
0
.Ofi
(K-l)
I
I
I
0.10
0.15
0.20
0 !5
1/Temperature (K“) Figure 31.
Quenching of 1.593 eV feature with increasing temperature.
occurring at energies
20.4
- 21.3
meV below the main feature and also by an interesting local mode of 36.5
meV which was attributed to the presence of Nacd in the defect center.68 The feature at 1.470 eV, followed by up to seven LO-phonon replicas spaced by 21.3 meV, has been assigned to a donor-to-acceptor the (Vc&J
pair recombination
transition from the ground state of the iodine donor to
acceptor state complex formed by adjacent Vcd and Ire sites. This assignment was made because
the PL emission intensity increased proportionally with increasing dopant concentration. in emission intensity was also observed for a sample grown with no Cd overpressure. to a larger concentration
A significant increase
This result was attributed
of acceptor defects which provide an increased path for recombination
indirectly confirms the effectiveness
of excess Cd in decreasing V,
and, therefore,
associated point defects and complexes.
This analysis results in an ionization energy of 0.125 eV for the (V&*-IT,+) acceptor complex. Confirmation
of the transition energies
observed
in the PL spectra has also been obtained by
monitoring the intensity of the 1.470 eV line as a function of temperature.
The luminescence
intensity can be
fitted as a function of inverse temperature by three activation energies; 15 meV at low temperatures, intermediate
temperatures,
and 125 meV above 140°C.
Estimates of the Huong-Rhys
67 meV at
factor from the LO-
phonon replica spectra range between 1.5 and 2 and are significantly less than the phonon coupling strengths for chlorine and bromine, reported as 2.2 and 2.9, respectively.
This reduction in phonon coupling is attributed to a
reduced lattice distortion since the Te and iodine have such similar size.
Thus, the lower level of (Vcd-1~~)
defects and reduced phonon coupling both support the argument of iodide being an effective donor in CdTe and related alloys.
198
5.2
C. J. Summers et al. Iodine Dopine of HgCdTe For this study ethyliodide doped HgCdTe layers were grown by CBE at 180°C on 10 pm thick
CdTe buffer layers deposited on (21 l)B oriented GaAs substrates. seem and 0.16 seem, respectively.
The Hg-PCVS
The DipTe and DmCd flow rates were 0.8
control pressure was 0.725 torr, for a beam equivalent
pressure of 1.4 x 10e4torr. These conditions gave an x-value of 0.241 and produced a growth rate of 2.3 pm/hr. The ethyliodide previously.27
was incident on the growth surface without cracking as for the CdTe doping discussed
The conductance
of the orifice was calibrated with the ethyliodide
from the product of the capacitance
manometer
pressure and the characteristic
flow rate being determined decay time of the orifice.
In
addition, the beam equivalent pressure was measured directly for larger flow rates. Figure 32 shows the electron concentration iodine-doped
as a function of sample temperature
for a range of
HgCdTe layers and undoped CBE HgCdTe layers. The low temperature electron concentration
varied over three orders of magnitude and indicated that iodine is an effective n-type dopant for HgCdTe. maximum electron concentration MBE grown material. concentration
was 5 x 1018cmm3and is the highest reported n-type carrier concentration
The lowest electron
concentration
sample was undoped
of 3.4 x 10” cme3. Figure 33 shows the temperature dependence
The in
and had a 20 K carrier
of the carrier concentration
and
mobility for a sample doped to 1.8 x lOI cme3. The peak mobility was 50,000 cm’V’s_’ which is the highest reported mobility at this carrier concentration measurements
for MBE HgCdTe material.
It should be noted that all of these
were made on as-grown material, without any low temperature, Hg-saturated anneals to remove
Hg vacancies as is usually given to MBE layers before measurements.
Therefore, the electron mobilities are
expected to improve after such treatments.
Figure 32.
Dependence of electron concentration on temperaturefor iodinedoped Hgi.,Cd,Te samples with x = 0.24.
Chemical Beam Epitaxy
199
Dependence of electron concentration and mobility on temperature for iodine doped Hgr.,Cd,Te (x = 0.24) with n = 1.8 x 10” cm-3.
Figure 33.
Figure 34 shows the electron concentration at 20 K as a function of the ethyliodide flow rate. The carrier concentration
increased as the square root of the ethyhodide flow and only deviated from this behavior at
high doping levels.
These results am in agreement with the previously discussed dependence
of n-type doping
on ethyliodide flux for CdTe.
5.3
P-Tvw Dovim of CdTe Due to the difftculty in p-type doping HgCdTe, a number of different
investigated
techniques
to activate As-doped HgCdTe layers. For example, As has been incorporated
layers of a CdTe/HgTe multilayer and then interdiffused
have been
only into the CdTe
into the adjacent HgTe layers, and laser illumination
has been used to promote high As doping in CdTe at low growth temperatures.65
Arias et aL6’ and Wu et aL6’
have reported that Cd-rich off-stoichiometric
growth flux conditions are required for the effective activation of
As and In, respectively.
of iodine,
stoichiometry.63
The activation
however,
is apparently
insensitive
to the surface
Also, epitaxial growth temperatures have to be minimized to obtain high dopant incorporation.
Thus, laser illumination
has been used with reduced growth temperatures
to maintain adequate CdTe adatom
surface mobilities. Because low temperature growth has been remarkably effective for incorporating
arsenic atoms
into CdTe,” Maruyama et al.” undertook studies to reduce the growth temperature by the addition of an excess Cd flux to compensate
for the preferential desorption of Cd atoms.
or accurate Cd-rich off-stoichiometric layers at high temperatures
Thus, stoichiometric
surface conditions,65
growth conditions can be used to achieve both high quality CdTe buffer
and high concentration Asdoped
At the higher temperatures
layers at low temperatures.
the CdTe growth rate was observed to increase with increasing Cd
flux and attained a constant value for CdRe flux ratios exceeding
1.8. This saturation of the growth rate at
C. J. Summers
200
et a/.
/'
10% E
./'
.
_. /'
1016 o.ooo1
I 0.01
* - 8a.
. .I 1
. - ..
. : :: 100
EthyliodideFluwrate, Fi (TOIT%‘) Figure 34.
Electron concentration (measured at 20 K) as a function of ethyliodide flow rate. Fitted line corresponds to square root dependence of carrier concentration on flow rate.
high excess Cd fluxes implies that the Cd desotption stoichiometric
was offset by the increased Cd flux, thus yielding a
surface for the CdTe layers as confirmed by RHEED. After buffer layer growth at 300°C with a Cme
was reduced to below 200°C and the growth conditions technique, as depicted in Figure 35. For temperatures
flux ratio of 1.8 the CdTe growth temperature
studied by the same RHEED intensity oscillation
less than 200°C no change was seen in the growth rate
using excess Cd. However, the RHEED patterns were shown to strongly depend on the growth conditions and Cd/Te flux ratio.
Streaky RHEED patterns were taken to indicate two dimensional
morphology
and good crystallinity,
accompanied
by twin-induced
experiments
growth, smooth surface
while spotty patterns were indicative of three dimensional
growth, and
diffraction features, which signaled the onset of crystallinity degradation.
These
were used to define a window of growth flux ratios at a given substrate temperature that yielded
high material quality. From this study optimum growth conditions were obtained for Cd!Te flux ratios between 1.02 and 1.18 at 180°C and between decreasing
temperature,
demonstrating
1.06 and 1.12 at 170°C. As shown, this window decreases in size with the need for precise flux and temperature
control at lower growth
temperatures. The CdTe layers were grown under different growth conditions and their properties evaluated by photoluminescence,
SIMS, and Hall effect measurements.
Amine was pyrolyzed to generate a large flux of
dimer arsenic, as comparedto the tetmmerAs species obtained in the conventional AS sublimation process.
Chemical Beam Epkaxy
Buffer:J,
14. =l .O
Growth Temperature (“C) 1.7
r
1.6 1.6 F 1.4 3
1.3 z ') 1.2 1.1 1.0 0.9 150
I
160
I
I
I
I
170
180
190
200
Growth Temperature (“C) b) Figure 35.
Ctystahinity of CdTe, estimated from RBEED patterns, as a function of the growth temperature and Cite flux ratios with (a) J&Jr. = 1.0 and (b) JcdJT, = 1.8.
Figure 36 shows a 10 K PL spectrum obtained for an As doped CdTe layer.
The four peaks
between 8200 and 8600 8, confirmed arsenic incorporation on the Te sublattice, where. the donor-acceptor (DAP) peak at 8200 8, corresponded
pair
to As atoms occupying Te sites and the remaining peaks are longitudinal-
optical (LO) phonon replicas of this peak.
From this study dimer arsenic was shown to incorporate
tellurium sublattice with a higher efficiency than was obtained with tetramer.
on the
Undoped CdTe layers grown
under similar conditions did not show any peaks around 8200 A. SIMS profile measumments
showed that the arsenic concentration was as large as 2.5 x lOI7 cm-‘,
for a large arsenic flux of 1.1 x 10e7torr BEP at 180°C.
The abrupt reduction of the arsenic signal at the
interface indicated that the diffusion of arsenic was very low at the CdTe growth temperature. of the As concentration
The dependance
on the cracked amine partial pressure is shown in Figure 37. The maximum arsenic
C.
202
1
I
A
Summers
et al.
I
I
I
CdTe:As
Ad(
j
J.
T=lOK PATHS= 4.8x10w7torr Jcd/J Te = 1.68 T~200”C
/
7t
10
8000
8400
Wavelength Figure 36.
8800 ( 8)
Photoluminescence specttum of a CdTe:As layer obtained at 10 K.
concentration
was 3.0 x lOi cmm3 for an arsine BEP of 1.3 x 10.’ torr for growth at 170°C.
concentration
incorporated
The As
in CdTe grown at 170°C was equivalent to that grown at 18O”C, despite the lower
amine pressure at 170°C. Thus, the incorporation efficiency of As was remarkably improved for 170°C growth. Doping using solid arsenic for the same growth conditions and for equivalent As tetramer flux, produced an As concentration
at the SIMS detection limit of 2 x 10” cme3. This indicates that the incorporation
efficiency of
dimer As is two orders of magnitude higher than that using As tetramers. However, carrier concentrations were considerably
room temperature
Hall effect measurements
and mobilities (3.5 x lOi5 cm-3, 81 cm%%-’
showed typical p-type values for the
and 1.7 x 1015cm-‘, 52 cm*V’s-‘) which
lower than measured by SIMS. The high resistivity was attributed to autodoping from the
CdTe substrates as the SIMS measurements
also showed high concentrations
of bromine, which acts as a donor
in CdTe. It is also possible that untracked or partially cracked amine was incorporated onto Te sites and caused self-compensation. As tetramers
These results indicate that As climers obtained by pyrolyzing amine are a better source than
for high-efficiency
arsenic doping, but that more work remains to be done to achieve high
electrical activity. P-type doping of Cd&Zn,Te
using atomic nitrogen supplied by a dc glow plasma source has
recently been reported7’ with hole concentrations doping
efftciency
concentration
decreased
significantly
of up to l$’
cm”
as the Cd mole fraction
of only 8 x 1015 cme3 was achieved for homogeneously
being obtained in ZnTe. was increased doped CdTe.
However, the
and a maximum
hole
Pulse doping (alternating
doped and undoped layers) was used to achieve a factor of ten increase in doping, to lOI7 cmm3for CdTe.
Chemical Beam Epitaxy
I
-
-
I
203
I
I
l Tg=180”C o Tg=170”C
j
0
I
lo-'0 10-9 10-8 1o-7 10-6 AsH3 pressure (torr) Figure 37.
Dependence of As concentrationon amine beam equivalent pressure.
6. CRE GROWTH OF RELATED MATERIALS: CdTe AND ZnU-Ig.Cd)Te
In this section only Te-based compounds grown by CBE are reviewed.
Excellent review articles
on the CBE growth of the wider bandgap Se and S based materials have been published by Yoshikawa and Konagai.r2*73 Rajavel and Zinck have recently extended the CBE technique to the growth of CdTe, CdZnTe and ZnTe.r4*7s*76 These growths were performed
in a Vacuum Generators
system in which the DmCd and
DeZn precursors were cracked in tantalum based cracker cells. In this study, diethyltelluride as the Te precursor
and was cracked in a quartz cracker to prevent unwanted
precursors were completely DeTe was decomposed
dissociated
into monomer
into the elemental species and hydrocarbon
(DeTe) was used
corrosion.
Both group II
by-products,
and dimer tellurium, partially alkylated tellurium
whereas the
compounds,
and
hydrocarbon by-products. prior to growth, surface kinetic studies were performed on (001) GaAs substrates.
The growth
kinetics of ZnTe and CdTe were studied by measuring the variation in growth rate as a function of substrate temperature and II/VI ratio using RI-TEED intensity oscillations.
Figure 38 shows the variation in growth rate
measured for a fixed Te flux as a function of the DeZn pressure and substrate temperature.
As shown, the
curves have the expected behavior with the growth rate increasing rapidly with DeZn flux at low flux values and then plateauing for higher values. The saturated maximum in the growth rate defines a Te limited growth rate regime,
which above 33O“C decreases
with increasing
incorporation
coefficient of the Te species under Zn-rich growth conditions are unity below 330°C but become
increasingly less than unity at higher temperatures.
temperature.
This result indicates
that the
A similar study also was performed for CdTe and showed
that the Te sticking coefficient became less than unity above 280°C. These results are similar to those reported
204
C. J. Summers
et
al.
by Benz et al. except that the critical temperature is significantly lower, presumably because of the presence of dimer Te and other Te species which have a lower activation energy than monomer Te.”
The difference
in
critical temperature between CdTe and ZnTe is attributed to the higher bond strength between Zn and Te.77 The physical properties measurements
of these 2 - 3 pm thick films as determined
from X-ray and SIMS
indicated the rocking curve linewidths were 200 - 220 arc-set for ZnTe and 480 - 580 arc-set for
I-
T,=315”C x Ts = 330 . T, = 350 . T, = 365 n Ts = 385 q
0.8
1.0
1.2
1.4
1.6 L
Figure 38.
1.8
2.0
2.2
2.4
(torr)
Variation in growtb rate as a function of substrate temperature and DeZn/DeTe flux ratio.
CdTe with lower values of 210 - 230 arc set being obtained for thicker (6 - 7 Frn) films. The PL spectra of the ZnTe samples were dominated by strong near band edge emissions as is shown in Figure 39. Free excitonic recombination
was observed at 2.382 eV and lower energy features which repeat at 26 meV intervals below the
free excitonic peak at 2.356 and 2.330 eV, were attributed to phonon replicas of the free exciton. Strong bound excitonic
recombinations
associated
with gallium donors and arsenic and zinc However, in the absence of
vacancy (Va) acceptors were observed at 2.379,2.374
and 2.368 eV, respectively.
PL excitation
The strongest bound excitonic feature was the arsenic
data, these assignments
are tentative.
bound excitonic transition which had a FWHM of 3 meV. vacancy complexes.
No evidence was found for the presence of zinc
Films grown under different B/W ratios showed similar features, however, the relative
intensities of the excitonic features varied.
These results demonstrate that the structural and optical properties
of CBE grown ZnTe films equal those prepared by other epitaxial growth techniques.
Chemical Beam Epitaxy
205
T
-
ZnTelGaAs T=5K
As”,X
vz,“,x
I
Ga”.X
I
J1,;_ X
I
I
I
I
I
I
1
2.30
2.32
2.34
2.36
2.38
2.40
Energy (eV) Figure39.
Photoluminescence spectra of ZnTe at 5 K.
SIMS measurements
on these samples indicated the presence of Ga and As at a 5 x 10” cme3
level, presumably due to contamination contamination
from the substrate.
High levels of Si, Al, and Br due to source material
were also observed and C and 0 identified at SIMS background levels of - 1Or7cmm3. The CdTe samples were grown using a ZnTe buffer layer to ensure (001) orientation.
crystalline quality sample was observed to exhibit a rich PL structure. CdTe film is shown in Figure 40. recombination
The highest
The PL spectrum of a representative
A strong free exciton peak was observed
at 1.596 eV and associated
from its fust excited state at 1.603 eV and its upper polariton branch at 1.598 eV. These free
excitonic features attest to the high quality of the CBE grown (001) CdTe/GaAs films. Also, a sharp (FWHM = 1 meV) and intense bound excitonic feature was observed at 1.590 eV and was attributed to excitons bound to arsenic acceptors.
Similar results were obtained for CdTe growth using DeCd.
Cdt,Zn,Te
(x - 0.04) films also were grown at 320°C on 0.2 pm thick ZnTe buffer layers
deposited on (001) GaAs substrates.
Films 8 - 9 pm thick exhibited x-ray FWHMs of 210 - 250 arc-s and 3 pm
thick films FWHMs of 400 - 450 arc-sec. bound excitonic recombination
The near band-edge region of the PL spectrum was dominated by
features associated with acceptors and donors at 1.615 and 1.619 eV, respectively of the free exciton was also observed at 1.622 eV.
and the
Donor acceptor pair recombination
was
observed at 1.5686 eV, and two phonon replicas with a spacing of 0.021 eV were observed at lower energies. A weak defect related band appeared at 1.494 eV. The surface morphology
of the (Cd,Zn)Te films prepared
206
C. J. Summers et al.
under near-stoichiometric
growth conditions
were smooth and nearly featureless as determined
by Nomarski
contrast microscopy. Studies of higher x-value alloys also show that the crystalline quality of the CBE material was very good. For example, a Cdi.,Zn,Te
(x = 0.17) grown on a (001) GaAs substrate exhibited a X-Ray FWHM
of 350 arc-sec. Also, PL spectra at 5 K exhibited strong donor bound excitonic transitions.
A”,X 1
I.580
(001) CdTe/GaAs T, = 280°C T=5K
I
I
I
1.585
1.590
1.595
I 1mO
I 1.605
1.610
Energy (eV)
Figure 40.
Photoluminescence
spectrum of a CdTe layer grown by CBE.
HgZnTe growth by CBE using elemental Zn also has been reported by Benz et al3 and Summers et al6
Figure 41 shows the room temperature
FUR spectra obtained for two Hgi.,Zn,Te
layers grown at
185°C. The cutoff wavelengths correspond to zinc mole fractions of 0.59 and 0.33.
7. MANUFACTURING
ISSUES
Of particular interest is the application of CBE as a flexible manufacturing systems.
technique for infrared
Given the material issues previously discussed for HgCdTe, it is important that a growth technique be
repeatable, precise, flexible and scalable to become a viable manufacturing tool. Wagner et al. have assessed the performance of CBE as a flexible manufacturing measurements
of the optical, electrical and compositional
measurements
of system performance
potential advantages of CBE.
techniques from
properties of CBE grown HgCdTe layers2’
RHEED
were also reported along with theoretical analysis which indicate the
Chemical Beam Epitaxy
1
3
5
7
9
11
13
Wavelength
207
15
17
19
21
(pm)
Room temperature FTIR spectra obtained for hvo HgZrrTe samples grown by CBE
Fipre41.
As previously mentioned,
a major advantage of CBE over conventional
MBE is the increased
stability possible with pressure based flow controllers as opposed to conventional thermal evaporation sources. This is especially important for the HgCdTe material system where the host elements and the group V and VII dopants have high vapor pressures. a small variation in temperature especially
important
For these solid sources, very stringent temperature control is required since
causes a relatively large change in the growth fluxes.
at the low growth temperatures
High flux control is
used for HgCdTe growth as any excess flux of the
constituent elements will not desorb from the growth surface and, therefore, will be incorporated
as defects.
The ease of computer control, ability to change sources externally and potential for selective area epitaxy and in situ processing
also make CBE an effective
manufacturing
demonstrated
the selective area epitaxy of CdTe by CBE?*
7.1
Characterization
achieved
In fact, Benson et al. have already
of CBE Grown HeCdTe
To experimentally undertaken
technique.
assess the potential of the CBE system, a series of HgCdTe growths was
over several months and under constant growth conditions over alloy composition.
measurements
These properties
were determined
to assess the control that could be by infrared transmission
and SIMS
to measure the alloy composition variations from run-to-run, laterally across the wafer and along
the growth direction. To transmission
determine
the
alloy
composition
and run-to-run
spectra were measured on each sample from 2 - 15 um.
variation,
room
temperature
FfIR
Figure 42 shows selected transmission
C. J. Summers et al
208
Hg,,CdxTe R = 0.242 80
80
4
FTIR transmission curves for three CBE gown
Figure 42.
10
8
Wavedgth
(urn)
HgCdTe samples whose cutoff wavelengths correspond
to the average and maximum variations of a series of fourteen layers.
curves obtained
for samples
whose compositions
obtained from a series of fourteen growths. corresponding
to the composition
correspond
to the average and the maximum
variations
The vertical lines in Figure 42 represent the wavelength (5.93 pm)
of the mean x-value and the mean plus or minus one standard deviation in x-
value. The data indicate a standard deviation of 0.17 pm. The cut-on wavelength was obtained from each of these curves by assuming that it corresponds The x-value was then calculated
to 50% of the average transmission
using Hansen and S&nit’s
expression
value at long wavelengths.
relating x-value, temperature
and
energy gap.” Figure 43 shows the range of x-values obtained from the above analysis for a series of 14 consecutively
grown samples.
As shown, the samples exhibit a mean composition
deviation of f 0.0043, a variation of less than 1.8%. It should be emphasized over a period of two weeks and were accomplished previous experiments.
of x = 0.241 and a standard
that these runs were performed
with no recalibration of the system and no feedback from
Also, growths performed months later with the same growth conditions yielded layers
with x-values within Ax of the mean x-value of 0.241. More recently, after implementing allowing
longer
compositional
substrate
of 14 growth
runs have produced
of the lateral variations in the HgCdTe composition
was carried out using
temperature
stabilization
times,
a series
computer control and
variations off 0.0016 for x = 0.229 material. An initial assessment
SIMS measurements.
In these measurements
the sampled area was 0.5 mm2 and several sets of data were taken
Chemical Beam Epitaxy
209
_I~1
Std. CM.: 0.004 Std. DeWMean:1.85%
I
I
0.2301: : : : : : :: :: : : : :: :: :: :: :: : : : : : : : : :: : : : : : : c 30 50 40 60
Run Number Figure 43.
Range of
compositions obtained from a series
of fourteen consecutively grown HgCdTe samples
over a 5 mm area. The studies indicated a variation of - 3% in the average lateral compositional
uniformity for
the x = 0.241 layers. FTIR scans taken on the x = 0.229 material indicated a lateral compositional
variation of
0.0005 over a 1.5 x 1.5 cm* sample. The issue of uniformity along the growth direction was also investigated. measuted
for several samples with the x-value being determined
shows the results obtained for three samples.
SIMS profiles were
using the Bubolac technique.s”
Figure 44
As shown, two of these layers exhibit sharp HgCdTeKdTe
interfaces while the non-ideal interface of the third sample was attributed to not allowing sufficient time for the sample to equilibrate at the growth temperature.
The average x-value of these layers was x = 0.241 and agrees
very well with the results obtained for the x-values using FIIR. shows a systematic increase in the x-value from the CdTekIgCdTe
Closer examination of the variation in x-value interface to the surface, and that this slope is
the major contribution to the variation observed in layer composition. Figure 45a shows the x-value as a function of depth for one of the layers shown in Figure 44. Note that at the CdTeJHgCdTe interface the x-value decreases from x = 1 to x - 0.24. There is also an increase in x-value evident as one approaches the surface of the layer. It is believed that this change is due to either the emissivity change of the substrate surface when HgCdTe growth is initiated or to measurement
inaccuracies in
the SIMS technology. Analysis of the variation in composition (neglecting the interface regions).
was first carried out across the HgCdTe layer thickness
The data obtained for these layers yield x = 0.240 f 0.0022, x = 0.235 f
0.0040 and x = 0.24 f 0.0056. Next, in order to minimize the long term variation in composition and measurement
due to growth
errors the data was fit with a low order (quadratic) polynomial (Figure 45b). The tit was then
subtracted from the original compositional
profile leaving a profile of the short term compositional
variation
210
C. J. Summers
et
al.
-r
1.2 (211)B Hg,.$d,Te/CdTe/GaAs 1.0 +z 2 9
0.8
& s ‘Z .8 E
0.6
0.4
0.2
0
4
Depth (,~rn)~ Figure 44.
SIMS composition profile of three HgCdTe samples.
(Figure 45~).
Analyzing
these profiles yielded variations
in the x-value of f 0.0005, 0.0008 and 0.001,
respectively, for the layers discussed above. This analysis shows that the short term changes in x-value that can be attributed principally to the control over the host gas delivery system (Cd + Te) and that potentially the control of the x-value can be better than f 0.00075.
This is very close to the predicted value from the simple model calculations
described in
Section 1. As a measure of the doping stability achievable with gas source doping technology, SIMS profiles of I-doped CdTe layers also were characterized.
From several layers results were obtained which indicate that
doping variations of less than 1% could be achieved on a routine basis.
8. SUMMARY
A detailed review has been presented of the science and technology issues of the CBE of HgCdTe and related compounds. The HgCdTe CBE system described features a specially designed pumping and purging system, a Hg-PCVS,
custom flow controller
systems for both the host elements and doping species, and special gas
cracker cells for efficient pyrolysis of the precursor gases. used to accurately calibrate system performance
A special RHEED system was also developed and
and to investigate
and optimize
substrate preparation
and
surface growth kinetics. Structural, optical, and electrical measurements
show that high quality HgCdTe alloys with x-
values between 0.0 and 0.4 can be achieved by the CBE technique at growth temperatures ranging from 120 to
Chemical
Beam Epitaxy
211
I 0.25
-
0.20
2 0
025
4.0
0.0
.
5
0.24
8 Q
0.23
..=
E
O
0
8.0
0002 .
0.000
4mo2
I I
I
I
I
I
I”
I ;
C) I
0.0
I 2.0
I
I 4.0
I
6.0
Depth (urn) Figure
185°C.
4%.
SIMS compositional profile of HgCdTe sample: (a) x-value as a function of depth for a HgCdTe layer grown at 180°C. (b) quadratic polynomial fit to data of (a). (c) short term compositional variation profile obtained by subtracting (b) from (a).
HgCdTe layers have been grown with background electron concentrations
low temperature mobilities of up to 9 x 10’ cm*Ns.
These HgCdTe properties equal or exceed those reported
for MBE grown material. The material characterization impurity contamination
as low as 3 x 1014cme3 and
results presented indicate that carbon or organometallic
did not significantly affect the electrical properties of CBE grown HgCdTe, making this
technique viable for infrared detector materials. N-type doping of CdTe and HgCdTe was accomplished
using a chemical beam of ethyliodide.
The films had electron concentrations
ranging from 4.4 x 1Or4 to 5 x lOI* cmm3;some of the highest values
achieved in MBE CdTe and HgCdTe.
Additionally, the highest electron mobilities were reported for the lightly
and heavily doped CdTe layers and the heavily doped HgCdTe layers. Iodine was shown to exhibit an electrical activity between 50% and 100% and to be a slow diffuser.
The near-edge 10 K luminescence
exhibited sharp bound exciton peaks and no long-wavelength
luminescence
spectra of CdTe:I
indicating that CBE iodine doping
C. J. Summers et al.
212
of CdTe is an effective
method to achieve high n-type conductivity
in CdTe and HgCdTe with the well
controlled doping profiles required for advanced device structures. A single study was conducted concentrations
on p-type doping of CdTe using amine. Although
high As
(> 5 x IO’s cme3) were observed in CdTe by SIMS, the electrical activation was low (10” - lOI
cmm3). Preliminary
N-plasma doping experiments
although low activation efficiency
indicated that N may be a suitable p-type dopant for CdTe
may be a problem and again no results were reported for HgCdTe.
This
problem with p-type doping is widespread throughout the HgCdTe community and is believed to result from impurities in poor substrate material. The inability to efficiently dope HgCdTe alloys p-type requires continued research. Additional cutoff wavelengths
superlattice
structures with
in the 2 - 5 pm region.
In addition, capabilities
research reported included the growth of HgCdTe-CdTe
recent studies of the CBE system performance
for the flexible manufacturing
obtained using this growth technique
have demonstrated
its excellent
of advanced HgCdTe material structures and devices.
demonstrate
the exciting potential of chemical
growth of advanced device concepts in HgCdTe, such as the development
The results
beam epitaxy for the
of avalanche photodiode
infrared
detectors for advanced imaging applications and infrared laser systems.
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BIOGRAPHY Christopher J. Summers (M’82) was born in Oxford, England, on August 3, 1940. He received the B.S. and Ph.D. degrees in physics from Reading Univeristy, Reading, England, 1962 and 1966, respectively. After holding postdoctoral fellowship positions at Reading University and Bell Telephone Laboratories, he joined GTE Laboratories in 1970 as a member of the technical staff. In 1972 he moved to McDonnell Douglas Research Laboratories, where he became a Senior Research Scientist. He joined the Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, in 1981 and is currently a GTRI Fellow and Head of the Quantum Microstructures Branch and the Director of the Phosphor Technology Center of Excellence. His current research interests include optoelectronic properties of heterostructures and superlattices, the metalorganic and molecular beam epitaxy of II-VI and III-V semiconductors, and the growth and characterization of phosphor materials. Rudolph Cl. Benz, II was born in Detroit, Michigan, on February 22, 1962. He received the B.S. degree with honors in Engineering Science from the Pennsylvania State University in 1984, and the M.S. and Ph.D degree in physics from the Georgia Institute of Technology in 1988 and 1992, respectively. He is currently a research scientist at the Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta. His research interests include the growth and characterization of II-VI and III-V semiconductors by molecular beam epitaxy, the surface kinetics of epitaxial growth, and the application of GaAs charge transfer devices to solid state imager applications. Brent K. Wagner was born in Pottstown, PA, on June 22, 1962. He received his B.S.h. degree in Engineering Science at the Pennsylvania State University in 1984. He then received his M.S. degree in Physics and his Ph.D. degree in Electrical Engineering at the Georgia Institute of Tehnology in 1987 and 199 1, respectively. He is currently a research scientist at the Georgia Tech Institute, Georgia Insitute of Technology in Atlanta, Georgia. His research interests include the thin film growth and characterization of II-VI and III-V semiconductors and phosphor materials and the application of heterostructures to optoelectronic devices.