Influence of temperature ramp on the materials properties of GaSb grown on ZnTe
using molecular beam epitaxy
Jin Fan, Lu Ouyang, Xinyu Liu, Ding Ding, Jacek K. Furdyna, David J. Smith, and Yong-Hang Zhang
Citation: Journal of Vacuum Science & Technology B 30, 02B122 (2012); doi: 10.1116/1.3681280
View online: http://dx.doi.org/10.1116/1.3681280
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/30/2?ver=pdfcov
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Influence of temperature ramp on the materials properties of GaSb grown
on ZnTe using molecular beam epitaxy
Jin Fan and Lu Ouyang
Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287 and Department of Physics,
Arizona State University, Tempe, Arizona 85287

Xinyu Liu
Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556

Ding Ding
Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287 and School of Electrical,
Computer and Energy Engineering, Arizona State University, Tempe, Arizona 85287

Jacek K. Furdyna
Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556

David J. Smith
Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287 and Department of Physics,
Arizona State University, Tempe, Arizona 85287

Yong-Hang Zhanga)
Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287 and School of Electrical,
Computer and Energy Engineering, Arizona State University, Tempe, Arizona 85287

(Received 21 October 2011; accepted 9 January 2012; published 15 February 2012)
This paper reports high-quality GaSb grown on ZnTe using molecular beam epitaxy with a temperature
ramp during growth, and investigates the influence of the temperature ramp on material properties.
During growth, in situ reflection-high-energy electron diffraction shows rapid and smooth transition
from ZnTe surface reconstruction to GaSb surface reconstruction. Post-growth structural characterization
using x-ray diffraction and transmission electron microscopy reveals smooth interface morphology and
low defect density. Strong photoluminescence emission is observed up to 200 K. The sample grown
with a temperature ramp from 360 to 470  C at a rate of 33  C/min showed the narrowest bound exciton
C 2012 American Vacuum Society.
emission peak with a full width at half maximum of 15 meV. V
[DOI: 10.1116/1.3681280]

I. INTRODUCTION
A new material platform consisting of 6.1 Å semiconductors grown on GaSb substrates has recently been proposed
for optoelectronic devices.1,2 This family of materials contains both II-VI (MgZnCdHg)(SeTe) and III-V (InGaAl)(AsSb) compound semiconductors, which have direct
bandgaps spanning the entire energy spectrum from farinfrared (0 eV) up to ultra-violet (3.4 eV). Due to the
high cost and limited size of commercial GaSb and ZnTe
substrates, ZnTe epilayers grown on GaAs and Si have been
proposed and successfully demonstrated as virtual substrates.3,4 Realization of the growth of high-quality GaSb on
ZnTe virtual substrates should enable monolithic integration
of InAs- and GaSb-based semiconductor devices, such as
mid-wavelength infrared (IR) laser diodes and longwavelength IR photodetectors composed of InAs/InAsSb
and InAs/(In)GaSb type-II superlattices,5 on large low-cost
GaAs or Si substrates. Although some preliminary studies of
the growth of GaSb on ZnTe have been reported,6,7 realization of high-quality crystalline materials of GaSb on ZnTe
still remains challenging.
In the research reported here, a temperature ramp during
the growth of GaSb on ZnTe has been used in an effort to
a)

Electronic mail: yhzhang@asu.edu

02B122-1

J. Vac. Sci. Technol. B 30(2), Mar/Apr 2012

achieve better material quality. To investigate the influence
of the growth temperature ramp on the structural and optical
properties of GaSb on ZnTe, a set of samples was grown on
ZnTe/GaSb (001) substrates under different growth conditions using molecular beam epitaxy (MBE). During MBE
growth, in situ reflection-high-energy electron diffraction
(RHEED) was used for growth monitoring and optimization.
High-resolution x-ray diffraction (XRD) measurements were
performed to determine the structural quality of the GaSb
epilayers, and transmission electron microscopy (TEM) was
used to study the surface morphology of the GaSb epilayers
and misfit dislocations at GaSb/ZnTe interfaces. Photoluminescence was also applied to characterize the optical properties of GaSb epilayers.
II. EXPERIMENTAL DETAILS
The epitaxial growth was carried out using an MBE system consisting of II-VI and III-V chambers connected with
an ultrahigh-vacuum (UHV) transfer module. The vacuum
of the transfer chamber was typically maintained at about
5  109 Torr to prevent significant contamination during
sample transfer. The growth temperatures (Tg) were measured with a thermocouple positioned on the back of the substrate holder. For the growth of the samples described in this
paper, typically a thin ZnTe epilayer was first grown on a

1071-1023/2012/30(2)/02B122/5/$30.00

C 2012 American Vacuum Society
V

02B122-1

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02B122-2 Fan et al.: Influence of temperature ramp on the materials properties of GaSb grown on ZnTe

GaSb (001) substrate, as described previously.2,3 After the
growth of ZnTe in the II-VI chamber, the wafer was transferred to the III-V chamber for the GaSb growth. Since the
commonly used temperature for the growth of GaSb
(Tg ¼ 470  C) is considerably higher than that used for ZnTe
(Tg ¼ 320  C), the surface of ZnTe layer is likely to be
severely degraded if the growth of GaSb on ZnTe is initiated
at such a high temperature. To overcome this problem,
growth of a thin GaSb transition layer under a temperature
ramp was first carried out before the remaining GaSb epilayer was deposited at the normal growth temperature. To
further investigate the influence of different temperature
ramps on the material quality, separate GaSb epilayers were
grown on ZnTe under different growth conditions. The
growth parameters used for these samples are summarized in
Table I.
During growth, the beam equivalent pressure (BEP) ratios
of Ga to Sb were adjusted by monitoring the surface reconstructions observed using RHEED. As an example, RHEED
patterns for sample # 3 are shown in Fig. 1. The RHEED patterns showed the typical (2  1) reconstruction for the ZnTe
surface before growth of the GaSb transition layer. At the
initial stages of GaSb growth, the RHEED measurements
showed spotty and unclear patterns. The typical (1  3)
GaSb surface reconstruction started to appear after 10 s. A
bright and streaky RHEED pattern with clear (1  3) surface
reconstruction became clearly visible after  60 s, indicating
a smooth transition from the ZnTe surface reconstruction to
the GaSb surface reconstruction.
The cross-sectional TEM samples were prepared using
mechanical polishing and dimpling followed by ion-beam
thinning. To minimize any artifactual milling damage, liquid
nitrogen and low-energy (2.5–3 keV) ion beams were used
during the sample preparation. Images were recorded using a
JEM-4000EX TEM operated at 400 keV with a structural resolution of 1.7 Å. The high-resolution XRD x2h scans
were performed using a PANalytical X’Pert PRO MRD xray diffractometer with multicrystal monochromator. The
Ka1 line of copper (1.54 Å) was used as the incident beam.
For characterization of optical properties, PL measurements
were carried out using the 780 nm line of a laser diode for
excitation. A Fourier transform infrared spectrometer configured with a quartz beam splitter and liquid-nitrogen-cooled
InSb detector was used for detection.
III. RESULTS AND DISCUSSION
After completion of growth, high-resolution XRD measurements were performed on all samples in the vicinity of

02B122-2

the (004) diffraction peak of the GaSb substrate. Figures 2(a)
and 2(b) show XRD patterns for samples #1 and #3. These
patterns show clear diffraction peaks from the ZnTe epilayer,
the GaSb epilayer, and the GaSb substrate. For sample #1,
the nominal thicknesses of the GaSb and ZnTe epilayers
were 490 and 133 nm, respectively, as determined by the
MBE growth rates. The thickness of GaSb epilayer is also
calculated to be 490 nm based on the period of the XRD Pendellösung thickness fringes. For comparison, a simulated
x2h curve is also plotted, which shows good agreement
with the experimental data. It is also apparent in Fig. 2(a)
that the diffraction peak of GaSb epilayer is on the right side
of the GaSb substrate peak, which indicates that the vertical
lattice parameter (a\) of the GaSb epilayer is smaller than
that of the GaSb substrate. The simulation result indicates
that the ZnTe epilayer is partially relaxed ( 8%). Thus, the
GaSb epilayer is subjected to a tensile strain when it is
grown on the partially relaxed ZnTe layer, leading to the
smaller vertical lattice constant. For sample #3, the thicknesses of GaSb and ZnTe epilayers are determined to be 380
and 300 nm, respectively, based on the XRD simulation,
matching the nominal thicknesses derived from the growth
rates. The simulated x2h curve shows that the diffraction
fringes are a combination of Pendellösung thickness fringes
from both GaSb and ZnTe epilayers, indicating high-quality
of GaSb and ZnTe single-crystal epitaxial layers with
smooth interfaces, uniform thicknesses, and low defect
densities.
Cross-sectional TEM has been used to investigate the
ZnTe/GaSb and GaSb/ZnTe interface morphology, especially interfacial misfit dislocations. As visible in Figs. 3(a)
and 3(b), low-magnification TEM images of samples #3 and
#4 demonstrate excellent crystallinity as well as smooth
morphology for both ZnTe/GaSb and GaSb/ZnTe interfaces.
This observation clearly establishes that use of the GaSb
transition layer grown with a temperature ramp prior to normal growth effectively prevents the GaSb/ZnTe interface
from being damaged. Furthermore, the images reveal no
misfit dislocations or stacking faults at either of the ZnTe/
GaSb or GaSb/ZnTe interfaces, indicating that they are
highly coherent with very low defect density, as expected
due to the very small lattice mismatch between ZnTe and
GaSb ( 0.13%). In addition, the thicknesses of GaSb and
ZnTe layers were also directly measured from the TEM
images. For sample #3, Fig. 3(a) shows that the GaSb and
ZnTe layers are 380 and 300 nm thick, respectively, which
confirms the XRD simulation results shown in Fig. 2(b).
PL measurements were carried out to investigate the
optical properties of the GaSb epilayers. The PL spectra of

TABLE I. Growth parameters for GaSb samples.
Sample No.
1
2
3
4

Growth temperature

Temperature ramp

Ramping rate

Ga/Sb BEP ratio

Growth rate

380  C
470  C
470  C
470  C

NA
380–470  C
360–470  C
320–470  C

NA
27  C/min
33  C/min
45  C/min

1:5
1:5
1:5
1:5

0.8 lm/h
0.8 lm/h
0.8 lm/h
0.8 lm/h

J. Vac. Sci. Technol. B, Vol. 30, No. 2, Mar/Apr 2012

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02B122-3 Fan et al.: Influence of temperature ramp on the materials properties of GaSb grown on ZnTe

02B122-3

FIG. 1. RHEED patterns for growth of GaSb on top of ZnTe. Patterns in upper and lower rows are referred to [110] and [1
10] axes, respectively.

all samples, as measured at 13 K, are shown in Figs.
4(a)–4(d). Low-temperature PL of GaSb has been well studied by many authors,8–13 and there are about 20 reported
transitions in the range of 680–810 meV.8–11 Among these
transitions, three main PL lines are often observed and these
have been discussed in optical characterization measurements: (i) A PL line with maximum at 796 meV, denoted as

FIG. 2. (Color online) XRD x-2h curves measured in the vicinity of the
(004) diffraction peak of GaSb substrate for: (a) sample #1 and (b) sample
#3.

FIG. 3. Low-magnification TEM images for (a) sample #3 and (b) sample
#4.

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02B122-4 Fan et al.: Influence of temperature ramp on the materials properties of GaSb grown on ZnTe

“BE” or “BE4,” which is considered as an emission of an
exciton bound to a nonspecified neutral acceptor.12 (ii) A PL
line with maximum at 777 meV, denoted as the “A” line,
which is ascribed to recombination at a native acceptor level
(A) via band-acceptor (BA) or donor-acceptor pair (DAP)
transitions.8,13 (iii) A PL line with maximum around
758 meV, denoted as the “B” line, which is interpreted as a
transition from another acceptor level (B).8,13
As shown in Fig. 4, sample #1 has very weak emissions
from 793 to 807 meV, which are ascribed to the transitions
from bound excitons.9,11–13 The dominant PL feature is the
emission in the range of 570–780 meV with peaks positioned
around 637 and 728 meV. Since the growth of GaSb for sample #1 is carried out at lower temperature (Tg ¼ 380  C),
rather than the normal temperature (Tg ¼ 470  C), this broad
emission is therefore attributed to optical transitions related
to defects generated during the low-temperature growth. For
sample #2, the intensity of emission from the bound exciton
is increased by about 20 times relative to that of sample #1.
A broad emission is observed in the range of 570–780 meV.
Within this range, emission peaks are found around 777 and
758 meV, with intensities as strong as that from the bound
exciton. Thus, this broad emission is attributed to optical
transitions from acceptor “A” and “B,” and other growthrelated defects. For sample #3, which contains the GaSb
transition layer grown under Tramp ¼ 360–470  C, the PL
spectrum shows a narrow peak at 793 meV from the bound
exciton with full width at half maximum (FWHM) of
15 meV. Similarly, a broad emission is observed between
650 meV and the bound exciton peak, which is attributed to
emissions with the same origins as sample #2. Meanwhile, it
is also noticed that this emission is greatly depressed in intensity and energy range (650–780 meV), which suggests a

02B122-4

large decrease in the density of impurities and defects. For
sample #4, the main PL feature is in the range of
570–850 meV. The emissions from acceptor “A” and bound
“B” excitons are not well resolved in this case. Since the
GaSb epilayers in this study were subjected to tensile strain
when they were being grown on ZnTe layers, the change in
bandgap of the strained GaSb can be estimated using the
Luttinger–Kohn model and unitary transformation
method.14–16 The calculations show that the bandgaps of the
GaSb epilayers are decreased due to tensile strain by
2.8 meV for sample #1 and by 3.4 meV for samples #2–#4.
The experimental result shows the bound exciton peak
energy of sample #3 (793 meV) is 3 meV smaller than that of
bulk GaSb (796 meV), which is in a good agreement with
the calculation (3.4 meV). For sample #1, however, the PL
spectrum does not show the expected shift in emission
energy, which will be further investigated.
By comparing the PL spectra between sample #1 and
samples #2 – #4, the latter show highly increased PL intensities, which indicates that use of the GaSb transition layer
grown with a temperature ramp significantly improves the
overall optical properties of GaSb. From closer comparison
among samples #2, #3, and #4, it is also apparent that different temperature ramps affect optical properties differently.
When the starting point of the temperature ramp is close to
the growth temperature of ZnTe, the GaSb/ZnTe interface is
expected to be less damaged while the optical properties of
GaSb will be more deteriorated due to defects generated during the low-temperature growth. On the other hand, when
the starting point of the temperature range is close to the
growth temperature of GaSb, the ZnTe surface is more damaged during the initial GaSb growth so that the optical properties of GaSb are adversely affected due to the interfacial

FIG. 4. (Color online) PL spectra measured at 13 K with excitation density of 16 W/cm2 for (a) sample #1, (b) sample #2, (c) sample #3, and (d) sample #4.
J. Vac. Sci. Technol. B, Vol. 30, No. 2, Mar/Apr 2012

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02B122-5 Fan et al.: Influence of temperature ramp on the materials properties of GaSb grown on ZnTe

02B122-5

tions or stacking faults were observed at the interfaces.
Strong PL emission from GaSb is observed from 13 to
200 K. The PL spectra show that the proposed GaSb transition layer grown on ZnTe while using a temperature ramp
significantly improves the overall optical properties of GaSb.
ACKNOWLEDGMENTS

FIG. 5. Temperature-dependent PL spectra measured from 13–200 K with
excitation density of 16 W/cm2 for sample #3.

defects. Thus, it can be concluded that a temperature ramp
starting from a reasonable compromise temperature, which
is neither too close to the ZnTe growth temperature nor to
the GaSb growth temperature, will shield the GaSb/ZnTe
interface from severe damage while getting the temperature
close enough to the normal GaSb growth temperature. As a
result, excellent optical properties of GaSb can be achieved.
Temperature-dependent PL spectra of sample #3 are
shown in Fig. 5. As the temperature is increased, the band
edge-related PL peak from GaSb shows redshift due to the
decrease in the bandgap energy, while the FWHM of the PL
peak becomes broader as expected. In addition, the intensity
of emission from defects and impurities gradually decreases
and disappears above 140 K, which suggests that the nonradiative recombination mechanism is activated.
IV. SUMMARY
The MBE growth of high quality GaSb layers on ZnTe/
GaSb (001) composite substrates has been demonstrated.
High-resolution XRD results show clear Pendellösung thickness fringes from both GaSb and ZnTe epilayers and simulations fit the experimental data very well. TEM images show
excellent crystallinity and smooth morphology for both
ZnTe/GaSb and GaSb/ZnTe interfaces. No misfit disloca-

The work at ASU was partially supported by Science
Foundation Arizona, Contracts SRG 0339-08 and AFOSR
Grant No. FA9550-10-1-0129. ASU and Notre Dame were
also jointly supported by the Air Force Research Laboratory/
Space Vehicles Directorate (Contract No. FA9453-08-20228) and an NSF grant (Grant No. ECCS-1002072). The
authors gratefully acknowledge the use of facilities in the
John M. Cowley Center for High Resolution Electron Microscopy, the Center for Solid State Electronics Research,
and the LeRoy Eyring Center for Solid State Science at
Arizona State University.
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