Structural properties of InAs/InAs1–xSbx type-II superlattices grown by molecular
beam epitaxy
Lu Ouyang, Elizabeth H. Steenbergen, Yong-Hang Zhang, Kalyan Nunna, Diana L. Huffaker, and David J. Smith
Citation: Journal of Vacuum Science & Technology B 30, 02B106 (2012); doi: 10.1116/1.3672026
View online: http://dx.doi.org/10.1116/1.3672026
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Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Structural properties of InAs/InAs1–xSbx type-II superlattices grown
by molecular beam epitaxy
Lu Ouyanga)
Department of Physics, Arizona State University, Tempe, Arizona 85287

Elizabeth H. Steenbergen and Yong-Hang Zhang
School of Electrical, Computer and Energy Engineering and Center for Photonics Innovation, Arizona State
University, Tempe, Arizona 85287

Kalyan Nunna and Diana L. Huffaker
California NanoSystems Institute, University of California, Los Angeles, California 90095

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

(Received 20 September 2011; accepted 3 December 2011; published 21 December 2011)
Strain-balanced InAs/InAs1xSbx type-II superlattices (SLs) have been proposed for possible longwavelength infrared applications. This paper reports a detailed structural characterization study of
InAs/InAs1xSbx SLs with varied Sb composition grown on GaSb (001) substrates by modulated
and conventional molecular beam epitaxy (MBE). X-ray diffraction was used to determine the SL
periods and the average composition of the InAs1xSbx alloy layers. Cross-section transmission
electron micrographs revealed the separate In(As)Sb/InAs(Sb) ordered-alloy layers within individual InAs1xSbx layers for SLs grown by modulated MBE. For the SLs grown by conventional
MBE, examination by high-resolution electron microscopy revealed that interfaces for InAs1xSbx
deposited on InAs were more abrupt, relative to InAs deposited on InAs1xSbx: this feature was
attributed to Sb surfactant segregation occurring during the SL growth. Overall, these results establish that strain-balanced SL structures with excellent crystallinity can be achieved with proper
C 2012
design (well thickness versus Sb composition) and suitably optimized growth conditions. V
American Vacuum Society. [DOI: 10.1116/1.3672026]

I. INTRODUCTION
Type-II superlattices (SLs) have attracted much interest
from the semiconductor industry since first being proposed.1
Antimonide-based type-II SLs have been recognized as possible low-cost alternatives to the HgCdTe materials system
for infrared (IR) applications, due to several key advantages
including lower tunneling current,2 greater flexibility in
bandgap engineering,3 and reduced Auger recombination.3
Extensive investigations of InAs/(In)GaSb type-II SLs have
been carried out, including theoretical calculations of the
band structure4 and minority carrier lifetimes,5 and significant success has been achieved for device performance in
mid-, long- and very-long-wavelength infrared (VLWIR)
ranges.6–9 Strain-balanced InAs/InAs1xSbx SLs have been
proposed as another possible alternative to HgCdTe,10 and
have already shown great promise for mid-IR laser and photodetector structures,11 with photoluminescence emission in
the range of 5–10 lm being achieved for SL structures containing Sb concentrations of 1427%.12 Recently, InAs/
InAs1xSbx SLs grown by metalorganic chemical vapor
depostition (MOCVD) have been studied in more detail:
these SLs showed excellent structural properties and strong
optical response.13,14 The absence of gallium in these InAs/
InAs1–xSbx SLs is expected to simplify the SL interfaces and
a)

Author to whom correspondence should be addressed; electronic mail:
louyang@asu.edu

02B106-1

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

hence the growth process,13 and also result in longer carrier
lifetimes,15,16 as very recently demonstrated.17
In the growth of mixed As/Sb alloys by conventional
solid-source molecular beam epitaxy (MBE), the As2 and
Sb2 beam flux ratio is normally used to control the average
group-V composition of the epitaxial material. However,
due to large differences in the incorporation coefficients of
As2 and Sb2, accurate control of composition in the
InAs1xSbx alloys becomes challenging. Growth by modulated MBE involves control of As and Sb incorporation by
rapidly alternating the As2 and Sb2 beam flux, using the timing of shutter operation to control the group-V composition.18 This growth technique could possibly provide more
precise control and reproducibility of the group-V alloy
composition in the InAs1xSbx layers of InAs/InAs1xSbx
SLs. Improvements in composition control and crystalline
quality of AlAsxSb1x and In(Ga)AsxSb1x alloys grown by
modulated MBE have been reported.11,19–23 Further advantages of modulated MBE are that the technique could provide protection against composition drift and achieve more
abrupt interfaces,23 which are important factors to take into
consideration since the transition wavelength and recombination efficiency are expected to be strongly influenced by
the compositional abruptness at the interfaces.24,25
High quality InAs/InAs1xSbx SLs require sharp and
defect-free interfaces between InAs and InAs1xSbx layers.
In this study, strain-balanced InAs/InAs1xSbx type-II SLs
on GaSb (001) substrates with different Sb compositions, but

1071-1023/2012/30(2)/02B106/5/$30.00

C 2012 American Vacuum Society
V

02B106-1

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02B106-2 Ouyang et al.: Structural properties of InAs/InAs1xSbx type-II superlattices

FIG. 1. (Color online) Schematic illustration of the InAs/InAs1xSbx SL
structure design for: (a) ordered-alloy and (b) random alloy.

the same layer thicknesses, were designed for LWIR detectors of different wavelengths. Transmission electron microscopy (TEM) was used to investigate the microstructure of
InAs/InAs1xSbx SLs with ordered and random InAs1xSbx
alloy layers grown by modulated and conventional MBE,
respectively. X–ray diffraction (XRD) was used to determine
the average composition of the InAs1xSbx alloy layers and
the SL periods. Further information including details of the
SL structure design and measurements of the optical properties of these samples is being reported elsewhere.26
II. EXPERIMENTAL DETAILS
The SL samples were grown in a solid-source MBE system, using modulated and conventional MBE growth techniques. All samples nominally consisted of a 0.2-lm-thick
GaSb buffer layer grown on an n-type GaSb (001) substrate
at 500  C, followed by deposition of 7-nm-thick InAs1xSbx
layers alternating with 18-nm-thick InAs layers for 20 periods, at a growth temperature of 435  C. Finally, a 0.1-lmthick GaSb capping layer was deposited at 480  C. Figure 1
shows a schematic of the sample structures. The modulated
and conventional MBE growths are expected to yield ordered and random InAs1xSbx alloy layers, respectively.
For the modulated MBE growth, the In shutter was kept
open throughout the entire period of growth of the
InAs1xSbx layers, while the As and Sb shutters were alternately opened and closed for very short periods. The overall
Sb composition would then be controlled by the Sb-shutter
duty-cycle: Sbshutter-time/(Asshutter-time þ Sbshutter-time). Each
of these ordered InAs1xSbx alloy layers consisted of sixperiod In(As)Sb/InAs(Sb) multiple quantum well (MQW)
layers. The relevant growth parameters are listed in Table I.
The high-resolution XRD measurements were performed
using a PANalytical X’pert Pro MRD, and the average Sb

02B106-2

compositions of the InAs1xSbx alloys were determined by
comparing simulations to (004) high-resolution XRD measurements. Cross-sectional specimens were prepared for TEM
observation using mechanical polishing and dimpling followed by argon ion-milling. The mechanical polishing and
dimpling typically reduced the sample thicknesses to
1012 lm, and small holes in the films were then made by
ion-milling at low energy (2.53 keV), using a liquid-nitrogen-temperature cooling stage to minimize any thermal or
ion-beam damage.27 The TEM characterization studies were
mostly carried out using a JEM-4000EX high-resolution
electron microscope operated at 400 keV and with a structural resolution of 1.7 Å. All samples were prepared for observation along {110}-type zone-axis projections so that the
direction of the electron beam would be aligned perpendicular to the growth surface normal.
III. RESULTS AND DISCUSSTION
The InAs/InAs1xSbx SLs samples that are described in
this paper, which include two ordered alloy SLs and two random alloy SLs, are summarized in Table I. The SL periods
and the average Sb compositions of the two random
InAs1xSbx alloy samples, as determined by high-resolution
XRD measurements are also listed. The XRD patterns of
Samples A and B are shown in Fig. 2, together with the corresponding simulations offset below each experimental profile.
The most intense SL peak for Sample A no longer corresponds to the zero-order peak because of the large SL period.
The XRD of the thick individual InAs1xSbx and InAs layers
show the envelope modulation of the SL peaks on either side
of the substrate peak, causing some satellite peaks to be more
intense than the zero-order SL peak. From the separation of
the substrate peak from the zero-order SL peak, the average
SL strain in the growth direction is determined to be 0.18%
for Sample A. In comparison, Sample B has broader peaks
than Sample A, suggesting the presence of increased defect
density, which was later confirmed in the TEM microstructure studies.
Cross-sectional electron micrographs of Sample A, as for
example shown in Fig. 3, revealed excellent crystallinity and
a very low density of growth defects, and indicated an SL
periodicity of 245 Å, which was in close agreement with the
results of the XRD simulation. Higher magnification images
of the SL, such as Fig. 4, showed sharp interfaces between
the individual InAs1xSbx and InAs layers, and interfacial
misfit dislocations were not observed. Moreover, the
ordered–alloy structure, consisting of six-period In(As)Sb/
InAs(Sb) MQW layers, is clearly visible within each

TABLE I. Summary of SL samples reported in this investigation.
Sample

MBE growth

Sb shutter-time (second)

Sb shutter duty-cycle

Flux ratio Sb/(SbþAs)

SL period (nm)

xsb

A
B
C
D

Modulated
Modulated
Conventional
Conventional

3
4
—
—

35%
47%
—
—

—
—
0.35
0.37

24.5
24.1
24.6
24.6

—
—
0.28
0.29

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02B106-3 Ouyang et al.: Structural properties of InAs/InAs1xSbx type-II superlattices

02B106-3

FIG. 4. Higher-magnification image clearly showing well-ordered In(As)
Sb/InAs(Sb) MQW structure within the individual InAs1xSbx layers of
Sample A.

FIG. 2. (Color online) High-resolution XRD (004) x-2h profiles and corresponding simulations (offset below each measurement) for Samples A and B,

structure was only well-defined within the first InAs1xSbx
layer and progressively disappeared within the later
InAs1xSbx layers, as shown in Fig. 5(b). Defective regions
were visible within the SL structure that propagated upwards
similar to the behavior of self-aligned quantum dots in
lattice-mismatched systems, but opposite to the conventional
domelike appearance. Most of these defects originated from
the first SL layer, and are possibly due to the longer Sb shutter time used for this sample compared to Sample A. In

InAs1xSbx layer. These layers are very well defined, with
the darker lines corresponding to the In(As)Sb layers.
In the case of Sample B, which is another SL sample
grown by modulated MBE when the growth conditions were
not yet fully optimized, low magnification images revealed a
more defective SL system. As shown in Fig. 5(a), the substrate/buffer interface is clearly visible, and it is not flat,
although this lack of flatness does not apparently cause
any defects in the SL layers. However, the ordered-alloy

FIG. 3. Cross-sectional electron micrograph showing entire SL structure of
Sample A, and confirming very low defect density.

FIG. 5. (a) Low magnification image of Sample B showing typical defective
region: (b) Higher-magnification image showing the presence of the In(As)Sb/
InAs(Sb) MQW structure apparently only within the first InAs1xSbx layer.

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02B106-4 Ouyang et al.: Structural properties of InAs/InAs1xSbx type-II superlattices

02B106-4

FIG. 6. Low magnification image of Sample C showing the entire structure.

addition, some extensive {111}-type stacking faults were
observed propagating upwards through several SL periods to
the top of the SL layer. The disappearance of the welldefined ordered-alloy structure within subsequent
InAs1xSbx layers is possibly due to intermixing of As and
Sb atoms induced by the strain associated with propagating
defects.
Low magnification images of Sample C, which was a random alloy SL that was again not grown under fully optimized conditions, revealed the entire structure including the
GaSb (001) substrate and buffer layer, 20-period SL layers,
and the GaSb capping layer, as shown in Fig. 6. Growth
defects, such as {111}-type stacking faults, are clearly visible. Most of these defects originated at the substrate/buffer
interface, but some originated in the middle of the buffer
layer, and many were present within the SL layers.
In the case of Sample D, cross-section electron micrographs typically showed no evidence of any defects across
the entire field of view and demonstrated that excellent crystalline quality of the InAs/InAs0.71Sb0.29 SL could be
achieved. The entire structure showing 20 well-defined SL
periods is visible in Fig. 7. The substrate/buffer interface
was again observed not to be flat, but no defects in the buffer

FIG. 7. Cross-sectional electron micrograph of Sample D demonstrating the
well-defined and defect-free InAs/InAs0.71Sb0.29 SL region.

FIG. 8. (Color online) (a) High-resolution lattice image showing individual
InAs and InAs0.71Sb0.29 layers for sample D; (b) Filtered image revealing
different interface abruptness between layers, which is attributed to segregation of the Sb surfactant during growth; (c) Line profile average across both
InAs-on-InAsSb and InAsSb-on-InAs interfaces, as indicated by blue dashed
area boxed above, showing the differences in interface abruptness.

layer have been caused by this lack of flatness. (It is worth
noting here that such homoepitaxial interfaces are not always
visible in the TEM images of samples grown under optimal
conditions when there is complete desorption of the surface
oxide layer present on the GaSb substrate surface.)
High-resolution lattice images of Sample D, such as Fig.
8(a), clearly show the individual InAsSb and InAs layers. As
confirmed by the Fourier-filtered image of the indicated area
shown in Fig. 8(b), it is also apparent that the interfaces of
the InAs0.71Sb0.29 layers grown on InAs are more abrupt
than those of InAs layers grown on InAs0.71Sb0.29. Figure
8(c) is a line profile averaging across both InAs-on-InAsSb
and InAsSb-on-InAs interfaces, as indicated by the blue
dashed area boxed on Fig. 8(b). It is clear that the intensity
drops rapidly at the InAsSb-on-InAs interface, while the intensity increases much more slowly at the InAs-on-InAsSb
interface. This interface asymmetry in the SL layers is most

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02B106-5 Ouyang et al.: Structural properties of InAs/InAs1xSbx type-II superlattices

likely related to Sb segregation, whereby some unintentional
Sb is initially incorporated into the InAs layers during
growth, as reported previously.20,28
IV. SUMMARY
The structural properties of strain-balanced InAs/
InAs1xSbx SLs grown by MBE on GaSb (001) substrates,
have been studied using XRD and TEM. Excellent atomicscale structural quality of the InAs/InAs1xSbx SLs has been
demonstrated by TEM. Well-defined ordered–alloy In(As)Sb/
InAs(Sb) MQW layers within individual InAs1xSbx layers,
were observed for samples grown by modulated MBE. However, the ordering disappeared when defects propagating
through the SL layers appeared during growth. For samples
grown by conventional MBE, high-resolution images
revealed that interfaces for InAs1xSbx grown on InAs layers
were sharper than for InAs grown on InAs1xSbx layers,
most likely due to some Sb surfactant segregation effect
which warrants further investigation. Overall, the microstructural results are highly promising for the future growth of
InAs/InAs1xSbx SLs designed for operation at specific
wavelengths.
ACKNOWLEDGMENTS

The work at ASU was primarily supported by the MURI
Grant No. W911NF-10-1-0524 administered by U.S. Army
Research Office, and monitored by William W. Clark. The
authors gratefully acknowledge the use of facilities in the
John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University.
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