Structural and optical characterization of type-II InAs/InAs1−xSbx superlattices grown
by metalorganic chemical vapor deposition
E. H. Steenbergen, Y. Huang, J.-H. Ryou, L. Ouyang, J.-J. Li, D. J. Smith, R. D. Dupuis, and Y.-H. Zhang
Citation: Applied Physics Letters 99, 071111 (2011); doi: 10.1063/1.3625429
View online: http://dx.doi.org/10.1063/1.3625429
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/99/7?ver=pdfcov
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APPLIED PHYSICS LETTERS 99, 071111 (2011)

Structural and optical characterization of type-II InAs/InAs12xSbx
superlattices grown by metalorganic chemical vapor deposition
E. H. Steenbergen,1,2 Y. Huang,3 J.-H. Ryou,3 L. Ouyang,1,4 J.-J. Li,1,2 D. J. Smith,1,4
R. D. Dupuis,3 and Y.-H. Zhang1,2,a)
1

Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287, USA
School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe,
Arizona 85287, USA
3
Center for Compound Semiconductors and School of Electrical and Computer Engineering, Georgia Institute
of Technology, Atlanta, Georgia 30332-0250, USA
4
Department of Physics, Arizona State University, Tempe, Arizona 85287, USA
2

(Received 9 June 2011; accepted 25 July 2011; published online 18 August 2011)
Strain-balanced type-II InAs/InAs1–xSbx superlattices with various compositions (x ¼ 0.22, 0.23,
0.37) and different layer thicknesses (tInAs ¼ 7 nm, tInAsSb ¼ 3.3, 2.3, 2.0 nm, respectively) have been
grown by metalorganic chemical vapor deposition on GaSb substrates. X-ray diffraction revealed
narrow satellite peaks (full-width-half-maximum of <100 arc sec), indicative of uniform superlattice
periodicity and excellent crystallinity, which was also corroborated by cross-sectional transmission
electron microscopy observations. Despite relaxation, low-temperature photoluminescence
measurements showed peaks at 6.7 lm and 5.8 lm, while photoconductance results showed strong
C 2011 American Institute
spectral response up to 200 K, when the photoresponse onset was 8.6 lm. V
of Physics. [doi:10.1063/1.3625429]
Type-II superlattices (SLs) have been extensively investigated for infrared applications since their initial proposal1,2
and the first InAs/GaSb SL experimental demonstration.3
These SLs enable energy transitions that are smaller than the
bandgaps of the constituent materials, even far beyond the
smallest bandgap of any unstrained bulk III-V material, which
is 9 lm for InAs0.39Sb0.61 at 77 K.4 The following advantages
make III-V SL photodetectors viable alternatives for expensive
HgCdTe infrared detectors: greater control of the alloy composition and layer thickness, resulting in more uniform materials
and cutoff wavelengths;5 stronger bonds and structural stability;6 less expensive, closely lattice-matched substrates, i.e.,
GaSb;7 mature III-V growth and processing technology;7
lower band-to-band tunneling due to larger electron effective
mass;5 and strain engineering in combination with larger effective masses reducing Auger recombination.5,7–9 Recently, mid
and long-wavelength infrared (LWIR) focal plane arrays using
InAs/(In)GaSb SLs have been demonstrated by several
groups.10–14 The performance of InAs/(In)GaSb SLs is
approaching that of HgCdTe,7 but the minority carrier lifetime
of the InAs/(In)GaSb SLs is limited by Shockley-Read-Hall
(SRH) recombination and the background carrier concentration is considerably higher than that of HgCdTe.15
InAs/InAs1xSbx SLs represent another alternative for
infrared laser16 and detector applications due to possible lower
SRH recombination,17 and the absence of gallium, which simplifies the SL interfaces and the growth process.18 An ideal
theoretical comparison of a 10-lm InAs/InAs1–xSbx SL with
an 11-lm InAs/InxGa1xSb SL on GaSb substrates revealed
that the performance of the InAs/InxGa1–xSb SL only slightly
exceeds that of the InAs/InAs1–xSbx SL so that the real distinction between choice of materials will come from practical,
growth-related variations.17 With the major improvements in
molecular beam epitaxy (MBE) and metalorganic chemical
a)

Electronic mail: yhzhang@asu.edu.

0003-6951/2011/99(7)/071111/3/$30.00

vapor deposition (MOCVD) technologies in the last couple of
decades, it is an ideal time to investigate the InAs/InAs1–xSbx
SL system experimentally. MOCVD technology compared
to MBE has very high throughput, which is good for mass
production, and thus is worth investigating despite it being
a challenge to grow high-quality antimonides compared to
MBE at present.
Photoluminescence (PL) near 10 lm has been reported
for a 4 lm thick InAs0.62Sb0.38/InAs0.54Sb0.46 SL grown by
MBE on GaSb using a graded SL for a strain-balancing buffer
layer,19 a 2 lm thick InAs/InAs0.61Sb0.39 SL grown by MBE
on GaAs with an InAs0.80Sb0.20 buffer,20,21 and most recently
a 6-period InAs/InAs0.733Sb0.267 SL grown by MOCVD on an
InAs0.91Sb0.09 buffer on GaSb.22 To be suitable for infrared
detectors, thick, high-quality materials are necessary, which
can be achieved via strain-balancing the individual SL layers
on the substrate to avoid misfit dislocations. GaSb is the ideal
substrate for strain-balancing InAs/InAsSb SLs due to its lattice constant being between that of the two layers, eliminating
the need for complicated metamorphic buffer layers and thus
simplifying the growth process.18 As the Sb concentration in
the InAs1xSbx layer increases, the strain of the layer on GaSb
increases making the growth more difficult; but reaching
LWIR wavelengths (8–12 lm) requires higher Sb concentrations to maintain larger electron-hole wave function overlaps
for stronger absorption. We report here a detailed study of the
structural and optical properties of thick (50 or 100 periods),
InAs/InAs1xSbx type-II SLs with varying Sb compositions
(x ¼ 0.22, 0.23, 0.37) grown by MOCVD on GaSb substrates.
To design the InAs/InAs1xSbx SL transition wavelengths for infrared applications, the band edge alignment
with strain effects was first calculated. Then strain-balanced
thicknesses for the InAs and InAs1xSbx layers strained on
GaSb were determined using the thickness-weighted method
to achieve zero average strain in the growth direction.
Finally, the Kronig-Penney model was solved to obtain the

99, 071111-1

C 2011 American Institute of Physics
V

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071111-2

Steenbergen et al.

Appl. Phys. Lett. 99, 071111 (2011)

TABLE I. Structural details for the superlattices studied in this work.
Thickness(nm)
Sample
A: 3-2006
B: 3-2008
C: 3-2009

g

x 6 1(%)

InAs

InAsSb

Number of
periods

Calculated E
at 0 K(meV)

Photoresponse onset
at 10 K(meV)

PL peak position
at 6 K(meV)

22
23
37

7.0
7.0
7.0

3.3
2.3
2.0

100
50
50

224
250
171

177
214
130

185
213
–

energy states and wave functions, from which the lowest
energy transition and the wave function overlaps were calculated. Table I displays the calculated transition energies.
The InAs/InAs1xSbx SLs were grown by MOCVD at
500  C on 2 in. (100) n-GaSb substrates following deposition
of a 50 nm GaSb buffer layer using growth conditions
reported previously.18 The prior Sb compositions of the sample structures were based on growth calibrations18 and are
different from the details shown in Table I, which were
determined by analysis of x-ray diffraction (XRD) measurements. The layer thicknesses are still assumed to be the same
values from the growth calibrations. The (004) and (224)
x2h coupled high-resolution XRD patterns were recorded
using a PANalytical X’Pert Pro MRD. The simulation results
closely agree with the experimental data, as shown in Fig. 1
for the (004) patterns. Sample A shows intense satellite
peaks with narrow full width at half maximum (FWHMs) of
less than 100 arc sec, indicating the high degree of crystallinity and uniform periodicity of this SL structure. Some variation in the InAs1xSbx composition, however, is evident
from the peaks’ shapes. The simulation used varying Sb
compositions for different portions of the 100 periods. The
most intense SL satellite peaks correspond to an average
x ¼ 0.22 in the InAs1xSbx layer for the majority of the periods, while the broader, less intense, periodic shoulder peaks
are simulated well with just a few periods containing
x ¼ 0.35. The average relaxation of the SL was 74%, as
determined from (224) x2h coupled scans. The satellite
peaks of sample B are broader than those of sample A, and
the simulated pattern of sample B in Fig. 1 uses 83% relaxa-

tion and x ¼ 0.23 derived from (224) x2h coupled scans.
The XRD patterns of sample C exhibited 100% relaxation.
Specimens for cross-sectional transmission electron microscopy (TEM) observation were prepared by standard mechanical polishing, dimpling, and argon-ion-milling at
reduced energy (2–2.5 keV), with the sample held at liquidnitrogen temperature to minimize thermal and ion-beam
damage. Figure 2 shows a cross-sectional TEM image of
sample A, which confirms the very high crystallinity of this
specimen. In contrast, samples B and C showed the presence
of considerable growth defects, especially {111}-type stacking faults, originating at either the substrate/buffer interface
or the buffer/SL interface, and propagating well into the SL
region. These defects presumably contribute significantly
towards broadening the FWHM of the XRD satellite peaks.
The low temperature PL spectra for samples A and B
are shown in Fig. 3. The data were acquired with a doublemodulation Fourier transform infrared spectrometer setup to
suppress the background 300 K blackbody radiation noise
near 10 lm. However, the increasing background noise is
still visible in the spectra near the MCT detector cutoff at
12 lm. The 532 nm pumping laser was modulated at 60 kHz.
Using a Lorentzian fit to the data, the peak position, intensity, and FWHM for sample A are 185 meV, 822 a.u., and
20 meV and for sample B are 213 meV, 872 a.u., and 32 meV.
The PL intensity is comparable for both samples measured
under the same conditions, but the FWHM of sample B’s spectrum is 60% larger than that of sample A, due to the higher
density of defects, as deduced from the TEM, and the higher
degree of relaxation in sample B.

FIG. 1. (Color online) High-resolution (004) XRD patterns and simulations
(offset below each measurement) for samples A and B.

FIG. 2. Cross-sectional transmission electron micrograph of sample A demonstrating the excellent crystallinity of the InAs/InAsSb superlattice grown
on a GaSb (100) substrate.

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071111-3

Steenbergen et al.

Appl. Phys. Lett. 99, 071111 (2011)

In summary, InAs/InAs1xSbx SLs grown by MOCVD
were studied as an alternative material to HgCdTe for infrared
photodetectors. Strong, narrow satellite peaks in the x-ray
diffraction pattern and minimal defects in the transmission
electron micrograph revealed the excellent structural properties of this strain-balanced InAs/InAs1xSbx SL. In addition,
intense photoluminescence spectra and photoconductivity
spectral responses up to 200 K at 8.6 lm were observed. The
structural properties plus the strong optical responses, despite
relaxation occurring, warrant further investigation of these
superlattice materials grown by MOCVD for infrared photodetector applications.

FIG. 3. (Color online) Photoluminescence spectra at 6 K for samples A and
B. The inset shows the type-II band alignment between InAs and InAs1xSbx.

The spectral photoconductivity of all three samples was
measured using a BioRad FTIR at increasing temperatures
from 10 K until the signal disappeared—up to 250 K for
sample A, 77 K for sample B, and just at 10 K for sample C.
A bias current of 4 mA for sample A and 0.5 mA for sample
B was applied at indium contacts on the top surface of the
samples. The individual photoconductivity signals were
corrected for the background with a reference spectrum.
Figure 4 shows the temperature-dependent spectral photoresponse for sample A with a strong response up to 200 K
(onset at 8.6 lm) and sample B with response up to 60 K
(onset at 5.9 lm). To determine the onset of photoresponse,
a linear fit was made to the steep segment of the response as
it approached zero, and the x-intercept of the linear function
was taken as the photoresponse onset.
Table I shows the comparison of the PL peak position
and the photoresponse onset. They agree very well for sample
B. For sample A, the PL peak position is higher in energy
than the absorption onset, due to the band filling effect caused
by the relatively high pump power intensity. Taking the SL
relaxation into account, the calculated values follow the same
trend as the measured values, although there is a discrepancy
due to the measurement uncertainty of x (61%) and the layer
thicknesses.

FIG. 4. (Color online) The temperature-dependent spectral photoresponse of
sample A, showing strong signals up to 200 K and out to 8.6 lm (145 meV),
and sample B, showing signals up to only 60 K and out to 5.9 lm (210 meV).

The authors are grateful for the support of ARO MURI
programs W911NF-06-1-0353 and W911NF-10-1-0524 and
an AFOSR Grant (FA9550-10-1-0129). Many experiments
were performed at AFRL/RXPS with the support of G.
Brown and B. Ullrich. E.H.S. appreciates the DOD SMART
and SFAz scholarships. R.D.D. is thankful for the support of
the Steve W. Chaddick Endowed Chair in Electro-Optics
and the Georgia Research Alliance.
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