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Mid-infrared InAs/GaSb strained layer superlattice detectors with nBn design grown on a
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2010 Semicond. Sci. Technol. 25 085010
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IOP PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGY
Semicond. Sci. Technol. 25 (2010) 085010 (4pp) doi:10.1088/0268-1242/25/8/085010
Mid-infrared InAs/GaSb strained layer
superlattice detectors with nBn design
grown on a GaAs substrate
E Plis 1 , J B Rodriguez 2 , G Balakrishnan 1 ,YDSharma 1 ,HSKim 1 ,
T Rotter 1 and S Krishna 1,3
1 Center for High Technology Materials, Department of Electrical and Computer Engineering,
University of New Mexico, Albuquerque NM 87106, USA
2 Institut d’Electronique du Sud, UMR 5214 CNRS, Université Montpellier 2, Place Eugène Bataillon,
34095 Montpellier Cedex 5, France
E-mail: skrishna@chtm.unm.edu
Received 9 April 2010, in final form 12 April 2010
Published 23 July 2010
Online at stacks.iop.org/SST/25/085010
Abstract
We report on a type-II InAs/GaSb strained layer superlattice (SLS) photodetector (λcut-off
∼4.3 μm at 77 K) with nBn design grown on a GaAs substrate using interfacial misfit
dislocation arrays to minimize threading dislocations in the active region. At 77 K and 0.1 V
of the applied bias, the dark current density was equal to 6 × 10 −4 Acm −2 and the maximum
specific detectivity D ∗ was estimated to 1.2 × 10 11 Jones (at 0 V). At 293 K, the zero-bias D ∗
was found to be ∼10 9 Jones which is comparable to the nBn InAs/GaSb SLS detector grown
on the GaSb substrate.
1. Introduction
Detectors based on type-II InAs/GaSb strained layer
superlattices (SLSs) are being actively pursued as an
alternative to mercury cadmium telluride and quantum well
infrared technologies for the fabrication of high-performance
infrared (IR) imagers. Flexibility of controlling the electronic
band structure by variation of composition and thickness of the
constituent layers [1, 2], reduced Auger recombination rates
[3] and improved quantum efficiency (QE) due to allowed
normal incidence absorption together with enhanced material
quality [4] enables the fabrication of high-performance focal
plane arrays (FPAs) operating in the mid-wave IR (MWIR) [5]
and long-wave IR [6] spectral regions.
There are two constraints limiting the progress of present
day SLS technology. First, relatively high levels of noise due to
generation–recombination (G–R) and trap-assisted tunneling
processes in the depletion region of the detector as well as
leakage through surface states degrade the overall device
performance. Various designs such as the M-structure [7],
W-structure [8], graded bandgap [9] and p-π-M-n design [10]
utilize the possibility of independent adjustment of conduction
3 Author to whom any correspondence should be addressed.
and valence band positions provided by the type-II band
alignment and successfully suppress different dark current
mechanisms. Recently proposed [11] and implemented on
type-II SLS [12] nBn design concept with its counterparts [13]
further extends this heterojunction engineering technique. It
uses both a wide-band gap barrier to block majority carriers
and a buried device structure effectively eliminating surface
currents. In addition, it excludes the SRH G–R component
of the dark current since it is intended to operate with n-type
layers in a flatband or with little depletion. InAs/GaSb MWIR
SLSs single element detectors with nBn design show reduced
dark current density by two orders of magnitude (at 77 K)
compared to a conventional photodiode [14].
The second constraint, which is the limiting factor in
realizing large format SLS FPAs, is the lack of a high-quality,
optically transparent larger area (4 inch diameter and beyond)
substrates. Even though the GaSb substrate technology has
made a significant progress in the past few years, the substrates
are still not ‘epi-ready’ with a non-optimized oxide layer and
macroscopic defects present on the growth surface. Moreover,
the absorption coefficient of GaSb substrates is in the
100 cm −1 range for IR radiation beyond 5 μm [15]. Thus,
removal of the GaSb substrate is required for FPA applications.
Moreover, the size of commercially available GaSb substrates
0268-1242/10/085010+04$30.00 1 © 2010 IOP Publishing Ltd Printed in the UK & the USA

Semicond. Sci. Technol. 25 (2010) 085010 E Plis et al
is less than 3 inch in diameter due to limited demand in the
semiconductor industry. To overcome this issue the use of
alternative substrates like GaAs [16–18] forInAs/GaSb SLS
growth has been proposed. The absorption coefficient of
GaAs is nearly two orders of magnitude lower than that of
GaSb. In addition, large format epi-ready GaAs substrates
are commercially available (6 inch in diameter) which is
advantageous for the cost reduction in the mass production
of IR FPAs.
Unfortunately, due to an ∼7.8% lattice mismatch between
GaAs and GaSb the direct growth of GaSb on GaAs gives rise
to a high misfit dislocation density (10 9 cm −2 )[19] atthe
GaSb/GaAs interface and throughout the epitaxial film. In
order to realize high-performance InAs/GaSb SLS detectors
on GaAs substrates, it is necessary to develop growth methods
to accommodate strain and decrease the dislocation density.
Among them is a low-temperature nucleation technique
[20] utilized for the demonstration of type-II SLS MWIR
photodiode with p-i-n design grown on GaAs substrates. In
this paper, we demonstrate an InAs/GaSb SLS detector with
nBn design (λcut-off ∼ 4.3 μm, 77 K) grown on a GaAs
substrate with a significantly different growth scheme termed
as the interfacial misfit dislocation (IMF) array technique
[21, 22].
2. Experimental procedure
The detector structures were grown using a solid source
molecular beam epitaxy (MBE) VG-80 system on semiinsulating
epi-ready (1 0 0) 3 inch GaAs substrates. First, a
1 μm GaSb buffer layer was deposited on top of the GaAs
substrate with the IMF technique. This growth mode involves
establishing a (2 × 8) Sb reconstruction on GaAs (1 0 0)
followed by direct growth of GaSb. As was reported in
previous publications [21, 22] such a process establishes an
array of 90 ◦ interfacial misfit dislocations at the GaSb/GaAs
interface allowing the growth of spontaneously relaxed and
extremely low-defect density GaSb (1 × 10 6 cm −2 )onGaAs.
Then a 500 nm thick bottom contact layer consisting of 8 ML
InAs/8 ML GaSb SLS with Si-doped InAs layers (n = 4 ×
10 18 cm −3 ) was grown followed by a 100 nm Al0.2Ga0.8Sb
non-intentionally doped (n.i.d.) barrier layer. Next, an
∼2.5 μm thick (520 periods) n.i.d. absorption region was
grown with the same superlattice composition and thickness
as the bottom contact layer. The SLS absorption region
is residual n-type with a carrier concentration in the low
10 16 cm −3 at room temperature [23, 24]. The structure was
capped by a 100 nm thick top contact layer made of the
same material as the bottom contact layer. The SLS growth
conditions were similar to the procedure reported previously
[12]. The inset to figure 1 shows an atomic force micrograph
(AFM) of the detector structure over a 20 × 20 μm 2 area
and root-mean-square (rms) surface roughness of 2 nm. The
material of the barrier layer has a wider bandgap than absorbing
and contact layers in order to provide nearly zero valence
band offset and a large conduction band offset. As a result, the
majority carrier (electrons) current between the two electrodes
2
Figure 1. Normalized temperature-dependent spectral response
from the InAs/GaSb SLS detector with nBn design. The inset
shows an AFM of the full detector structure over 20 × 20 μm 2 areas.
(This figure is in colour only in the electronic version)
is blocked by the large energy offset, while there is no barrier
for photogenerated minority carriers (holes).
The material was processed into normal incidence 410 ×
410 μm2 single pixel devices with a 300 μm diameter circular
aperture. The processing was initiated by mesa definition
etching in an inductively coupled plasma reactor with BCl3
gas. Subsequently, ohmic contacts were made by depositing
Ti/Pt/Au on the bottom and top contact layers of the detectors.
After contact metallization, the fabricated devices were wire
bonded to a leadless chip carrier for further characterization.
The 50% cut-off wavelength of the processed devices
was obtained from spectral response measurements performed
using a Fourier transform IR spectrometer with a glow-bar
black body source. Normalized spectral response was obtained
by dividing the photocurrent of the SLS detector with that
obtained using a standard deuterated triglycine sulfate thermal
detector. The responsivity and QE of devices were measured
using a Mikron blackbody at 500 ◦C. Specific detectivity D∗ of the device was evaluated using the following equation:
D ∗ = Ri · � Ad · �f / iN, (1)
where Ri is the measured responsivity, Ad is the electrical area
of the diode, and iN is the noise current obtained by integrating
the noise spectrum in a bandwidth of �f beyond the corner
frequency.
3. Results and discussion
The detector 50% cut-off wavelength was found to be 4.3 μm
at 77 K (Vb = 0.5 V). Figure 1 shows the normalized spectral
response of the detector at different temperatures. It should
be noted that the bias was taken as positive when the voltage
polarity was positive on the top contact. The applied bias did
not significantly affect the behavior of the spectral response
curves.
Current voltage (IV) characteristics were measured for
temperatures ranging from 77 to 293 K and are shown in

Semicond. Sci. Technol. 25 (2010) 085010 E Plis et al
Table 1. Performance comparison of detectors grown on different substrates with conventional p-i-n or nBn designs.
Parameter nBn on GaAs nBn on GaSb [12] p-i-n on GaAs [16] p-i-n on GaSb [25]
λ50%cut−off (μm) 4.3 (77 K) 5.1 (250 K) 5.2 (300 K) ∼4.04 (77 K) ∼4.5 (260 K) 4.2 (77 K) 4.6 (250 K)
Dark current density (A cm −2 ) 6× 10 −4 at Vb = 0.1 V 2 × 10 −3 at Vb =−0.5 V 3 × 10 −4 at Vb =−0.3 V 2 × 10 −5 at Vb = –0.3 V
(77 K)
Dark current density (A cm −2 ) 3.6 at Vb = 0.1 V 2 at Vb =−0.5 V 3 at Vb =−0.5 V 0.18 at Vb = –0.1 V
(293 K)
QE (%) (77 K, λ = 4.0 μm) 15 at Vb = 3V 20atVb =−0.3 V 17 at Vb = 0V 18atVb =−0.5 V
Zero-bias D ∗ at 77 K (Jones) 10 11 at 4 μm N/A ∼3 × 10 11 at 4 μm 1.9 × 10 12 at 4 μm
Zero-bias D ∗ at 293 K (Jones) 10 9 at 4 μm 10 9 at 4.5 μm ∼7 × 10 8 at 4 μm 1.9 × 10 9 at 4 μm
Figure 2. Dark current density versus applied bias at different
temperatures for the nBn detector with 410 × 410 μm 2 mesa size.
The normal operating bias for the nBn detector is the forward bias
voltage which is defined as a positive voltage applied to the bottom
contact.
figure 2. At 77 K and 0.1 V of the applied bias, the dark current
density was 6 × 10 −4 Acm −2 corresponding to a differentialresistance-area
product at zero bias (R0A) of 268 � cm 2 .At
293 K and the same applied bias, the dark current density was
3.6 A cm −2 with R0A of 2.2 × 10 −2 � cm 2 . The measured
values of R0A in our device are worse compared with those
for the recently reported InAs/GaSb p-i-n detectors grown on
GaAs substrates with a 4 μm GaSb buffer layer [16]. Since
the nBn detector is a minority carrier photoconductor, R0A is
not a very good metric for such devices. Comparison of the
measured values of dark current densities under operational
bias conditions for our device, the nBn detector grown on
GaSb [12] and p-i-n detectors grown on GaAs [16] and GaSb
[25] substrates is shown in table 1. At 77 K, our detector
demonstrates performance comparable with nBn detectors
grown on GaSb [12] and p-i-n detectors grown on the GaAs
substrate [16]. However, it showed a factor of ∼30 higher dark
current density than p-i-n detectors grown on GaSb reported
by our group earlier [25]. We attribute it to the longer cutoff
wavelength of our device and possible imperfection of
the IMF technique. Currently, an optimization of the IMF
technique for the SLS detectors is being undertaken in our
group.
At 77 K and zero bias the device exhibited QE of 11%
(figure 3). The maximum value of QE was equal to 15% at
Vb = 3 V. Corresponding values of responsivity were
3
Figure 3. Quantum efficiency and responsivity measured for the
InAs/GaSb nBn detector at 77 K and 4 μm.
Figure 4. Noise current and detectivity obtained from the
InAs/GaSb nBn detector at 77 K using a calibrated blackbody at
500 ◦ C.
0.35 A W −1 (zero bias) and ∼0.5 A W −1 (Vb = 3 V). It should
be noted that QE remains constant in the (0–0.6) V bias region
and it slightly increases at 0.6 V. This bias dependence of QE is
probably due to the fact that not all the carriers are collected at
lower bias voltages, which is attributed to the presence of small
valence band offset. Further optimization of composition and
the doping level of the ternary barrier layer are required to
achieve a zero-energy valence band offset between the barrier
and SLS forming the absorption region of the device.
Finally, the specific detectivity D ∗ of the device was
evaluated at 4 μm and the results are summarized in figure 4.

Semicond. Sci. Technol. 25 (2010) 085010 E Plis et al
At 77 K the maximum D ∗ was found under zero bias and equal
to 1.2 × 10 11 Jones which is similar to the performance of other
MWIR type-II SLS grown on GaAs substrates [16] (∼3 ×
10 11 Jones). At 293 K, D ∗ under zero bias was equal to
∼10 9 Jones, which is slightly higher than D ∗ demonstrated
by InAs/GaSb SLS detectors with p-i-n design grown on
the GaAs substrate [16] (7× 10 8 Jones) and comparable
with performance of nBn detectors grown on the GaSb
substrate [12] (10 9 Jones). Detailed performance comparison
of detectors grown on different substrates with conventional
p-i-n or nBn designs is shown in table 1.
4. Conclusions
In conclusion, we report an InAs/GaSb SLS detector with
nBn design operating in the MWIR wavelength range (λcut-off
∼4.3 μm at 77 K) grown on a GaAs substrate with the IMF
technique. QE between 11% and 18%, and specific detectivity
D ∗ ∼10 11 Jones at 77 K and 4 μm were demonstrated. At
293 K, D ∗ under the same conditions was equal to ∼10 9 Jones
which is comparable to the p-i-n InAs/GaSb SLS detectors
grown on the GaAs substrate and the nBn InAs/GaSb SLS
detector grown on GaSb substrates, and within a factor of
2 lower than D ∗ measured for the MWIR p-i-n detector grown
on the GaSb substrate.
Acknowledgments
The authors would like to acknowledge support from AFOSR
Contract FA9550-09-1-0231 and AFRL Contract FA9453-07-
C-0171.
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