We present a model for the spectral external quantum efficiency (EQE) to extract the minority carrier diffusion length (Ln) of a unipolar nBp InAs/GaSb Type-II superlattice (T2SL) mid-wave infrared (MWIR) detector. The detector consists of a 4 μm thick p-doped 10ML InAs/10ML GaSb SL absorber with a 50% cut-off wavelength of 5 μm at 80 K and zero bias. The n-type doped InAs/AlSb SL barrier in the structure was included to reduce the GR dark current. By fitting the experimentally measured EQE data to the theoretically calculated QE based on the solution of the drift-diffusion equation, the p-type absorber was found the have Ln = 10 ± 0.5 μm at 80K, and Ln = 12 ± 0.5 μm at 120K and 150K. We performed the absorption coefficient measurement at different temperatures of interest. Also, we estimated the reduced background concentration and the built-in potential by utilizing a capacitance-voltage measurement technique. We used time-resolved-photoluminescence (TRPL) to determine the lifetime at 80K. With the result of the model and the lifetime measurement, we calculated the diffusion coefficient and the mobility in the T2SL detector at various temperatures. Also, we studied the behavior of different dark current mechanisms by fitting the experimentally measured and simulated dark current density under different operating temperatures and biases.
We report high quantum efficiency (QE) MWIR barrier photodetectors based on the InAs/GaSb/AlSb type II superlattice (T2SL) material system. The nBp design consists of a single unipolar barrier (InAs/AlSb SL) placed between a 4 μm thick p-doped absorber (InAs/GaSb SL) and an n-type contact layer (InAs/GaSb SL). At 80K, the device exhibited a 50% cut-off wavelength of 5 μm, was fully turned-ON at zero bias and the measured QE was 62% (front side illumination with no AR coating) at 4.5 μm with a dark current density of 8.5×10-9 A/cm2 . At 150 K and Vb=50 mV, the 50% cut-off wavelength increased to 5.3 μm and the quantum efficiency (QE) was measured to be 64% at 4.5 μm with a dark current of 1.07×10-4 A/cm2 . The measurements were verified at multiple AFRL laboratories. The results from this device along with the analysis will be presented in this paper.
In this work, we compare the performance of three MWIR unipolar barrier structures based on the InAs/GaSb Type-2 strained layer superlattice material system. We have designed, fabricated, and characterized pBiBn, pBn, and pBp detector structures. All the structures have been designed so that the cut off wavelength is around 5 microns at 100 K. We fabricated single-pixel devices and characterize their radiometric performance. In addition, we have characterized the degradation of the performance of the devices after exposing the devices to 63 MeV proton radiation to total ionizing dose of 100 kRad (Si). In this report, we compare the performance of the different structures with the objective of determining the advantages and disadvantages of the different designs. This work was supported by the Small Business Innovation Research (SBIR) program under the contract FA9453-14-C-0032, sponsored by the Air Force Research Laboratory (AFRL).
We report on the development of dual-band InAs/GaSb type-II strained layer superlattices (T2SL) detectors with barrier designs at SK Infrared. Over the past five years, we demonstrated mid-wave/long-wave (MW/LWIR, cut-off wavelengths are 5 μm and 10.0 μm), and LW/LWIR (cut-off wavelengths are 9 μm and 11.0 μm) detectors with nBn and pBp designs. Recent results include a high performance bias-selectable long/long-wavelength infrared photodetector based on T2SL with a pBp barrier architecture. The two channels 50% cut-off wavelengths were ~ 9.2 μm and ~ 12 μm at 77 K. The “blue” and “red” LWIR absorbers demonstrated saturated QE values of 34 % and 28 %, respectively, measured in a backside illuminated configuration with a ~ 35 μm thick layer of residual GaSb substrate. Bulk-limited dark current levels were ~ 2.6 x 10-7 A/cm2 at + 100 mV and ~ 8.3 x 10-4 A/cm2 at - 200 mV for the “blue” and “red” channels, respectively.
We present an approach to realize antimonide based superlattices on silicon substrates without using conventional
Indium-bump hybridization. In this approach, PIN based superlattice detectors are grown on top of a 60 nm Al0.6Ga0.4Sb
sacrificial layer on a GaSb host substrate. Following the growth, the individual pixels are transferred using our epitaxiallift
off technique, which consists of a wet-etch to undercut the pixels followed by a dry-stamp process to transfer the
pixels to a silicon substrate prepared with a gold layer. Structural and optical characterization of the transferred pixels
was done using an optical microscope, scanning electron microscopy and photoluminescence. The interface between the
transferred pixels and the new substrate was abrupt and no significant degradation in the optical quality was observed.
An Indium-bump-free membrane detector was then fabricated using this approach. Spectral response measurements
provided a 100% cut-off wavelength of 4.3 μm at 77 K. The performance of the membrane detector was compared to a
control detector on the as-grown substrate. The membrane detector was limited by surface leakage current. The proposed
approach could pave the way for wafer-level integration of photonic detectors on silicon substrates, which could
dramatically reduce the cost of these detectors.
In this work, we report on the design, fabrication, and characterization of MWIR unipolar barrier photodetectors based on InAs/GaSb Type-II superlattice. We have designed, fabricated, and characterized band-structure engineered MWIR photodetectors based on the pBiBn architecture. The devices have been characterized using the most relevant radiometric figures of merits. At 200 K, the peak value of detectivity is 1.2 x 1011 Jones at an applied bias voltage of -0.5 V.
The realization of high operating temperature (HOT) midwave infrared (MWIR) photodetectors would significantly relax the requirements imposed on the cooling system, which would lead to a reduction in the size, weight, and cost of the detection system. One of the most attractive material systems to develop HOT photodetectors is InAs/GaSb Type II Superlattice (T2SL). This is due the ability of T2SL materials to engineer the band structure of the device, which can be exploited to make devices with unipolar barriers. It has been shown that the use of unipolar barriers can dramatically reduce the dark current levels of the device, which is essential to realize HOT photodetectors. In this work, we report on the performance of a unipolar barrier mid wave infrared detector based on type-II InAs/GaSb strained layer superlattice for high operating temperatures. The device architecture is the double-barrier heterostructure, pBiBn design. Under an applied bias of -10 mV and an operating temperature of 200 K, the best performing devices show a dark current density of 4.9×10-4 A/cm2. At 200 K, the measured zero-bias specific detectivity was 4.4×1010 Jones.
Midwave infrared (MWIR) photodetectors that do not require cryogenic cooling would significantly reduce the complexity of the cooling system, which would lead to a reduction in the size, weight, and cost of the detection system. The key aspect to realize high operating temperature (HOT) photodetectors is to design device structures that exhibit significantly lower levels of dark current compared to the existing technologies. One of the most attractive material systems to develop HOT photodetectors is InAs/GaSb Type II Strained layer Superlattice (SLS). This is due the ability of Type II SLS materials to engineer the band structure of the device, which can be exploited to make devices with unipolar barriers. It has been shown that, compared to the traditional homojunction SLS devices, band-gap engineered unipolar barrier SLS devices can obtain significantly lower levels of dark current. In this work, we report on the design, growth, and fabrication of mid wave infrared detectors based on type-II InAs/GaSb strained layer superlattice for high operating temperatures. The device architecture is the double-barrier heterostructure, pBiBn design. Under an applied bias of -10 mV and an operating temperature of 200 K, the tested devices show a dark current density of 4 x 10-3 A/cm2 and a quantum efficiency of 27%. At 4.5 μm and 200 K, the devices show a zero-bias specific detectivity of 4.4 x 1010 Jones.
In past decade, T2SL detectors with promising performance have been reported by various institutions thanks to the extensive modeling efforts, improvement of T2SL material quality, and development of advanced low-dark-current architectures with unipolar barriers (Xbn, CBIRD, pBiBn, M-structure, etc). One of the most demanding challenges of present day T2SL technology is the suppression of surface leakage currents associated with the exposed mesa sidewalls, which appear during the definition of device optical area. Typical FPA pixels have large surface/volume ratio and their performance is strongly dependent on surface effects. In order to overcome the limitation imposed by surface leakage currents, a stable surface passivation layer is needed. In this paper we report on InAs/GaSb T2SL detectors operating in the LWIR spectral region (100% cut-off wavelength of ~10 μm at 77K) passivated with epitaxially grown ZnTe. In order to compensate for the high conductivity of ZnTe passivation it was doped with chlorine to 1 × 1018cm−3 concentration. Dark current measurements reveal the significant reduction of noise current after ZnTe passivation.
Detectivity of mid-wave infrared (MWIR) detectors based on InAs/GaSb type II strained layer superlattices (T2SLs) can be significantly enhanced at select wavelengths by integrating the detector with a back-side illuminated plasmonic coupler. The application of a simple metal-T2SL structure directly on the GaSb substrate can result in radiation losses into the substrate due to the low refractive index of T2SL layer. However, insertion of a higher refractive index material, such as germanium (Ge), into the metal-SLS structure can confine the surface plasmon waveguide (SPW) modes to the surface. In this work, metal (Au)-Ge-T2SL structures are designed with an approximately 100 nm thick Ge layer. The T2SL layer utilized a p-i-n detector design with 8 monolayers (MLs) InAs/8 MLs GaSb. A plasmonic coupler was then realized inside the 300 μm circular apertures of these single element detectors by the formation of a corrugated metal (Au) surface. The T2SL single element detector integrated with an optimized plasmonic coupler design increased the quantum efficiency (QE) by a factor of three at an operating temperature of 77 K and 3 to 5 μm illumination wavelength, compared to a reference detector structure, and each structure exhibited the same level of dark current.
The objective of this study is to optimize the absorption in the active region of InAs/GaSb T2SL photodetectors for the
realization of high-performance MWIR devices. Two sets of MWIR (λ100% cut-off ~ 5.5μm at 77K) T2SL detectors were
realized; one set with varied detector absorber thickness, the other set with varied T2SL period. The T2SL material
quality was evaluated on the basis of room temperature photoluminescence (RTPL) and the high-resolution X-ray
diffraction (HRXRD) data. Then the device performance was compared using spectral response, dark current and
responsivity measurements. Finally, quantum efficiency was calculated and employed as a metric for the definition of
the optimal T2SL period and active region thickness. For the first part of the study, a homojunction pin architecture
based on 8 monolayers (MLs) InAs/8MLs GaSb T2SL was used. The thickness of the non-intentionally doped absorber
layers were 1.5μm, 2.5μm, and 3.5μm. For the second part of the study, unipolar barrier (pBiBn) devices were grown.
The thickness of the absorber region and the T2SL constituent InAs layer thicknesses were kept the same (1.5 μm and 8
MLs, respectively) whereas the T2SL constituent GaSb thickness was varied as 6 MLs, 8 MLs, and 10 MLs. We have
found that the pin detector with 2.5 μm thick absorber and the pBiBn detector with 8 ML InAs/ 8 ML GaSb T2SL
composition are, within the scope of this study, optimal for the realization of MWIR single-element devices and FPAs
with corresponding architectures.
Current infrared imaging systems monitor emission from a given scene over a broad spectral range, which results with "black and white" images. As a result, there is ever increasing emphasis on the development of new, on the pixel level, infrared imaging technology that can provide spectral information. Attempts at creating a robust imaging system with spectral information have been made through a network of external optics, which results with a high cost and large system package. Here, we propose a metamaterial design that resonantly couples to an infrared photodetector for enhanced performance.
Over the last several years, owing to the implementation of advanced device architectures, antimony-based type-II
superlattice (T2-SL) infrared (IR) photodetectors and their focal plane arrays (FPAs) have achieved significant
advancements. Here we present our recent effort towards the development of high operating temperature (HOT)
mid-IR (MWIR) photodetectors, which utilizes an interband cascade scheme with discrete InAs/GaSb SL absorbers,
sandwiched between electron and hole barriers. This low-noise device architecture has enabled background-limited
operation above 150 K (300 K, 2π field-of-view), as well as above room temperature response in the mid-IR region.
The detector yields a dark current density of 1.10×10-7 A/cm2 (1.44×10-3 A/cm2) at -5 mV, and a Johnson-limited D*
of 2.22×1011 cmHz1/2/W (1.58×109 cmHz1/2/W) at 150 K (room temperature) and 3.6 μm, respectively. In this
presentation, we will discuss the operation principles of the interband cascade design and our most recent progress
in MWIR photodetectors toward high operating temperatures.
Infrared (IR) detector technologies with the ability to operate near room temperature are important for many
applications including chemical identification, surveillance, defense and medical diagnostics. Reducing the need for
cryogenics in a detector system can reduce cost, weight and power consumption; simplify the detection system
design and allow for widespread usage. In recent years, infrared (IR) detectors based on uni-polar barrier designs
have gained interest for their ability to lower dark current and increase a detector's operating temperature.
Our group is currently investigating detectors based on the InAs/GaSb strain layer superlattice (SLS) material
system that utilize barrier heterostructure engineering. Examples of such engineering designs include pBp, nBn,
PbIbN, CBIRD, etc. For this paper I will focus on LW (long wave) pBp structures. Like the built-in barrier in a p-n
junction, the heterojunction barrier blocks the majority carriers allowing free movement of photogenerated minority
carriers. However, the barrier in a pBp detector, in contrast with a p-n junction depletion layer, does not significantly
contribute to generation-recombination (G-R) current due to the lack of a depletion region across the narrow band
gap absorber material. Thus such detectors potentially work like a regular photodiode but with significantly reduced
dark current from G-R mechanisms.
The mechanism of photoconductive (PC) gain has not been fully characterized in such device architectures and
in many recent studies has been assumed to be unity. However, studies conducted with similar device structures
have shown the presence of PC gain. In this report we will measure and analyze the impact of PC gain in detectors
utilizing single unipolar barriers such as the case of pBp detectors.
We present an investigation of the quantum confined energy levels in a mid-wave infrared and long-wave infrared
InAs/GaSb type II strained-layer superlattice (SLS) photodetector by computing the first derivative of the absorption
spectra from 80K to 250K , with respect to the wavelength. Energy levels of both the fundamental transition and two
other higher orders are identified for the SLS. The temperature evolution of each of these bands was also characterized
by fitting the energy transitions to the Varshni equation, which showed that in general, the higher-energy transitions have
a greater change in bandgap with temperature than the lower-energy ones. The transition energies appeared linearly
dependent on the InAs layer thickness, and had a weaker dependence on the GaSb layer thickness. A feature that
vanished at higher temperatures was also observed, which is due to a GaSb characteristic, rather than the superlattice.
Infrared (IR) detectors operated in the space environment are required to have high performance while being subjected to
a variety of radiation effects. Sources of radiation in space include the trapped particles in the Van Allen belts and
transient events such as solar events and galactic cosmic rays. Mercury cadmium telluride (MCT)-based IR detectors
are often used in space applications because they have high performance and are generally relatively tolerant of the space
environment when passivated with CdTe; often, the readout-integrated circuit is far more susceptible to radiation effects
than the detector materials themselves. However, inherent manufacturing issues with the growth of MCT have led to
interest in alternative detector technologies including type-II strained-layer superlattice (T2SLS) infrared detectors with
unipolar barriers. Much less is known about the radiation tolerance properties of these SLS-based detectors compared to
MCT. Here, the effects of 63 MeV protons on variable area, single element, dual-band InAs/GaSb SLS detectors in the
pBp architecture are considered. When semiconductors devices are irradiated with protons with energies of 63 MeV the
protons are capable of displacing atoms within their crystalline lattice. The SLS detectors tested here utilize a pBp
architecture, which takes advantage of the higher mobility electrons as the minority photocarrier. These detectors are
also dual-band, implying two absorbing regions are present and separated by the unipolar barrier. The absorbers have
cutoff wavelengths of roughly 5 and 9 μm allowing for mid-wave (MWIR) and long-wave (LWIR) infrared detection,
respectively. The radiation effects on these detectors are characterized by dark current and quantum efficiency as a
function of total ionizing dose (TID) or, equivalently, the incident proton fluence.
Long-wave infrared (LWIR) detector technologies with the ability to operate at or near room temperature are very
important for many civil and military applications including chemical identification, surveillance, defense and medical
diagnostics. Eliminating the need for cryogenics in a detector system can reduce cost, weight and power consumption;
simplify the detection system design and allow for widespread usage. In recent years, infrared (IR) detectors based on
uni-polar barrier designs have gained interest for their ability to lower dark current and increase a detector's operating
temperature.
Our group is currently investigating nBn and pBp detectors with InAs/GaSb strain layer superlattice (SLS)
absorbers (n) and contacts (n), and AlGaSb and InAs/AlSb superlattice electron and hole barriers (B) respectively. For
the case of the nBn structure, the wide-band-gap barrier material (AlGaSb) exhibits a large conduction band offset and a
small valence band offset with the narrow-band-gap absorber material. For the pBp structure (InAs/AlSb superlattice
barrier), the converse is true with a large valence band offset between the barrier and absorber and a small or zero
conduction band offset. Like the built-in barrier in a p-n junction, the heterojunction barrier blocks the majority carriers
allowing free movement of photogenerated minority carriers. However, the barrier in an nBn or pBp detector, in contrast
with a p-n junction depletion layer, does not contribute to generation-recombination (G-R) current.
In this report we aim to investigate and contrast the performance characteristics of an SLS nBn detector with that of
and SLS pBp detector.
We present our efforts on development of high performance low noise, long-wave infrared (LWIR)
and multicolor detectors based on the InAs/GaSb strained layer material (SLS) material system. The LWIR
SLS detector with PbIbN architecture showed improved performance over the conventional PIN design due to
unipolar current blocking layers. At 77K and Vb=-0.25V, a responsivity of 1.8 A/W, dark current density of
1.2 mA/cm2, quantum efficiency of 23% and shot noise limited detectivity (D*) of 8.7×1010 Jones (λc = 10.8
μm) has been observed. Dual band response was registered with 50% cut-off wavelengths of 5μm and 10μm
from an SLS detector with the pBp design. The responsivity equal to 1.6 A/W (at λ = 5 μm and Vb = +0.4 V)
and 1.8 A/W (at λ = 9 μm and Vb = -0.7 V) for MWIR and LWIR absorbers was achieved with corresponding
values of specific detectivity 5 x 1011 Jones and 2.6 x 1010 Jones, respectively. The maximum values of
quantum efficiency were estimated to 41% (MWIR) and 25% (LWIR) at Vb = +0.4V and Vb = -0.7V.
Moreover, the diffusion-limited behavior of dark current at higher temperatures was observed for the MWIR
absorber for pBp detector. Finally, three-color response was registered from three contact device with nBn
architecture for SWIR and MWIR and heterojunction PIbN architecture for LWIR detection (NbNbiP). At
77K, the cut-off wavelength for SWIR, MWIR and LWIR regions have been observed as 3.0 μm, 4.7 μm, and
10.1 μm respectively. At the same temperature, D* of 1.4 × 1010 Jones, 1.8 × 1010 Jones and 1.5 × 109 Jones
for SWIR, MWIR and LWIR signals has been observed.
KEYWORDS: Sensors, Laser sintering, Resistors, Infrared sensors, Infrared detectors, Field effect transistors, Amplifiers, Etching, Superlattices, Signal to noise ratio
Recent experiments on conventional p-on-n and n-on-p Type II superlattices (SLS) infrared detectors still indicate larger than
theoretically predicted dark current densities, despite the well known suppression of the Auger recombination mechanism.
Rather, dark current in SLS is thought to still be limited by trap-assisted tunneling in the depletion region and surface leakage
currents resulting from lack of fully passivated mesa sidewalls. An emerging infrared detector technology utilizing a
unipolar, single-band barrier design, the so-called nBn architecture, potentially suppresses these remaining noise current
mechanisms. In this report, measurements of the noise current spectral density of a mid-wave infrared nBn detector,
composed of a type-II InAs/GaSb strain layer superlattice (SLS) absorber (n) and contact (n) layers with an AlGaSb barrier
(B), under low-temperature, low-background conditions are presented. Here, noise was measured using a transimpedance
amplifier incorporating a dewar-mounted feedback resistor RF and source-follower MOSFET, both held at 77 K. This
configuration confines high detector impedance issues to the dewar, minimizes Johnson noise due to the electronics, and
enhances bandwidth by reducing stray capacitance. Features of the detector's noise spectrums at different bias are examined.
We report on dual-band (mid-/long-wave infrared) InAs/GaSb strained layer superlattice detector
with nBn and pBp architectures. Two band response was registered with 50% cut-off wavelengths of 5μm
(both nBn and pBp detectors) and 9μm (nBn)/10μm (pBp). The maximum peak responsivity of MWIR
absorber equal to 1.6 A/W (at λ = 5 μm and Vb = +1 V) and LWIR absorber equal to 1.2 A/W (at λ = 10 μm
and Vb = -1 V) for nBn detector, with the corresponding values of D* were 1.2 x 1011 Jones and 1.2 x 1010Jones for MWIR and LWIR absorbers, respectively (77 K). The maximum values of quantum efficiency were
estimated to 36% (MWIR) and 15% (LWIR) at Vb = +1V and Vb = -1V. For pBp detector, the responsivity
equal to 1.6 A/W (at λ = 5 μm and Vb = +0.4 V) and 1.8 A/W (at λ = 9 μm and Vb = -0.7 V) for MWIR and
LWIR absorbers was achieved with corresponding values of specific detectivity 5 x 1011 Jones and 2.6 x 1010Jones, respectively. The maximum values of quantum efficiency were estimated to 41% (MWIR) and 25%
(LWIR) at Vb = +0.4V and Vb = -0.7V. Moreover, the diffusion-limited behavior of dark current at higher
temperatures was observed for MWIR absorber for pBp detector. The overall performance of the dual-band
InAs/GaSb SLS detectors with investigated designs showed comparable (nBn design) and superior (pBp
design) performance to the QWIP detectors both in MWIR and LWIR bands and comparable performance to
MCT detectors in MWIR band (nBn and pBp detector designs).
Relationship between V/III beam equivalent pressure (BEP) flux ratios during the molecular beam epitaxial
(MBE) growth of long-wave infrared InAs/GaSb strained layer superlattice (SLS) material, crystalline quality of asgrown
material, and devices' signal (responsivity) and noise (dark current) characteristics was investigated. It was found
that the V/III ratio is a critical factor affecting the dark current, cut off wavelength and the responsivity of the device.
Modest change of As/In BEP flux ratio (from 5.5 to 7) resulted in red-shift of cut-off wavelength by 0.6 μm.
Temperature-dependent dark current measurements revealed more than two orders of magnitude difference in dark
current densities of detectors grown with different As/In BEP flux ratios. The highest responsivity and QE values, equal
to 0.75 A/W and 10% (74K, 9 μm, -0.4V), were demonstrated by the device with highest dark current density and notoptimal
structural properties. The observed dependences of devices' signal (responsivity) and noise (dark current)
characteristics in conjunction with the structural properties and the growth conditions of SLS material suggest that the
good structural properties of grown detector material as well as low noise would not necessary result in improved device
performance.
The use of Spatial Light Modulators (SLM), Liquid Crystal Devices for atmospheric turbulence simulation in
optical system has increased in the recent years. These devices allow the development of test-beds that can be
used to simulate, analyze and improve optical components or systems in a controlled laboratory environment
before further implementation on the eld. Most research has been performed at visible wavelengths with
the use of a vast array of atmospheric turbulence simulation algorithms. We present preliminary work on an
atmospheric simulation test bed which uses an algorithm developed at NRL with a transmissive high denition
Liquid Crystal Device SLMs for applications in the short-wavelength infrared, with the main focus of interested
at 1550nm. Preliminary results are shown for the application to a high denition re
ective Liquid Crystal Device
SLM for the same wavelength.
IR detectors operated in a space environment are subjected to a variety of radiation effects while required to have very
low noise performance. When properly passivated, conventional mercury cadmium telluride (MCT)-based infrared
detectors have been shown to perform well in space environments. However, the inherent manufacturing difficulties
associated with the growth of MCT has resulted in a research thrust into alternative detector technologies, specifically
type-II Strained Layer Superlattice (SLS) infrared detectors. Theory predicts that SLS-based detector technologies have
the potential of offering several advantages over MCT detectors including lower dark currents and higher operating
temperatures. Experimentally, however, it has been found that both p-on-n and n-on-p SLS detectors have larger dark
current densities than MCT-based detectors. An emerging detector architecture, complementary to SLS-technology and
hence forth referred to here as nBn, mitigates this issue via a uni-polar barrier design which effectively blocks majority
carrier conduction thereby reducing dark current to more acceptable levels.
Little work has been done to characterize nBn IR detectors tolerance to radiation effects. Here, the effects of gamma-ray
radiation on an nBn SLS detector are considered. The nBn IR detector under test was grown by solid source molecular
beam epitaxy and is composed of an InAs/GaSb SLS absorber (n) and contact (n) and an AlxGa1-xSb barrier (B). The
radiation effects on the detector are characterized by dark current density measurements as a function of bias, device
perimeter-to-area ratio and total ionizing dose (TID).
We report on the investigation of lateral diffusion of minority carriers in InAsSb based photodetectors with
the nBn design. Diffusion lengths (DL) were extracted from temperature dependent I-V measurements. The
behavior of DL as a function of applied bias, temperature, and composition of the barrier layer was
investigated. The obtained results suggest that lateral diffusion of minority carriers is not the limiting factor
for InAsSb based nBn MWIR detector performance at high temperatures (> 200K). The detector with an As
mole fraction of 10% in the barrier layer has demonstrated values of DL as low as 7 μm (Vb = 0.05V) at 240K.
Our group is investigating nBn detectors based on bulk InAs(1-x)Sb(x) absorber (n) and contacts (n) with an AlAs(1-x)Sb(x)
barrier (B). The wide-band-gap barrier material exhibits a large conduction band offset and small valence band offset
with respect to the narrow-band-gap absorber material. An important matter to explore in this design is the barrier
parameters (material, composition and doping concentration) and how they effect the operation of the device. This paper
investigates AlAs(1-x)Sb(x) barriers with different compositions and doping levels and their effect on detector
characteristics, in particular, dark current density, responsivity and specific detectivity.
Over the last several years the development of type-II Strained Layer Superlattice (SLS) infrared photodetectors has
yielded devices that may offer plausible alternative technology to conventional mercury cadmium telluride (MCT)-based
photodetectors. Prevailing theory predicts that SLS-based detector technologies will have several potential advantages
over MCT technologies, including lower dark currents and higher operating temperatures. However, experimentally it
has been found that conventional p-on-n and n-on-p SLS detectors have high dark current and thus, do not reach
theoretically predicted performance benchmarks. The two prevailing contributors to this high dark current are the
generation-recombination (GR) current and surface leakage currents, the latter resulting from the mesa sidewall
exposure. A recently emerging technology that utilizes a uni-polar barrier design nBn has been shown to reduce dark
current, while keeping the inherent advantages of SLS. Specific advantages of SLS over MCT include wavelength
tunability, improved uniformity, and operability potentially at a reduced manufacturing cost. This report presents some
recent experimental findings for the electrical and optical response of an nBn detector composed of an InAs/GaSb SLS
absorber (n) and contacts (n) with an AlGaSb barrier (B). Results include the intrinsic determination of the diffusion
current, and the GR current for the nBn device. Also presented is the optical response of the InAs/GaSb nBn detector at
77K over a broad range of operating biases. Dark current measurements over the 10K-300K temperature range were
undertaken to extract the activation energies in the heterostructure.
We report on the testing of a set of InAs/GaSb multicolor strained-layer superlattice photodetectors and Dotin-
Well detectors grown with InAs dots in InGaAs/GaAs wells fabricated by the Center for High Technology
Materials at the University of New Mexico. These devices are 2-color devices sensitive to near-IR and mid-IR
wavelengths. The wavelength sensitivities of these devices are a function of the applied forward and reverse bias.
We present measurements of the dark current and relative spectral response of these photodetectors measured
at both cryogenic and room temperatures.
The development of InAsSb detectors based on the nBn design for the mid-wave infrared (MWIR) spectral region is
discussed. Comparisons of optical and electrical properties of InAsSb photodetectors with two different barrier material,
namely, AlAs 0.15Sb0.75 (structure A) and AlAs0.10Sb0.9 (structure B) are reported. The dark current density in the
AlAs0.15Sb0.85 is lower possibly due to the larger valence band offset. Clear room temperature spectral responses is
observed and a specific detectivity (D*) of 1.4x1012 and 1.01x1012 cmHz1/2/W at 0.2 V, and a responsivity of 0.87 and
1.66 A/W under 0.2 V biasing at 77 K and 3.5 μm, assuming unity gain, was obtained for structures A and B,
respectively.
We report on surface passivation studies for type-II InAs/GaSb superlattice (SL) PIN detectors designed to
operate in the mid-wave infrared (MWIR) region and the long wavelength infrared (LWIR) spectrum. The two SL
structures were grown by molecular beam epitaxy and processed into mesa diodes using standard lithography. A simple
spin on photoresist, SU-8, was used to passivate the sample after a wet etch. Optical and electrical measurements were
then undertaken on the two devices. The dark current density of a single pixel device with SU-8 passivation is reduced
by four orders of magnitude and by a factor of eight compared to devices without any passivation for the MWIR and
LWIR pin detectors, respectively, at 77K.
We report heterojunction bandgap engineered long wave infrared (LWIR) photodetectors based on type-II InAs/GaSb
strained layer superlattices (SLS) which show significant improvement in performance over conventional PIN devices.
For this study, a device with unipolar barriers but same absorber region as PIN has been studied and compared. Unipolar
barriers reduce the tunneling currents and SRH recombination current in the active region due to reduced electric field
drop across the active region, while maintaining the photocurrent level. Moreover, they also reduce the diffusion current
by blocking the minority carriers from the two sides of the junction. We report three orders of magnitude reduction in the
dark current with the use of unipolar barriers. The reduction in the dark current results in significant improvement in
signal to noise ratio, resulting in measured specific detectivity of 2×1010 (cm-√Hz)/W and dark current density of 8.7
mA/cm2 at -0.5 V applied bias, for the 50% cutoff wavelength of 10.8μm.
We report on surface passivation using SU-8 for type-II InAs/GaSb strained layers superlattice (SLS) detectors
with a PIN design operating in mid-wave infrared (MWIR) spectral region (λ50% cut-off ~ 4.4 μm). Material growth and
characterization, single pixel device fabrication and testing, as well as focal plane array (FPA) processing are described.
High quality strain-balanced SLS material with FWHM of 1st SLS satellite peak of 36 arcsec is demonstrated. The
electrical and optical performance of devices passivated with SU-8 are reported and compared with those of unpassivated
devices. The dark current density of a single pixel device with SU-8 passivation showed four orders of magnitude
reduction compared to the device without any passivation. At 77K, the zero-bias responsivity and detectivity are equal to
1.1 A/W and 4 x 1012 Jones at 4μm, respectively, for the SU-8 passivated test pixel on the focal plane array.
The development of type-II InAs/GaSb superlattice (SL) detectors with nBn and pin designs for the long wave
infrared (LWIR) spectral region are discussed. First, SL growth optimization for LWIR region is explained,
then the structures based on LWIR nBn and pin are presented. Comparison of optical characterization for the
identical SL structures based on the nBn and pin designs is reported. Dark current density of 0.001 A/cm2 at
100 mV for nBn as compared to 0.2 A/cm2 for pin devices shows a reduction of dark current density using the
nBn design. At 77 K, the peak responsivity and detectivity are measured to be 5.86 A/W and 3.08 × 1010 Jones
for the nBn and 1.49 A/W and 4.2 × 109 Jones for the pin based design, respectively.
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