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.
InAs/GaSb type-II strained-layer superlattice (T2SLS) materials are being considered for space-based infrared detector applications. However, an inadequate understanding of the role of carrier transport, specifically the vertical mobility, in the radiation tolerance of T2SLS detectors remains. Here, progress towards a vertical transport study of proton-irradiated, p-type InAs/GaSb T2SLS materials using magnetoresistance measurements is reported. Measurements in the growth direction of square mesas formed from InAs/GaSb superlattice material were performed using two distinct contact geometries in a Kelvin mode setup at variable magnetic fields, ranging from -9 T to 9 T, and temperatures, ranging from 5 K and 300 K. The results here suggested multi-carrier conduction and a field-dependent series resistance from the contact layer were present. The implications of these results and the plans for future magnetoresistance measurements on proton-irradiated T2SLS materials are discussed.
Metallic metamaterial structures are used in nanophotonics applications in order to localize and enhance an incident
electromagnetic field. We have theoretically and experimentally studied resonant coupling between plasmonic modes of
an SRR array and a quantum dot-in-a-well (DWELL) heterostructure. The near-field distribution from the SRRs on the
GaAs substrate was first modeled by electromagnetic simulations and optimized SRR dimensions for maximum nearfield
coupling at the peak absorption were extracted. The DWELL sample with a ground state emission peak at 1240 nm
was grown by molecular beam epitaxy on a semi-insulating GaAs substrate. The sample was uniformly covered with an
array of SRRs, and patterned by standard electron-beam-lithography. In order to study the near field coupling of the
plasmonic structure into the DWELL, optical characterization was performed on the SRR-DWELL heterostructure,
including room temperature photoluminescence, and transmission measurement.
We propose a facile approach to fabricate graphene nano-objects (GNOs) using interference lithography (IL) and direct
self-assembly of nanoparticles. Uniformly spaced parallel photoresist (PR) lines and periodic hole arrays are proposed as
an etch mask for producing graphene nanoribbons (GNRs), and graphene nanomesh (GNM), respectively. In a different
experiment, the PR line arrays are transferred to uniform oxide channels, and silica nanoparticle dispersions with an
average size of 10 nm are spun on the patterned surface, leaving a monolayer uniform nanoparticle assembly on the
graphene. Following the particle deposition, the graphene is removed in the narrow spacing between the particles, using
the O2 plasma etch, leaving ordered graphene quantum dot (GQD) arrays. The IL technique and etch process enables
tuning the GNOs dimensions.
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.
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.
Electronic transport parameters in a nominally P+/π/P+ InAs/GaSb type-II superlattice vertical photoconductor
structure for long-wavelength infrared detectors have been characterized employing magnetic field dependent
resistivity and Hall-effect measurements, and high-resolution mobility spectrum analysis. Carrier transport parameters
from both the P+ and nominally π regions were obtained over the 80 to 300K temperature range. At
300 K, the minority carrier electrons in the nominally π region was found to be characterized by a mobility and
concentration of 11,000 cm2/Vs and 1.1×1017 cm-3, respectively. Taking into account our previously reported
room-temperature vertical electron transport parameters,1 the vertical to lateral mobility and carrier concentration
ratios have been determined to be 0.19 and 5.5×10-4 , respectively. A miniband energy gap of 192±8 meV
was estimated from the thermal activation of the minority carrier electrons in the lightly doped InAs/GaSb
superlattice region.
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.
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.
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.
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.
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.
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