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.
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.
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.
In this work, we report on the measurement of vertical transport parameters in p-doped InAs/GaSb type-II
superlattices for long-wavelength infrared detectors. Variable magnetic eld geometrical magnetoresistance mea-
surements have been employed to extract the vertical transport parameters, since the Hall-eect technique cannot
be employed in the vertical transport conguration. The room-temperature magnetoresistance measurements
were performed employing a kelvin-mode set up, at electric elds not exceeding 25 V/cm and at magnetic eld
intensities up to 12 T. The measured magnetoresistance, shown to exhibit multiple-carrier conduction charac-
teristics, were analyzed using a high-resolution mobility spectrum analysis technique. It is shown that, at room
temperature, the electrical conductivity of the sample is due to four distinct carriers, associated with the major-
ity carrier holes, sidewall inversion layer electrons, and two minority carrier electrons likely associated with two
distinct conduction band levels.
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.
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.
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 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.
We report on the performance of multi-stack quantum dots in a well (DWELL) detectors. Present-day QD detectors are
limited by low responsivity and quantum efficiency (QE). This can be attributed to the low absorption efficiency of
these structures due to the small number of QD stacks in the detector. In this paper we examine the effect of the number
of stacks on the performance of the detector. In particular, we investigate the InAs/GaAs/AlGaAs D-DWELL (Dots-in-double-well) design, which has a lower strain per DWELL stack than the InAs/InGaAs/GaAs DWELLs thereby enabling
the growth of many more stacks in the detector. The purpose of the study detailed in this paper is to examine the effects
of varying the number of stacks in the InAs/InGaAs/GaAs/AlGaAs D-DWELL detector, on its device performance. The
numbers of stacks grown using solid source molecular beam epitaxy (MBE), were 15, 30, 40, 50, and 60. Once
fabricated as single pixel devices, we carried-out a series of device measurements such as spectral response, dark current,
total current, responsivity along with computing the photoconductive gain and the activation energies. The goal of these
experiments is to not only study the single pixel detector performance with varying number of stacks in a D-DWELL
structure, but to also understand the effect of the transport mechanism in these devices.
We report the formation and growth characteristics of an interfacial misfit (IMF) array between AlSb and Si and their application to III-Sb based quantum well (QW) light-emitting devices including edge-emitting laser diodes and verticalcavity surface emitting lasers (VCSELs) monolithically grown on a Si (001) substrate. A III-Sb epi-structure is grown monolithically on the Si substrate via a thin (≅50 nm) AlSb nucleation layer. A 13% lattice mismatch between AlSb and Si is accommodated by using the IMF array. We demonstrate monolithic VCSELs grown on Si(001) substrates operating under room-temperature with optically-pumped conditions. A 3-mm pump spot size results in peak threshold excitation density of Ith= 0.1 mJ/cm2 and a multimode lasing spectrum peak at 1.62 μm. Moreover, broad-area edgeemitters consisting of GaSb/AlGaSb QWs are demonstrated under pulsed conditions at 77K with a threshold current density of ≅2 kA/cm2 and a maximum peak output power of ≅20 mW for a 1mm-long device. A use of 5° miscut Si substrates enables both IMF formation and suppression of an anti-phase domain, resulting in a drastic suppression of dislocation density over the III-Sb epi-layer and realization of electrically-injected laser diodes operating at 77 K. The current-voltage (I-V) characteristics indicate a diode turn-on of 0.7 V, which is consistent with a theoretical built-in potential of the laser diode. This device is characterized by a 9.1 Ω forward resistance and a leakage current density of 0.7 A/cm2 at -5 V and 46.9 A/cm2 at -15 V. This IMF technique will enable the realization of III-Sb based electrically injected VCSELs operating at the fiber-optic communication wavelength monolithically grown on a Si platform.
We present a 1.54 μm, 77 K, pulsed GaSb quantum well (QW) laser diode grown monolithically on a Si(100)-5°
substrate. The III-Sb device is grown on an AlSb nucleation layer on Si with the 13% mismatch accommodated by a
self-assembled 2D array of pure 90° dislocations. We demonstrate the simultaneous formation of this interfacial misfit
dislocation (IMF) array along with antiphase domain suppression in the growth of AlSb on 5° miscut Si (001) substrate.
The lomer dislocation spacing in the IMF (~ 3.46 nm) corresponds to the 13% mismatch between AlSb and Si and is
also well matched to the step length of the 5° miscut Si (001) substrate. The resulting bulk material has both very low
defect density (~7 × 105/cm2) and very low APD density (~ 103/cm2) confirmed by transmission electron and atomic force microscope images. The GaSb QW based laser diodes are grown on this high quality AlSb layer and the resulting
devices operate at 77 K under pulsed conditions (2 μsec pulse-width and a 0.1% duty cycle) with an emission
wavelength of 1.54 μm and a threshold current density of 2 kA/cm2 for a 100 μm x 2mm device. The maximum peak
power from the device is ~ 20 mWatts.
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