HgCdTe has been called the ideal infrared detector material for good reason: high absorption coefficients and very long Shockley-Read-Hall (SRH) recombination lifetimes lead to the highest performance infrared detectors today for space applications. III-V materials, such as InAsSb, are currently limited by short SRH recombination lifetimes due to defects, and their performance is still relatively lacking for space applications where sensitivity requirements are extremely high. However, the performance of III-V superlattice infrared detectors has improved such that it is sufficient for tactical applications, which can now take advantage of the manufacturing benefits of III-V (greater uniformity and yield). With the growing NewSpace movement, there is a need for higher-volume, lower-cost infrared detectors capable of operating in space for applications such as environmental monitoring, space-based weather, and planetary science. One way to increase volume and lower cost is to grow the detectors on large-format substrates, such as 6-inch silicon or GaAs, but lattice-matched large substrates are not available for HgCdTe or InAsSb. Here a comparison between mid-wavelength infrared HgCdTe and InAsSb infrared detectors grown on non-lattice-mismatched substrates and designed for increased proton radiation tolerance, as compared to previous designs on mismatched substrates, is given. The comparison of these recent HgCdTe photodiode and InAsSb bariode designs for space applications shows that the InAsSb bariode has an order of magnitude better dark current density proton radiation tolerance while the HgCdTe photodiode has an order of magnitude better quantum efficiency proton radiation tolerance operating at 130 K. Therefore, the choice of detector material and architecture is not clear and will depend on the required performance for a specific space application.
To improve the performance of photodiodes based on narrow-bandgap InAs/GaSb type-II strained layer superlattices (T2SLs), knowledge of the vertical minority carrier transport is necessary. For this purpose, the key parameters influencing vertical minority-carrier electron transport in an nBp MWIR detector were studied: diffusion length, lifetime, mobility. The detectors were designed with p-type, 10/10 ML, InAs/GaSb T2SL absorbers, targeting a 50% cut-off wavelength of 5.0 µm at 80 K. The nBp structure is attractive because the junction field predominately drops across a relatively wide-gap InAs/AlSb SL barrier, which reduces the expected generation-recombination dark current. Measurements of the electron beam-induced current (EBIC), combined with minority carrier lifetime results from microwave reflectance measurements, enabled the determination of the minority carrier diffusion length (Le) and mobility in the growth direction as a function of temperature. The Le was extracted at each temperature by fitting the EBIC data to analytical expressions for carrier collection efficiency. The EBIC measurements were also repeated at different electron-beam energies to vary the distribution of minority carriers near the surface to gauge the surface recombination velocity. Microwave reflectance allowed for accurate measurement of the minority carrier lifetime over a large dynamic range of excess carrier concentrations, enabling a separation of recombination mechanisms. The lifetime and extracted diffusion length data were then used to estimate the diffusion coefficient and mobility versus temperature by applying the Einstein diffusion relationship.
Accurate p-type doping of the active region in III-V infrared detectors is essential for optimizing the detector design and overall performance. While most III-V detector absorbers are n-type (e.g., nBn), the minority carrier devices with p-type absorbers would be expected to have relatively higher quantum efficiencies due to the higher mobility of their constituent minority carrier electrons. However, correctly determining the hole carrier concentration in narrow bandgap InAsSb may be challenging due to the potential for electron accumulation at the surface of the material and at its interface with the layer grown directly below it. Electron accumulation layers form high conductance electron channels that can dominate both resistivity and Hall-effect transport measurements. Therefore, to correctly determine the bulk hole concentration and mobility, temperature- and magnetic-field-dependent transport measurements in conjunction with Multi-Carrier Fit analysis were utilized on a series of p-doped InAs0.91Sb0.09 samples on GaSb substrates. The resulting hole concentrations and mobilities at 77 K (300 K) were 1.6 x 1018 cm-3 (2.3 x 1018 cm-3) and 125 cm2 V-1 s-1 (60 cm2 V-1 s-1), respectively, compared with the intended Be-doping of ~2 x 1018 cm-3.
Strain-balanced InAs/InAsSb superlattices can be tuned to absorb and emit across the mid- to long-wave infrared, and exhibit appropriate minority carrier lifetimes for high performance infrared photodetectors. The optical quality of this material has been shown to improve with the use of Bi as a surfactant. Specifically, InAs/InAsSb superlattices grown at 425 °C and 430 °C exhibit improved photoluminescence intensity for Bi/In flux ratios up to 1.0%, and optical quality improves further with increasing growth temperature and increasing Bi/In flux ratios up to 5.0%. The identification of optimal growth conditions for InAs/InAsSb superlattices with Bi surfactant, as well as further exploration of the impact of Bi surfactant is an important component to further developing and optimizing this infrared material system.
Several strain-balanced InAs/InAsSb superlattices are grown using molecular beam epitaxy at temperatures ranging from 425 °C to 475 °C using Bi/In flux ratios ranging from 0.0% to 10.0%. The structural and optical properties of the samples are evaluated using X-ray diffraction, secondary ion mass spectrometry, and photoluminescence spectroscopy. Analysis of the mass spectrometry data indicates that surfactant Bi incorporates into the InAs/InAsSb material system with a sticking coefficient of 0.3% at 450 °C, yielding dopant-level concentrations for typical Bi/In surfactant flux ratios. Analysis of the integrated photoluminescence intensity indicates that photoluminescence efficiency is greatest with a 1.0% Bi/In flux ratio for growth at 425-430 °C, and a 5.0% Bi/In flux ratio for growth at 450-475 °C. The improvement in photoluminescence efficiency is associated with a longer Shockley-Read-Hall lifetime in the superlattices grown with Bi surfactant.
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.
Type-II strained layer superlattices (SLS) are an active research topic in the infrared detector community and applications for SLS detectors continue to grow. SLS detector technology has already reached the commercial market due to improvements in material quality, device design, and device fabrication. Despite this progress, the optimal superlattice design has not been established, and at various times has been believed to be InAs/GaSb, InAs/InGaSb, or InAs/InAsSb. Building on these, we investigate the properties of a new mid-wave infrared SLS material: InGaAs/InAsSb SLS. The ternary InGaAs/InAsSb SLS has three main advantages over other SLS designs: greater support for strain compensation, enhanced absorption due to increased electron-hole wavefunction overlap, and improved vertical hole mobility due to reduced hole effective mass. Here, we compare three ternary SLSs, with approximately the same bandgap (0.240 eV at 150 K), comprised of Ga fractions of 5%, 10%, and 20% to a reference sample with 0% Ga. Enhanced absorption is both theoretically predicted and experimentally realized. Furthermore, the characteristics of ternary SLS infrared detectors based on an nBn architecture are reported and exhibit nearly state-of-the-art dark current performance with minimal growth optimization. We report standard material and device characterization information, including dark current and external quantum efficiency, and provide further analysis that indicates improved quantum efficiency and vertical hole mobility. Finally, a 320×256 focal plane array built based on the In0.8Ga0.2As/InAs0.65Sb0.35 SLS design is demonstrated with promising performance.
The last two decades have seen tremendous progress in the design and performance of mid-wavelength infrared (MWIR) type-II superlattices (T2SL) for detectors. The materials of focus have evolved from the InAs/(In)GaSb T2SL to include InAs/InAsSb T2SLs and most recently InGaAs/InAsSb SLs, with each materials system offering particular advantages and challenges. InAs/InAsSb SLs have the longest minority carrier lifetimes, and their best nBn dark current densities are <5X Rule ’07 at high temperatures, while those of InAs/GaSb SLs and InGaAs/InAsSb SLs are <10X Rule ’07. The quantum efficiency of all three SL detectors can still be improved, especially by increasing the diffusion length beyond the absorber length at low temperatures. Evidence of low temperature carrier localization is greatest for the two SLs containing ternary layers; however, the interface intermixing causing the localization is present in all three SLs. Localization likely does not affect the high temperature detector performance (>120 K) where these SL unipolar barrier detectors are diffusion-limited and Auger-limited. The SL barrier detectors remain diffusion-limited post proton irradiation, but the dark current density increases due to the minority carrier lifetime decreasing with increased displacement damage causing an increase in the trap density. For these SL detectors to operate in space, the continued understanding and mitigation of point defects is necessary.
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).
Significant attention has recently been given to photoluminescence (PL) spectra and lifetime measurements on InAs/InAsSb superlattices, as high quality optical material with long carrier lifetimes are required for infrared detectors. The standard sample structure for PL measurements includes energy barriers to block photo-generated carriers from being lost through non-radiative recombination at interfaces between the superlattice and the surface or between the superlattice and the buffer/substrate. However, defect, surface, and/or interface states in AlSb, a commonly used barrier material, are known to contribute carriers to InAs quantum wells. Due to the similarity of the AlSb interface with the InAs/InAsSb superlattice, the effects of the barriers on the electrical and optical properties of the superlattice were investigated. Structures with AlSb barriers at the top and bottom of the superlattice, with no AlSb barriers, and with an AlSb barrier only at the top of the superlattice structure were studied. Hall Effect measurements revealed little change in the sheet carrier concentration at 10 K due to the barriers, but significant increases in low temperature mobility and a two-dimensional-like mobility temperature dependence were observed when barriers were present. Further high magnetic field measurements are necessary, however, to understand the transport properties of these samples due to the likelihood that multiple carriers are present. The photoluminescence (PL) spectra were almost identical regardless of the barriers, except for a 15% increase in intensity with the AlSb barrier between the buffer layer and the superlattice. The surface AlSb barrier had little effect on the intensity. The barriers are therefore recommended for PL measurements to increase the signal intensity; however, they complicate the analysis of single-field Hall Effect measurements.
Recently, a new strategy used to achieve high operation temperature (HOT) infrared photodetectors including III-V compound materials (bulk materials and type-II superlattices) and cascade devices has been observed. Another method to reduce detector’s dark current is reducing volume of detector material via a concept of photon trapping detector. The barrier detectors are designed to reduce dark current associated with Shockley-Read (SR) processes and to decrease influence of surface leakage current without impeding photocurrent (signal). In consequence, absence of a depletion region in barrier detectors offers a way to overcome the disadvantage of large depletion dark currents. So, they are typically implemented in materials with relatively poor SR lifetimes, such as all III-V compounds. From considerations presented in the paper results that despite numerous advantages of III-V barrier detectors over present-day detection technologies, including reduced tunneling and surface leakage currents, normal-incidence absorption, and suppressed Auger recombination, the promise of a superior performance of these detectors in comparison to HgCdTe photodiodes, has not been yet realized. The dark current density is higher than that of bulk HgCdTe photodiodes, especially in MWIR range. To attain their full potential, the following essential technological limitations such as short carrier lifetime, passivation, and heterostructure engineering, need to be overcome.
The material properties of p-type InAs/InAsSb superlattices are of interest for infrared photodiodes, but
InAs/InAsSb superlattices are residually n-type and p-type superlattices have not been investigated thus far. This study
examines the material properties of a mid-wavelength infrared InAs/InAsSb superlattice design doped with Be
concentrations from 0.5-7x1016 cm-3. High-resolution x-ray diffraction revealed slight structural variation throughout the
~500 nm thick superlattice layer, but the RMS surface roughness was reasonable. Hall Effect measurements, taken at
10 K to remove any conduction effects from the undoped GaSb substrate, revealed the superlattice converting from ntype
to p-type at Be:3x1016 cm-3. The maximum hole mobility achieved at the two highest Be doping levels was
~24,000 cm2/Vs, which is high for mid-wavelength infrared superlattices. The doped superlattices all had
photoluminescence (PL) peaks 12 - 34 meV lower in energy than the undoped sample, and the PL peak FWHMs
increased as the average superlattice mismatch increased, as expected. Comparing the photoresponse to the PL allowed
the Be acceptor binding energy in the superlattice (13 meV) to be determined, which agreed with the reported Be
acceptor binding energy in InAs.
Infrared detector arrays operating in space must be able to withstand defect-inducing proton radiation without performance degradation. Therefore, it is imperative that the proton-radiation hardness of infrared detector materials be investigated. Photoluminescence (PL) is sensitive to defects in materials, and thus can be used to quantify the effects of proton-radiation-induced defects. The excitation intensity-dependent PL was used to examine of a set of InAs/InAsSb superlattices before and after 63-MeV-proton irradiation. A proton dose of 100 kRad(Si) was applied to a different piece of each superlattice sample. The low-temperature excitation intensity dependent PL results reveal minimal increases in the carrier concentration, non-radiative recombination, and the PL full-width half-maximum. These results suggest that InAs/InAsSb superlattices are quite tolerant of proton irradiation and may be suitable for space infrared detector arrays.
The stability of colloidal PbS quantum dot (QD) films deposited on various substrates including glass and GaAs was
studied. Over a period of months, the QD film sample was re-tested after being left unprotected in air under ambient
conditions. Despite exposure to 532 nm laser excitation and cooling to cryogenic temperatures, the initial
photoluminescence (PL) remained stable between tests. We also retested a set of samples that had remained under
ambient conditions for over 2 years. To track potential changes to the QDs over time, X-ray photoelectron
spectroscopy (XPS), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), optical microscopy,
UV-Vis-NIR spectrophotometry and atomic force microscopy (AFM) were employed. Evidence points towards
oxidation enforced shrinking of the active QD volume causing a blue shift of the absorption and photoluminescence.
The presented studies are important for reliability expectations of light emitters based on PbS QDs.
Temperature-dependent minority carrier lifetimes of InAs/InAs1-xSbx type-II superlattices are presented. The longest lifetime at 11 K is 504 ± 40 ns and at 77 K is 412 ± 25 ns. Samples with long periods and small wave function overlaps have both non-radiative and radiative recombination mechanisms apparent, with comparable contributions from both near 77 K, and radiative recombination dominating at low temperatures. Samples with short periods and large wave function overlaps have radiative recombination dominating from 10 K until ~200 K. The improved lifetimes observed will enable long minority carrier lifetime superlattices to be designed for high quantum efficiency, low dark current infrared detectors.
Optical and structural properties of InAs/InAsSb type-II superlattices (T2SL) and their feasibility for mid- and longwavelength
infrared (MWIR and LWIR) photodetector applications are investigated. The InAs/InAsSb T2SL structures
with a broad bandgap range covering 4 μm to 12 μm are grown by molecular beam epitaxy and characterized by highresolution
x-ray diffraction and photoluminescence (PL) spectroscopy. All of the samples have excellent structural
properties and strong PL signal intensities of the same order of magnitude, indicating that non-radiative recombination is
not dominant and the material system is promising for high performance MWIR and LWIR detectors and multiband
FPAs.
This paper describes structural properties of strain-balanced InAs/InAs1-xSbx type-II superlattices (SLs) with random and
modulated InAs/InAs1-xSbx alloy layers as grown on GaSb(001) substrates either by molecular beam epitaxy (MBE) or
metalorganic chemical vapor deposition. The SL periods and the average Sb compositions of the InAs/InAs1-xSbx alloys are
determined by comparison of simulations with (004) high-resolution X-ray diffraction (XRD) measurements. The most
intense SL peaks no longer correspond to the zero-order peak because of the large SL periods, and XRD studies of thick
individual InAs/InAs1-xSbx and InAs layers show envelope modulations of the SL peaks on either side of the substrate peak,
causing some satellite peaks to be more intense than the zero-order SL peak. From the substrate - zero-order SL peak
separations, the average SL strain in the growth direction is revealed to be less than ~0.2%. Calculated bandgap energies
agree closely with photoluminescence peaks for mid-wavelength and long-wavelength infrared samples. Cross-sectional
electron micrographs reveal the entire structure including the GaSb substrate and buffer layer, the SL periods, and the
GaSb cap layer. Growth defects are occasionally visible, some originating at the substrate/buffer interface, some starting
in the middle of the buffer layer, and some located only just within the SL. Higher magnification images of the SLs
grown by MBE reveal that interfaces for InAs/InAs1-xSbx deposited on InAs are considerably more abrupt than those of InAs
deposited on InAs/InAs1-xSbx with the most likely reason being segregation of the Sb surfactant during layer growth.
InAs/InAs1-xSbx strain-balanced superlattices (SLs) on GaSb are a viable alternative to the well-studied InAs/Ga1-xInxSb
SLs for mid- and long-wavelength infrared (MWIR and LWIR) laser and photodetector applications, but the InAs/InAs1-xSbx SLs are not as thoroughly investigated. Therefore, the valence band offset between InAs and InAs/InAs1-xSbx, a critical
parameter necessary to predict the SL bandgap, must be further examined to produce InAs/InAs1-xSbx SLs for devices
operational at MWIR and LWIR wavelengths. The effective bandgap energies of InAs/InAs1-xSbx SLs with x = 0.28 -
0.40 are designed using a three-band envelope function approximation model. Multiple 0.5 μm-thick SL samples are
grown by molecular beam epitaxy on GaSb substrates. Structural characterization using x-ray diffraction and atomic
force microscopy reveals excellent crystalline properties with SL zero-order peak full-width-half-maximums between 30
and 40 arcsec and 20 x 20 μm2 area root-mean-square roughnesses of 1.6 - 2.7 A. Photoluminescence (PL) spectra of
these samples cover 5 to 8 μm, and the band offset between InAs and InAs/InAs1-xSbx is obtained by fitting the PL peaks to
the calculated values. The bowing in the valence band is found to depend on the initial InAs/InSb valence band offset
and changes linearly with x as CEv_bowing = 1.58x - 0.62 eV when an InAs/InAs1-xSbx bandgap bowing parameter of 0.67 eV is
assumed. A fractional valence band offset, Qv = ΔEv/ΔEg, of 1.75 ± 0.03 is determined and is practically constant in the
composition range studied.
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