Developing uncooled photodetectors at mid-wavelength infrared (MWIR) is critical for various applications including remote sensing, heat seeking, spectroscopy, and more. In this study, we demonstrate room-temperature operation of nanowire-based photodetectors at MWIR composed of vertical selectivearea InAsSb nanowire photoabsorber arrays on large bandgap InP substrate with nanoscale plasmonic gratings. We accomplish this by significantly suppressing the nonradiative recombination at the InAsSb nanowire surfaces by introducing ex-situ conformal Al2O3 passivation shells. Transient simulations estimate an extremely low surface recombination velocity on the order of 10 3 cm/s. We further achieve room-temperature photoluminescence emission from InAsSb nanowires, spanning the entire MWIR regime from 3 µm to 5 µm. A dry-etching process is developed to expose only the top nanowire facets for metal contacts, with the sidewalls conformally covered by Al2O3 shells, allowing for a higher internal quantum efficiency. Based on these techniques, we fabricate nanowire photodetectors with an optimized pitch and diameter and demonstrate room-temperature spectral response with MWIR detection signatures up to 3.4 µm. The results of this work indicate that uncooled focal plane arrays at MWIR on low-cost InP substrates can be designed with nanostructured absorbers for highly compact and fully integrated detection platforms.

There exists a long-term need for foreign substrates on which to grow GaSb-based optoelectronic devices. We address this need by using interfacial misfit arrays to grow GaSb-based thermophotovoltaic cells directly on GaAs (001) substrates and demonstrate promising performance. We compare these cells to control devices grown on GaSb substrates to assess device properties and material quality. The room temperature dark current densities show similar characteristics for both cells on GaAs and on GaSb. Under solar simulation the cells on GaAs exhibit an open-circuit voltage of 0.121 V and a short-circuit current density of 15.5 mA/cm2. In addition, the cells on GaAs substrates maintain 10% difference in spectral response to those of the control cells over a large range of wavelengths. While the cells on GaSb substrates in general offer better performance than the cells on GaAs substrates, the cost-savings and scalability offered by GaAs substrates could potentially outweigh the reduction in performance. By further optimizing GaSb buffer growth on GaAs substrates, Sb-based compound semiconductors grown on GaAs substrates with similar performance to devices grown directly on GaSb substrates could be realized.

The optical properties of InGaAs/GaAs surface quantum dots (SQDs) and buried QDs (BQDs) are investigated by photoluminescence (PL) measurements. The integrated PL intensity, linewidth, and lifetime of SQDs are significantly different from the BQDs both at room temperature and at low temperature. The differences in PL response, measured at both steady state and in transient, are attributed to carrier transfer between the surface states and the SQDs.

Growth of GaSb with low threading dislocation density directly on GaAs may be possible with the strategic strain relaxation of interfacial misfit arrays. This creates an opportunity for a multijunction solar cell with access to a wide range of well-developed direct bandgap materials. Multijunction cells with a single layer of GaSb/GaAs interfacial misfit arrays could achieve higher efficiency than state-of-the-art inverted metamorphic multi-junction cells while forgoing the need for costly compositionally graded buffer layers. To develop this technology, GaSb single junction cells were grown via molecular beam epitaxy on both GaSb and GaAs substrates to compare homoepitaxial and heteroepitaxial GaSb device results. The GaSb-on-GaSb cell had an AM1.5g efficiency of 5.5% and a 44-sun AM1.5d efficiency of 8.9%. The GaSb-on-GaAs cell was 1.0% efficient under AM1.5g and 4.5% at 44 suns. The lower performance of the heteroepitaxial cell was due to low minority carrier Shockley-Read-Hall lifetimes and bulk shunting caused by defects related to the mismatched growth. A physics-based device simulator was used to create an inverted triple-junction GaInP/GaAs/GaSb model. The model predicted that, with current GaSb-on-GaAs material quality, the not-current-matched, proof-of-concept cell would provide 0.5% absolute efficiency gain over a tandem GaInP/GaAs cell at 1 sun and 2.5% gain at 44 suns, indicating that the effectiveness of the GaSb junction was a function of concentration.

Exploring the potentiality of enhancing the performance of avalanche photo diodes (APDs) using novel nanoscale structures is highly attractive for overcoming the bottleneck of avalanche probability. This work demonstrates, for the first time, multiplication enhancement of electroninitiated photocur rent due to impact ionization in InAs quantum dots (QDs) within a GaAs APD structure. A fivelayer stacked 2.25 MLs InAs QD/50 nm GaAs spacer multiplication structure integrated into a separated absorption, charge, and multiplication GaAs homo APD results in up to six times higher multiplica tion factors in comparison to a reference device without QD over a tempera ture range of 77-300 K. In addition, extremely low excess noise factor in close proximity to that of silicon is also observed with an effective k eff factor below 0.1. This demonstration is of fundamental interest and relevant for future ultra efficient avalanche detector applications.

We extract the carrier mobility-lifetime products for epitaxially grown GaSb and demonstrate the spectral response to gamma rays of a GaSb p–i–n photodiode with a 2-µm-thick absorption region. Under exposure from 55Fe and 241Am radioactive sources at 140 K, the photodiode exhibits full width at half maximum energy resolutions of 1.238 ± 0.028 and 1.789 ± 0.057 keV at 5.89 and 59.5 keV, respectively. We observe good linearity of the GaSb photodiode across a range of photon energies. The electronic noise and charge trapping noise are measured and shown to be the main components limiting the measured energy resolutions.

and multiplication avalanche photodiodes (SAM-APDs) composed of photoabsorbers and large bandgap multiplication regions are advantageous for achieving highefficient and low-noise photodetection with high-energy resolution. [11][12][13] Such a detector architecture can be also applied to energy-sensitive gamma-ray and X-ray detection. A GaAs/AlGaAs SAM-APD comprised of a 4.5 µm absorption region and a staircase-like multiplication region has been developed for X-ray photodetection. [14] Meanwhile, a similar GaAs/ Al 0.8 Ga 0.2 As SAM-APD structure has been also demonstrated to detect soft X-rays at 5.9 keV with a full-width half-maximum (FWHM) of 1.08 keV. [15] However, its spectroscopic characterizations show an undesired secondary peak located at energy lower than the 5.9 keV peak. This is because of 1) different degrees of impact ionization experienced by the electrons and holes; [16][17][18] 2) insufficient layer thickness ratio; [19] and 3) inadequate difference of pair creation energy (PCE) between GaAs and AlGaAs. [20,21] Indeed, the absorption efficiency can be improved by simply increasing the thickness of the absorption layer. Unfortunately, the commonly used (Al)GaAs semiconductors are not able to concurrently offer high-Z (i.e., high atomic number) absorption and large PCE difference within SAM structures for highenergy X-ray or gamma-ray detection.Alternatively, the antimony-based (Sb-based) SAM-APD structure with high-Z GaSb and large bandgap AlAsSb is promising. GaSb absorbers can provide a much higher probability than GaAs to stop X-ray and gamma-ray photons at a given energy due to a relatively higher Z. [22] In addition, the GaSb/AlAsSb material system provides a larger dissimilarity in both PCE and absorption efficiency. [23,24] This allows for a significant suppression of spurious photopeaks, which are generated outside the intended absorption regions. The combination of these unique capabilities shows promise for achieving X-ray and gamma-ray detectors with high spectroscopic performance. To prove the concept, we develop GaSb/AlAsSb SAM-APDs composed of 2 µm GaSb absorbers and AlAsSb digital alloy multiplication regions to detect X-ray and gamma-ray photopeaks under irradiation from 241 Am sources. Due to the reduced high-peak electric Demonstrated are antimony-based (Sb-based) separate absorption and multiplication avalanche photodiodes (SAM-APDs) for X-ray and gamma-ray detection, which are composed of GaSb absorbers and large bandgap AlAsSb multiplication regions in order to enhance the probability of stopping highenergy photons while drastically suppressing the minority carrier diffusion. Well-defined X-ray and gamma-ray photopeaks are observed under exposure to 241 Am radioactive sources, demonstrating the desirable energy-sensitive detector performance. Spectroscopic characterizations show a significant improvement of measured energy resolution due to reduced high-peak electric field in the absorbers and suppressed nonradiative recombination on surfaces. Additionally, the GaSb/AlAsSb SAM...

This work develops a (NH4)2S/Al2O3 passivation technique for photodiode-basedGaSb/AlAsSb heterostructure. Surface-sulfurated GaSb/AlAsSb heterostructure mesas show a significant suppression of reversed-bias dark current by 4 -5 orders of magnitude after they are further passivated by Al2O3 layers. So the mesa sidewalls treated with (NH4)2S/Al2O3 layers can effectively inhibit the shunt path of dark carriers. The activation energies for both bulk and surface components are extracted from temperature-dependent current-voltage characteristics, which suggest that the bulk characteristics remain unchanged, while Fermi-level pinning at surfaces is alleviated. Additionally, temperature coefficients of the breakdown voltage are extracted, confirming that the breakdown process is confined entirely in the large bandgap AlAsSb regions. This study shows that the implementation of (NH4)2S/Al2O3 passivation can lead to room-temperature GaSb-based photodiodes and GaSb/AlAsSb-based avalanche photodiodes for highly efficient photodetection..