Optical characterization of AlAsSb digital alloy and random alloy on GaSb

III-(As, Sb) alloys are building blocks for various advanced optoelectronic devices, but the growth of their ternary or quaternary materials are commonly limited by spontaneous formation of clusters and phase separations during alloying. Recently, digital alloy growth by molecular beam epitaxy has been widely adopted in preference to conventional random alloy growth because of the extra degree of control offered by the ordered alloying. In this article, we provide a comparative study of the optical characteristics of AlAsSb alloys grown lattice-matched to GaSb using both techniques. The sample grown by digital alloy technique showed stronger photoluminescence intensity, narrower peak linewidth, and larger carrier activation energy than the random alloy technique, indicating an improved optical quality with lower density of non-radiative recombination centers. In addition, a relatively long carrier lifetime was observed from the digital alloy sample, consistent with the results obtained from the photoluminescence study

GaSb on GaAs interfacial misfit solar cells

The GaAs/GaSb interface misfit design can achieve comparable efficiency to conventional inverted metamorphic multijunction cells at up to 30% cost reduction. In this pre-liminary work, GaSb single junctions were grown via molecular beam epitaxy on both GaSb and GaAs substrates to compare and fine tune the interfacial misfit growth process. Current vs voltage results show that the best homo-epitaxial cell achieved 5.2% under 35-sun concentration. TEM did not reveal any threading dislocations in the hetero-epitaxial cells, however, device results indicated higher non-radiative recombination than expected, likely due to unpassivated surface states. Improvements to cell processing will be explored and more characterization is planned to determine the cause of degraded hetero-epitaxial cell performance

Energy-sensitive GaSb/AlAsSb separate absorption and multiplication avalanche photodiodes for X-Ray and gamma-ray detection

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 high‐energy photons while drastically suppressing the minority carrier diffusion. Well‐defined X‐ray and gamma‐ray photopeaks are observed under exposure to 241Am 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‐APDs clearly exhibit energy response linearity up to 59.5 keV with a minimum full‐width half‐maximum of 1.283 keV. A further analysis of the spectroscopic measurement suggests that the device performance is intrinsically limited by the noise from the readout electronics rather than that from the photodiodes. This study provides a first understanding of Sb‐based energy‐sensitive SAM‐APDs and paves the way to achieving efficient detection of high‐energy photons for X‐ray and gamma‐ray spectroscopy

Hybrid type-I InAs/GaAs and type-II GaSb/GaAs quantum dot structure with enhanced photoluminescence

We investigate the photoluminescence (PL) properties of a hybrid type-I InAs/GaAs and type-II GaSb/GaAs quantum dot (QD) structure grown in a GaAs matrix by molecular beam epitaxy. This hybrid QD structure exhibits more intense PL with a broader spectral range, compared with control samples that contain only InAs or GaSb QDs. This enhanced PL performance is attributed to additional electron and hole injection from the type-I InAs QDs into the adjacent type-II GaSb QDs. We confirm this mechanism using time-resolved and power-dependent PL. These hybrid QD structures show potential for high efficiency QD solar cell applications

Room-temperature midwavelength infrared InAsSb nanowire photodetector arrays with Al2O3 passivation

Developing uncooled photodetectors at midwavelength 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 selective-area 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 103 cm/s. We further achieve room-temperature photoluminescence emission from InAsSb nanowires, spanning the entire MWIR regime from 3 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

Optical properties of bimodally distributed InAs quantum dots grown on digital AlAs0.56Sb0.44 matrix for use in intermediate band solar cells

High-quality InAs quantum dots (QDs) with nominal thicknesses of 5.0–8.0 monolayers were grown on a digital AlAs0.56Sb0.44 matrix lattice-matched to the InP(001) substrate. All QDs showed bimodal size distribution, and their optical properties were investigated by photoluminescence (PL) and time-resolved PL measurements. Power dependent PL exhibited a linear relationship between the peak energy and the cube root of the excitation power for both the small QD family (SQDF) and the large QD family (LQDF), which is attributed to the type-II transition. The PL intensity, peak energy, and carrier lifetime of SQDF and LQDF showed very sensitive at high temperature. Above 125 K, the PL intensity ratio increased continuously between LQDF and SQDF, the peak energy shifted anomalously in SQDF, and the longer carrier radiative lifetime (≥3.0 ns at 77 K) reduced rapidly in SQDF and slowly in LQDF. These results are ascribed to thermally activated carrier escape from SQDF into the wetting layer, which then relaxed into LQDF with low-localized energy states

GaSb Thermophotovoltaic Cells Grown on GaAs by Molecular Beam Epitaxy Using Interfacial Misfit Arrays

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

Significant suppression of surface leakage in GaSb/AlAsSb heterostructure with Al2O3 passivation

This work develops a (NH4)2S/Al2O3 passivation technique for photodiode-based GaSb/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

GaSb solar cells grown on GaAs via interfacial misfit arrays for use in the III-Sb multi-junction cell

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 multi-junction solar cell with access to a wide range of well-developed direct bandgap materials. Multi-junction 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

Behaviors of beryllium compensation doping in InGaAsP grown by gas source molecular beam epitaxy

We report structural properties as well as electrical and optical behaviors of beryllium (Be)-doped InGaAsP lattice-matched to InP grown by gas source molecular beam epitaxy. P type layers present a high degree of compensation on the order of 1018 cm−3, and for Be densities below 9.5×1017 cm−3, they are found to be n type. Enhanced incorporation of oxygen during Be doping is observed by secondary ion mass spectroscopy. Be in forms of interstitial donors or donor-like Be-O complexes for cell temperatures below 800°C is proposed to account for such anomalous compensation behaviors. A constant photoluminescence energy of 0.98 eV without any Moss-Burstein shift for Be doping levels up to 1018 cm−3 along with increased emission intensity due to passivation effect of Be is also observed. An increasing number of minority carriers tend to relax via Be defect state-related Shockley-Read-Hall recombination with the increase of Be doping density