We used Raman scattering and time-resolved photoluminescence spectroscopy to investigate the molecular-beam-epitaxy (MBE) growth parameters that optimize the structural defects and therefore the internal radiative quantum efficiency of MBE-grown GaAs/AlGaAs double heterostructures (DH). The DH structures were grown at two different temperatures and three different As/Ga flux ratios to determine the conditions for an optimized structure with the longest nonradiative minority carrier lifetime. Raman scattering measurements show an improvement in the lattice disorder in the AlGaAs and GaAs layers as the As/Ga flux ratio is reduced from 40 to 15 and as the growth temperature is increased from 550 to 595 °C. The optimized structure is obtained with the As/Ga flux ratio equal to 15 and the substrate temperature 595 °C. This is consistent with the fact that the optimized structure has the longest minority carrier lifetime. Moreover, our Raman studies reveal that incorporation of a distributed Bragg reflector layer between the substrate and DH structures significantly reduces the defect density in the subsequent epitaxial layers.

: Thin layers of single-crystal, epitaxial semiconductor tin (alpha-Sn) were grown by molecular beam epitaxy (MBE) on cadmium telluride (CdTe) substrates. X-ray diffraction and Raman scattering measurements confirm that the thin layers of alpha-Sn are slightly strained, which supports theoretical prediction that alpha-Sn is a 3-D topological insulator (TI). Future studies will aim at the growth of alpha-Sn with improved transport characteristics using high-quality CdTe buffer layers, studies of the 3-D TI characteristics of alpha-Sn, and the MBE growth of stanene.

Effects of concentrated solar radiation on photovoltaic performance are investigated in well-developed GaAs quantum dot (QD) solar cells with 1-Sun efficiencies of 18%–19%. In these devices, the conversion processes are enhanced by nanoscale potential barriers and/or AlGaAs atomically thin barriers around QDs, which prevent photoelectron capture to QDs. Under concentrated radiation, the short circuit current increases proportionally to the concentration and the open circuit voltage shows the logarithmic increase. In the range up to hundred Suns, the contributions of QDs to the photocurrent are proportional to the light concentration. The ideality factors of 1.1–1.3 found from the VOC-Sun characteristics demonstrate effective suppression of recombination processes in barrier-separated QDs. The conversion efficiency shows the wide maximum in the range of 40–90 Suns and reaches 21.6%. Detailed analysis of I-V-Sun characteristics shows that at low intensities, the series resistance decreases inversely proportional to the concentration and, at ∼40 Suns, reaches the plateau determined mainly by the front contact resistance. Improvement of contact resistance would increase efficiency to above 24% at thousand Suns.

: A novel technique is used to determine the minority carrier lifetimes, interface recombination velocity, and radiative recombination constant from time-resolved photoluminescence measurements on a set of 3 molecular beam epitaxy (MBE)-grown gallium arsenide (GaAs)/aluminum gallium arsenide (AlGaAs) double heterostructures (DHs) and published theory. This technique is used to determine that a distributed Bragg reflector between the substrate and the DHs increases the GaAs nonradiative lifetime. The fractional increase in the nonradiative lifetime varies with the MBE growth parameters.

The unintentional background electron population and associated interface and surface conductivity in a heterostructure of InAs0.58Sb0.42 with a bandgap of 0.144 eV and AlInSb was studied with multi-carrier Hall-effect analysis. A free electron bulk concentration at 77 K was found with a density of 2.4 × 1015 cm−3and mobility of 140 000 cm2 V−1 s−1. A surface electron accumulation layer was observed with a density of 5.5 × 1011 cm−2 and mobility of 4500 cm2 V−1 s−1 that is consistent with predictions of surface Fermi level pinning. Another accumulation layer was identified at the interface with the AlInSb of 4 × 1011 cm−2 with a mobility of 37 000 cm2 V−1 s−1. The origin of the defects and the implications for device structures are discussed.

: We are investigating the use of Raman spectroscopy of tin as an analytical tool for discerning specific allotropic differences in ultra-thin tin films, and discerning differences between the tin and the growth substrates of interest. We have acquired spectra from barium difluoride (BaF2), crystalline silicon dioxide (SiO2), as well as ultra-thin tin semiconductor and tin metallic allotropes, and we are developing a fundamental understanding of the spectra. The research has identified that BaF2 is an excellent passivation layer for two-dimensional tin (stanene) that will enhance the resolution of the Raman spectrum of future tin layers and facilitate the molecular beam epitaxy (MBE) growth of stanene.

Minority carrier lifetimes in undoped and Beryllium-doped Type-2 Ga-free, InAs/InAsSb strained layer superlattices (SLS) with energy gaps as low as 0.165 eV were determined from photoluminescence kinetics. The minority carrier lifetime of 450 ns at 77 K in the undoped SLS confirms a high material quality. In similarly-grown structures that were p-doped to N A = 6 × 1016 and 3 × 1017 cm−3, electron lifetimes of τ n = 45 ns and 8 ns were measured. The 6 × 1016 cm−3 doping level is a factor of 6 greater than the typical background doping level in long-wave infrared (LWIR) Ga-containing InAs/GaSb SLS with similar bandgap and electron lifetime. This suggests that LWIR photodetectors with InAs/InAsSb SLS absorbers can be designed with smaller minority carrier concentrations and diffusion dark current densities. A relatively slow decrease of the lifetime with doping suggests a minor role of Auger recombination in the studied Ga-free SLS at T = 77 K with p-doping up to mid-1017 cm−3 level.

: In this report, we present results of an ongoing study aimed at measuring and optimizing the nonradiative lifetime and the internal radiative quantum efficiency of molecular beam epitaxy (MBE)-grown gallium arsenide (GaAs) solar cells that are grown at the U.S. Army Research Laboratory (ARL) using different substrate growth temperatures and arsenic (As)/gallium (Ga) flux ratios to determine the growth parameters that maximize the bulk GaAs minority carrier nonradiative lifetime. We report a significant increase in the nonradiative lifetime and the internal radiative quantum efficiency of MBE-grown GaAs/aluminum gallium arsenide (AlGaAs) double heterostructure (DH) structures grown at ARL with a growth temperature of 595 C and an As/Ga flux ratio = 15. Our results show that the nonradiative lifetime and internal radiative quantum efficiency of the DH structures grown using these parameters are comparable to those of the highest quality reported MBE-grown GaAs.

The band gap energy of the alloy InAsSb has been studied as a function of composition with special emphasis on minimization of strain-induced artifacts. The films were grown by molecular beam epitaxy on GaSb substrates with compositionally graded buffer layers that were designed to produce strain-free films. The compositions were precisely determined by high-resolution x-ray diffraction. Evidence for weak, long-range, group-V ordering was detected in materials exhibiting residual strain and relaxation. In contrast, unstrained films having the nondistorted cubic form showed no evidence of group-V ordering. The photoluminescence (PL) peak positions therefore corresponds to the inherent band gap of unstrained, unrelaxed, InAsSb. PL peaks were recorded for compositions up to 46% Sb, reaching a peak wavelength of 10.3 mu m, observed under low excitation at T = 13 K. The alloy band gap energies determined from PL maxima are described with a bowing parameter of 0.87 eV, which is significantly larger than measured for InAsSb in earlier work. The sufficiently large bowing parameter and the ability to grow the alloys without ordering allows direct band gap InAsSb to be a candidate material for low-temperature long-wavelength infrared detector applications. DOI: 10.1103/PhysRevB.86.245205

The strong bandgap bowing in the InAsxSb1−x alloy system allows it to potentially be used for infrared photodetection in the middle and long wavelength range. The authors have used compositionally graded metamorphic buffer layers to accommodate the misfit strain between InAsxSb1−x alloys and GaSb and InSb substrates in order to reach the long wave infrared range. In this work, we present the characterization of metamorphically grown InAsxSb1−x films that demonstrate strong photoluminescence in the spectral range from 5 to 9 μm.

Thick InAsBi layers were grown for photoluminescence (PL) characterization. The As to In overpressure ratio was carefully characterized and adjusted to achieve Bi-droplet-free surfaces. A closed loop feedback system was used to maintain the As overpressure during a 5-h deposition sequence. Despite a high degree of control of the growth parameters, evidence for local phase separation was observed in the PL spectra.

The band gap energy of the alloy InAsSb has been studied as a function of composition with special emphasis on minimization of strain-induced artifacts. The films were grown by molecular beam epitaxy on GaSb substrates with compositionally graded buffer layers that were designed to produce strain-free films. The compositions were precisely determined by high-resolution x-ray diffraction. Evidence for weak, long-range, group-V ordering was detected in materials exhibiting residual strain and relaxation. In contrast, unstrained films having the nondistorted cubic form showed no evidence of group-V ordering. The photoluminescence (PL) peak positions therefore corresponds to the inherent band gap of unstrained, unrelaxed, InAsSb. PL peaks were recorded for compositions up to 46$%$ Sb, reaching a peak wavelength of 10.3 \ensuremath{\mu}m, observed under low excitation at $T=13$ K. The alloy band gap energies determined from PL maxima are described with a bowing parameter of 0.87 eV, which is significantly larger than measured for InAsSb in earlier work. The sufficiently large bowing parameter and the ability to grow the alloys without ordering allows direct band gap InAsSb to be a candidate material for low-temperature long-wavelength infrared detector applications.

Infrared (IR) detector materials based on III-V semiconductors are an affordable alternative to HgCdTe, which is the current material of choice for most high performance IR focal plane array systems fielded in the Army. Based on the assumption that III-V compounds do not have an inherently small bandgap enabling operation out to 10 mu m, the long wave infrared (LWIR) range, there has been substantial research investment in superlattice (SL) approaches. For this reason, quantum structures such as quantum well IR photodetectors (QWIPs) and typeII strained-layer superlattices (SLS) are grown to take advantage of confinement effects and to induce an effective bandgap in the desired range. The drawbacks of these approaches include inherently low quantum efficiencies and operating temperature for QWIPs, and very short minority carrier lifetimes for SLS detectors. In this presentation, we will discuss approaches for developing III-V, direct bandgap, dilute N and Bi alloys grown by molecular beam epitaxy (MBE) for LWIR applications.

GaInSb and AlGaInSb compositionally graded buffer layers grown on GaSb by MBE were used to develop unrelaxed InAs1-XSbXepilayers with lattice constants up to 2.1 % larger than that of GaSb. The InAsSb buffer layer was used to grow InAs0.12Sb0.88 layer on InSb. The structural and optical characterization of 1-μm thick InAs1-xSbx layers was performed together with measurements of the carrier lifetime.

We have examined the growth of strained layer superlattice (SLS) structures for the purpose of characterizing and improving the minority carrier lifetime. Structures with different SL periods but with same absorption wavelength were first studied. Despite a doubling of the number of interfaces per thickness unit, no significant change was seen in the carrier lifetime. This observation points away from the interfaces as the location of lifetime limiting defect centers. To gain further insights into the spatial location of the defect centers, a series of binary InAs and GaSb layers grown with different substrate temperatures, were studied. We found that higher growth temperatures were beneficial for both binaries, although the improvement for GaSb was less than that of InAs. The substrate temperature was also varied in SLS structures and characterized with high-resolution x-ray diffraction. By using the peak width from the SLS zero-order diffraction as a figure of merit, we found a shallow growth window of ∼±20° around an optimum temperature of 440°C. Outside this temperature window the material quality deteriorated very rapidly. Unfortunately, the substrate temperatures that would provide an improvement in the binary lifetimes fall mainly above the SLS growth window, thus limiting this parameter as a means of improving lifetimes in the SLS. A model that qualitatively relates bulk and SLS lifetimes through native defects is proposed and strategies for improving the lifetimes are discussed.

A new type of waveguide-based demultiplexer is made in a GaAs-AlGaAs system with a grating-assisted codirectional coupler structure to pick out the wanted wavelength. Two waveguides that are made vertically coupled are fabricated using a special backside processing method. This proposed multichannel demultiplexer is operated in the near-infrared wavelength range.

This paper describes the design, growth and fabrication characterization of novel multi-wavelength QWIP wafers based on InP material systems. We designed, grew, fabricated and characterized AlGaInAs/GaInAs QWIPs suitable for operation at 3-5 μm, and 8-12 μm spectral range. We fabricated mid-wave IR 320 x 250 focal plane arrays, hybridized them with Si -readout circuits and performed radiometric and imaging tests. Excellent imaging results of the mid-wave IR focal plane arrays with an operability of 88% and mean NEDT of 0.09K have been achieved. To our knowledge, this is the first imaging with InP based QWIPs focal plane array.