This paper reports a study of Shockley-Read-Hall, radiative, and Auger recombination processes in a series of molecular beam epitaxy grown InAs/InAsSb mid-wavelength infrared and long-wavelength infrared type-II superlattice samples using temperature- and excitation -density-dependent photoluminescence measurements, which are carried out from 12 to 77 K with excitation densities from 5 mW/cm2 to 20 W/cm2. A theoretical model is applied to describe the relation between integrated photoluminescence intensity and excitation density. Shockley-Read-Hall, radiative, and Auger recombination coefficients are extracted by fitting this relation. The results show that the Shockley-Read-Hall recombination lifetimes in all InAs/InAsSb type-II superlattice samples are longer than 100 ns, specifically the lifetime in a long-wavelength infrared sample reaches 358 ns at 77 K, in good agreement with the previously reported result of 412 ns measured using time-resolved photoluminescence on a similar sample.
Ga-free InAs/InAsSb type-II superlattice (T2SL) nBn photodetectors with very low dark current are fabricated and
characterized. The typical device without antireflection coating and surface passivation has a cut-off wavelength of 13.2
micrometers, quantum efficiency (QE) of 2.5% and a background limited operating temperature of 70 K. Our analysis
shows that the anticipated highest operating temperature of a 10.6 micrometer cut-off Ga-free T2SL nBn device can be
108 K, with a potential to reach 135 K if 20% QE or lower noise is achieved.
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.
With much effort devoted to the improvement of material and electrical designs, high-quality GaAs single-junction solar
cell performance is getting close to its theoretical limit. To further improve device performance, it is critical to find the
optimal optical designs for single-junction solar cells. In this work, planar single-junction solar cells are investigated
using a semi-analytical model, where combinations of smooth, textured, non-reflective, and reflective surfaces are
explored. Statistical ray tracing is used to obtain the optical properties of planar structures and the impact of critical
design parameters such as junction thickness, together with material quality and solar concentration on the device
performance is analyzed. The combination of textured and reflective surfaces shows the best performance by effectively
increasing the photon and carrier densities, which leads to higher open-circuit voltages and conversion efficiencies. It is
expected that the GaAs single-junction cells can practically achieve ~30% conversion efficiency under one sun AM1.5G,
with optimal optical structures, the state-of-art material quality, and properly designed doping profile. Even higher
efficiency of ~38% is possible via concentration of 1000 suns.
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