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.
This paper reports the improvements and limitations of MBE grown 1.3μm GaAsSb/GaAs single QW lasers. At room
temperature, the devices show a low threshold current density (Jth) of 253 Acm-2, a transparent current density of 98
Acm-2, an internal quantum efficiency of 71%, an optical loss of 18 cm-1 and a characteristic temperature (T0) = 51K. The
defect related recombination in these devices is negligible and the primary non-radiative current path has a stronger
dependence on the carrier density than the radiative current contributing to ~84% of the threshold current at RT. From
high hydrostatic pressure dependent measurements, a slight decrease followed by the strong increase in threshold current
with pressure is observed, suggesting that the device performance is limited to both Auger recombination and carrier
leakage.
The injection and temperature dependence of the spontaneous emission quantum efficiency of molecular beam epitaxy
grown InGaAs/GaAs quantum wells is determined using excitation dependent photoluminescence (PL) measurements.
The PL measurements were performed at temperatures from 50 to 300 K using a HeNe pump laser with powers ranging
from 0.6 to 35 mW. The quantum efficiency is inferred from the power law predicted by the rate equations that links
pump power and integrated PL signal. The peak spontaneous emission quantum efficiency of molecular beam epitaxy
(MBE) grown InGaAs/GaAs triple quantum wells is determined to be 0.941 at 300K with an overall best value of 0.992
at 100 K.
The fundamental mechanisms of electroluminescence (EL) refrigeration in heterostructure light emitting diodes, is
examined via carrier energy loss (and gain) during transport, relaxation, and recombination, where the contribution of
electrons and holes are treated separately. This analysis shows that the EL refrigeration process is a combination of
thermoelectric cooling that mainly occurs near the metal/semiconductor contacts and radiative recombination which
mainly occurs in the active region. In semiconductors such as GaAs, electrons and holes make different contributions
to the refrigeration processes as a result of their different densities of states.
A theoretical study of photoluminescence refrigeration in semiconductors has been carried out using a model that takes into account photon recycling and includes the rate equations for both carriers and photons. General expressions for cooling efficiency, cooling power density, and the cooling condition are derived. The investigation of the photoluminescence refrigeration in an intrinsic GaAs slab shows that net cooling is accessible when quantum efficiency and luminescence extraction are high, and that photon recycling contributes strongly to photoluminescence refrigeration when the luminescence extraction efficiency is small.
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