Strain-balanced InAs/InAsSb superlattices can be tuned to absorb and emit across the mid- to long-wave infrared, and exhibit appropriate minority carrier lifetimes for high performance infrared photodetectors. The optical quality of this material has been shown to improve with the use of Bi as a surfactant. Specifically, InAs/InAsSb superlattices grown at 425 °C and 430 °C exhibit improved photoluminescence intensity for Bi/In flux ratios up to 1.0%, and optical quality improves further with increasing growth temperature and increasing Bi/In flux ratios up to 5.0%. The identification of optimal growth conditions for InAs/InAsSb superlattices with Bi surfactant, as well as further exploration of the impact of Bi surfactant is an important component to further developing and optimizing this infrared material system.
Several strain-balanced InAs/InAsSb superlattices are grown using molecular beam epitaxy at temperatures ranging from 425 °C to 475 °C using Bi/In flux ratios ranging from 0.0% to 10.0%. The structural and optical properties of the samples are evaluated using X-ray diffraction, secondary ion mass spectrometry, and photoluminescence spectroscopy. Analysis of the mass spectrometry data indicates that surfactant Bi incorporates into the InAs/InAsSb material system with a sticking coefficient of 0.3% at 450 °C, yielding dopant-level concentrations for typical Bi/In surfactant flux ratios. Analysis of the integrated photoluminescence intensity indicates that photoluminescence efficiency is greatest with a 1.0% Bi/In flux ratio for growth at 425-430 °C, and a 5.0% Bi/In flux ratio for growth at 450-475 °C. The improvement in photoluminescence efficiency is associated with a longer Shockley-Read-Hall lifetime in the superlattices grown with Bi surfactant.
Fully microscopic many-body models are used to determine important material characteristics of GaAsBi and InAsBi
based devices. Calculations based on the band anti-crossing (BAC) model are compared to first principle density
functional theory (DFT) results. Good agreement between BAC-based results and experimental data is found for
properties that are dominated by states close to the bandgap, like absorption/gain and photo luminescence. Using the
BAC model for properties that involve states in the energetic region of the BAC defect level, like Auger losses and free
carrier absorption results in a sharp resonance in the dependence of these quantities for Bismuth concentrations for which
the bandgap becomes resonant with the spin-orbit splitting or the BAC-splitting of the light and heavy hole bands. DFT
calculations show that the BAC model strongly over-simplifies the influence of the bismuth atoms on the bandstructure.
Taking into account the more realistic results of DFT calculations should lead to a reduction of the sharp resonance and
lead to enhancements or suppressions for other Bismuth concentrations and spectral regions.
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 optical emission and gain properties of Ga(AsSb) quantum-islands are investigate. These islands form during growth
in a self-organized process in a series of Ga(AsSb)/GaAs/(AlGa)As heterostructures, resulting in an additional in-plane
hole confinement of several hundreds of meV. The shape of the in-plane confinement potential is nearly parabolic and thus
yields almost equidistant hole energy levels. Transmission electron microscopy reveals that the quantum islands are 100nm
in diameter and exhibit an in-plane variation of the Sb concentration of more than 30 %. Up to seven bound hole states
are observed in the photoluminescence spectra. Time-resolved photoluminescence data are shown as function of excitation
density, lattice temperature, and excitation photon energy and reveal fast carrier capture into and relaxation within the
quantum islands. Furthermore, the optical gain is measured using the variable stripe-length method and the advantages of
such structures as active laser material are discussed.
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.
KEYWORDS: Photovoltaics, Solar cells, Quantum efficiency, Solar energy, Energy efficiency, Light emitting diodes, Luminescence, Electroluminescence, Active optics, Sun
It has been proposed recently that thermally assisted electroluminescence may in principle provide a means to convert solar or waste heat into electricity. The basic concept is to use an intermediate active emitter between a heat source and a photovoltaic (PV) cell. The active emitter would be a forward biased light emitting diode (LED) with a bias voltage, Vb, below bandgap, Eg (i.e., qVb < Eg), such that the average emitted photon energy is larger than the average energy that is required to create charge carriers. The basic requirement for this conversion mechanism is that the emitter can act as an optical refrigerator. For this process to work and be efficient, however, several materials challenges will need to be addressed and overcome. Here, we outline a preliminary analysis of the efficiency and conversion power density as a function of temperature, bandgap energy and bias voltage, by considering realistic high temperature radiative and non-radiative rates as well as radiative heat loss in the absorber/emitter. From this analysis, it appears that both the overall efficiency and net generated power increase with increasing bandgap energy and increasing temperature, at least for temperatures up to 1000 K, despite the fact that the internal quantum yield for radiative recombination decreases with increasing temperature. On the other hand, the escape efficiency is a crucial design parameter which needs to be optimized.
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.
Room-temperature continuous wave operation of Antimonide-based long wavelength VCSELs has been demonstrated, with 1.2mW power output at 1266nm, the highest figure reported so far using this material system. Single mode powers of 0.3mW at 10°C and 0.1mW at 70°C and side-mode suppression ratios up to 42dB have also been achieved. Preliminary reliability test results have shown so far that the devices can work normally without obvious degradation after stress testing at up to 125°C for thousands of hours.
High-mobility InAs single quantum well with symmetrical AlSb and asymmetrical AlSb and Al0.8Ga0.2Sb barriers were grown on GaAs (100) by MBE. Magneto-transport studies revealed enhancement of sufficient effective g-factor in a quantizing magnetic field. This enhancement is quite sensitive to the layer composition of the epitaxially-grown structures. The implications of these results for the implementation of InAs-based spintronics structures are discussed.
We demonstrated the integration of 32 by 32 p-i-n photodiode arrays with Si-dummy chips for potential use in massively parallel short-distance optical interconnects. Individual devices in 32 by 32 InGaAs/InP photodetector arrays were successfully tested and demonstrated a small signal modulation speed above 10 GHz under dark condition, corresponding to an aggregate data transmission capacity in excess of Tera-bit/s.
We demonstrate the integration of vertical-cavity surface- emitting laser (VCSEL) arrays with Si-dummy chips for potential use in short-distance parallel optical interconnects. An 8 X 8 flip-chip bonded InGaAs VCSEL array was successfully modulated at data rates up to 0.8 Gbit/s/channel, corresponding to an aggregate data transmission capacity in excess of 50 Gbit/s. A 2 X 4 VCSEL array was indirectly flit-chip bonded to a Si substrate via a transparent glass carrier and package- limited data rates of 0.4 Gbit/s/channel were achieved. The large signal modulation bandwidth of these devices exceeded 2 Gbit/s. The electrical driving characteristics of the devices were found to be compatible with 3.3 V CMOS technology.
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