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.
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.