Composite materials for photovoltaics: A realistic aim?

Layer-type microcrystalline powders of WS₂, MoS₂, WSe₂ and MoSe₂, which were tested for photoeffects with contact free microwave conductivity measurements, were incorporated into a nanostructured TiO₂ matrix in an attempt to obtain macroscopic photocurrents. Even though only 10% of the microcrystals were found to be active in contact with an iodide/iodine solution, photocurrents of the order of 100 μA/cm² to 1 mA/cm² were measured. The photoelectrochemical behavior of microcrystals was studied with space resolved photocurrent and photovoltage measurement techniques and it was attempted to understand the mechanism of current generation via the TiO₂/microcrystal/electrolyte interaction. Critical and still unresolved problems are recognized to be the control and optimization of photoeffects in microcrystals and the adjustment of doping levels of the two material faces. Only small photopotentials could be observed up to now, probably due to an insufficient and inhomogeneous doping of microcrystals. More research will be needed to determine whether this strategy could lead to realistic photovoltaic systems.     

Magnetoluminescence properties of Hg1−xCdxTe epitaxial layers and superlattice structures grown by metalorganic molecular beam epitaxy

We present a study of the electro-optical properties ofHg _1- xCd_xTe epitaxial layers and Hg_1-x Cd_xTe/CdTe (0.28 < x < 0.30) superlattice structures by x-ray diffraction, lateral transport and photo- and magneto-luminescence measurements. Systematic studies of the excitation intensity and magnetic field dependence of the photoluminescence revealed direct evidence of an excitonic contribution to the observed luminescence in Hg_{1- x}Cd_xTe epitaxial layers. Similar investigations of the superlattice structures indicated that excitonic corrections were required to adequately fit the luminescence data. Optical gains of 80 cm⁻¹ were obtained for an excitation intensity of 100 kW/cm² indicating suitable electro-optical properties for making efficient mid-infrared laser diodes.     

Spectroscopy of exciton states of InAs quantum molecules

Tunnel-coupled pairs of InAs quantum dots (quantum molecules) were formed by molecular beam epitaxy in a GaAs matrix. Optical and structural properties of the obtained quantum molecules were studied. Four molecular exciton states forming a photoluminescence spectrum were revealed. The photoluminescence decay times indicate the possibility of interlevel radiative recombination from the second excited state, which is of particular importance for designing mid-infrared devices.     

Effect of high-temperature annealing on GaInP/GaAs HBT structures grown by LP-MOVPE

We have investigated the effect of high-temperature annealing on device performance of GaInP/GaAs HBTs using a wide range of MOVPE growth parameters for the C-doped base layer. Carbon doping was achieved either via TMG and AsH₃ only or by using an extrinsic carbon source. High-temperature annealing causes degradation of carbon-doped GaAs in terms of minority carrier properties even at doping levels of p=1 × 10¹⁹ cm⁻³. The measured reduction in electron lifetime and luminescence intensity correlates with HBT device results. It is shown that the critical temperature where material degradation starts is both a function of doping method and carbon concentration.     

Effect of nanobridges on the emission spectra of a quantum dot-quantum well tunneling pair

Emission in the narrow spectral range 950–1000 nm is obtained at the nanobridge optical transition involving experimentally and theoretically observed hybrid states in the InGaAs system, i.e., quantum dot-nanobridge-quantum well. It is experimentally shown that the oscillator strength of the new transition sharply increases in the built-in electric field of a pin junction. In the mode of weak currents in the system under study, the nanobridge transition is the dominant electroluminescence channel. At current densities >10 A cm², nanobridge “burning” is observed, after which the system becomes a “quasi-classical” quantum dot-quantum well tunneling pair separated by a barrier.     

Imaging Catastrophic Optical Mirror Damage in High-Power Diode Lasers

We image catastrophic optical mirror damage (COMD) in red- and infrared-emitting high-power broad-area diode lasers by combining highly COMD-selective thermography, near-field imaging, scanning electron microscopy, and cathodoluminescence. All techniques exhibit strong correlations in terms of COMD location and strength and allow for an unambiguous decision about COMD occurrence. In particular, temperatures en route to and during COMD are measured, and the concept of a critical facet temperature that induces thermal runaway is supported.     

DRIP-XII Conference 2007

Mn²⁺-doped ZnS nanoparticles were synthesized by a hydrothermal method. The reactive conditions are researched, such as the ratios of the reaction concentrations [S²⁻]/[Zn²⁺], the concentration of Mn²⁺ ions, the reaction temperatures and the pH value. The concentration doped-Mn is from 1 mol% to 20 mol% and the hydrothermal synthesis temperatures are from 70 °C to 110 °C. The structure and luminescent properties of ZnS:Mn nanoparticles are researched by using XRD, SEM and PL.