Optical studies of thermalization mechanisms ina-Si:H

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Abstract
Thermalization mechanisms of photoexcited carriers in a-Si:H have been studied by monitoring the spectral shift with delay of the main luminescence band as a function of temperature. The weak temperature dependence of the shift at early times precludes any multiple-trapping process based on thermal excitation to transport states, but is consistent with thermalization by direct hopping between localized states. A hopping model recently proposed to account for low-temperature transport phenomena is shown to provide a good description of the early-time spectral shift. This represents the first unified description of optical and transport measurements at low temperatures. The shift rates suggest that only one of the carriers is thermalizing or that excitons dominate the early-time dynamics. The acceleration of the shift rate per decade at later times is found to correlate with the onset of thermally activated recombination processes, implying that depletion of higher-energy states by recombination is responsible.