Extended-state mobility and its relation to the tail-state distribution ina-Si:H

The differing interpretations of drift-mobility data for electrons in a-Si:H have been examined. On the one hand, there are the analyses which involve the ``thermalization approximation'' (TA). One such analysis, based on the density-of-states profile deduced by Spear from field-effect data, leads to the unexpectedly high value of the extended-state mobility μₑₓₜ₌₅₀₀ cm² V^-1 s^-1. Another analysis, which assumes an exponential distribution of tail states, results in the more commonly accepted value μₑₓₜ₌₁₃ cm² V^-1 s^-1. In this work, we show that both models are inconsistent with experimental data when a more thorough examination is made of the field dependence and the activation energy of the drift mobility μd as predicted by those models. On the other hand, we examined Spear's analysis, which does not involve the TA. We show that a thorough analysis of multiple-trapping transport in a tail-state distribution as proposed by Spear leads to more dispersion than he experimentally observed. Our examination is based on a newly developed computational procedure, which analyzes multiple-trapping transport by discretizing the continuous distribution of localized states. Apart from the relevance with respect to electron transport in a-Si:H, this work shows how a relatively simple but accurate analysis of drift mobility and transient photocurrents can be performed with a much wider applicability than the analyses based on the TA.