Space-charge depletion studies of deep states in glassy semiconductors

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Abstract
The dark decay of open-circuit surface voltage on a composition series of capacitively charged a-Se-Te films has been analyzed. The bulk process controlling dark decay is demonstrated to be thermal emission of holes from discrete states deep in the mobility gap. The hole sweepout process causes the progressive development of negative deeply trapped space charge in the bulk which is experimentally manifested in the distinct shape of the associated dark discharge curves. The rate of dark discharge and the detailed shape of the dark decay function dV/dt vs t vary systematically with alloy composition. Phenomenological parameters of the dark decay can be related to an underlying Gaussian emission center distribution in the gap by numerical simulation techniques. It is demonstrated that progressive Te addition to the binary alloy broadens this distribution and shifts it toward the valence-band mobility edge.