Variation of photoconductivity with doping and optical degradation in hydrogenated amorphous silicon

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Abstract
A simple model for photoconductivity in a-Si:H is shown to be capable of describing the variations of photoconductivity observed both with doping and with optical degradation. The model consists of a trivalent recombination center with consideration only of transitions connecting the centers and extended states, and of exponential conduction and valence-edge tail states with occupancy described simply by the locations of the dark or quasi-Fermi levels. The model describes the changes in the magnitude of photoconductivity with doping or optical degradation as arising primarily from changes in the density of effective recombination centers caused by shifts in the location of the dark Fermi level, and only secondarily from changes in the total density of dangling bonds. The model also describes the change in the exponent for the variation of photoconductivity as a power of the excitation rate; this exponent changes from a value of 0.50, when the density of electrons trapped in conduction tail states is proportional to the density of neutral recombination centers, to a value of 1.0 when an appreciable density of neutral recombination centers exists in thermal equilibrium. Both qualitative and semiquantitative agreement are demonstrated between the predictions of the model and published experimental data on doping and optical degradation.