On the photoconductivity of amorphous chalcogenide semiconductors

An analysis is presented of the photoconducting properties of amorphous semiconductors containing valence alternation pairs (VAPs) of chalcogen atoms. It is shown that the presence of VAPs leads to quasi-equilibrium between the electron and hole populations for all intensities of the incident light. Formulae are derived for the activation energies of the steady state photoconductivity, and also for the photoconductivity decay time, for both low and high levels of illumination, i.e. for the high-temperature linear region and the intermediate-temperature square-root region, respectively. The model can account for the variety of experimental results, especially in the linear region, obtained for different specimens of similar materials if it is assumed that VAPs dominate the recombination process in the square-root region, while at higher temperatures other states may play an appreciable role in recombination processes. However, the plausibility of the VAP model can be tested only by comparing the states identified in this way with those revealed by other experiments on the same or closely similar samples.