ac conductivity and ac photoconductivity in amorphous and crystalline insulators

Abstract

Measurements of ac conductivity and ac photoconductivity are reported on representative amorphous and crystalline insulators. Low-frequency measurements have been argmented by photomicrowave experiments. In the dark some of these materials exhibit the familiar ${\omega }^s$ frequency dependence but under illumination all materials studied exhibit frequency-independent ac conductivity in the range 20 Hz-100 kHz. The amorphous chalcogenides studied do not exhibit photomicrowave response; however, strong response is observed in these films following crystallization in situ. Frequency-independent ac conductivity is usually associated with transport in extended states. In ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$, however, transient measurements are interpreted most easily in terms of hopping transport. A phenomenological model which reconciles these observations is presented. Aspects of the relationship of steady-state ac to dc electrical measurements and the relationship of ac measurements to the time-of-flight technique are analyzed.