The influence of a temperature dependent bandgap on the energy scale of modulated photocurrent experiments

Abstract

Amorphous semiconductors and chalcogenide glasses exhibit a high density of localized states in their bandgap as a consequence of structural defects or due to a lack of long range order. These defect states have a strong influence on the electronic transport properties. Thus, many theories attribute the “resistance drift” or the “threshold switching” effects, both observed in amorphous phase change alloys, to defects within the bandgap. The energetic distribution of states within the bandgap can be probed via modulated photocurrent (MPC) experiments that enable a spectroscopy of the relative density of these defect states by varying the modulation frequency at various temperatures T. It is also a common feature that the bandgap decreases with temperature. Nevertheless, the consequences of a shrinking bandgap with increasing temperature have been neglected in the classical analysis of MPC experiments. In this paper, we propose to add correction terms to the classical MPC energy scaling to take the temperature dependence of the bandgap of the studied material into account to improve the accuracy of the determination of the defect distribution. We illustrate the efficacy of our proposed corrections by applying it to the study of disordered materials such as hydrogenated amorphous silicon a-Si:H, a-GeTe and a-Ge2Sb2Te5.