Defect-related photoinduced absorption in amorphous silicon

Abstract

Photoinduced optical absorption in thin films of hydrogenated amorphous silicon (a-Si : H) is studied over the energy range related to electronic transitions involving the silicon dangling-bond defects, by using the dual-beam Constant Photocurrent Method (CPM). The subgap absorption in the range 0.8–1.4 eV is observed to increase with bias light intensity. Since CPM measures only transitions contributing to the photocurrent, the subgap absorption spectrum in undoped a-Si:H is dominated by transitions between singly occupied D 0 or doubly occupied D − defect states and conduction band extended states. The photoinduced changes in the subgap absorption are consistent with numerical calculations with a recombination model for a-Si : H which yield dangling bond electronic occupations as a function of photogeneration rate. The results suggest that the observed changes in the optical absorption spectra are mainly determined by changes in the occupation of defects induced by the bias-light.