Photoconductivity spectroscopy of hydrogenated amorphous silicon

Abstract

Abstract We have developed a numerical model that can give a realistic description of the photoconductivity of amorphous semiconductor films even in the case of subgap excitation and in the presence of surface states. We have compared the results of this model with the spectral photoconductivity, the mobility-lifetime product and the absorption coefficient measured on hydrogenated amorphous silicon (a-Si: H) samples of different thicknesses. We find that the widely accepted description, in which the deep defects are divided in two classes (acceptor like and donor like) and the capture cross-sections of charged defects are much larger than those of the neutral defects, is unable to reproduce a number of experimental results. For instance the well known fact that, in a-Si: H thick films, the absorption coefficients derived from spectral photoconductivity and from photothermal deflection spectroscopy are in fair agreement can be reproduced only if the capture cross-sections have not too different values. Our model also shows that the weak dependence on the sample thickness of the absorptivity derived from the photoconductivity spectrum cannot be explained if the surface is schematized as a thin layer different from the bulk only because of a larger defect density. The simulations suggest that this weak dependence could be due to band-bending effects.