Investigation of the occupation function of localized states in a-Si: H under infrared stimulated photoexcitation

Abstract

The occupation function of localized states in a-Si : H under infrared-stimulated photoexcitation is directly calculated from the observed infrared-stimulated photocurrent spectra. We find that, at short times during infrared-stimulated photoexcitation, the occupation function for the ‘shallow’ trap states exhibits a Boltzmann distribution; but for the ‘deep’ trap states it is energy independent but time dependent. In the intermediate-energy region its shape is neither a Boltzmann nor a Fermi-type distribution. As time evolves, the system progressively approaches and reaches its steady state, at which the occupation function becomes a complete Fermi function with a trap quasi-Fermi energy E _th. Our calculation has the great advantage that it does not require knowledge of the density of localized states in the material. This approach also presents a useful way of judging the correctness of the physical model and to estimate the degrees of accuracy in the analysis of photocurrent spectra when using approximate expressions for the occupation function of localized states.