Investigation of the localised state distribution in amorphous Si films

Abstract Field effect techniques have been used to determine the distribution function N (ϵ3) of the localised states in amorphous Si films prepared by glow discharge decomposition of silane. It was found possible to sweep the surface potential through about 0.5 eV and to determine N (e) to within 0.18 eV of the extended states. N (e) curves show a pronounced structure which largely depends on the substrate temperature during deposition of the films (400–630 K). Localised state densities increase with decreasing substrate temperatures. The equilibrium Fermi level generally lies close to a peak in the distribution and N (eF) ∼ 17 cm−3 eV−1. The analysis of the field effect experiments is described in some detail and relevant information from drift mobility and conductivity measurements are discussed. In particular it is found that the distribution of occupied states calculated from the N (e) curves agrees with predictions from the transport experiments. This supports our contention that the N (e) curves represent a volume rather than a surface property of the films.