Effect of midgap states in intrinsic hydrogenated amorphous silicon on sub-band-gap photoconductivity

Abstract

Photoconductivities, generated by photon energies less than the optical gap, in intrinsic hydrogenated amorphous silicon were measured over a wide range of carrier generation rates. Detailed numerical analysis of the corresponding sub-band-gap absorption was used to model the nature, densities, and distribution of midgap states. These derived gap state parameters are consistent with the electron lifetimes and their dependence on the generation rates. The analysis shows how the spectral response of the sub-band-gap photoconductivity (absorption) depends not only on the densities of midgap states but also on their location relative to the band edges. The position of these defect states can be obtained from sub-band-gap photoconductivity measurements providing that the corresponding occupation by electrons is taken into account.