DENSITY-OF-STATE DISTRIBUTION AND VARIABLE-RANGE-HOPPING TRANSPORT INAMORPHOUS-SILICON PREPARED BY ION-BOMBARDMENT

Photothermal deflection spectroscopy measurements have been performed on amorphous silicon (a-Si) films produced by ion bombardment of cryst alline silicon (c-Si). The analysis of the sub-bandgap optical absorpt ion data provides, for the first time, detailed information about the localized density-of-states (LDOS) distribution in a system that exhib its variable-range hopping transport at the Fermi energy. We find that in a-Si the Fermi energy is pinned within a relatively non-uniform di stribution of positive-correlation-energy states. Qualitatively, our f indings support previous suggestions that the pre-exponential or sigma 0 problem in variable-range hopping might be related to LDOS non-unifo rmities around (E(F)). Comparing electronic transport data on a-Si pro duced by ion bombardment, to expectations based on the Mott theory, we show that the overall effect of LDOS non-uniformity and intra-site el ectron correlation is a significant reduction of the inter-site tunnel ling distances. Possible reasons for this reduction are discussed.