State distribution in hydrogenated microcrystalline silicon

Abstract

We have been able to determine the density of states map in the band gap of a semiconductor by the measurement of the phototransport properties of its majority and minority carriers. In particular we found that the carrier recombination in single-phase hydrogenated microcrystalline silicon $(\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H})$ is significantly different from the one in hydrogenated amorphous silicon $(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H})$ and that it is controlled only by its two band tails. The comparison of the observed temperature dependence of the phototransport properties of this material with model simulations further suggests that, while the conduction-band tail has an exponential distribution of states, the valence-band-tail states have a Gaussian-like distribution. This, in turn, meets the challenge of the determination of the analytical shape of the density of states distribution from experimental data. Our experimental procedure implies then that this distribution is associated with the route through which the transport and phototransport take place and thus we conclude that both the recombination and transport in undoped single-phase $\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ take place in the disordered layer that wraps the crystallites. We further conclude that, from the transport and phototransport points of view, the single-phase $\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ is, in general, different from both polycrystalline silicon and $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$ The polycrystalline-silicon-like behavior, when found, appears to be an asymptotic case in which the crystallites are large enough, while the $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ behavior prevails only when there is a significant content of its phase within the system.