Transport properties of a-Si1-xCx:H films investigated by the moving photocarrier grating technique

The photoconductivity, electron and hole drift mobility, recombination lifetime, and optical properties of hydrogenated amorphous silicon-carbon alloys have been systematically studied as a function of the optical gap. Samples have been prepared by glow-discharge decomposition of silane-methane mixtures with high hydrogen dilution. The methane concentration in the gas phase has been varied between 0 and 66%, obtaining samples with optical gaps between 1.72 and 2.09 eV. Transport parameters have been measured by using the moving photocarrier grating technique. We have found a large decrease in the electron drift mobility with optical gap, especially when small amounts of carbon are incorporated into the silicon matrix. The hole drift mobility is correlated with the Urbach energy, as predicted by the multiple-trapping model. The recombination lifetime shows a maximum for a gap energy close to 1.85 eV, which we assign to competition between a widening of the band tails and an increase in the density of defects acting as recombination centers. We have observed a power-law dependence of the transport parameters on the generation rate. The characteristic power-law exponents are presented as a function of the optical gap.