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

The photoconductivity, electron and hole drift mobility, recombination life time, and optical properties of hydrogenated amorphous silicon-carbon alloy s have been systematically studied as a function of the optical gap. Sample s have been prepared by glow-discharge decomposition of silane-methane mixt ures with high hydrogen dilution. The methane concentration in the gas phas e has been varied between 0 and 66%, obtaining samples with optical gaps be tween 1.72 and 2.09 eV. Transport parameters have been measured by using th e 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 mobilit y is correlated with the Urbach energy, as predicted by the multiple-trappi ng 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 cen ters. We have observed a power-law dependence of the transport parameters o n the generation rate. The characteristic power-law exponents are presented as a function of the optical gap.