Surface quenching of optically generated carriers in thin-film hydrogenated amorphous silicon: Picosecond transient-grating experiments

Subnanosecond-timescale transient-grating experiments are used to investigate the dynamics of optically generated charge carriers in thin-film hydrogenated amorphous silicon (a-Si:H) at room temperature. By examining the time dependence of the transient-grating signal as a function of fringe spacing for small fringe spacing, the diffusion constant in the plane of the film (D?) is determined. At large fringe spacing, the grating decay becomes independent of fringe spacing and is highly nonexponential. The functional form and rate of the large-fringe-spacing decay change with sample thickness. A model for the effect of surface quenching on the transient-grating decay is presented. The calculations are in good agreement with the data and yield a diffusion constant (D_⊥) for motion perpendicular to the thin-film plane. D_⊥=0.4×{}10^-2 cm²/sec, while D?=1.0×{}10^-2 cm²/sec.