Light-induced D diffusion measurements in hydrogenated amorphous silicon: Testing H metastability models

We measure light-induced D tracer diffusion in hydrogenated amorphous silicon samples under conditions at which thermal diffusion is negligible. Under high-intensity (9 W cm^-2), red-light soaking at 135 ^∘{}C, the D diffusion coefficient is DD=1.3×10^-18cm²s^-1 and the rate of D emission from Si-D to transport is 3.5×10^-5s^-1. We also find an upper bound of DD=3×10^-20cm²s^-1, the light-induced diffusion coefficient at 65 ^∘{}C. Previous experiments had revealed only ``light-enhanced'' diffusion between from 200 to 300 ^∘{}C, a regime in which thermal diffusion is also significant. Our 135 ^∘{}C result extends the range of the 0.9-eV activation energy for this diffusion; our 65 ^∘{}C upper bound is consistent with the extrapolation of the higher temperature data. We also measure metastable defect creation at 65 and 135 ^∘{}C to test models of light-induced metastability that involve emission of H from Si-H bonds to an H transport level. This class of models can be limited, but not excluded, by our data. The H emission parameter of the H collision model of metastability is also estimated.