Kinetics of optically generated defects in hydrogenated amorphous silicon

The time dependence of optically induced degradation in hydrogenated amorphous silicon (a-Si:H) has been measured at various temperatures. It is found that the degradation follows the stretched-exponential law with the stretching parameter β and the relaxation time constant τ independent of the sample temperature. This result is explained by the fact that the defects in an amorphous solid are distributed exponentially in energy and the absorbed photon transfers its energy into a local mode of the defect to raise the local temperature to a value of ${\mathit{T}}_{\mathit{x}}$=408 K. We conclude that the sample temperature will not affect the process of defect conversion if it is smaller than ${\mathit{T}}_{\mathit{x}}$. We further predict that if it is larger than ${\mathit{T}}_{\mathit{x}}$, the process will show the temperature dependence. The prediction is consistent with the previous measurements. Thus, our results may serve as a solution to the hitherto unresolved issue of the temperature dependence of the creation of dangling bonds and establish its underlying mechanism. The observed independence of the relaxation parameters β and τ on the sample temperature does not favor the hydrogen-diffusion weak-bond model for the optically induced degradation in a-Si:H, in which the hydrogen atom moves a microscopic distance to break a weak bond. We also show that the ${\mathit{t}}^{1/3}$ time dependence can only describe a certain range of our measurements.