Femtosecond photomodulation spectroscopy of a-Si:H and a-Si:Ge:H alloys in the midinfrared

We present the results of a low-intensity femtosecond pump and probe study of hydrogenated amorphous silicon (a-Si:H) and its germanium alloys (a-Si:Ge:H) in the probe spectral regions of 0.52-0.81 and 1.19-1.48 eV. We find that the picosecond photoinduced change in the energy and time dependence of the complex dielectric constant is consistent with a model for the injected carrier relaxation which combines extended-to-tail and tail-to-tail thermal carrier transitions, and which uses an optical probe sensitivity to carriers in the localized tail states. The measurements on a-Si:H are in quantitative agreement with our model over the entire probe spectral range and from subpicosecond to nanosecond probe delay times; however, we find only qualitative agreement between the model and the pump-probe results of two series of a-Si:Ge:H alloys prepared under different conditions. Fits to the subpicosecond data of both the a-Si:H and a-Si:Ge:H samples indicate that the carriers are initially trapped into the tail states with a characteristic attempt frequency at least eight times higher than the rate at which they are subsequently trapped into deeper tail states.