The transport of 29 cm-1 phonons in hydrogenated amorphous silicon

The scattering of 29 cm−1 phonons in hydrogenated amorphous silicon (a-Si:H) at 2 K is investigated by means of optically generated heat pulses and the ruby phonon detector. The transport is shown to be governed by elastic spatial diffusion and a diffusion coefficient of ∼1 cm2/s. Resonant scattering by soft oscillators, as well as Rayleigh scattering by microvoids or by the disordered network itself account for the experimental findings. In optically excited a-Si:H, the transmission of 29 cm−1 phonons is reduced by inelastic scattering, proportional to the optical excitation density in a-Si:H and recovers on a 40-ns time scale. These results are discussed in terms of interaction of 29 cm−1 phonons with optically generated long-lived very high-frequency phonons in a-Si:H.