Dynamics of vibrations in a mixed amorphous-nanocrystalline Si system

We present results of pulsed Raman and phonon-induced luminescence experiments on a mixed amorphous-nanocrystalline silicon system (a-nc-Si:H). With these experiments, the decay and transport of nonequilibrium phonons in a-nc-Si:H was examined and compared with the behavior of phonons of the same frequencies in a-Si:H and c-Si. From the Raman measurements, we find that in the spectral region of the TO vibrations in the crystallites (505--520 cm^-1), phonons have shorter decay times than the TO phonons in a-Si:H, but longer than in c-Si. In addition, the lifetimes increase with decreasing frequency, from less than 10 ns at 515 cm^-1 to ~30 ns at 505 cm^-1. We further show that phonons with a frequency of ~150 cm^-1 in a-nc-Si:H have longer lifetimes than in a-Si:H (τ<10 ns). Finally, the diffusion of 29-cm^-1 phonons through the a-nc-Si:H and a-Si:H material was examined in phonon-induced luminescence experiments. Transport through the a-nc-Si:H film appears to be much slower than through the a-Si:H layer. We explain these results as effects of phonon confinement, and relate them to the extremely long phonon lifetimes found in Raman experiments on a-Si(:H).