Light scattering by acoustic phonons in unhydrogenated and hydrogenated amorphous silicon

Brillouin and Raman spectroscopies have been applied to dense undoped films, either multiple-energy Si-implanted a-Si layers or device-grade a-Si:H layers deposited by glow-discharge. The frequencies of the surface (Rayleigh) and longitudinal acoustic Brillouin peaks were compared to those measured on crystalline {111} and {100} silicon (c-Si) surfaces, yielding longitudinal sound velocities of 6.1 km/s in a-Si and 8.9 km/s in a-Si:H, and surface phonon velocities of 4.1 km/s in a-Si and 4.85 km/s in a-Si:H. We confirm the existence of a local hydrogen-related mode around 210 cm^-1 in a-Si:H and propose a careful analysis of the low-energy Raman scattering by acoustic phonons: the normalised TA Raman intensity in a-Si, with a threshold around 30 cm^-1, reached above 50 cm^-1 the scattered light level detected in a-Si:H. A low-frequency background of excitations, involving among other possibilities multiphonon processes and in particular LA ± TA combination modes, was confirmed to exist in plasma-deposited a-Si:H.