Frequency gaps for acoustic phonons ina-Si:H/a-SiNx:H superlattices

We investigate the frequency gaps for zone-folded LA and TA phonons propagating perpendicular to the layers in a-Si:H/a-${\mathrm{SiN}}_{\mathrm{x}}$:H superlattices using Raman and phonon transmission spectroscopy. Raman spectra in the forward scattering configuration were used to determine the magnitude of the zone-center gaps and the symmetry of the zone-center modes delimiting these gaps. The zone-boundary gaps were measured in the backscattering geometry by choosing the superlattice period and the excitation wavelength so as to couple the light to the zone-boundary phonons. The phonon transmission spectra show minima corresponding to frequency gaps in the dispersion relation of TA phonons. Other minima appear at those frequencies where the LA and TA zone-folded branches cross. They are associated with additional gaps in the phonon dispersion relation for modes with mixed LA and TA character with wave vectors deviating from the superlattice normal. The phonon blocking within the frequency gaps is degraded as a result of phonon scattering. By comparing the transmission spectra of samples with a different number of layers we conclude that for low phonon energies (<20 ${\mathrm{cm}}^{\mathrm{-}1}$) the scattering takes place mainly at the first superlattice interfaces near the substrate.