Characterization ofa-Si:H/a-Ge:H superlattices by Raman scattering

We have investigated the vibrational spectra of a-Si:H/a-Ge:H superlattices by Raman scattering. For high frequencies (>100 ${\mathrm{cm}}^{\mathrm{-}1}$) the intensity of the Raman spectrum is proportional to the density of vibrational states and shows contributions from the Si—Si and Ge—Ge bonds within the superlattice layers and from the Si—Ge bonds in the interface region. From the relative intensities of these modes we determine the individual layer thicknesses and the width of the interface. These results are discussed in terms of the growth conditions. For low frequencies (<80 ${\mathrm{cm}}^{\mathrm{-}1}$) the Raman spectra show peaks corresponding to the excitation of folded-zone longitudinal acoustic phonons. The dispersion relation for these phonons is determined by measuring the peak positions as a function of laser energy and of the superlattice period. The result is well described by a continuum elastic model. The linewidth of these peaks increases with phonon frequency, indicating that the dispersion relation acquires a non-negligible imaginary component due to the loss of phase coherence of the acoustic vibrations.