Substrate and capping layer effects on the phonon spectrum of ultrathin superlattices

GemSin ultrathin superlattices have been grown on (001) Si substrates and partly capped with Si. Raman scattering from acoustic phonons in the long-wavelength region shows unexpectedly intense broad peaks that develop into an increased number of sharper intense peaks on capping. We identify these peaks with resonant phonon modes and use a linear chain model to expose the importance of substrate-superlattice-capping layer interactions in these multilayer structures.