Phonons in semiconductor superlattices studied by in-plane Raman scattering

In-plane Raman scattering, via cleaved or polished sample edges, has been applied to GaAs/AlAs superlattices for investigating the angular anisotropy predicted for optical phonons in polar heterostructures. For both extreme short-period and long-period superlattices, the angular dispersion can be described within a simple dielectric continuum model, whereas this model fails for intermediate layer thicknesses. For intermediate thicknesses, mode mixing and effects of bulk phonon dispersion produce frequency gaps in the angular dispersion. For non-polar Si/Ge superlattices, in-plane Raman scattering enables a comprehensive investigation of interface roughness. The experimental features for the mechanical interface mode and the Si-like confined modes are well explained by microscopic calculations assuming two intermixed alloy monolayers at each interface.