In-plane Raman scattering of [001]-grown Si/Ge superlattices

We present a micro-Raman study of short-period [001] Si/Ge superlattices. The submicron spatial resolution of this technique enables in-plane scattering geometries, thus overcoming the severe limitations of conventional Raman set-ups for backscattering from the growth surface, where only longitudinal modes can be observed. We were able to study the complete phonon spectrum consisting of confined longitudinal optical and transverse optical and folded longitudinal acoustic and transverse acoustic modes. The scattering wavevector was found to be negligible for the confined optical modes, whereas it affects the frequencies of the folded acoustic modes. In contrast to the bulk, the selection rules are determined by the wavevector of the superlattice periodicity and not by the scattering wavevector.