In-plane Raman scattering of (001)-Si/Ge superlattices: Theory and experiment

Using a micro-Raman setup, in-plane Raman scattering was performed on short-period (001)-Si/Ge superlattices, and the complete phonon spectrum (longitudinal and transverse, optical and acoustic modes) could be studied. In the relevant wave-vector range, the in-plane dispersion was found to affect the frequencies of folded-acoustic modes, while being negligible for confined optical modes. The comparison with theoretical spectra, calculated by means of first-principles interatomic force constants, shows that the observed deviations of ``unfolded'' confined optical modes from the bulk dispersions can be attributed to interface roughness and is well described by a simple alloy layer model (2--3 intermixed SiGe atomic layers at interfaces).