Vibrational properties of Si/Ge superlattices: Theory and in-plane Raman scattering experiments

Phonons in short-period (001)-Si_nGe_n Superlattices (SL's) have been studied both theoretically, by a first principles approach including strain and interface intermixing, and experimentally by micro-Raman spectroscopy where in-plane scattering geometries allow the observation of both longitudinal (L) and transverse (T) modes. Experimental data are found to deviate considerably from theoretical predictions for SL's with ideally sharp interfaces, both in frequency of higher-order confined Si-like modes and in the polarization dependence of the SiGe-like "interface" peak (lineshape and L-T splitting). Supercell calculations representing interface intermixing within a simple model by 2-3 monolayers of SiGe alloy at the interfaces are found to reproduce these major experimental findings. Measurements of Raman resonance profiles of various SL phonon modes strongly confirm their calculated different spatial localization.