VIBRATIONAL PROPERTIES OF SI GE SUPERLATTICES - THEORY AND INPLANE RAMAN-SCATTERING EXPERIMENTS/

Phonons in short-period (001)-Si(n)Ge(n) Superlattices (SL's) have bee n studied both theoretically, by a first priniciples approach includin g strain and interface intermixing, and experimentally by micro-Raman spectroscopy where in-plane scattering geometries allow the observatio n 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 co nfined Si-like modes and in the polarization dependence of the SiGe-li ke ''interface'' peak (lineshape and L-T splitting). Supercell calcula tions representing interface intermixing within a simple model by 2-3 monolayers of SiGe alloy at the interfaces are found to reproduce thes e major experimental findings. Measurements of Raman resonance profile s of various SL phonon modes strongly confirm their calculated differe nt spatial localization.