Interpretation of Raman spectra of Ge/Si ultrathin superlattices

We present a study of Raman scattering by phonons in Geₘ{Si}ₙ$-type ultrathin superlattices. We calculate the Raman spectra of ideal unstrained structures and then successively include strain and interface smudging. The calculations reveal interesting systematics about quasiconfinement. It is shown that quasiconfinement, strain, and interface smudging have to be treated concurrently in the interpretation of observed spectra. The nature of the ``Ge-Si''-like mode often seen in experimental spectra is elucidated. Using the information base built up from the study of theoretical spectra, we analyze experimental spectra of superlattices grown on Si(100) and Ge(100) substrates to obtain quantitative information useful for structural characterization of the samples studied.