Vibrational properties of Ge- and Sb-adsorbed Si(001) surfaces

Abstract

We report on vibrational properties obtained within the adiabatic bond-charge model for $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstructed monolayer Ge- and Sb-adsorbed Si(001) surfaces. The phonon spectra of these surfaces are compared in detail with that of the clean $\mathrm{Si}(001)(2\ifmmode\times\else\texttimes\fi{}1)$ surface presented in a similar recent study. We observe that acoustic-phonon modes on the covered surfaces are mainly localized in the adsorbate layers, while high-energy optical phonon modes are localized in the top few substrate layers. This phenomenon is attributed to the large mass difference between Sb (Ge) and Si atoms. Adsorption of these overlayers is shown to result in characteristic new peaks in the phonon density of states, which, together with differences in polarization of certain gap phonon modes, provide a distinct signature for each overlayer system.