Phonons in a surface with a mass defect: As:Si(111)(1×1)

Using inelastic helium scattering, we have made the first measurements of surface-phonon dispersion relations in a ``capped'' semiconductor, As:Si(111)(1×{}1). The simple, single-layer surface modification introduced by the As atoms gives rise to two surface acoustic modes. We suggest that such surfaces present a unique opportunity for studying the effect on the surface lattice dynamics of surface modification (i.e., a nearly ideal mass defect) and that the results may provide general insight into the dynamical behavior of clean as well as adsorbate-covered surfaces.