Elastic screening of surface vibrations: Surface phonons on As:Si(111)(1×1)

We analyze the surface-phonon spectrum of As:Si(111)(1×1), modeling the system as a single atomic As monolayer bound to a Si substrate, where the substrate is treated as a semi-infinite elastic continuum. The As surface atoms are treated as local oscillators, with oscillator parameters for the As-Si bonding calculated from self-consistent local-density-functional calculations of the total surface energy. The results are in semiquantitative agreement with recent helium scattering measurements of surface phonons on As:Si(111), and indicate the importance of substrate dynamics in determining the dispersion of the surface modes. In particular, our analysis suggests that the observed phonons can be interpreted most simply as surface mass-defect modes of an elastic substrate.