ANGLE-RESOLVED ELECTRON-ENERGY-LOSS STUDY OF AL/SI(111)

We have performed a high-resolution electron-energy-loss study of the Si(111)-(square-root 3 x square-root 3) R30-degrees:Al surface and have measured the dispersions of several surface phonons along the GAMMAKBAR direction. We interpret these using two different simple lattice dynamical models, both employing force constants from an ab initio electronic-structure calculation of Northrup. The first yields quantitative information at q = 0; the second is a semiquantitive estimate of the dispersion. These models have no adjustable parameters. The good agreement with the experiment for most of the observed modes indicates that a single theory can explain both the electronic structure and vibrational modes of this surface and also illustrates both the usefulness and limitations of simple models of lattice dynamics.