Angle-resolved electron-energy-loss study of Al/Si(111)

We have performed a high-resolution electron-energy-loss study of the Si(111)-(√3 × √3 )R30°:Al surface and have measured the dispersions of several surface phonons along the ΓK¯ 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 semiquantitative 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.