Surface Vibrational Excitations on Si(001) 2 × 1

We present the first realistic study of the surface-phonon spectrum of a reconstructed semiconductor surface. The extensive reconstruction of the Si(001)2 ×{} 1 surface is found to localize a rich spectrum of surface modes including a band localized nearly completely on a rocking motion of surface dimers and an unanticipated optical mode localized in the first two subsurface layers. With use of a tight-binding theory for structural energies, the model incorporates the effects of bond rehybridization at the surface.