SURFACE PHONONS OF THE SI(111)-7X7 RECONSTRUCTED SURFACE

We have determined the phonon spectrum of bulk silicon and the Si(111)-7X7 reconstructed surface by performing molecular-dynamics simulations based on the Car-Parrinello fictitious Lagrangian and a tight-binding Hamiltonian. The calculations are performed on parallel distributed memory computers. The phonon-dispersion relation of bulk silicon is obtained from a 128-atom simulation by the fast Fourier transform (FFT) of the ionic velocity data. The vibrational spectrum is reproduced with a resolution of 0.1 THz. Dynamical simulations of the Si(111)-7 X 7 surface consisting of a 400-atom cell are performed over 8 ps and surface phonons are extracted from the ionic velocity-velocity autocorrelation function using the FFT. Localized vibrations on adatoms are found at 15 and 16.3 THz, which we identify as the localized phonons at adatomic sites as observed experimentally by electron-energy-loss (EEL) spectroscopy. We also find surface phonons on the adatomic sites in a lower-frequency range (6-10 THz), which have been previously observed by EEL. We observe a highly symmetrical collective mode of the first-layer atoms at 12.5 THz. This mode involves out-of-phase breathing modes of the two subcells, into which the supercell is divided by the dimer row. Three surface-free atoms in one subcell are incoming while the rest of the atoms are outgoing.