VIBRATIONAL-MODES ON SI(111)1X1-AS STUDIED BY ELECTRON-ENERGY-LOSS SPECTROSCOPY

The dispersion of several surface phonons on Si(111)1 x 1-As has been measured along the ($) over bar Gamma ($) over bar M and ($) over bar Gamma ($) over bar K direction using electron-energy-loss spectroscopy . In the acoustical surface-phonon range, good agreement with previous inelastic He-scattering data and calculations of the surface-phonon d ispersion in the bond-charge model is found. Two new optical surface-p honon modes with frequencies of 355 and 520 cm(-1) at ($) over bar Gam ma were followed throughout the surface Brillouin zone. The 355-cm(-1) mode is found to be dipole active on Si(111)1 x 1-As, which is interp reted as being due to a decoration of this mode by the As atoms. The 5 20-cm(-1) mode is identified with the Lucas mode.