Surface phonons of hydrogen-terminated semiconductor surfaces .3. Diamond(001) monohydride and dihydride

Surface-phonon spectra of hydrogen-terminated diamond (001) surfaces, resulting from our semiempirical total-energy approach, are presented and discussed in comparison with mode frequencies determined experimentally from electron energy-loss (EEL) spectroscopy data. We have studied both the H:C(001)-(2x1) monohydride and 2H:C(001)-(1x1) dihydride surfaces employing appropriate slab geometries. A large number of surface-induced phonon branches are found for both systems. The physical origin and nature of salient H-induced, as well as, substrate-surface related vibrational modes, are analyzed in detail by a study of spectral densities combined with investigations of vibrational modes of appropriate surface molecules. Both CHx bond-stretching modes, as well as bond-bending modes, are clearly identified, and their calculated frequencies show very good agreement with the mode-frequencies determined at the Gamma point by specular EEL measurements. We present full surface-phonon dispersions along high-symmetry lines of the surface Brillouin zones. They should prove very useful for an interpretation of experimental results from off-specular EEL investigations of these technologically important systems.