Anisotropic surface phonon dispersion of a deuterium-terminated Si(110)-(1 × 1) surface studied by high-resolution electron-energy-loss spectroscopy and first-principles calculations: Isotope effect

The surface phonon dispersion of a deuterium-terminated Si(110)-(1 × 1) surface [D:Si(110)-(1 × 1)] is investigated by using high-resolution electron-energy-loss spectroscopy (HREELS) and first-principles calculations based on the density functional theory (DFT) with the local density approximation (LDA). The characteristics of D:Si(110)-(1 × 1) are unique compared to those of H:Si(110)-(1 × 1) (Matsushita et al., 2015) in terms of the resolved vibrational modes. By the HREELS, one-dimensional surface phonons consisting of D–Si stretching vibrations are observed above the bulk-phonon band energy edge of 64.5 meV. Ten modes are observed below this value, classified as surface, surface resonant, and bulk phonons according to the calculated energy dispersion as well as the depth profile of spectral density and displacement vectors. In particular, five D–Si bending modes are observed out of the seven theoretically predicted modes. The bending modes are strongly coupled with the displacements across the D and five Si layers. The DFT-LDA surface phonon dispersion is in good agreement with the experimental results except a few frequency/dispersion mismatches, as the structure optimized by DFT-LDA mismatches with the previous scanning tunneling microscopy (STM) results (Matsushita et al., 2015). D:Si(110)-(1 × 1) elucidates the nature of covalently bonded phonons and their characteristics both experimentally and theoretically.