Surface phonons in thin aluminum oxide films: Thickness, beam-energy, and symmetry-mixing effects.

Abstract

The surface-phonon (Fuchs-Kliewer) modes of thin \ensuremath{\alpha}-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ films prepared on a Ru(001) substrate have been measured with high-resolution electron-energy-loss spectroscopy. An understanding of the differences in phonon spectra for thick cyrstals versus thin films is derived from calculations using dielectric theory and the infrared optical constants for \ensuremath{\alpha}-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$. In addition to the three characteristic phonon modes assigned previously at \ensuremath{\sim}400, \ensuremath{\sim}650, and \ensuremath{\sim}900 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for \ensuremath{\sim}10-\AA{} aluminum oxide films, our experiments and theoretical modeling confirm the presence of an additional mode at \ensuremath{\sim}800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the splitting of the 400-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ feature into two peaks at about 350 and 500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for our somewhat thicker 30-\AA{} well-annealed \ensuremath{\alpha}-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ films. The primary beam-energy dependence for the \ensuremath{\sim}900-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ phonon is found experimentally to be ${\mathit{E}}^{\mathrm{\ensuremath{-}}0.9}$ while the dielectric theory predicts ${\mathit{E}}^{\mathrm{\ensuremath{-}}0.8}$, in contrast to the well known ${\mathit{E}}^{\mathrm{\ensuremath{-}}0.5}$ dependence for bulk ionic crystals. The 800- and 900-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ features are related to the high-frequency surface-phonon branches corresponding to the 650-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ bulk TO modes and are expected from dielectric theory for an ideal alumina layer on metal support. The 350-, 500-, and 650-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ modes are related to the low-frequency surface-phonon branches, which are allowed due to a (\ensuremath{\omega},k)-dependent dispersion-related symmetry-mixing process. Successful modeling of these latter modes is carried out under the assumption of a ``self-supported'' alumina film.