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

The surface-phonon (Fuchs-Kliewer) modes of thin {α}-Al₂{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 {α}-Al₂{O}₃. In addition to the three characteristic phonon modes assigned previously at ~400, ~650, and ~900{cm}^{{{-}}1}for ~10- aluminum oxide films, our experiments and theoretical modeling confirm the presence of an additional mode at ~800{cm}^{{{-}}1}$ and the splitting of the 400-${cm}^{{{-}}1}$ feature into two peaks at about 350 and 500 ${cm}^{{{-}}1}for our somewhat thicker 30- well-annealed α-{Al}₂O₃ films. The primary beam-energy dependence for the {~}900-cm^-1 phonon is found experimentally to be E^-0.9 while the dielectric theory predicts E^-0.8, in contrast to the well known E^-0.5 dependence for bulk ionic crystals. The 800- and 900-cm^-1 features are related to the high-frequency surface-phonon branches corresponding to the 650-cm^-1 bulk TO modes and are expected from dielectric theory for an ideal alumina layer on metal support. The 350-, 500-, and 650-cm^-1 modes are related to the low-frequency surface-phonon branches, which are allowed due to a ({ω},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.