SURFACE PHONONS IN THIN ALUMINUM-OXIDE FILMS - THICKNESS, BEAM-ENERGY, AND SYMMETRY-MIXING EFFECTS

The surface-phonon (Fuchs-Kliewer) modes of thin alpha-Al2O3 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 crystals versus thin films is derived from calculations using dielectric theory and the infrared optical constants for alpha-Al2O3. In addition to the three characteristic phonon modes assigned previously at approximately 400, approximately 650, and approximately 900 cm-1 for approximately 10-angstrom aluminum oxide films, our experiments and theoretical modeling confirm the presence of an additional mode at approximately 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-angstrom well-annealed alpha-Al2O3 films. The primary beam-energy dependence for the approximately 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 (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.