ELECTRONIC AND VIBRATIONAL PROPERTIES OF HYDROXYLATED AND DEHYDROXYLATED THIN AL2O3 FILMS

The electronic and vibrational properties of planar thin-films (20-30 angstrom) of aluminum oxide on a Ru(001) substrate were studied using X-ray photoelectron (XPS), Auger, and high resolution electron energy loss (HREEL) spectroscopies. Hydroxylated Al2O3 films, prepared in water/oxygen mixtures or produced by exposure of dehydroxylated Al2O3 films to water vapor, exhibit O-H stretching vibrations at approximately 3730 cm-1, characteristic of isolated hydroxyl groups. It is particularly significant that the hydroxyl group stability on planar Al2O3 films, oxidized at pressures between several Torr and 1 atm, is shown to be comparable to that for alumina powders. This is taken as a strong indication that the planar oxide structure is similar to that of powder alumina surfaces. XPS and Auger spectra for the thin-film aluminas indicate Auger parameters and valence band densities-of-state comparable to those of powdered aluminas. The aluminum Auger parameter shifts from gamma-like to alpha-like at least 200 K below transition temperatures for micron thick films, and 350 K below the bulk transition temperatures. Our 30 angstrom films are substantially thinner, and so the Auger parameter may be indicative of a low temperature phase transformation, but is certainly characteristic of a change in the local cation environment from a mixture of tetrahedral and octahedral cation occupation to only octahedral coordination. The surface phonon spectra of aluminas containing no excess Al0 produce the characteristic three-peak vibrational spectrum. Calculations performed using the dielectric theory of electron energy loss spectroscopy confirm this characteristic phonon loss spectrum to be composed of both longitudinal (LO) and transverse (TO) surface optic modes.