The vibrational properties of thin films of ${\mathrm{Al}}_2$${\mathrm{O}}_3$ on Au, Al, and Si have been studied by means of infrared reflection absorption spectroscopy (IRAS). The films have been characterized by x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), transmission electron microscopy, and low-angle x-ray experiments. The latter two techniques have shown that the amorphous films transform to crystalline $\gamma -{\mathrm{Al}}_2{\mathrm{O}}_3$ after tempering above a critical temperature. XPS and UPS indicate that the films are stoichiometric and continuous down to the monolayer range. IRAS experiments of ${\mathrm{Al}}_2$${\mathrm{O}}_3$ films on Au with thicknesses ranging between 3 and 200 Å show a striking thickness dependence of the vibrational frequency of the Al-O longitudinal mode. A similar thickness dependence is found for ${\mathrm{Al}}_2$${\mathrm{O}}_3$ films on Al. This observation can be explained by the use of a simple model which is based on the fact that the restoring forces at the surface are smaller than in the bulk. The transition from amorphous ${\mathrm{Al}}_2$${\mathrm{O}}_3$ to crystalline $\gamma -{\mathrm{Al}}_2{\mathrm{O}}_3$ is also reflected in the IRAS experiments of ${\mathrm{Al}}_2$${\mathrm{O}}_3$ on Si and Al.

• Received 18 July 1983

DOI:https://doi.org/10.1103/PhysRevB.29.4691