Structure of thin aluminium-oxide films determined from valence band spectra measured using XPS

Very thin (<10 nm) aluminium-oxide films were produced by dry, thermal oxidation of a bare Al(4 3 1) substrate in the temperature range from 373 to 773 K at a partial oxygen pressure of 1.33×10−4 Pa. The development of the structure of the grown oxide films has been determined from the analysis of the measured valence band (VB) spectra recorded from the oxidised metal using X-ray photoelectron spectroscopy. Two principal components (PCs) were recognised in the `oxide-filmʹ upper valence band (UVB) spectra, as resolved from the measured VB spectra of the oxidised metal. On the basis of high-resolution transmission electron microscopy (HREM), one PC can be associated with amorphous aluminium oxide and the other PC with crystalline γ-Al2O3. From the relative contribution of the amorphous and γ-Al2O3 basic spectra to the resolved oxide-film UVB spectra, the development of the structure of the aluminium-oxide films as a function of temperature and time, as well as of thickness, is established. With increasing oxidation temperature a gradual transition from amorphous aluminium oxide to crystalline γ-Al2O3 occurs, as confirmed by the HREM analysis in the present work. Up to 1 nm the aluminium-oxide films are to a high-degree amorphous, above this thickness the aluminium-oxide films are predominantly crystalline.