Structural ordering of ultra-thin, amorphous aluminium-oxide films

The structural ordering of ultra-thin (<5 nm) Al-oxide films, as grown by dry, thermal oxidation for various oxidation times within the temperature range of 373-773 K, has been investigated by chemical state analysis using X-ray photoelectron spectroscopy (XPS). To this end, the changes in chemical environment of the Al and O ions in the oxide film during the transformation from amorphous Al-oxide to γ-Al2O3 with increasing oxidation temperature and oxide-film thickness has been deduced from the corresponding shifts of the Al and O Auger parameters. It is concluded that the transition from amorphous Al-oxide to γ-Al2O3 is realised by a gradual development of long-range order in the dense-random-packing-of-spheres network of oxygen ions, accompanied by a densification of the oxide film. During the transformation, neither a significant short-range ordering of Al cations nor a redistribution of Al cations over tetrahedral and octahedral interstices of the oxygen sublattice is observed.