Composition and chemical state of the ions of aluminium-oxide films formed by thermal oxidation of aluminium

The chemical composition and the chemical state of the Al and O ions of thin aluminium-oxide films grown by the dry, thermal oxidation of a bare Al(4 3 1) substrate at a partial oxygen pressure of 1.33×10−4 Pa in the temperature range of 373–773 K were studied using X-ray photoelectron spectroscopy. At low temperatures (T⩽573 K) an aluminium deficient (as compared to Al2O3), amorphous oxide film of limiting thickness develops. At higher temperatures (T>573 K) an Al-enriched, amorphous oxide film is formed, which gradually attains the stoichiometric composition of Al2O3 and becomes crystalline γ-Al2O3. It was shown for all oxide films grown that the oxygen content of these films is constant, whereas the Al content increases with temperature and time. The amorphous oxide films of limiting thickness are relatively enriched in Al (as compared to Al2O3) at their very surface. This Al-enriched surface oxide does not occur for the crystalline γ-Al2O3 oxide films. As evidenced by the approximately constant chemical state of the Al ions and the Al–O binding state in the oxide films (independent of both the oxidation temperature and time), the short range ordering of the amorphous Al-oxide films closely resembles that of the crystalline γ-Al2O3 films. The major part of the observed shifts in Al 2p and O 1s binding energies, and Al KL23L23 kinetic energy, of the oxide-film component with both increasing oxidation temperature and thickness are related to shifts of the Fermi level within the band gap of the oxide. These shifts of the Fermi level are induced by defect states within the band gap due to an Al-enrichment in the developing oxide film at its very surface (at low temperatures) or near the metal/oxide interface (at high temperatures).