Ultrathin AlOx-TiOy film formation by controlled oxidation of titanium deposited on polycrystalline aluminum surfaces

The interaction of oxygen with titanium deposited on polycrystalline aluminum surfaces has been studied at room temperature and low oxygen pressures, using AES, XPS, and ARXPS. The growth of titanium on the aluminum surfaces occurs in two stages: formation of a uniform TiAlx layer up to similar to4 monolayers (ML), followed by the formation of metallic titanium islands 10 ML thick that grow over the TiAlx layer previously formed. The oxidation of Ti/Al interfaces shows the formation of an aluminum intermediate oxidation state, Al2+, in addition to Al3+, which is attributed to the formation of Al-O-Ti cross-linking bonds at the interface between an outer aluminum oxide-Ti4+ mixed layer and a film beneath it composed by a mixture of titanium suboxides. The Al2O3 concentration in the oxide film decreases as the Ti deposition time previous to oxidation increases, as a consequence of the thickening of the TiAlx layer up to 4 ML in the first stage of Ti growth and subsequent formation of metallic Ti islands in the second stage. The Al2+ concentration reaches a maximum at the end of the first stage of Ti growth, decreasing afterward because of metallic Ti islands oxidation. The analysis of the Ti 2p band shows Ti oxidation for Ti/Al interfaces of both the first and second stage of titanium growth. However, for Ti/Al interfaces of the first stage, the ratio between Ti4+ and titanium suboxides, the lack of the oxygen signal characteristic of pure Ti oxidation, and the formation of an oxygen species in the high binding energy side of the O is band, related to the titanium oxidation of the TiAlx compound, suggest the formation of an aluminum titanate-like compound.