Promoted oxidation of aluminum thin films using an alkali metal catalyst

Abstract

We investigate the effect of sodium overlayers on the oxidation of aluminum thin films by means of core level and valence band photoemission spectroscopy using synchrotron radiation. The presence of Na enhances the oxidation rate of the aluminum by nearly 4 orders of magnitude with formation, at room temperature, of alumina likely as α-Al2O3. The sodium catalyst is removed by thermal desorption at temperatures below 300 °C leading to the formation of an aluminum oxide having a larger binding energy than stoichiometric alumina. It indicates the formation at the surface of an aluminum oxide having higher oxidation states as Al2O3+x. Unlike catalytic oxidation of elemental semiconductors such as silicon, formation of Na related oxides are shown to play an important role in the alkali metal promoted aluminum oxidation. Interestingly, Na appears a more efficient catalyst for Al thin film than on single crystal surface. The micromechanisms of promotion are found to be significantly different from that of alkali metal promoted oxidation of semiconductor surfaces.