Initial stages of surface and subsurface oxidation of Al(100) studied by photoelectron spectroscopy and low energy ion scattering

Abstract

In this work the initial stages of surfaces and subsurface oxidation of Al(1 0 0) were investigated by X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS) and ultraviolet photoelectron spectroscopy (UPS). By varying the He+ incident kinetic energy the depth sensitivity of LEIS was tuned for differentiating between surface versus subsurfaces oxidation processes. It is shown that the oxidation of Al(1 0 0) surface maybe separated into three sequential stages: (i) up to ∼2 L dissociative chemisorption takes place, (ii) between 2 and ∼120 L both chemisorption and subsurface oxidation processes occur concurrently and (iii) for larger exposures a thick oxide layer is formed reaching saturation, according to XPS, at about 300 L. XPS line shape analysis shows that at the initial stages of adsorption, associated with the first two fast stages, a non-stochiometric surface oxide is formed with an O/Al ratio of ∼0.5 increasing to the stochiometric Al2O3 value of 1.5 upon formation of a stable oxide layer. It was found that upon oxidation a strong reduction in the total integral intensity of the scattered He+ ions occurs, which is correlated with a decrease of the work function. It is suggested that the decrease of the work function results in an enhanced neutralization rate of elastically scattered He ions. Annealing of the oxidized surface to 400 °C results in stabilization of Al-oxide and a partial restoration of the work function.