A high-resolution photoemission study of nanoscale aluminum oxide films on NiAl(110).

Using high-resolution soft X-ray photoemission, Al 2p, we have been able to quantify the relative populations of tetrahedrally (Al(tet)) and octahedrally (Al(oct)) coordinated Al(3+) in three distinct phases of nanoscale aluminum oxide films on NiAl(110). We have hence determined the bulk alumina phases that the nanoscale films most resemble. Adsorption of oxygen at room temperature produces a layer which predominately (90%) contains Al(tet) and is analogous to the amorphous bulk phase of alumina. Annealing this layer results in an Al enrichment of the oxide layer, through the diffusion of metal from the substrate, and an increase in the relative amount of Al(oct), producing a gamma-alumina-like layer with a relative Al(oct)/Al(tet) occupancy of 28 +/- 3%/72 +/- 3%. Oxygen adsorption at 823 K also produces a gamma-like phase, with a relative Al(oct)/Al(tet) occupancy of 27 +/- 3%/73 +/- 3%, although this layer is thicker than that formed at room temperature. Both oxidation methods produce gamma-alumina layers that display poor translational order. However, these poorly ordered layers have a relative Al(oct)/Al(tet) occupation similar to that of well-ordered oxide films produced using different oxidation conditions in previous studies. Both gamma layers undergo partial decomposition upon annealing to 1273 K, producing an alpha-alumina-like oxide, which contains only Al(oct), and is highly deficient in Al. There are significant oxide-free areas within the alpha-alumina oxide layer, which is characteristic of crystallite formation. Repeated cycles of oxidation and annealing to 1273 K do not produce a homogeneous film, but they do make the alpha-like oxide more Al rich.