Growth of γ - Al₂O₃ (111) on an ultra - thin interfacial Al₂O₃ layer / NiAl(110)

The oxidation of NiAl(110) was investigated in the temperature regime between 300 K and 1300 K using LEED (low energy electron diffraction), TPD (temperature programmed desorption) and HREELS (high resolution electron energy loss spectroscopy). The adsorption of N₂O and O₂ up to saturation on NiAl(110) at 300 K only leads to the selective oxidation of surface aluminium without reconstructions. Stepwise annealing of the oxygen-saturated sample from 600 K to 1300 K in UHV (ultra-high vacuum) results in firstly the onset of randomly oriented then finally fairly well-ordered, 5 Å Al₂O₃ film with quasi-hexagonal periodicity. Ordered thicker oxide films of 18-30 Å seem to be grown on this interfacial oxide layer by direct oxidation of sample at elevated temperatures between 1150 and 1300 K because of the LEED pattern consisting of new broad hexagonal spots and the previous 5 Å spots. Although the periodicity of surface oxygen arrays shows no significant change from an hexagonal close-packing, the O-O distance changes from ~3.0 Å for the 5 Å film to ~2.9 Å for thicker oxides. With the appearance of Auger parameter, for the 5 Å film, which is significantly different from those of γ- and α-like Al₂O₃ samples, the 5 Å film can be described better as an interfacial oxide layer. The observation of three symmetric phonon peaks can be also a supporting evidence for this phase assignment since thicker oxide films on the same NiAl(110) show somewhat different phonon structure much closer to that of the γ-Al₂O₃ The adsorption/desorption of methanol further proves the preparation of less-defective and/or oxygen-terminated Al₂O₃ films showing ordered phase transitions with the change of oxide thickness between 5 Å to 30 Å.