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STM/STS, AES and XPS have been combined to study ultra-thin films of alumina grown on γ-TiAl(111) surfaces. The clean surface of the alloy is characterized by a (2x4) superstructure. The uptake of oxygen at 650°C (pO2=10-7 mbar) shows a saturation level at ~800 L corresponding to a pure alumina layer grown by selective oxidation of the alloy. The equivalent thickness of the oxide film is estimated to ~0.5 nm (~2 oxygen layers). The growth of the oxide film is confirmed by STS which reveals a gap of electronic states not measured on the non oxidized surface. STM imaging reveals nanostructuring at two different length scales, similar to that observed on the Al2O3/Ni3Al(111) surface (Hamm et al, Phys. Rev. Lett 97 (2006) 126106). At a shorter length scale, two variants at 8.5±1° of an hexagonal lattice (1.85±0.1 nm) are observed. They form a (√37x√37)R+/-25.3° superstructure with respect to the hexagonal metal lattice (0.295±0.015 nm). This superstructure is assigned to the coalescence of metal vacancies injected by the growth of the oxide and nanostructuring the metal surface below the oxide film. The superstructure contains numerous defects joining the nods of the hexagonal lattice and suggesting larger islands of vacancies than in the well-ordered areas. At a longer length scale, two variants of a larger hexagonal lattice (6.4±0.1 nm) are observed. They are (2√3x2√3)R30° larger than the √37 lattice thus forming a (2√111x2√111)R+/-55.3° superstructure with respect to the metal lattice. Measurements performed on a partially reduced surface obtained after prolonged annealing at 650°C under UHV give differences of apparent height measured between the 2√111 nanostructured islands and the reduced islands of ~0.4 nm consistent with the estimated thickness of the oxide layer. This superstructure is assigned to the coincidence cell formed by the O sub-lattice of the oxide layer with the atomic lattice of the substrate. Based on the measured value of 0.295 nm for the metal lattice, a value of 0.282 nm is obtained for the O sub-lattice, in excellent agreement with that expected (0.279 nm from bulk parameters) for a (111) oriented γ-like alumina layer. |