Formation of a well-ordered ultrathin aluminum oxide film on Ni(1 1 1): Determination of its thickness, composition and structure

A well-ordered ultrathin alumina film on top of Ni(1 1 1) has been obtained by an oxidation at 300 K of a thin Ni3Al(1 1 1) layer epitaxially grown on Ni(1 1 1) and subsequent annealing at 1000 K. The formation of this film was studied by Rutherford backscattering spectrometry (RBS) under channeling conditions, nuclear reaction analysis (NRA), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The absolute amounts of both types of atoms participating to the oxide film are, respectively, (2.3 ± 0.3) × 1015 Al/cm2 and (3.3 ± 0.3) × 1015 O/cm2. Hence it appears that this film, with a stoichiometry very close to Al2O3, has an oxygen content corresponding nearly to two compact planes of oxygen in bulk crystalline alumina (about 3.0 × 1015 O/cm2). A commensurate (5√3 × 5√3)R30° superstructure with a lattice parameter of 2.16 nm, can be deduced from the LEED pattern. This superstructure differs from the one observed by other authors for alumina films (of similar thickness and composition) formed on bulk-Ni3Al(1 1 1), a surface with symmetries and interatomic distances comparable to that of Ni(1 1 1). This difference in structure is most probably connected to the absence, in our case, of any Al atom not strongly bound to O atoms at the interface between the ordered alumina film and the Ni substrate: the Al atoms not involved in the oxide film have diffused deeply in the bulk of the Ni substrate during the high temperature annealing stage needed for alumina ordering.