Influence of argon ion bombardment on the oxidation of nickel surfaces

The effect of argon ion bombardment on the oxidation of nickel films was studied using X-ray photoelectron spectroscopy (XPS). In the absence of any ion beams, exposure of nickel surfaces to oxygen leads to the moderately rapid formation of a thin (3-5 monolayers thick) nickel oxide overlayer. At room temperature the oxygen uptake stops once this limit is reached, but at higher temperatures the siow growth of a thicker oxide is seen. The NiO growth kinetics can be phenomenologically described by a diffusion coefficient for either oxygen or (more likely) nickel ions through the forming oxide film of the order of 2 x 10 -18 cm 2 s -1 at 625 K. The simultaneous impingement of argon ions on the surface during oxygen exposures was found to enhance the oxidation process, and ion beam current densities as low as 0.01 μA cm -2 were found to be sufficient to induce nickel oxidation past the 3-5 ML limit at room temperature. The oxidation rate was found to be roughly proportional to both the ion flux and the square of the oxygen pressure, suggesting that the Ar + bombardment oxidation enhancement may be due to an increase in diffusivity through the NiO surface film induced by local heating around the ion impact areas. The build-up of an NiO film during this Ar + -ion/oxygen treatment was also found to slow down at higher temperatures, presumably because of the combined effect of a higher probability for desorption of molecular oxygen from the surface and a higher atomic-oxygen mobility into the bulk. The oxide films prepared at low temperatures appear to be quite disordered, and display an extra feature in the Ni 2p XPS spectra around 853.2eV which could be assigned to partially reduced nickel, Ni x+ , x<2. Annealing of those films to temperatures above 400 K leads to the possible ordering of the surface and to the disappearance of the signal for the Ni x+ species in XPS, and further heating above 600 K leads to the diffusion of oxygen atoms into the bulk and to the partial reduction of the surface nickel to its metallic state.