INFLUENCE OF ARGON ION-BOMBARDMENT ON THE OXIDATION OF NICKEL SURFACES

The effect of argon ion bombardment on the oxidation of nickel films w as studied using X-ray photoelectron spectroscopy (XPS). In the absenc e of any ion beams, exposure of nickel surfaces to oxygen leads to the moderately rapid formation of a thin (3-5 monolayers thick) nickel ox ide overlayer. At room temperature the oxygen uptake stops once this l imit is reached, but at higher temperatures the slow growth of a thick er oxide is seen. The NiO growth kinetics can be phenomenologically de scribed 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 t he surface during oxygen exposures was found to enhance the oxidation process, and ion beam current densities as low as 0.01 mu A cm(-2) wer e found to be sufficient to induce nicker oxidation past the 3-5 ML li mit at room temperature. The oxidation rate was found to be roughly pr oportional 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 him induced by l ocal 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 hi gher temperatures, presumably because of the combined effect of a high er probability for desorption of molecular oxygen from the surface and a higher atomic-oxygen mobility into the bulk. The oxide films prepar ed at low temperatures appear to be quite disordered, and display an e xtra feature in the Ni 2p XPS spectra around 853.2 eV which could be a ssigned to partially reduced nickel, Ni-x+, x<2. Annealing of those fi lms 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 i n XPS, and further heating above 600 K leads to the diffusion of oxyge n atoms into the bulk and to the partial reduction of the surface nick el to its metallic state.