Oxidation of molybdenum surfaces by reactive oxygen plasma and O2+ bombardment: an auger and XPS study

The oxidation of molybdenum at room temperature with oxygen plasma or a beam of O 2 + ions has been studied with Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS). Auger electron spectroscopy shows a progressive modification of the N 2,3 VV lineshape and the development of new features close to the intense M 4,5 NN Auger transitions of metallic Mo at 160, 186 and 221 eV. The effects are greater with the oxygen plasma than with the O 2 + bombardment. X-ray photoelectron spectroscopy shows that the incorporation of oxygen into the molybdenum gives rise to the formation of Mo 6+ , Mo 4+ and a form of Mo n+ (n < 4). The (Mo 4+ + Mo n+ )/Mo 6+ ratio was higher after O 2 + ion bombardment than by treatment with the oxygen plasma, in which case the concentration of the Mo 4+ and Mo 6+ oxidation states was very similar. The analysis of the O 1s and O KLL Auger spectra under the different conditions showed that whenever a high concentration of the Mo 6+ species is present, the O-Mo bond is more covalent in character. To evaluate the influence of chemical and ballistic effects in the two oxidation processes, additional experiments were carried out with MoO 3 . This oxide was bombarded with Ar + and O 2 + until a steady-state surface composition was reached. In both cases, the formation of a considerable amount of Mo 4+ + Mo n+ (n < 4) and Mo° species (i.e. reduction of Mo 6+ initially present in the sample) was detected. These results suggest that the high concentration of Mo 4+ + Mo n+ found upon bombardment of Mo 0 with O 2 + is rather produced by ballistic effects, which cause the reduction of the initially formed Mo 6+ .