Oxidation of Molybdenum surfaces by reactive oxygen plasma and O2+-ion bombardment. An Auger an XPS study

L. D. LOPEZ CARREÑO; G. BENITEZ; L. VISCIDO; J.M. HERAS; F. YUBERO; J.P. ESPINÓS; A.R. GONZÁLEZ-ELIPE

SURFACE AND INTERFACE ANALYSIS

JOHN WILEY & SONS

Año: 1998 vol. 26 p. 235 - 241

0142-2421

  The oxidation of molybdenum at room temperature with oxygen plasma or a beam of O2+ ions has been studied with Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS). Auger electron spectroscopy shows a progressive modification of the N2,3VV   lineshape and the development of new features close to the intense M4,5NN   Auger transitions of metallic Mo at 160, 186 and 221 eV. The effects are greater with the oxygen plasma than with the O2+ bombardment. X-ray photoelectron spectroscopy shows that the incorporation of oxygen into the molybdenum gives rise to the formation of mo6+, Mo4+ and a form of Mon+  (n < 4). The (Mo4+ + Mon+)/Mo6+ ratio was higher after O2+ ion bombardment than by treatment with the oxygen plasma, in which case the concentration of the Mo4+ and Mo6+ oxidation states was very similar. The analysis of the O ls and O KLL Auger spectra under the different conditions showed that whenever a high concentration of the Mo6+ 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 MoO3 . This oxide was bombarded with Ar+ and O2+ until a steady-state surface composition was reached. In both cases, the formation of a considerable amount of Mo4+ + Mon+ (n < 4) and Mo0 species (i.e. reduction of Mo6+ initially present in the sample) was detected. These results suggest that the high concentration of Mo4+ + Mon+ found upon bombardment of Mo0 with O2+ is rather produced by ballistic effects which cause the reduction of the initially formed Mo6+.