THE INITIAL INTERACTIONS OF OXYGEN WITH POLYCRYSTALLINE TITANIUM SURFACES

The interactions of gaseous oxygen and different types of polycrystall ine titanium surfaces were studied at room temperature within the expo sure range of 0-1000 L. Combined measurements utilizing direct recoils spectrometry (DRS), Auger electron spectroscopy (AES), X-ray photoele ctron spectroscopy (XPS), and work function variations enabled the dis tinction between processes occurring on the topmost atomic layer and t hose associated with subsurface incorporation of oxygen. Also, the dif ferent chemical forms (oxidation states) developing during the exposur e course were identified. The results were compared for three types of surfaces, each prepared by a different cleaning procedure. It has bee n concluded that: (i) Oxygen initially accumulates on the topmost atom ic layer, regardless of the type of the studied surface. No preferred subsurface occupation has been observed. (ii) The kinetics of initial accumulation (up to a complete surface coverage) are similar for all t he different types of surfaces. (iii) Mixtures of different oxidation states of titanium (0, +2, +3, +4) are present during the whole course of exposure. Qualitatively, increasing proportions of the higher vale nce states are displayed for higher oxygen exposures. However, the qua ntitative estimates of their relative amounts indicate a strong depend ence on the type of surface, with preferred high oxidation (+4) states obtained for high temperature annealed samples (as compared with room temperature sputtered surfaces). (iv) Topmost oxygen atoms which term inate the oxides surfaces are less negatively charged than the underly ing (i.e., subsurface) ''oxidic'' atoms. These results may account for some of the controversies presented in the literature.