OXIDE FORMATION ON FE AND TI THIN-FILMS AND ON FE THIN-FILMS MODIFIED WITH ULTRATHIN LAYERS OF TI

Reactions of O2 and H2O to form thin oxides on Fe, Ti and Ti-modified Fe thin films have been studied by XPS, following O2 and H2O exposures in the range 0-600 L, to form oxides with thicknesses of <40 angstrom. XPS lineshape analysis is used, utilizing a special combination of reflection electron energy-loss spectroscopy (REELS) and nonlinear least-squares fitting routines to model the intrinsic and extrinsic energy losses that accompany photoemission for the Ti and Fe 2p lines. During the formation of the thinnest oxides, this approach yields a unique picture of the composition of the oxide, while for the thicker layers, there is little significant difference between this fitting approach and (1) fitting approaches using an integral background approach or (2) direct deconvolution methods using the REELS data. Both Fe and Ti are quite reactive to O2, yielding a surface oxide that is apparently an FeO-dominated FeO/Fe3O4 bilayer on Fe surfaces, and a surface oxide that is predominantly TiO2 on the Ti surfaces. Pure Fe and Ti surfaces are unreactive to H2O. Predosing of these surfaces with low levels of O2 (5 L) does not increase the reactivity appreciably toward H2O. Ti dispersed on the Fe surface as an adatom layer, with an equivalent thickness of 3 angstrom, greatly suppresses the reactivity of Fe toward O2, while the Ti is oxidized primarily to TiO2. In contrast to the pure Ti layers, these Ti adatom layers are quite reactive toward H2O, yielding oxides (no evidence for hydroxide) in a uniform distribution of oxidation states (Ti+2, Ti+3 and Ti+4).