Room temperature isotope tracing in transition metal oxides probed by LEIS

The understanding of low temperature oxygen interaction and diffusion in ultrathin oxide films is a significant challenge. In this work, we analyse the oxygen-oxide interaction through isotope exchange depth profiling (IEDP) with low energy ion scattering (LEIS). The high sensitivity of LEIS enables the correlation of isotope exchange kinetics at surfaces and in-depth penetration with high accuracy. This method was applied to films of zirconium oxide (ZrO2) and molybdenum oxide (MoO3) of 2 to 10 nm, reactively deposited via DC magnetron sputtering. After deposition, the samples were transferred in vacuum and analysed prior and after exposure to atomic 16O and 18O generated by a plasma gun (1015 atoms.cm-2.s-1) at room temperature. With the diffusion profiles obtained, diffusion modes and parameters were determined based on models stated in literature. In both stoichiometrically deposited oxides, the total penetration depth was not influenced by the layer thickness. However, isotope diffusion was shown to be strongly influenced by the presence of metallic and sub-stoichiometric oxide species. In all cases, a higher isotope diffusion constant was observed in the outermost few monolayers at the surface, with no indication of bulk diffusion through the film. These factors indicate that the oxygen penetration is driven by the formation of a space-charge layer in the oxide surface, a consequence of band bending of the oxide induced by adsorbed atomic oxygen species.