Abstract
The diffusion of oxygen in thin films of ZrO2 and MoO3 was investigated with atomic 18O as a tracer using low energy ion scattering sputter depth profiling. 3 nm amorphous and 20 nm polycrystalline films were prepared by reactive magnetron sputtering and exposed to atomic oxygen species at room temperature. Exposure results in a fast diffusion of oxygen to a limited depth of ~1 nm and ~2.5 nm for ZrO2 and MoO3, respectively, and surface exchange limited to a maximum of 65% to 75%. The influence of the crystalline structure of the films on exchange and diffusion was negligible. We propose that the transport of oxygen in oxides at room temperature is dominated by a field-induced drift, generated by the chemisorption of reactive oxygen species. The maximum penetration of oxygen is limited by the oxide space charge region, determined by the oxide electrical properties. We applied a drift–diffusion model to extract values of surface potential and kinetic parameters of oxygen exchange and diffusion. The developed experimental analysis and modelling suggest that the electric field and consequent distribution of charged species are the main factors governing exchange rates and species diffusion in an oxide thin film at room temperature.
Highlights
With the advance in nanometer-thick film synthesis, the application of transition metal oxides has exponentially grown
Since the detection of crystallinity in few nm thick films by X-ray diffraction (XRD) may be difficult due to the small volume of material, the 3 nm ZrO2 film was investigated by transmission electron microscopy, which confirmed the amorphous nature of this oxide film
Our study demonstrates the room temperature oxygen exchange and diffusion in amorphous and polycrystalline films of zirconium (ZrO2) and molybdenum (MoO3)
Summary
With the advance in nanometer-thick film synthesis, the application of transition metal oxides has exponentially grown As thin films, they are employed in a wide range of functions, from electrodes in solid oxide fuel cells [1] and charge injection layers in organic electronics [2], to protective layers against corrosion and for increase of components lifetime [3]. In case of exposure to atomic or radical species, the absence of the molecular dissociation step might enable the interaction to proceed to the lattice transport phase This transport might affect the system in which an oxide is employed, for example by inducing changes in the oxide properties or by affecting layers that are supposed to be protected by the oxide (especially relevant when oxides are employed as diffusion barrier materials). A better understanding of the nature of the interaction between atomic oxygen and oxide at low temperatures is of immediate importance for relevant technological applications of oxides
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