Diffusion of oxygen in nanoscale-thin transition metal films

The work presented in this thesis aims to deepen the understanding of the interaction between oxygen and thin films of transition metals (TM) and TM oxides at low temperatures (298 K to 473 K), together with the development of knowledge related to the low energy ion scattering (LEIS) technique. The results described build on an extensive experimental analysis, performed on thin films of TM (and their oxides) from groups IV to VIII of the periodic table, with various thicknesses and crystalline structures. Regarding LEIS analysis, the work presented has contributed to the advance of LEIS characterization by both: (i) demonstrating the influence of surface composition on low energy ion neutralization, proposing a rule for selection of reference samples for the correct quantitative analysis of compound surfaces and (ii) by validating its application as a non-destructive in-depth analysis of sample composition for oxide films on metal. Regarding fundamental knowledge on oxygen-thin films interaction, key physical aspects that dictate species migration in metals and oxides at low temperatures are demonstrated. A key finding was the demonstration that at low temperatures, the surface concentration of reactive atomic species is the determining factor inducing metal oxidation and oxygen diffusion in oxides. In both cases, the adsorption of atomic oxygen induces charge transfer processes between gas and solid phases, leading to the development of a contact-potential and consequent field-induced diffusion of species in the films. The analysis developed allows for putting forward the hypothesis that these observations on field-induced diffusion can be extended to other acceptor-type adsorbents, such as nitrogen: if radical species are directly in contact with the metal or oxide layer and enough coverage is present, a field-assisted diffusion will be established. The presented results contribute to the better characterization and understanding of oxygen-thin films interaction, and might have a significant impact, not only in the design of materials and synthesis of thin films, but also for the development of processes where reactive species are put in direct contact with thin layers

The influence of oxygen on the neutralization of slow helium ions scattered from transition metals and aluminum surfaces

Low energy ion scattering (LEIS) was employed for the analysis of thin films of Mo, Ru, Hf, Al and their oxides. Measurements with different He+ energies showed that the characteristic velocities for neutralization of the transition metal atoms change when the metal binds with oxygen. However, such behavior was not observed for aluminum. We suggest that the increased neutralization in oxidized Ru, Hf and Mo originates from the presence of the O 2s band. This band is in resonance with the He 1s level, which allows for a quasi-resonant neutralization mechanism (qRN). On the other hand, a decrease of the strong Auger neutralization for metallic Al upon oxidation may compensate for the increase in neutralization by qRN, leading to similar neutralization behavior of Al in both states. We also demonstrate the dependence of characteristic velocity on oxygen content and discuss how this effect can be used to select proper reference samples for quantitative surface analysis by LEIS

Room temperature oxygen exchange and diffusion in nanometer-thick ZrO2 and MoO3 films

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