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