Combined strain and composition-induced effects in the metal-insulator transition of epitaxial VO films

The role of epitaxial strain, thermal strain, and bulk (strain-free) lattice parameters in the metal-insulator transition (MIT) and the structural phase transition (SPT) of VO is investigated for the case of epitaxial films grown on (001)-oriented TiO substrates. Temperature-resolved X-ray reciprocal space mapping has been used to determine the absolute state of strain as well as the bulk lattice parameters of VO at 100 °C. For the thinnest film (15 nm), the state of strain is dominated by the film/substrate lattice mismatch yielding an in-plane tensile strain which, in turn, shifts both the MIT and the SPT towards lower temperatures. Conversely, for the thickest film (100 nm), the epitaxial strain is relaxed, so that the state of strain is dominated by the VO/TiO thermal expansion mismatch which is responsible for a compressive in-plane strain. In all cases, a swelling of the strain-free VO unit-cell is observed which indicates the presence of interfacial oxygen vacancies and/or Ti diffusion into the VO films. The presence of oxygen vacancies stabilizes the metallic rutile phase and counterbalances the action of thermal strain on the MIT and the SPT and degrades the electric properties for the thinnest film. For the thickest film, the resistivity ratio is 6.4 × 10.