Three-phase metal-insulator transition and structural alternative for a VO2 film epitaxially grown on Al2O3(0001)

Abstract

We report on the crystallographic phases and their epitaxial relationships observed during the metal-insulator transition (MIT) of a VO 2 film deposited on Al 2O 3(0001). A key feature of this work is to establish two possible in-plane epitaxial relationships for the VO 2 film, introducing an "on-axis" and a "diagonal" model. These models have distinctive signatures in reciprocal spaces in the form of multiplet reflections and can, therefore, be easily differentiated by diffraction. They serve as a basis for interpreting the results gained by high-resolution X-ray diffraction, and complemented by Raman spectroscopy, on a 420-nm-thick VO 2 film grown by microwave plasma-assisted reactive sputtering on Al 2O 3(0001). We address its orientation and follow its structure through the MIT. The film is oriented according to the diagonal model and exhibits a MIT involving three phases with different temperature domains of (co)existence. The room- and higherature phases are, respectively, identified as the monoclinic M1 and rutile R polymorphs of VO 2. Additionally, we observe that when the sample resistivity starts to decrease, the insulating M1 phase turns into an intermediary phase, which then partially transforms into the metallic R phase when the MIT is complete. The intermediary phase is similar to the M2 polymorph of VO 2, according to its lattice parameters, Raman signature, and its known proximity with the M1 and R phases in tensile conditions. The mixing of the M2 and R phases is still detected at 110 ° C, i.e., far above the MIT; the M2 polymorph is being probably stabilized at the interface between the film and the substrate, where the interfacial strain is higher.