Epitaxial Growth of V_2O_3 Thin Films on c-Plane Al_2O_3 in Reactive Sputtering and Its Transformation to VO_2 Films by Post Annealing

摘要

Epitaxial growth of thin vanadium sesquioxide (V_2O_3) films on c-plane sapphire (c-Al_2O_3) substrates was achieved with reactive magnetron sputtering under restricted oxygen flow. Even with a film thickness of approximately 12nm, highly c-axis textured growth of corundum V_2O_3 was realized because of the smaller mismatch of V_2O_3 against corundum Al_2O_3. Post annealing in O_2 atmosphere for as-grown V_2O_3 films caused phase transformation to oxidized crystalline phases. At a moderate annealing temperature of 450℃, the V_2O_3 thin films transformed to VO_2 films, which show a resistivity change of over three orders of magnitude. The X-ray photoelectron spectroscopy spectra for the annealed VO_2 film showed a single charge state of V~(4+), indicating a homogeneous crystalline structure, in contrast to the inhomogeneous feature with mixed charge states of V in addition to V~(3+) for as-grown V_2O_3 film. This method is promising to prepare thin VO_2 films with metai-insulator transition in productive reactive sputtering and to examine crystalline phase transformation mechanisms, including phase coexistence.