Large modification in insulator-metal transition of VO₂ films grown on Al₂O₃ (001) by high energy ion irradiation in biased reactive sputtering

High energy ion irradiation in biased reactive sputtering enabled significant modification of insulator-metal transition (IMT) properties of VO₂ films grown on Al₂O₃ (001). Even at a high biasing voltage with mean ion energy of around 325 eV induced by the rf substrate biasing power of 40 W, VO₂ film revealed low IMT temperature (T_IMT) at 309 K (36 °C) together with nearly two orders magnitude of resistance change. Raman measurements from −193 °C evidenced that the monoclinic VO₂ lattice begins to transform to rutile-tetragonal lattice near room temperature. Raman spectra showed the in-plane compressive stress in biased VO₂ films, which results in shortening of V–V distance along a-axis of monoclinic structure, a_M-axis (c_R-axis) and thus lowering the T_IMT. In respect to that matter, significant effects in shortening the in-plane axis were observed through transmission electron microscopy observations. V2p_3/2 spectra from XPS measurements suggested that high energy ion irradiation also induced oxygen vacancies and resulted for an early transition onset and rather broader transition properties. Earlier band gap closing against the temperature in VO₂ film with higher biasing power was also probed by ultraviolet photoelectron spectroscopy. Present results with significant modification of IMT behavior of films deposited at high-energy ion irradiation with T_IMT near the room temperature could be a newly and effective approach to both exploring mechanisms of IMT and further applications of this material, due to the fixed deposition conditions and rather thicker VO₂ films.