Electronic excitation induced controlled modifications of semiconductor-to-metal transition in epitaxial VO2 thin films

We report controlled modifications in the semiconductor-to-metal transition characteristics of VO2 single-crystal thin films induced by swift heavy ion (SHI) irradiation with varying ion fluences. At very high energies of ions (200 MeV Au), the electronic stopping (similar to 2009 eV/angstrom) dominates over nuclear stopping (similar to 16 eV/angstrom). Under these extreme electronic excitation conditions caused by electronic stopping and the passage of SHIs through the entire thickness of the film, creation of certain unique type of defects and disordered regions occurs. X-ray diffraction, Raman spectroscopy, infrared transmission spectroscopy, x-ray photoelectron spectroscopy (XPS), and electrical measurements were performed to investigate the characteristics and role of these defects on structural, optical, and electrical properties of VO2 thin films. XPS and electrical resistivity measurements suggest that the ion irradiation induces localized defect states that appear to correlate well with the creation of disordered regions in the VO2 thin films. The high-energy heavy-ion irradiation changes the transition characteristics drastically from a first-order to a second-order transition (electronic-Mott type). The low-temperature conductance data for these ion-irradiated films fit well with the quasiamorphous model for resistivity of VO2, where ion irradiation is believed to create mid-bandgap defect states.