Oxygen stoichiometry controlled sharp insulator-metal transition in highly oriented VO2/TiO2 thin films

The insulator-metal transition (IMT) in vanadium dioxide (VO2) which occurs above room temperature (67 °C) is highly sensitive to atomic defects caused by oxygen stoichiometry. The strained growth and the degree of oxygen deficiency in VO2 epitaxial films result in lowering of transition temperature below room temperature as well as the broadening of transition parameters such as transition width and hysteresis width, which limit its application potential. Here we demonstrate the growth of highly oriented strain-relaxed VO2 thin films on (001)-oriented TiO2 substrates at various oxygen partial pressures, exhibiting the narrow transition and hysteresis width. The cross-sectional transmission electron microscopy and x-ray diffraction analyses of the films reveal the highly oriented growth of insulating monoclinic VO2. The IMT parameters associated with temperature-dependent phase transition vary with the oxygen partial pressure used during the deposition. The presence of multiple and mixed valence states of vanadium in the films was confirmed by Raman and XPS analyses. We have achieved a narrow transition width (2.3 °C) and hysteresis width (1.2 °C) through controlling the oxygen stoichiometry during the growth of VO2/TiO2 films.