Study of Metal-Insulator Transition in Strongly Correlated Vanadium Dioxide Thin Films

Vanadium dioxide (VO2) has attracted considerable interest during the past six decades because of a first order phase transition that occurs at 340 K, together with the possibility of modifying the transition temperature. This transition is accompanied by abrupt changes in the electrical conductivity, optical transmittance, and reflectance in the infrared region, which can be of benefit in many applications of sensing and switching. Utilizing the reactive bias target ion beam deposition (RBTIBD) growth technique, the growth and characterization of pure-VO2 and doped-VO2 thin films on various substrates was investigated.
The electronic and structural characterizations of vanadium dioxide thin films deposited under various growth conditions, i.e. substrate temperature and O2 flow rate, were investigated. As the O2 flow rate increased, the <010> lattice parameter for monoclinic VO2 was reduced and coincidently distinctive changes in the metal-semiconductor transition (MST) and transport behaviors were observed despite the identical valence state of vanadium confirmed by X-ray absorption and photoemission spectroscopy. The effect of the oxygen partial pressure on the monoclinic structure and electronic structure of thin films, and consequently the MST is discussed.
The effect of macro-strain arising from substrate clamping on MST and transport anisotropy was investigated in epitaxial VO2 films grown on various orientations of TiO2 single crystal substrates. One remarkable result is that highly strained epitaxial thin films were rutile in the insulating state as well as in the metallic state. These highly strained films undergo an electronic phase transition without the concomitant Peierls transition. It also shows that a very large tensile strain along the c-axis of rutile VO2 resulted in a phase transition temperature of ~ 433 K, much higher than those in previous reports. The conductivity mapping of thin films also reveal the origin of transport anisotropy arising from the strain from substrate clamping. Extremely large conductivity anisotropy was discovered on VO2/TiO2 (100) thin film.
RBTIBD was used to explore the chemical doping (Al3+ and Mn4+) of VO2 thin films deposited on c-plane sapphire. Two approaches of doping technique were employed. The lower bias pulse frequency results in better crystallinity, enhancing the resistive change and the sharpness of the MST, while the high frequency doping degrades film crystallinity and suppresses the MST. Both type of dopants were found to improve the MST, whereas the MST of films doped with Al3+ is more improved than those doped with Mn4+. Doping with Mn4+ does not raise the transition temperature while doping with Al3+ slightly increases the transition temperature, as the Al concentration increases. Evidence of hole doping from Al3+ was also found in Hall effect for the first time.