Effects of sputtering process on the thermochromic function of vanadium dioxide thin films

Vanadium oxide is known to be semiconductive and thermochromic with a very selective amount of oxygen in order to form the required monoclinic crystal structure (nonconductive, M phase) at room temperature and transform to a tetragonal phase (conductive, R phase) above 68 °C. In this study, vanadium oxide thin films are deposited by sputtering under different oxygen flow rates to include various amounts of oxygen, which is then followed by rapid thermal annealing to become crystalline with a variety of properties and functions. The properties and functions of annealed vanadium oxide are examined by x-ray diffraction for crystal structures, Raman spectrometer for crystal vibrational modes, four-point probe for electrical resistivity, and UV-Vis-NIR spectrometers for optical properties. All these characterizations help us to determine the range of oxygen supply under which thermochromic VO2 films can form. Results indicate that in our current setup, the lower oxygen (<0.25 SCCM) produces vanadium-rich films, the medium (0.5-1.5 SCCM) results in semiconductive films, and the higher (>2 SCCM) creates insulated oxides. Among the semiconductive films, the one deposited under 1.0 SCCM O2 supply after annealed becomes thermochromic as confirmed by the hysteresis changes of optical transmittance and electrical conductivity under thermal cycles between 25 and 95 °C.