High-quality Tungsten-doped Vanadium Dioxide Thin Films Fabricated in an Extremely Low-oxygen Furnace Environment

Abstract: This work reports the fabrication and characterization of high-quality tungsten-doped vanadium dioxide (WxV1-xO2, x = 0~3 at. %) by thermal oxidation of sputtered tungsten-vanadium alloyed thin films with different atomic percentages and high-temperature annealing in an extremely low oxygen atmosphere (5 to 20 ppm) along with reduction of surface over-oxides in high vacuum (1 mPa). Oxidation parameters such as temperature, time and nitrogen purging rate are first optimized for obtaining high quality undoped VO2 thin film. Insulator-to-metal (IMT) phase transition behavior of VO2 thin films fabricated in a low-O2 environment is characterized with temperature dependent spectral infrared transmittance and electrical resistivity measurements, where there is 15% higher infrared transmittance change and additional 1 order change in resistivity in comparison with VO2 thin films fabricated in a O2-rich environment. Grazing angle X-ray diffraction scan confirms no presence of higher oxides in the VO2 oxidized in low-O2 environment, which improves its quality significantly. Comprehensive studies on thermal annealing and vacuum reduction for tungsten doped VO2 thin films are also carried out to find the optimal fabrication conditions. With the tungsten at. % measured by X-ray photoelectron spectroscopy, the optimal WVO2 thin films fabricated through this streamlined oxidation, annealing and reduction processes in extremely low-O2 furnace environment exhibit lowered IMT temperature at -23{\deg}C per at.% of tungsten dopants from 68{\deg}C without doping. This low-cost and scalable fabrication method could facilitate the wide development of tunable WVO2 coatings in thermal and energy applications.