Lowering insulator-to-metal transition temperature of vanadium dioxide thin films via co-sputtering, furnace oxidation, and thermal annealing

Abstract

Thermochromic vanadium dioxide thin films have attracted much attention recently for constructing variable-emittance coatings upon their insulator-metal phase transition for dynamic thermal control. However, fabrication of high-quality vanadium dioxide thin films in a cost-effective way is still a challenge. In addition, the phase transition temperature of vanadium dioxide is around 68 °C, which is higher than most of terrestrial and extraterrestrial applications. In this study, we report the fabrication and characterization of tungsten-doped vanadium dioxide thin films with lowered phase transition temperatures via co-sputtering, furnace oxidation, and thermal annealing processes for wider application needs. Doping is achieved by co-sputtering of tungsten and vanadium targets while the doping level is varied by carefully controlling the sputtering power for tungsten. Doped thin film samples of 30 nm thick with different tungsten atomic concentrations are prepared by co-sputtering onto undoped silicon wafers. Optimal oxidation time of 4 h is determined to reach full oxidation in an oxygen-rich furnace environment at 300 °C. A systematic thermal annealing study is carried out to find the optimal annealing temperature and time. By using an optical cryostat coupled to an infrared spectrometer, the temperature-dependent infrared transmittance of fully annealed tungsten-doped vanadium dioxide thin films is measured in a wide temperature range from −60 to 100 °C. The phase transition temperature is found to decrease at 24.5 °C per at. % of tungsten doping, and the thermal hysteresis between heating and cooling shrinks at 5.5 °C per at. % from the fabricated vanadium dioxide thin films with tungsten doping up to 4.1 at. %.