Study of low temperature rf-sputtered Mg-doped vanadium dioxide thermochromic films deposited on low-emissivity substrates

•Thermochromic VO2 was deposited by sputtering at low substrate temperature and low deposition rate.•The thermochromic properties of VO2 were studied by doping with Mg.•Temperature dependence transmittance revealed optimum Mg doping.•Mg doping increased luminous transmittance and reduced critical temperature.•Excess Mg destroyed thermochromic properties.

Undoped VO2 was fabricated by RF sputtering from metallic target on low emissivity glass substrates at a record low growth temperature. The structure of the films was examined by X-Ray Diffraction whereas temperature dependence transmittance was employed for monitoring the film's thermochromic properties. Thermochromic VO2 could be achieved at the substrate temperature of 300 °C which is the lowest growth temperature of VO2 phase by sputtering reported in literature without any additional post-deposition annealing treatment of the films. The thermochromic properties obtained for undoped VO2 were: luminous transmittance of 36.2%, metal-to-insulator transition temperature at 55.7 °C and infrared transmittance modulation, ΔTIR, (transmittance difference in the infrared, at λ = 2000 nm, when heated from 25 °C to 90 °C) by 20.5% and solar transmittance modulation ΔTsol = 5.2%. Subsequently, magnesium (Mg) was introduced in the structure of thermochromic VO2 up to 2.4 at.%, and the Mg-induced changes in thermochromisity were examined. By increasing the amount of Mg in VO2 structure the visible transmittance increased up to 70%. Improvement in thermochromic characteristics was observed for atomic percentage of Mg up to 0.3% above which the films tend to be non-thermochromic. The optimum atomic percentage of Mg (0.3 at.%) leads to thermochromic VO2 with the characteristic lowest phase transition temperature at 49.2 °C, luminous transmittance ΤIum = 46.6% and solar transmittance modulation ΔΤsol = 2.8% and ΔΤIR = 13%. Overall, 70 nm thick thermochromic VO2 films deposited at the low substrate temperature of 300 °C and low deposition rate 0.75 nm/min when doped with Mg up to 0.3 at.% resulted in vanadium dioxide with improved thermochromic properties is regarded as particularly promising for smart windows applications on flexible substrates.