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

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, ΔT_IR, (transmittance difference in the infrared, at λ = 2000 nm, when heated from 25 °C to 90 °C) by 20.5% and solar transmittance modulation ΔT_sol = 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 Τ_Ium = 46.6% and solar transmittance modulation ΔΤ_sol = 2.8% and ΔΤ_IR = 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. - Highlights: • 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.