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

Abstract

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=2000nm, 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 2.4at.%, 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 (0.3at.%) 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, 70nm thick thermochromic VO2 films deposited at the low substrate temperature of 300°C and low deposition rate 0.75nm/min when doped with Mg up to 0.3at.% resulted in vanadium dioxide with improved thermochromic properties is regarded as particularly promising for smart windows applications on flexible substrates.