Low thermal budget annealing for thermochromic VO2 thin films prepared by high power impulse magnetron sputtering

Thermochomic VO2 thin films are fabricated by using high power impulse magnetron sputtering at room temperature. In order to increase the crystallinity and transparency of VO2 thin films, first a buffer layer of TiO2 is deposited on the glass substrate. Then TiO2/VO2 stacks are post-annealed at a temperature of 500 degrees C for 3 min. The transmittance as functions of film thickness and O-2/Ar ratio during film deposition is discussed. It is found that the O/V atomic ratio decreases with increasing O-2/Ar ratio, which is due to serious inter-diffusion inside stacked layers. At high O-2/Ar ratio, columnar-shaped crystals are suggested, which belongs to V2Ti3O9:Si quaternary oxide. The depth profiles of binding energies measured by X-ray photoelectron spectroscopy are compared with different O-2/Ar ratios. Good endurance property for a thermal stress cycle of 25 degrees C/85 degrees C is obtained under the O-2/Ar ratio of 6%. The laminated film shows satisfactory optical properties with an excellent solar regulation efficiency (Delta T-sol = 10.4%) and an applicable luminous transmittance (T-lum, = 35.2%) in a low-temperature state.