Highly Reduced Phase Transition Hysteresis of Vanadium Dioxide Thin Films in Multilayer Structure with Titanium Dioxide
Abstract
Herein, we present the electrical, structural, and optical characteristics of pristine VO2, VO2/TiO2, and TiO2/VO2/TiO2 thin films deposited on a conventional glass substrate via magnetron sputtering. To obtain a crystallized structure, the as-deposited films were annealed in a tube furnace at 450 and 550 °C in an oxygen atmosphere at 20–25 mTorr for 90 min. The prepared films were characterized by four-point probe resistivity, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet–visible–near-infrared spectrophotometry, and field-emission transmission electron microscopy. The microstructural analyses revealed that using TiO2 as a buffer and the TiO2/VO2/TiO2 sandwich structure contributed to the improvement in VO2 crystallinity. In particular, the (011) diffraction peak parameters of VO2, such as crystallite size, increased when the d-spacing and microstrain of the films decreased. The atomic fraction of the VO2 phase in the TiO2/VO2/TiO2 sample increased from 11 to 19 at. % after annealing at 450 °C. In addition, the multilayer film exhibited relatively increased optical transmittance near the infrared region and showed a reduction in the hysteresis loop width (HLW) from 21 to 10 °C at a transition temperature of 65 °C in relation to those of pure VO2 and bilayer VO2/TiO2 films. Upon increasing the annealing temperature to 550 °C, the bilayer film showed the highest temperature-dependent infrared transmittance variation (ΔTIR) of ∼37% at a wavelength of 2000 nm. In addition, the TiO2/VO2/TiO2 sample showed the lowest HLW (3 °C) with a ΔTIR of ∼30%. The direct film fabrication on conventional glass substrates, relatively low HLW, and increase in optical transmittance in the near-infrared region can contribute to the production of cost-effective, fine-tuned, energy-saving smart windows and infrared switches.