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 VO 2 , VO 2 /TiO 2 , and TiO 2 /VO 2 / TiO 2 thin films deposited on a conventional glass substrate via magnetron sputtering. To obtain a crystallized structure, the asdeposited 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 TiO 2 as a buffer and the TiO 2 /VO 2 /TiO 2 sandwich structure contributed to the improvement in VO 2 crystallinity. In particular, the (011) diffraction peak parameters of VO 2 , such as crystallite size, increased when the d-spacing and microstrain of the films decreased. The atomic fraction of the VO 2 phase in the TiO 2 /VO 2 /TiO 2 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 (H LW ) from 21 to 10 °C at a transition temperature of 65 °C in relation to those of pure VO 2 and bilayer VO 2 /TiO 2 films. Upon increasing the annealing temperature to 550 °C, the bilayer film showed the highest temperature-dependent infrared transmittance variation (ΔT IR ) of ∼37% at a wavelength of 2000 nm. In addition, the TiO 2 /VO 2 /TiO 2 sample showed the lowest H LW (3 °C) with a ΔT IR of ∼30%. The direct film fabrication on conventional glass substrates, relatively low H LW , 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.