Characteristics of electrically-induced phase transition in tungsten-doped vanadium dioxide film

The phase transition characteristics of tungsten-doped vanadium dioxide film driven by an applied voltage are studied in the paper. A high-quality film is successfully deposited on an FTO (F:SnO2) transparent conductive glass substrate by direct current magnetron sputtering and post-anneal processing. First of all, an FTO substrate is placed in the chamber of magnetron sputtering system after being cleaned and dried. Then W-doped vanadium film is fabricated on the substrate with V-W alloy target with 1.4% W by mass fraction. In the process of magnetron sputtering, the operating pressure is kept at 3.0 x 10(-1) Pa, and the operating voltage and current are 400 V and 2 A, respectively. Finally, W-doped VO2 film with a thickness of about 310 nm is prepared by being annealed at 400 degrees C in air atmosphere for 2.5 h. In order to explore the crystal structure, element constituents, surface morphology, roughness and photoelectric properties of W-doped vanadium dioxide film, it is respectively characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM) and semiconductor parameter analyzer. The XPS analysis confirms that there are no other elements except vanadium, oxygen, carbon and tungsten on the surface of W-doped VO2 film. The XRD patterns illustrate that tungsten-doping exerts an influence on the crystal lattice of VO2, but the film still prefers the orientation (110). The SEM and AFM images display that the film with low roughness has a compact structure and irregular crystal particles. Tungsten-doping is found to be able to improve the surface morphology of VO2 thin film significantly. In addition, a remarkable change in electrical resistivity and a narrow thermal hysteresis loop are also obtained in the metal-semiconductor phase transition. Further, the influences of tungsten-doping on the phase transition properties of the film are analyzed. The experiment demonstrates that the threshold voltage at which the phase transition of W-doped VO2 film occurs is 4.2 V at room temperature when the film is driven by an applied voltage ranging from 0 V to 8 V. It can be observed clearly that the current changes abruptly by two orders of magnitude. As the ambient temperature rises, the threshold voltage of phase transition drops and the current varies slightly. The optical transmittance curves show the distinct differences under applied voltage at different temperatures. It is found that the infrared transmittance difference reaches up to a maximal value of 27% at 50 degrees C during phase transition, while it increases by only 23% at 20 degrees C in a wavelength range of 1100-1500 nm. All these outstanding features indicate that W-doped VO2 film has excellent properties of electrically-induced phase transition. Compared with undoped-VO2 film, the W-doped VO2 film is predicated to have a wide range of applications in the high-speed optoelectronic devices due to its advantages of lower phase transition temperature, resistivity and threshold voltage.