Properties Of VO_x Thin Films Deposited By Reactive Magnetron Sputtering

Abstract:Vanadium oxide (VOx) thin film has attracted much attention from researchers due to its special performance occurring at phase transitions. For example, transmittance, resistivity and magnetic susceptibility of VOX may change abruptly during the phase transformation. Therefore, VOx thin film has great potential applications in many fields. However, the vanadium oxide system has many phases, it is difficult to obtain a vanadium oxide film that has only one phase.This work investigates the influence of the experimental parameters such as partial pressure of oxygen, sputtering power, deposition pressure, and bias voltage on the vanadium oxide films, which are deposited on K9optical glass substrates by reactive magnetron sputtering, and some of the samples are doped with tungsten. Profiler has been employed to measure thickness of the thin films, and the deposition rate is calculated based on the deposition time. X-ray diffraction (XRD) is adopted to characterize the crystalline structure of the VOx films. Atom force microscopy (AFM) is used to observe the morphology of the sample surface and its roughness. The optical constants such as transmittance, refractive index and extinction coefficient are measured by a spectrophotometer and an ellipsometer, respectively. The mechanical properties of the nano-hardness and elastic modulus are obtained by the Nano Indenter system. Tungsten-doped vanadium oxide films has also been investigated in this work. Changes in phase structure and light transmittance of the doped films are comparatively investigated with those undoped.The experimental results show that all the tungsten oxide films are crystalline but their phases change with deposition parameters such as oxygen partial pressure, sputtering power, and deposition pressure. As the oxyten partial pressure is either higher than0.1Pa or less than0.01Pa, there has only one weak peak of V2O5(001) on the XRD pattern. Several sharp and obvious peaks can be observed and the results indicate that the films deposited under an appropriate pressure have more than one crystalline structures. The crystalline structure of the films can be improved by increasing sputtering power.Deposition pressure also affects the crystalline structure of the films. The films are amorphous as the pressure is lower than0.5Pa, and they are crystalline while the pressure higher than0,5Pa. The morphology of the films depends on both oxygen partial pressure and sputtering power. The roughness of the surface increases to some extent and then decreases either with increasing oxygen partial pressure or with increasing sputtering power.The optical properties of the vanadium oxide films depend on the deposition parameters, too. Inletting oxygen (oxygen partial pressure greater than OPa) significantly enhances the transmittance which is more than80%at a partial pressure of0.02Pa and the VOX film has little absorptance. Increasing either the sputtering power or the working pressure makes the transmittance of the film increase. Large sputtering power supplies enhanced particle energy, which improves the film qulity by reducing the defects. It can be seen that the films deposited with a negative bias have higher transmittance than those without a bias. At a higher deposition pressure, the thickness of the films decreases and as a result, the transmittance is improved. The oxygen partial pressure has little effects on refractive index and absorptance of the films. The refractive index of the film increases with sputtering power. The absorptance of the samples is low at a sputtering power of80W,100W and120W.Hardness of the vanadium oxide film samples is low the average value of which is less than10GPa. Nano-hardness of the films changes with oxygen partial pressure, sputtering power as well as deposition pressure. At high oxygen partial pressures and sputtering powers, the films are dense and both the hardness and the elastic modulus are enhanced. While increasing working pressure results in a loosen structure and both hardness and modulus of VOx film decreases.All of the tungsten-doped vanadium oxide films are amorphous and absorptive to visible light.But transmittance of the films can be improved by enlarging oxygen-argon ratio, by increasing deposition pressure or by decreasing the sputtering power.