Effects Of Rare Earth(Gd、Y)-doping On Structures And Properties Of Vanadium Oxide Thin Films

Vanadium oxides have been attracted lots of attention as typical semiconductor materials having the metal-insulator phase transition(MIT). Vanadium dioxide(VO2) can be used in a variety of applications such as smart windows, optical switches, optical storage, due to the outstanding change of optical and electrical properties during the MIT at about 68℃, a temperature near room temperature. Different practical applications require the different MIT features of VO2, therefore the modulation of VO2 MIT features has become one of hot topics.In this dissertation, vanadium oxide thin films doped with rare earth(RE) were prepared by means of reactive direct current magnetron sputtering technique. The effects of RE-doping, substrate temperature on the MIT properties, compositions, morphologies and structures of vanadium oxide thin films were systematically investigated by means of a four point-point probe instrument, X-ray diffraction, Raman spectroscope, Scanning electron microscope, X-ray electron energy spectrum etc. The main results are summarized as follows:(1) The influence of Gd-doping on the phase transition characteristics and microstructures of vanadium oxide thin films was investigated. The experimental results show that Gd-doping reduced the MIT temperature and hysteresis width of the thin films, the phase temperature and hysteresis width of VGd-0 is 65.4℃ and 13.1℃, while the phase characteristics of VGd-9 is 54.5℃ and 6.3℃, weakening the MIT characteristics of the VO2 films. As the concentration of Gd-doping is increased up to 4.1at%, the Gd-doped VO2 thin films show the metallic feature and the phase transition disappears entirely. Gd-doping inhibits the growth of VO2 grains. The grain sizes of thin films decrease with the increase of the Gd concentration, Gd exists in the form of Gd3+ in the Gd-doped VO2 thin films. Gd-doping does not change the crystalline structure of the VO2 thin films, and the monoclinic polycrystalline structures is still maintained. Even the Gd concentration in enhanced up to 4.1at%, there is no tetragonal phase existing in the Gd-doped thin films with metallic features. The films composed of polycrystalline monoclinic VO2 shows metal properties.(2) On the basis of the above research about the MIT characteristics of Gd-doped VO2 thin films, we further investigate the effect of the substrate temperature on the phase transition characteristics and microstructures of Gd-doped vanadium oxide thin films. The substrate temperature can significantly affect the phase transition characteristics and microstructures of Gd-doped vanadium oxide thin films. The crystalline structure of Gd-doped vanadium oxide did not change with the increase of the substrate temperature. The thin films are still composed of polycrystalline monoclinic VO2. But the inhibition of Gd-doping on the phase transition characteristics and microstructures of the thin films was significantly enhanced with the increase of the substrate temperature.(3) The effect of the substrate temperature on the phase transition characteristics and microstructures of Y-doped vanadium oxide thin films was investigated. The properties of Y-doped vanadium oxide thin films depend greatly on the substrate temperature. Two different monoclinic VO2(P21/c and C2/m) was detected in the thin films at the lower substrate temperature(50℃) and the films almost do not have the phase transition. At the suitable substrate temperature(100℃), the thin films are composed of polycrystalline monoclinic VO2(P21/c). The MIT features are obvious, and the films have a narrow hysteresis width. As the substrate temperature is increased up to 150℃, the resistivity of the films becomes much lower, and the phase transition disappears entirely. At the same time, the inhibition of Y-doping on grain size of the films also depends greatly on the substrate temperature: the higher substrate temperature, the smaller grain size, thus the rougher surface.