| Vanadium oxide(VOx) films can be used in a wide variety of applications such as photoelecric switching devices, storage devices, terahertz modulators and smart windows, due to the excellent thermally driven insulator-metal phase transition(MIT) near room temperature. However, different practical applications require different MIT properties, especially the hysteresis width. For example, for switching-type applications, a smaller hysteresis width is preferred in order to acquire larger response, higher reliability and efficiency. For storage-type applications, however, a larger hysteresis width is needed. Previous studies revealed that the MIT properties of VOx films are related to the corresponding microstructures of the deposited films and the process conditions. Thus, the preparation as well as the microstructure and MIT properties of VOx films has become one of the hot topics in functional materials in recent years. In this dissertation,nano-VOx films with phase transition characteristics were prepared by means of reactive direct current magneton sputtering technique. The effects of key experimental conditions, substrate types and doping on the the chemical compositons, structures, morphologies and MIT properties of VOx films were systematically investigated with the help of various analytic instruments. In addition, the MIT of VOx films prepared on conducting glass substrates can be triggered not only by heating plate but also by Joule heat produced by electrical current through the conductive sublayer. Interestingly, the experimental results show that the MIT properties triggered by Joule heat are different from those triggered by heating plate. The main contents include four parts as follows.1. The methods for controlling the morphology and hysteresis width of nano-VOx films prepared by magnetron sputtering were achieved. The influences of key experimental conditions on the chemical compositons, structures, morphologies and MIT properties of VOx films were systematically investigated and conclusions were drawn as follows:(1)the process conditions(reactive oxygen flow rates, substrate temperature and annealing temperature) profoundly affect the surface morphologies(grain shapes, grain sizes and grain boundaries) and MIT properties(transition temperature, transition amplitude and hysteresis width);(2)reactive oxygen flow rates, substrate temperature and annealing temperature modulate the surface morphologies which affect the hysteresis width of VOx films. In other words, the hysteresis widths of VOx films mainly depend on the surface morphologies of the deposited films. Better symmetry of grain shape, smaller grain size and/or more obvious grain boundaries will result in larger hysteresis width. Otherwise, the hysteresis width is smaller. In addition, Zr doped VOx(VOx:Zr) films were prepared in this dissertation. The experimental results indicate that in the process of film growth Zr was doped into VOx films and acted as nucleating centers, which can also effectively modulate the surface morphologies and hysteresis width of VOx films. However, in the case of VOx:Zr films, the hysteresis widths depend on the additional nucleating-defect density introduced by doping rather than the surface morphology of the deposited films.2. The effects of transversal grain size on the hysteresis width of nano-VOx films were intensively investigated. A series of VOx films with similar surface morphologies comprising spheroidal nanoparticles were obtained by changing the sputtering time or in-situ annealing time, and their MIT properties were investigated. The outstanding contributions of this work are:(1)the methods for controlling the grain size but not changing the spheroidal grain shapes were obtained, which enables us to effectively investigate the grain size dependence of the hysteresis loops and thereby develop a deeper understanding of the materials physics associated with the phase transition;(2)the hysteresis width and the transversal grain size of VOx films are closely related, namely the larger the mean transversal grain size, the smaller the hysteresis width;(3)more importantly, the experimental results show that the hysteresis width is inversely proportional to the mean transversal grain size of VOx films with smooth and compact surface. The observed grain size dependence of hysteresis width was explained quantitatively using the theoretical calculation based on the previous model. Such a direct and clear-cut relationship between the transversal grain size and the hysteresis width of VOx films was established for the first time.3. Properties of VOx films in terahertz(THz) range were studied. Firstly, the impact of film thickness on the chemical compostions, structures, morphologies and MIT properties at THz range was investigated. The experimental results show that the THz modulation ability increases with increase in film thickness in a certain range. Secondly, Si doped VOx(VOx:Si) films were prepared and the experimental results show that Si doping profoundly affects the preferred crystallographic orientation, surface morphology and the MIT properties at THz range. Interestingly, the Si doping in a proper range can hugely reduce the hysteresis width without compromising the THz modulation efficiency, which indictates that VOx:Si films are promising potential applications in THz switching and modulators.4. Preparation and MIT properties of nano-VOx films on transparent conducting glass substrates were investigated. The main work in this part includes two parts as follows:(1)VOx films were prepared on both indium-tin oxide(ITO) glass substrate and Al doped zinc oxide(AZO) glass substrate, and were thermally-triggered MIT by two heating methods, namely heating plate and Joule heat. Interestingly, the experimental results show that the MIT properties triggered by Joule heat are different from those triggered by heating plate, suggesting a new method for controlling MIT properties of VOx films;(2)VOx films were prepared on AZO conducting glass substrates, and the effects of sputtering time, reactive oxygen flow rates and annealing oxygen flow rates on the transmittance and MIT properties of the deposited films were investigated. Moreover, comparative studies of VOx films deposited on AZO conducting glass and glass substrates were performed. Interestingly, compared with the sample VOx/glass, the sample VOx/AZO/glass processed at the same experimental conditions not only shows a higher visible light transmittance at room temperature but also shows a larger transmission modulation ratio in near infrared light range across the MIT. This work experimently indicates that VOx/AZO/glass is more suitable for applications in smart windows. |
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