| VO2 crystals have attracted many researchers’attention due to its excellent properties of electricity and optics. When the temperature rises from room temperature to around 68℃(Tt), a metal-insulator transition (MIT) will happen in VO2 crystals. The reduce of resistance during the transition can be 5 magnitudes. Besides, the transmittance and absorptivity of optics also have sharp changes. Generally, almost at the same temperature range, a structural phase transition (SPT) happens along with the MIT in VO2 crystals. The monoclinic VO2 (phase M1) at the temperature below the transition point (Tt) will transfer to rutile (phase R). However, the question about whether the SPT is the necessity to induce MIT or not in VO2 is still under debate.VO2 crystals, especially VO2 films have a wide application prospect, including smart windows, storage devices, optical switch devices, infrared detectors and so on. Recently, by the step of the deeper investigation on VO2 epitaxial films, interfacial strain engineering has been recognized as an effective approach to tune the MIT of epitaxial VO2 films. However, the strain-involved structural evolution during the MIT process is still not clear, which prevents comprehensively understanding and utilizing interfacial strain engineering in VO2 films. In this work, we have systematically studied the epitaxial VO2 film grown on TiO2 (001) single crystal substrate and the structural transition at the boundary of MIT region (from room temperature to 100 degrees Celsius).This dissertation is divided into four parts:the first chapter is the introduction of the relative background knowledge; the second chapter narrates the principle, devices and methods of sample preparation and characterization; the third chapter presents the data of experiments and the analysis of results; the forth chapter gives a brief conclusion and puts forward some further investigations.In chapter one, the background knowledge of VO2 crystals was introduced. This chapter also contained the application fields of VO2 and the research progress of VO2 films. First, the properties of several types of vanadium oxide were displayed; then we described the attractive MIT and SPT properties of VO2 and the research about the mechanism of MIT; at last the methods to prepare VO2 films were introduced as well as the properties of structure and the control of epitaxial strain/stress in VO2 films.In chapter two, the details of depositing VO2 epitaxial film on TiO2 (001) crystal using magnetron sputtering were described. The choices of some important factors when sputtering were discussed therein. Besides, we introduced the methods to characterize the electrical and structural properties of VO2 epitaxial film in this chapter, including the theory and devices. Home-made device system was utilized to character the electrical properties of VO2 and in situ high solution x-ray diffraction technique was used to investigate the structural properties of VO2.In chapter three, the data of the experiments were displayed and analyzed. In the figures of resistance-temperature curves and the θ-2θ scanning curves, a structural phase transition along with MIT was found and the two phases were coexistent with competing and squeezing at the transition range. Moreover, the result of RSM experiment at room temperature indicates that the phase of VO2/TiO2 epitaxial film at low temperature is tetragonal but not the extend rutile as previous thoughts or other phases found in VO2 crystals. That means the structural phase transition of VO2/TiO2 epitaxial film is from tetragonal phase to rutile phase when heating. This result implies that the transition of monoclinic phase to rutile phase is not the necessity to trigger MIT. In addition, we answered the question about the evolution mechanism of the tetragonal and rutile coexistence when the structural phase transition happens in this chapter. These discoveries not only provide some clues on the MIT mechanism study of VO2 from the structural point of view, but also favor the interface engineering assisted VO2-based devices and applications in the future.In chapter four, the sample preparation and characterization experiments as well as the results and conclusions were summarized. Otherwise, several questions about the details of tetragonal phase which need further investigation were proposed in this chapter. |
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