Fabrication And Mechanism Of Metal-Insulator Transition Of VO₂ Epitaxial Thin Films

Vanadium dioxide (VO2) is an advanced functional material with metal-insulator phase transition. It transforms from the low-temperature insulator to high-temperature metal at ～68℃, accompanying with abrupt changes of electrical, optical and magnetic properties. Furthermore, the metal-insulator transition (MIT) can be tuned by temperature, electric field, light, ionic liquid, strain and so on, which leads it to wide potential applications. Especially, the VO2 in thin-film form is an important candidate for developing novel optoelectronic devices. In recent years, although there are many investigations on the VO2 thin films, lots of questions are still not fully understood, such as the different mechanisms of phase transition in bulk VO2 and thin film VO2, the modulating mechanism of MIT by ionic liquid or strain, and the applications in the THz range. To address the above questions, we fabricated a seires of VO2 films on Al2O3 and TiO2 substrates, and then studied the modulation of MIT by organic liquids and strain, and the mechanism of MIT in the VO2 thin films. The contents and results of this dissertation are as follows.In chapter one, we introduced the research progress on the MIT of the VO2 thin films and pointed out the unresolved questions. Firstly, we introduced the mechanisms of MIT briefly. Then we reviewed the research progress on crystal structures, properties and mechanism of phase transition and potential applications of VO2 thin films. Thirdly, we introduced the THz technology and the advantages of the VO2-based thin film prototype devices at THz region. Lastly, the contents of this dissertation were proposed.In chapter two, we introduced the important experimental techniques utilized in our studies, including the fabrication techniques of the VO2 thin films, high resolution X-ray diffraction, electrical and optical properties characterizations of VO2 and so on. All these techniques provide important tools for this work.In chapter three, we set up home-made and temperature-controlled THz time domain spectroscopy (THz-TDS) system. We introduced the mechanism of THz emission and detection in section one. Then we introduced the procedures and most important aspects of developing THz-TDS system. At last, we reported some results on optical properties at THz for the VO2 thin films with this THz-TDS system.In chapter four, we studied the resistance switching of epitaxial VO2/Al2O3 heterostructure at room temperature induced by organic liquids. Due to the potential applications of VO2-based chemical sensors, we selected cyclohexane, n-butanol and ethylene glycol to modulate the electronical properties of VO2 films. The modulation depth of the cyclohexane, n-butanol and ethylene glycol are 31%,3.8% and -7.7%, respectively, which is not affected by voltage and environmental humidity. Moreover, the resistance will restore its initial state when organic liquids are wiped off. From the results of the X-ray photoelectron spectroscopy (XPS) studies, we ascribed the reason to their different redox capability of these organic liquids. The organic liquids with different redox capability change the electronic structure of VO2 thin films amd thus modulate the resistance.In chapter five, we studied the strain effects on VO2/Al2O3 epitaxial thin films. We fabricated a series of VO2 films with different thickness and studied the relationship between transition temperature and strain. We found that the larger of the strain, the lower of the transition temperature, which indicates that strain is an effective way to modulate the MIT of VO2 thin films. However, an anomalous strain behavior appeared in our films, which is due to the different surface growth modes. Lastly, with temperature-dependent in situ XRD and THz-TDS, we studied the relationship between MIT and electronic transition and structural phase transitions in the VO2 thin films. We also observed a transmission modulation at THz via experiment measurement and theoretical calculation.Based on the above results that the thickness affects the MIT of the VO2 thin films, we fabricated 300 nm and 14 nm VO2 films on the small mismatch TiO2 substrates by RF magnetron sputtering techniques and studied the mechanism of MIT. With Synchrotron Radition XRD, we revealed the particular transition route-from "tetragonal-like" to "rutile" in 300 nm VO2/TiO2 thick films, which suggestes that the structural phase transition should play an important role in the MIT. On the other hand, we found that there is no structural phase transition across the MIT in the ultra-thin (-14 nm) VO2 thin film with the help of temparature-dependent XRD, Raman spectroscopy and UPS. All these results indicate that the epitaxial strain may suppress the structural phase transition during MIT. Therefore, the electronic phase transition is the only reason to induce the MIT for ultra-thin VO2/TiO2 epitaxial thin films.