VO₂Epitaxial Film Preparation,Study Of Growth Mechanism And Phase Transition Temperature Modulating

Vanadium dioxide (VO2) is a system that still attracts a tremendous interest because of its metal-insulator transition near the room temperature, accompanied by abrupt and large changes in optical, electrical and magnetic properties. The particular characteristics of the VO2indicate many applications in different fields such as energy-saving material, memory devices and optical switchs. Vanadium oxides contain multivalent states such as V2+, V3+, V4+, V5+, and exists in a Magneli phase VnO2n-1(3<n<9) due to ordered point defects. The oxygen environment in the film preparation process is a key parameter to compose the rich VO2phase diagram. At present, the form of VO2that applys in toelectronic devices is in film condition. As a consequence the preparation of a high quality VO2epitaxial film is very meaningful for improving and optimizing the applications. Understanding the growth behavior of a VO2epitaxial film is mandatory to synthesize a high quality film. At present the bottleneck that limits VO2application is its higher critical temperature, and a continuous control of its phase transition temperature is mandatory. Based on the above consideration, the researches performed and the results achieved are summarized as follow:(1) We systematically studied the role of the oxygen pressure in the VO2film growth. For the first time we successfully prepared a2-inch VO2epitaxial film using oxide plasma Molecular Beam Epitaxy (MBE). The role of oxygen defect in the VO2phase transition process was also analyzed. We proposed a new model to explain how oxygen defects decrease the critical temperature.(2) The in-plane lattice matching relation of a VO2/Al2O3epitaxial film was studied by j-scan XRD and the epitaxial property was explained in the domain lattice matching. We also performed high resolution synchrotron radiation XRD experiment on the VO2/Al2O3epitaxial film discovering a fine diffraction structures in j-scan XRD patterns. The growth behavior was explained according to a new interfacial model, which was validated by the results of the grazing incidence φ-scan XRD. The result of the VO2film deposition can be extended also to other six-fold rotation symmetry substrates.(3) We proceeded on the research on VO2phase transition temperature control via interfacial strain and voltage. We prepared VO3/TiO2films with different thickness and correlated it with the critical temperature. The critical temperature decreases as the film becomes thinner. The correlation with the interfacial strain was also demonstrated thanks to the Reciprocal Space mapping (RSM) method and the density of state near the Fermi surface of these different thickness films was calculated by using the VASP software. From the analysis of the interfacial strain data and theoretical calculations we discovered that the electronic orbital occupancy is strongly affected by the interfacial strain, which changes the electron-electron correlation and shifts the phase transition temperature. We also tuned the critical temperature by applying an external voltage on VO2films growth on a piezoelectric material (PMN-PT). The lattice constant of the substrate PMN-PT was changed by applying an external voltage, which also changes the lattice of the growth VO2film through an interfacial strain. The critical temperature can be the controlled almost continuously with this method.Besides, we also prepared VO2/GaN p-n junction films that change the critical temperature in a similar way using the same procedure.