Engineering On The Anisotropic Metal-insulator Transition In VO₂ Epitaxial Thin Films

Vanadium dioxide(VO2)has received extensive attention due to its closing to room temperature(about 68 ?)metal-insulator transition(MIT),accompanying with the giant changes of the optical and magnetic properties.The MIT of VO2 originates from the coupling and competition among lattice,orbit and charge,making it one of the most important research directions on condensed matter physics.On the other hand,VO2 MIT behavior is susceptible to temperature,strain,light,electricity,magnetism and other field modulation.So,VO2 have broad application prospects in energy-saving,electronic,intelligent and other fields materials.Particularly,with the advantages of high quality and stability,phase change repeatability,easy integration with micro-mechanical and micro-processing technology,VO2 become one of the best candidate materials for the development of new electricity and photon devices.However,there still are many unresolved scientific problems in VO2 MIT research until now,such as the preparation of high-quality VO2 thin films,strain modulation of VO2 MIT and its mechanism,micro-nano-scale VO2 metal-insulator transition behavior and its mechanism.Aiming at these problems,high quality VO2 epitaxial films were prepared on titanium dioxide(TiO2)substrate by magnetron sputtering technology,and the regulation process of epitaxial strain on MIT was studied.We found that different orientations and thicknesses of VO2 epitaxial films were different,resulting in different changes in MIT transition temperature;and the metal and insulation phase coexistence and competition led to MIT transition temperature range widened.On the other hand,we found the anisotropic MIT behavior of VO2 epitaxial film,and discussed the mechanism of this phenomenon:the size and orientation of the structure domain determines the level of MIT anisotropy.The first chapter firstly reviews the characteristics of VO2 and its application prospects.VO2 characteristics are thermal induced high-grade and steep,electrical and thermal properties.This feature makes VO2 popular in energy saving,intelligent materials such as intelligent windows,infrared detectors,laser protection glasses and so on.This paper focuses on the modulation of the MIT of the VO2 film,so we introduce the relevant physical background knowledge,making it easy to understand the following work.Finally,the contents and methods for this studying were introduced.The second chapter mainly introduces the preparation process of VO2 epitaxial film.The diversity of vanadium oxide makes the preparation of VO2 epitaxial film very harsh.In this chapter,we first introduce some common preparation methods of VO2 thin films and their advantages and disadvantages.Then we introduce the magnetron sputtering method used in this paper.High quality VO2 epitaxial film samples laid an important foundation for subsequent experiments.The third chapter mainly introduces the characterization of microstructure and properties of VO2 epitaxial thin films.First of all,this chapter intruduces the electrical testing and crystal structure characterization technology of the VO2 epitaxial film in detail.In addition,we analyzed the composition information of the samples by photoelectron spectroscopy(XPS).And the orbital changes of VO2 epitaxial films were explored by soft X-ray absorption(XAS)technique.The surface morphology of VO2 epitaxial films was characterized by atomic force microscope(AFM)for appearance information.The fourth chapter mainly studies the regulation of metal-insulator transition of VO2 epitaxial thin films.First,different VO2 epitaxial films were prepared by different orientation substrates.It was found that the transition behavior of different orientations of VO2 epitaxial thin films was different.And,the anisotropy transition properties of(110)-oriented VO2 epitaxial thin films were more obvious than(001)-and(011)-direction.Then,we selected the VO2/(110)-TiO2 epitaxial film with the largest anisotropy to study the effect of thickness on the metal-insulator transition of VO2 epitaxial thin films.It was found that the transition behavior of metal-insulator changed regularly with the changing of thickness.The results of microstructural characterization show that the orientation and thickness dependent metal-insulator transition behavior are closely related to the domain structure and strain state of VO2 epitaxial films.By the change of orientation,the increase of VO2 c-axis strain leads to the increase of MIT transition temperature,and the increase of size and orientation of the domain structure leads to the increase of anisotropy of metal-insulator transition.With the increase of film thickness,the c-axis strain of(110)VO2 occurs relaxation and the metal-insulator transition temperature decreases;however,the metal-insulator transition anisotropy increases,which is mainly owing to the thickness dependent of the domain size and orientation.Chapter 5 summarizes the main contents of this paper and points out the shortcomings.In addition,this chapter also puts forward the prospect of the study on the regulation of metal-insulator transition in VO2 epitaxial thin films.