Study On The Deposition And Mechanisms Of Metal-Insulator Transitions Of V₂O₃ Thin Films

V₂O₃ is a typical material with abundant metal-insulator transition phenomena.At ambient conditions,pure V₂O₃ is a paramagnetic metal(PM).Cr doping transforms PM phase into paramagnetic insulator(PI)phase with constant crystal structure symmetry,and this PM-PI phase transition is also considered as the standard Mott-Hubbard phase transition in condensed matter physics.When the temperature is lowered to about 160 K,V₂O₃ will change from the PM phase to the antiferromagnetic insulating(AFI)phase,accompanied by a structural phase transition.Although the metal-insulator phase transition in V₂O₃ has received extensive research,there is still a great controversy about the PM-PI phase transition,and the understanding of the PM phase and its PM-AFI phase transition is still unclear.In this thesis,V₂O₃ was chosen as the research object,and high quality V₂O₃ epitaxial films were successfully prepared on Al₂O₃ single crystal substrates by pulsed laser deposition,and the modulation of the metal-insulator phase transition mechanism was investigated.The whole thesis is divided into five chapters,the main contents are concluded as follows:Chapter 1 mainly introduces the research background and the research progress of metal-insulator phase transition in V₂O₃.Firstly,the metal-insulator phase transition and the phase transition mechanism are introduced,then the metal-insulator phase transition in V₂O₃ is introduced,and finally the research content of this thesis is presented.Chapter 2 details the V₂O₃thin film preparation,structural characterization and physical property measurement methods used in this thesis,including pulsed laser deposition(PLD),X-ray diffraction(XRD),reciprocal space mapping(RSM),X-ray photoelectron spectroscopy(XPS),physical property measurement system(PPMS),and Raman spectroscopy.Chapter 3 focuses on the PLD preparation of V₂O₃/Al₂O₃ epitaxial films,and a series of high quality films are obtained by optimizing parameters such as substrate temperature and substrate orientation.The lattice/thermal expansion coefficient mismatch between V₂O₃ and Al₂O₃ was utilized to achieve a wide range of continuous regulation of stress in(001)V₂O₃/Al₂O₃ films.Chapter 4 focuses on the stress regulation and phase transition mechanism of the metal-insulator phase transition in V₂O₃ thin films.The PM phase is transformed into PI phase in(001)V₂O₃/Al₂O₃ films by in-plane tensile stress which enhanced trigonal distortion,and the electrical transport properties and Raman spectroscopy results indicate that stress in V₂O₃ thin films has the same effect on PM-PI phase transition as Cr doping in(V_1-xCr_x)₂O₃ single crystals.Comparative experiments in V₂O₃ and(V_1-xCr_x)₂O₃films showed that the local structural distortion/disorder caused by Cr doping only plays a role in increasing the resistivity of PM and PI phases.Raman spectroscopy also reveals that the orbital occupation state of the PM phase in V₂O₃ films changes significantly with the trigonal distortion and directly determines the PM-AFI phase transition properties,revealing that the stress is regulated by a different mechanism than that of Ti doping and applied hydrostatic pressure.In Chapter 5,the relationship between structural and electronic phase transitions in V₂O₃films is investigated.The corundum-monoclinic structural phase transition and PM-AFI phase transition are characterized by variable temperature Raman spectroscopy and electrical transport properties,respectively.The experimental results show that the structural and electronic phase transitions occur simultaneously during the warming and cooling,and the physical origin of the controversial relationship between the two in existing studies are discussed.Finally,a summary and outlook of the whole text.