| Vanadium dioxide(VO2)is a thermally induced phase transition metal oxides.In the vicinity of 341 K,VO2 will occur from metallic phase at a high temperature(rutile structure,R)to semiconductor phase at a low temperature(monoclinic structure,M)of the transition(MIT)under the external temperature field driven,the phase change is reversible,accompanied by optical(especially the infrared light),electrical and magnetic properties of the reversible mutations.These prompted a keen interest for VO2 in terms of preparation process,physical properties,characteristics and mechanism of metal insulator transition(MIT),practical applications,etc,and carried out extensive research.Grow a Vanadium dioxide(Super)thin films that have an atomically smooth surface with defined thickness from sub-monolayer(nano islands)to dozens of monolayer is the basis for deep understanding of VO2 metal insulator transition(MIT)mechanism.Despite many years of study,but so far no high quality single crystal Vanadium dioxide(ultra)thin films have been reported.Differing from the usual experiments conducted ex-situ in the atmosphere for VO2 study,in this paper we carry out the growth of VO2 thin films and the study of their MIT characters for the first time in an ultrahigh vacuum system that combines Electron beam evaporation technology(EBE)for thin film growth and in-situ surface characterization methods(including LEED-low energy electron diffraction,XPS-X-ray photoelectron spectroscopy and STM-scanning tunneling microscopy).The main research contents and results are as follows:Firstly,in the clean Si(100)single crystal substrate,we use EBE method under different growth conditions(substrate temperature,oxygen pressure and flux current)to prepared a series of VO2 thin films,it was basically identified the changes of the VO2 components under different growth conditions.The results showed that,with increasing oxygen pressure,the higher vanadium oxide valence state was formed;the more slowly deposition rate,the more fully combination between the vanadium atom and oxygen;the lower temperature of the substrate,the more oxygen combined with vanadium atom in the substrate surface,and the higher vanadium oxide valence state was formed.Secondly,in the clean TiO2(110)single crystal substrate,studied different growth patterns and different deposition rate effects on the VO2 film morphology.The results showed that,step by step growth 1000s VO2 film,it obtained rough surface and mainly some disorganized clusters.Whereas,one step growth 1000s VO2 film,it obtained orderly chain structure and more conducive to growth epitaxial VO2 thin film.When the flux current at 20 or 30 nA can obtain high-quality epitaxial VO2 film.Finally,we used to XPS technology to detect the density of electronic states near the Fermi level changes of VO2 film due to the MIT(metal insulator transition)phase transition.The results show that,when at room temperature 293K,it had no distribution of electron states above Fermi level,thus belongs to the insulation state(or semiconductor state);when at 380K,it had distribution of electron states above Fermi level,thus belongs to the metallic state.These directly prove the VO2 film MIT phase transition occurred,XPS successful capture the MIT phase transition process.All of these works provides a good start and work foundation for the future research,such as the microscopic mechanism of VO2 film MIT,determine VO2 film pseudomorphic growth critical thickness,search for MIT phase change size effect. |
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