The Preparation Of Vanadium Dioxide Thin Films With Enhanced Semiconductor-Metal Transition Performance

Vanadium dioxide?VO₂?exhibits a first-order semiconductor-metal phase transition?SMT?at 68°C,which is accompanied by the structural transformation from a high-temperature tetragonal rutile to a low-temperature monoclinic.Great changes in electrical resistivity and optical transmittance of VO₂ occur because of SMT.Due to those unique properties,VO₂ films are expected to be applied in various optical and electrical devices,such as optical switching,smart window and storage devices.Directed metal oxidation process with rapid thermal annealing is a common method for preparing VO₂ thin films,which separates the oxidation process from the sputtering process.This method reduces the requirements for preparation conditions,and offers many advantages,such as short process times,high controllability and repeatability,etc.However,the phase transition properties of VO₂ films prepared by this method are poor,which is difficult to meet the requirements of practical applications.Therefore,the study of the influence of preparation conditions and heat treatment conditions on the composition,crystallization and phase transformation properties of VO₂ film and the optimization of phase transition properties of VO₂ film have become an important issue.In this paper,the effect of the initial state of metal vanadium film on the phase transition properties of VO₂ film was first studied.The metal vanadium film was prepared by magnetron sputtering.During the sputtering process,the initial state of the vanadium film was changed by adjusting the sputtering power.It was found that the increase of sputtering power is beneficial to the increase of the size of the metallic vanadium particles.Subsequently,the metal vanadium film was oxidized by rapid thermal annealing,and the oxidation conditions were the same.It was found that the vanadium oxide after annealing of the metal vanadium with larger particle size was mainly VO₂,and the vanadium oxide after annealing of the metal vanadium with smaller particle size contained V₂O₅,which indicated that the metallic vanadium with a smaller particle size is excessively oxidized.According to the temperature-resistance curve,the phase transition properties of vanadium oxide were analyzed.It was found that the phase transition magnitude of vanadium oxide film showed a trend of increasing first and then decreasing when the size of metallic vanadium particles increased,which is related to the VO2 content in vanadium oxide.When the average particle radius of metal vanadium is 8.8nm,the phase transition magnitude of the VO₂ film reaches the maximum,about 20.5 times,but it still cannot meet the practical application requirements.In order to further improve the phase transition magnitude of VO₂ film,the VO₂ film was heat treated in a tube furnace.It was found that the grain size of VO₂ increased significantly after heat treatment,and the phase transformation magnitude of the film increased from 90 to 1694.The phase transition performance of the VO₂ film can be effectively improved because the grain size is increased and the grain boundary density is lowered.In this study,the phase transition properties of VO₂ film were optimized by adjusting the initial state of metal vanadium film and increasing the heat treatment process of VO₂ thin film tube furnace,and finally VO₂ film with phase transition magnitude exceeding three orders of magnitude was obtained,which is helpful for the improvement of the VO₂ film preparation process.