Study On HiPIMS Preparation Technology And Phase Transition Temperature Adjusting Mechanism Of VO₂ Film

VO₂ thin films can be exploited in many applications due to the unique metal-insulator transition?MIT?characteristics.However,the high phase transition temperature?68 °C?limits its use in certain fields,such as laser protection and s mart windows.Reducing of the phase transition temperature of VO₂ thin films has become one of the hotspots in this area.However,the reduction of MIT temperature was often accompanied with the significant deterioration on resistance and transmittance changes,and it is hard to fabricate high-quality VO₂ thin film with a low MIT temperature.In this thesis,VO₂ thin films were prepared by high power impulse magnetron sputtering?Hi PIMS?,and the phase transition properti es were adjusted effectively.The influences of oxygen flow rate,peak current,bias voltage and deposition time during the Hi PIMS process on microstructure of the films were studied,respectively.At room temperature,the lattice distortions of the thin films deposited with varied oxygen flow rates were different.However,their grain size,roughness and thickness were similar.The room temperature lattice distortions were basically the same with the increase of peak currents from 25 A to 62 A,and the grain size was larger than that of the thin film with a peak current of 25 A.When the peak current exceeded 58 A,the preferential growth of the films was obvious and the amount of diffraction peaks was reduced.Using a two-step method,a pulse bias was applied on the non-conductive quartz glass substrate successfully.The grain size of the as-deposited films was reduced significantly with the magnitude of the bias voltage,while the film composition and thickness did not change basically.Especially,the grain size of the VO₂ thin film with a bias magnitude of-250 V was only 11.9 nm,and it's crystal orientation was completely changed,and the degree of crystallization was the lowest.In addition,a VO₂?M?thin film was prepared by applying a pulse bias directly on a conductive ITO substrate.When the bias voltage was-200 V,VO₂ thin film could be prepared at 300 °C.Additionally,at low deposition temperature,the crystal structure of the film was changed to VO₂?B?after reducing the deposition time.When the substrate temperature was increased to 485 °C,ultrathin VO₂ film could be fabricated by a one-step method.Based on the analysis of the film resistance-temperature curves and the transmittance spectra at different temperatures,the phase trans ition characteristics of the films deposited under different parameters were studied.It can be seen that the phase transition temperature could be effectively adjusted by the change of the oxygen flow rate and substrate bias,and the MIT temperature of the undoped polycrystalline VO₂ thin film could be reduced to 32 and 31.5 °C,respectively.A high quality polycrystalline VO₂ thin film with low phase transition temperature?49.2 °C?could be prepared by increasing the peak current in HiPIMS process.Simultaneously,the phase transition temperature could be reduced with little deterioration on resistance and transmittance change as well.In addition,the deposition time had little effect on the MIT temperature,and the ultrathin film with a deposition time of 2 min showed conspicuous MIT characteristics.In addition,the regulation mechanism of the phase transition temperature of the as-deposited films was studied.At a high temperature,the crystal structure of the film was transformed to that of the standard VO₂?R?.The lattice distortion at room temperature was the main reason for the different MIT temperatures of the films deposited at varied oxygen flow rates.The smaller the interplanar spacing to the high temperature VO₂?R?,the lower the transition temperature.The ab-initio molecular dynamics and density functional calculations further confirmed that the proper lattice distortion could reduce the phase transition temperature and the energy gap.The phase transition of the films was a coordinated deformation of planes with different orientation.The larger the crystal structure changes,the more the change on resistance.The lattice distortion of the films did not change with the peak discharge current.And the reason for the decrease of the phase transition te mperature was related to not only the decrease of the grain size,but also the less crystalline orientation of the film,in which the reverse deformations did not exist during the phase transition process.The significant change on the phase transition temperature of the films deposited at different bias voltages was mainly related to the decrease of the grain size due to the lattice distortion.For the thin film deposited on ITO glass at 300 °C,the change on the crystal structure was suppressed during the phase transformation process,indicating that the MIT process was a typical Mott one.Additionally,density functional theory simulation proved that the MIT process of VO₂?M?could be realized by the change of the electronic interaction.