Fabrication Of Ti_xV_1-xO₂ Thin Films And Analysis Of Bands Structure

VO₂,a strong electron correlation material has been widely studied,undergoes a metalinsulator transition near room temperature（ 340 K）and at atmospheric pressure.When the temperature is lower than the phase transition temperature,the VO₂ is in the monoclinic phase with the P21/c space group.When the temperature is higher than the phase transition temperature,the VO₂ is in the tetragonal rutile phase with the P42/mnm space group.The response time of VO₂ phase transition is extremely short and reaches picosecond order of magnitude.Meanwhile,the phase transition is accompanied by a great change of electrical and optical properties.These features make it widely used in electronics,optical devices,sensors,thermal windows and other fields.In general,the bulk VO₂ phase transition will produce dislocations,with the poor repeatability.While the VO₂ thin film can reach more than 104 times phase transformation.The films VO₂ will be more practical application.The phase transition temperature,hysteresis width and other properties of VO₂ thin film can be changed by ions-doping method.Although the phase transition mechanism of vanadium dioxide is still debated,however,studies shown that the orbital electron occupancy of VO₂ plays an important role in the phase transformation of vanadium dioxide.Therefore,this paper aims to control the phase transition temperature of vanadium dioxide by Ti4+ ion substitution V⁴⁺ ion,and successfully improve the phase transition temperature of vanadium dioxide,raise at 1.64 K per Ti at.%.Which is achieved by controlling electron occupation of the energy band（orbital）to tailor the phase transition temperature VO₂.This study will be a guide to the development of thermal devices which based on VO₂ materials.The main contents and achievements of this paper are as follows:（1）Preparation of Ti-doped vanadium dioxide by co-sputtering oxidation coupling method.By different materials（metal V target and Ti target）in two magnetron targets simultaneously sputtering on the c-plane of sapphire substrate.By maintaining the sputtering power of the V target and adjusting the Ti target sputtering power,Ti_xV_1-xO₂ thin films with different doping concentrations can be obtained after sufficient oxidation.The method has the advantages of simple preparation process,low cost,high quality VO₂ and doped VO₂ thin film（film resistance difference of 3 4 orders）,and controllable film thickness and doping concentration.The film samples were analyzed by TEM,XRD,XPS,ultraviolet-infrared spectroscopy and temperature-dependent resistant measurement.（2）Taking the Ti_xV_1-xO₂ thin film as an example,the electronic occupation of the 3d orbital of vanadium dioxide was studied,and the phase change temperature was controlled.A series of Ti_xV_1-xO₂ thin films with different doping concentrations were prepared and their resistance-temperature curves showed that the phase transition temperature increases at 1.64 K per Ti at.%.The corresponding high-temperature rutile phase occupancy of the d-orbital analyzed by the UV-IR spectra.The experimental results show that the substitution of Ti⁴⁺ ions for V4 + ion decreases the d-orbital electronic occupancy and increases the phase transition temperature.Reaching the purpose of controlling the orbital electronic occupation to modify the phase transition temperature.It will be a great help for the development of thermal devices based on the VO₂.（3）First-principles calculations of d-orbital electron density of states with different doping contents of Ti_xV_1-xO₂.The simulation results show that with the increase of the doping content,the Fermi level energy decreases,the d-orbital electronic occupancy decrease,Which is in agreement with the experimental results.