Study The Effect Of Different Substrates On The Phase Transition Properties Of VO₂ Thin Films

Metal-Insulator transition（MIT）in strongly correlated electron systems has attracted much attention in materials science.Among the MIT materials,VO₂ is of wide interest to researchers as its phase transition occurs near or slightly above room temperature.During the MIT,the resistivity,the infrared transmittance and the thermodynamic properties change significantly accompanied by a substantial change in the lattice symmetry,thus VO₂ can be used in a variety of applications such as electronic switching devices,thermal sensors,smart windows or even memristor neurons.The research contents and results of this dissertation are as follows:（1）The VO₂ thin films were successfully prepared by the sol-gel method with the optimal process parameters:air pressure of 90～180 Pa,annealing temperature of 520°C,and annealing time of 60 min.The phase transition properties of the VO₂ thin films can be regulated by the control of the process parameters.（2）In order to investigate the effect of lattice mismatch on the phase transition properties,Mg F₂ and TiO₂,which have similar lattice structures to VO₂,were selected as the growth substrates.As the decrease of lattice mismatch between Mg F₂ and TiO₂substrates and VO₂ thin films,their phase transition temperature and thermal hysteresis width are also reduced.（3）During the insulator-metal phase transition,the relaxation of the electric resistance at a fixed temperature arises from the existence of an energy barrier relevant to the first-order transition.We propose a model with four fitting parameters to estimate the energy barrier responsible for the hysteresis of the thermally driven Mott phase transition.The fitting results are in good agreement with experimental data.（4）The VO₂ thin film shows the non-volatile resistance changes only during the heating process in the hysteretic temperature regime,which is tuned by the applied electric pulses.The experimental data and the corresponding circuit-model simulation suggest that the non-uniform distribution of the applied field and different energy barriers for interconversion between different phases are the key factor to tune the resultant resistance of the sample.