Preparation And Ferroelectric Properties Of La-HfO₂ Thin Films

Ferroelectric thin films are widely used in all kinds of RF devices,non-volatile memory,drivers and sensors based on ferroelectric thin films because of their excellent performance.The previous study of ferroelectric thin films mainly focused on traditional perovskite ferroelectric materials,such as Pb?Zr,Ti?O₃,BaTiO₃ and so on.However,these conventional ferroelectrics materials suffer from various problems including poor Si-compatibility,environmental issues related to Pb,large physical thickness,low resistance to hydrogen,and small bandgap,which greatly affect the lifetime and reliability of the devices.Therefore,developing new lead-free ferroelectric materials is an urgent task for researchers.In recent years,remarkable room temperature ferroelectricity has been observed in doped HfO₂-based materials.This material has the characteristics of high dielectric constant,wide bandgap?5.5eV?,and good thermal stability,meanwhile it can be compatible with Si.The problems faced by traditional perovskite ferroelectric materials are effectively overcome.Various dopants such as Si,Al,Zr,Y,Sr,La,and Gd were reported to induce ferroelecticity in HfO₂ films.Compared with other doped films,La-HfO₂ thin films have larger remanent polarization,which would be very promising for memory applications.The main contents of this thesis are as follows:?1?La-HfO₂ thin films were prepared by pulsed laser deposition,and the effects of oxygen partial pressure and deposition temperature on the structure and properties of the films were studied.It was found that the films exhibited obvious room temperature ferroelectricity without annealing.By changing the oxygen partial pressure during the growth process,the noncentral symmetric orthogonal phaseorthogonal phase ratio can effectively increase and make the thin films show better ferroelectric properties.?2?La-HfO₂ thin films were annealed in nitrogen atmosphere.The effects of annealing on the structure and ferroelectricity of HfO₂-based ferroelectric thin films were analyzed,the phase transition mechanism of ferroelectric thin films based on HfO₂ was studied,and the favorable conditions for the transition from tetragonal phase to orthogonal phase are found.The experimental results show that annealing in nitrogen atmosphere can effectively increase the proportion of orthogonal phases in the films,and the maximum saturation polarization can reach 40.4?C/cm²,remanent polarization?2Pr?is 40?C/cm²,which can be compared with traditional perovskite ferroelectric materials.?3?La-HfO₂ thin films were annealed in oxygen atmosphere.The effect of oxygen atmosphere on the annealing process was explored,and the leakage current conduction mechanism of La-HfO₂ thin films annealed in oxygen atmosphere was analyzed.The results show that the annealing process in oxygen atmosphere can effectively reduce the oxygen vacancy,decrease the leakage current density,enhance the room temperature ferroelectricity,and increase the hysteresis loop rectangle ratio.