Phase-field Theory Study On The Effect Of Oxygen Vacancy On Domain Structure And Phase Transition Of Hafnium Oxide Based Ferroelectric Thin Films

As a new type of ferroelectric thin-film material,hafnium oxide has attracted extensive attention because of its excellent ferroelectric properties since it was found to be ferroelectric in 2011.However,HfO₂-based ferroelectric thin films still face two urgent problems:wake-up effect and fatigue failure.According to relevant research,oxygen vacancy,stress and strain,and other factors will affect the ferroelectric properties of ferroelectric thin films.In practical application,the influence on the ferroelectric properties of thin films is not single,but the cooperative effect of multiple physical fields,such as oxygen vacancy,strain effect,interface effect,and other factors affect the ferroelectric properties of ferroelectric materials.Most of the existing experimental and theoretical studies are about the influence of a single factor on the ferroelectric properties of the films,which can not truly take the synergistic effect of oxygen vacancy and strain on the phase transition process and domain evolution mode of the whole film.Most of the existing phase-field simulation methods are traditional ferroelectric materials,and there are few studies on hafnium oxide-based ferroelectric thin films.To clarify the effects of single oxygen vacancy,strain,and oxygen vacancy synergy on the phase transition and domain inversion of hafnium oxide based ferroelectric thin films,we studied the effects of single oxygen vacancy,oxygen vacancy,and strain（uniform strain,miscut substrate strain）on the phase transition and domain inversion of hafnium oxide based ferroelectric thin films by using the finite element phase-field theory.The specific research contents are as follows:（1）Based on the multiphase coexistence phase-field model of hafnium oxide,the phase-field model of multiphase coexistence of hafnium oxide-based ferroelectric thin films considering oxygen vacancy is established ferroelectric phase and non-ferroelectric phase considering the effect of oxygen vacancy are simulated successfully.Under the influence of an external electric field,the evolution and phase transition of domain structure are realized:（2）The effects of different oxygen vacancy concentrations on the phase transition and domain flipping properties of hafnium oxide thin films were studied.According to the simulation,it can be found that the effect of oxygen vacancy concentration on the ferroelectricity of thin films has two sides.When the oxygen vacancy concentration reaches 1×10^-4,the residual polarization of the thin film reaches the maximum,and when the oxygen vacancy concentration is more than 1×10^-4,the residual polarization of the thin film decreases with the increase of oxygen vacancy concentration.（3）Based on the multiphase coexistence model of hafnium oxide-based ferroelectric thin films considering the effect of oxygen vacancy,a multiphase coexistence model considering the synergistic action of strain（uniform strain and miscut substrate strain）and oxygen vacancy is established.The results show that when a uniform strain is applied to the thin film,the tensile strain will increase the residual polarization of the thin film under different oxygen vacancy concentrations,and the compressive strain will reduce the residual polarization of the thin film under different oxygen vacancy concentration.The uniform strain does not change the optimal range of oxygen vacancy regulation on the ferroelectricity of thin films.When the miscut substrate strain is applied to the thin film when the substrate tilt angle is 6°,the residual polarization of the thin film will be increased,the optimal regulation interval of oxygen vacancy concentration will shift to the right,and the optimal oxygen vacancy concentration will be changed from 1×10^-4 to 3×10^-4.