Study On The Phase Transition Dynamics Of HfO₂-based Ferroelectric Films Under Ultrafast Electric Fields

As a new type of ferroelectric material,hafnium oxide-based ferroelectric material is considered to be one of the future development directions of ferroelectric materials because of its strong size reduction ability and high compatibility with CMOS process.Ferroelectric memory based on this type of ferroelectric material is considered to be able to overcome the disadvantages of the traditional ferroelectric memory,such as poor retention performance and poor size reduction ability,and promote the revolutionary development of the ferroelectric memory industry.However,in the current study,we have found that HfO₂-based ferroelectric memory still has reliability problems such as wake-up effect and fatigue failure.These problems seriously affect the stability and retention of data storage reliability of HfO₂-based ferroelectric memory,the reason of these problems is that the HfO₂-based ferroelectric memory is easy to stimulate the generation and movement of micro defects under the action of ultrafast electric field,which causes changes in the number of reversible ferroelectric domains and changes in the dynamic behavior of ferroelectric domain phase transitions.In order to explore the microscopic nature of the wake-up effect and fatigue failure of HfO₂-based ferroelectric thin films,a phase field dynamics model of HfO₂-based ferroelectric thin films with ultrafast time-scale was established in this paper,and the transformation behavior and the evolution of the flipping properties of ferroelectric phase under the action of ultrafast electric field were studied.The specific research contents are as follows:(1)An ultrafast time-scale multiphase coexistence phase field model of HfO₂-based ferroelectric thin films is established,and the dynamic response of ferroelectric domain walls under ultrafast electric pulses is preliminarily analyzed using this model.By comparing it with the simulation results of the relaxation phase field model,we find that the ferroelectric domain wall motion under the dynamic phase field model has inertia,which proves that the model can simulate the dynamic behavior of ferroelectric domain in ultrafast time scale.(2)Using the phase field model built in this paper,we analyzed the influence of electric pulse amplitude,pulse duration and multi-pulse width on the phases of HfO₂-based films.The simulation results show that the ferroelectric phase has different transition rules under high and low amplitude pulses:under low-amplitude electric fields,the volume fraction of ferroelectric c-domain changes linearly and steadily with time,while under high-amplitude pulsed electric fields,the volume fraction of ferroelectric c-domain changes nonlinearly with time.Based on the simulation results,the expressions of ferroelectric domain wall displacement and velocity with respect to time and amplitude under low amplitude pulse electric field are obtained.Studies on the duration of a single pulse show that if the pulse duration is less than 1.2 ns,the ferroelectric c-domain will undergo a severe reverse phase transition.When the pulse duration is longer than 1.8 ns,the ferroelectric c-domain content remains and cumulatively increases.(3)We have established the phase field dynamics model of HfO₂-based ferroelectric thin films with point charge defects.The influence of the phase on the HfO₂ based ferroelectric film with point charge defect under the ultrafast electric field is studied,and the influence of the generation,diffusion and increase of the point charge on the domain reversal properties of HfO₂.Simulation results show that the local internal electric field generated by the point charge defects cannot be effectively offset by increasing the amplitude of the electric pulse.Under the action of periodic pulse,the narrower the pulse width,the higher the content of non-ferroelectric phase in the defect films containing point charge.Under the action of AC electric field,the increase of point charge makes more and more ferroelectric domains in the films are pining and become"dead domains",which reduces the reversal performance of ferroelectric films and causes fatigue failure of the films.In addition,it is found that the content of non-ferroelectric phase does not change monotonously with the increase of point charge,and the defect dipole may increase the content of non-ferroelectric phase.Finally,it is found that the diffusion of the point charge defect from the interface to the film will increase the remnant polarization and improve the ferroelectric properties of the film.