Study On HfO₂ Phase Transition Mechanism Based On Group Theory And First Principle Correlation Phonon Soft Mode

Since the discovery of ferroelectricity in the high dielectric material HfO₂ in 2011,it has been regarded as a new generation of ferroelectric materials because of its good compatibility with CMOS process,good retention performance,and high integration due to its ferroelectricity when the thickness is less than 10nm.However,due to the"wake-up"effect and fatigue failure in HfO₂ based ferroelectric thin films,HfO₂ based ferroelectric memories have not been well applied.In the past studies,it was found that the essence of affecting the properties of HfO₂based ferroelectric thin films is that the ferroelectric phase of HfO₂ is metastable at room temperature and pressure,and the coexistence of multiple phases in HfO₂ ferroelectric thin films makes its ferroelectric phase unable to exist stably.In the past,people have done a lot of research on the stability of its metastable ferroelectric phase and made some progress,but the research does not involve how to change the non-polar phase into the ferroelectric phase.Therefore,in this paper,the phase transition path of HfO₂ is discussed through first principle calculation and soft mode theory,and the results are verified by external stress.At the same time,the phase stability and phase transition type of HfO₂ are also analyzed.Based on the phase transition path calculated theoretically,this study has carried out a more comprehensive search for the phase transition path through the change of group analysis symmetry,and analyzed the domain structure generated in the process of symmetry change.The details are as follows:（1）Through the first-principles calculations,the soft mode-phase transition model is established.The calculated results show that the phase-transition caused by the displacement of the atoms produced by the phonon soft-mode is consistent with the experimental results.Through this result,this study gets phase-transition pathways of HfO₂.At the same time,due to the cell-doubling phenomenon,the study also compared the different phase-transition modes of traditional perovskite materials and new HfO₂ material.（2）To verify the reliability of the model,this study changes the environments by controlling the external stress and then calculates the phonon spectrum of HfO₂ material under hydrostatic pressure.the results show that when the phase-transition occurs,the original phonon soft-mode in the original high-symmetry phase structures disappears,while the low-symmetry phase structures produce a new phonon soft-mode after the phase-transition,and the original stable phase structures change into an unstable phase structures due to the phonon soft-mode,This phenomenon provides a reliable proof for this model.（3）Through controlling the external stress,it is found that the energy difference between the phases of HfO₂ material is decreasing with the increasing stress.When the applied external stress reaches 20GPa,a stable ferroelectric PO phase is obtained in this study.At the same time,the volume change of each phase at the phase transition point is analyzed,and different phase transition types in HfO₂ are obtained.（4）According to the phase-transition pathways obtained from the theoretical calculation,this study improves the phase-transition pathways of HfO₂ material by reducing the symmetry space-group.Since there is no parent-sub group relationship betweenPca2₁ andP2₁/c,this study gets two different phase-transition pathways:C-PO and C-M.Because the ferroelectric PO phase has only upward and downward polarization,finally this study gets six domain structures through group-theory analysis.