Thermodynamic And Kinetic Stability Of Hafnium Oxide Ferroelectric Metastable Phase

As a new ferroelectric material,hafnium oxide is expected to become the key to developing the next generation of ferroelectric memory because of its excellent CMOS process compatibility.However,what restricts its development is that the prepared ferroelectric thin films are polycrystalline and polyphase films,which will lead to the uneven performance of the designed devices and affect the yield.The main reason for polycrystalline multiphase is that its ferroelectricity comes from the ferroelectric phase,but the ferroelectric phase is unstable at room temperature and pressure.People have done a lot of research on how to stabilize the ferroelectric phase,and there are many influencing factors.However,no matter how people adjust the external conditions,the film will always exist in the form of multiphase.And most of the existing theoretical research focuses on how to change the energy of the hafnium oxide metastable phase to make it thermodynamic stable.In this paper,not only the stability of the Hf O₂ferroelectric metastable phase is studied from the perspective of thermodynamics,but also the physical mechanism of Hf O₂ ferroelectric metastable phase stability is analyzed from the perspective of solid-state phase transition dynamics.The dynamic stability of the Hf O₂ ferroelectric metastable phase doped with oxygen vacancy is studied by using phase transition dynamics,and the correctness of our calculation is predicted from the experimental results.The specific research contents are as follows:（1）The results show that the oxygen vacancy can not stabilize the ferroelectric metastable phase of hafnium oxide thermodynamically.The improvement of the ferroelectric properties of the films does not depend on the ferroelectric phase transition,but the introduction of oxygen vacancy can enhance the polarization of the ferroelectric phase.We calculated the polarization enhancement caused by oxygen vacancies at different positions,found the polarization enhancement caused by oxygen vacancies at other parts,calculated and simulated the migration and diffusion behavior of oxygen vacancies in the film,and analyzed the results.From the results,the oxygen vacancy can form an ordered structure,which leads to the film’s failure.（2）After applying equibiaxial strain to each phase of hafnium oxide and studying the effect of pressure on the stability of hafnium oxide ferroelectric metastable phase,we can know that stress can directly stabilize the ferroelectric phase,but the stability conditions are very harsh.After introducing oxygen vacancy,the strain conditions of stabilizing hafnium oxide ferroelectric metastable phase can be reduced.（3）By considering the stability of the ferroelectric metastable phase of hafnium oxide from the perspective of dynamics,the possible phase transition paths of T-PO,T-M and PO-M are calculated through the mapping relationship between crystal axis and atom,and the three ways of dynamic stability are determined.From the analysis of the results,we can know that the phase change barrier between T-PO and T-M is relatively small,which explains why ferroelectricity can appear only after rapid annealing,And the extremely high potential barrier of PO-M will also stabilize the ferroelectric phase.（4）After the phase transition path is obtained,the effect of Zr doping on the dynamic stability of hafnium oxide is calculated.From the results,we can know that when the Zr doping concentration is about 50%,the potential barrier of T-PO will be lower than that of t-m.Combined with the experimental results,we can know that the calculated results are in line with the experimental results;then,we figured the effect of oxygen vacancy on the phase transition kinetics of hafnium oxide and predicted that the ferroelectric properties of the films would reach the best when the oxygen vacancy concentration was about 3.8 f.u.%。...