Research On Ferroelectric Domain Wall Migration Of ABO₃ And HfO₂ Based On First-principle Method

A memory using ferroelectric thin film as a storage medium is a kind of non-volatile memory device,which has great potential for application.The structural,electrical and kinetic properties of the domain walls of ferroelectric thin films are the fundamental factors that determine the physical properties and electrical failure of ferroelectric thin films.Therefore,the in-depth study of the physical properties of the domain wall is the key to understanding the performance of ferroelectric thin films and promoting the development of high-density memory.Especially for the new ferroelectric domain-wall memories,the electrical properties and movement laws of the domain walls are the basis for their application.Therefore,in-depth study the dynamical evolution of ferroelectric thin domain wall is very significant to high-density ferroelectric memory.In this paper,the domain wall structure and its dynamics evolution characteristics of ferroelectric thin film were studied by using the principle of the first principle,the regularities of strain and doping control on ferroelectric were also explored.The research results obtained in this thesis include the following aspects:(1)Study on domain walls and evolution of simple perovskite ferroelectric materials.First-principles calculation method was used in this paper to study the motion mechanism of the 180° domain wall of traditional perovskite.It is found that the movement process between two adjacent equilibrium positions(the(100)plane centered on A(Ba,Pb))is the basic step of lateral migration of domain walls.For BaTiO3(PbTiO3),the energy barrier of 180O domain wall moving across one unit cell is 5.6 meV(30.4 meV),and the activation energy of the domain wall motion derived from the barrier agrees well with the experimental values.In addition,the effect of oxygen vacancies and plane strain on the domain wall motion is also studied in this paper.The results show that,for BaTiO3(PbTiO3),when the domain wall across the oxygen vacancy,there will be a potential well of 23 meV(299 meV).Therefore,the domain wall needs corresponding energy to get rid of the constraint of oxygen vacancy,which is one of the important reasons for the polarization fatigue of ferroelectric film.It is also found that under the plane compressive stress,the energy barrier of the domain wall motion increases,and the migration barrier decreases with the plane tensile stress.(2)Study on domain walls and evolution of hafnium oxide-based ferroelectric materials.We studied the polarization reversal method of HfO2 ferroelectric phase and its regulation rules of strain and doping.Based on this,the structure and dynamic evolution of the 180° domain wall of HfO2(separately along a-axis and b-axis)were also studied.The results show that the polarization switching potential barrier of HfO2 is only 0.4 eV,which is 0.32eV for PbTiO3 with the same volume;Appropriate strain/doping can reduce the energy difference between ferroelectric phase/and monoclinic phase m,to some extent,which is significance for stabilizing the HfO2 ferroelectric phase.In addition,different domain wall structure systems were simulated in this paper and obtained the most stable domain wall structure(type III),which is located on the(010)plane,thG domain wall energy is-15.6 mJ/cm2.The energy barrier for moving across one unit cell along the b-axis is also the smallest,about 0.042 eV.According to the energy barriers of domain wall migration,we speculate that the electric field that should be applied to make the domain wall of this structure move sideways should be 0.83 to 0.42 MV/cm.