Aberration Corrected Transmission Electron Microscopy Studies On Structures And Defects Of BiFeO₃ Films

The multiferroic BiFeO3 films have attracted substantial attention due to the requirements of miniaturization and improved performances.To further facilitate the application of ferroelectric films,it is definitely crucial to illuminate the relationship between structures and properties.Here,we designed and fabricated a series of BiFeO3 films by using pulsed laser deposition.Then,the domain/domain walls,phase structures,defects and their coupling behaviors were studied in depth by the state-of-the-art aberration corrected transmission electron microscopy.Recently the polar topological domains have received wide interests due to the potential application in high-density storages.To reveal the polar topological structures in lead-free BiFeO3 films,we have deposited the BiFeO3/GdScO3/BiFeO3 multilayered films on orthogonal TbScO3(010)o substrates by reducing the symmetry of substrates.The periodic 109° domain patterns were obtained in the BiFeO3 layers sandwiched by two orthogonal structures.The scanning transmission electron microscopy further revealed that the 109°domain walls alternately terminated within the BiFeO3 films on the one end and at the heterointerfaces on the other end,thereby forming the line-like distribution of triangular areas near the TbScO3/BiFeO3 and BiFeO3/GdScO3 interfaces.The combination of experimental and theoretical results proved that the triangular areas are the polar orthogonal phases.Thus,the vortices with continuous polarization rotation across the rhombohedral BiFeO3 and orthogonal BiFeO3 phases were obtained.The large compressive strain,stronger interfacial oxygen octahedral coupling from substrates and the localized electric fields all contributed to the formation of polar orthogonal phase.Besides,the polar vortices were also stabilized in the BiFeO3 layers with symmetrical electrical boundary condition.This study investigates the polar vortices with two-phase coexistence in multiferroic BiFeO3 films,which would facilitate the design of high density memories.To systematically illuminate the influences of different mechanical and electrical boundary conditions on the polar topological nanodomains,we have fabricated BiFeO3/GdScO3/BiFeO3/TbScO3(010)o multilayered films with different thicknesses.The domain structures in the two BiFeO3 layers were investigated.For the BiFeO3 layers clamped by orthorhombic structures,different domain patterns were revealed in the BiFeO3 layers with different thicknesses,such as the 109°domain patterns and 180°domain patterns.However,for the BiFeO3 layers closed to the surface,periodic 109°domain structures were observed in the BiFeO3 layers with different thicknesses.It is expected that the localized electric fields exist at the termination of both 109° domain walls and 180° domain walls,thereby contributing to the formation of polar topological structures.The scanning transmission electron microscopy further revealed that the vortices with two-phase coexistence and the flux-closures constituted by several domain walls form at the termination of 109° domain walls in the BiFeO3 layers clamped by orthorhombic structures.Besides,the semi-vortices were observed at the termination of 180° domain walls in this BiFeO3 layers.However,different distributions of flux-closure nanodomains were observed in the BiFeO3 layers neighboring the surfaces,which were determined by the domain patterns in another BiFeO3 layers.This work has systematically discussed the definite influences of different boundary conditions on the formation of multiple topological nanodomains in BiFeO3 multilayered films,which could offer guidance for designing and regulating other topological polar domains.As mentioned,there is an interaction between domains and phase structures in BiFeO3 films.To reveal the structural relationship between domain walls and phase transitions in BiFeO3 films,we deposited two kinds of BiFeO3 ultrathin films grown on NdGaO3(010)o substrates,including the BiFeO3 monolayer and BiFeO3/GdScO3 multilayer.The large anisotropic biaxial compressive strain was imposed on BiFeO3 films by NdGaO3(010)o substrates.The combination of high angle annular dark field and annular bright field imaging illuminated the transition from dense 180° domain walls to antiferroelectric phase with the thickness decrease of monolayer BiFeO3 films.Increasing the oxygen octahedral coupling effect by capping one GdScO3 layer further contributed to the stabilization of antiferroelectric phase in BiFeO3 layers.By comparing the lattice information of sparse 180° domain walls,dense 180° domain walls and antiferroelectric phase,the structural distortion in one 180°domain wall pair shared the similar behaviors as one modulation period of the antiferroelectric phase,providing the possibility of antiparallel A-site ionic displacement in antiferroelectric phase.The results of first-principles calculations further confirmed the critical role of interfacial oxygen octahedral coupling during this phase transition.This work has stepwise stabilized the antiferroelectric phase in strained pure BiFeO3 ultrathin films,which illuminate the nature of antiferroelectric phase transition.Our work would offer guidance for designing high-performance energy-storage devices.Thereby,the domain and phase structures in ferroelectric films contribute much to the physical properties of ferroelectric films,which determine the development of ferroelectric-based electronic devices to some extent.Nevertheless,the inevitable growth defects in ferroelectric oxide films would weaken the service performance,limiting the further applications.Thus,it is crucial to understand the influence of charged defects within films on the polarization distribution and ferroelectric domains.Thus,we fabricated BiFeO3 thin films with different concentrations of oxygen vacancies by tuning the annealing oxygen pressure.The high concentration of oxygen vacancies tended to cluster and became ordered as stepped oxygen vacancy plates.The positively charged vacancy plates were proved to influence the polarization distribution of surrounding BiFeO3 due to their built-in electric fields,serving as the tail-to-tail charged domain walls.At high annealing oxygen pressures,the distribution of oxygen vacancy plates was sparse and no coupling existed between each other.For one oxygen vacancy plate,the transformation from 71°to 109°charged domain walls happened by expanding the vacancy plates.Further decreasing the annealing oxygen pressure,the introduction of oxygen vacancies with higher concentration shortened the horizontal distribution distances and increased the interaction between two neighboring vacancy plates.As a result,the 71°-109° charged domain wall pairs were stabilized.The influences of oxygen vacancy plates on the polarization distribution and domain patterns of BiFeO3 films were further confirmed by phase field simulations.This work illuminates the rich modulation behaviors of oxygen vacancy defects on ferroelectric polarization and domain structures,which in turn could provide implications for designing potential electronics devices.