| Ferroelectric thin films are widely utilized in micro-electronic devices such as high energy density capacitors,non-volatile ferroelectric memory etc.,due to their excellent electromechanical performance.The electromechanical properties have close relation with micro-scale electrical domain structures,which provides the guidelines on design,preparation and property optimization for achieving high performance through controllable tuning their electrical domain structures in ferroelectric thin films.The misfit strain,external field,and oxygen vacancy et al.,are seemed to be the possible paths to tune the electrical domain structures,wherein the external field is considered to be the most convenient method.Piezoresponse force microscopy(PFM)have become the one of the important tools to control the electrical domain structures down to the micrometer or even nanometer scale,by applying of a DC voltage in the probe tip.However,the highly localized electrical fields induced by the probe tip can not only switch the polarization directions of the electrical domains,but also lead to the electrical transportation of oxygen vacancy in ferroelectric thin films.Since the highly localized electrical fields result in the complex interactions between electrical domains and oxygen vacancy transportations,and it would finally bring a huge challenge in control and analysis of the electromechanical properties in ferroelectric thin films.In this thesis,we firstly carried out the study on the influence of electrical domain structures with misfit strains.Furthermore the dynamic analysis of the electrical domain switching have been investigated under the highly non-uniform external fields for typical 90°and 180°electrical domain patterns.Finally,the transportation dynamics of oxygen vacancy were investigated for different electrical domain structures,and the complex dynamic interaction between the domain structure and oxygen vacancy have been studied under the electrical field.The core research contents and results are presented as follows.(1)Based on our established phase field model for ferroelectric thin films,the evolution of the electrical domain structures have been studied the different biaxial misfit strains under different electrical boundary conditions.The simulation results show that the biaxial tensile strains along the in-plane directions are conducive to the growth of in-plane 90°domains,while the biaxial compressive strain along the in-plane directions are conductive to the growth of out-of-plane 180°domains.And the higher depolarization energy on the film surface can be resulted by the open circuit electrical boundary conditions in comparison with the short circuit electrical boundary conditions,which makes the distributions of the polarizations on the surface are seems to be more disordered under biaxially compressive strains.(2)Taking Pb Ti O3 and Ba(Zr0.08Ti0.92)O3films as examples,the dynamic process of domain switching induced by the charge probe tip have been simulated and analyzed under the PFM experiment.The results demonstrate that the highly and locally non-uniform electric field causes the nucleation of the c domain by switching the polarizations along the in-plane to out-of-plane direction.Moreover,the domain wall migrates to the surrounding areas and the c domain grows to occupy larger areas under the electrically external loadings.(3)Based on our established transportation model of oxygen vacancy,migration mechanism of oxygen vacancies have been analyzed on several typical domain structures,and further the interactions between the polarizations and oxygen vacancy have been studied.The results show that the built-in electric fields are the main driving force for the transportation of oxygen vacancies in different domain structures.And the charged probe tip can induce both the nucleation of the domain structures and transportation of oxygen vacancies.The distributions of the oxygen vacancies are effected by the domain structures and external electrical fields. |
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