Phase-field Simulations Of Domain Structures And Physical Properties In High Index Oriented Ferroelectric Films

Ferroelectric thin films have broad application prospects in functional electronic devices due to their rich physical properties.Analyzing and controlling the phase,domain structure and related physical properties of ferroelectric thin films have important research value for deeply understanding the physical mechanism of ferroelectric films and constructing new ferroelectric nanodevices in the future.Experimental results show that(111)-oriented ferroelectric films have more complex multi-domain structures and strange physical properties than(001)-oriented ferroelectric films.However,systematic understanding of the high index oriented domain structures and their property controlling from theoretical point of view are still lacking.In this dissertation,taking the classical tetragonal ferroelectric PbTiO3(PTO)as the research system,the effects of isotropic and anisotropic misfit strain on the ferroelectric phases and domain structures of(111)-oriented PTO thin films were systematically studied by using phase-field simulations,and the in-plane thermal conductivity of(111)-oriented PTO thin films could be effectively controlled by domain structure engineering.The main contents are as follows:By means of phase-field simulation and stereographic projection point analysis,the temperature-isotropic misfit strain multi-domain phase diagram of(111)-oriented ferroelectric PTO films was constructed firstly.The results show that the epitaxial strain can effectively control the orientations of polarization vectors,which uniquely determine the domain variant patterns,domain wall orientations and densities of PTO films.In particular,two types of tetragonal-like domain patterns with distinct structural characteristics,ferroelectric properties and thermal conductivities can form under different strain states.Their electrical properties and thermal conductivity were also found to be different.These results deepen our understanding of the domain structures of high index oriented ferroelectric thin films,provide theoretical guidance for the understanding of experimental data and the design of specific domain configurations,and open a new way for the design of high index ferroelectric nanodevices.The effects of anisotropic strain on the ferroelectric phases and domain structures of(111)-oriented PTO thin films were then studied by means of Transmission Electron Microscopy and phase-field simulation.Firstly,we observed two kinds of tetragonal domains in PTO films grown on GdScO3(GSO)substrate,and revealed the mechanism of formation and evolution of domain structure from the perspective of energy competition.Secondly,various ferroelectric phases of(111)-oriented PTO films under anisotropic strain were determined by stereographic projection point analysis,and the misfit strain-misfit strain phase diagram at room temperature was also constructed.The phenomenon of ferroelectric domain variant separation induced by anisotropic strain was found.In addition,the typical ferroelectric phases and their domain structures under different anisotropic strains were analyzed in detail.These results fill the gap in the study of ferroelectric phase and domain structure of high index oriented ferroelectric thin films under anisotropic strain,and lay a theoretical foundation for further understanding and engineering these novel structures.It is of great significance to control the thermal conductivity of ferroelectric materials in the field of phonology and thermoelectricity.However,the previous researches on the thermal conductivity of ferroelectric films are mostly focused on low index oriented films,but the research on domain structures of high index oriented films is still insufficient.Therefore,we grew the PTO films on the(111)c-oriented KTO and(101)o-oriented GSO substrates respectively.Combining with the Transmission Electron Microscopy characterization and phase-field simulation analysis,it was found that when the domain structures with four kinds of tetragonal ferroelectric variants were formed in the PTO films,in-plane thermal conductivity in the film is anisotropic.Then,the domain structures and thermal conductivities of(111)-oriented PTO films were calculated systematically by using phase-field simulation,including the changing the electric boundary conditions and film thickness of the PTO films grown on GSO substrate,and tuning the anisotropic misfit strains along the special path in the larger strain range.The results show that when the tetragonal domains with four ferroelectric variants were formed,the films had anisotropic in-plane thermal conductivity,and the anisotropy can be enhanced or weakened by adjusting the volume fraction of T1 and T2 domains.These results open up a new way to understand and accurately tune the thermal conductivity of ferroelectric domain structure,which is of guiding significance for the design and development of high index ferroelectric nanodevices with excellent performances.