Experimental And Theoretical Investigation On Poly-domain Switching Of The PZT Ferroelectric Thin Film

Ferroelectric thin films have received considerable attention in the modernmicroelectrics and optoelectronic technology because of their excellent physical properties,such as high dielectric, pyroelectric, piezoelectric and electrooptic properties. Thosephysical properties and applications are directly based on the domain structure anddynamics. Experimentally, piezoresponse force microscopy (PFM) is the primary method tostudy ferroelectric thin films’ domain structures, characteristics and electrical properties.Theoretically, the nonlinear thermodynamic theory is the critical way to determine thedielectric and piezoelectric properties of these films. Currently, the poly-domain structuresare observed by PFM, but the single domain structures is modeled by thermodynamic theory.That situation cannot properly explain the real questions of domain switching and phasetransition in ferroelectric thin films. In this thesis, the local nanoscale poly-domainswitching in Pb(Zr1-xTix)O3(PZT) films was observed by PFM, and then we used a modifiedthermodynamic theory to describe the poly-domain states. The method combining thetheoritical analysis with the experimental observation together may properly investigate thepoly-domain switching. Finally, we also investigated the effects of external stress,component content and misfit strain on the dielectric and piezoelectric responses of PZTthin films. The main contents are shown as follows:1. We characterized the microstructure and ferroelectric properties of Pb(Zr0.52Ti0.48)O3ferroelectric thin film, which grown on the cubic substrate. The film exhibits goodcrystalization and excellent ferroelectric properties. The local nanoscale poly-domainswitching within several grains of PZT film were observed by PFM under the differentloading forces. The opposite color contrast was analyzed within the selected grains inpiezoelectric phase images. It is proved that the increasing external forces can lead to the90°domain switching in PZT thin films.2. The nonlinear thermodynamic theory, which is employed to study the single-domainstates in thin film growing on the cubic substrate, is modified to describe the poly-domainstates under external stress field. We introduce the ac-/aa*, ac+/aa*and c/ac/aa*poly-domain states to describe the force-induced domain switching at the simplification ofuniform stress distribution. The misfit strain-external stress phase diagrams and polarizationcomponents-external stress curves are constructed by nonlinear thermodynamic theories.And the experimental observations are consistent with the simulation results. Moreover, the threshold of the external stress force-generated domain switching3is near-1.20GPa atfixed misfit strain0.001evaluated by X-ray diffraction. The dielectric and piezoelectricresponses could be improved by applying an appropriate external stress to the PZT film.3. We examined the equilibrium polarization states and dielectric properties ofsingle-/poly-domain epitaxial PZT thin films grown on tetragonal substrates by themodified thermodynamic theory. We constructed three-dimensional phase diagrams of twodirectional misfit strains and Ti component content by minimizing the thermodynamicpotential. And the further study confirmed that some conventional poly-domain states existmore stably in ferroelectric thin films than the single-domain states. The appearance anddisappearance of “phase filed” in phase diagrams are influenced both by misfit strains andcomponent content. The “turning point” in the size change tendency of “phase field” isclose to the morphotropic phase boundary (MPB), which is situated around Ti contentx=0.40–0.50. Moreover, the dielectric and piezoelectric responses of are very sensitive tomisfit strain and component content.