Domain structures in ferroelectric lead zirconate titanate based thin films

Abstract

Lead zirconate titanate (PZT) thin films have received much attention due to their excellent ferroelectric, pyroelectric and piezoelectric properties, making them excellent candidates for next generation high-precision micro/nano electromechanical systems, integrated circuit resonators and actuators. In these applications, the functional property is determined by ferroelectric and ferroelastic domain wall movement (i.e., electromechanical response). It has been shown that electromechanical response enhancement can be achieved by fabricating multilayered heterostructures. Moreover, new polar phases can arise from interfacial effects in these multilayers. In this dissertation, tetragonal (T) PbZr0.30Ti0.70O3 / rhombohedral (R) PbZr0.55Ti0.45O3 / La0.67Sr0.33MnO3 (LSMO) heterostructures were grown by pulsed laser deposition. A (001) oriented SrTiO3 (STO) substrate was used to achieve high quality epitaxial growth. The T and R films were both ~100 nm thick and the LSMO layer (bottom electrode) was ~20 nm thick. Ferroelastic herringbone c/a1/c/a2 domains were found in the T layer. In contrast, reducing the T layer thickness to ~20 nm, a-axis oriented domains were observed. Additionally, local regions (~50 nm wide) were observed exhibiting fine c/a non-equilibrium domains. Cross-sectional transmission electron microscopy revealed crystallographic relationships between the domains and domain walls. Systematic piezoresponse force microscopy studies revealed complex interactions between the electrical and mechanical fields. These bilayered samples exhibited enhanced electromechanical properties due to the presence of extremely mobile ferroelastic domains in the T layer, where the local symmetry within the domain walls differed from the bulk. These domains were formed by elastic interactions between T and R layers. Atomic-resolution imaging was used to further investigate how domains interacted near, or at, the interfaces. The location of interface between adjacent layers was determined by electron energy loss spectroscopy. Aberration-corrected scanning transmission electron microscopy imaging, followed by post processing, was used to map the polar displacements. Unconventional polarization rotation was found near the T/R interfaces and the domain wall boundaries. Moreover, a new phase was identified, presumably formed to accommodate electric fields for local and interfacial structures. The results presented provide a fundamental insight into the relationships between the ferroelectric/ferroelastic domain structures and physical mechanisms that govern ferroelastic domain behaviours.