Abstract
The Ta content effects on the ferroelectric properties of strontium bismuth tantalate SrBi2Ta2O9 (SBT) thin films synthesized using metalorganic decomposition (MOD) and spin coating techniques were investigated. The physical properties of these SBT samples were strongly dependent upon the Ta ratio. Polarization measurements revealed that Ta-deficient SBT exhibited a relatively low coercive field (2Ec = 87 kV/cm) and a high remanent polarization (2Pr = 15 μC/cm2); the value of 2Pr decreased as the Ta ratio in SBT increased. The improved ferroelectric properties of the Ta-deficient SBT samples may have resulted from the uniformly well-grown bismuth-layered-structured (BLS) phases of the films and their highly preferential orientation along the a and b axes. Author suggests that the incorporation of Ta vacancies plays an important role in enhancing the crystallinities and microstructures of Ta-deficient SBT films. Further, the seeding effect on the crystallization behaviors of the SrBi2Ta2O9 Aurivillius phase on top of the ultra thin (40nm) buffer/seeding layer, SrBi2Ta2xO9 (x=0.9, 1.0, or 1.1), was studied. The morphology and crystallographic orientation of complete SBT heterostructures were found to be highly dependent on the buffer layers with various Ta contents. The uniform microstructure and the preferred polar a-axis orientation within the SrBi2Ta1.8O9 buffered SrBi2Ta2O9 thin film were promoted; meanwhile, the optimized ferroelectric properties were achieved with 2Pr values about 19.7 μC/cm2, which was 93% greater than that of the stoichiometric buffered one. Since the off-stoichiometric buffer layer is too thin to influence the total composition of the seeded SBT layer, this heterostructure behaves moderate leakage property as compared with the stoichiometric SBT film. The off-stoichiometric thin buffer/seeding layer displays as a new method to improve the microstructure, polar a-axis orientation, and electric characterizations of SrBi2Ta2O9 thin films.In the second section of this thesis, the effects of constraint during annealing on crystal structure and electric properties of the Pb (Zr0.6Ti0.4) O3 ferroelectric films were investigated. The external stress was applied though bending a circular section on the substrate, which effectively led to the variety of crystallographic orientation, structure, P-E behavior, dielectric, and fatigue properties in PZT. Meanwhile, the compression-annealed films enhanced the remanent polarization and the dielectric constant by ~68% and by ~70%, respectively. The observed variations of the ferroelectric behaviors with constraint–annealing can be reasonably interpreted by the crystal structure, especially phase construction and texture, which is dependent on the stress state. Either a tensile or a compressive mechanical constraint was also applied during annealing on SrBi2Ta2O9 (SBT) ferroelectric thin films with different thickness to investigate the effects of both thickness and stress on the physical and electric properties. Both the tensile and compressive stresses exerted pronounced effects on the volume fraction of the pyrochlore phase (ratio of perovskite vs. pyrochlore) and the grain growth mechanisms of the SBT films. These variations of structures became more significant in the relatively thinner films. The effects of the stress correlated with the crystallization temperatures. The stress applied to an SBT sample during its crystallization led to the creation of distinct microstructures and constituted phases of the film. Since the electrical properties of ferroelectrics were strongly depended on their microstructure, domain structure, and constituted phases, the SBT samples exhibited distinct ferroelectric behavior with the condition of various stressing and thickness.