Grain-Boundary Segregation and Non-linear Electrical Properties in $SrTiO_3$

Abstract

Owing to the unique physical properties, strontium titanate has been used for various applications in electronic devices such as dielectric, pyroelectric, semicondicting, and sensor materials. In particular, a number of electrical properties of SrTiO3 have been shown to be strongly dependent on the characteristics of physical atomic structures and chemical composition at the interfaces. Consequently, understanding of grain boundary segregation and microstructure should be necessary to control of electrical properties in SrTiO3. In present thesis, effects of the acceptor segregation, second phase, and liquid film on non-linear electrical properties of SrTiO3 have been investigated using EDS with nanoprobe size attached to FETEM (field emission transmission electron microscopy) and AFM (atomic force microscopy). In chapter III, the current-voltage characteristics with acceptor segregation at grain boundaries have been investigated in H2-sintered SrTiO3. Al-added SrTiO3 was sintered in H2 and then annealed in air for selective oxidation of grain boundaries. The samples showed nonlinear current-voltage characteristics, and both the breakdown voltage and nonlinearity coefficient increased with Al concentration. An energy dispersive spectroscopy analysis revealed that Al ions were segregated at the grain boundary, suggesting the formation of Schottky barriers at the boundary. The present results thus point toward the possibility of modifying the grain boundary composition in SrTiO3 as well as fabricating effective switching devices by H2-sintering. In chapter IV, nonlinear current-voltage characteristic of SrTiO3 has been improved greatly by inducing non-equilibrium segregation of Fe-acceptor at grain boundaries. When Fe-added SrTiO3 samples with different Fe additions (0, 0.2, 0.5, 1, 2 and 3 mol %) were sintered at 1350 oC in H2, metallic Fe particles were precipitated at grain boundaries and triple junctions. During subsequent air-annealing at 1100 oC, the meta...