Abstract
Abstract Orientation control of lanthanide (Nd, La)-substituted bismuth titanate thin films (Bi4-xLnxTi3O12, BLnT) for nonvolatile ferroelectric random access memory applications was investigated in this research. The BLnT thin films were prepared on Pt/TiOx/SiO2/Si using chemical solution deposition (CSD) technique. The crystal structure of BLnT thin films was found to be changed with the different doping content of lanthanide elements, annealing temperatures and crystallization schemes. A probable grain growth model was provided to explain the orientation-dependent grain growth of BLnT thin films. The surface structure, dielectric, leakage current and ferroelectric properties of different oriented BLnT thin films were also examined in this work. The different lanthanide element doping of BLnT films was also used to demonstrate the orientation-dependent grain growth model of thin films. Regarding to Nd-substituted bismuth titanate, Bi4-xNdxTi3O12 (BNdT) thin films were crystallized with layered perovskite phase and no second phase was observed in films when the annealing temperature was above 640℃, and the grain size was considerably increased as the annealing temperature increased. At low crystallization temperature, the films tend to be dominated with off-c-axis grains, but become to crystallize with c-axis-oriented at higher temperature due to the change of nucleation at the electrode interface. In addition, Nd addition seems to retard the grain growth of BNdT thin films. The electric behaviors of BNdT films are also dependent on the different annealing temperature and Nd doping content. For example, the best ferroelectric properties were observed for Bi3.25Nd0.75Ti3O12 films crystallized at 680℃ with 2Pr and Ec to be 38 μC/cm2 and 98 kV/cm respectively. Crystallization behavior of Bi3.5Nd0.5Ti3O12 thin films was found to change with the crystallization schemes applied. Enhanced a-axis-oriented crystal growth occurred when Bi3.5Nd0.5Ti3O12 films derived with layer-by-layer crystallization. In contrast, the films derived with 12-layer crystallization were dominated by random diffractions. Examination of structural evolution of Bi3.5Nd0.5Ti3O12 films has indicated that, owing to the geometrical effect, the growth of (117)-oriented crystals was restricted by the layer thickness, while the growth of a-axis-oriented crystals was not. BNdT films derived with layer-by-layer crystallization generally show high remanent polarization, fatigue resistance, and better endurance. Compared to BNdT films, similar trend was also found for the La-substituted bismuth titanate (Bi4-xLaxTi3O12, BLT) thin films in terms of the crystallization temperature and La addition. In addition, layer-by-layer crystallization also applies to BLT films, but the ferroelectric response was not optimized as much as BNdT, suggesting the delicate effect of lanthanide addition to the bismuth titanate with different element.