10 - Development of novel ferroelectric materials

Abstract

Two kinds of ferroelectric oxides have been recognized as a candidate for the memory materials, which include lead zirconate titanate (PZT) with perovskite structure and the bismuth layer-structured ferroelectrics (BLSF) with layered perovskite structure such as SrBi2Ta20 9 (SBT) and Bi4Ti3012 (BiT). Layered ferroelectric SBT and BiT have been widely studied as candidate materials for ferroelectric memories because of their high fatigue endurance. SBT, however, shows a relatively low Pr of 7–10 μC/cm2, which leads to a difficulty in establishing high-density ferroelectric memories using SBT. The substitution of La into BiT allows one to obtain ferroelectric films at a relatively low deposition temperature of 650°C with a larger Pr (20 ∼C/cm2), but this Pr value is still lower than that of PZT films. As lead-free materials with sufficient ferroelectric properties are preferred to protect the environment and the ecosystem, novel Pb-free ferroelectrics with a larger Pr have become a necessity for next-generation high-density ferroelectric memories. Crystal structure of bismuth layer-structured ferroelectrics (BLSF) are also discussed where the Bi202 layers act as insulating paraelectric layers and control the electronic responses such as electrical conductivity, band gap, etc. while the ferroelectricity arises mainly in the perovskite blocks. The crystal structure of BLSFs is briefly characterized by m, and the dielectric and ferroelectric anisotropy strongly depends on the value of m. Superlattice-structured BLSFs, discovered by Kikuchi et al. have also received a renewed interest as a promising candidate for ferroelectric materials. Dielectric measurements of superlattice-structured BiT–BBTi single crystals grown by a self-flux method, whose structure was confirmed by X-ray diffraction and transmission electron microscope analyses, show that the Tc of the BiT–BBTi is 540°C, which is 40°C higher than that of stoichiometric BiT–BBTi ceramics.