Ferroelectric Thin Films for Nonvolatile Memories. Especially for Bismuth Layer-Structured Compound.

Abstract

Ferroelectric thin film materials, which are expected to find their applications in future nonvolatile memory devices, are reviewed. In those applications high speed, large capacity nonvolatile memory is required because electronic devices become portable and use battery drives. Memories that use ferroelectric thin films are proposed to fulfill those requirements. Ferroelectric materials for ferroelectric memories are then reviewed as the main theme of this paper, centering on two types. One type has perovskite structures containing lead (Pb) and the other group is made up of bismuth layerstructured materials. The former group, represented by lead zirconium titanate (PZT), has been frequently studied in ceramics research. This group, however, has two problems: fatigue, deterioration due to polarization switching and environmental problems resulting from its lead content. The latter group also has two problems: high crystallization temperature and low process-resistance especially in annealing under a reducing atmosphere. By partially overcoming the above problems, ferroelectric memories have begun to be applied to low-capacity memories. Higher capacity devices are now awaiting finding a compete solution to the above problems.