Development of Bismuth Layer-structured Oxide Devices with Ferroelectric and Conductive Intergrowth Layers

对于目前作为铁电存储器件材料而受到关注的层状结构氧化物，研究了具有两种钙钛矿层的共生结构铁电体的设计和同时显示铁电和导电性能的层状结构材料的实现。首先考察了非化学计量比和晶格缺陷对晶体结构和铁电性能的影响。发现钽酸锶铋中的非化学计量比（钙钛矿晶格中的A位缺陷）使居里温度升高，原子位置移动，导致大的反射极化。在钛酸铋陶瓷中掺入少量钒，使其极化强度提高了3倍左右。得到的反射极化与无取向陶瓷的单晶值计算值相当。对于共生材料的形成，两个钙钛矿晶格的尺寸匹配是必要的。结果表明，某些共生结构铁电体比组分铁电体表现出更大的反射极化。由堆叠不同钙钛矿层引起的应力被认为是导致晶格畸变和原子位移增加的原因。对制备钙钛矿型导电层进行了试验，发现Mn取代Bi_5Ti_3FeO_3中的Fe<15>可有效地使p型电子电导率提高4个数量级以上。从X射线衍射分析，铁电和导电钙钛矿层的共生结构的材料被证实形成，虽然在样品中含有少量的第二相。研究结果表明，通过控制非化学计量比和晶格缺陷，可以显著改善铁电性能，并为共生结构的形成和不同电功能的融合设计提供了指导原则。

For Bismuth layer-structured oxides which is now paid attention as a ferroelectric memory device material, studies were conducted on design for intergrowth-structure ferroelectrics with two kind of perovskite layers and on realization of a layer-strucrured material showing both ferroelectric and conductive properties simultaneously. At first, effects of nonstoichiometry and lattice defects on crystal structure and ferroelectric property were examined. Nonstoichiometry (A-site deficiency in perovskite lattice) in strontium bismuth tantalate was found to give increases in Curie temperature and shift of atom positions, resulting in a large remanent polarization. By doping small amount of V into Bismuth titanate ceramics, remanent polarization increased about 3 times of non-doped material. Obtained remanent polarization was as much as the value calculated from single-crystal value for non-oriented ceramics. For the formation of intergrowth materials, the size matching of two perovskite lattice is necessary. It was found that some intergrowth-structure ferroelectrics show larger remanent polarization than those of component ferroelectrics. Stress induced by stacking different perovskite layers was assumed to be responsible for increases in lattice distortion and atom shift. Trial for making conductive perovskite layers was carried out, and substitution of Mn for Fe in Bi_5Ti_3FeO_<15> was found to be effective for an increase in p-type electronic conductivity more than four orders of magnitude. From X-ray diffraction analysis, an intergrowth-structure material with ferroelectric and conductive perovskite layers was confirmed to be formed though minor second phases are contained in samples. From these results, it was found that a marked improvement of ferroelectric property can be achieved by control of nonstoichiometry and lattice defects, and the guiding principle was obtained for design of intergrowth-structure formation and fusion of different electrical functions.