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MATERIALS RESEARCH

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Preparation of epitaxial SrBi2Nb2O9 and SrBi2Ta2O9 thin films by the coating-pyrolysis process T. Nagahama,a),b) T. Manabe, I. Yamaguchi, T. Kumagai, and S. Mizutab) National Institute of Materials and Chemical Research, Higashi 1-1, Tsukuba, Ibaraki 305-8565, Japan

T. Tsuchiyab) Department of Materials Science and Technology, Science University of Tokyo, Yamazaki 2641, Noda, Chiba 278-0022, Japan (Received 6 January 1998; accepted 20 April 1998)

Epitaxial and polycrystalline thin films of bismuth layer-structured ferroelectrics, SrBi2Nb2O9 (SBN) and SrBi2Ta2O9 (SBT), were prepared on single-crystal SrTiO3(001) and polycrystalline yttria-stabilized zirconia substrates, respectively, by the coating-pyrolysis process. The epitaxial relationship of the films and substrates was SBN, SBT (001)//SrTiO3(001) and SBN, SBT [100]//SrTiO3[100],[010], where pseudotetragonal indices were adopted for SBN and SBT. The lattices of the epitaxial films were found to be slightly strained owing to stress from the substrate. Atomic force microscopy observations showed that the epitaxial films as well as polycrystalline films consisted of round-shaped, islandlike grains of submicrometer size.

I. INTRODUCTION

In recent years, much attention has been paid to ferroelectric thin films for nonvolatile random-access memory (NvRAM) applications,1 for which Pb (Zr,Ti)O3 (PZT) has been considered to be one of the most promising candidates.2 However, some serious problems with PZT have been revealed concerning with reliability—e.g., polarization fatigue—and environmental contamination of lead, although the former has been somewhat reduced by using oxide electrodes instead of the commonly used platinum. Meanwhile lead-free bismuth layer-structured ferroelectrics (BLSF) such as SrBi2Nb2O9 (SBN) and SrBi2Ta2O9 (SBT) have been noticed because of their fairly high dielectric constants and Curie temperatures and, above all, essentially fatigue-free properties with polarization reversals.3 BLSF have large electric anisotropy due to their anisotropic crystal structures, which consist of (Bi2O2)2+ layers between the pseudoperovskite blocks along the c direction. Since the number of pseudoperovskite blocks along the c axis is even, SBN and SBT have large spontaneous polarization in the a–b plane but no polarization along the c axis.4 Thus a-axis-oriented SBN and SBT

a)

Permanent address: Department of Materials and Science and Technology, Science University of Tokyo, Yamazaki 2641, Noda, Chiba 278-0022, Japan. b) Cooperation program between Agency of Industrial Science and Technology and Science University of Tokyo. 3090

http://journals.cambridge.org

J. Mater. Res., Vol. 14, No. 7, Jul 1999 Downloaded: 16 Mar 2015

thin films are thought to be preferable for construction of conventional-type memory devices [Fig. 1(a)]. Another type of memory structure, which is a planar type using c-axis-oriented BLSF films as schematically shown in Fig. 1(b), is proposed by Suzuki et al.5,6 If ferroelectricelectrode lat