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ABSTRACT We have succeeded in an epitaxial growth of bismuth tungstate (Bi 2WO 6, BWO) thin films, one of the bismuth layer-structure ferroelectrics (BLSF), on SrTiO 3 (001) single crystal substrates by a spin coating process. BLSF are known to be one of the promising materials for ferroelectric random access memory (FeRAM) devices. A homogeneous coating solution was prepared with tungsten hexachloride and bismuth 2-ethylhexanoate as raw materials, and 2(2-methoxyethoxy) ethanol and formamide as solvents. The as-coated thin films were sintered at temperatures from 500 to 800"C for lh in air. BWO crystallized at temperatures above 500°C. Any crystal phase was not observed in the thin films except for (00!) phases of BWO in the XRD patterns. It was confirmed that the thin film was growing epitaxially by measurement of XRD pole figure. The crystallographic relationship of the film and substrate was BWO(001)//STO(001), BWO[1 10]//STO[100]. INTRODUCTION

Bismuth layer structure ferroelectrics (BLSF) in the system

2 (Bi 202) +(An,.BnO 3,

2

)2-

(n=1 to 5) have a structure in which the (Bi2O2 ) " layers and quasi-perovskite units are piled up along the crystallographic c-axis [1-6]. Bi2WO 6 (BWO) is the simplest representative of the BLSF family (n=l). It is polar at low temperature but undergoes a nonferroelectric reconstructive phase transition near 950 to 960°C [7,8]. In a ferroelectric thin film, the degradation of ferroelectric properties by the size effect becomes a problem. The size effect is caused by small crystal grains. The crystal grains become small when the film thickness is decreasing. It also has been reported even in BaTiO 3, PbTiO 3, and Pb(ZrxTi.x)O 3 thin films [9,10]. It may be possible to solve the problem by producing these ferroelectrics as epitaxial films. Preparation of epitaxial Bi2WO 6 thin films has been reported by pulsed laser deposition (PLD) [7]. The BWO thin film shows epitaxial growth along the [100] direction on a [100] oriented Nb-doped SrTiO 3 substrate. The sol-gel process has received much interest due to a potential of precise control of chemical stoichiometry, homogeneity, low temperature processing and decreasing costs [1115]. Preparation of epitaxial thin films by sol-gel deposition has also been reported [16]. In this study, an epitaxially grown film of BWO was prepared by sol-gel deposition, and the evaluation of its properties was carried out. At present, DRAMs are used as memories for computers. DRAMs lose information, when the power of the computer is cut. A ferroelectric memory has a working speed which is equivalent to that of the DRAM, and it does not lose information, since the ferroelectric property is utilized. The write is excellent with 1012 to 1013 cycles without degradation. 377

Mat. Res. Soc. Symp. Proc. Vol. 623 © 2000 Materials Research Society

Ferroelectric memories are expected as a memory for computer in the next generation [7,17]. The ferrolelctric properties of the bismuth layer structure ferroelectrics (BLFS) are very promising for the application t