Ferroelectric and Dielectric Properties of Chemical-Solution-Derived Bismuth Lanthanum Titanate Thin Films with Various
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Ferroelectric and Dielectric Properties of Chemical-Solution-Derived Bismuth Lanthanum Titanate Thin Films with Various Bismuth Oxide Template layers Dinghua Bao, Naoki Wakiya, Kazuo Shinozaki and Nobuyasu Mizutani Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo, 152-8552, Japan ABSTRACT (Bi,La)4Ti3O12 (BLT) thin films with various Bi2O3 template layers were prepared on Pt/Ti/SiO2/Si substrates by a chemical solution deposition method. Both of the BLT films with a thin Bi2O3 template layer and those without a Bi2O3 layer had a highly c-axis oriented growth, while both of the BLT films with a thin Bi2O3 bottom layer and those with a Bi2O3 intermediate layer were highly c-axis oriented. It was found that the use of Bi2O3 template layers improved significantly the ferroelectric properties of BLT thin films. In addition, the thin films with a thin Bi2O3 template layer showed good dielectric properties. All the capacitors with Bi2O3 template layers showed high polarization fatigue resistance and good retention properties. INTRODUCTION In recent years, bismuth lanthanum titanate, (Bi,La)4Ti3O12 (BLT) has received much attention as one of the most important candidate materials for nonvolatile memory applications due to its excellent fatigue-free property. However, like other Bi-layered perovskite materials such as SrBi2Ta2O9 (SBT), BLT also contains volatile element Bi. It has been reported that the control of Bi composition during the crystallization process is important to obtain large remanent polarization values for SBT thin films [1]. In order to prevent the evaporation of Bi from film surface and diffusion of Bi into the bottom electrode during the crystallization process, excess Bi is usually required to achieve stoichiometry for these kinds of the Bi-containing thin films. Most researchers controlled the Bi content by adding excess Bi in precursor solutions for chemical solution deposition or in target materials for sputtering or pulsed laser deposition (PLD) [2,3], which is similar to the method used in preparation of Pb-based ferroelectric thin films for the compensation of Pb evaporation [4,5]. Recently, there have been several other attempts on controlling the bismuth composition. Aizawa and coworkers used a face-to-face annealing method to prevent evaporation of Bi for obtaining a higher remanent polarization of SBT thin films, in which an SBT film deposited on a substrate is directly put with the film side down on the other SBT film during the crystallization process [1]. The study done by Dinu and coworkers suggested that the formation of ferroelectric phases is favored at a lower substrate temperature by
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incorporating Bi2O3 template layers into SBT thin films in the form of Bi2O3/SBT/Bi2O3 sandwich structure prepared by PLD [6]. Zanetti, et al. adopted an SrBi2Nb2O9/Bi2O3 PLD sequential process to prepare SrBi2Nb2O9 thin films which also effectively avoided the loss of bismuth in thin films [7]. Park et al. reported that
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