Origin of Lowered Crystallization Temperature in SBT-BTT Ferroelectric Thin Films
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Origin of Lowered Crystallization Temperature in SBT-BTT Ferroelectric Thin Films Woo-Chul Kwak, Gopinathan M. A. Kumar, Se-Yon Jung, Seung-Joon Hwang and Yun-Mo Sung Department of Materials Science & Engineering, Daejin University Pochun-koon, Kyunggi-do 487-711 Korea (South) ABSTRACT The crystallization kinetics of SBT and SBT-BTT thin films formed by sol-gel technique on Pt substrate was studied. Phase formation and crystal growth are greatly affected by the film composition and crystallization temperature. Isothermal kinetics analysis was performed on x-ray at 10 intervals. diffraction (XRD) patterns of the thin films heated in the range 730 to 760 Activation energy and Avrami exponent values were determined for the fluorite to Aurivillus phase formation. A reduction of ~55 kJ/mol in activation is observed for the SBT-BTT system. A comparison has been made and the possible crystallization mechanism is discussed.
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INTRODUCTION Strontium bismuth tantalate (SBT) is a one of the most promising materials for ferroelectric random-access memory applications because of its excellent fatigue, low coefficient field performance, and low leakage current densities on Pt substrates [1, 2]. However, the application of SrBi2Ta2O9 thin film has been limited due to the high processing temperature (>700 ). In order to reduce the formation temperature, several attempts such as cation doping [3], composition and heating condition control [4, 5] and so on have been made. Recently, S. B. Desu et al. [6, 7] have investigated the solid solution of SBT system, and showed decreased crystallization temperature and increased remanent polarization value. However, the basic reason for the reduction in the processing temperature was not investigated in detail. In this study, an attempt has been made to reveal the origin of lowered crystallization temperature in SBT-BTT ferroelectric thin films compared to that of SBT. Isothermal kinetic analysis has been carried out on x-ray diffractometry results, in order to find out the activation energy and Avrami exponent values involved in the transformation of fluorite to Aurivillus phase in the SBT and SBT-BTT systems.
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EXPERIMENTAL PROCEDURE Sr-isoproxide (Aldrich Chemical, Milwaukee, WI, USA), Bi-t-amyloxide, Ta-ethoxide, and Ti-isopropoxide (High-purity chemical Co., Osaka, Japan) were used as the precursor for SBT and SBT-BTT synthesis and these metal alkoxide were handled in a glove box under dried Ar and atmosphere. Sr-isoproxide was dissolved in 2-methoxyethanol by refluxing at 130 distilled via thermal and vacuum processes. Bi-amyloxide was added to the Sr- methoxyethoxide solution followed by refluxing at 60 and Sr-Bi double methoxyehhoxide solution was distilled again. Ta-ethoxide was mixed with 2-methoxyethanol by refluxing at 130 and the solution was distilled. This Ta-methoxyethoxide solution was added to the Sr-Bi double methoxyethoxide solution and the final Sr-Bi-Ta triple methoxyethoxide solution was prepared. In the preparation of SBT-BTT sol, Ti-isoproxide was dissolved in T
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