Enhancement of Aurivillius Phase Formation Kinetics in SBT Thin Films using Nanoparticle Seeding
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Enhancement of Aurivillius Phase Formation Kinetics in SBT Thin Films using Nanoparticle Seeding Yun-Mo Sung, Woo-Chul Kwak, Se-Yon Jung, and Seung-Joon Hwang Department of Materials Science & Engineering, Daejin University, Pochun-si, Kyunggi-do 487-711, Korea (South) ABSTRACT Pt/Ti/SiO2/Si substrates seeded by SBT nanoparticles (~60-80 nm) were used to enhance the phase formation kinetics of Sr0.7Bi2.4Ta2O9 (SBT) thin films. The volume fractions of Aurivillius phase formation obtained through quantitative x-ray diffraction (Q-XRD) analyses showed highly enhanced kinetics in seeded SBT thin films. The Avrami exponents were determined as ~1.4 and ~0.9 for unseeded and seeded SBT films, respectively, which reveals different nucleation modes. By using Arrhenius–type plots the activation energy values for the phase transformation of unseeded and seeded SBT thin films were determined to be ~264 and ~168 kJ/mol, respectively. This gives a key reason to the enhanced kinetics in seeded films. Microstructural analyses on unseeded SBT thin films showed formation of randomly oriented needle-like crystals, while those on seeded ones showed formation of domains comprised of directionally grown worm-like crystals.
INTRODUCTION Strontium bismuth tantalate (SBT) thin films deposited on Pt electrodes exhibit almost polarization fatigue free operation for up to ~1012 switching cycles, little tendency to imprint and relatively low leakage, making SBT capacitors one of the strongest candidates for application in the first generation of NvFRAMs.1-3 However, the high temperature processing has been a major limiting factor.4,5 Therefore, for these systems it is quite necessary that the processing temperature be lowered below 800°C, since at high temperatures, the reaction at the bottom electrode/thin film interface causes degradation of the electrical properties. Several optimized processes including the rapid thermal annealing (RTA) and annealing of SBT thin films under the oxygen flow, have been successful to lower the crystallization temperature approximately to 650oC6-10 and to improve the properties of SBT thin films as well. Another effective method for controlling the reaction kinetics as well as the microstructure would be to introduce crystallographically preferred nucleation sites in the form of seed particles into a system. Transformation temperatures can be significantly lowered using seed materials. However, the effect of Aurivillius seeds on the crystallization kinetics of SBT thin films has not been reported so far. The detailed information of crystallization kinetics of thin films is of great importance because of its strong bearing on the preparation and application of them. In the present work Sr0.7Bi2.4Ta2O9 thin films with fluorite phase were prepared on unseeded and SBT nanoparticle-seeded Pt/Ti/SiO2/Si substrates. Activation energy and Avrami exponent values for the fluorite-to-Aurivillius phase transformation were determined using the isothermal kinetic analyses. The phase formation behavior of the SBT t
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