An Effective Way to Suppress the Pyrochlore Phase Formation in SBT Thin Films
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An Effective Way to Suppress the Pyrochlore Phase Formation in SBT Thin Films Se-Yeon Jung, Woo-Chul Kwak, Seung-Joon Hwang, and Yun-Mo Sung Department of Materials Science & Engineering, Daejin University Pochun-si, Kyunggi-do 487-711, Korea (South) ABSTRACT Sr0.7Bi2.4Ta2O9 (SBT) thin films were deposited on Pt/Ti/SiO2/Si substrates with and without a seed layer of ~40 nm thickness using sol-gel and spin coating methods. The influence of seed layer on the phase formation characteristics of SBT thin films was investigated using x-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. Formation of pyrochlore as well as Aurivillius phase was observed in both the unseeded and seeded SBT films heated at 740oC. However, it was revealed that Aurivillius phase formation was enhanced in seeded SBT thin films and pyrochlore phase formation was highly suppressed. In this study, two possible mechanisms for the suppression of pyrochlore phase formation were proposed from the perspectives of activation energy difference for Aurivillius and pyrochlore phase formation and Bi-ion diffusion to pyrochlore phase.
INTRODUCTION Among ferroelectric materials strontium bismuth tantalate (SrBi2Ta2O9: SBT) has been intensively studied for non-volatile ferroelectric random access memory (NvFRAM) applications due to its relatively low voltage operation, low leakage current, fast switching and fatigue-free property with Pt bottom electrodes.1-3 SBT has the Aurivillius structure, comprising of alternating one Bi2O2 layer and two SrTa2O7 perovskite layers along c-axis.4,5 However, the pyrochlore phase, defined as a bismuth-deficient and non-ferroelectric phase, has been often observed during the processing of SBT thin films.6,7 Refinement carried out by Rodriguez et al.6 indicates that the stoichiometry for the pyrochlore compound in the SBT system is Sr0.2(Sr0.5Bi0.7)Ta2O6.75. Seeding has been employed as a highly effective way to enhance the kinetics of a specific crystalline phase formation in a material for many years.8-11 In this study very thin SBT seed layer (~40 nm) was introduced between Pt bottom electrode and SBT thin film and the role of the seed layer on the phase formation characteristics of SBT thin films was studied. Two possible mechanisms were proposed for the suppression of pyrochlore formation in SBT thin films grown on seed layers.
Experimental The composition of SBT selected was Sr0.7Bi2.4Ta2O9, since it has been known that SBT with an off-stoichiometric composition (Sr1-xBi2+yTa2O9) shows significantly improved ferroelectric properties due to lattice distortion.12 For the preparation of Sr0.7Bi2.4Ta2O9 (SBT) sols, Srisoproxide (Aldrich Chemical, Milwaukee, WI), Bi-tri-amyloxide and Ta-penta-ethoxide (HighPurity Chemical Co., Osaka, Japan) were used as the starting chemicals and all of these alkoxides were handled in a glove box under dried Ar atmosphere. 2-methoxyethanol was used
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as the solvent in the SBT sol preparation. Sr-isopropoxide was dissolved in 2-methoxyethanol by refluxing
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