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A Sr0.7Bi2.4Ta2O9 (SBT) seed layer approximately 40 nm thick was formed on Pt/Ti/SiO2/Si substrates, and SBT thin films with the same chemical composition were deposited on the substrates with and without seed layer 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 and scanning electron microscopy analyses. Formation of pyrochlore as well as Aurivillius phase was observed in both the unseeded and seeded SBT films heated at 740 °C. 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 between Aurivillius and pyrochlore phase formation, and Bi-ion diffusion to pyrochlore phase.

Among ferroelectric materials strontium bismuth tantalate (SrBi2Ta2O9; SBT) has been intensively studied for nonvolatile ferroelectric random access memory applications due to its relatively low voltage operation, low leakage current, fast switching, and fatigue-free properties with Pt bottom electrodes.1–3 SBT has the Aurivillius structure, composed of alternating one Bi2O2 layer and two SrTa 2 O 7 perovskite layers along c axis.4,5 However, the pyrochlore phase, defined as a bismuth-deficient and nonferroelectric 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 used 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. The composition of SBT selected was Sr0.7Bi2.4Ta2O9, since it has been known that SBT with an off-stoichiometric composition (Sr 1−x Bi 2+y Ta 2 O 9 ) shows significantly

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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 8, Aug 2003

http://journals.cambridge.org

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improved ferroelectric properties due to lattice distortion.12 For the preparation of Sr0.7Bi2.4Ta2O9 (SBT) sols, Sr-isoproxide (Aldrich Chemical, Milwaukee, WI), Bi-tri-amyloxide, and Ta-penta-ethoxide (High-Purity 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. In the SBT sol preparation, 2-methoxyethanol was used as the solvent. Sr-isopropoxide was dissolved in 2-methoxyethanol by refluxing it at 130 °C followed by distillation. Biamyloxi