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Crystal Structures, Ferroelectric Properties, and Chemical Reactions of SrBi2Ta2O9: Solid-State Chemistry of SBT Materials for FeRAMs

Published online by Cambridge University Press:  10 February 2011

Y Shimakawa  and
Y Kubo
Y Shimakawa
Affiliation:
shimak@frl.cl.nec.co.jp
Y Kubo
Affiliation:
Fundamental Research Laboratories, NEC Corporation, Tsukuba 305–8501, Japan
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Abstract

We discuss some aspects of solid-state chemistry concerning ferroelectric SrBi2Ta2O9, (SBT). Crystal structure analysis by high-resolution neutron diffraction revealed considerable structural distortion in the SBT materials. This distortion, which consists of large atomic displacements of constituent ions, causes large ferroelectric spontaneous polarization. The structural distortion is more pronounced in Sr-deficient-and-Bi-excess material Sr0.8Bi2.2Ta2O9 than in stoichiometric SBT, leading to the larger remanent polarization. The enhanced structural distortion is explained in terms of an “ionic size effect” at the A-cation site. This effect was apparently observed in a series of compounds of ABi2Ta2O9 (A=Ca, Sr, and Ba) with different size A-site cations. On the other hand, analysis of chemical reactions of SBT gave important information on the degradation of the SBT capacitors and on the reaction between the SBT materials and a Pt electrode during device fabrication. The degradation of SBT in an H2- containing reducing atmosphere and its recovery by subsequent annealing in oxygen have been reproduced experimentally by using bulk ceramics and analyzed by thermogravimetric and x-ray- diffraction measurements. SBT decomposes into Bi metal or Bi-Pt alloy in H2 by oxygen dissociation. In the presence of Pt, the SBT degradation and the re-crystallization processes differ from those for SBT alone.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 596: Symposium Y – Ferroelectric Thin Films VIII , 1999 , 131
Copyright
Copyright © Materials Research Society 2000

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