The effect of excess bismuth on the ferroelectric properties of SrBi 2 Ta 2 O 9 thin films
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The effect of excess bismuth on the ferroelectric properties of SrBi2 Ta2 O9 thin films Tze-Chiun Chen, Tingkai Li, Xubai Zhang, and Seshu B. Desua) Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0237 (Received 29 February 1996; accepted 6 February 1997)
The effect of excess bismuth on the ferroelectric properties of SrBi2 Ta2 O9 (SBT) thin films having a perovskite-like layered structure was investigated for excess bismuth contents ranging from 0% to 100%. For the first time, a limited solid solution of SBT and Bi2 O3 was shown to exist when the amount of excess Bi was less than 50%. The formation of a solid solution enhanced the grain size and a-b plane orientation of the films, resulting in substantial improvement in the ferroelectric hysteresis properties of the films. On the other hand, when the amount of excess Bi exceeded 50%, Bi2 O3 appeared as a second phase which led to high leakage current and poor ferroelectric hysteresis curves. 30 –50% excess Bi content was found to be the optimum composition with respect to grain size, crystallographic orientation, and single phase formation. Within this range, SBT films exhibit low leakage current density (,1029 Aycm2 ) and maximum remanent polarization (2Pr ,12 mCycm2 ).
I. INTRODUCTION
In recent years, interest in ferroelectric materials for nonvolatile random access memory applications has intensified dramatically.1–3 Ferroelectric thin films have been applied onto silicon integrated circuits to provide high speed, high read/write endurance, radiation hard, and low power consumption nonvolatile memories.4 Some of the candidate materials for these applications are perovskite ferroelectrics such as PbZr12x Tix O3 (PZT) or doped PZT chosen for their high Curie temperature and promising electrical properties such as large remanent polarization and low switching field. However, perovskite ferroelectrics, in general, are known to suffer from serious degradation problems such as fatigue, aging, and leakage current that affect the lifetime of the devices.5–13 Recently, alternate ferroelectric oxides belonging to the family of layered perovskites, e.g., SrBi2 Ta2 O9 , were identified as promising candidates for nonvolatile memory applications. SrBi2 Ta2 O9 (SBT) thin films were found to exhibit no fatigue up to 1012 switching cycles, very good retention properties, and low leakage current densities on Pt electrodes.14–17 The layered structure of SBT is comprised of a stacking of two perovskite-like units of O –Ta–O chains between Bi2 O2 layers along the pseudotetragonal caxis while Sr occupies the cubo-octahedral sites in the perovskite layer.18 It is noteworthy that the continuous extension of the O–Ta–O chains along the c-axis is interrupted not only by the presence of Bi2 O2 layers a)
Author to whom correspondence should be addressed. J. Mater. Res., Vol. 12, No. 6, Jun 1997
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