The effect of excess bismuth on the ferroelectric properties of SrBi₂Ta₂O₉ thin films

The effect of excess bismuth on the ferroelectric properties of SrBi₂Ta₂O₉ (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 Bi₂O₃ 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%, Bi₂O₃ 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 (˜10⁻⁹ A/cm²) and maximum remanent polarization (2Pr ˜12 µC/cm²).