Role of bismuth precursor in crystallization of SrBi2Ta2O9 thin films

Crystallization of SrBi₂Ta₂O₉ (SBT) thin films was studied as a function of viscosity of bismuth precursor and baking temperature, in order to fabricate capacitors with improved ferroelectric properties. SBT thin films were deposited on to Pt substrates using a chemical solution deposition (CSD) technique. Post-deposition anneal at 750 °C for 1 h in oxygen atmosphere revealed a significant influence of baking temperature and the viscosity of bismuth precursor on the microstructure and the ferroelectric properties of SBT thin films. A high baking temperature (350 °C) and a low viscosity of bismuth precursor (8 cp) yielded larger amounts of Bi₂O₃ secondary phase, smaller SBT grains (104 nm), and lower remanent polarization (P_r=2.0 7c/cm²). Additionally, these films exhibited a very high rate of ageing (>45% reduction in P_r after 7 days). A modified CSD process is suggested, which could suppress the formation of Bi₂O₃ secondary phase. Films fabricated using modified CSD technique exhibited a much larger grain size of 165 nm, higher P_r of 7.2 7c/cm², and significantly improved ageing characteristics (<1% reduction in P_r after 7 days). A qualitative model to describe the ageing in SBT-based capacitors is also suggested.