Doping The SrBi <sub> 4 </ Sub> Ti <sub> 4 </ Sub> The O <sub> 15 </ Sub> Thin Film Ferroelectric Properties

Various doped bismuth-layered perovskite ferroelectric SrBi₄Ti₄O₁₅ (SBTi) thin films have been prepared successfully by using a sol-gel method. The microstructrue, the ferroelectric properties and the fatigue endurance have been studied. This will be useful for the application of Ferroelectric Random Access Memory (FeRAM).The SBTi precusor solutions were prepared and the ferroelectric thin films were synthesized on Si /SiO₂ /Ti /Pt substrates. The analysis of XRD, SEM and AFM indicated that the films have good crystal tropism and surface morphology, and no any second phase appeares. Amorphous SBTi thin films prepared by sol-gel method begin crystallized at 650℃and attained a complete perovskite phase at annealing temperature of 700℃. The XRD pattern also reveals that the present SBTi films exhibit good ferroelectric properties for the annealing temperature above 750℃. The grain size of the SBTi films annealed at 750℃is rod-like and the average grain size in diameter is about 500-600 nm. The measurement of the ferroelectric properties of the SBTi films demonstrate that the remnant polarization (2Pr) and coercive field (E_c) are 25.3μC/cm² and 124.3 kV/cm respectively under an applied electric field of 317 kV/cm. Furthormore, the SBTi films show little change of Pnv and ?Pnv up to 3×10⁹ switching cycles, which suggest the excellent fatigue-endurance characteristics of the films.The ferroelectric results of the SrBi_4-xLa_xTi₄O₁₅, (x=0.00, 0.10, 0.25) show that appropriate La-doping can improve ferroelectric properties in SBTi thin films. The 2Pr of the SBLT-0.10 thin film reaches a maximum of 46.0μC/cm², which is twice as large as that of the SBTi thin film. While the 2Pr of the SBTi ceramics reaches the maximum value at the La content of 0.25. The difference may attribute to the relatively high degree of c-axis orientation.In Sr-deficient and Bi-excess SBTi thin film, the substitution of Bi³⁺ for Sr2+ leads to an obviously increase in 2Pr. The dielectric constant peak moves towards higher temperature after substitution. Then the increase of 2Pr may relate to the larger structural distortion, the decrease of defect concentration, and the increase in b-axis orientation.