Oriented Growth And Dynamic Hysteresis Scaling Of Ferroelectric Bi₄Ti₃O₁₂ Thin Films Of Layered-perovskite Structure

Ferroelectric Bi4Ti3O12 of layered-perovskite has high Curie temperature and large spontaneous polarization Ps lying close to the a-axis. Rare-earth doped Bi4Ti3O12 thin films is one of the candidate materials for nonvolatile ferroelectric memory application because of their high fatigue endurance on metal electrodes. Thus it is significant to prepare a-axis (or near a-axis) oriented (the a/b-axes orientation degree exceed 95%) ferroelectric thin films to obtain large remanent polarization, especially on metal bottom electrodes. Unfortunately, it is difficult to grow such uniformly oriented films directly on metal electrodes.Recently, we prepared Bi4Ti3O12 thin films of high a/b-axes orientation on standard (111)Pt/Ti/SiO2/Si(100) substrates by the sol-gel method. When the Bi4Ti3O12 thin films were crystallized with rapid heating, an in-situ electric field was applied to induce the oriented Bi4Ti3O12 grains nucleate. The effects of applied electric field, heating rates and annealing temperatures on a/b-axes orientation degree were investigated. It was found that the applied electric field has significant influence on the orientation of Bi4Ti3O12 thin films if the Curie temperature of a ferroelectric is higher than its crystallization temperature. The direction of spontaneous polarization of ferroelectric grains tends to consist with that of the applied electric field. Highly a/b-axes oriented Bi4Ti3O12 thin films were obtained with remanent polarization 2Pr of 63μC·cm-2 when the films were prepared under the in-situ electric field of 2 kV, with heating rate of 60℃/s and at annealing temperature of 750℃. The volume fraction of the a/b-axes oriented grains in the films was estimated to be 62%.The scaling behavior of the dynamic hysteresis on a/b-axes oriented and randomly oriented Bi4Ti3O12 thin films was investigated. The scaling relations of hysteresis area against frequency f and field amplitude E0 for the saturated loops of a/b-axes oriented Bi4Ti3O12 thin films were∝f0.018E00.6 when f＜1/τe and ∝f-0.026E00.6 when f＞1/τe, while that of randomly oriented Bi4Ti3O12thin films was∝f-0.106E01.667 in the measured frequency range. The character timeτe of domain reversal decreased with increasing E0, andτe of saturated hysteresis was 1.1 ms for a/b-axes oriented thin films. Butτe can't be observed in the measured frequency range for randomly oriented thin films. This indicates the difference of domain reversal between a/b-axes and randomly oriented grains. The of randomly oriented thin films decays more quickly with f and grows more quickly with E0 than that of a/b-oriented thin films, meaning that randomly oriented grains dissipate less energy and need more time in domain reversal process than that of a/b-oriented.