Polarization switching at room temperature of undoped BiFeO₃ thin films crystallized at temperatures between 400 ≤ T ≤ 500 °C

Pure BiFeO₃ perovskite thin films have been prepared on Pt-coated silicon substrates by chemical solution deposition at temperatures below 500 °C. Precursor solutions with and without Bi(III) excess have been used. Perovskite films without crystalline secondary phases, as detected by X-ray diffraction analysis, are obtained at the lowest temperature limit of 400 °C. However, the scanning electron micrographs of these films show surface microstructures formed by well defined grains surrounded by a fine grained phase, suggesting the appearance of a volume fraction of crystals in an early stage of crystallization. The films prepared with Bi(III) excess have better defined ferroelectric hysteresis loops than those without any excess, especially for the films annealed at 400 °C, which can be attributed to an improved connectivity of the ferroelectric phase. This together with the fact that leakage current densities in the films decrease with decreasing the processing temperature, make that the BiFeO₃ films prepared with Bi(III) excess and annealed at 400 and 450 °C can be poled at room temperature, obtaining an effective switching of the ferroelectric polarization with the electric field. Remanent polarization values of P_R ~ 10 and ~60 μC/cm² with coercive fields of E_C ~ 205 and 235 kV/cm were obtained for the films prepared at 400 and 450 °C, respectively. The demonstration of the functionality at room temperature of these low temperature processed undoped BiFeO₃ thin films increases the interest in these materials for their integration in multiferroic devices.