Ferroelectric Properties of Cr-Doped BiFeO₃ Films Crystallized below 500 °C

Cr-doped BiFeO₃ (BFCO) thin films were formed on Pt/Ti/SiO₂/Si(100) substrates by chemical solution deposition, in which the nominal Fe/Cr ion ratio was changed from 100/0 to 40/60. The spin-coated, dried, and prefired films were finally crystallized in air, nitrogen, and oxygen flow mainly at temperatures below 500 °C. X-ray diffraction analysis revealed that polycrystalline grains of BiFeO₃ (BFO) were formed in the films with ratios smaller than 80/20 after crystallization at 450 °C. It was also found from cross-sectional energy dispersive X-ray spectroscopy-scanning transmission electron microscopy (EDX–STEM) that Fe and Cr atoms mainly existed in different crystallites. On the basis of these results, a growth model in which the low-temperature crystallization of BFO is triggered by Bi₇CrO_12.5 crystallites is proposed. The leakage current density and remanent polarization (P_r) in BFCO films increased with increasing Fe ratio, and a large P_r of 60 µC/cm² was obtained in a 450 °C-crystallized BFCO film with the 80/20 ratio measured at 1.6 MV/cm and 10 kHz. In the 500 °C-crystallized BFCO films with a Cr doping ratio of more than 50 at. %, a P_r close to the initial value was obtained even after 10⁸ switching cycles owing to the reduced leakage current density.