Stable self-polarization in lead-free BiO3 thick films

The BiFeO3-based film is one of the most promising candidates for lead-free piezoelectric film devices. In this work, the 1 [Formula: see text]m-thick Bi([Formula: see text][Formula: see text][Formula: see text])O3 (BFMT) films are grown on the ITO/glass substrate using a sol-gel method combined with spin-coating and layer-by-layer annealing technique. These films display a large saturated polarization of 95 [Formula: see text]C/cm2, and a remanent polarization of 70 [Formula: see text]C/cm2. Especially, the films are self-poled caused by an internal bias field, giving rise to asymmetric polarization-electric field ([Formula: see text]) loops with a positive shift along the [Formula: see text]-axis. A stable self-polarization state is maintained during the applied electric field increasing to 1500 kV/cm and then decreasing back. The weak dependence of [Formula: see text] loops on frequency (1–50 kHz) and temperature (25–125[Formula: see text]C) indicate that the internal bias field can be stable within a certain frequency and temperature range. These results demonstrate that the self-polarized BFMT thick films can be integrated into devices without any poling process, with promising applications in micro-electro-mechanical systems.