Enhanced ferroelectric and improved leakage of BFO-based thin films through increasing entropy strategy

BiFeO3 (BFO) has received much attention as a lead-free ferroelectric film due to its large theoretical remnant polarization, but it suffers from a large leakage current, resulting in poor ferroelectric properties. Herein, a series of BFO-based thin films were deposited on fluorine-doped tin oxide (FTO) substrates by the sol-gel method, and the effects of the mixed substitution of the elements Co, Cu, Mn (B-site) and Sm, Eu, La (A-site) on the crystal structure, ferroelectricity, and leakage current of the BFO-based thin films were investigated. It is confirmed that the substitution of individual elements in the BFO-based films caused lattice distortion by X-ray diffraction. Sm and Eu substitutions make the lattice distorted to a pseudo-cubic structure, while La is biased toward pseudo-tetragonal. Piezoelectric force microscopy confirms that the prepared films can achieve reversible switching of ferroelectric domains by nearly 180°. The ferroelectric hysteresis loops shows that the order for the polarization contribution is: Cu>Co>Mn (B-site), Sm>La>Eu (A-site). And the current density voltage curves indicate that the order for leakage contribution is: Mn<Cu<Co (B-site), La<Eu<Sm (A-site). The scanning electron microscopy shows that the introduction of Cu element favors the formation of dense grains, and the grain size distribution statistics prove that La element favors the reduction of grain size, which leads to the increase of grain boundaries and the reduction of leakage. Finally, we prepared Bi0.985Sm0.045La0.03Fe0.96Co0.02Cu0.02O3 (SmLa-CoCu) thin film with a qualitative leap in the remnant polarization from 25.5 (Bi0.985Sm0.075FO3) to 98.8 µC/cm2 (SmLa-CoCu) through the synergistic action of Sm, La, Co, and Cu elements. And the leakage current is also significantly reduced from 160 to 8.4 mA/cm2 at a field strength of 150 kV/cm. Thus, the present study focuses on the notion of enhancing ferroelectricity and decreasing leakage current, based on chemical engineering increasing entropy strategy, which provides a promising path for the advancement of ferroelectric devices.