As a promising lead-free ferroelectric, BiFeO3 has a very large intrinsic polarization of ∼100 μC/cm2, enabling its great potential in electronic applications especially in a film format. In this sense, reliable ferroelectric properties are desired; however, pure-phase BiFeO3 films are notorious for their large leakage current, especially of those processed by using the sol–gel methoda facile and industrially scalable method for film preparation. In this study, a protection layer, which can be easily integrated in the sol–gel process, is used to ensure the acquirement of remnant polarization of ∼65 μC/cm2 in ∼200 nm BiFeO3 thin films, whereas O2 annealing can enhance that to ∼120 μC/cm2 in ∼400–700 nm films. Reliable ferroelectricity of BiFeO3 films on Si wafers within a wide thickness range was thus achieved. The obtained ferroelectricity is among the best-achieved properties to date of BiFeO3 films for both thin and intermediate thicknesses, including both chemically and physically derived. These results are helpful to advance potential use of sol–gel-processed BiFeO3 films in electromechanical devices with different desired thicknesses.

Crystalline defects appear when the perfect order of the lattice or ideal arrangements of atoms, molecules, or ionic groups is destroyed, which is inevitable during crystal growth, thereby impacting material functionalities, either in a reinforcing or unwanted way. For functional ferroic materials, natural interfaces called domain walls form, which separate regions of different orientations of a specific order (such as magnetic, ferroelastic, or ferroelectric) in the material. These, including phase boundary, grain boundary, and/or domain boundary, can be reckon as two-dimensional defects. During functioning of ferroic materials, phase transformation and/or dynamic motion of domain walls occur with external stimuli like electric fields or stress. Therefore, domain engineering, phase boundary construction, and grain engineering has long been the most considered effective strategies to enhance the performance of ferroic materials. Meanwhile, lower-dimensional defects, including point defects, defect dipole, and line defects, are another crucial dimension to be considered to tune functionality, since their motion greatly interact with the domain wall dynamics under external stimuli. In all, defect engineering (here we refer to one- and two-dimensional defects) and its coupled motion with order parameters is an interesting topic that has attracted significant attention for functional oxides.