Manipulating the Ferroelectric Domain States and Structural Distortion in Epitaxial BiFeO ₃ Ultrathin Films via Bi Nonstoichiometry

Exploring and manipulating domain configurations in ferroelectric thin films are of critical importance for the design and fabrication of ferroelectric heterostructures with a novel functional performance. In this study, BiFeO ₃ (BFO) ultrathin films with various Bi/Fe ratios from excess Bi to deficient Bi have been grown on (La _0.7 Sr _0.3 )MnO ₃ (LSMO)-covered SrTiO ₃ substrates by a laser molecular beam epitaxy system. Atomic force microscopy and piezoresponse force microscopy measurements show that both the surface morphology and ferroelectric polarization of the films are relevant to Bi nonstoichiometry. More significantly, a Bi-excess thin film shows an upward (from substrate to film surface) uniform ferroelectric polarization, whereas a Bi-deficient thin film exhibits a downward uniform polarization, which means the as-grown polarization of BFO thin films can be controlled by changing the Bi contents. Atomic-scale structural and chemical characterizations and second-harmonic generation measurements reveal that two different kinds of structural distortions and interface atomic configurations in the BFO/LSMO heterostructures can be induced by the change of Bi nonstoichiometry, leading to the two opposite as-grown ferroelectric polarizations. It has also been revealed that the band gap of BFO thin films can be modulated via Bi nonstoichiometry. These results demonstrate that Bi nonstoichiometry plays a key role on the ferroelectric domain states and physical properties of BFO thin films and also open a new avenue to manipulate the structure and ferroelectric domain states in BFO thin films.