Institute of Physics , Chinese Academy of Sciences , Beijing 100190 , China.
University of Chinese Academy of Sciences , Beijing 100049 , China.
ACS Appl Mater Interfaces. 2018 Dec 19;10(50):43792-43801. doi: 10.1021/acsami.8b15703. Epub 2018 Dec 7.
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 (BFO) ultrathin films with various Bi/Fe ratios from excess Bi to deficient Bi have been grown on (LaSr)MnO (LSMO)-covered SrTiO 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.
探索和操纵铁电薄膜中的畴结构对于设计和制造具有新颖功能性能的铁电异质结构至关重要。在这项研究中，通过激光分子束外延系统在覆盖有（LaSr）MnO（LSMO）的 SrTiO 衬底上生长了具有不同 Bi/Fe 比的 BiFeO（BFO）超薄膜，从过量 Bi 到缺 Bi。原子力显微镜和压电力显微镜测量表明，薄膜的表面形貌和铁电极化都与 Bi 非化学计量比有关。更重要的是，Bi 过剩薄膜表现出向上（从衬底到薄膜表面）均匀的铁电极化，而 Bi 缺乏薄膜则表现出向下均匀的极化，这意味着可以通过改变 Bi 含量来控制 BFO 薄膜的本征极化。原子尺度的结构和化学特性以及二次谐波产生测量表明，通过改变 Bi 非化学计量比可以在 BFO/LSMO 异质结构中诱导两种不同的结构畸变和界面原子构型，从而导致两种相反的本征铁电极化。还揭示了 BFO 薄膜的带隙可以通过 Bi 非化学计量比进行调制。这些结果表明，Bi 非化学计量比对 BFO 薄膜的铁电畴状态和物理性质起着关键作用，也为操纵 BFO 薄膜的结构和铁电畴状态开辟了新途径。
