Bandgap engineering strategy through chemical strain and oxygen vacancies in super-tetragonal BiFeO3 epitaxial films

The wide band gap of bismuth ferrate-based materials limits their application in optoelectronic devices, and how to achieve band gap modulation in the simplest manner is the primary requirement for their industrialization. BiFeO3 (BFO) films can achieve narrower bandgaps than those under substrate strain, interphase strain, or chemical strain. In this study, we have achieved simultaneously increased tetragonality and reduced bandgaps in the tetragonal-like phase BFO-based films through chemical strain. Co substitution can be controlled by both stoichiometric ratios and atomic deposition rates. The tetragonality can increase up to a large c/a of 1.239, and the bandgap can decrease to 1.45 eV from 2.21 eV. The oxygen vacancies are closely related to the decrease of the band gap. Density functional theory calculations indicate that the introduction of Co and oxygen vacancies essentially reduced the conduction band bottom, resulting from the hybridization energy levels of Co 3d and O 2p, respectively. This paper provides a new strategy for the regulation of the optical band gap of BFO and further reveals its underlying mechanism.