Role of terbium doping in controlling oxygen vacancies and enhancing conductive performance in BiFeO3 thin films

The influence of Tb doping on structural and electrical properties of BiFeO3 thin films is investigated, combining the experimental observations and density functional theory. With such an approach we aimed to elucidate the influence of Tb dopant and defects in the form of oxygen vacancies on electronic structure and electrical conductivity of BiFeO3 films. X-ray diffraction confirmed that the rhombohedral (R3c) crystal structure remains stable in the whole doping range without any impurity phases. Relatively large leakage current was generated in pure BiFeO3 and 5% and 20% Tb-doped films. Significant reduction of electrical conductivity was seen only in 10% Tb-doped film. Conductive atomic force microscopy (C-AFM) allowed us to investigate local electrical conduction properties at the microscale level. The 10% Tb doped film exhibited the smallest conductive surface confirming that this sample has the smallest conductivity, whereas the current maps revealed that the conduction takes place across spatially inhomogeneous grain boundaries with enhanced concentration of defects. The Raman and XPS measurements have shown that oxygen vacancies are the dominant defects in Bi1-xTbxFeO3 films. The higher concentration of oxygen vacancies, found in pure, 5% and 20% Tb-doped films can be responsible for higher conductivity of these films. A reduction in oxygen vacancy concentration was registered in 10% Tb-doped film which is in accordance with much lower conductivity of this sample. Density functional theory calculations provide atomic-level insights into the electronic transport mechanisms and are consistent with experimental findings. Undoped and low level Tb-doped samples (∼5%) are conductive due to n-type dopants in the form of oxygen vacancies which are primarily localized on surface. For intermediate Tb doping (~10%), the balance between the effects of oxygen vacancies and dopant-induced states results in the highest electrical resistivity. Increased conductivity at higher doping levels (∼20%) can be attributed to the denser population of Tb states around the Fermi level, which can overlap, presenting dispersive, conducting states. This work underlines the complex interplay of doping concentration, oxygen vacancies, and electronic transport, suggesting that 10% of Tb is an optimal dopant concentration for enhancing the electrical performances of BiFeO3 thin films.