Saturated hysteresis loops and conduction mechanisms in Mn-doped BiFeO₃ thin films derived from sol–gel process

Mn-doped BiFeO₃ (BFO) thin films with nominal composition of BiFe_1−x Mn _x O₃ (x = 0.00, 0.01, 0.03, 0.05, 0.07) were deposited on (111)Pt/Ti/SiO₂/Si substrates via a simple sol–gel spin-coating method with rapid thermal annealing process. The BFO films with different Mn dopant contents were well crystallized in the perovskite structure and their overlapped (110) diffraction peaks shifted toward higher angles with the increase in Mn content, indicating a slight distortion in the lattice structure. Improved microstructure with smaller grain size and diminished structural defects can be observed in the films of x around 0.03. X-ray photoelectron spectroscopy analysis confirmed the coexistence of 2+ and 3+ electronic states for Fe element and proper substitution of Mn for Fe can decrease the amount of Fe²⁺ while excess doping results in increasing Fe²⁺ content. The intrinsic ferroelectric polarization was hard to be measured in the pure BFO film due to high leakage contribution, whereas the x = 0.03, 0.05 and 0.07 films exhibited well-saturated rectangular shape-like ferroelectric hysteresis loops, and more importantly, perfectly closed hysteresis loops were obtained for the x = 0.03 film with a 2Pr value of 85.2 µC/cm². The leakage current density in high electric field region was dramatically decreased by Mn doping, e.g. decreased to 3.3 × 10⁻⁴ A/cm² at electric filed intensity of 170 kV/cm for the x = 0.03 film. Detailed leakage current characteristic analysis suggested that the dominant conduction mechanism in the pure BFO film was the space charge limited conduction at medium/high electric fields, which was associated with the space charges originated by oxygen vacancies; however, the leakage current of the x = 0.03 film was dominated by the Schottky mechanism in medium/high electric field region.