Comparative study of Mn3+ and Mn2+ doping effects on structure and electrical properties of BiFeO3 thin films

BiFeO3 (BFO) thin films doped differently with 5 mol% Mn3+ and 5 mol% Mn2+ were prepared on (111)Pt/Ti/SiO2/Si substrates by a sol–gel method with rapid thermal annealing. Systematic analyses about the two different doping effects and their underlying mechanisms were performed. Pure and doped BFO films were well crystallized in perovskite structure, whilst doping of Mn3+ and Mn2+ resulted in slight distortion in the lattice structure. After doping, smaller grain size, diminished structural defects and denser microstructure were confirmed. Both Mn3+ and Mn2+ dopants could reduce the concentration of oxygen vacancies in BFO films, whereas Mn3+doping exhibited better ability. As a result, the Mn3+ doped film showed the highest dielectric permittivity among those three samples, εr=105 at 100 kHz; meanwhile, the Mn3+ doped film showed a superior 2Pr value of 79.5 µC/cm2, compared to 63.2 µC/cm2 for the Mn2+ doped film and unachievable intrinsic ferroelectric polarizations in the undoped BFO film because of high leakage contribution. The leakage current density in medium/high electric field region was dramatically decreased by Mn doping, accompanied with a change of conduction mechanism from the space charge limited conduction to the Schottky mechanism.