Solution processing of morphotropic phase boundary BiFeO3-PbTiO3 thin films with reduced conductivity for high room temperature switchable polarization

The (1 – x)BiFeO₃-xPbTiO₃ (BFO-PTO) perovskite solid solution has great potential for being used in practical devices, as it exhibits significant ferroelectric response at its morphotropic phase boundary (MPB). However, the significant conduction, particularly high electrical leakage currents that BFO-PTO films show at room temperature, deteriorates their functionality. This is mainly associated with the presence of multivalent iron along with A-site and oxygen vacancies. Here, solution-derived BFO-PTO thin films have been crystallized at low temperature on Pt/TiO₂/SiO₂/Si substrates, by a rapid thermal annealing process to minimize Bi and Pb volatilization. X-ray analyses have revealed that textured films are obtained with a pseudocubic perovskite structure and without the formation of detectable second phases. Microstructural studies indicated a columnar growth of the films with grain size well above the nanometric range, which, therefore, should not produce an appreciable reduction of the ferroelectric response due to size effects. Because of the relatively low content of charged defects produced in these BFO-PTO films during processing, ferroelectric hysteresis loops can be measured at room temperature. The highest value of remnant polarizations (2P_R = 58 μC/cm²) was obtained for the 0.65BiFeO₃-0.35PbTiO₃ (65BFO-35PTO) films, which suggests that this film composition lies in the proximity of the MPB where the coexistence of a highly textured <100> tetragonal phase and a rhombohedral one seems to occur.