Ferroelectric properties of lanthanum-doped bismuth titanate thin films grown by a sol-gel method

Lanthanum-doped bismuth titanate, Bi3.25La0.75Ti3O12 (BLT), thin films were grown on Pt(111)/Ti/SiO2/Si(100) substrates by a sol-gel spin coating process followed by annealing at 550-700 °C for crystallization of the thin films. From the X-ray diffraction (XRD) studies, it was found that the ratio of c-axis oriented grains in the annealed BLT thin films strongly depends on the annealing temperature. The remanent polarization (2Pr) and the coercive field (2Ec) values of the BLT thin film capacitor annealed at 650 °C for 30 min were approximately 70 μC/cm2 and 132 kV/cm at electric field of 200 kV/cm, respectively. The current-voltage characteristics were found to be an Ohmic conduction at low voltage region and a space charge conduction at high voltage region. The dipole polarization and the leakage current of the BLT thin film capacitor were interpreted by introducing charge traps and charge injections. Also, the BLT thin film capacitor annealed at 650 °C exhibited a good fatigue endurance under bipolar pulse up to 4.5×1010 read/write cycles. From the results, the lanthanum-doped BIT thin film should be considered seriously for an environmentally safe lead-free ferroelectric material with an excellent ferroelectricity.