The structure and degradation mechanism of ferroelectric SrBi2 Ta2O9 thin films

Ferroelectric bismuth layer oxide SrBi₂Ta₂O ₉ (SBT) thin films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition technique. The films were deposited at the substrate temperature of 540°C and annealed at 800°C for 1 hr in oxygen ambient. The composition of mixed powder was Sr_0.8Bi_2.4Ta_2.0O₉, but the film annealed 800°C display Sr_0.7Bi_2.4Ta _2.0O₉ measured by electron probe micro analyzer (EPMA). The crystalline fluorite phase was maintain in the annealing temperature from 540 to 750°C and SBT phase appeared at 750°C, crystallized at 800°C, which was observed in the X-ray diffraction patterns and scanning electron microscopy images. The grains, which were spherical-like, increased from about 50 to 250 nm in diameter with increasing annealing temperature from 540°C to 800°C. The SBT film annealed at 800°C showed 2P_r=12.6μ C/cm² , 2E_c=125 kV/cm at applied voltage of 5 V, and the hysteresis loops became saturated at 2 V. The fatigue characteristics of SBT thin films with various applied voltage and frequency have also been investigated. It revealed that the polarization fatigue occurred with decreasing switching voltage and frequency. This significant polarization fatigue was originated due to partial switching when the switching voltage was lower than saturation voltage (~3 V). The signal/noise ratio had maximum value of 5 at 3 V, which was capable of low-voltage operation in NVFRAM device