The structure and degradation mechanism of ferroelectric SrBi2Ta2O9 thin films

Ferroelectric bismuth layer oxide SrBi2Ta2O9 (SBT) thin films mere prepared on Pt/Ti/SiO2/Si substrates by pulsed laser deposition technique. The films were deposited at the substrate temperature of 540 degreesC and annealed at 800 degreesC for 1 hr in oxygen ambient. The composition of mixed powder was Sr0.8Bi2.4Ta2.0O9, but the film annealed 800 degreesC display Sr0.17Bi2.4Ta2.0O9 measured by electron probe micro analyzer (EPMA). The crystalline fluorite phase was maintain in the annealing temperature from 540 to 750 degreesC and SET phase appeared at 750 degreesC, crystallized at 800 degreesC, 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 degreesC to 800 degreesC. The SET film annealed at 800 degreesC showed 2P(r) = 12.6 mu C/cm(2), 2E(c) = 125kV/cm at applied voltage of 5 V, and the hysteresis loops became saturated at 2V. The fatigue characteristics of SET 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 (similar to3 V). The signal/noise ratio had maximum value of 5 at 3 V, which was capable of low-voltage operation in NVFRAM device.