The effect of imprint on remanent piezoelectric properties and ferroelectric aging of PbZr0.52Ti0.48O3 thin films

Ferroelectric films suffer from both aging and degradation under high ac-field drive conditions due to loss of polarization with time. In this study, the roles of defect chemistry and internal electric fields on the long-term stability of the properties of piezoelectric films were explored. For this purpose, lead zirconate titanate (PZT) films with a Zr/Ti ratio of 52/48 doped with Mn- (PMZT) or Nb- (PNZT) were deposited on Pt coated Si substrates by the sol-gel method. It was demonstrated that the magnitude of the internal field is much higher in PMZT films compared to PNZT films after poling in the temperature range of 25-200 degrees C under an electric field of -240kV/cm. The development of the internal field is thermally activated, with activation energies from 0.5 +/- 0.06 to 0.8 +/- 0.1eV in Mn doped films and from 0.8 +/- 0.1 to 1.2 +/- 0.2eV in Nb doped films. The different activation energies for imprint suggests that the physical mechanism underlying the evolution of the internal field in PMZT and PNZT films differs; the enhanced internal field upon poling is attributed to (a) alignment of oxygen vacancy-acceptor ion defect dipoles ((MnTi ''-VO)x, (MnTi '-VO)') in PMZT films, and (b) thermionic injection of electron charges and charge trapping in PNZT films. In either case, the internal field reduces back switching, enhances the remanent piezoelectric properties, and dramatically improves the aging behavior. PMZT films exhibited the greatest enhancement, with reduced high temperature (180 degrees C) aging rates of 2%-3%/decade due to improved stability of the poled state. In contrast, PNZT films showed significantly larger high temperature aging rates (15.5%/decade) in the piezoelectric coefficient, demonstrating that the fully poled state was not retained with time.