FATIGUE MODELING OF LEAD ZIRCONATE TITANATE THIN-FILMS

Polarization of ferroelectrics decreases as the number of polarization reversals increase. This phenomenon is known as fatigue. Understanding fatigue mechanisms is important for improving the reliability of ferroelectric thin films. A quantitative fatigue model is proposed for lead zirconate titanate (PZT) thin film capacitors on the basis of effective one-directional movement of defects, defect entrapment at electrode-PZT interfaces (and/or at grain and domain boundaries) and the resultant polarization loss at those surfaces under an alternating pulse. An asymmetric polarization may occur on alternating polarity because of asymmetry in electrode-ferroelectric interfaces, and/or non-uniform defect distribution in the bulk. The internal field difference owing to the asymmetric polarization causes an effective one-directional movement of defects such as oxygen vacancies, lead vacancies, mobile impurity ions, etc. These defects can be trapped at electrode-PZT interfaces, grain boundaries, and/or domain boundaries because of their lower potential energies at those sites than those in PZT bulk. This entrapment in turn results in the loss of polarization. In accordance with this fatigue source, a fatigue equation is derived which consists of three constants: initial saturation polarization, piling constant, and decay constant. The predicted values from the model agree very well with the experimental data.