Effects of cation-substitution on the ferroelectric properties of sol-gel derived PZT thin film for FRAM application

Cation-substituted PZT thin films are more desirable for ferroelectric memory applications due to their lower coercive field, higher resistivity and lower dielectric memory-aging rate compared to PZT thin films. The resulting effect of cation-substitution can be explained on the basis of defect chemistry in a perovskite lattice. In this study, La and Nb were chosen as substituents and their effects on fatigue behavior and leakage properties of PZT-base thin films were investigated. La- or Nb-substituted PZT thin films were deposited by the sol-gel processing method on Pt electrode. The Zr/Ti ratio was fixed as 40/60 with the tetragonal perovskite phase. The excess Pb added onto the starting precursor was fixed to 15 wt.%. Each sol-gel process condition and heating process were optimized based upon its thermal analysis result. The surface microstructure, crystallinity, ferroelectric properties, and leakage characteristics were investigated. It is shown through C-V and I-V characteristics that both A-site (La) and B- site (Nb) substituent play a role in development of electrical properties. However, cation substitution induces lattice site defect to maintain a charge neutrality. In case of Nb, Pb-deficient pyrochlore phase was formed due to A-site vacancies (Pb vacancies). PNZT films showed the degradation of ferroelectric properties.