Fatigue and switching in ferroelectric memories: Theory and experiment

Abstract

A theoretical model of fatigue in ferroelectric thin‐film memories based upon impact ionization (e.g., Ti⁺⁴ to Ti⁺³ conversion in PbZr_1−xTi_xO₃), resulting in dendritic growth of oxygen‐deficient filaments, is presented. The predictions of spontaneous polarization versus switching cycles P_s(N) are compared with both Monte Carlo simulations for a two‐dimensional Ising model and with experimental data on small‐grain (40 nm) sol‐gel PZT films. Excellent agreement between theory and experiment is obtained. In addition to modeling the P_s(N) curves, the theory developed explains the observed linear proportionality between switching time t_s(N) and polarization P_s(N) during fatigue; other models of aging do not account for this. Earlier theories of switching are also extended to include finite grain sizes, surface nucleation, triangular drive pulses, and dipolar forces. Good agreement with sol‐gel PZT switching data is obtained.