Electrical properties of ferroelectric thin-film capacitors with hybrid (Pt,RuO₂) electrodes for nonvolatile memory applications

For Pb(Zr_xTi_1-x)O₃ (PZT) thin-film capacitors to be used in ferroelectric nonvolatile memories, they must have low polarization fatigue and low leakage currents. PZT capacitors fabricated in our laboratory with RuO₂ electrodes exhibit excellent polarization fatigue characteristics, but they have large and variable leakage currents (typically 10^-3-10^-4 A/cm² at 1 V). On the other hand, PZT capacitors with Pt electrodes have low leakage currents (typically <10^-9 A/cm² at 1 V), but they undergo severe polarization fatigue. New (Pt,RuO₂) hybrid electrodes which result in PZT capacitors that combine the excellent fatigue behavior of RuO ₂/PZT/RuO₂ with the low leakage currents of Pt/PZT/Pt capacitors have been developed. The hybrid electrodes studied are of two main types: one type consisted of Pt/RuO₂ or RuO₂/Pt double layers, while the other consisted of a codeposited Pt-RuO₂ layer. All capacitors discussed here had an RuO₂/PZT/hybrid electrode/MgO heterostructure. It will be shown that capacitors with negligible fatigue (up to 10¹¹ switching cycles) and with leakage currents that are two to four orders of magnitude lower than those of RuO₂/PZT/RuO₂ capacitors can be achieved. In addition, the capacitors with hybrid electrodes have very small retention loss.