Programming-Pulse Dependence of Ferroelectric Partial Polarization: Insights From a Comparative Study of PZT and HZO Capacitors

Abstract

The programming of partial polarization in ferroelectric (FE) hafnium zirconate (HZO) capacitors is shown to depend on the delay between FE reset and the polarization write pulse (delay-before-write), revealing a subtle history dependence. Programming is substantially independent of delays inserted after the write and before the read (delay-before-read), with no decay in partial polarization observed from $1~\mu \text{s}$ up to 260 ms. We further show that nonsaturation in FE polarization can be introduced if the pulse protocol is not strictly symmetric, where strictly symmetric means that the integral of the voltage waveform over a single measurement cycle is zero. The delay-before-write dependence in HZO is compared with lead-zirconium-titanate (PZT) capacitors used in FE random access memory (FRAM) which shows much weaker dependence. The delay dependence is attributed to the dynamics of subcritical nuclei formation in nucleation limited switching. For the first time, the decay of subcritical nuclei in HZO is shown to be observable up to 1 s, and an upper bound on their volume is estimated to be ~10 nm3, corresponding to $\sim {4}\times {4} \times {4}$ unit cells.