Nucleation suppression by charge screening on grain boundaries: a kinetic model for bulk imprint in polycrystalline ferroelectric thin films

TL;DR

This work addresses imprint in ferroelectric memories by proposing a bulk imprint mechanism based on grain-boundary charge screening (GBCS) and a phase-field model. It shows that intense local fields at grain boundaries can tune domain-nucleation barriers, reproducing the observed logarithmic time dependence $\Delta E_c = E_0 \ln(1 + t/t_0)$, thermal acceleration, and asymmetric hysteresis branch shifts. The model couples depletion-charge kinetics with trap states to grains via a polycrystalline geometry, revealing how grain size $h_g$, trap density $N_t$, film thickness $h_f$, and trap energy $\Delta E_t$ modulate $E_0$ and imprint dynamics. The results provide a coherent mechanism that aligns with experimental trends and offers guidance for material design and electrode/interface engineering to mitigate imprint, with future work extending to fatigue and elastic-energy effects.

Abstract

The imprint effect, a significant reliability challenge in ferroelectric memories, manifests as a shift in the coercive field during retention and endurance tests, ultimately degrading the usable memory window. \rv{While traditional models attribute imprint primarily to charge screening at the interface between the dead layer and the ferroelectric film, the contribution from grain boundaries has been largely overlooked. This work advances a bulk imprint mechanism by establishing a phase-field model, which demonstrates that the tuning of domain nuclei near grain boundaries via charge screening consistently explains the imprint process and aligns with key experimental trends.} These findings provide novel insights into the imprint process and advance the understanding of reliability issues in ferroelectric memory devices.

Figures (4)

• Figure 1: Schematics for imprint induced by (a) interface charge screening, and (b) grain-boundary charge screening.
• Figure 2: (a)-(f) The polarization field and electric potential before and after imprint for different polarization states; (g)(h) the trap charge density after imprint; (i)(j) the shift of hysteresis loop during the imprint process.
• Figure 3: The probability distribution of the size of the domain nucleus (a,b,c) and the strength of local electric field (d,e,f) for the base case of the bulk imprint model.
• Figure 4: Temporal evolution of coercive field shift ($\Delta E_c$) during imprint of P state ($\Delta E_c<0$) and N state ($\Delta E_c>0$), for six cases. The shaded area representing the standard deviation for 5 random polycrystal systems, while the dotted lines show the fitting using $\Delta E_c = E_0 \ln(1+t/t_0)$. The units for $E_0$ and $t_0$ are MV/cm and $\tau$, respectively.