In-depth Understanding of Polarization Switching Kinetics in Polycrystalline $\mathbf{Hf}_{0.5} \mathbf{Zr}_{0.5} \mathbf{O}_{2}$ Ferroelectric Thin Film: A Transition From NLS to KAI

To achieve deep insights into the polarization switching kinetics in ferroelectric $\text{Hf}0.5\text{Zr}0.5\mathrm{O}_{2}$ (HZO), the intrinsic switching characteristics are measured accurately by using a novel pulse sequence to eliminate the charge trapping, polarization unsaturation and imprint effects. By NLS fittings, the average switching time and the concentration of pinning sites are extracted, with main focus on their dependences on the external electric field, the electrode size $l$ and the temperature $T$. Although the average switching time shows weak dependence on $T,$, it decreases sharply with $l$ scaling and sub-ns switching behavior is predicted at $l\leq 3.89\mu \mathrm{m}$. More importantly, our observations intensely imply that a transition happens from NLS to KAI model when $T$ is below 161K or $l$ is approaching the grain size (10~30nm). This surprised but reasonable transition indicates that, as scaling the ferroelectric memory size or for operations under cryogenic temperatures, HZO ferroelectrics can switch much faster and more uniform.

I. Introduction

Owing to the robust ferroelectricity uncovered in HfO2-based high-k transition metal oxide, the nonvolatile ferroelectric memory is entering an unprecedented period of rapid development [1], [2]. Due to its anisotropic crystal structure and the existence of the so-called spacer, the polarization switching process of Hf02 is reported to be different from that of the conventional perovskite-based ferroelectrics [3]. The polarization switching kinetics can be used to predict the switching process and the switching uniformity. The classical Kolmogorov-Avrami-Ishibashi (KAI) model, which accurately describes the switching kinetics of perovskite-based single crystal, fails to reach consistency with the kinetics of HfO2-based ferroelectrics due to a constant definition of the switching time. At the same time, the nucleation limited switching (NLS) model considering a distribution of switching time is just suitable for Hf02-based ferroelectrics because of the intrinsic nature of polycrystal [4]. Recently, the switching kinetics of HfO2-based ferroelectrics were focused on the wake-up, the fatigue and the sub-loop behavior, which revealed regular distributions of switching time at different stages [5], [6]. Besides, the polycrystalline properties in Hf02-based thin films possibly result in abnormal switching kinetics at rather small electrode sizes and low temperatures. However, these limiting factors are not well discussed in detail.