Stacking direction dependent Raman mode, wake-up alleviation, and coercive-field reduction in HfO2/ZrO2 superlattices

Abstract

Ferroelectric superlattices (SLs) are emerging as a cutting-edge solution to various challenges, such as lowering the coercive field, increasing polarization and dielectric constant, mitigating wake-up, and enhancing endurance. However, a comprehensive atomic-scale understanding and predictive design framework for these SLs still requires further investigation. Herein, based on group theory and first-principles simulations, we explored stacking direction dependent properties of HfO2/ZrO2 (HZO) SLs and established a strong relationship between Raman-mode frequencies and the SL stacking direction, which further highlights that Raman modes can be used as effective descriptors for uncovering the local ordering in such SLs. Additionally, we discovered that a cell-doubling distortion mode intrinsic to the [010] HZO SL leads to the emergence of an intermediate orthorhombic Pmn21-like phase during the transitions between the antiferroelectric P42/nmc tetragonal and ferroelectric Pca21 orthorhombic phases. This intermediate phase remarkably reduces the energy barrier between the two states, playing a pivotal role in achieving nearly wake-up-free ferroelectricity. Moreover, the reduction in this barrier decreases the energy required for ferroelectric polarization switching, lowering the coercive field while retaining the polarization magnitude of bulk HfO2. Our findings provide an opportunity for the development of next-generation low-voltage memory devices.