Unveiling the ferroelectricity of hafnium-based thin films: Multifactorial coupling

Hafnium-based thin films have garnered attention for their compatibility with CMOS processes and impressive ferroelectric properties, yet the origin of their ferroelectricity remains unclear; it is generally believed to stem from the Pca21 space group phase (O-phase), which is inherently unstable in its natural state. Research indicates that factors such as dopants, oxygen vacancies, and stress are crucial for stabilizing the O-phase. Although a single factor may be insufficient to stabilize the O-phase, the combined action of multiple factors is considered to be a viable approach to achieving high-performance hafnium-based thin films. 

A team lead by Prof. Yanping Jiang from School of Physics and Optoelectronic Engineering, Guangdong University of Technology and Prof. Yichun Zhou from School of Advanced Materials and Nanotechnology, Xidian University, systematically investigated the individual and combined effects of the V_O, uniaxial strain, and the E_e on the crystal energy of hafnia-based. The increase of V_O could reduce the energy differences between ferroelectric and monoclinic phase, but could not render the ferroelectric phase as the most stable one. Since the P_s shows a dependency on the concentration and charge state of V_O, it indicates that there is a coupling effect between the V_O and E_e. When both V_O and uniaxial strain are present, the uniaxial strain can independently stabilize the antiferroelectric phase and promote the stabilization of the ferroelectric phase, and the increase in the V_O concentration and charge state reduces the strain demand in stabilizing the ferroelectric phase. In addition, the uniaxial compressive strain increases the P_s of the ferroelectric phase, which will enhance the effect of the E_e on the phase stability. These indicate that the stabilization of ferroelectric phase in hafnia is a typical mechanical-electrical-chemical coupling situation. When considering V_O, uniaxial strain and E_e simultaneously, it will achieve the purpose of stabilizing the ferroelectric phase easily. This work provides an explanation for the typical wake-up effect and theoretical guidance to obtain and stabilize ferroelectric phase in hafnia. 

编辑概述

揭秘铪基薄膜铁电性：多因素耦合决定结构稳定性

铪基薄膜因其与CMOS工艺的兼容性及优异的铁电性能受到关注，但其铁电性起源尚不明确，一般认为这种性质源自于空间群Pca21相（O相），然而O相在自然状态下并不稳定。研究显示，掺杂剂、氧空位及应力等因素对稳定O相至关重要。尽管单一因素难以稳定O相，多因素联合作用被认为是实现高性能铪基薄膜的可行途径。

来自广东工业大学物理与光电工程学院蒋艳平副教授和西安电子科技大学周益春教授领导的团队，系统研究了氧空位（V_O）、单轴应变和外部电场（E_e）对铪基晶体能量的单独及耦合影响。研究发现，虽然增加V_O的数量能减少铁电相和单斜相之间的能量差距，但它并不能单独确保铁电相成为最稳定的形态。由于自发极化（P_s）与V_O的浓度和电荷状态密切相关，这一现象揭示了V_O与E_e之间的耦合作用。当氧空位与单轴应变共同作用时，单轴应变可独立稳定反铁电相，并提升铁电相的稳定性。同时，V_O浓度和电荷状态的提升可以减少为稳定铁电相所需的应变。此外，单轴压缩应变能够提高铁电相的P_s，从而增强外部电场对相稳定性的影响力。这些发现指出，在铪基材料中铁电相的稳定是一个典型的机械-电-化学耦合过程。当同时考虑氧空位、单轴应变和外部电场时，能够更容易地实现铁电相的稳定。该研究为解释铪基材料中的“唤醒”现象提供了新的解释，并为实现及维持铪基材料中的铁电相提供了理论指导。