研究顯示，摻雜劑、氧空位及應力等因素對穩定O相至關重要。盡管單一因素難以穩定O相，多因素聯合作用被認為是實現高性能鉿基薄膜的可行途徑。

來自廣東工業大學物理與光電工程學院蔣艷平副教授和西安電子科技大學周益春教授領導的團隊，系統研究了氧空位（V_O）、單軸應變和外部電場（E_e）對鉿基晶體能量的單獨及耦合影響。

Fig. 3 Effect of charged VO on crystals.

研究發現，雖然增加V_O的數量能減少鐵電相和單斜相之間的能量差距，但它并不能單獨確保鐵電相成為最穩定的形態。由于自發極化（P_s）與V_O的濃度和電荷狀態密切相關，這一現象揭示了V_O與E_e之間的耦合作用。

Fig. 4 Effects of uniaxial strain and VO coupling on crystal energies.

當氧空位與單軸應變共同作用時，單軸應變可獨立穩定反鐵電相，并提升鐵電相的穩定性。同時，V_O濃度和電荷狀態的提升可以減少為穩定鐵電相所需的應變。此外，單軸壓縮應變能夠提高鐵電相的P_s，從而增強外部電場對相穩定性的影響力。

這些發現指出，在鉿基材料中鐵電相的穩定是一個典型的機械–電–化學耦合過程。當同時考慮氧空位、單軸應變和外部電場時，能夠更容易地實現鐵電相的穩定。

Fig. 6 Effect of mechanical-electrical-chemical coupling on crystal energies.

該研究為解釋鉿基材料中的“喚醒”現象提供了新的解釋，并為實現及維持鉿基材料中的鐵電相提供了理論指導。該文近期發表于npj Computational Materials 9: 219 (2023).

Fig. 7 Phase diagram of hafnia.

Editorial Summary

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.

Fig. 8 Phase diagram of HZO.

A team led 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_Oand 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. This article was recently published in npj Computational Materials 9: 219 (2023).

原文Abstract及其翻譯

Mechanical-electrical-chemical coupling study on the stabilization of a hafnia-based ferroelectric phase (基于鉿氧化物鐵電相穩定性的機械–電–化學耦合研究)

Fenyun Bai,?Jiajia Liao,?Jiangheng Yang,?Yanping Jiang,?Xingui Tang,?Qiuxiang Liu,?Zhenhua Tang?&?Yichun Zhou?

Abstract?The metastable polar orthorhombic phase is believed to be the origin of the ferroelectricity of hafnia-based films. The adjustment of stain, oxygen vacancies and dopant during film deposition and the wake-up electric cycling are common strategies to induce the ferroelectricity in hafnia. However, none of them could independently render the ferroelectric phase to be the most stable phase from the theoretical calculation results. The exact external conditions to stabilize orthorhombic phase still remain elusive. In this paper, we investigate the effects of the type, distribution, concentration, and charge state characteristics of oxygen vacancies and the uniaxial strain on the crystal’ energy, dielectric constant and spontaneous polarization (P_s); In addition, the impact of the applied electric field parallel to the P_s on the crystal’ energy is explored by first-principles calculations. It is challenging to independently stabilize the ferroelectric phase of hafnia-based films by a single component owing to the rather strict conditions. Surprisingly, the ferroelectricity can be easily obtained when simultaneously considering the effects of oxygen vacancies, uniaxial strain, and applied electric fields, suggesting the extremely important mechanical-electrical-chemical coupling effects. This work provides an explanation for the typical wake-up phenomenon in hafnia and a guidance for film applications.

摘要 鉿基薄膜的鐵電性被認為源于其亞穩態的極性斜方相。在薄膜的沉積過程中，調節應變、氧空位和摻雜元素，以及“喚醒”的電循環，是促進鉿基薄膜產生鐵電性的通行做法。然而，從理論計算來看，這些因素單獨作用都無法使鐵電相變為最穩定的構型。斜方相穩定所需的確切外部條件至今仍不明確。本研究深入分析了氧空位的種類、分布、濃度以及電荷狀態的特征，以及單軸應變對晶體能量、介電常數和自發極化（P_s）的影響。此外，我們還利用第一性原理計算，探討了沿自發極化方向的電場對晶體能量的作用。由于實驗條件的嚴苛性，通過單一因素來獨立穩定鉿基薄膜的鐵電相頗具挑戰。出乎意料的是，當氧空位、單軸應變和外加電場的效應同時作用時，鐵電性卻可以較為容易地實現。這一發現凸顯了機械、電學和化學耦合效應的極端重要性。我們的工作為理解鉿基薄膜中常見的“喚醒”現象提供了新的視角，并為未來薄膜材料的應用提供了寶貴指導。