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HfO₂最早因其高介电常数在半导体工业中备受关注。自2011年被发现具有铁电性以来，其研究热点逐步转向这种铁电材料在CMOS兼容设备中的应用。然而，HfO₂的极性相在热力学上具有亚稳特性，体相图中并没有极性相，主要以非极性的单斜相为基态。这种特性严重限制了HfO₂作为功能性铁电材料的实际应用，因为只有在特定的动力学条件或外延应变作用下才能稳定极性铁电相。因此，如何稳定铁电相HfO₂，一直是该领域的一大挑战。文章中已有诸多有关如何稳定铁电相HfO₂的报道，包括氧空位、掺杂剂的种类和浓度、表面能最小化、猝灭动力学和力学效应等，但结果都不尽人意。
Fig. 1 | Structures of the considered 2/2 X/Hf oIII SL’s superlattices.
来自卢森堡科学技术研究所（LIST）材料研究与技术系的Binayak Mukherjee等人，提出了一种创新的超晶格设计，将HfO₂与其他简单氧化物构成多层超晶格，通过调节超晶格的成分和堆叠方向，实现了铁电相的热力学稳定性。通过第一性原理计算，作者发现，可以通过组合选择极性、反极性或混合极性结构，在实现热力学稳定性的同时，提升材料的极化表现。
Fig. 2 | Structure and sublayer polarization of Ge/Hf and Ti/Hf mixed SLs.
这一工作为未来的铁电薄膜器件提供了重要的设计思路，并为开发新一代高效稳定的铁电材料铺平了道路。该文近期发表于npj Computational Materials 10: 153 (2024)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
First-principles predictions of HfO2-based ferroelectric superlattices
Binayak Mukherjee, Natalya S. Fedorova & Jorge Íñiguez-González
The metastable nature of the ferroelectric phase of HfO2 is a significant impediment to its industrial application as a functional ferroelectric material. In fact, no polar phases exist in the bulk phase diagram of HfO2, which shows a dominant non-polar monoclinic ground state. As a consequence, ferroelectric orthorhombic HfO2 is stabilized either kinetically or via epitaxial strain. Here, we propose an alternative approach, demonstrating the feasibility of thermodynamically stabilizing polar HfO2 in superlattices with other simple oxides. Using the composition and stacking direction of the superlattice as design parameters, we obtain heterostructures that can be fully polar, fully antipolar or mixed, with improved thermodynamic stability compared to the orthorhombic polar HfO2 in bulk form. Our results suggest that combining HfO2 with an oxide that does not have a monoclinic ground state generally drives the superlattice away from this non-polar phase, favoring the stability of the ferroelectric structures that minimize the elastic and electrostatic penalties. As such, these diverse and tunable superlattices hold promise for various applications in thin-film ferroelectric devices.
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