The wurtzite structure is one of the most frequently found crystal structures in modern semiconductors and its inherent spontaneous polarization is a defining materials property. Despite this significance, confusion has been rampant in the literature with respect to the orientation of the spontaneous polarization inside the unit cell of the wurtzite structure, especially for the technologically very relevant III-N compounds (AlN, GaN, and InN). In particular, the spontaneous polarization has been reported to either point up or down for the same unit cell orientation, depending on the literature source—with important implications for, e.g., the carrier type and density expected at interfaces of heterostructures involving materials with the wurtzite structure. This perspective aims to resolve this ambiguity by reviewing available reports on the direction of the energetically preferred polarization direction in the presence of external electric fields as well as atomically resolved scanning transmission electron microscopy images. While we use ferroelectric wurtzite Al1−xScxN as a key example, our conclusions are generalizable to other compounds with the same crystal structure. We demonstrate that a metal-polar unit cell must be associated with an upward polarization vector—which is contrary to long-standing conventional wisdom.

中文翻译：

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纤锌矿结构晶体中自发极化方向的澄清


纤锌矿结构是现代半导体中最常见的晶体结构之一，其固有的自发极化是一种决定性的材料特性。尽管具有这一重要性，但文献中关于纤锌矿结构晶胞内自发极化方向的困惑仍然很严重，特别是对于技术上非常相关的 III-N 化合物（AlN、GaN 和 InN）。特别是，据报道，对于相同的晶胞方向，自发极化可以向上或向下，具体取决于文献来源，这对于例如涉及纤锌矿材料的异质结构界面处预期的载流子类型和密度具有重要意义结构。该观点旨在通过审查有关外部电场存在下的能量优选偏振方向的现有报告以及原子分辨的扫描透射电子显微镜图像来解决这种模糊性。虽然我们使用铁电纤锌矿 Al1−xScxN 作为关键示例，但我们的结论可以推广到具有相同晶体结构的其他化合物。我们证明金属极性晶胞必须与向上的极化矢量相关——这与长期以来的传统观点相反。