超铁电候选体LiZnSb在GaSb（111）B上外延生长过程中的多晶型控制
Control of polymorphism during epitaxial growth of hyperferroelectric candidate LiZnSb on GaSb (111)B
摘要：
铁电器件面临的一个主要挑战是去极化场，它与铁电薄膜的极限范围内的长程极性序竞争并经常破坏它们。最近的理论预测表明，一类新的材料，称为超铁电，这应该是强大的对退极化场，使铁电性下降到单层极限。在这里，我们展示了超铁电候选材料之一的六方晶系LiZnSb的外延生长，在GaSb（111）B衬底上的分子束外延。由于所有三种原子物种的高挥发性，我们发现，LiZnSb可以生长在一个吸附控制的窗口，使用过量的锌通量。在该窗口内，所需的极性六方相相对于竞争的立方多晶型物是稳定的，如通过X射线衍射和透射电子显微镜测量所揭示的。第一性原理计算表明，对于适度量的外延应变和适度浓度的Li空位，立方LiZnSb相的形成能比六方相低，但每个分子式单元仅低几毫电子伏。因此，我们认为动力学在低温下稳定所需的六方相方面发挥着作用。我们的研究结果提供了一个路径，实验证明铁电性和超铁电性在一类新的三元金属间化合物。
英文摘要：
A major challenge for ferroelectric devices is the depolarization field, which competes with and often destroys long-range polar order in the limit of ultrathin films. Recent theoretical predictions suggest a new class of materials, termed hyperferroelectics, that should be robust against the depolarization field and enable ferroelectricity down to the monolayer limit. Here we demonstrate the epitaxial growth of hexagonal LiZnSb, one of the hyperferroelectric candidate materials, by molecular-beam epitaxy on GaSb (111)B substrates. Due to the high volatility of all three atomic species, we find that LiZnSb can be grown in an adsorption-controlled window, using an excess zinc flux. Within this window, the desired polar hexagonal phase is stabilized with respect to a competing cubic polymorph, as revealed by X-ray diffraction and transmission electron microscopy measurements. First-principles calculations show that for moderate amounts of epitaxial strain and moderate concentrations of Li vacancies, the cubic LiZnSb phase is lower in formation energy than the hexagonal phase, but only by a few meV per formula unit. Therefore we suggest that kinetics plays a role in stabilizing the desired hexagonal phase at low temperatures. Our results provide a path towards experimentally demonstrating ferroelectricity and hyperferroelectricity in a new class of ternary intermetallic compounds.
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