Abstract：
Bottom-up growth offers precise control over the structure and geometry of semiconductor nanowires (NWs), enabling a wide range of possible shapes and seamless heterostructures for applications in nanophotonics and electronics. The most common vapor-liquid-solid (VLS) growth method features a complex interaction between the liquid metal catalyst droplet and the anisotropic structure of the crystalline NW, and the growth is mainly orchestrated by the triple-phase line (TPL). Despite the intrinsic mismatch between the droplet and the NW symmetries, its discussion has been largely avoided because of its complexity, which has led to the situation when multiple observed phenomena such as NW axial asymmetry or the oscillating truncation at the TPL still lack detailed explanation. The introduction of an electric field control of the droplet has opened even more questions, which cannot be answered without properly addressing three-dimensional (3D) structure and morphology of the NW and the droplet. This work describes the details of electric-field-controlled VLS growth of germanium (Ge) NWs using environmental transmission electron microscopy (ETEM). We perform TEM tomography of the droplet-NW system during an unperturbed growth, then track its evolution while modulating the bias potential. Using 3D finite element method (FEM) modeling and crystallographic considerations, we provide a detailed and consistent mechanism for VLS growth, which naturally explains the observed asymmetries and features of a growing NW based on its crystal structure. Our findings provide a solid framework for the fabrication of complex 3D semiconductor nanostructures with ultimate control over their morphology.
摘要：
自下而上的生长提供了对半导体纳米线（NWs）的结构和几何形状的精确控制，使广泛的可能的形状和无缝异质结构的应用在纳米光子学和电子学。最常见的气-液-固（VLS）生长方法具有液态金属催化剂液滴与晶体NW的各向异性结构之间的复杂相互作用，增长主要由三相线(TPL)协调。尽管液滴和NW对称性之间存在固有的不匹配，由于它的复杂性，它的讨论在很大程度上被避免了，这导致了多种观察到的现象，例如NW轴向不对称或TPL的振荡截断，仍然缺乏详细的解释。引入液滴的电场控制引发了更多的问题，如果没有正确解决NW和液滴的三维（3D）结构和形态，就无法回答。这项工作描述了使用环境透射电子显微镜（ETEM）对锗（Ge）NW进行电场控制的VLS生长的细节。我们在不受干扰的生长过程中对液滴NW系统进行TEM层析成像，然后在调节偏置电势的同时跟踪其演化。使用3D有限元方法(FEM)建模和晶体学考虑，我们为VLS增长提供了详细且一致的机制，这自然地解释了基于其晶体结构的生长的NW的观察到的不对称性和特征。我们的发现为制造复杂的3D半导体纳米结构提供了坚实的框架，并最终控制其形态。
