Zhang Qian, Voorhees Peter W, Davis Stephen H
Department of Engineering Sciences and Applied Mathematics, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3125, USA.
Department of Engineering Sciences and Applied Mathematics, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3125, USA; Department of Materials Science and Engineering, Northwestern University, 2225 Campus Drive, Evanston, Illinois 60208-3030, USA.
Beilstein J Nanotechnol. 2017 Feb 24;8:506-513. doi: 10.3762/bjnano.8.54. eCollection 2017.
Heterostructured GaAs-AlGaAs core-shell nanowires with have attracted much attention because of their significant advantages and great potential for creating high performance nanophotonics and nanoelectronics. The spontaneous formation of Al-rich stripes along certain crystallographic directions and quantum dots near the apexes of the shell are observed in AlGaAs shells. Controlling the formation of these core-shell heterostructures remains challenging. A two-dimensional model valid on the wire cross section, that accounts for capillarity in the faceted surface limit and deposition has been developed for the evolution of the shell morphology and concentration in Al Ga As alloys. The model includes a completely faceted shell-vapor interface. The objective is to understand the mechanisms of the formation of the radial heterostructures (Al-rich stripes and Al-poor quantum dots) in the nanowire shell. There are two issues that need to be understood. One is the mechanism responsible for the morphological evolution of the shells. Analysis and simulation results suggest that deposition introduces facets not present on the equilibrium Wulff shapes. A balance between diffusion and deposition yields the small facets with sizes varying slowly over time, which yield stripe structures, whereas deposition-dominated growth can lead to quantum-dot structures observed in experiments. There is no self-limiting facet size in this case. The other issue is the mechanism responsible for the segregation of Al atoms in the shells. It is found that the mobility difference of the atoms on the {112} and {110} facets together determine the non-uniform concentration of the atoms in the shell. In particular, even though the mobility of Al on {110} facets is smaller than that of Ga, Al-rich stripes are predicted to form along the {112} facets when the difference of the mobilities of Al and Ga atoms is sufficiently large on {112} facets. As the size of the shell increases, deposition becomes more important. The Al-poor dots are obtained at the apices of {112} facets, if the attachment rate of Al atoms is smaller there.
具有异质结构的GaAs - AlGaAs核壳纳米线因其显著优势以及在制造高性能纳米光子学和纳米电子学方面的巨大潜力而备受关注。在AlGaAs壳层中观察到沿特定晶体学方向自发形成富铝条纹以及在壳层顶端附近形成量子点。控制这些核壳异质结构的形成仍然具有挑战性。针对AlGaAs合金中壳层形态和浓度的演变，已经开发了一个在纳米线横截面上有效的二维模型，该模型考虑了刻面表面极限中的毛细作用和沉积。该模型包括一个完全刻面的壳 - 气相界面。目的是了解纳米线壳层中径向异质结构（富铝条纹和贫铝量子点）的形成机制。有两个问题需要理解。一个是负责壳层形态演变的机制。分析和模拟结果表明，沉积引入了平衡Wulff形状上不存在的刻面。扩散和沉积之间的平衡产生了尺寸随时间缓慢变化的小刻面，从而产生条纹结构，而以沉积为主导的生长可导致实验中观察到量子点结构。在这种情况下不存在自限性刻面尺寸。另一个问题是负责壳层中Al原子偏析的机制。发现{112}和{110}面上原子的迁移率差异共同决定了壳层中原子的不均匀浓度。特别是，即使Al在{110}面上的迁移率小于Ga，但当Al和Ga原子在{112}面上的迁移率差异足够大时，预计会沿{112}面形成富铝条纹。随着壳层尺寸的增加，沉积变得更加重要。如果Al原子在{112}面顶端的附着率较小，则会在这些顶端获得贫铝点。
