Correlation between structural and optical properties of single semiconductor nanowires with core-shell heterostructure

半导体纳米线已成为研究基础物理和制造新型器件的重要材料基础。在纳米线几何结构中，应变可以在自由侧壁处弹性松弛，因此不同材料的异质结构可以以晶体质量生长，这在平面薄膜中是不可能获得的。同时，纳米线的准一维形状、纳米尺寸和高表面体积比可以产生意想不到的效果。为了理解、预测和控制纳米线的物理性质，必须阐明感兴趣的性质如何依赖于纳米线的结构。该项目的总体目标是将单个砷化镓纳米线的光学和结构特性与径向（In,Ga）As/GaAs量子阱联系起来。对于这种纳米线异质结构，由于晶格失配引起的应变在（in,Ga）As量子阱和GaAs核心和外壳之间共享，因此与平面异质结构相比，形成位错的临界厚度更大。此外，纳米线异质结构的光学跃迁预计会有所不同，因为量子阱的应变状态被修改，能带结构受到由于费米水平在纳米线侧壁上钉住而引起的侧向能带弯曲的影响，并且整个结构本质上是三维的。雄心勃勃的实验目标是随后测量同一条纳米线的光学和结构特性，并对具有系统结构变化的一系列样品进行此类测量。样品将通过分子束外延生长，光学性质将通过光和阴极发光光谱研究，结构将通过x射线纳米衍射分析。为了更深入地了解结构-性能的相关性，实验将辅以连续弹性和多波段k。光学跃迁的P计算。我们将识别观测到的发射线，将它们分配给结构特征，并阐明这些线如何随In含量和/或量子阱厚度变化。

Semiconductor nanowires have emerged as an important materials basis both for studying fundamental physics and for fabricating novel devices. In the nanowire geometry, strain can elastically relax at the free sidewalls, and thus heterostructures of dissimilar materials can be grown in a crystalline quality that is impossible to obtain in planar thin films. At the same time, the quasi one-dimensional shape, nanometric size, and high surface-to-volume ratio of nanowires can lead to unexpected effects. In order to understand, predict, and control the physical properties of nanowires, it is imperative to elucidate how the properties of interest depend on the nanowire structure. The overall objective of this project is to correlate the optical and structural properties of single GaAs nanowires with radial (In,Ga)As/GaAs quantum well. For such nanowire heterostructures a larger critical thickness for the formation of dislocations is expected than in their planar counterparts, because the strain induced by lattice mismatch is shared between (In,Ga)As quantum well and GaAs core and outer shell. Also, the optical transitions are anticipated to be different for the nanowire heterostructure, because the strain state of the quantum well is modified, the band structure is affected by lateral band bending due to Fermi level pinning at the nanowire sidewalls, and the nature of the entire structure is intrinsically three-dimensional.The ambitious experimental goal is to measure subsequently for one and the same single nanowire the optical and structural properties, and carry out such measurements for series of samples with systematic variations in structure. Samples will be grown by molecular beam epitaxy, the optical properties will be investigated by photo- and cathodoluminescence spectroscopy, and the structure will be analyzed by x-ray nanodiffraction. In order to obtain a deep understanding of the structure-property correlation, experiments will be complemented by continuum elasticity and multiband-k.p calculations of the optical transitions. We will identify the observed emission lines, assign them to structural features, and elucidate how these lines vary with In content and/or quantum well thickness.