Charge carrier dynamics under the influence of extreme strain gradients realized in bent semiconductor nanowires

应变工程是定制半导体电子结构的一种成熟方法。在传统的平面薄膜异质结构中，晶格失配会导致均匀的双轴应变。这个概念的推广就是非均匀应变。这种应变场在三维纳米结构中形成，具有应变梯度。然而，关于它们对半导体物理的影响仍有许多悬而未决的问题。该项目的总体目标是深入了解极端应变梯度影响下的载流子动力学。为了实现这种梯度，我们将以接近弹性极限的可控方式弯曲独立的半导体纳米线。我们的重点是由直接半导体砷化镓组成的纳米线，我们已经证明了这种弯曲。为此，晶格不匹配的壳只沉积在纳米线的一侧。应变梯度会导致影响受激载流子动力学的各种现象。首先，变形势相互作用导致带隙中的梯度作为准电场。其次，应变引起压电极化。第三，应变梯度引起挠曲电极化。后一种效应对载流子动力学的影响迄今在很大程度上被忽略了。我们的目标是揭示这些现象的相互作用，并确定迄今为止未知的砷化镓的相关挠曲电系数。为了系统地研究载流子动力学，我们将在专门的样品系列上进行光和阴极发光实验，并辅以k·p模拟。对于发光数据的分析，关于纳米线的应变和结构的精确信息是至关重要的。这些参数将由纳米x射线衍射测定。然而，强弯曲晶体上的x射线衍射代表了未知的领域，因此我们将首先开发相应的方法。这一成果将具有独立于载流子动力学研究的科学价值。最后，通过结合纳米x射线分析和同步激光脉冲，我们将研究电荷载流子密度的变化如何通过反向压电和挠曲电效应影响纳米线的弯曲。此外，我们将激发奈米线产生强烈的机械振荡，从而使其动态弯曲。这样，我们将产生一个连续变化的弯曲半径，为进一步的实验。

Strain engineering is an established approach to tailor the electronic structure of semiconductors. In conventional planar thin film heterostructures, a lattice mismatch leads to homogeneous biaxial strain. A generalization of this concept is inhomogeneous strain. Such strain fields form in three-dimensional nanostructures with strain gradients. However, there are still many open questions regarding their influence on semiconductor physics. The overall goal of this project is to gain a deep understanding of the charge carrier dynamics under the influence of extreme strain gradients.To realize such gradients, we will bend free-standing semiconductor nanowires in a controlled way close to the elastic limit. We focus on nanowires consisting of the direct semiconductor GaAs, for which we have already demonstrated such a bending. For this purpose, a lattice-mismatched shell is deposited on only one side of the nanowires.Strain gradients lead to various phenomena that influence the dynamics of excited charge carriers. First, the deformation potential interaction leads to a gradient in the band gap that acts as a quasi-electric field. Second, strain induces a piezoelectric polarization. Third, the strain gradient causes a flexoelectric polarization. The influence of the latter effect on charge carrier dynamics has so far been largely ignored. We aim to unravel the interplay of these phenomena and to determine the hitherto unknown relevant flexoelectric coefficient of GaAs. To systematically study the charge carrier dynamics, we will perform photo- and cathodoluminescence experiments on dedicated sample series, supplemented by k·p simulations. For the analysis of the luminescence data, precise information about strain and structure of the nanowires is crucial. These parameters will be determined by nano x-ray diffraction. X-ray diffraction on strongly bent crystals, however, represents uncharted territory, so that we will first develop a corresponding methodology. This achievement will be of scientific value in its own right, independent of the study of charge carrier dynamics.Finally, by combining nano x-ray analysis with synchronized laser pulses, we will investigate how a change in charge carrier density influences the bending of nanowires via the converse piezo- and flexoelectric effects. In addition, we will excite nanowires to strong mechanical oscillations and thus bend them dynamically. In this way, we will generate a continuous variation of the bending radius for further experiments.