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既定方法。在传统的平面薄膜异质结构中，晶格失配导致均匀的双轴应变。这个概念的一个推广是非均匀应变。这样的应变场在具有应变梯度的三维纳米结构中形成。然而，关于它们对半导体物理的影响，仍然有许多悬而未决的问题。本项目的总体目标是深入了解极端应变梯度影响下的电荷载流子动力学。为了实现这种梯度，我们将以接近弹性极限的受控方式弯曲独立的半导体纳米线。我们专注于由直接半导体GaAs组成的纳米线，我们已经证明了这种弯曲。为了达到这个目的，晶格失配的壳层只沉积在纳米线的一侧。应变梯度导致各种影响激发载流子动力学的现象。首先，变形势相互作用导致带隙中的梯度，其充当准电场。第二，应变引起压电极化。第三，应变梯度引起挠曲电极化。到目前为止，后者对载流子动力学的影响在很大程度上被忽略了。我们的目标是解开这些现象的相互作用，并确定迄今为止未知的GaAs的相关挠曲电系数。为了系统地研究电荷载流子动力学，我们将对专用样品系列进行光致发光和阴极发光实验，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。这些参数将通过纳米X射线衍射确定。然而，强弯曲晶体的X射线衍射是一个未知领域，因此我们将首先发展一种相应的方法。这一成就将在其本身的科学价值，独立的电荷载流子dynamics.Finally的研究相结合的纳米X射线分析与同步激光脉冲，我们将调查如何在电荷载流子密度的变化影响弯曲的纳米线通过匡威压电和flexoelectric效应。此外，我们还将激发纳米线产生强烈的机械振荡，从而动态地使其弯曲。通过这种方式，我们将生成弯曲半径的连续变化，以供进一步的实验。

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.