Charge Carrier Transport Analysis in Radial and Axial Charge-Separating Junctions of III/V Semiconductor Nanowires

现在有各种各样的纳米线设备可以在实验室样品的水平上获得。特别是，那些由化合物半导体材料组成的材料通过在纳米线拓扑中实现同质和异质结构，扩展了电子和光子能力的光谱。然而，总体性能仍然远远低于预期，并决定性地阻碍了商业实施。例如，这适用于将光转换为电能或产生电动泵浦的光发射器甚至激光的能力。到目前为止，纳米线研究忽略了对这些技术固有局限性的研究。然而，为了打开纳米线器件的新市场，甚至取代现有市场上基于平面结构的现有半导体器件，需要进行实质性的质量改进，而不仅仅是数量上的改进。该项目涉及光与纳米线相互作用的内在限制，如径向和轴向纳米线结构中的光-电流转换。该项目旨在找出器件结构中光电性能限制的根源，包括微观分析。这项工作计划需要宏观设备数据和空间高分辨率显微镜数据之间的密切关联。这些器件结合了用于电荷分离的轴向和径向纳米线异质结，包括由MOVPE制造的基于GaAs和InGaP的同质结和异质结。界面形成和复合路径之间的相互关系将通过系统内四尖测量、扫描探针显微镜和光学方法相结合来研究。特别是，我们结合高分辨率扫描隧道显微镜记录了应用于轴向和径向纳米线结构的局部电流-电压特性。光电学性质的研究是使用条纹相机系统进行的。这两种测量技术及其定位都固有地局限于纳米结构，因此需要物理模型。在这里，建模是由仿真软件包SilVaco Atlas执行的。该项目的目的是确定纳米线生长、器件设计、界面形成和表面钝化与纳米线中不同电荷分离pn结的质量之间的定性和定量的相互关系。因此，将提出并论证在光-纳米线相互作用中显著提高光电性能的概念。

A wide variety of nanowire devices are nowadays available at the level of laboratory samples. In particular, those consisting of compound semiconductor materials have extended the spectrum of electronic and photonic capabilities by implementing homo- and heterostructures in nanowire topologies. However, the overall performance remains far below expectations und hampers decisively a commercial implementation. This applies, for example, to the capability to convert light to electrical energy or to produce electrically pumped light emitters or even LASERs. Up to now, nanowire research has neglected the investigation of inherent limitations of these technologies. However, in order to open up new markets for nanowire devices or to even replace established semiconductor devices based on planar structures in existing markets, substantial qualitative rather than only quantitative improvements are required. This project deals with the inherent limitations in the interaction between light and nanowire such as light-current conversion in radial as well as axial nanowire structures.The project aims to identify the origin of the limitations in optoelectronic performance within the device structure, including microscopic analysis. The work program entails an intense correlation between macroscopic device data and spatially highly-resolved microscopy data. The devices incorporate axial and radial nanowire heterojunctions for charge separation, consisting of GaAs- and InGaP-based pn-homo- and heterojunctions, which are fabricated by MOVPE. The interrelation between interface formation and recombination paths is to be investigated by a combination of in-system 4-tip measurements, scanning probe microscopy and optical methods. In particular, we record local current-voltage characteristics applied to axial versus radial nanowire structures combined with high-resolution scanning tunneling microscopy. The investigation of optoelectronic properties is carried out using a streak-camera system. Both, measurement techniques and their localization are inherently limited in nanostructures, hence requiring physical modeling. Here, modeling is performed by the simulation software package Silvaco Atlas. The performance data feeding into conversion efficiencies are obtained taking generation and recombination mechanisms into account as well as minority and tunneling transport across the pn junctions.The aim of the project is the identification of the qualitative and quantitative interrelation between nanowire growth, device design, interface formation, and surface passivation with respect to the quality of different charge-separating pn-junctions in the nanowires. Thereby, concepts for a significant increase of the optoelectronic performance in the light-nanowire interaction will be proposed and demonstrated.