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

目前，各种各样的纳米线器件在实验室样品水平上是可用的。特别是那些由化合物半导体材料组成的材料，通过在纳米线拓扑结构中实现同质和异质结构，扩展了电子和光子能力的频谱。然而，整体性能仍远低于预期，并阻碍了商业实施。例如，这适用于将光转换为电能或产生电泵浦光发射器甚至激光器的能力。到目前为止，纳米线的研究忽略了对这些技术固有局限性的研究。然而，为了为纳米线器件开辟新的市场，甚至取代现有市场上基于平面结构的半导体器件，需要大量的定性改进，而不仅仅是定量改进。该项目涉及光与纳米线相互作用的固有限制，例如径向和轴向纳米线结构中的光电流转换。该项目旨在确定器件结构中光电性能限制的起源，包括微观分析。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强烈相关性。该器件采用轴向和径向纳米线异质结进行电荷分离，由基于GaAs和ingap的pn-同质结和异质结组成，由MOVPE制造。结合系统内四端测量、扫描探针显微镜和光学方法，研究了界面形成和复合路径之间的相互关系。特别是，我们结合高分辨率扫描隧道显微镜记录了轴向和径向纳米线结构的局部电流电压特性。利用条纹相机系统对其光电特性进行了研究。这两种测量技术及其定位在纳米结构中都受到固有的限制，因此需要物理建模。本文采用仿真软件包Silvaco Atlas进行建模。考虑了产生和重组机制以及跨pn结的少数输运和隧道输运，获得了转化效率的性能数据。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之间的定性和定量相互关系，以及纳米线中不同电荷分离pn结的质量。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.