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

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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.