Semiconductor Nanowires: Characterization and surface modification

The topic of III-V nanowires is still, after more than two decades, a growing and lively research area. The areas of application are wide and contain such important topics as energy harvesting, cheap and efficient lighting, high efficiency detectors and new types of electronics. III-V materials offer properties superior to the widely used Si. They can have considerably higher carrier mobility which paves the way for high-speed electronics and a great flexibility in band gap which is fundamental for optoelectronics. Producing these materials in the form of nanowires presents additional advantages as it gives the opportunity to tailor the material by altering the crystal structure and material composition in ways not possible in larger structures as well as integrating them with existing standard materials platforms.This combination of great materials in a small size has demonstrated potential for improved solar cells, high-speed low power transistors as well as energy efficient and flexible LEDs. But the promises come with challenges. The quality of the III-V surfaces is a significant factor for determining device performance, potentially both improving and limiting functionality. Further, the relevance of surfaces properties increases with decreasing sizes as the surface to bulk ratio goes up. This thesis focuses on characterization of semiconducting III-V surfaces with a special focus on nanowires, exploring recently developed methods, types of nanowires and nanowire synthesis.Native oxides forming on the surfaces of III-V materials in air are adverse and impede the development of top-tier devices. The surfaces have to be cleaned of the detrimental oxides and protected as part of the manufacturing of components. In the present work X-ray photoelectron spectroscopy (XPS) has been used to improve the understanding of different cleaning methods including the self-cleaning effect of atomic layer deposition (ALD) of high-k oxides.Aerotaxy growth of nanostructures, where nanowires grow in a carrier gas, has arisen as a more cost effective and scalable production method than epitaxial growth on a solid substrate. Yet little is known about both the mechanism involved in the growth as well as about the quality of the resulting nanostructure surfaces. Here XPS and scanning probe microscopy (SPM) have been used to unravel the effect of doping on the surfaces of aerotaxy nanowires. Further, also a technique using small-angle X-ray scattering (SAXS) for in-situ characterization of the aerotaxy seed particles has been developed.SPM has been a revolutionary tool for surface science since its invention. Central for the function of an SPM is the tip, which is usually made out of metal and formed by a simple electrochemical etching procedure. In this thesis the development of a high resolution GaN nanowire probe for SPM is described. It combines the optical and electronical advantages of GaN with the well-controlled tip formation at the end of a nanowire