Atomic Scale Characterization of III-V Nanowire Surfaces

Abstract: This dissertation focus on the atomic-scale characterization of the surface properties and electronic structure of III–V semiconductor nanowires (NWs). Since the early 2000s, the fabrication and characterization of III–V NWs has been a very significant topic within material science due to their potential for applications in lighting, energy harvesting, and electronics. A prominent feature of the NW geometry is the ability to form heterostructures—both radially and axially—for shaping the electronic landscape in the NW. The heterostructures can consist of variations in the crystal structure, material composition, or a combination of the two. A pre-requisite for the fabrication of advanced heterostructures in NWs is an intricate knowledge concerning the atomic-scale surface properties as well as the material and crystal structure dependent electronic properties. However, little is known about the surface properties of III–V NWs and how the electronic structure behaves at the heterostructure interface at the atomic level.In this dissertation I describe the work on the electronic properties of crystal structure heterojunctions in GaAs and InAs NWs, down to the atomic bilayer level. Scanning tunneling microscopy and spectroscopy (STM/S) was used to probe the surface structure and electronic properties of NW surfaces with atomic resolution. Also, the effects of material overgrowth on GaAs and InAs NW surfaces was studied at the atomic scale. The studies allowed for the identification of nucleation sites, incorporation processes, and growth modes. Furthermore, quantum-size effects have been investigated within crystal phase structures in NWs.Due to the large surface–to–volume ratio of nanostructures, the optical and electronic performance of NW devices is often impeded by surface defects. The quantity, origin and individual influence of such defects is unknown. I have used atomic-scale STM imaging of externally biased NW devices operando, as well as x-ray photoelectron spectroscopy (XPS) measurements, to investigate how surface oxides and atomic-scale defects affect the conductivity of NWs.