Dynamics of a Droplet that Assists III-V Nanowire Growth

Abstract: Control of the process of crystal growth has for decades been achieved by addressing the growth temperature and material supply of the growth species. Directional control of both crystal growth and etching has, for example, been achieved by utilizing a liquid droplet to assist the process. This is a common approach to achieve crystal growth of nanostructures, as in the case of Au-assisted III-V semiconductor nanowires. Although controlled droplet-assisted growth has been studied in depth, less attention has been given to the droplet composition and how it dynamically interacts with the nanowire. This thesis explores the fundamental limits for one-directional droplet-assisted crystal growth. This is studied first by intentionally displacing the droplet from the facet at which it originally assisted the crystal growth. The process and cause for displacement is studied for Au-assisted GaAs and InAs nanowires by combining experimental observations and theoretical modeling of the droplet wetting. In addition, it is shown that the final position of the droplet can be controlled by tailoring the surfaces of the nanowire, which in turn is used for design of branched structures. Furthermore, this thesis focuses on the droplet dynamics and the formation of a truncation at the droplet-nanowire interface, as the geometry of the droplet wetting of the top facet approaches the fundamental limit. The studies of this thesis are conducted using metal-organic chemical vapor deposition (MOCVD), both in- and outside an environmental transmission electron microscope (ETEM). Ex-situ analysis of droplet displacement allows us to investigate the statistics of the process, to understand trends of the droplet wetting. On the other hand, performing MOCVD inside an ETEM enables real-time studies of the dynamic processes during growth, such as observations of the droplet wetting angle or the facet truncation. Using a combination of theoretical modeling, high-temperature X-ray energy dispersive spectroscopy and direct imaging during growth, we measure and estimate the previously inaccessible gallium and arsenic concentration in the droplet, as well as the surface energies of the Au-Ga droplet and the GaAs nanowire sidewalls. These findings could in turn be used to further improve our understanding of the atomic arrangement at the crystal surfaces and interfaces during growth. Such an understanding could lead to improved control and design of crystal nanostructures.