Core-Shell Heterostructured Nanowires on Foreign Substrates for use in Opto-Electronic Applications.

摘要

The Au nanoparticle-assisted growth of coaxially heterostructured III-V compound semiconductor nanowires (NWs) on foreign substrates, through gas-source molecular beam epitaxy, was explored. The structural properties of GaP/GaAsP NW systems were characterized through extensive electron microscopy-based techniques, leading to several key findings. A core-multishell NW architecture was defined, based upon the combination of seed-mediated axial growth and sidewall diffusion-mediated radial growth regimes. The formation of undesirable stacking faults, characterized as zincblende insertions within an otherwise wurtzite crystal, was effectively eliminated through the control of seed supersaturation. Thus, a novel method for the elimination of stacking faults and achievement of phase-purity in NW heterostructures was realized. The non-uniformity of group V adatom incorporation within NW segments composed of ternary III-V compounds was also investigated. The effects of preferential P-incorporation and passivating GaP shell layers on the optical properties of GaAsP core segments were determined through spectroscopic characterization techniques.;Furthermore, the growth of GaAs NWs on several foreign substrates, including single crystalline Si substrates, stainless steel foils, glass substrates with polycrystalline Si buffer layers, carbon-nanotube (CNT) composite films, and highly-ordered pyrolytic graphite (HOPG), was explored. Growth on HOPG was shown to proceed according to a Ga-assisted mechanism, for which a qualitative growth model was proposed. Au-nanoparticle mediated growth was achieved on all other surfaces. The CNT composite films were selected as the most suitable substrate for the fabrication of NW-based opto-electronic devices. Flexible photovoltaic cells were fabricated using this nano-hybrid material, demonstrating a conversion efficiency of 0.32% under mechanical flexure up to a bend radius of 12.5 mm. The GaAs NWs were confirmed to be the active light-harvesting medium, while the CNT films served as flexible substrates and back-side electrodes. This work is presented as a novel route towards the realization of low-cost, flexible NW-based opto-electronic device applications.