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

Abstract

The Au nanoparticle-assisted growth of coaxially heterostructured III-V compound semiconductor nanowires (NWs) on foreign substrates, through gassource 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 nonuniformity of group V adatom incorporation within NW segments composed of ternary III-V compounds was also investigated. The effects of preferential Pincorporation 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 lowcost, flexible NW-based opto-electronic device applications.