Fabricating Si[111] Nanostructures on Graphene by Aluminum-Induced Crystallization for High Yield Vertical III-V Semiconductor Nanowire Growth

Abstract

III-V semiconductor nanowire-graphene photovoltaics is an emerging technology that has the potential of highly efficient, flexible and ultra-thin solar cells. This thesis has explored aluminum-induced crystallization(AIC) of amorphous silicon on graphene and graphite surfaces, with the aim of increasing the nucleation yield of nanowires. It is demonstrated that thin films of aluminum and amorphous silicon, separated by an oxide membrane, produces [111]-oriented crystalline silicon after annealing at 470-500°C, well below the crystallization temperature of silicon, for glass, kish graphite and graphene substrates. The crystal orientation and elemental composition of the samples are characterized by X-ray diffraction(XRD), electron backscatter diffraction(EBSD) and energy-dispersive X-ray spectroscopy(EDX). The crystallized silicon forms semi-continuous films on glass and graphene, while it forms dendrite structures on kish graphite substrates. Silicon crystallization is achieved with both sputtering and electron beam evaporation of aluminum, and it is suggested that the aluminum microstructure is the determining factor for whether silicon crystallization occurs or not. The developed process for AIC of amorphous silicon is combined with the one-shot exposure electron beam lithography(EBL) technique to pattern silicon nanodot arrays on few-layer graphene substrates, showing high yields and controllable dot diameter sizes of 80-160 nm. The processes developed in this thesis would have a high potential for the high-density nucleation of vertically aligned, self-catalyzed gallium arsenide(GaAs) nanowires on graphene, enabling the fabrication of an ultra-thin solar cell with GaAs nanowires as the photoactive component and graphene as the bottom electrode.