Chemical and molecular beam epitaxy of III-V nanowires on silicon for photovoltaic application

Nanowires, due to their unique structure and carrier transport abilities, have sparked huge interest in the semiconductor industry. An array of nanometric size wires inserted between the p and n conductivity regions of a conventional solar cell or core shell type p-n junction nanowires synergized with semiconductor nanocrystals can lead to faster carrier collection, thereby improving device performance. This work investigates the growth of GaAs and InP semiconductor nanowires on silicon (111) using Chemical Beam Epitaxy (CBE) and Molecular Beam Epitaxy (MBE). Uniform gold nanoparticles acting as growth centers in the Vapor Liquid Solid mode of growth were generated by using the cheap and rapid technique called Nanosphere Lithography (NSL). Variation of the experimental parameters during NSL resulted in honeycomb and hexagonal patterns of gold nanoparticles. A high degree of selectivity was obtained for CBE grown nanowires whereas the MBE grown GaAs nanowires revealed the formation of a thick polycrystalline wetting layer at the interface. The CBE grown InP nanowires mostly maintained the honeycomb structure although they were found to be oriented contrary to the expected <111> direction. SEM analysis of GaAs nanowires grown by CBE showed that during growth, the nanowires may coalesce with each other resulting in unique structures such as bipods, tripods and multipods. High resolution TEM analysis of single GaAs nanowires revealed periodic formation of contrasting materials. Diffraction patterns recorded at these dark contrast areas confirmed the formation of hexagonal wurtzite single crystal structures interspaced with cubic zincblende single crystal structures. These nanowires can be used for photovoltaic applications or as light emitting devices. In addition, the formation of superlattices of different crystal structures can pave the way for novel quantum confined optoelectronic devices.