III-V Semiconductor Vertical and Tilted Nanowires on Silicon Using Chemical Beam Epitaxy
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III-V Semiconductor Vertical and Tilted Nanowires on Silicon Using Chemical Beam Epitaxy Gokul Radhakrishnan1,2, Alex Freundlich1,2, Joe Charlson1, and Bodo Fuhrmann3 1 Electrical and Computer Engineering Department, University of Houston, Houston, TX, 77204-5004 2 Center for Advanced materials, Houston, TX, 77204-5004 3 Interdisciplinary Center of Materials Science, Martin Luther University of Halle, Halle, D-06120, Germany ABSTRACT Nowadays nanostructures play a vital part in the rapidly expanding areas of photovoltaics. The ability of nanowires to transfer photo-generated carriers rapidly across a solar cell has lead to our interest in growth of nanowires. Currently Vapor Liquid Solid (VLS) epitaxy is the most common method used to grow vertically aligned nanowires. A metal particle such as gold is used to form a liquid alloy eutectic with the material of a substrate or with material supplied in the vapor phase. In growing semiconductor wires using metal droplets, it has been shown that the wires grow in the (111) direction and have clean facets. Furthermore these wires generally present an undesirable larger pyramidal base at the bottom and there is also evidence of surface migration of the metal catalyst. Thus far, most of the effort in the development of vertical III-V semiconductor nanowires has been limited to homo-polar combinations (e.g. InAs on InP). The ability to fabricate III-V nanowires on silicon could however pave the way toward the monolithic integration of III-V nanostructured solar cells with Si. Here we demonstrate the growth of GaAs and InP nanowires on silicon (111) using gold as the metal seed particle. An ordered array of gold nano dots was patterned on the surface of a silicon substrate using self-assembled polystyrene nanospheres as the Au evaporation template. The size of the gold dots range from 40 nm to 150 nm and the pitch is about 500 nm. The growth of the nanowires was performed by chemical beam epitaxy under a vapour phase environment. Scanning electron microscopy, photoluminescence and Raman spectroscopy were used to characterize these nanowires. These nanowires exhibit high crystallinity and there is an absence of the pyramidal base at the bottom of the nanowire using this technique. Furthermore the study also shows evidence of pre-growth motion of some of the gold particles causing coalescence of nanowires and leading to the development of nanopods and tilted (off-normal) nanowires. Finally in light of their optical properties the relevance of these wires to photovoltaic applications is discussed.
INTRODUCTION The most successful approach towards nanowire generation is by using the vaporliquid-soild (VLS) mechanism [1]. The mechanism is based on breaking the isotropic symmetry of the crystal to a so called anisotropic crytal growth. A metal particle such as gold is used to form a liquid alloy eutectic with the material of the substrate or with material supplied in the vapor phase. The supersaturated liquid alloy leads to 1D
nanowire formation in one preferential d
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