Nanowire growth

A semiconductor nanowire is a solid rod with a diameter less than 100–200 nm composed of one or several semiconductor materials.  A lower limit is hard to define – wires of 5 nm in diameter are realistic options for this technology. Nanowires can be synthesized using many techniques, and a comprehensive review on this subject is given by Xia et al. [4]. The particular type of nanowire we focus on here is typically grown from a metal particle, the size of which determines the diameter of the wire. The growth is in general described by the vapor-liquid-solid (VLS) growth mechanism [5,6], although in some systems evidence for a vapor-solid-solid (VSS) mechanism has been observed [7,8]. In both of these cases, the crystal growth rate in one dimension at a given temperature is greatly enhanced with respect to the typical two dimensional growth rate, resulting in structures with a large aspect ratio. Nanowires can also be grown without a metal seed, for example in the presence of an oxide [9] or by masking a substrate [10].
Many techniques for epitaxial crystal growth are currently used to produce nanowires, such as molecular/chemical beam epitaxy, vapor phase epitaxy/ chemical vapor deposition, and laser ablation. The particular method used influences the potential material compositions, doping (intentional and unintentional), crystal quality (microstructure and chemical purity), and growth rate. It is therefore important to establish the relative merits of the various growth techniques. Nanowires are of interest to both the scientific and technical communities because of the possibilities for greatly reduced device dimensions, radial epitaxial capping, and axial growth of lattice mismatched structures. To be of real interest to industry, however, it is necessary to solve not only generic issues related to epitaxy (crystal quality, doping, and reproducibility), but also obtain control of those properties unique to nanowires, such as wire position, direction, and shape. The main challenges for nanowire growth can be divided into four areas: control of growth, heterostructures, doping, and integration with Si.
	Figure 2. Animation of the growth of III-V semiconductor nanowires. Seed particles are placed on a crystalline substrate and placed in an epitaxial system. At the right conditions, the crystal only grows underneath the seed particles. The composition of the grown crystal can be altered during growth, resulting in heterostructures.
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