Epitaxial Growth and Design of Nanowires and Complex Nanostructures
Sammanfattning: This thesis describes the epitaxial growth of III-V semiconductor nanowires using Au seed particles, and the design of more complex three-dimensional branched structures from these wires. Growth was performed by metallorganic vapour phase epitaxy, in which precursor molecules for the semiconductor material components are introduced in a low-pressure vapour. Nanowires grow epitaxially (with controlled crystal orientation) on a semiconductor substrate; diameter control is achieved via the Au particles, while length is controlled by growth parameters. Particle-assisted nanowire growth is used extensively today to achieve well-controlled structures. The current understanding of this growth mechanism was developed over forty years ago, and is known as the Vapour-Liquid-Solid (VLS) mechanism. This model indicates that a liquid alloy is formed between the seed particle and the growth material(s), and growth proceeds by precipitation from a supersaturated particle. The enhanced growth rate compared to the bulk growth from the vapour is typically attributed to preferential decomposition of precursor materials at or near the particle surface. Recently, however, several inconsistencies have been observed between this model, which was developed for Au-assisted Si whiskers (micro-scale wires), and particle-assisted growth of other materials. In particular, there have been many reports of nanowire growth at temperatures too low for a liquid alloy to form. As well, nanowire growth has been reported for systems where no precursor molecules are used, and thus growth enhancement cannot be explained by preferential decomposition. Other reports have given evidence that such preferential decomposition does not necessarily occur when precursors are used. The first part of this thesis presents the current understanding of particle-assisted growth, both generally and for the specific materials and growth systems used here. Semiconductor nanowires present the possibility for numerous applications, and many simple device components have been demonstrated. The development of practical applications of such prototypes may rely on the ability to assemble nanowires into more complex structures. The second part of this thesis presents techniques for the production of three-dimensional branched nanowire structures, including methods to achieve controlled structure and morphology. The assembly of branched structures into large-scale interconnected networks is also presented.
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