Silicon nanowires: Growth, transport and device physics

Silicon is the second most abundant element in the earth's crust and has been the backbone of the information technology revolution. It is the most well-studied material in all of solid-state chemistry and physics and has been used to make a variety of devices including transistors, resonators, and solar cells. Nanowires could provide advantages over bulk silicon; however, there are many fundamental challenges that must be overcome in order to use them in high-performance, reproducible devices. The first chapter of this dissertation gives an introduction to nanoscience with an emphasis on the working principles of the nanowire devices that are discussed later and the problems that face nanowire implementation. Chapter two demonstrates that platinum nanoparticles can be substituted for gold as the nanowire growth catalyst without sacrificing crystalline quality, epitaxial growth or electrical properties. Replacing gold with a clean-room compatible material such as platinum is important to allow for nanowire integration into microfabricated devices. Chapter three focuses on making horizontal surround-gate field effect transistors for capacitance-voltage measurements. These devices are used to extract the dopant profile and density of interface states from individual nanowires, showing results consistent with planar control samples and simulations. The results are encouraging because they suggest low surface recombination velocities (similar to bulk planar wafers) should be possible as long as the nanowire surface is smooth and well-faceted. Chapter four demonstrates two low-cost, scalable methods for fabricating silicon nanowire photovoltaics. Because of the rough surface induced by the electroless etching process and the poor junction quality from the nanocrystalline chemical vapor deposition film, the efficiency of cells made with the first approach is relatively low at about 0.5%. The second approach, using an assembly of silica beads, deep reactive ion etching and diffusion, gives much smoother nanowires and has a single-crystalline p-n junction. The efficiency of these solar cells improves by almost an order of magnitude, up to 3.6% for a 25 mum thin silicon absorbing layer. These studies represent a significant step towards realizing the benefits of integrated silicon nanowire devices.