UC Berkeley

Silicon's chemical stability, high natural abundance (as the second most common element in the earth's crust), mechanical stiffness, and semiconducting behavior have made it the subject of extensive scientific investigation and the material of choice for both the microelectronics and microelectromechanical device industries. The success of Moore's Law that demands continual size reduction has directed it to a central place in emerging nanoscience and nanotechnology as well. Crystalline nanowires (NWs) are one nanostructured form that silicon may take that has sparked significant interest as they can exhibit considerable confinement effects and high surface-to-volume ratios, but may be interfaced simply along one direction for the determination of material properties and implementation into new technologies. The expense and difficulty involved in the creation of semiconductor nanowires using the "top down" fabrication techniques of the microelectronics industry has promoted an explosion of chemical synthetic "bottom up" techniques to produce high quality crystalline nanowires in large quanitities. Nevertheless, bottom up synthesized Si NWs retain a new set of challenges for their successful integration into reliable, high-performance devices, which is hindered by an incomplete understanding of the factors controlling their material properties.

The first chapter of this dissertation introduces the motivation for studying semiconductor NWs and the benefits of limiting the scope to silicon alone. A brief survey of the current understanding of thermal conductivity in silicon nanowires provides prime examples of how confinement effects and surface morphology may dramatically alter nanowire properties from their bulk crystal counterparts. The particular challenges to bottom up silicon nanowire device integration and characterization are noted, especially related to Si nanowires that are grown epitaxially on crystal silicon substrates, and Raman spectroscopy is introduced as a promising optical characterization and metrology tool for semiconductor nanowire based devices.

Chapter two describes the vapor-liquid-solid (VLS) mechanism for the synthesis of very high quality, single-crystal silicon nanowires using Au and Pt catalyst nanoparticles. A new technique is presented for the simplified synthesis of branched silicon nanowires based on the migration of Au catalyst during an hydrogen anneal intermediate between growth stages, and the faceting behavior at synthetic stages is revealed by the analysis of electron microscope images. Synthesis of solid and porous Si nanowires based on Ag mediated electrochemical silicon etching is described as well.

The third chapter specifies new processing techniques developed with future device integration of epitaxially VLS-grown Si nanowires in mind. Epitaxially bridging nanowires are shown to provide an excellent platform for single-wire electrical and mechanical property measurements. Galvanic displacement through block copolymer micelle/homopolymer surface templates is demonstrated as a means to deposit catalyst nanoparticles with controlled sizes and areal densities in a variety of geometries and with registration to photolithographic patterns. Ex situ boron doping by the direct hydrogen reduction of boron tribromide is shown to achieve active concentrations exceeding 10¹⁹ cm⁻³ with high axial uniformity, while avoiding the adverse impact on nanowire morphology that is often observed with in situ boron doping of silicon nanowires.

Chapter four describes the characteristics of Raman spectroscopy that are relevant to studying individual semiconductor nanowires. Careful spectral measurements show that the anharmonic dependence of Raman spectra on temperature for individual Si nanowires remains unchanged from the bulk crystal for diameters down to 30 nm, regardless of surface morphology. Using this result, a new technique for measuring the thermal conductivity of individual semiconductor nanowires is then outlined based on Raman thermal mapping of individual cantilevered nanowires.

Finally, the dissertation is concluded with suggestions for possible future experiments. One avenue is to probe more deeply the morphology of faceted silicon nanowires and nanotrees and its impact on their transport physics. Another possible route for further study would be to explore new characterization and metrological applications of Raman spectrocopy for semiconductor nanowires.