| dc.description.abstract | The III-V nitride materials system offers tunable electronic and optical properties that can be tailored for specific electronic and optoelectronic applications by varying the (In,Ga,Al)N alloy composition. While nitride thin films tend to suffer from high dislocation densities due to the lattice mismatch with growth substrates, nanowires can be grown dislocation-free on highly mismatched substrates including silicon. Furthermore, axial and radial junction configurations offer unique nanoscale device architectures that enable more optimal device design. In order to realize the potential benefits of III-V nitride nanowires, precise control of nanowire synthesis is required. This thesis describes the development of experimental techniques and theoretical models that guide the synthesis of Ill-V nitride and other compound semiconductor nanowires with control over material structure, morphology, and composition. First, GaN nanowires were synthesized with control over nanowire orientation, morphology, and defect density. Substrate orientation was used to control whether nanowires grew preferentially in the polar [0001] direction or the nonpolar [1-100] direction. Film deposition on the nanowire sidewalls was effectively minimized by reducing the Ga precursor flux and internanowire spacing. Using nonpolar-oriented GaN nanowires with uniform diameter, the diameter-dependent growth rate was modeled to demonstrate that growth is limited by nucleation at the perimeter of the seed/nanowire interface. Finally, Ni- and Au-seeded GaN nanowires were directly compared, and the higher growth rate and reduced defect density in Ni-seeded nanowires were consistent with a reduced seed/nanowire interfacial energy. Next, nonpolar-oriented InN/InGaN axial heterostructure nanowires were grown by introducing Ga precursors during InN nanowire growth. The formation of GaN shells placed an upper limit on the allowable Ga precursor flux. Shell deposition was minimized by operating at higher temperature and pressure. However, a reduction in the local supply of Ga to the seed particle also limited InGaN formation. Therefore, brief high-flux pulses were used at lower pressure to form InN/InGaN axial heterostructures with minimal shell formation. Electron tomography and energy dispersive X-ray spectroscopy were used to analyze the Ga-driven driven changes in nanowire morphology and composition, respectively. The reduction in nanowire diameter upon the introduction of Ga was found to be driven by changes in seed particle composition. A flow-controlled approach was developed to modulate the diameter along individual nanowires, which can enable unique properties including enhanced light trapping in nanowire arrays and increased phonon scattering in thermoelectrics. In InN nanowires, a reduction in V flow produced segments with larger diameters and slower growth rates. A reduction in III flow in GaN nanowires also produced segments with slower growth rates, but thinner diameters. These trends are a consequence of the separate pathways traveled by the III and V sources to the site of reaction, enabling control over the incorporation rate of III source into the seed particle and the extraction rate of III source out of the seed particle, respectively. Based on these promising results, models were developed to explore the potential for template-free nanowire diameter modulation via particle-mediated growth. The results from diameter-modulated InN and GaN nanowires were evaluated considering contributions of seed particle volume, wetting angle, and three-dimensional morphology to the observed diameter changes. To achieve large diameter ratios using liquid seed particles, significant changes in both seed particle volume and wetting angle are necessary. Furthermore, the model was used to evaluate the surface energy and morphology of the liquid/solid interface. The interface was found to not be flat, contrary to common assumptions, which has significant implications for nanowire growth models. Finally, we extended the flow-controlled diameter modulation approach to GaAs nanowires, demonstrating that the technique is generally applicable to particle-mediated compound semiconductor nanowires. Both the III and V sources were varied during growth, producing similar trends in diameter and growth rate as with III-V nitride nanowires. Notably, three different types of [111]B-oriented nanowires were observed and had discrete differences in diameter modulation, growth rate, and cross-sectional shape, which were attributed to differences in seed particle phase. By controlling growth conditions during nanowire nucleation, each of the three types of nanowires were preferentially produced, indicating that the seed particle phase can be controllably varied in compound-forming seed alloys. Together, these results provide a foundation for fabricating III-V nitride and other nanowires with control over material structure, morphology, and composition. Experimental techniques and theoretical models were developed that enable control over growth direction, tapering, growth rate, defect density, composition, and diameter. These tools are helpful in achieving nanowires with rationally tailored properties for functional nanowire-based devices. | en_US |
