III-Nitride materials, which consist of AlN, GaN, InN, and their alloys have become the cornerstone of the third generation compound semiconductor. Planar IIINitride materials are commonly grown on sapphire substrates which impose several limitations such as challenging scalability, rigid substrate, and thermal and lattice mismatch between substrate and material. Semiconductor nanowires can help circumvent this problem because of their inherent capability to relieve strain and grow threading dislocation-free without strict lattice matching requirements, enabling growth on unconventional substrates. This thesis aims to investigate the microscopic characteristics of the nanowires and expand on the possibility of using transparent amorphous substrate for III-nitride nanowire devices. In this work, we performed material growth, characterization, and device fabrication of III-nitride nanowires grown using molecular beam epitaxy on unconventional substrates. We first studied the structural imperfections within quantum-disks-in-nanowire structure grown on silicon and discovered how growth condition could affect the macroscopic photoluminescence behavior of nanowires ensemble. To expand our work on unconventional substrates, we also used an amorphous silica-based substrate as a more economical substrate for our nanowire growth. One of the limitations of growing nanowires on an insulating substrate is the added fabrication complexity required to fabricate a working device. Therefore, we attempted to overcome this limitation by 5 investigating various possible GaN nanowire nucleation layers, which exhibits both transparency and conductivity. We employed various nucleation layers, including a thin TiN/Ti layer, indium tin oxide (ITO), and Ti3C2 MXene. The structural, electrical, and optical characterizations of nanowires grown on different nucleation layers are discussed. From our work, we have established several key processes for transparent nanowire device applications. A nanowire LED emitting at ∼590 nm utilizing TiN/Ti interlayer is presented. We have also established the growth process for n-doped GaN nanowires grown on ITO and Ti3C2 MXene with transmittance above 40 % in the visible wavelength, which is useful for practical applications. This work paves the way for future devices utilizing low-cost substrates, enabling further cost reduction in III-nitride device fabrication.

GaAs nanowires (NW) have been grown on Si (111) substrates by the self-assisted vapor-liquid-solid (VLS) mechanism using molecular beam epitaxy (MBE). Substrates were prepared with nano-patterned oxide templates using electron beam lithography (EBL) in order to achieve position controlled NW growth. Early experiments uncovered several key issues with regards to the patterning process. Cross-sectional lamella prepared using the focused-ion beam (FIB) technique were used to study the NW-substrate interface using transmission electron microscopy (TEM). Undesirable growth outcomes were found to be caused in part by an unintended residual layer of oxide. Uniform NW dimensions were then obtained by improving the pattern transfer method. The effects of deposition parameters on the growth results were then explored in further experiments. The first systematic study of the axial and radial growth rates of vertical NWs in the positioned array was conducted. It was proposed that the observed expansion of the Ga droplet in Ga-rich growth conditions results in a slight inverse tapered morphology, promoting significant radial growth. While the growth rates were shown to be approximately constant in time, their measured values were found to increase with increasing pattern pitch and decrease with increasing hole diameter. A phenomenological model was then developed based on the principle of mass conservation. A fit to the experimental data was obtained by calculating the collection of growth material supplied by a secondary flux of both gallium and arsenic species desorbing from the oxide surface between the NWs, subsequently impinging on the liquid droplet and NW sidewalls. The reduction of this contribution due to shading of the incident and scattered flux by neighboring NWs in the array was able to account for the differences in final NW morphologies observed with increasing pattern pitch. This model demonstrates the significant impact of secondary adsorption in patterned self-assisted NW growth. / Thesis / Doctor of Science (PhD)

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Molecular beam epitaxy (MBE) grown metal has been a renewed area of interest recently in order to achieve high quality metal films or nanostructures for plasmonics. Recently MBE grown silver films have been shown to possess optical constants closer to that of intrinsic silver leading to lower losses and thus allowing for higher quality plasmonics. MBE has also been used to grow silver nanocrystals and indium droplets, or islands, for plasmonics. These self-assembled nanostructures can be grown in close proximity to quantum confined structures such as InAs/GaAs quantum dots or InGaAs/GaAs quantum wells in a single process, without post-processing and fabrication, allowing for increased plasmonic enhancement due to the improved interface between the semiconductor and plasmonic structures.In this dissertation, widely tunable plasmonic resonances of indium islands will be discussed and plasmonic enhancement results will be presented and compared to those of nanoantennas constructed from standard fabrication processes. The coupling between near-surface quantum confined structures, both fabricated and self-assembled, will be compared to the coupling in typical dielectric cavities, such as photonic crystal nanobeams. Beyond the plasmonic possibilities of indium islands, indium becomes superconducting at 3.4 K. With the proximity effect allowing for electrons in materials in contact with a superconductor to occupy a superconducting like state, allowing for the possibility for a hybrid superconductor/semiconductor optical source. The observation of superconductivity in indium islands will be presented and considerations for a superconductor/semiconductor source will be discussed.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Master / Master of Philosophy

published_or_final_version / abstract / Physics / Master / Master of Philosophy