Catalyst Mediated Growth and Optical Properties of Group III-V Nanowires: Au catalyzed GaAs and Self-catalyzed InP

In this work, the first half will cover gold catalyzed gallium arsenide nanowire growth via vapor-liquid-solid (VLS) process using molecular beam epitaxy (MBE). And, the second half will discuss self-catalytic growth of indium phosphide nanostructures via either VLS or vapor epitaxy process using metal organic vapor phase epitxay (MOVPE).Part I: Au Catalyzed Growth of GaAs Nanowires The correlation between prepatterned catalyst films and GaAs nanowire growth was studied, using Au/GaAs as a model system, for the identification of the initial growth conditions on nanowire densities. GaAs nanowires are preferentially grown at 490 oC using solid-source molecular beam epitaxy via VLS process with e-beam patterned Au dots as catalysts. The resulting nanowire morphologies and the fractional surface densities are determined as a function of the electron beam dose, dot size, and inter-dot spacing using scanning electron microscopy. We report the micro Raman studies of Au catalyzed GaAs nanowire bundles. Strong dependence of Raman spectra shift on the nanowire bundles are attributed to structural defects, residual stress, aspect ratio, spatial density and random growth orientation, resulting from variations of catalyst thickness and pattern size. The surface optical phonon modes of GaAs nanowires are also systematically investigated using micro Raman spectroscopy as a function of the fill factor and average diameter. Part II: Self-Catalyzed Growth of InP Nanostructures We report the effect of morphology of nanostructures on optical properties. Self-catalyzed InP nanostructures (nanocones, nanopillars) were grown on both indium phosphide (111)B and silicon (100), (111) single crystal substrates using MOVPE. Micro Raman scattering experiment was employed to understand the effect of morphology and substrate on the optical properties of as-grown structures. Due to crystal symmetry breaking in one-dimensional nanostructures, three phonon vibration modes (LO, TO, SO) were clearly resolved. Their broadening and downshift of Raman spectra were found to be morphology dependent (i.e. surface-to-volume ratio and crystal plane size). The LO phonon was found to depend on surface-to-volume ratio of nanostructures, implying that high scattering volume and surface electric fields with defect density leads to enhancing LO phonon excitation in InP nanocones