Here, we report a direct synthesis approach for obtaining GaN nanowires with control on growth directions: <0001> or
c-direction, and <10-10> or a-direction, on amorphous substrates. The direct nitridation of Ga droplets using either
dissociated ammonia or N2/H2 plasma resulted in GaN nanowires with <0001> growth direction; and the vapor transport
of controlled (low) amounts of Ga flux in the presence of dissociated ammonia resulted in GaN nanowires with <10-10>
growth direction. In both cases, the resulting GaN nanowires have diameters as small as 20 nm and lengths exceeding
one hundred microns. Photoluminescence measurements showed that the bandgap of <10-10> wires blue-shifted by 50
meV from the wires with <0001> direction. Homo-epitaxial growth studies onto the pre-synthesized a-direction GaN
nanowires led to belt or ribbon shaped morphologies. Homo-epitaxial growth onto c-direction wires developed micro
hexagonal prism morphologies. The island growth morphologies observed on the hundred micron long, sub 30 nm size
nanowires suggest that the surface transport of adatoms on c-direction wires exhibit ballistic transport or "one-dimensional"
transport with mean distances over several tens of microns.
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