  Manipulation of surface architecture of semiconducting nanowires with a
control in surface polarity is one of the important objectives for nanowire
based electronic and optoelectronic devices for commercialization. We report
the growth of exceptionally high structural and optical quality nonpolar GaN
nanowires with controlled and uniform surface morphology and size distribution,
for large scale production. The role of O contamination (~1-10^5 ppm) in the
surface architecture of these nanowires is investigated with the possible
mechanism involved. Nonpolar GaN nanowires grown in O rich condition show the
inhomogeneous surface morphologies and sizes (50 - 150 nm) while nanowires are
having precise sizes of 40(5) nm and uniform surface morphology, for the
samples grown in O reduced condition. Relative O contents are estimated using
electron energy loss spectroscopy studies. Size-selective growth of uniform
nanowires is also demonstrated, in the O reduced condition, using different
catalyst sizes. Photoluminescence studies along with the observation of
single-mode waveguide formation, as far field bright violet multiple emission
spots, reveal the high optical quality of the nonpolar GaN nanowires grown in
the O reduced condition.
