  We present a conceptually novel approach to achieve selective area epitaxy of
GaN nanowires. The approach is based on the fact that these nanostructures do
not form in plasma-assisted molecular beam epitaxy on structurally and
chemically uniform cation-polar substrates. By in situ depositing and
nitridating Si on a Ga-polar GaN film, we locally reverse the polarity to
induce the selective area epitaxy of N-polar GaN nanowires. We show that the
nanowire number density can be controlled over several orders of magnitude by
varying the amount of pre-deposited Si. Using this growth approach, we
demonstrate the synthesis of single-crystalline and uncoalesced nanowires with
diameters as small as 20 nm. The achievement of nanowire number densities low
enough to prevent the shadowing of the nanowire sidewalls from the impinging
fluxes paves the way for the realization of homogeneous core-shell
heterostructures without the need of using ex situ pre-patterned substrates.
