  We demonstrate the top-down fabrication of ordered arrays of GaN nanowires by
selective area sublimation of pre-patterned GaN(0001) layers grown by hydride
vapor phase epitaxy on Al$_{2}$O$_{3}$. Arrays with nanowire diameters and
spacings ranging from 50 to 90 nm and 0.1 to 0.7 $\mu$m, respectively, are
simultaneously produced under identical conditions. The sublimation process,
carried out under high vacuum conditions, is analyzed \emph{in situ} by
reflection high-energy electron diffraction and line-of-sight quadrupole mass
spectromety. During the sublimation process, the GaN(0001) surface vanishes,
giving way to the formation of semi-polar $\lbrace1\bar{1}03\rbrace$ facets
which decompose congruently following an Arrhenius temperature dependence with
an activation energy of ($3.54 \pm 0.07$) eV and an exponential prefactor of
$1.58\times10^{31}$ atoms cm$^{-2}$ s$^{-1}$. The analysis of the samples by
low-temperature cathodoluminescence spectroscopy reveals that, in contrast to
dry etching, the sublimation process does not introduce nonradiative
recombination centers at the nanowire sidewalls. This technique is suitable for
the top-down fabrication of a variety of ordered nanostructures, and could
possibly be extended to other material systems with similar crystallographic
properties such as ZnO.
