  Ultrathin GaN nanowires (NWs) are attractive to maximize surface effects and
as building block in high-frequency transistors. Here, we introduce a facile
route for the top-down fabrication of ordered arrays of GaN NWs with aspect
ratios exceeding $10$ and diameters below $20\,$nm. Highly uniform thin GaN NWs
are first obtained by using electron beam lithography to pattern a Ni/SiN$_x$
hard mask, followed by dry etching and wet etching in hot KOH. The SiN$_x$ is
found to work as an etch stop during wet etching in hot KOH. Arrays with NW
diameters down to $(33 \pm5)\,$nm can be achieved with a yield exceeding
$99.9\,\%$. Further reduction of the NW diameter down to $5\,$nm is obtained by
applying digital etching which consists in plasma oxidation followed by wet
etching in hot KOH. The NW radial etching depth is tuned by varying the RF
power during plasma oxidation. NW breaking or bundling is observed for
diameters below $\approx 20\,$nm, an effect that is associated to capillary
forces acting on the NWs during sample drying in air. This effect can be
principally mitigated using critical point dryers. Interestingly, this
mechanical instability of the NWs is found to occur at much smaller aspect
ratios than what is predicted for models dealing with macroscopic elastic rods.
Explicit calculations of buckling states show an improved agreement when
considering an inclined water surface, as can be expected if water assembles
into droplets. The proposed fabrication route can be principally applied to any
GaN/SiN$_{x}$ nanostructures and allows regrowth after removal of the SiN$_{x}$
mask.
