  We experimentally investigate whether crystal polarity affects the growth of
GaN nanowires in plasma-assisted molecular beam epitaxy and whether their
formation has to be induced by defects. For this purpose, we prepare smooth and
coherently strained AlN layers on 6H-SiC(0001) and SiC(000$\bar{1}$) substrates
to ensure a well-defined polarity and an absence of structural and
morphological defects. On N-polar AlN, a homogeneous and dense N-polar GaN
nanowire array forms, evidencing that GaN nanowires form spontaneously in the
absence of defects. On Al-polar AlN, we do not observe the formation of
Ga-polar GaN NWs. Instead, sparse N-polar GaN nanowires grow embedded in a
Ga-polar GaN layer. These N-polar GaN nanowires are shown to be accidental in
that the necessary polarity inversion is induced by the formation of Si$_{x}$N.
The present findings thus demonstrate that spontaneously formed GaN nanowires
are irrevocably N-polar. Due to the strong impact of the polarity on the
properties of GaN-based devices, these results are not only essential to
understand the spontaneous formation of GaN nanowires but also of high
technological relevance.
