Spontaneous Nucleation
and Growth of GaN Nanowires:
The Fundamental Role of Crystal Polarity
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Abstract
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(0001̅) 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<sub><i>x</i></sub>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