GaN NWs grown by PA-MBE crystallize in the wurtzite structure with the polar 〈0001〉 axis parallel to the growth direction. The  and  directions (known as Ga- and N-polar orientations, respectively) are thus non-equivalent. In the case of the growth of epitaxial films, it is well established that the polarity of the layer has a strong impact on their properties since it determines the direction of both spontaneous and piezoelectric polarization fields, affects the morphology, and influences the incorporation of dopants, impurities, as well as the formation of native point defects. Despite these facts, the role of the polarity in the nucleation and growth of GaN NWs is unknown. Another important open question is whether or not defects are responsible for the nucleation of GaN NWs.
In this work we experimentally investigate whether the crystal polarity affects the growth of GaN NWs in PA-MBE 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(0001) substrates to ensure a well-defined polarity and an absence of structural and morphological defects. If planar GaN layers were grown on these templates, GaN would adopt the polarity of the AlN buffer layer. Here, on N-polar AlN, a homogeneous and dense N-polar GaN NW array forms, evidencing that GaN NWs form spontaneously in the absence of defects [Fig.1 (a)]. On Al-polar AlN, we do not observe the formation of Ga-polar GaN NWs. Instead, sparse N-polar GaN NWs grow embedded in a Ga-polar GaN layer [Fig.1(b)]. These N-polar GaN NWs 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 NWs 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 NWs but also of high technological relevance.