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Semiconductor nanowires (NWs) are promising components for future applications in nanoelectronics, including spintronics and nano-optoelectronics.[1–4] GaN has great potential owing to its direct bandgap, high mobility and internal polarization, as well as the large variation of its optical gap induced by alloying with In and Al. GaN NWs have been obtained by different synthesis methods using chemical vapor deposition (CVD) or molecular-beam epitaxy (MBE) procedures, with or without catalyst.[5–15]

To avoid catalyst-induced contaminations that might alter the electronic properties of the material, catalyst-free growth is preferable. However, the nucleation and growth mechanisms of GaN wires in the catalyst-free procedure are still under debate. Two mechanisms are usually invoked for the nucleation and the growth of NWs. One is based on the Ga-droplet formation followed by the well-established vapor–liquid …