The Mechanism of Ni-Assisted GaN Nanowire Growth.

Despite the numerous reports on the metal-catalyzed growth of GaN nanowires, the mechanism of growth is not well understood. Our study of the nickel-assisted growth of GaN nanowires using metalorganic chemical vapor deposition provides key insights into this process. From a comprehensive study of over 130 nanowires, we observe that as a function of thickness, the length of the nanowires initially increases and then decreases. We attribute this to an interplay between the Gibbs-Thomson effect dominant in very thin nanowires and a diffusion induced growth mode at larger thickness. We also investigate the alloy composition of the Ni-Ga catalyst particle for over 60 nanowires using energy dispersive X-ray spectroscopy, which along with data from electron energy loss spectroscopy and high resolution transmission electron microscopy suggests the composition to be NiGa. At the nanowire growth temperature, this alloy cannot be a liquid, even taking into account melting point depression in nanoparticles. We hence conclude that Ni-assisted GaN nanowire growth proceeds via a vapor-solid-solid mechanism instead of the conventional vapor-liquid-solid mechanism.