Nanoscale compositional analysis of Ni-based seed crystallites associated with GaN nanowire growth

Scanning transmission electron microscopy has been used to investigate the growth mechanism of gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy. The composition of nickel seeds used to promote nanowire-type growth has been studied. These seeds remain at the end of the nanowires after growth as Ni-based metallic crystallites. The concentration of gallium, nickel, oxygen and nitrogen in the seeds were quantified by analysing electron energy-loss spectra. The seed crystallites at the ends of the GaN nanowires are found to comprise of a metallic core of mainly nickel and gallium, with a surrounding oxide shell consisting of Ni, Ga, and O, attributed to the post-growth atmospheric exposure of the nanowires. Only a background concentration of nitrogen was found in the metallic nickel seed cores.