Nucleation, Growth, and Bundling of GaN Nanowires in Molecular Beam Epitaxy: Disentangling the Origin of Nanowire Coalescence

We investigate the nucleation, growth,
and coalescence of spontaneously formed GaN nanowires in molecular
beam epitaxy combining the statistical analysis of scanning electron
micrographs with Monte Carlo growth models. We find that (i) the nanowire
density is limited by the shadowing of the substrate from the impinging
fluxes by already existing nanowires, (ii) shortly after the nucleation
stage, nanowire radial growth becomes negligible, and (iii) coalescence
is caused by bundling of nanowires. The latter phenomenon is driven
by the gain of surface energy at the expense of the elastic energy
of bending and becomes energetically favorable once the nanowires
exceed a certain critical length.