  We investigate the axial and radial growth of GaN nanowires upon a variation
of the Ga flux during molecular beam epitaxial growth. An increase in the Ga
flux promotes radial growth without affecting the axial growth rate. In
contrast, a decrease in the Ga flux reduces the axial growth rate without any
change in the radius. These results are explained by a kinetic growth model
that accounts for both the diffusion of Ga adatoms along the side facets
towards the nanowire tip and the finite amount of active N available for the
growth. The model explains the formation of a new equilibrium nanowire radius
after increasing the Ga flux and provides an explanation for two well known but
so far not understood experimental facts: the necessity of effectively N-rich
conditions for the spontaneous growth of GaN nanowires and the increase in
nanowire radius with increasing III/V flux ratios.
