We investigated the axial and radial growth of GaN NWs 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 toward the NW tip and the finite amount of active N available for the growth. The model explains the formation of a new equilibrium NW radius after increasing the Ga flux and provides an explanation for two well-known but so far not fully understood experimental facts: the necessity of effectively N-rich conditions for the spontaneous growth of GaN NWs and the increase in NW radius with increasing III/V flux ratio.
Consequently, the results presented here provide a guide for modifying in situ the morphological properties of group-III nitride NW heterostructures. They furthermore demonstrate that to fully understand the diffusion-induced growth of compound semiconductor NWs it is necessary to consider the actual finite amount of the element that is present in abundance.