Large anisotropic adatom kinetics on nonpolar GaN surfaces: Consequences for surface morphologies and nanowire growth

The diffusion barriers of Ga and N adatoms on the nonpolar a- and m-plane surfaces have been studied employing density-functional theory calculations. Our calculations reveal a strong in-plane anisotropy of the diffusion barriers for both surfaces: for the a-plane surface larger diffusion lengths parallel to the c axis are observed. For the m-plane surface the in-plane anisotropy is reversed, and significantly smaller barriers are found for diffusion normal to the [0001] direction. These results are consistent with the experimentally observed in plane growth anisotropy of the nonpolar GaN surfaces. Moreover, they indicate that the large aspect ratio achieved for c-axial GaN nanowires is not a consequence of a strong anisotropy in the adatom mobilities but is rather related to surface thermodynamics and nucleation.