  We discuss the microstructural origin of enhanced radial growth in magnesium
(Mg) doped gallium nitride (GaN) nanorods (NRs) using electron microscopy and
\textit{first-principles} Density Functional Theory calculations.
Experimentally, we find the Mg incorporation increases surface coverage of the
grown samples and the height of NRs decreases as a consequence of an increase
radial growth rate. We also observed the coalescence of NRs becomes prominent
and the critical height of coalescence decreases with the increase in Mg
concentration. From \textit{first-principles} calculations, we find the surface
free energy of Mg doped surface reduces with increasing Mg concentration in the
samples. The calculations further suggests a reduction in the diffusion barrier
of Ga adatoms along [11$\overline{2}$0] on the side wall surface of the NRs,
possibly the primary reason for the observed enhancement in the radial growth.
