  We address the mechanism of early stages of growth and shape transition of
the unique nanowall network (NwN) nanostructure of GaN by experimentally
monitoring its controlled growth using PA-MBE and complementing it by
\textit{first-principles} calculations. Using electron microscopy, we observe
the formation of tetrahedron shaped (3 faced pyramid) islands at early stages
of growth, which later grows anisotropically along their edges of the
(20$\overline{2}$1) facets, to form the wall like structure. The mechanism of
this crystal growth is discussed in light of surface free energies of the
different surfaces, adsorption energy and diffusion barrier of Ga ad-atoms on
the (20$\overline{2}$1) facets. By \textit{first-principles} calculations, we
find that the diffusion barrier of ad-atoms decreases with decreasing width of
facets, and is responsible for the anisotropic growth and formation of the
nanowall network. This study suggest that formation of NwN is a archetype
example of structure dependent attachment kinetic (SDAK) instability induced
shape transition in thin film growth.
