  Selective area growth of GaN nanostructures has been performed on full 2"
c-sapphire substrates using Si3N4 mask patterned by nanoimprint lithography
(array of 400 nm diameter circular holes). A new process has been developed to
improve the homogeneity of the nucleation selectivity of c-oriented hexagonal
prismatic nanostructures at high temperature (1040\circ C). It consists of an
initial GaN nucleation step at 950 \circ C followed by ammonia annealing before
high temperature growth. Structural analyses show that GaN nanostructures are
grown in epitaxy with c-sapphire with lateral overgrowths on the mask. Strain
and dislocations are observed at the interface due to the large GaN/sapphire
lattice mismatch in contrast with the high quality of the relaxed crystals in
the lateral overgrowth area. A cathodoluminescence study as a function of the
GaN nanostructure size confirms these observations: the lateral overgrowth of
GaN nanostructures has a low defect density and exhibits a stronger near band
edge (NBE) emission than the crystal in direct epitaxy with sapphire. The shift
of the NBE positions versus nanostructure size can be mainly attributed to a
combination of compressive strain and silicon doping coming from surface mask
diffusion.
