Atmospheric pressure metalorganic vapor phase epitaxy is used to perform selective regrowth of GaN on a silicon nitride patterned mask, capping a GaN epitaxial layer deposited on (0001) sapphire substrate. The basic pattern is constituted by a 15 μm period hexagonal array of hexagonal openings in the mask, these openings being circumscribed into 10 μm diam circles. We investigate the effect of Mg incorporation on the growth anisotropy of the localized GaN islands by varying the ratio [Mg]/[Ga] of bis-methylcyclopentadienyl-magnesium and trimethylgallium partial pressures. Both undoped and Mg-doped GaN hexagonal pyramids, delimited by C
(0001) and R
(1 101) facets, are achieved with a good selectivity. It is found that the GaN growth rates VR
measured in the R〈101〉
〈0001〉 directions respectively, are drastically affected by the Mg incorporation. By adjusting the Mg partial pressure in the growth chamber, the VR/VC
ratio can be increased so that the delimiting top C
facet does not vanish as usually observed in undoped GaN localized growth, but by contrast expands. © 1998 American Institute of Physics.