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 &mgr;m period hexagonal array of hexagonal openings in the mask, these openings being circumscribed into 10 &mgr;m 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 byC(0001) andR(1 1¯01) facets, are achieved with a good selectivity. It is found that the GaN growth ratesVRandVC,measured in theR⟨11¯01⟩andC⟨0001⟩ directions respectively, are drastically affected by the Mg incorporation. By adjusting the Mg partial pressure in the growth chamber, theVR/VCratio can be increased so that the delimiting topCfacet does not vanish as usually observed in undoped GaN localized growth, but by contrast expands. ©1998 American Institute of Physics.