We carried out atomic-scale observations of Mg-ion-implanted GaN by transmission electron microscopy (TEM) and atom probe tomography (APT) to clarify the crystallographic structures of extended defects and Mg agglomerations that form during post-implantation annealing. The complementary TEM and APT analyses have shown that Mg atoms agglomerate at dislocations that bound extended defects. The concentration of Mg is higher at the dislocations with a larger Burgers vector. This indicates that Mg agglomeration is caused by the pressure at the dislocations. Mg concentration in highly Mg-rich regions is 1 at. %, which exceeds the solubility limit of Mg in GaN. We investigated isothermal and isochronal evolution of the defects by TEM, cathodoluminescence analysis, and positron annihilation spectroscopy. The results indicated that the intensity of donor–acceptor pair emission increases with the annealing temperature and duration and reaches a maximum after elimination of the extended defects with highly Mg-rich regions. These results strongly suggest that such extended defects reduce the acceptor formation and that they as well as the previously reported compensating centers, such as N-related vacancies, can inhibit the formation of p-type GaN. The mechanism by which the extended defects reduce acceptor formation is discussed.

Herein, transmission electron microscopy direct observations are used to examine the time evolution of dislocation loops in Mg‐ion‐implanted GaN during annealing in N2 atmospheres of 2.0 and 0.3 GPa. It is indicated in the results that the diffusion of the native defects, for both interstitials and vacancies, is retarded during annealing at the higher pressure. Furthermore, secondary ion mass spectrometry shows that annealing at the higher pressure retards the migration of Mg and increases Mg‐acceptor concentration in the ion‐implanted region. These findings provide a design principle of the postimplantation annealing process to activate ion‐implanted Mg in GaN.