Atomic resolution analysis of extended defects and Mg agglomeration in Mg-ion-implanted GaN and their impacts on acceptor formation

We used atomic-scale direct observations by transmission electron microscopy (TEM) and atom probe tomography to clarify the crystallographic structures of extended defects and Mg agglomeration that form in Mg ion-implanted GaN during post-implantation annealing. The analysis showed that Mg atoms agglomerate at dislocations that bound extended defects. The concentration of Mg was higher at the dislocations with a larger Burgers vector. This indicates that the cause for the Mg agglomeration should be the pressure at the dislocations. The Mg concentration in the highly Mg-rich regions was 1 atomic %, 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 the duration increase, 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 can be a cause for inhibiting the formation of p-type GaN, as well as the previously reported compensating centers, such as N-related vacancies. The mechanism of reducing the acceptor formation by the extended defects is discussed.