  Recent experiments suggest that Mg condensation at threading dislocations
induce current leakage, leading to degradation of GaN-based power devices. To
study this issue, we perform first-principles total-energy electronic-structure
calculations for various Mg and dislocation complexes. We find that threading
screw dislocations (TSDs) indeed attract Mg impurities, and that the electronic
levels in the energy gap induced by the dislocations are elevated towards the
conduction band as the Mg impurity approaches the dislocation line, indicating
that the Mg-TSD complex is a donor. The formation of the Mg-TSD complex is
unequivocally evidenced by our atom probe tomography in which Mg condensation
and diffusion through [0001] screw dislocations is observed in p-n diodes.
These findings provide a novel picture that the Mg being a p-type impurity in
GaN diffuses toward the TSD and then locally forms an n-type region. The
appearance of this region along the TSD results the reverse leakage current.
