  Using first-principles calculations, we explored all the 21 defect-pairs in
GaN and considered 6 configurations with different defect-defect distances for
each defect-pair. 15 defect-pairs with short defect-defect distances are found
to be stable during structural relaxation, so they can exist in the GaN lattice
once formed during the irradiation of high-energy particles. 9 defect-pairs
have formation energies lower than 10 eV in the neutral state. The vacancy-pair
VN-VN is found to have very low formation energies, as low as 0 eV in p-type
and Ga-rich GaN, and act as efficient donors producing two deep donor levels,
which can limit the p-type doping and minority carrier lifetime in GaN. VN-VN
has been overlooked in the previous study of defects in GaN. Most of these
defect-pairs act as donors and produce a large number of defect levels in the
band gap. Their formation energies and concentrations are sensitive to the
chemical potentials of Ga and N, so their influences on the electrical and
optical properties of Ga-rich and N-rich GaN after irradiation should differ
significantly. These results about the defect-pairs provide fundamental data
for understanding the radiation damage mechanism in GaN and simulating the
defect formation and diffusion behavior under irradiation.
