Various forms of defect complexes are studied in Eu-doped GaN by first-principles calculations with the framework of density functional theory. We consider defect complexes formed by dopant Eu and vacancies or insterstitals with the neutral charge states in GaN, i.e., Eu-Ga-V-Ga, Eu-Ga-V-N, Eu-Ga-N-i, and Eu-Ga-O-i, where Eu-Ga represents the Eu site substituting for Ga site, and V-Ga, V-N, N-i and O-i are Ga vacancies, N vacancies, interstitial N and interstitial O, respectively. The formation energies are calculated for all defect configurations. The formation energies of native Ga vacancies are found to be higher compared to those of N vacancies, while the tetrahedral interstitials are energetically more stable than the octahedral interstitial. The Eu-4f electrons introduce a magnetic moment of about 6.15 mu(B) in the case of single dopant Eu, i.e., Eu-Ga, which is probably suppressed in the presence of octahedral interstitial N around Eu. And Ga vacancies and octahedral interstitial O can contribute the magnetic moments to the systems, while other intrinsic defects do almost not promote the formation of local magnetic moments.
