Gallium self-interstitial relaxation in GaAs:: An ab initio characterization

Ga interstitials in GaAs (I-Ga) are studied using the local-orbital ab initio code SIESTA in a supercell of 216+1 atoms. Starting from eight different initial configurations, we find five metastable structures: the two tetrahedral sites in addition to the 110-split([Ga-As]), 111-split([Ga-As]), and 110-split([Ga-Ga]). Studying the competition between various configuration and charges of I-Ga at T=0 K, we find that predominant gallium interstitials in GaAs are charged +1, neutral, or at most -1 depending on doping conditions and prefer to occupy the tetrahedral configuration where it is surrounded by Ga atoms. Our results are in excellent agreement with recent experimental results concerning the dominant charge of I-Ga, underlining the importance of finite size effects in the calculation of defects.