  We present an ab initio study of dopant-dopant interactions in
beryllium-doped InGaAs. We consider defect formation energies of various
interstitial and substitutional defects and their combinations. We find that
all substitutional-substitutional interactions can be neglected. On the other
hand, interactions involving an interstitial defect are significant. Specially,
interstitial Be is stabilized by about 0.9/1.0 eV in the presence of one/two
BeGa substitutionals. Ga interstitial is also substantially stabilized by Be
interstitials. Two Be interstitials can form a metastable Be-Be-Ga complex with
a dissociation energy of 0.26 eV/Be. Therefore, interstitial defects and
defect-defect interactions should be considered in accurate models of Be doped
InGaAs. We suggest that In and Ga should be treated as separate atoms and not
lumped into a single effective group III element, as has been done before. We
identified dopant-centred states which indicate the presence of other charge
states at finite temperatures, specifically, the presence of Beint+1 (as
opposed to Beint+2 at 0K).
