Nitrogen pairs, triplets, and clusters in GaAs and GaP

The electronic and atomic structure of substitutional nitrogen pairs, triplets, and clusters in GaP and GaAs is studied using the multiband empirical pseudopotential method with atomistically relaxed supercells. A single nitrogen impurity creates a localized a₁(N) gap state in GaP, but in GaAs, the state is resonant above the conduction-band minimum. We show how the interaction of multiple a₁ impurity levels, for more than one nitrogen, results in a nonmonotonic relationship between energy level and impurity separation. We assign the lowest (NN1) line in GaP to a [2,2,0] oriented pair, the second (NN2) line to a triplet of nitrogen atoms, and identify the origin of a deeper observed level as an [1,1,0] oriented triplet. We also demonstrate that small nitrogen clusters readily create very deep levels in both GaP and GaAs.