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Stability of deep donor and acceptor centers in GaN, AlN, and BN

We examine the atomic and electronic structure of substitutional Be, Mg, and C acceptor impurities and of Si, Ge, S, and O donor impurities in GaN, AlN, and BN through first-principles calculations. The small bond lengths in III-V nitrides are found to inhibit large lattice relaxations around impurities and, with a few exceptions, this leads to a significant stabilization of effective-mass states over deep centers despite the large band gaps of these materials. In particular, we find that Ge and S impurities do not have stable or even metastable DX centers in GaN, AlN, or BN, independent of crystal structure. Similarly, the effective-mass states of Be, Mg, and C acceptor impurities in GaN are found to be more stable than the corresponding deep AX centers. We propose that persistent photoconductivity in Mg-doped GaN arises from the bistability of a N-site vacancy that is accompanied by a charge-state change from +3e to +1e.