DOPING PROPERTIES OF C, SI, AND GE IMPURITIES IN GAN AND ALN

Doping properties of substitutional C, Si, and Ge impurities in wurtzi te GaN and AIN were studied by quantum molecular dynamics. We consider ed incorporation of impurities on both cation and anion sublattices. W hen substituting for cations, Si and Ge are shallow donors in GaN, whi le Ge becomes a deep donor in AlN. Both impurities are deep accepters on the N site. Substitutional C-cation is a shallow donor in GaN, but a deep one in AlN; C-N is a relatively shallow acceptor in both materi als. Two effects that potentially quench doping efficiency were invest igated. The first one is the transition of a donor from a substitution al position to a DX-Like configuration. In crystals with a wurtzite sy mmetry, there are two possible variants of a DX-like state, and they h ave substantially different properties. In GaN, DX- states of both Si and Ge are unstable, or metastable, and thus they do not affect doping efficiency. In contrast, they are stable in AIN, and therefore neithe r Si nor Ge is a dopant in this material. Estimates obtained for AlxGa 1-xN alloys show that the crossover composition for DX stability is mu ch lower for Ge (x congruent to 0.3) than for Si (x congruent to 0.6). The second effect quenching the doping efficiency is self-compensatio n, i.e., simultaneous incorporation of impurity atoms on both cation a nd anion sublattice. This effect may be enhanced by the formation of n earest-neighbor donor-acceptor pairs. The calculated binding energies of such pairs are large, about 1 eV, influencing self-compensation in some cases. Finally, the computed formation energies are used to ident ify growth conditions under which all these impurities may be efficien t dopants in wide-band-gap nitrides.