A study was made, of the electronic structure, energetics, and migration of H and H-complexes, on the basis of first-principles total-energy calculations. The latter revealed a number of features which were very different to those which were exhibited by H in more traditional semiconductors such as Si or GaAs. These included a very large negative-U effect (of about 2.4eV), an instability of the bond-center site, high energies of H molecules, and an unusual geometry of the Mg-H complex. All of these features were shown to be a result of the particular properties of the present material, such as its strongly ionic nature and the high strength of the Ga-N bond. A simple model was proposed, for the negative-U behavior, which was expected to be valid for H in any semiconductor.

J.Neugebauer, C.G.Van de Walle: Physical Review Letters, 1995, 75[24], 4452-5