Relaxation effects on the negatively charged Mg impurity in zincblende GaN

The electronic structure of Mg impurity in zincblende (c-)GaN is investigat ed by using the ab initio full potential linear-augmented plane-wave method and the local density-functional approximation. Fun geometry optimization calculations, including nearest and next-nearest neighbor displacements, ar e performed far the impurity in the neutral and negatively charged states. A value of 190 +/- 10 meV was obtained for the Franck-Condon shift to the t hermal energy, which is in good agreement with that observed in recent low temperature photoluminescence and Hall-effect measurements. We conclude tha t the nearest and next-nearest neighbors of the Mg impurity replacing Ga in c-GaN undergo outward relaxations which play an important role in the dete rmination of the center acceptor energies.