The results above are in good agreement with the pseudopotential calculation by Mattila. In the present work we study the substitutional C, Zn and Mg impurities. The importance of the C impurity follows from the fact that carbon atoms may be unintentionally incorporated in nitrides during growth. C Ga introduces a resonant state above the CBM in GaN, and C Al creates in AlN an effective-mass donor level nearly degenerate with the conduction band. They are the most commonly used acceptors in the III-nitrides.
Magnesium creates even shallower acceptor states.
As results from our GF calculations the Mg Ga in GaN is close to the VBM for the neutral charge state. For Mg as well as Zn impurities we find outward relaxations around the defects. Below we report the pressure dependence of some defect levels as calculated by means of the LMTO GF method. The level shifts are therefore derived without taking relaxations into account. This approximation may lead to somewhat inaccurate pressure coefficients in cases where large atomic displacements are found near the defects.