Computational study of the effect of Al and In on the formation energies and acceptor levels of Mg and C dopants in GaN

The effect of aluminium and indium on the formation energies and acceptor l evels of magnesium and carbon dopants in GaN have been calculated using a d ensity functional approach. Single aluminium and indium atoms are incorpora ted into the lattice at a minimal distance from the acceptor species. The f ormation energies are obtained as a function of the position of the Fermi l evel. In the absence of aluminium or indium, magnesium in a charge neutral state is found to have a formation energy of 1.1 eV, whereas carbon has a f ormation energy of 2.6 eV. In the presence of indium, the magnesium formati on energy rises to 1.4 eV, whereas the addition of aluminium. has no effect . For carbon, the presence of aluminium. and indium increases the formation energy by 0.4 and 0.3 eV, respectively. Furthermore, the calculations pred ict that the magnesium acceptor level becomes more shallow by the addition of aluminium (from 0.14 to 0.01 eV above the valence band maximum), but is made deeper by indium (from 0.14 to 0.27 eV above the valence band maximum) . The carbon acceptor level is found to remain approximately unchanged with aluminium and indium doping. These results are compared to experimental da ta and the effect of various approximations in the calculations is discusse d.