The thermodynamic doping limits of GaN and ZnO were assessed on the basis of point defect calculations performed using the embedded cluster approach and employing a hybrid non-local density functional for the quantum mechanical region. Using this approach, a staggered (type-II) valence band alignment between the two materials was deduced, with the N 2p states contributing to the lower ionization potential of GaN. With respect to the stability of free electron and hole carriers, redox reactions resulting in charge compensation by ionic defects were found to be largely endothermic (unfavourable) for electrons and exothermic (favourable) for holes, which was consistent with the efficacy of electron conduction in these materials

Electron and Hole Stability in GaN and ZnO. A.Walsh, C.R.A.Catlow, M.Miskufova, A.A.Sokol: Journal of Physics - Condensed Matter, 2011, 23[33], 334217