Limits to doping in oxides

The chemical trends of limits to doping of many semiconducting metal oxides is analyzed in terms of the formation energies needed to form the compensating defects. The n-type oxides are found to have high electron affinities and charge neutrality levels that lie in midgap or the upper part of their gap, whereas p-type oxides have small photoionization potentials and charge neutrality levels lying in the lower gap. The doping-limit energy range is found to vary with the bulk free energy of the compound.