n-type doping of oxides by hydrogen

First-principles total-energy calculations suggest that interstitial hydrogen impurity forms a shallow donor in SnO2, CdO, and ZnO, but a deep donor in MgO. We generalize this result to other oxides by recognizing that there exist a “hydrogen pinning level” at about 3.0±0.4 eV below vacuum. Materials such as Ag2O, HgO, CuO, PbO, PtO, IrO2, RuO2, PbO2, TiO2, WO3, Bi2O3, Cr2O3, Fe2O3, Sb2O3, Nb2O5, Ta2O5, FeTiO3, and PbTiO3, whose conduction band minimum (CBM) lie below this level (i.e., electron affinity>3.0±0.4 eV) will become conductive once hydrogen is incorporated into the lattice, without reducing the host. Conversely, materials such as BaO, NiO, SrO, HfO2, and Al2O3, whose CBM lie above this level (i.e., electron affinity<3.0±0.4 eV) will remain nonconductive since hydrogen forms a deep impurity.