n-type doping of oxides by hydrogen

Abstract

First-principles total-energy calculations suggest that interstitial hydrogen impurity forms a shallow donor in ${\mathrm{SnO}}_2,$ 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 $\text{3.0±0.4\hspace{0.17em}}\mathrm{eV}$ below vacuum. Materials such as ${\mathrm{Ag}}_2\mathrm{O},$ HgO, CuO, PbO, PtO, ${\mathrm{IrO}}_2,$ ${\mathrm{RuO}}_2,$ ${\mathrm{PbO}}_2,$ ${\mathrm{TiO}}_2,$ ${\mathrm{WO}}_3,$ ${\mathrm{Bi}}_2{\mathrm{O}}_3,$ ${\mathrm{Cr}}_2{\mathrm{O}}_3,$ ${\mathrm{Fe}}_2{\mathrm{O}}_3,$ ${\mathrm{Sb}}_2{\mathrm{O}}_3,$ ${\mathrm{Nb}}_2{\mathrm{O}}_5,$ ${\mathrm{Ta}}_2{\mathrm{O}}_5,$ ${\mathrm{FeTiO}}_3,$ and ${\mathrm{PbTiO}}_3,$ whose conduction band minimum (CBM) lie below this level (i.e., electron $\mathrm{affinity}\text{>3.0±0.4\hspace{0.17em}}\mathrm{eV})$ will become conductive once hydrogen is incorporated into the lattice, without reducing the host. Conversely, materials such as BaO, NiO, SrO, ${\mathrm{HfO}}_2,$ and ${\mathrm{Al}}_2{\mathrm{O}}_3,$ whose CBM lie above this level (i.e., electron $\mathrm{affinity}\text{<3.0±0.4\hspace{0.17em}}\mathrm{eV})$ will remain nonconductive since hydrogen forms a deep impurity.