Ultrahigh Concentration Hydrogen Doping into TiO₂

We investigate ultrahigh concentration doping of hydrogen (H) into rutile-TiO₂ (100) single crystals by low-energy (2.5 keV) hydrogen ion beam irradiation at low temperature (LT). While the hydrogen concentration was limited to H_0.3TiO₂ at 300 K, <i>in situ</i> nuclear reaction analysis (NRA) revealed ultrahigh concentration doping of hydrogen up to H_1.2TiO₂ by the LT irradiation at 50 K. The large (∼8.2%) expansion of the out-of-plane lattice constant suggests that hydrogen occupies interstitial sites in rutile TiO₂. Hydrogens of early stage irradiation act as electron donors and induce a large increase in conductivity, which is consistent with theoretical studies in the dilute limit. The nature of excess H was investigated <i>in situ</i> by transport and photoemission measurements. After LT excess H doping and postannealing to room temperature, unusual electrical transport properties were observed while maintaining the ultrahigh H concentration. <i>In situ</i> photoemission measurements show that the excessively doped hydrogens by LT irradiation generate a deeper in-gap state (IGS) of metastable nature. Density functional theory predicts the formation of double neighboring interstitial hydrogens as a possible mechanism for the deeper IGS.