Ultrahigh concentration hydrogen doping into TiO

Lim, G.-C.*; Fukutani, Katsuyuki; 8 of others*

We investigate ultrahigh concentration doping of hydrogen (H) into rutile-TiO(100) single crystals by low-energy (2.5 keV) hydrogen ion beam irradiation at low temperature (LT). While the hydrogen concentration was limited to HTiO at 300 K, in situ nuclear reaction analysis (NRA) revealed ultrahigh concentration doping of hydrogen up to HTiO 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 TiO. 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 in situ 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. In situ 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.