Reorientation kinetics of hydroxyl groups in anatase TiO2.

The reorientation kinetics of hydrogen in a variety of complexes in the anatase polymorph of TiO2 was investigated by means of stress-induced dichroism. For the hydrogen-defect resulting in an O-H vibrational mode with a frequency of 3389 cm-1, the energy barrier separating adjacent equivalent in-plane sites of hydrogen was determined to be independent of the isotope and equal to 0.74 ± 0.02 eV, whereas the attempt frequency was found to be (1.10 ± 0.20) × 1012 and (0.75 ± 0.15) × 1012 s-1 for hydrogen and deuterium, respectively. The defect with vibrational modes at 3412 and 3417 cm-1 previously assigned to isolated hydrogen did not reveal alignment under the stress up to room temperature, which indicates that the barrier of hydrogen motion is above 0.9 eV.

Unambiguous identification of the role of a single Cu atom in the ZnO structured green band.

High quality and purity single crystal ZnO samples doped with single isotopes of (63)Cu and (65)Cu, with equal concentrations of both these isotopes, and with natural Cu using a wet chemical atomic substitution reaction and anneal were studied using low temperature optical spectroscopy. Our data on the zero phonon line of the structured green band in ZnO confirm unambiguously the involvement of a single Cu atom in this defect emission. These data allow us to confirm the main features of the assignment proposed by Dingle in 1969 and to comment further on the defect structure.

Identification of hydrogen molecules in ZnO.

Hydrogen molecules in ZnO are identified by their local vibrational modes. In a Raman study, interstitial H2, HD, and D2 species were found to exhibit local vibrational modes at frequencies 4145, 3628, and 2985 cm-1, respectively. After thermal treatment of vapor phase grown ZnO samples in hydrogen atmosphere, most hydrogen forms shallow donors at the bond-centered site (HBC). Subsequently, HBC migrates through the crystal and forms electrically inactive H2. These results imply that the "hidden" hydrogen in ZnO [G. A. Shi et al., Appl. Phys. Lett. 85, 5601 (2004)10.1063/1.1832736] occurs in the form of interstitial H2.