Properties of hydrogen in ZnO
Final Report Abstract
The influence of uniaxial stress on local vibrational modes of hydrogen in ZnO has been studied by IR absorption spectroscopy. The defects considered are a center tentatively assigned to isolated hydrogen trapped at the antibonding site of the ZnO lattice; a defect related to hydrogen trapped by a substitutional Li atom; and the Zn vacancy passivated by two hydrogen atoms. In all cases the uniaxial stress splitting agrees with the proposed microscopic structures of the defects. From the stress splittings it was determined that the hydrostatic part of the stress tensor is negative for the defects formed by the antibonding hydrogen and close to zero for those centers that comprise the bond-centered hydrogen. The influence of uniaxial stress on the vibrational mode of the Cu-H complex at 3192 cm-1 in ZnO has been studied. It was shown that the split patterns are consistent with the stretching mode of a bond-centered hydrogen located in the basal plane between substitutional Cu and O. Quantitative analysis of the stress effects reveals two low energy modes with frequencies of 25 and 49 cm-1. Upon substituting deuterium for hydrogen they shift to 22 and 36 cm-1, respectively. The Cu-H complex in ZnO consists of Cu on Zn site and a hydrogen atom bound to a nearby O atom with the O-H bond oriented in the basal plane of the hexagonal lattice to the c axis. The motion of hydrogen in the Cu-H complex has been studied by the stressinduced dichroism. Stress applied at room temperature along [1010] results in an alignment of the Cu-H bond. The reorientation process was found to be thermally activated with the activation energy of 0.52 ± 0.04 eV. The connection of the hydrogen movement in the Cu-H complex with the hydrogen diffusion in ZnO has been discussed and consequences for the existence of interstitial hydrogen at room temperature are presented. Local vibrational modes of a copper-dihydrogen complex CU-H2 in ZnO have been identified by IR absorption spectroscopy. Two O-H modes at 3347 and 3374 cm-1 are observed after Cu in-diffusion at 1200 °C and subsequent hydrogenation at 725 °C in a sealed ampoules filled with an H2 gas. Isotope substitution experiments revealed that the defect consists of two equivalent hydrogen atoms. From uniaxial stress measurements, the defect was found to have monoclinic symmetry. Based on these results, the observed local modes are ascribed to a defect consisting of a substitutional Cu atom and two adjacent bond-centered hydrogen atoms each of which is bound to the oxygen atoms in the basal plane of the defect. The hydrogen motion around the Cu atom was found to be thermally activated with an activation energy of 0.29±0.04 eV. The apparent thermal stability of Cu-H2 depends on the Cu doping profile and the interaction with a hydrogen species invisible in IR absorption. It was also shown that the defect giving rise to the 3326 cm-1 IR absorption line could not be aligned under uniaxial stress up to 220 K, implying that the barrier for hydrogen motion in these complexes is above 0.7 eV. Based on this, the assignment of the 3326 cm-1 line as HAB is questioned. VznH- is proposed as a tentative model of the defect giving rise to the 3326 cm-1 signal.
Publications
- Effect of uniaxial stress on vibrational modes of hydogen in ZnO, Phys. Rev. B 73, 035208 (2006)
E.V. Lavrov and J. Weber
- Uniaxial stress study of the Cu-H complex in ZnO, phys. stat. sol. (b) 243, 2657 (2006)
E. V. Lavrov and J. Weber
- Hydrogen motion in the Cu-H complex in ZnO, Phys. Rev. B 75, 205202 (2007)
F. Börrnert, E. V. Lavrov, and J. Weber
- Hydrogen motion in ZnO, Physica B 401-402, 366 (2007)
E.V. Lavrov, F. Börrnet, and J. Weber
- Copper-dihydrogen complex in ZnO, Phys. Rev. B 77, 155209 (2008)
E. V. Lavrov, J. Weber, and F. Börrnet
