Effect of uniaxial stress on vibrational modes of hydrogen in ZnO

The influence of uniaxial stress on local vibrational modes of hydrogen in ZnO has been studied by IR absorption spectroscopy. The defects considered are two centers tentatively assigned to isolated hydrogen trapped at the bond-centered and antibonding sites, 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. Recently, it was suggested [S. Limpijumnong and S. B. Zhang, Appl. Phys. Lett. 86, 151910 (2005)] that the sign of the hydrostatic pressure shift of the local vibrational mode is characteristic for the position of the hydrogen involved in the defect. From our stress splittings we determine 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.