Bistability of a hydrogen defect with a vibrational mode at 3326 cm-1 in ZnO

Results from illumination-sensitive infrared absorption and photocurrent spectroscopy measurements on the hydrogen defect in ZnO characterized by a local vibrational mode with a frequency of 3326 cm are reported. It is shown that photoexcitation with (h) over bar omega >= 2.7 eV results in two new axial oriented O-H stretch vibrational modes at 3170 and 3589 cm, which appear at the expense of the previously reported azimuthal oriented 3326- and 3358-cm modes. We propose a tentative configurational-coordinate energy diagram of the defect and discuss its microscopic origin.