The results of a combined study of Raman scattering, IR absorption, photoluminescence, and photoconductivity on ZnO are presented. Two shallow donors-hydrogen at the bond-centered lattice site, H BC , and hydrogen bound in an oxygen vacancy, H O -were identified. Donor H BC has an ionization energy of 53 meV. The recombination of an exciton bound to H BC gives rise to the 3360:170:2 meV photoluminescence line. The 1s-2p donor transition at 330 cm Γ1 is detected in the Raman scattering and photoconductivity spectra. The stretch mode of the associated O-H bond is detected in IR absorption at 3611 cm Γ1 . The H O donor in ZnO has an ionization energy of 47 meV. The excitonic recombination at H O leads to the previously labeled I 4 line at 3362.8 meV. Photoconductivity and Raman spectra reveal the 1s-2p donor transition at 265 cm Γ1 .
It is shown that H BC migrating through the ZnO lattice forms electrically inactive interstitial H 2 . Vibrational modes of H 2 , HD, and D 2 were identified at 4145, 3628, and 2985 cm Γ1 , respectively. These results suggest that interstitial H 2 is responsible for the ''hidden'' hydrogen in ZnO.