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)] occurs in the form of interstitial H₂.