A systematic study of various, nominally undoped ZnO single crystals, either hydrothermally grown or melt-grown, was performed. The crystal quality was assessed by X-ray diffraction, and a comprehensive estimation of the detailed impurity and hydrogen contents by inductively coupled plasma mass spectrometry and nuclear reaction analysis, respectively, was made also. High precision positron lifetime experiments showed that a single positron lifetime was observed in all crystals investigated, which clustered at 180 to 182ps and 165 to 167ps for hydrothermally grown and melt-grown crystals, respectively. Furthermore, hydrogen was detected in all crystals in a bound state with a high concentration (at least 0.3at%), whereas the concentrations of other impurities were very small. From ab initio calculations it was suggested that the existence of Zn-vacancy–hydrogen complexes was the most natural explanation for the given experimental facts at present. Furthermore, the distribution of H at a metal/ZnO interface of a melt-grown crystal, and the H content of a hydrothermally grown crystal upon annealing and time afterward was monitored, as this was most probably related to the properties of electrical contacts made at ZnO and the instability in p-type conductivity observed at ZnO nanorods in literature. All experimental findings and presented theoretical considerations support the conclusion that various types of Zn-vacancy–hydrogen complexes exist in ZnO and need to be taken into account in future studies, especially for hydrothermally grown materials.

Identification of Zn-Vacancy–Hydrogen Complexes in ZnO Single Crystals: a Challenge to Positron Annihilation Spectroscopy. Brauer, G., Anwand, W., Grambole, D., Grenzer, J., Skorupa, W., Čížek, J., Kuriplach, J., Procházka, I., Ling, C.C., So, C.K., Schulz, D., Klimm, D.: Physical Review B, 2009, 79[11], 115212