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. G.Brauer, W.Anwand,

D.Grambole, J.Grenzer, W.Skorupa, J.Čížek, J.Kuriplach, I.Procházka, C.C.Ling,

C.K.So, D.Schulz, D.Klimm: Physical Review B, 2009, 79[11], 115212