Zinc oxide (ZnO) is a wide-band-gap semiconductor with potential optical, electronic, and mechanical applications. First-principles investigations [C. G. Van de Walle, Phys. Rev. Lett. 85, 1012 (2000)] predicted that hydrogen impurities act as shallow donors in ZnO. IR spectroscopy [M. D. McCluskey et al., Appl. Phys. Lett. 81, 3807 (2002)] showed that a local vibrational mode at $3326.3{\mathrm{cm}}^{-1}$, at liquid-helium temperatures, corresponded to an $\mathrm{O}-\mathrm{H}$ type bond. The microscopic structure of this hydrogen complex, however, was not determined. In this Brief Report, the structure and stability of $\mathrm{O}-\mathrm{H}$ complexes are discussed. The second excited state of the $\mathrm{O}-\mathrm{H}$ stretch mode was found at $6389{\mathrm{cm}}^{-1}$, allowing us to compare the experimental results with the harmonic calculations of Van de Walle. Results from high-pressure and polarized IR spectroscopy strongly suggest that hydrogen occupies an antibonding location with an $\mathrm{O}-\mathrm{H}$ bond oriented at an angle of 111° to the $c$ axis. By correlating the IR absorbance strength with free-electron concentration, it was established that the $\mathrm{O}-\mathrm{H}$ complexes are shallow donors. However, the $\mathrm{O}-\mathrm{H}$ donors are unstable, decaying significantly after several weeks at room temperature. The kinetics of the dissociation follow a bimolecular decay model, consistent with the formation of ${\mathrm{H}}_2$ molecules.

• Received 7 July 2005

DOI:https://doi.org/10.1103/PhysRevB.72.113201