Structure and stability of O-H donors in ZnO from high-pressure and infrared spectroscopy

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 , Appl. Phys. Lett. 81, 3807 (2002)] showed that a local vibrational mode at 3326.3 cm(-1), at liquid-helium temperatures, corresponded to an O-H type bond. The microscopic structure of this hydrogen complex, however, was not determined. In this Brief Report, the structure and stability of O-H complexes are discussed. The second excited state of the O-H stretch mode was found at 6389 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 O-H bond oriented at an angle of 111 degrees to the c axis. By correlating the IR absorbance strength with free-electron concentration, it was established that the O-H complexes are shallow donors. However, the O-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 H-2 molecules.