Stability and structure of hydrogen defects in zinc oxide

Zinc oxide (ZnO) is a wide band gap semiconductor with potential optical, electronic, and mechanical applications. First-principles investigations predicted that hydrogen impurities act as shallow donors in ZnO. IR spectroscopy showed that hydrogen exhibits a local vibrational mode at 3326.3 cm?1 , at liquid-helium temperatures, corresponding to an O-H type bond. 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 110.6o from 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 H2 molecules. Attempts at chemical vapor transport (CVT) growth of ZnO crystals were successful. Results varied from Zn-rich, orange-colored samples to nearly colorless ZnO polycrystals. Further analysis revealed an unknown contaminant that became apparent after annealing in hydrogen, resulting in an IR absorption peak at 3150 cm?