Infrared absorption from OH− ions adjacent to lithium acceptors in hydrothermally grown ZnO

An intense infrared absorption band has been observed in a hydrothermally grown ZnO crystal. At $12\mathrm{K}$ , the band peaks near $3577.3{\mathrm{cm}}^{-1}$ and has a half width of $0.40{\mathrm{cm}}^{-1}$ , and at $300\mathrm{K}$ , the band peaks at $3547{\mathrm{cm}}^{-1}$ and has a half width of $41.3{\mathrm{cm}}^{-1}$ . This absorption band is highly polarized, with its maximum intensity occurring when the electric field of the measuring light is parallel to the $c$ axis of the crystal. Photoinduced electron-paramagnetic-resonance experiments show that the crystal contains lithium acceptors (i.e., lithium ions occupying zinc sites). Lithium and $\mathrm{O}{\mathrm{H}}^{-}$ ions are present in the crystal because lithium carbonate, sodium hydroxide, and potassium hydroxide are used as solvents during the hydrothermal growth. In the as-grown crystal, some of the lithium acceptors will have an $\mathrm{O}{\mathrm{H}}^{-}$ ion located at an adjacent axial oxygen site (to serve as a passivator), and we assign the $3577.3\text{-}{\mathrm{cm}}^{-1}$ band observed at $12\mathrm{K}$ to these neutral complexes. Our results illustrate the role of hydrogen as a charge compensator for singly ionized acceptors in ZnO.