Defects in hydrothermal grown ZnO single crystals were studied as a function of annealing temperature by using positron annihilation, X-ray diffraction, Rutherford back-scattering, Hall and cathodoluminescence measurements. Positron lifetime measurements revealed the existence of Zn vacancy-related defects in the as-grown state. The positron lifetime decreased upon annealing above 600C, which implied the disappearance of Zn vacancy-related defects, and then remained constant up to 900C. The Rutherford back-scattering and X-ray rocking curve measurements revealed an improvement in crystal quality due to annealing above 600C. Although the crystal quality, as monitored by X-ray diffraction measurements, was further improved by annealing at above 1000C, the positron lifetime began to increase. This was due either to the formation of Zn vacancy-related defects, or to a change in the Zn vacancy charge state occupancy as a result of Fermi level movement. The electron concentration increased continuously with increasing annealing temperature up to 1200C; thus indicating the formation of excess donors such as O vacancies or Zn interstitials. The cathodoluminescence measurements revealed that the ultra-violet emission was greatly enhanced over the same temperature range. The experimental results showed that the ZnO crystal quality, electrical and optical characteristics were improved by post-growth annealing at 600 to 1200C. The disappearance of Zn vacancy-related defects contributed to the initial stage of improved crystal quality.

Post-Growth Annealing of Defects in ZnO Studied by Positron Annihilation, X-ray Diffraction, Rutherford Back-Scattering, Cathodoluminescence and Hall Measurements. Chen, Z.Q., Yamamoto, S., Maekawa, M., Kawasuso, A., Yuan, X.L., Sekiguchi, T.: Journal of Applied Physics, 2003, 94[8], 4807-12