Thermal evolution of defects in as-grown and electron-irradiated ZnO studied by positron annihilation

Vacancy-type defects in as-grown ZnO single crystals have been identified using positron annihilation spectroscopy. The grown-in defects are supposed to be zinc vacancy (V-Zn)-related defects, and can be easily removed by annealing above 600 degrees C. V-Zn-related defects are also introduced in ZnO when subjected to 3 MeV electron irradiation with a dose of 5.5x10(18) cm(-2). Most of these irradiation-induced V-Zn are annealed at temperatures below 200 degrees C through recombination with the close interstitials. However, after annealing at around 400 degrees C, secondary defects are generated. A detailed analysis of the Doppler broadening measurements indicates that the irradiation introduced defects and the annealing induced secondary defects belong to different species. It is also found that positron trapping by these two defects has different temperature dependences. The probable candidates for the secondary defects are tentatively discussed in combination with Raman scattering studies. After annealing at 700 degrees C, all the vacancy defects are annealed out. Cathodoluminescence measurements show that V-Zn is not related to the visible emission at 2.3 eV in ZnO, but would rather act as nonradiative recombination centers.