Introduction and annealing behavior of defects in Al(+)-implanted ZnO have been studied using an energy variable slow positron beam. Vacancy clusters are produced after Al(+)-implantation. With increasing ion dose above 10(14) Al(+)/cm(2) the implanted layer is amorphized. Heat treatment up to 600degreesC enhances the creation of large voids that allow the positronium formation. The large voids disappear accompanying the recrystallization process by further heat treatment above 600degreesC. Afterwards, implanted Al impurities are completely activated to contribute to the n-type conduction. The ZnO crystal quality is also improved after recrystallization.