Evolution of voids in Al+-implanted ZnO probed by a slow positron beam

Undoped ZnO single crystals were implanted with aluminum ions up to a dose of 10(15)Al(+)/cm(2). Vacancy defects in the implanted layers were detected using positron lifetime and Doppler broadening measurements with slow positron beams. It shows that vacancy clusters, which are close to the size of V-8, are generated by implantation. Postimplantation annealing shows that the Doppler broadening S parameter increases in the temperature range from 200degreesC to 600degreesC suggesting further agglomeration of vacancy clusters to voids. Detailed analyses of Doppler broadening spectra show formation of positronium after 600 degreesC annealing of the implanted samples with doses higher than 10(14) Al+/cm(2). Positron lifetime measurements further suggest that the void diameter is about 0.8 nm. The voids disappear and the vacancy concentration reaches the detection limit after annealing at 600-900degreesC. Hall measurement shows that the implanted Al+ ions are fully activated with improved carrier mobility after final annealing. Cathodoluminescence measurements show that the ultraviolet luminescence is much stronger than the unimplanted state. These findings also suggest that the electrical and optical properties of ZnO become much better by Al+ implantation and subsequent annealing.