Undoped ZnO single crystals were implanted with Al ions up to a dose of 1015Al+/cm2. Vacancy defects in the implanted layers were detected by using positron lifetime and Doppler broadening measurements with slow positron beams. This showed that vacancy clusters, which were close to the size of V8, were generated by implantation. Post-implantation annealing showed that the Doppler broadening S parameter increases in the temperature range from 200 to 600C suggesting further agglomeration of vacancy clusters to voids. Detailed analyses of Doppler broadening spectra showed formation of positronium after 600C annealing of the implanted samples with doses higher than 1014Al+/cm2. Positron lifetime measurements further suggested that the void diameter was about 0.8nm. The voids disappeared and the vacancy concentration reached the detection limit after annealing at 600 to 900C. Hall measurements showed that the implanted Al+ ions were fully activated with improved carrier mobility after final annealing. Cathodoluminescence measurements showed that the ultra-violet luminescence was much stronger than the unimplanted state. These findings also suggested that the electrical and optical properties of ZnO become much better by Al+ implantation and subsequent annealing.

Evolution of Voids in Al+-Implanted ZnO Probed by a Slow Positron Beam. Chen, Z.Q., Maekawa, M., Yamamoto, S., Kawasuso, A., Yuan, X.L., Sekiguchi, T., Suzuki, R., Ohdaira, T.: Physical Review B, 2004, 69[3], 035210