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

Abstract

Undoped ZnO single crystals were implanted with aluminum ions up to a dose of ${10}^{15}{\mathrm{Al}}^{+}/{\mathrm{cm}}^2.$ 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 ${\mathrm{V}}_8,$ are generated by implantation. Postimplantation annealing shows that the Doppler broadening S parameter increases in the temperature range from $200°\mathrm{C}$ to $600°\mathrm{C}$ suggesting further agglomeration of vacancy clusters to voids. Detailed analyses of Doppler broadening spectra show formation of positronium after $600°\mathrm{C}$ annealing of the implanted samples with doses higher than ${10}^{14}{\mathrm{Al}}^{+}/{\mathrm{cm}}^2.$ 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 $600–900°\mathrm{C}.$ Hall measurement shows that the implanted ${\mathrm{Al}}^{+}$ 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 ${\mathrm{Al}}^{+}$ implantation and subsequent annealing.