Interaction of nitrogen with vacancy defects in N+-implanted ZnO studied using a slow positron beam

ZnO crystals were implanted with N+, O+, and Al+, and co-implanted with O+/N+ and Al+/N+ ions. Positron annihilation measurements indicate introduction of vacancy clusters upon implantation. In the N+-implanted and Al+/N+ co-implanted samples, these vacancy clusters are only partially annealed at 800 degrees C, as compared with their entire recovery in the O+- and Al+-implanted samples at 800-900 degrees C, suggesting a strong interaction between nitrogen and vacancy clusters. However, in the O+/N+ co-implanted sample, most vacancy clusters disappear at 800 degrees C. Probably oxygen scavenges nitrogen to enhance the annealing of the vacancy clusters. Upon further annealing at 1000-1100 degrees C, nitrogen also forms stable complexes with thermally generated vacancies. These nitrogen-related vacancy complexes need high-temperature annealing at 1200-1250 degrees C to be fully removed.