ZnO single crystals implanted with O~+ and B~+ ions were studied by positron annihilation and Raman scattering measurements. Positron annihilation results show that vacancy clusters are generated by implantation. For the B~+-implanted sample, the vacancy clusters have a sufficient increase in size and evolve into microvoids after annealing up to 500 ℃. These microvoids need a high temperature of 900-1000 ℃ to be annealed out. However, for the O~+-implanted sample, the size of the vacancy clusters shows only a slight increase during annealing process, and they are removed at much lower temperature of 700-800 ℃. The different annealing process is supposed to be due to the chemical effect of boron impurities. Raman measurements reveal the production of oxygen vacancies by implantation. In the B~+-implanted sample they have high thermal stability up to 700 ℃, while in the O~+-implanted sample they are annealed out early at 400 ℃. It is thus suggested that the boron impurities might form complexes with oxygen interstitials and stabilize oxygen vacancies, which favors the vacancy agglomeration process.
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