We have applied positron annihilation spectroscopy combined with sheet resistance measurements to study the electrical isolation of thin ZnO layers irradiated with 2 MeV ${\text{O}}^{+}$ ions at various fluences. Our results indicate that Zn vacancies, the dominant defects detected by positrons, are produced in the irradiation at a relatively low rate of about $2000{\text{cm}}^{-1}$ when the ion fluence is at most ${10}^{15}{\text{cm}}^{-2}$ and that vacancy clusters are created at higher fluences. The Zn vacancies introduced in the irradiation act as dominant compensating centers and cause the electrical isolation, while the results suggest that the vacancy clusters are electrically inactive.

• Received 24 April 2008

DOI:https://doi.org/10.1103/PhysRevB.78.035125