The electronic properties of zinc oxide (ZnO) seem to be exceptionally resistant to deterioration caused by electron irradiation. The microscopic processes behind the high radiation hardness of ZnO need to be fully understood in order to uncover all the potential applications of the material. The role of vacancies in both self-compensation and the high radiation hardness can be determined by positron annihilation spectroscopy. We have studied both as-grown and 2-MeV electron irradiated single crystal ZnO bulk samples, which were nominally undoped and exhibited n-type conductivity. Recent positron studies have shown that the dominant compensating center in both the as-grown and irradiated material is the zinc vacancy (V_Zn), presumably in its doubly negative charge state [1]. The introduction rate of the zinc vacancies in electron irradiation is two orders of magnitude lower than that of gallium vacancies in gallium nitride (GaN), which is a clear manifestation of the radiation hardness. The electron irradiation produces also neutral oxygen vacancies (V_O), and their concentration can be estimated to be significantly higher than that of V_Zn. The irradiation-induced defects anneal completely out of the samples in two main stages between 100 ^oC and 300 ^oC, after which the concentration of the compensating centers is the same as in the as-grown material. Shining monochromatic light on the irradiated samples during the positron measurements at 20 K causes an increase of the average positron lifetime at photon energy 2.3 eV, indicating that also negative ion type defects are present, and that they as well as the irradiation-induced zinc vacancies have an ionization level close to 2.3 eV. No such effect was observed after the annealings or in as-grown ZnO.

[1] F. Tuomisto, V. Ranki, K. Saarinen, and D.C. Look, Phys. Rev. Lett. 91, 205502 (2003).

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