Slow positrons have been used to study ZnO layers grown on a-axis sapphire and irradiated by 2 MeV O+ ions to fluences from 10(12) cm (2) to 10(17) cm (2). At low fluences Zn vacancies are observed, and their introduction rate is estimated as 2000 cm (1). At the highest fluences of 10(16)-10(17) cm (2) vacancy clusters are formed. The extent of the primary damage and its recovery is discussed. (c) 2008 Elsevier B.V. All rights reserved.