Positron annihilation spectroscopy was used to study the introduction and recovery of point defects in electron-irradiated n-type ZnO. Irradiation (Eel = 2MeV, fluence = 6 x 1017/cm2) was performed at room temperature, and isochronal annealing was performed at 300 to 600K. Monochromatic illumination of the samples during low-temperature positron measurements was used to identify the defects. Two kinds of vacancy defect were distinguished: Zn and O vacancies. These were either isolated or belonged to defect complexes. In addition, negative-ion-type defects were observed which were attributed to O interstitials or O antisites. The Zn vacancies and negative ions acted as compensating centers and were introduced at a concentration [VZn] ≈ cion of 2 x 1016/cm3. The O vacancies were introduced at a 10-times higher concentration, [VO], of 3 x 1017/cm3 and were suggested to be isolated. The O vacancies were observed to be neutral at low temperatures, and an ionization energy of 100meV could be fitted with the help of temperature-dependent Hall data; thus indicating their deep-donor nature. The irradiation-induced defects fully recovered after annealing at 600K; in good agreement with electrical measurements. The Zn vacancies recovered in 2 separate stages, thus indicating that the Zn vacancies were part of 2 different defect complexes. The O vacancies annealed out simultaneously with the Zn vacancies at a later stage; with an activation energy of 1.8eV. The negative ions annealed out between the 2 annealing stages of the vacancies.

Introduction and Recovery of Point Defects in Electron-Irradiated ZnO. F.Tuomisto, K.Saarinen, D.C.Look, G.C.Farlow: Physical Review B, 2005, 72[8], 085206