Defects in electron irradiated ZnO single crystals

Point defects were introduced in ZnO single crystals by means of irradiation with several fluences of 10 MeV electrons giving rise to irradiation doses between 60 and 240 Gy. Irradiation defects were subsequently studied by positron annihilation lifetime spectroscopy (PALS), a sensitive technique for investigating open volume defects and photoluminescence (PL). After irradiation, samples were annealed in an air atmosphere from 100 to 1000 ∘C in order to follow the evolution of irradiation defects and their effect on the emission properties. Positron lifetimes and intensities measured by PALS as a function of radiation doses show that the defects generated act as effective positron traps. The PL spectra for all the samples analysed consist of a near-band-edge (NBE) emission centred at 369 nm and a broad deep-level (DL) emission around 550 nm. The main observation is that the most intense PL emission is found for the higher electron dose used, i.e. 240 Gy. This effect is similar to that observed after processes leading to an improvement of the crystal quality, but it should be interpreted as arising from the annihilation of the non-radiative recombination centres, which strongly enhances the radiative recombination mechanisms. The trends observed using both experimental techniques were discussed in terms of the possible origin, nature and state of charge of the radiation induced defects involved.