Defects in Electron irradiated ZnO single crystals.

M.A.HERNÁNDEZ-FENOLLOSA; L.C.DAMONTE; B.MARÍ

Strasbourgh, Francia

Simposio; E-MRS Symposium: G: ZnO and related materials, Strasbourgh, junio 2005.; 2005

ZnO is being the object of intense studies owing to its attractive applications on UV optoelectronic. Intrinsic and induced point defects in ZnO lead to the generation of bounded states with important effects on the material properties. A complete knowledge of point defect structure is needed to understand its optical behaviour. We have used two experimental techniques highly sensitive to defects: positron annihilation lifetime spectroscopy (PALS) well adapted for open volume defects studies and photoluminescence (PL) which directly probes the optical activity of the centres. ZnO single crystals commercially provided were exposed to different 10 MeV electrons fluences giving rise to irradiation doses between 60 and 240 Gy. After irradiation, samples were annealed in air atmosphere from 100 to 600ºC 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 generated defects act as effective positron traps. The PL spectra for all the analyzed samples 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 an annihilation of the non radiative recombination centres which strongly enhances the radiative recombination mechanisms. The trends observed by both experimental techniques were discussed in terms of the possible origin, nature and state of charge of the radiation induced defects involved.