THE INFLUENCE OF PROCESSING CONDITIONS ON POINT-DEFECTS AND LUMINESCENCE-CENTERS IN ZNO

Positron Lifetime spectroscopy and cathodoluminescence were employed t o study luminescence centers in ZnO. The samples were high-purity poly crystalline ceramics sintered at temperatures ranging from 800 to 1400 degrees C for 2 to 40 h. Scanning electron microscopy shows that as a nnealing temperatures and/or times increase, the average grain size in creases and can reach 30 mu m for samples sintered at 1200 degrees C. At the same time, the positron bulk Lifetime preaches theoretically es timated single-crystal values, while the integrated luminescence inten sity increases significantly. A further increase of the sintering temp erature beyond 1200 degrees C results in a decrease in the luminescenc e intensity, in good agreement with the only weak luminescence observe d in single-crystalline material. The positron lifetime spectra clearl y show the existence of one dominant vacancy-type defect, most likely a complex involving V-Zn, or the divacancy, V-Zn V-O, independent of s ample thermal history. The concentration of this center steadily decre ases with increasing sintering temperature. It is concluded that the y ellow luminescence centers are related to charged zinc vacancies trapp ed in the grain boundary regions. We propose that the observed broadne ss of the spectra Likely originates from the modification of the elect ronic configuration of the luminescence centers due to their complex e nvironment. A direct connection between the positron and the luminesce nce results could not be established; instead, they appear to reflect two relatively independent aspects of the samples. It could be shown, however, that positron annihilation measurements can be used effective ly to monitor the evolution of the microstructure of the samples, in g ood agreement with scanning electron micrographs.