Peculiarities of erbium incorporation into ZnO microrods at high doping level leading to upconversion and the morphology change. Influence on excitonic as well as shallow donor states

Heavily Er-doped zinc oxide (ZnO) microrods with nominal compositions, ZnO:Er(2, 10, 30%), were prepared by the hydrothermal growth method. The crystallographic phases present in the as grown and annealed materi-als were determined. The presence of the hexagonal Wurtzite ZnO phase in the form of the hexagonal shape nanoplatelets and microrods was confirmed. Moreover, the Er2O3 phase in the form of nanosheets has been revealed as well. Erbium is partly incorporated into the ZnO hosts. It was thinning upon erbium doping level contributing to erbium oxide. This effect was even more pronounced upon annealing in air. Moreover, the in-fluence of erbium doping and annealing on paramagnetic shallow donor centers has been studied. There were two types of these centers, both affected by the erbium presence. The increase of the neutral zinc vacancies amount upon erbium doping has been proven. Annealing in air moderated the charge distribution between them. Moreover, upconversion processes between the two erbium ions were observed and explained. The surface distribution of excitonic emission centers was studied. The larger the erbium content the weaker the exciton luminescence. The smaller the ZnO microrods the stronger the exciton emission.