Influence of post annealing treatments on the luminescence of rare earth ions in ZnO:Tb,Eu/Si heterojunction

Highlights: • Co-doped ZnO:Tb, Eu film grown by magnetron sputtering on Si substrate. • Low photoluminescence and high electroluminescence emissions of Tb³⁺ and Eu³⁺ in ZnO. • Oxyapatite phase formation starting at 1173 K and containing both rare earth ions. • Efficient energy transfer from Tb³⁺ to Eu³⁺ in oxyapatite. (Tb, Eu)–co-doped ZnO films with about 3 at.% total doping rate were grown by magnetron sputtering on Si substrate. Post annealing treatments were performed at 973–1373 K in continuous nitrogen flow to investigate the transformation of microstructural and optical characteristics by means of X-ray diffraction, transmission electron microscopy, photoluminescence and electroluminescence. For annealing temperatures lower than 1073 K, segregation of Eu and Tb was observed mainly at the film/substrate junction. For temperatures higher than 1173 K, additional phases appeared, namely, Zn₂SiO₄ and rare earth silicates. For the highest temperature investigated (1373 K), only silica and rare earth silicates remained in the film due to Zn evaporation. PL measurements indicated a very intense Eu emission associated with the presence of rare earth silicate inclusions. Energy transfer from Tb towards Eu was evidenced in this secondary phase. At last, based on these preliminary works, a (Tb, Eu)–co-doped ZnO/Si electroluminescent structure was produced and showed very promising results paving the way for very thin ZnO based light emitting diodes.