Abstract
In (Tb, Eu) co-doped ZnO films grown on p-Si and Al2O3 substrates, the annealing at 900 °C was found to result in the diffusion and segregation of rare earth (RE) dopants to ZnO grain boundaries and film surface. The segregation of RE dopants to film/substrate interface was observed in the film on p-Si and explained by the drift of positively charged RE ions in the electric field of n-ZnO/p-Si junction. The annealing promoted also formation of additional crystal phases, namely Tb2O3, TbO2, TbxEuySiO7, Zn2SiO4 and ZnAl2O4. In the photoluminescence spectra, it caused the appearance of narrow and broader Eu3+ emission lines. The broader lines emerged only for the film on p-Si substrate and were ascribed to Eu3+ in TbxEuySiO7 inclusions formed at the ZnO/p-Si interface. The narrow lines were attributed to Eu3+ ions in Tb2O3 inclusions located at ZnO grain boundaries and film surface. The energy transfer from Tb3+ to Eu3+ was demonstrated in the annealed films only. A theoretical approach that allowed to describe the RE ion surrounding and to calculate the energy spectra of Tb3+ and Eu3+ in various site positions was developed. The crystal field parameters were also estimated.
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