Correlation of structural properties with energy transfer of Eu-doped ZnO thin films prepared by sol-gel process and magnetron reactive sputtering.

We investigated the structural and optical properties of Eu-doped ZnO thin films made by sol-gel technique and magnetron reactive sputtering on Si (100) substrate. The films elaborated by sol-gel process are polycrystalline while the films made by sputtering show a strongly textured growth along the c-axis. X-ray diffraction patterns and transmission electron microscopy analysis show that all samples are free of spurious phases. The presence of Eu²⁺ and Eu³⁺ into the ZnO matrix has been confirmed by x-ray photoemission spectroscopy. This means that a small fraction of Europium substitutes Zn²⁺ as Eu²⁺ into the ZnO matrix; the rest of Eu being in the trivalent state. This is probably due to the formation of Eu₂O₃ oxide at the surface of ZnO particles. This is at the origin of the strong photoluminescence band observed at 2 eV, which is characteristic of the ⁵D₀→⁷F₂ Eu³⁺ transition. In addition the photoluminescence excitonic spectra showed efficient energy transfer from the ZnO matrix to the Eu³⁺ ion, which is qualitatively similar for both films although the sputtered films have a better structural quality compared to the sol-gel process grown films. [ABSTRACT FROM AUTHOR]