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 Eu 2 + and Eu 3 + into the ZnO matrix has been confirmed by x-ray photoemission spectroscopy. This means that a small fraction of Europium substitutes Zn 2 + as Eu 2 + into the ZnO matrix; the rest of Eu being in the trivalent state. This is probably due to the formation of Eu 2 O 3 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 D 5 0 → F 7 2 Eu 3 + transition. In addition the photoluminescence excitonic spectra showed efficient energy transfer from the ZnO matrix to the Eu 3 + 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.