Substrate-dependent luminescent properties of thin-film phosphors: Comparative study of Eu3+-doped ZnO films deposited on SiO2, Si, and sapphire substrates

Photoluminescence from Eu3+-doped ZnO (ZnO:Eu) thin films deposited on SiO2 substrates was compared with that of films on Si(100) and sapphire C-plane substrates in terms of correlation with crystallinity. Common to the three kinds of substrate, deposition with H2O vapor followed by post annealing resulted in reasonably intense Eu3+ emissions upon bandgap excitation. For the films on SiO2, the emission intensity increased for Eu contents up to 3 at.%, whereas concentration quenching prevailed at Eu contents above 1 at.% on Si and sapphire substrates. Such differences demonstrate the advantages of SiO2 substrate over Si and sapphire. Its tolerance against high-density doping was due to moderately c-axis-oriented ZnO crystals on SiO2. The amorphous network of SiO2 may absorb strain in the ZnO crystal when Zn2+ sites are substituted with Eu3+ ions, and thus, degradation in the crystallinity of the ZnO host is suppressed. Another difference between the films on Si and SiO2 substrates was in the defect emission spectra superimposed on the Eu3+ emission. Only a single broad deep-level emission peak centered at 600 nm was observed for films on Si substrates, whereas multiple peaks at 520, 590, and 690 nm appeared for films on SiO2. Thus, the defect species created by annealing of ZnO host films on Si were different from those on SiO2 substrates, and this led to the distinct deep-level emission characteristics.