Photoluminescence of Europium in ZnO and ZnMgO thin films grown by Molecular Beam Epitaxy

In this work, luminescence properties of Europium doped ZnO and ZnMgO thin films grown on a-oriented (11-20) sapphire substrates by oxygen plasma assisted Molecular Beam Epitaxy method are studied. The concentrations and distributions of Eu atoms in the respective hosts were analysed using Secondary Ion Mass Spectrometry, which confirmed successful doping around the level of similar to 1%. Optical properties of the samples were investigated by room temperature photoluminescence (PL) and Photoluminescence excitation (PLE) measurements. The PL data suggest the co-existence of 2+ and 3+ ionic states of Eu in the as-grown samples. It has been observed that alloying Mg with ZnO increases the intensity of the D-5(0) -> F-2(J) intra-4f-shell emission of Eu3+ ions by an order of magnitude even in as-grown samples. The samples were also annealed at 800 degrees C in oxygen flux and re-measured to see changes in the PL. After annealing, all the emission lines due to the D-5(0) -> F-2(J) intra-4f-shell transitions of Eu3+ were strongly enhanced, whereas the bands attributed to Eu2+ were found to diminish or disappear completely. The results of PLE measurements point to efficient host to dopant energy transfer in the studied films.