Luminescence of (Mg,Zn)Al_2O_4:Tb mixed spinel thin films prepared by spin-coating

摘要

MgAl_2O_4 and ZnAl_2O_4 both have the spinel structure and similar lattice constants, but the bandgap of MgAl_2O_4 is about double that of ZnAl_2O_4, making it interesting to consider the mixed spinel (Mg_xZn_(1-x))Al_2O_4 as a possible host for luminescent ions. Prior to preparing thin films, the Mg:Zn ratio and Tb concentration were optimized for green luminescence from the ~5D_4 - ~7F_5 transition of Tb~(3+) ions using nanocrystalline samples prepared by combustion synthesis. Thin films with x = 0.75 and 0.5 mol% Tb were spin-coated on Si(100) substrates using a solution of the nitrates of Mg, Zn, Al and Tb in ethanol, with ethylene glycol as complexing agent. Samples about 200 nm thick were obtained by sequentially depositing 10 layers at 3000 rpm for 30 s. Samples were annealed for 1 h in air before measuring their luminescence properties. For the sample annealed at 600 ℃, x-ray diffraction showed the thin film had a strong (111) preferential orientation. Atomic force microscopy revealed a root means square roughness of 1 nm and Auger electron spectroscopy depth profiles showed a uniform layer with a sharp interface at the Si substrate. With an increase in annealing temperature up to 1000 ℃, the luminescence increased while the surface became slightly rougher and the layer-substrate interface more interdiffused. Annealing the samples at 1200 ℃ resulted in diffusion of Si through the layer and the formation of an additional phase. While the green Tb emission was slightly reduced, blue emission from the ~5D_3 level of Tb~(3+) was greatly enhanced in these samples.