Luminescence properties of europium titanate thin films grown by atomic layer deposition

Abstract

Luminescent europium titanium oxides (Eu_xTi_yO_z) have been deposited as thin films by atomic layer deposition (ALD) in the temperature range 225 to 375 °C with control of the stoichiometry from pure TiO₂ to pure Eu₂O₃, and their luminescent properties have been characterized. The current work demonstrates the feasibility of tailoring the luminescent properties of these films by varying ALD deposition parameters. The highest luminescence efficiency was observed for the amorphous phase while it decreases steadily with increasing annealing temperature up to 700 °C due to the formation of crystalline Eu₂Ti₂O₇. The limiting factor for luminescence at low Eu contents is the low concentration of emitting Eu³⁺ atoms, while the luminescence at high Eu contents is limited by a low UV absorption due to the low Ti⁴⁺ content in these samples. The precursors used for the Ti–O deposition were TiCl₄ and H₂O, while Eu(thd)₃ (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) and ozone were used for the deposition of Eu–O. The films have been characterized by spectroscopic ellipsometry, photoluminescence, X-ray diffraction, X-ray fluorescence and atomic force microscopy. The depositions gave smooth (RMS roughness < 0.4 nm) and amorphous films for 30% and more Eu–O cycles (9.5 cationic% Eu), while 10 and 20% gave rough and partly crystalline films. The refractive index, extinction coefficient and luminescence efficiency are shown to vary continuously with Eu content for 30% Eu–O cycles or more, while the shape of the luminescence spectrum is independent of Eu content in the same range. For the 10 and 20% Eu–O depositions, we observed a gradual change in the luminescence spectra, coinciding with an increase of the proportion of anatase phase with decreasing Eu content.