Luminescent properties of europium titanium phosphate thin films deposited by atomic layer deposition

Abstract

In the present contribution we assess the luminescent properties of amorphous europium titanium phosphate thin films. The films have been deposited by atomic layer deposition, using the precursor combinations Eu(thd)₃ (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) and O₃, TiCl₄ and H₂O, and Me₃PO₄, H₂O and O₃ at a deposition temperature of 300 °C. UV light is absorbed in the (TiO₆)⁸⁻ complex and subsequently transferred to Eu³⁺, from which characteristic visible red emission takes place. The inclusion of phosphate is expected to reduce the symmetry around Eu³⁺ compared to the pure oxide structure, which should increase the f–f transition rate and improve the luminescence efficiency. The as deposited samples display a high degree of photobleaching when subjected to UV or X-rays. The bleaching rate has been quantified for the as deposited samples and can be controlled by adjusting the Ti-content in the samples. Annealing the samples at 500–700 °C increases both the photostability and the luminescence of all compositions, while annealing at 800 °C and above quenches the Eu³⁺ luminescence. Annealing at 1000 °C results in a broad NIR emission centred around 820 nm for most of the samples. The structure, roughness, thickness and composition of the deposited films have been studied by X-ray diffraction, atomic force microscopy, ellipsometry, and X-ray fluorescence, respectively. The optical properties have been characterized with excitation and photoluminescence spectroscopy and spectroscopic ellipsometry, while field emission scanning electron microscopy has been used to determine the morphology of the annealed samples.