The nitrogen concentration effect on Ce doped SiO_xN_y emission: towards optimized Ce³⁺ for LED applications

Ce-Doped SiO_xN_y films are deposited by magnetron reactive sputtering from a CeO₂ target under a nitrogen reactive gas atmosphere. Visible photoluminescence measurements regarding the nitrogen gas flow reveal a large emission band centered at 450 nm for a sample deposited under a 2 sccm flow. Special attention is paid to the origin of such an emission at high nitrogen concentration. Different emitting centers are suggested in Ce doped SiO_xN_y films (e.g. band tails, CeO₂, Ce clusters, Ce³⁺ ions), with different activation scenarios to explain the luminescence. X-ray photoelectron spectroscopy (XPS) reveals the exclusive presence of Ce³⁺ ions whatever the nitrogen or Ce concentrations, while transmission electron microscopy (TEM) shows no clusters or silicates upon high temperature annealing. With the help of photoluminescence excitation spectroscopy (PLE), a wide excitation range from 250 nm up to 400 nm is revealed and various excitations of Ce³⁺ ions are proposed involving direct or indirect mechanisms. Nitrogen concentration plays an important role in Ce³⁺ emission by modifying Ce surroundings, reducing the Si phase volume in SiO_xN_y and causing a nephelauxetic effect. Taking into account the optimized nitrogen growth parameters, the Ce concentration is analyzed as a new parameter. Under UV excitation, a strong emission is visible to the naked eye with high Ce³⁺ concentration (6 at%). No saturation of the photoluminescence intensity is observed, confirming again the lack of Ce cluster or silicate phase formation due to the nitrogen presence.