The nitrogen concentration effect on Ce doped SiON emission: towards optimized Ce for LED applications.

Ce-Doped SiON films are deposited by magnetron reactive sputtering from a CeO 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 SiON films (e.g. band tails, CeO, Ce clusters, Ce ions), with different activation scenarios to explain the luminescence. X-ray photoelectron spectroscopy (XPS) reveals the exclusive presence of Ce 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 Ce ions are proposed involving direct or indirect mechanisms. Nitrogen concentration plays an important role in Ce emission by modifying Ce surroundings, reducing the Si phase volume in SiON 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 Ce 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.