Oxidation and crystallization behavior of calcium europium silicon nitride thin films during rapid thermal processing

Luminescent thin films were fabricated on silicon wafers using reactive magnetron sputtering of Ca, Si and Eu in Ar/N₂ atmosphere. In order to activate the luminescence, the as-deposited nitride films were heated to 1100 °C by a rapid thermal processing treatment. X-ray diffraction measurements reveal the crystal phases that form during thermal treatment. By recording scanning electron microscopy images of the surface and the cross-section of the film at different radial locations, the formation of different layers with a thickness depending on the radial position is revealed. Energy dispersive x-ray spectroscopy analysis of these cross-sections reveals the formation of an oxide top layer and a nitride bottom layer. The thickness of the top layer increases as a function of radial position on the substrate and the thickness of the bottom layer decreases accordingly. The observation of different 4f ⁶5d¹ → 4f ⁷ Eu^{2 +} luminescence emission bands at different radial positions correspond to divalent Eu doped Ca₃Si₂O₄N₂, Ca₂SiO₄ and CaSiO₃, which is in agreement with the phases identified by X-ray diffraction analysis. A mechanism for the observed oxidation process of the nitride films is proposed that consists of a stepwise oxidation from the as-deposited amorphous nitride state to crystalline Ca₃Si₂O₄N₂, to Ca₂SiO₄ and finally CaSiO₃. The oxidation rate and final state of oxidation show a strong temperature-time dependency during anneal treatment.