Oxidation of Cu3N thin films obtained from Cu annealed under NH3:O2 flow: A Raman and N-K-edge NEXAFS study

Cu3N layers prepared by annealing, under NH3:O2 flow, of Cu sputtered on Si, fused SiO2, and soda-lime-glass were studied by Raman and N-K-edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The annealing temperatures were ranging from 300° to 800°C. Benefiting from resonance effects on the Raman peak intensities when different excitation wavelengths are used, the contribution in the Raman signal of copper oxides, which provide peaks lying close to those of Cu3N, is more easily discriminated. The formation of Cu2O surface oxide is observed in almost all the samples even for annealing at low temperatures. CuO is generally formed at higher temperatures with the onset depending on the amount of O2 in the NH3:O2 mixture. However, high temperature is necessary to grow larger crystallites, as it is deduced from the decrease in the width of the high-frequency Raman peak (at ca. 650 cm−1) of Cu3N, in accordance with Scanning Electron Microscopy observations. The formation of CuO is accompanied by the creation of N2 trapped in the sample, which is directly detected by the NEXAFS measurements. N2 is most probably formed by the N atoms originating from the dissociation of the Cu-N bonds.