Effect of the thermal annealing on the structural, morphological and photoluminescent properties of ZnO/Si multilayers

In this work, multilayers (MLs) thin films composed of ten bilayers of ZnO/Si with high photoluminescence (PL) were deposited by RF-Sputtering and thermally annealed at different temperature. The composition, surface morphology, structural and PL properties of ZnO/Si MLs were studied as a function of the annealing temperature. The ZnO/Si ML structure was corroborated, in the as-deposited sample, by the analysis of its chemical composition in depth profile done by X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) images show a smooth surface of the ZnO/Si ML before thermal annealing, but the formation of surface nano-spheres with a diameter of about 63.4 ± 1.36 nm and 132.4 ± 2.44 nm after thermal annealing at 900 °C and 1000 °C, respectively. Si:ZnOy nanospheres are grown on a Zn:SiOx film after the thermal annealing, as analyzed by XPS in depth profile. X-ray diffraction (XRD) shows a high crystallinity with the temperature and the XRD peaks observed are related to Si–ZnO and Zn2SiO4 composites, especially at 1000 °C. The luminescent studies reveal that the samples have an intense and broad PL emission after thermal annealing, observed with the naked eye. Moreover, the presence of nanospheres combined with the different defects associated to ZnO and SiOx enhances the PL intensity up to 995-fold as compared to that ZnO/Si ML without thermal annealing. Therefore, this type of multilayer films are potential candidates for their application in light emitting devices.