Characterization of nanocrystalline ZnO grown on silicon substrates by dc reactive magnetron sputtering

The aim of the work was to study the effect of postgrowth thermal annealing processes on the characteristics of the zinc oxide films grown on silicon substrates by dc reactive magnetron sputtering. The growth temperature of the ZnO thin films was fixed at 230°C and then the samples were annealed in dry air atmosphere at 800°C for one hour. The surface of the ZnO samples was analyzed with a scanning electron microscope (SEM) and using an atomic force microscope (AFM). The structural properties were assessed by X-ray diffraction (XRD), Raman scattering, and Photoluminescence (PL) measurements. The XRD and Raman studies revealed the ZnO films crystallizes in the wurtzite structure with a certain amount of amorphous material in the as-grown films, after the thermal treatment a preferential orientation along the c-axis was observed. The films are constituted by crystallites of similar nanosize dimensions in spite of high temperature used in the annealing process. The most relevant result of this study was the observation of excitonic-like transitions in the PL response at the near band gap region at room temperature. The PL response also shows a broad defect-related green band at 516 nm. Both bands are clearly linked to the nanostructure and the point defects content of the ZnO films.