Polarization dependent NEXAFS and optical properties of ZnO films prepared by oblique angle magnetron sputtering

Paper presented at the 14th International Conference on Plasma Surface Engineering held in Garmisch-Partenkirchen (Germany) on September 15-19th, 2014.Wurtzite ZnO films were grown by dc reactive magnetron sputtering under oblique angle deposition (OAD) with deposition angles in the range 0–85o both using a single angle or in a zigzag configuration . The film microstructure and the surface morphology of the samples were explored by HRSEM while the crystalline structure and the composition were determined by XRD and RBS, respectively. Films were found to be stoichiometric with a (001) preferential orientation and consisting of an inclined columnar and porous structure, with columns tilting in the direction of the incident flux. Angle-dependent Near Edge X-ray absorption Fine Structure (NEXAFS) measurements were performed on the films to elucidate the chemical state and local electronic structure of the oriented ZnO columns. For the different samples, a clear anisotropy was observed at the O K- and Zn L3-edges consistent with the preferential orientation of the c-axis. The optical properties of the ZnO films (refractive index, n, extinction coefficient, k, transparency and bandgap) were also studied by spectroscopic ellipsometry and UV-VIS-NIR spectrophotometry. The experimental results reveal that the deposition angle, the Ar/O2 gas ratio and the distance between the target and the substrate play a significant role in the composition, crystalline structure and optical and electrical properties of the ZnO thin films. In particular, it has been found that it is possible to control the variation of the refractive index value from 1.9 to 1.5, which is due to the high porosity of the films as a result of the self-shadowing columnar structure produced during the process [1]