Preparation and characterization of high-performance direct current magnetron sputtered ZnO:Al films

High-performance aluminium-doped zinc oxide (ZnO:Al) thin films were deposited on glass substrate by direct current reactive magnetron sputtering from a Zn-Al metallic target (Al 3 wt.%). Films with a tangly string-like surface morphology, an average thickness of 837 nm, an optical transmittance up to 85% in the visible range and electrical resistivity down to 1.80 x 10-4 Ω cm were obtained. The dependences of the surface morphology, crystallinity, the electrical and optical properties of the films on substrate temperature, O2/Ar flow ratio, and sputtering power were investigated using X-ray diffraction, scanning electron microscopy, spectrophotometry, linear array four-point probe, and Hall-effect measurements. The comparison of the surface morphology, crystallinity, and interplanar stress of ZnO:Al films with those of ZnO films was also conducted. The properties are significantly and regularly influenced by the sputtering parameters. The optimal films were prepared with a substrate temperature of 250 deg. C, O2/Ar ratio of 10:40 and sputtering power of 55 W. Ionic replacement at the lattice-sites probably induces the decrease of the residual stress and even the reverse of its direction. The correlations between the electrical and the optical characteristics of ZnO:Al films are also discussed in this paper