DC-magnetron sputtering of ZnO:Al films on (00.1)Al2O3 substrates from slip-casting sintered ceramic targets

High (>350 °C) temperature DC-sputtering deposition of ZnO:Al thin films onto single-crystal (00.1) oriented Al2O3 (sapphire) substrates is reported, using a ultrahigh-density, low-resistivity and low-cost composite ceramic target produced by slip-casting (pressureless) sintering of ZnO-Al2O3 (AZO) powders. The original combination of high-angle θ-2θ (Bragg-Brentano geometry) X-ray diffraction with low angle θ-2θ X-ray reflectivity (XRR) techniques allows us to define the AZO target composition and investigate the structural properties and surface/interface roughness of as-sputtered ZnO:Al films; besides, the growth dynamics of ZnO:Al is unambiguously determined. The target turned out composed of the sole wurtzite ZnO and spinel ZnAl2O4 phases. X-ray diffraction analyses revealed highly (00.1)-oriented (epitaxial) ZnO:Al films, the material mean crystallite size being in the 13-20 nm range and increasing with temperature between 350 °C and 450 °C, while the film growth rate (determined via XRR measurements) decreases appreciably. XRR spectra also allowed to determine rms surface roughness <1 nm for present films and showed ZnO:Al density changes by only a few percent between 350 °C and 450 °C. The latter result disproves the often-adopted Thornton model for the description of the sputter-grown ZnO films and instead points out toward a reduction of the sticking coefficients of impinging species, as the main origin of film growth rate and grain size dependence with temperature. ZnO:Al films appear highly transparent, with visible-NIR integrated transmittance >97%.