ZnO, ZnO/Al, Al/ZnO/Al, and ZnO/Al/ZnO samples were deposited on c-plane sapphire and Si substrates by radio frequency magnetron sputtering (RFMS) using ZnO target. In order to form doped ZnO:Al thin films, these grown samples were annealed at temperatures of 450 degrees C for 1 h to let diffuse Al atoms into the ZnO. After annealing homogeneous Al, diffusion is observed for the sample having Al layer at the top and the bottom of the ZnO from the cross-sectional SEM images. The effects of Al diffusion on structural, optical, electrical, and magnetic properties of ZnO layers were investigated by using x-ray diffraction (XRD), optical transmittance, sheet resistance, and magnetic field dependence of magnetization (M(H)) measurements. After annealing, the optical transmissions of samples were higher than 60% in the visible and near-infrared region for all samples. The sheet resistance measurement results showed that the conductivity of Al/ZnO/Al deposited on sapphire was found to be 2.64 x 10(1) (omega)(-1) after annealing. The magnetism measurement results in that all samples show a weak ferromagnetic behavior except for the Al/ZnO/Al sample, which is attributed to the interface exchange coupling between the layers.