Transparent conductive metal oxides are interesting materials for various optoelectronic applications including solar cells and flat panel displays. This study focuses on the in situ deposition of aluminum-doped zinc oxide (AZO) thin layers on c-axis oriented sapphire substrates by dc sputtering and on the structural and electrical characterization. The films have a typical thickness of 90 nm and a roughness of 10 nm root mean square. An Al/Zn ratio of 2.4 at% Al was determined by X-ray photoelectron spectroscopy. X-ray diffraction shows a preferential growth in the (0002) c-axis direction. Films have an average transparency of 90% in the visible-light spectrum, a room-temperature resistivity of 3.7 × 10−3 Ω cm and a carrier mobility of 6.7 cm2 V−1 s−1.