Transparent conductive ZnO:A1 films were prepared in Ar gas by a planar magnetron sputtering system with facing targets, where strong internal magnets were contained in target holders to confine the plasma between the targets. A film resistivity of 4 x 10(-4) Ohm cm was attained at a substrate temperature of 200 degrees C. However, the film resistivity increased with increasing substrate temperature, due to a decrease of the carrier concentration. The reason for this phenomenon was the decrease of Zn donors in the film as native donors or an increase of Zn defects. When excess Zn atoms were supplied during film growth, a low resistivity of 2 x 10(-4) Ohm cm was obtained even at a substrate temperature of 250 degrees C.