The resistivity and the thermal stability of transparent conducting ZnO layers doped with aluminum have been correlated with the conditions of the sputtering process. Layers deposited at low rf power density (similar to1.3 to 2.6 W/cm(2)) exhibit a low resistivity of 9 X 10(-4) Ohm cm, predominantly due to a high concentration of intrinsic donor type defects. These donors are compensated during annealing at high temperature in a vacuum; the low resistivity increases and the layers are not thermally stable. At rf power densities of similar to3.2 W/cm(2) and more, the deposition conditions yield a high growth rate and the extrinsic aluminum dopant is incorporated on vacant cation sites, These substitutional donors are thermally stable therefore a low resistivity is retained after annealing at 550 degreesC.