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The effects of substrate temperature and near-substrate plasma density are studied for reactively sputtered dc magnetron aluminum doped zinc oxide (ZnO:Al). Plasma density is varied using an unbalanced magnetron along with external Helmholtz coils, and is characterized using flat and cylindrical Langmuir probes. The substrate ion-to-neutral flux ratio was varied from 0.2 to about 3.5 using this technique. The ZnO:Al films were characterized by resistivity, transmission, Hall effect, and theta-two theta x-ray diffraction. Under conditions of low plasma density, for substrate temperatures below 65 °C the substrate temperature does not have a significant influence on film quality, and the film quality is relatively poor. By 88 °C the film quality is rapidly improving with temperature, and at 135 °C high quality films are produced. For substrate temperatures below 65 °C, increased plasma density at the substrate has a significant beneficial effect on the optoelectronic quality of the films. At 105 °C this effect is diminished, and by 135 °C there appears to be no improvement in film quality with increased plasma density. It is shown that the improvement of film quality at low substrate temperatures with increased plasma density is not due to substrate heating by the plasma. Possible reasons for this improvement are suggested, including plasma activation of oxygen, and the implications for low temperature growth of ZnO:Al are discussed. © 2003 American Institute of Physics.