Thin films of Al-doped ZnO were prepared by reactive co-sputtering using independently controlled Zn and Al targets. Surface morphology of the films observed in a SEM revealed the nanometer columnar structure. The evolution of columnar structure was closely related to the involvement of Al during the film deposition process. The nanometer columns with a very strong preferred orientation along [002] direction was observed in a film prepared when the RF power ratio of Al and Zn targets was above one. As the power ratio of the targets kept at one, the column size was in the range from 37 to 62 nm when the RF power increased from 20 to 50 W. The refractive index of the film varied from 1.85 to 1.96 (λ = 500 nm) as the column size changed from 37 to 62 nm. All the as-grown films had high resistivity regardless of the amount of Al doping. A considerable reduction of the film resistivity was achieved by rapid thermal annealing at a temperature above 250^oC for 15 minutes in a nitrogen gas flow. The lowest sheet resistance obtained in our experiments was 4.78 × 10² Ohm/square. We also found that the sheet resistance of the annealed sample decreased as the Al doping increased. However, too much Al doping might increase the film resistivity because of considerable amount of aluminum oxide existed in the film. ESCA analysis gave the evidence that the amount of aluminum in the form of oxide reduced after annealing. This explains the activation of Al doping found in our experiments.

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