ZnO thin-film transistor (TFT) grown by rf magnetron sputtering in Ar/O2 atmosphere shows inferior turn-off characteristics compared to ZnO TFT grown by other methods. We thought that reactions between Zn and O2 might produce defects responsible for the poor turn-off behavior. In order to solve this problem, we studied sputtering growth in Ar/CO2 atmosphere at 450°C. During sputtering growth, we modulated substrate dc bias to control ion supply to the substrate. After growth ZnO was annealed in CO2 and O2 gas. With these methods, our bottom-gate ZnO thin-film transistor showed 4.7 cm2/Vsec mobility, 4×106 on/off ratio, and –2 V threshold voltage.