ZnO Thin‐Film Transistor Grown by rf Sputtering Using Carbon Dioxide and Substrate Bias Modulation

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.