Improved electrical performance of solution-processed zinc oxide-based thin-film transistors with bilayer structures

A bilayer thin-film transistor (TFT) structure with ZnO and Al-doped ZnO (AZO) was fabricated using a solution process. The film thickness and sintering atmosphere of ZnO and AZO were controlled and then evaluated by measuring their electrical characteristics. By changing the sintering atmosphere, carriers attributed to oxygen vacancies in the thin film increased. Moreover, the ZnO single-layer TFT sintered in N2 exhibited good electrical characteristics. In the ZnO/AZO bilayer TFT laminated with the ZnO sintered in N2 and high-resistance AZO thin films, electrical characteristics, such as the On/Off ratio and subthreshold swing, improved compared to those of the ZnO-TFT. The On/Off ratio of the ZnO/AZO-TFT using the AZO thin film sintered in an O2 atmosphere notably improved to 3.7 × 105 compared to that of the ZnO-TFT (1.7 × 104). In addition, the subthreshold swing in the ZnO/AZO-TFT was 0.36 V/dec, while the field-effect mobility was 2.2 × 10−1 cm2/Vs, thereby exhibiting the best electrical characteristics among the fabricated samples. According to the grazing-incidence X-ray diffraction measurement, the ZnO particle size and crystallinity of the ZnO/AZO bilayer were higher than those of the ZnO single layer. Therefore, improvement of the electrical characteristics was confirmed.