Effect of Al doping on performance of ZnO thin film transistors

Highlights: • Doping mechanism of Al in ZnO is investigated. • Saturation mobility (μ_SAT), subthreshold swing (SS) and on-to-off current ratio (I_on/I_off) obtain obvious improvement when Aluminum-doped Zinc Oxide (AZO) takes the place of ZnO to be the active layer of TFTs. • AZO thin films are demonstrated appropriate to work as the active layer of TFTs. In this work, we investigate the Aluminum-doped Zinc Oxide (AZO) thin films and their feasibility as the active layer for thin film transistors (TFTs). A comparison on performance is made between the AZO TFTs and ZnO TFTs. The electrical properties such as saturation mobility, subthreshold swing, and on-to-off current ratio are improved when AZO is utilized as the active layer. Oxygen component of the thin film materials indicates that Al is the suppressor for oxygen defect in active layer, which improves the subthreshold swing. Moreover, based on band structure analyzation, we observe that the carrier concentration of AZO is higher than ZnO, leading to the enhancement of saturation mobility. The microstructure of the thin films convey that the AZO films exhibit much smaller grain boundaries than ZnO films, which results in the lower off-state current and higher on-to-off current ratio of AZO TFTs. The AZO thin films show huge potential to be the active layer of TFTs.