Al-Doped ZnO Transistors Processed from Solution at 120 °C

A simple Al-doping method that is used to significantly enhance the operating characteristics of ZnO thin-film transistors processed from solution at temperatures down to 120 °C is reported. The two-step doping process relies on the dissolution of zinc oxide hydrate in ammonia hydroxide to form an aqueous Zn-ammine complex solution and the subsequent immersion of Al pellets into it at room temperature. The pellets are then removed, and the doped precursor solution is spin-coated onto the substrate followed by thermal annealing in air to form the n-doped ZnO:Al layers. By controlling the immersion time of the Al pellets in the precursor solution, the free electron concentration in ZnO can be tuned. The resulting ZnO:Al layers are shown to be polycrystalline with tuneable electrical properties. ZnO:Al-based transistors processed at 180 °C exhibit enhanced electron mobility when compared to intrinsic ZnO devices with the maximum values exceeding 5 cm² V⁻¹ s⁻¹. Even when the process temperature is reduced to 120 °C, the ZnO:Al transistors retain their excellent operating characteristics with a maximum electron mobility of 3 cm² V⁻¹ s⁻¹. This is amongst the highest values reported to date for soluton-deposited ZnO transistors processed at 120 °C in air.