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

Abstract

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.