High-performance Zinc Oxide Thin-Film Transistors For Large Area Electronics

Abstract

The increasing demand for high performance electronics that can be fabricated onto large area substrates employing low manufacturing cost techniques in recent years has fuelled the development of novel semiconductor materials such as organics and metal oxides, with tailored physical characteristics that are absent in their traditional inorganic counterparts such as silicon. Metal oxide semiconductors, in particular, are highly attractive for implementation into thin-film transistors because of their high charge carrier mobility, optical transparency, excellent chemical stability, mechanical stress tolerance and processing versatility. This thesis focuses on the development of high performance transistors based on zinc oxide (ZnO) semiconducting films grown by spray pyrolysis (SP), a low cost and highly scalable method that has never been used before for the manufacturing of oxide-based thin-film transistors. The physical properties of as-grown ZnO films have been studied using a range of techniques. Despite the simplicity of SP, as-fabricated transistors exhibit electrical characteristics comparable to those obtained from ZnO devices produced using highly sophisticated deposition processes. In particular, electron mobility up to 25 cm2/Vs has been achieved in transistors based on pristine ZnO films grown at 400 °C onto Si/SiO2 substrates utilising aluminium source-drain (S-D) electrodes. A strong dependence of the saturation mobility on the work function of S-D electrodes and the transistor channel length (L) has been established. Short channel transistors are found to exhibit improved performance as compared to long channel ones. This was attributed to grain boundary effects that tend to dominate charge transport in devices with L < 40 μm. High mobility, low operating voltage (<1.5 V) ZnO transistors have also been developed and characterised. This was achieved through the combination of SP, for the deposition of ZnO, and thermally stable solution-processed self-assembling monolayer gate dielectrics. Detailed study of the temperature dependence of the operating characteristics of ZnO transistors revealed a thermally activated electron transport process that was described by invoking the multiple trapping and release model. Importantly, ZnO transistors fabricated by SP are found to exhibit highly stable operating characteristics with a shelf lifetime of several months. The simple SPbased fabrication paradigm demonstrated in this thesis expands the possibilities for the development of advanced simple as well as multi-component oxide semiconductors far beyond those accessible by traditional deposition methods such as sputtering. Furthermore, it offers unprecedented processing scalability hence making it attractive for the manufacturing of future ubiquitous oxide electronics.