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.