Abstract
The use of orientated semiconducting nanowires as the active material in solution processable printable transistors is an area of research that can offer enormous potential in the field of high performance electronics. Lightweight, flexible and low cost components that are compatible with plastic substrates can further increase the appeal of this field. Currently, the most commonly used materials for field-effect transistors in large area electronics are polycrystalline, amorphous silicon and organic semiconductors. However, these classes of semiconductors face several limitations with regards to their compatibility in plastic electronics, either due to their high temperature processing (silicon) or low charge carrier mobility (organics). The current work investigates alternative semiconductors based on nanomaterials that can be used as active layers in large area electronics, with performances comparable to or exceeding those of amorphous silicon which can be processed at much lower temperatures. In this thesis, we explore semiconducting inorganic nanowires including silicon, germanium, and zinc oxide nanowires as the channel material in field-effect transistors to realise high performance printed electronics. Several main challenges, such as deposition of nanowire ‘inks’, ohmic contacts to nano wires, surface states, and the use of organic dielectrics are addressed as follows: (1) Development of two new nano wire deposition methods based on either spray coating or dip-coating has led to alignment control for nanowires and also high density coatings on various substrates. These techniques can offer scalability for large area surfaces at room temperature. (2) Solving problems of making near-ohmic contacts in nano wire transistors fabricated at low temperatures ensured efficient charge injection in devices. Optimised FETs with high work function metal electrodes and treated nano wire surfaces exhibited high output currents reaching 1mA, high on/off current modulation of ~107, and high hole mobility in the range of 5-26cm2/Vs. (3) Investigation of high Schottky barrier nano wire FETs led to better understanding of source contact barrier lowering by the gate field and the discovery of a new type of nanowire transistor operation that can offer improved power dissipation and near ideal current-voltage saturation characteristics with drain voltage saturation of less than 2V, even at large gate voltages. (4) The control of nanowire surfaces, especially at the nanowire-insulator interface was achieved using low-k organic dielectrics and self-assembled monolayers. This also resulted in the demonstration of high performance p-type transistors exhibiting 10μA output current, on/off current ratio of ~107 and the high field-effect mobility in the range of 5-20cm 2/V-s. (5) Finally, a high performance n-type ZnO nanowire transistor on a flexible plastic substrate with a low-k dielectric was demonstrated with output current of ~1uA, on/off current modulation of ~105, subthreshold voltage swing of ~0.26mV/dec and a field-effect mobility in the range of 49-65cm2/V-s. In short, this thesis delivers a new realisation of a concept in which solution processable high performance electronic devices may be fabricated using low temperature processing steps on various substrates including plastics. The approach is general to a broad range of nanowire material systems and can be applied to e-paper, flexible displays, chemical and biological sensors, RFID tags, memory elements and ambient intelligence devices.