ZnO nanowires and their application in field-effect transistors
Authors:   , Burke-Govey Conor Patrick 
Published: November 2021

<p>ZnO nanowires have shown great promise as a semiconducting material for a variety of different electronic applications at the nanoscale, and can be easily synthesised at low temperatures using the hydrothermal growth method. However, efforts to reliably produce field-effect transistors (FETs) using ZnO nanowires have been hampered by excessive charge carriers, requiring high temperature annealing (≥400°C) at the expense of the low-temperature synthesis before field dependence is achieved. This thesis presents hydrothermally synthesised ZnO nanowires which can effectively be used as FETs in dry and liquid environments without requiring any annealing or post-growth processing.  The role of polyethylenimine (PEI) in the hydrothermal growth of vertical ZnO nanowires is thoroughly investigated. PEI is a polymer used to increase the aspect ratio of ZnO nanowires, but the molecular weight of the polymer and interactions with other growth precursors are often overlooked. Using 4 mM of PEI(MW = 1300 g/mol) results in hierarchical nanowires, consisting of large primary nanowires which abruptly terminate in thinner secondary nanowires. The secondary nanowires, with lengths of up to 10 m and diameters below 50 nm, are synthesised during a PEI-mediated secondary growth phase, where Zn-PEI complexes continue to provide Zn²⁺ ions after the bulk of the precursors have been exhausted.  The PEI-mediated synthesis of hierarchical nanowires is used to fabricate FETs by laterally growing intersecting networks of nanowires from spaced pairs of ZnO/Ti films, which have been patterned on SiO₂/Si device substrates. All of these FETs show marked field dependence between VG = -10 V to 10 V, despite being used without annealing. Typical on-off ratios are between 10³ - 10⁵, with threshold voltages between -7.5 V to 5 V. This is a significant result, as the majority of ZnO nanowire FETs reported in the literature require high temperature annealing. Persistent photoconductivity measurements indicate that surface states on the nanowires contribute to the intrinsic field dependence of the devices.  Hierarchical nanowires are also synthesised by modular primary and secondary hydrothermal growths. FETs fabricated using these hierarchical nanowires show less field dependence than PEI-mediated hierarchical nanowires, with limited function ality when used in air. The best FET measured in air operates with an on-off ratio of 10⁴ and a threshold voltage of ~ 0 V. Devices which are field-independent in air can be reliably gated by measuring the FETs in a wet environment, using de-ionised water as a dielectric. A back-gated wet FET operates with an on-off ratio of 105 and a threshold voltage of ~ 8 V. Top-gated wet FETs operate with on-off ratios within 103 - 104, and threshold voltages within 0.4 - 0.9 V. These devices also have significantly low subthreshold swings, on the order of 80 mV/decade.  FETs are fabricated by contacting individual ZnO nanowires using electron-beam lithography, although only one vertical ZnO nanowire shows field dependence, with an on-off ratio of 10⁴ and a threshold voltage of -7 V. A PEI-mediated hierarchical nanowire is also contacted and shows field dependence, with an on-off ratio of 10² and a threshold voltage of -6 V. The poor on-off ratio is caused by high leakage currents of the device. The contacted nanowires undergo dissolution over time, disappearing from the substrates after 8 months, and also exhibit a conducting-to-insulating transition over 48 hours. This transition can be temporarily reversed by exposure to an electron beam. Neither of these effects are reported in the literature, and their causes are speculated on.  Finally, the thesis concludes with proposals for future work to further the advances made here.</p>