Zinc oxide nanowire field effect transistors for sensor applications


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A wide variety of tunable physio-chemical properties make ZnO nanowires a promising candidate for functional device applications. Although bottom-up grown nanowires are producible in volume, their high-throughput device integration requires control over dimensions and, more importantly, of precise placement. Thus development of top-down fabrication routes with accurate device positioning is imperative and hence pursued in this thesis. ZnO thin film transistors (TFT) were fabricated using solution based precursor zinc neodecanoate. A range of ZnO thin films were prepared by varying process parameters, such as precursor concentrations and annealing temperatures, and then analysed for their optical and electrical characteristics. ZnO TFTs prepared from a 15 % precursor concentration and annealing at 700 C exhibited best device performance with a saturation mobility of 0.1 cm2/V.s and an on/off ratio of 107. Trap limited conduction (TLC) transport was found to be dominant in these devices. A direct-write electron beam lithography (EBL) process was developed using zinc naphthenate and zinc neodecanoate precursors for the top-down synthesis of ZnO nanowires. Nanoscale ZnO patterns with a resolution of 50 nm and lengths up to 25 μm were fabricated. A linear mobility of 0.5 cm2/V.s and an on/off ratio of $\sim10^5$ was achieved in the micro-FETs with 50 μm channel width. Interestingly, on scaling down the ZnO channel width down to 100 nm, almost two orders of magnitude enhancement in the linear mobility was observed, which reached $\sim33.75cm^2$/V.s. Such increment in the device performance was attributed to the formation of larger grains and thus reduction in the grain-boundary scattering. Six volatile organic compounds (VOCs) were sensed at room temperature using the direct-write EBL fabricated ZnO devices under UV sensitisation. As the surface-to-volume ratio increases with the decreasing channel width (from 50 μm to 100 nm), sensing response of the ZnO devices becomes more significant. Ppm level detection of various VOCs was observed; with a 25 ppm level Anisole detection being the lowest concentration. Additionally, using 100 nm device, detection of 10 ppm NO2 was achieved at room temperature. The sensing response towards NO2 was found to be increased with UV illumination and sensor temperature. This led to exhibit $\sim171_2$.

Welland, Mark
ZnO, Nanowire, Electron beam lithography, Direct-write, Field effect transistors, Gas sensing
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
The candidate's PhD studies were funded by the Cambridge Trusts.