Defect Passivation in Nanowires and Demonstration of Nanowire Devices for use in Sensing Applications
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Oxide nanowires have gained recent importance as sensing elements due to the large concentration of intrinsic dopants causing high conductivity and very high sensitivity due to high mobility of carriers, highest among oxide materials. This leads these nanowires to demonstrate excellent sensing performance which is comparable to or even surpasses the best thin film counterparts. The large surface electric fields and charge accumulation unique to these nanowires is used for chemical sensing where the surface defect states are found to effectively bind analytes while simultaneously modulating the nanowire conductivity. In this thesis we investigate the defect passivation in oxide nanowires and demonstrate sensors using nanowire as the active material. As compared to elemental semiconductors, the growth behavior of oxide nanowires has not been very well understood. Hence a study of the nucleation in oxide nanowires along with an investigation into tapering observed in these nanowires grown using a carbothermal reduction technique will be first presented. Defects in poly and nano-crystalline oxides are responsible for reducing the carrier lifetime and quenching the radiative recombination channels. The nature of surface traps states in these oxide nanowires and how they effect the electrical and optical properties and importantly their positions in the forbidden bandgap have been investigated next using various optical techniques. Use of surface passivation to compensate the mid gap intrinsic defect states is studied next, further revealing how the interplay between surface oxygen and nanowire surface modifies the surface electronic properties. The need for cheap, simple and cryogen free operation nuclear sensors motivated us to fabricate Schottky contacted surface passivated nanowire sensors which was able to detect 137Cs Gamma radiation with high sensitivities. We further extended this study towards remote sensing of nuclear radiation using wireless sensing techniques using millimeter waves at 94 GHz in order to remotely interrogate changes in the irradiated nanowires. Lastly we present some preliminary sensor design ideas and detection techniques in our efforts of fabricating a terahertz radiation detector and justify the use of Silicon (Si) nanowires as the active material based on some ultrafast carrier dynamics study.