Doping of group IV semiconductor nanowires

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Game, Alexander
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University College Cork
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As Moore’s Law predicted in the 1960s, advancements in technology have led to an exponential increase in the numbers of transistors required per square inch of integrated circuits, leading to an ever pressing need for smaller transistors. In turn, there is a need for novel transistor architectures and materials, with the conventional Si FETs soon approaching the limits of modern technology. With the need for channel lengths and widths below 7 nm fast approaching, much research has turned to new materials and devices to fulfil these requirements when they are needed. With NWs being prominently used in studies of alternative device architectures, and a resurgence in research of Ge as a semiconductor for FET channels, Ge NWs show great promise as components for novel FET designs. GeSn also shows great potential over Si and Ge due to its direct bandgap allowing for lower energy devices. While most reported syntheses of Ge NWs use gas-based vapour-liquid-solid growths, some research has been reported on solution-based VLS growth of both Ge and Ge Sn NWs, although no literature to date has reported solution-based doping of VLS grown Ge or GeSn NWs. This thesis reports liquid-phase VLS growth and in-situ doping of Ge and GeSn NWs using a variety of dopant precursors. SEM and TEM were used to analyse the morphology of NWs grown. TEM, XRD and Raman Spectroscopy were used to analyse the crystal structures of the wires, including the presence of defects. Raman spectroscopy and EDX analysis were used to determine the atomic composition of the NWs. Electrical testing was also carried out on the NWs. Chapter 1 of this thesis outlines the advantages of the Ge and GeSn NWs over conventional FET materials and architectures, as well as introducing the mechanisms of the growth and doping of semiconductor NWs and summarising the existing literature of doping of NWs, particularly focusing on in situ doping. Chapter 2 outlines the experimental methodology for the synthesis of the Au NPs used for NW synthesis, as well as the syntheses of Ge and GeSn NWs, as well as detailing the equipment and chemicals used. Chapter 3 details how the dopant molecules impact the morphology of the NWs, with decreases in the diameters and lengths of NWs in most samples. The dopants are also shown to decease the NW yield, with most samples yielding cubic crystalline NWs grown in the (111) direction. Dopant precursors are also shown to have prominent effects on the Sn concentration of GeSn NWs, as well as having more pronounced effects on the crystallinity of the NWs. These results are followed by conclusions and an outline of potential future work in this field.
Nanowires , Germanium , Germanium-tin , Doping , Allyldiphenylphosphine , Allyltributylstannane , Diphenylgermane , Diphenylphosphine , Trioctylphosphine , In-situ doping
Game, A. 2020. Doping of group IV semiconductor nanowires. MRes Thesis, University College Cork.
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