Two-dimensional transition metal dichalcogenides as next generation semiconductor materials

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Coleman, Emma M.
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University College Cork
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In recent years 2D materials, and more specifically transition metal dichalcogenisdes (TMD) MoS2 and WS2 have been investigated in the field of semiconductors as they exhibit energy gaps which span from semi-metals through to wide band gap semiconductors. Due to their thickness on an atomic scale, their properties differ from that of their bulk counterpart. The tunability of TMD bandgaps between the monolayers and bulk allows for a broader range of physical, electronic and optical properties and can be applied to a variety of applications including sensors, photode- tectors, flexible devices, optoelectronics, electronic (logic and memory) devices and green energy. This thesis focuses on the possible use of TMDs for the 3D back end of line (BEOL) integration of logic and memory and it investigates some of these challenges to help bridge the gap in the knowledge to strive for the reality of 3D BEOL. One of the approaches examined in this thesis is the mechanical exfoliation of TMD crystals. The interface properties of WS2 or MoS2 and insulating oxides is investigated with fabricated inverted MOSCAP (metal oxide semiconductor capacitor) structures, where a heavily doped silicon substrate is used as the back gate. With a variety of different devices, the impact of boundary defects is discussed along with a proposed bulk defect effects. Another important line of work is the improvement of TMD growth. Chemical vapour deposition (CVD) is a popular TMD growth process. Though high quality TMDs have been produced by CVD, there is a lack of knowledge about how to grow at lower temperature, in order to stay within the thermal budget of 3D BEOL. We investigate CVD grown MoS2 at temperatures lower than 550 °C and perform Hall data on transistors which provides positive mobility values and carrier concentrations. The significance of this work also shows findings in the nanoelectronic operation of atomic layer deposition (ALD) WS2. Insight is gained from Hall effect analysis and temperature dependent electronic studies, which are lacking in literature. Typically, for WS2 thin-films, the mobility values reported to date have been field effect mobility values extracted from transistor characteristics. More specifically, data includes the similarity of contact and sheet resistance temperature dependence in the WS2 device, indicating that both are dominated by the WS2 hole concentration temperature dependence. Moreover, rarely reported change in the TMD material, such as sheet resistance, due to device fabrication, is attributed to the chemicals and thermal treatments required to form the electronic devices. Finally, the aim of investigating laser annealed MoS2 was to show an in-depth investigation into synthetizing crystalline MoS2 layers on wafer scale area, where MoS2 films deposited on Si and Si/SiO2 surfaces are explored. This method shows that within the correct parameter range, the process can produce crystalline MoS2 films with small domains size of around 3.5 nm, from an initially amorphous MoS2 film. Field effect transistors formed in laser annealed MoS2 show limited Ids variation with Vgs consistent with the small domain size.
2D materials , Semiconductors , Transition metal dichalcogenides
Coleman, E. M. 2023. Two-dimensional transition metal dichalcogenides as next generation semiconductor materials. PhD Thesis, University College Cork.