Towards quantum devices: a study on the fabrication and measurement of WS2 field effect transistors

Over the past few decades, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have garnered significant attention for their exceptional electrical performance and flexibility in functional modulation. These semiconducting materials, characterized by a layered structure with weak van der Waals interlayer interactions, present a bandgap ranging from 0.5 eV to 2.0 eV, making them promising candidates for future spintronic and valleytronic applications. Unlike graphene, TMDCs possess a substantial bandgap, making them suitable for transistor fabrication. A major application of TMDCs is in field-effect transistors (FETs), where their distinct properties, such as potential immunity to short-channel effects and reduced scattering due to their intrinsic 2D confinement, are utilized to potentially outperform conventional silicon-based devices. In this thesis, the variation of the quality of metal contacts as a function of the number of layers of each semiconductive channel is examined. Recent studies suggest that WS2 may exhibit promising electrical properties, therefore it is selected as the TMDC of interest. The core of this work is devoted to the fabrication of multilayer WS2 transistors on SiO2. The research then moves to the more challenging fabrication of monolayer WS2-based transistors on sapphire substrate. Electrical measurements highlight the poor quality of the contacts, pointing out the need of further research to manipulate WS2 monolayers grown by chemical vapor deposition. Photolithography is employed to produce source and drain contacts for the electrical characterization. The study emphasizes the strengths and criticalities of manipulating WS2 flakes obtained by mechanical exfoliation, with specific focus on the role of the chemicals used and arrangement of metal contacts. A fully mechanized transferring technique to transfer exfoliated flakes onto gold contacts is used, and electrical characterisation at room temperature is used to assess the device performance. Finally, AFM scanning allows the estimation of the thickness and number of layers of each flake, so that the study of the respective resistance trend is allowed. We used electrical measurements to extract transfer and output curves to measure intrinsic properties such as threshold voltage and on/off ratio. MOSFET mobility measurements are then detailed, illustrating methods to extract these parameters from experimental data. The field-effect mobility in the devices is found to be in the range of 11–14 cm²/Vs at source-drain voltage (Vds) of 6V, which is relatively low compared to values reported in other studies. Additionally, the devices exhibit non-linear output curves, with a few showing asymmetry around Vds = 0V. These observations underline the complexity of achieving consistent mobility performance across devices and highlight potential areas for further optimization. The non-linearity of output curves results from the Schottky barrier arising at the contact sides; this is expected because of the Schottky-Mott rule and the absence of a contact intermediate metal layer, e.g. indium, to decrease the barrier. Furthermore, asymmetric non-linearity may come from different Schottky barrier heights between source and drain interfaces. Achieving good ohmic metal contacts in TMDCs remains challenging, as the origins of interface phenomena, like Fermi level pinning and Schottky barrier height, are still under debate. We believe that the observations in this thesis can advance the understanding and practical appli