Electrical contact properties of ultrathin transition metal dichalcogenide sheets

The graphene discovery led to advances in exfoliation and synthetic techniques, and the lack of a bandgap in graphene has stimulated the research for new 2D semiconducting materials. Transition metal dichalcogenides (TMDCs), semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), have recently been isolated. TMDCs exhibit a unique combination of atomic-scale thickness, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, valleytronics and optoelectronics. According to optical measurements, single-layer WS2 sheets exhibit a direct band gap of at least 2.0 eV. Because of its strong spin-orbit coupling induced valence band splitting, WS2 shows spin-valley coupling, even in few-layer sheets , which may allow easier observation of the valley Hall effect than in the other TMDCs. The thesis reviews the theoretical background of TMDCs and their optoelectronic properties. It also reports on the fabrication of field-effect transistors based on few-layer sheets of WS2 and the investigation of their electronic transport properties. Particularly the project focuses on improving the interface between the metal contact and WS2 sheet, where annealing improves the contact transparency. Together with van der Pauw geometry, annealing allows four-terminal measurements to be performed and the pristine properties of the material to be recovered at room temperature, where the devices show n-type behaviour and a linear I-V curve. The promising improvements and the electronic properties shown in this thesis make WS2 interesting for future applications in valleytronic devices.