Electrical contacts on two-dimensional transition metal dichalcogenide semiconductors
Two dimensional (2D) transitional metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) have been demonstrated to be excellent semi-conductors for ultra-thin FETs. However, unusually high contact resistance and Fermi level pinning have been observed due to damage at the metal-2D TMD interface. Studies have shown that van der Waals (vdW) contacts formed by dry transfer of graphene and metals can avoid damage to atomically thin TMDs caused by metal deposition. My doctoral thesis focuses on study of metal-2D TMD interfaces through high resolution cross-section scanning transmission electron microscopy (STEM) observations of the atomic structure, X-ray photoelectron microscopy (XPS) to analyze the chemical environment, and electrical transport measurements to extract the Schottky barrier heights for electrons and holes. My results suggest that chemical reactions between metal contacts and 2D TMDs during deposition along with damage due to kinetic energy transfer are two main sources of defects at the metal-semiconductor interface that lead to the Fermi level pinning. To avoid this, I developed strategies based on soft metal, indium, as a buffer layer to realise ultraclean vdW contacts on atomically thin 2D MoS2, WS2, NbS2 and WSe2. Using electronic measurements, I have demonstrated that the contact resistance of indium electrodes is ~ 3000 Ω·µm for single layer and ~ 800 Ω·µm for few layered MoS2 – amongst the lowest observed for 3D metal electrodes evaporated on MoS2 and is translated into high performance field effect transistors (FETs) with mobility of ~ 170 cm2-V-1-s-1 at room temperature. Furthermore, I have also achieved clean vdW contacts with high work function metals such as Pd and Pt by optimization of evaporation conditions. FETs with ambipolar characteristics with higher hole currents can be achieved on MoS2 with Pd contacts and pure P-type characteristics can be achieved on WSe2 FETs with Pt contacts. The results of my doctoral work suggest that it is possible to make ultra-clean metal contacts on 2D TMD semiconductors to obtain both N- and P-type FETs.