Van der Waals contacts and layer-dependent Fermi level pinning at WSe₂/metal interfaces

Strong Fermi level pinning (FLP) at the interface between WSe₂ and metal electrodes can give rise to high Schottky barriers, which would degrade the performance of electronic device. Therefore, the weak FLP is desirable as it allows decreasing Schottky barrier height (SBH) and achieving the low contact resistance. Herein, we demonstrate that the van der Waals (vdW) contacts between WSe₂ and metals can greatly weaken the FLP and affect SBH. As compared to the direct contacts, the fewer metal-induced gap states and smaller interface dipoles are observed in WSe₂/metal vdW contacts, which leads to the reduced FLP effect. In addition, we found that the FLP strength is also dependent on the number of WSe₂ layers. Furthermore, we analyze the origin of the pinning factor deviation from Schottky-Mott limit in the WSe₂/metal vdW contacts and find that the deviation originates from the interface potential difference and Fermi-level shift. Benefiting from the weak FLP, the low n-type and p-type Schottky and Ohmic contacts can be obtained in the WSe₂/metal vdW contacts by choosing metal electrodes and number of WSe₂ layers. These findings illustrate that creating vdW contacts can be an effective approach for developing high performance ambipolar WSe₂-based electronic devices.