Insertion of an ultrathin Al₂O₃ interfacial layer for Schottky barrier height reduction in WS₂ field-effect transistors

We report an effective approach for reducing the Schottky barrier height (SBH) in the source and drain (S/D) contacts of WS₂ field-effect transistors (FETs) using an ultrathin Al₂O₃ interfacial layer between the metal and WS₂. Al₂O₃ with various thicknesses was deposited to study the effect of the insulator thickness on contact resistances. The reduction of the SBH by inserting an ultrathin insulator was confirmed via both calculations and experiments, while further increasing the thickness of the insulator introduces a tunneling barrier that leads to the decrease in the current. By analyzing the metal/WS₂ contact using four different metals, the density of gap states caused by defects at the interface is found to be the main reason for the Fermi level pinning, and this metal-insulator-semiconductor (MIS) contact structure shows its great advantage in the Fermi level depinning, a large improvement of the on-current, and enhanced field-effect mobilities in WS₂ FETs due to an Al₂O₃ capping layer. This work demonstrates that the SBH can be effectively modulated by the optimization of the inserted interfacial material and its thickness.