Van der Waals contacts between three-dimensional metals and two-dimensional semiconductors

As the dimensions of the semiconducting channels in fieldeffect transistors decrease, the contact resistance of the metalsemiconductor interface at the source and drain electrodes increases, dominating the performance of devices(1-3). Two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS2) have been demonstrated to be excellent semiconductors for ultrathin field-effect transistors(4,5). However, unusually high contact resistance has been observed across the interface between the metal and the 2D transition-metal dichalcogenide(3,5-9). Recent studies have shown that van der Waals contacts formed by transferred graphene(10,11) and metals(12) on few-layered transitionmetal dichalcogenides produce good contact properties. However, van der Waals contacts between a three-dimensional metal and a monolayer 2D transition-metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10-nanometre-thick indium metal capped with 100-nanometre-thick gold electrodes and monolayer MoS2. Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid solution after annealing at 200 degrees Celsius and that the interface between the gold-capped indium and the MoS2 is atomically sharp with no detectable chemical interaction between the metal and the 2D transition-metal dichalcogenide, suggesting van-der-Waals-type bonding between the gold-capped indium and monolayer MoS2. The contact resistance of the indium/gold electrodes is 3,000 +/- 300 ohm micrometres for monolayer MoS2 and 800 +/- 200 ohm micrometres for few-layered MoS2. These values are among the lowest observed for three-dimensional metal electrodes evaporated onto MoS2, enabling high-performance field-effect transistors with a mobility of 167 +/- 20 square centimetres per volt per second. We also demonstrate a low contact resistance of 220 +/- 50 ohm micrometres on ultrathin niobium disulfide (NbS2) and near-ideal band offsets, indicative of defect-free interfaces, in tungsten disulfide (WS2) and tungsten diselenide (WSe2) contacted with indium alloy. Our work provides a simple method of making ultraclean van der Waals contacts using standard laboratory technology on monolayer 2D semiconductors.