Direct Observation of 2D Electrostatics and Ohmic Contacts in 2D Heterojunctions*

Atomically thin semiconductors such as WS₂, MoS₂, WSe₂ and MoSe₂ are attractive candidates for making ultrathin field effect transistors and optoelectronics. However, owing to the saturated bonds of van der Waals surface, achieving Ohmic contacts which can largely improve their device performance is challenging. Here we introduce a way to prepare large-area graphene contacts to WS₂ using seedless chemical vapour deposition. The template-grown graphene/WS₂ heterojunctions are intriguing blocks for making WS₂ devices with Ohmic contacts. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping and electrostatics in graphene–WS₂ heterojunctions as-grown on sapphire and transferred to SiO₂ with and without thermal annealing. Both p–n and n–n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found, promising low-resistance Ohmic contacts. This indicates a more favorable band alignment for graphene–WS₂ than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic measurement and modelling demonstrate that the large depletion width of the graphene–WS₂ junction reflects the electrostatics of the one-dimensional junction between two-dimensional materials.