Ohmic versus Schottky Contact between Monolayer Tungsten Disulfide and Thin Bismuth Semimetal

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have emerged

as a promising candidate for replacing silicon-based electronic devices. Although wafer-scale

growth and fabrication of prototype 2D-TMD devices have already been realized, to obtain high-

performance devices the issue of high contact resistance between the 2D semiconductors and

their metallic contacts must be overcome. Here, we investigate the nature of contact between

monolayer tungsten disulfide (WS₂) and the semimetal bismuth (3-5 layers) as studied by low-

temperature scanning tunneling microscopy/spectroscopy (LT-STM/S.) By using barrier

resonances, we directly map out the spatial variation of work function in regions with and

without Bi overlayers. Such a work function study shows that the work function of Bi is about

the same as the electron affinity of ML-WS₂, suggesting an ohmic contact. Direct band

mapping, however, does not show substantial band bending as is required for the alignment of the

vacuum level. We found that the ML-WS₂/substrate interface also impacts the spatial variations

of the band structure. We further reveal dramatically different behavior between a vertical vs.

lateral Bi/WS₂ junction.