Zhiying Wang1,Yang Liu1,Song Liu1,Amirali Zangiabadi2,James Hone1
Columbia University1,University at Buffalo, The State University of New York2
Zhiying Wang1,Yang Liu1,Song Liu1,Amirali Zangiabadi2,James Hone1
Columbia University1,University at Buffalo, The State University of New York2
Achieving robust electrical contacts has emerged as a key challenge to realizing the promise of monolayer two-dimensional (2D) semiconductors, such as semiconducting transition metal dichalcogenides (s-TMDs) in next-generation electronic technologies. While recent work has reported breakthroughs in achieving low contact resistance using defective s-TMDs, there still exists a lack of fundamental understanding of the contact interfaces, especially, the role of defects within the s-TMD and external disorder in achieving the reported low contact resistance is not well understood.<br/><br/>In this work, we study bismuth (Bi) semimetal contacts to monolayer molybdenum diselenide (MoSe<sub>2</sub>), utilizing a device platform that combines ultrahigh-purity MoSe<sub>2</sub>, damage-and strain-free interfaces, and encapsulation of the channel within hexagonal boron nitride (hBN). The ultrahigh-purity MoSe<sub>2 </sub>crystal is characterized through dedicated STM imaging. Gaps etched in the hBN define the contact regions and stabilize the structure. We characterize the damage-free and strain-free metal-semiconductor junction with atomically sharp interface by cross-sectional STEM imaging. We quantify the contact characteristics using contact-front and contact-end measurements that go beyond the basic transfer length method. These measurements reveal large differences between sheet resistance inside the channel (R<sub>sh</sub>) and underneath the contacts (R<sub>sk</sub>), which are assumed to be equal in the standard transfer length method. We employ a charge transfer model to gain further insight into the microscopic mechanism of vertical transport across the van der Waals metal-s-TMDs interface. In deeply scaled contacts, we directly observed a steep increase in end resistance when the contact length downscaled to around 40 nm, in agreement with the experimental and theoretical model derived characteristic length. This integrated approach can be readily expanded to other clean semiconductors to allow better comparison between theory and experiment.