(184c) Tuning the Electronic and Structural Properties of MoS2-Metal Interface through in-Silico Screening of Metal Contacts

Sagar Udyavara^1*, Ryan Wu¹, Rui Ma², Yan Wang³, Manish Chhowalla^3,4, Turan Birol¹, Steven J. Koester², K.Andre Mkhoyan¹ and Matthew Neurock¹

¹Dept. of Chemical Engineering and Material Science, University of Minnesota, Minneapolis, MN 55455

²Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455

³Department of Material Science and Engineering, Rutgers University, Piscataway, NJ 08854

⁴Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK

Abstract:

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have recently generated a lot of interest due to its unique tunable electronic properties that make it suitable for electronic applications. Of these, 2D molybdenum disulfide (MoS₂) is of particular interest and acts as an excellent channel material for ultra-thin field effect transistors [1]. However, high contact resistance across the metal-MoS₂ interface due to Fermi level pinning continues to limit its full realization [2]. This contact resistance is linked to the structure of the interface which remains poorly understood. While previous theoretical studies have looked at such metal-MoS₂ contacts using the “metal-MoS₂-slab” approach, they however have failed to consider the structural changes at the interface upon metal deposition and the influence of those changes on the electronic properties of the interface [3]. Here, using a different “single-atom-addition” approach, we are able to characterize the resultant structural changes occurring at the interface upon deposition of different metals and also the subsequent effect on its electronic properties [4]. We initially looked at the Ti-MoS₂ interface which when characterized using atomic-resolution analytical scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) show a destruction of the pristine MoS₂ layers, formation of void pockets and penetration of metal atoms into the deeper MoS₂ layers. Calculations using the “single-atom-addition” approach validate these observations and show that early transition metals such as Ti and Sc, which have high affinity for sulfur react with the metal-MoS₂ interface leading to release of S from Mo along the interface, formation of M_xS_y clusters and pores in the MoS₂, which lead to the penetration of the metal atoms into the MoS₂ layers. Further, calculations suggest that such interfaces would have many localized states in the band gap of MoS₂ contributing into Fermi level pinning which show that the contact problem could be a fundamental limitation for such metals and cannot be overcome. We thus further looked into other different metal contacts and showed that this contact problem could be minimized by using metals like In which interact with the MoS₂ surface via weaker van der Waal’s type interactions, thus keeping the surface pristine. DFT calculations further revealed that such systems do not have many localized states present in the band gap due to which Fermi level pinning would likely be avoided, leading to lower contact resistances [5].

References:

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• Wu et al. An Inside Look at the Ti-MoS₂ contact in ultra-thin field effect transistor with atomic resolution, arXiv preprint arXiv:1807.01377
• Wang et al. Van der Waals contacts between three-dimensional metals and two-dimensional semiconductors, Nature 568, 70-74 (2019)