Electronic Structure, Stacking Arrangement and the Interaction Strength of Tungsten Disulfide at the Gold Contact

The nuanced interactions of a two-dimensional (2D) layered semiconductor/metal interface continue to be a central focus for study since device scaling demands the highest possible performance in the thinnest possible channel. Unlocking the intrinsically nanoscale nature of such a junction requires a detailed examination of the chemical and physical properties of the materials. Indeed, there is an intensive effort to understand and control the nature of attractive interactions between 2D layered semiconductors and metals and to examine its impact on the chemical bonding, electronic band structure, and contact resistance at the junction. Here, we present a photoelectron spectroscopy study on the interface between WS₂ films and gold, with a focus on the occupied electronic states near the Brillouin zone center (<i>i.e.</i>, the Γ point) and the work function, to illuminate the nature of the WS₂-Au interaction. To delineate the WS₂ spectra of a locally varying interaction strength with Au, we employ a microscopy approach and tailored sample structures, ranging from the WS₂/Au semi-epitaxial junctions to the suspended WS₂ regions. Spectroscopic analysis shows the shifts in the electron binding energy (<i>E</i>_B) of the occupied electronic states depending on the interaction strength. In the strong interaction limit, we have discovered a new source for contact heterogeneity – local stacking order. We hypothesize that these stacking perturbations will generally arise when the substrate-2D material interaction is stronger than the inter-2D layer interaction. Together, these findings show that optimizing 2D semiconductors’ electronic properties will require a careful control of the interface and conditioning steps for the underlying Au structure could directly impact the interface, in addition to the electron screening and defects that have been widely considered.<br/>This research was conducted in collaboration with Alex M. Boehm, Catalin D. Spataru, Cherrelle J. Thomas, and R. Guild Copeland at Sandia National Laboratories, and Jose J. Fonseca and Jeremy T. Robinson at Naval Research Laboratory. We acknowledge insightful discussions with R. Tandon and N. Bartelt. The work at Sandia National Laboratories was supported by Sandia’s LDRD program and the Center for Integrated Nanotechnologies user program, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science (DE-AC04-94AL85000). The work at the US Naval Research Laboratory was funded by the Office of Naval Research. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly-owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.