Abstract
Among various 2D materials, monolayer transition-metal dichalcogenide (mTMD) semiconductors with intrinsic band gaps (1–2 eV) are considered promising candidates for channel materials in next-generation transistors. Low-resistance metal contacts to mTMDs are crucial because currently they limit mTMD device performances. Hence, a comprehensive understanding of the atomistic nature of metal contacts to these 2D crystals is a fundamental challenge, which is not adequately addressed at present. In this paper, we report a systematic study of metal-mTMD contacts with different geometries (top contacts and edge contacts) by ab initio density-functional theory calculations, integrated with Mulliken population analysis and a semiempirical van der Waals dispersion potential model (which is critical for 2D materials and not well treated before). Particularly, In, Ti, Au, and Pd, contacts to monolayer and as well as and contacts are evaluated and categorized, based on their tunnel barriers, Schottky barriers, and orbital overlaps. Moreover, going beyond Schottky theory, new physics in such contact interfaces is revealed, such as the metallization of mTMDs and abnormal Fermi level pinning. Among the top contacts to , Ti and Mo show great potential to form favorable top contacts, which are both -type contacts, while for top contacts to , W or Pd exhibits the most advantages as an - or -type contact, respectively. Moreover, we find that edge contacts can be highly advantageous compared to top contacts in terms of electron injection efficiency. Our formalism and the results provide guidelines that would be invaluable for designing novel 2D semiconductor devices.
16 More- Received 27 June 2013
DOI:https://doi.org/10.1103/PhysRevX.4.031005
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Published by the American Physical Society
Popular Summary
Semiconductors are used throughout the electronics industry in transistors and integrated circuits. Recently, a new class of thin, two-dimensional monolayer semiconductor materials known as monolayer transition-metal dichalcogenides (mTMDs)—with a chemical formula of , where is a transition metal and is O, S, Se, or Te—has shown exceptional promise for building next-generation energy-efficient digital switches and ultrasensitive biosensors because of their inherently large and uniform band gaps of 1–2 eV. Furthermore, mTMDs have inherent flexibility and transparency, rendering them attractive to display electronics. These materials additionally have pristine surfaces without any out-of-plane dangling chemical bonds, which help reduce the interface traps as well as scattering of charge carriers, thereby boosting device performance. Ensuring low-resistance metal contacts to mTMD semiconductors is the primary hindrance to using this technology, however. We conduct a systematic study of various metal-mTMD contacts with different geometries (top contacts and edge contacts).
Studies have shown that the contact resistance at the metal-mTMD interface is 1–3 decades higher than that of metal-silicon contacts. These high resistances hinder the performance of mTMD transistors, prompting research to explore different metals and contact configurations. We investigate the nature of the physical contacts between monolayer and and In, Ti, Au, Pd, Mo, and W contacts using an ab initio density-functional theory specifically designed for two-dimensional layered materials. We not only provide a pathway to identify the best contact metals for these semiconductors but also reveal some new physics of the interfaces that, in turn, determine the carrier transport across such interfaces. Our calculations also show that edge contacts are favored over top contacts.
The formalism and the results in this work provide guidelines for novel two-dimensional semiconductor device design and fabrication, a field that is on the rise because of limitations in scaling silicon semiconductor technology. Our results show how two-dimensional monolayer semiconductors can be optimally designed to yield the highest electronic efficiencies.