skip to main content


This content will become publicly available on August 7, 2025

Title: Low Contact Resistance on Monolayer MoS 2 Field-Effect Transistors Achieved by CMOS-Compatible Metal Contacts
Contact engineering on monolayer layer (ML) semiconducting transition metal dichalcogenides (TMDs) is considered the most challenging problem towards using these materials as a transistor channel in future advanced technology nodes. The typically observed strong Femi level pinning induced in part by the reaction of the source/drain contact metal and the ML TMD frequently results in a large Schottky barrier height, which limits the electrical performance of ML TMD field-effect transistors (FETs). However, at a microscopic level, little is known about how interface defects or reaction sites impact the electrical performance of ML TMD FETs. In this work, we have performed statistically meaningful electrical measurements on at least 120 FETs combined with careful surface analysis to unveil contact resistance dependencies on the interface chemistry. In particular, we achieved a low contact resistance for ML MoS2 FETs with ultra-high vacuum (UHV, 3×10-11 mbar) deposited Ni contacts, ~500 ohm·μm, which is 5 times lower than the contact resistance achieved when deposited at high vacuum (HV, 3×10-6 mbar) conditions. These electrical results strongly correlate with our surface analysis observations. X-ray photoelectron spectroscopy (XPS) revealed significant bonding species between Ni and MoS2 under UHV conditions compared to HV. We also studied the Bi/MoS2 interface under UHV and HV deposition conditions. Different from the case of Ni, we do not observe a difference in contact resistance or interface chemistry between contacts deposited under UHV and HV. Finally, this article also explores the thermal stability and reliability of the two contact metals employed here.  more » « less
Award ID(s):
2002741
NSF-PAR ID:
10531719
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ;
Publisher / Repository:
American Chemical Society
Date Published:
Journal Name:
ACS Nano
ISSN:
1936-0851
Subject(s) / Keyword(s):
Contact interface engineering, low contact resistance, monolayer MoS2, field-effect transistors, CMOS-compatible, thermal stability, interface chemistry
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. The interface properties and thermal stability of bismuth (Bi) contacts on molybdenum disulfide (MoS2) shed light on their behavior under various deposition conditions and temperatures. The examination involves extensive techniques including X-ray photoelectron spectroscopy, scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). Bi contacts formed a van der Waals interface on MoS2 regardless of deposition conditions, such as ultrahigh vacuum (UHV, 3 × 10–11 mbar) and high vacuum (HV, 4 × 10–6 mbar), while the oxidation on MoS2 has been observed. However, the semimetallic properties of Bi suppress the impact of defect states, including oxidized-MoS2 and vacancies. Notably, the n-type characteristic of Bi/MoS2 remains unaffected, and no significant changes in the local density of states near the conduction band minimum are observed despite the presence of defects detected by STM and STS. As a result, the Fermi level (EF) resides below the conduction band of MoS2. The study also examines the impact of annealing on the contact interface, revealing no interface reaction between Bi and MoS2 up to 300 °C. These findings enhance our understanding of semimetal (Bi) contacts on MoS2, with implications for improving the performance and reliability of electronic devices. 
    more » « less
  2. Recently, the fabricated MoS2 field effect transistors (FETs) with 1T-MoS2 electrodes exhibit excellent performance with rather low contact resistance, as compared with those with metals deposited directly on 2H-MoS2 (Kappera et al 2014 Nat. Mater. 13 1128), but the reason for that remains elusive. By means of density functional theory calculations, we investigated the carrier injection at the 1T/2H MoS2 interface and found that although the Schottky barrier height (SBH) values of 1T/2H MoS2 interfaces can be tuned by controlling the stacking patterns, the p-type SBH values of 1T/2H MoS2 interfaces with different stackings are lower than their corresponding n-type SBH values, which demonstrated that the metallic 1T phase can be used as an efficient hole injection layer for 2H-MoS2. In addition, as compared to the n-type Au/MoS2 and Pd/MoS2 contacts, the p-type SBH values of 1T/2H MoS2 interfaces are much lower, which stem from the efficient hole injection between 1T-MoS2 and 2H-MoS2. This can explain the low contact resistance in the MoS2 FETs with 1T-MoS2 electrodes. Notably, the SBH values can be effectively modulated by an external electric field, and a significantly low p-type SBH value can be achieved under an appropriate electric field. We also demonstrated that this approach is also valid for WS2, WSe2 and MoSe2 systems, which indicates that the method can most likely be extended to other TMDs, and thus may open new promising avenues of contact engineering in these materials. 
    more » « less
  3. Tungsten transition metal dichalcogenides (W-TMDs) are intriguing due to their properties and potential for application in next-generation electronic devices. However, strong Fermi level (EF) pinning manifests at the metal/W-TMD interfaces, which could tremendously restrain the carrier injection into the channel. In this work, we illustrate the origins of EF pinning for Ni and Ag contacts on W-TMDs by considering interface chemistry, band alignment, impurities, and imperfections of W-TMDs, contact metal adsorption mechanism, and the resultant electronic structure. We conclude that the origins of EF pinning at a covalent contact metal/W-TMD interface, such as Ni/W-TMDs, can be attributed to defects, impurities, and interface reaction products. In contrast, for a van der Waals contact metal/TMD system such as Ag/W-TMDs, the primary factor responsible for EF pinning is the electronic modification of the TMDs resulting from the defects and impurities with the minor impact of metal-induced gap states. The potential strategies for carefully engineering the metal deposition approach are also discussed. This work unveils the origins of EF pinning at metal/TMD interfaces experimentally and theoretically and provides guidance on further enhancing and improving the device performance. 
    more » « less
  4. Two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are making impressive strides in a short duration compared to other candidates. However, to unlock their full potential for advanced logic transistors, attention must be given to improving the contacts or interfaces they form. One approach is to interface with a suitable low work function metal contact to allow the surface Fermi level (EF) movement toward intended directions, thereby augmenting the overall electrical performance. In this work, we implement physical characterization to understand the tin (Sn) contact interface on monolayer and bulk molybdenum disulfide (MoS2) via in situ x-ray photoelectron spectroscopy and ex situ atomic force microscopy. A Sn contact exhibited a van der Waals type weak interaction with the MoS2 bulk surface where no reaction between Sn and MoS2 is detected. In contrast, reaction products with Sn—S bonding are detected with a monolayer surface consistent with a covalentlike interface. Band alignment at the interface indicates that Sn deposition induces n-type properties in the bulk substrate, while EF of the monolayer remains pinned. In addition, the thermal stability of Sn on the same substrates is investigated in a sequential ultrahigh vacuum annealing treatment at 100, 200, 300, and 400 °C. Sn sublimated/desorbed from both substrates with increasing temperature, which is more prominent on the bulk substrate after annealing at 400 °C. Additionally, Sn significantly reduced the monolayer substrate and produced detectable interface reaction products at higher annealing temperatures. The findings can be strategized to resolve challenges with contact resistance that the device community is having with TMDs.

     
    more » « less
  5. Abstract

    Sc has been employed as an electron contact to a number of two-dimensional (2D) materials (e.g. MoS2, black phosphorous) and has enabled, at times, the lowest electron contact resistance. However, the extremely reactive nature of Sc leads to stringent processing requirements and metastable device performance with no true understanding of how to achieve consistent, high-performance Sc contacts. In this work, WSe2transistors with impressive subthreshold slope (109 mV dec−1) andION/IOFF(106) are demonstrated without post-metallization processing by depositing Sc contacts in ultra-high vacuum (UHV) at room temperature (RT). The lowest electron Schottky barrier height (SBH) is achieved by mildly oxidizing the WSe2in situbefore metallization, which minimizes subsequent reactions between Sc and WSe2. Post metallization anneals in reducing environments (UHV, forming gas) degrade theION/IOFFby ~103and increase the subthreshold slope by a factor of 10. X-ray photoelectron spectroscopy indicates the anneals increase the electron SBH by 0.4–0.5 eV and correspondingly convert 100% of the deposited Sc contacts to intermetallic or scandium oxide. Raman spectroscopy and scanning transmission electron microscopy highlight the highly exothermic reactions between Sc and WSe2, which consume at least one layer RT and at least three layers after the 400 °C anneals. The observed layer consumption necessitates multiple sacrificial WSe2layers during fabrication. Scanning tunneling microscopy/spectroscopy elucidate the enhanced local density of states below the WSe2Fermi level around individual Sc atoms in the WSe2lattice, which directly connects the scandium selenide intermetallic with the unexpectedly large electron SBH. The interface chemistry and structural properties are correlated with Sc–WSe2transistor and diode performance. The recommended combination of processing conditions and steps is provided to facilitate consistent Sc contacts to WSe2.

     
    more » « less