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1. Engineering the interface chemistry for scandium electron contacts in WSe 2 transistors and diodes

   https://doi.org/10.1088/2053-1583/ab2c44  

   Smyth, Christopher M. ; Walsh, Lee A. ; Bolshakov, Pavel ; Catalano, Massimo ; Schmidt, Michael ; Sheehan, Brendan ; Addou, Rafik ; Wang, Luhua ; Kim, Jiyoung ; Kim, Moon J. ; et al ( July 2019 , 2D Materials)

   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) andI_ON/I_OFF(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 theI_ON/I_OFFby ~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.

    

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2. Can Metals Other than Au be Used for Large Area Exfoliation of MoS 2 Monolayers?

   https://doi.org/10.1002/admi.202200106  

   Johnston, Ammon Cody ; Khondaker, Saiful I. ( March 2022 , Advanced Materials Interfaces)

   Au‐mediated exfoliation of 2D transition‐metal dichalcogenides (TMDs) has received significant attention due to its ability to produce large‐area monolayer (ML) flakes. This process has been attributed to strong TMD/Au binding energy (BE) as well as the uniform strain between the TMDs and Au. However, large‐area exfoliation of TMDs with other metals that have even stronger theoretical BE than Au/TMD is not successful, leading to question whether the BE plays any role in the exfoliation process. Here, successful demonstration of large‐area ML MoS₂using Cu, Ni, and Ag with various predicted strain, including Pd with almost no strain, but stronger BE than Au/MoS₂is demonstrated. Optical micrographs show MoS₂flakes with 100s of µm in size with a yield of several tens to hundreds of ML flakes per exfoliation. Photoluminescence and Raman spectroscopy confirm the ML nature of the flakes, while electrical transport measurements show mobilities of≈6 cm² V⁻¹ s⁻¹with a current on‐off ratio≈10⁸consistent with high‐quality ML MoS₂. Given that MoS₂can be exfoliated with metals that have strong BE irrespective of their strain values suggests that BE is the primary mechanism in successful exfoliation of large‐area ML MoS₂.

    

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3. Improvements in 2D p-type WSe2 transistors towards ultimate CMOS scaling

   https://doi.org/10.1038/s41598-023-30317-4  

   Patoary, Naim Hossain ; Xie, Jing ; Zhou, Guantong ; Al Mamun, Fahad ; Sayyad, Mohammed ; Tongay, Sefaattin ; Esqueda, Ivan Sanchez ( February 2023 , Scientific Reports)

   This paper provides comprehensive experimental analysis relating to improvements in the two-dimensional (2D) p-type metal–oxide–semiconductor (PMOS) field effect transistors (FETs) by pure van der Waals (vdW) contacts on few-layer tungsten diselenide (WSe₂) with high-k metal gate (HKMG) stacks. Our analysis shows that standard metallization techniques (e.g., e-beam evaporation at moderate pressure ~ 10^–5 torr) results in significant Fermi-level pinning, but Schottky barrier heights (SBH) remain small (< 100 meV) when using high work function metals (e.g., Pt or Pd). Temperature-dependent analysis uncovers a more dominant contribution to contact resistance from the channel access region and confirms significant improvement through less damaging metallization techniques (i.e., reduced scattering) combined with strongly scaled HKMG stacks (enhanced carrier density). A clean contact/channel interface is achieved through high-vacuum evaporation and temperature-controlled stepped deposition providing large improvements in contact resistance. Our study reports low contact resistance of 5.7 kΩ-µm, with on-state currents of ~ 97 µA/µm and subthreshold swing of ~ 140 mV/dec in FETs with channel lengths of 400 nm. Furthermore, theoretical analysis using a Landauer transport ballistic model for WSe₂SB-FETs elucidates the prospects of nanoscale 2D PMOS FETs indicating high-performance (excellent on-state current vs subthreshold swing benchmarks) towards the ultimate CMOS scaling limit.

    

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4. Dramatically Enhanced Valley‐Polarized Emission by Alloying and Electrical Tuning of Monolayer WTe 2 x S 2(1‐ x ) Alloys at Room Temperature with 1T′‐WTe 2 ‐Contact

   https://doi.org/10.1002/advs.202304890  

   Lin, Wei‐Hsiang ; Li, Chia‐Shuo ; Wu, Chih‐I ; Rossman, George R. ; Atwater, Harry A. ; Yeh, Nai‐Chang ( November 2023 , Advanced Science)

   Monolayer ternary tellurides based on alloying different transition metal dichalcogenides (TMDs) can result in new two‐dimensional (2D) materials ranging from semiconductors to metals and superconductors with tunable optical and electrical properties. Semiconducting WTe_2xS_2(1‐x)monolayer possesses two inequivalent valleys in the Brillouin zone, each valley coupling selectively with circularly polarized light (CPL). The degree of valley polarization (DVP) under the excitation of CPL represents the purity of valley polarized photoluminescence (PL), a critical parameter for opto‐valleytronic applications. Here, new strategies to efficiently tailor the valley‐polarized PL from semiconducting monolayer WTe_2xS_2(1‐x)at room temperature (RT) through alloying and back‐gating are presented. The DVP at RT is found to increase drastically from < 5% in WS₂to 40% in WTe_0.12S_1.88by Te‐alloying to enhance the spin‐orbit coupling. Further enhancement and control of the DVP from 40% up to 75% is demonstrated by electrostatically doping the monolayer WTe_0.12S_1.88via metallic 1T′‐WTe₂electrodes, where the use of 1T′‐WTe₂substantially lowers the Schottky barrier height (SBH) and weakens the Fermi‐level pinning of the electrical contacts. The demonstration of drastically enhanced DVP and electrical tunability in the valley‐polarized emission from 1T′‐WTe₂/WTe_0.12S_1.88heterostructures paves new pathways towards harnessing valley excitons in ultrathin valleytronic devices for RT applications.

    

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5. Contact Engineering of Layered MoS 2 via Chemically Dipping Treatments

   https://doi.org/10.1002/adfm.202000250  

   Bang, Seungho ; Lee, Sangyeob ; Rai, Amritesh ; Duong, Ngoc Thanh ; Kawk, Iljo ; Wolf, Steven ; Chung, Choong‐Heui ; Banerjee, Sanjay K. ; Kummel, Andrew C. ; Jeong, Mun Seok ; et al ( February 2020 , Advanced Functional Materials)

   The performance of electronic/optoelectronic devices is governed by carrier injection through metal–semiconductor contact; therefore, it is crucial to employ low‐resistance source/drain contacts. However, unintentional introduction of extrinsic defects, such as substoichiometric oxidation states at the metal–semiconductor interface, can degrade carrier injection. In this report, controlling the unintentional extrinsic defect states in layered MoS₂is demonstrated using a two‐step chemical treatment, (NH₄)₂S(aq) treatment and vacuum annealing, to enhance the contact behavior of metal/MoS₂interfaces. The two‐step treatment induces changes in the contact of single layer MoS₂field effect transistors from nonlinear Schottky to Ohmic behavior, along with a reduction of contact resistance from 35.2 to 5.2 kΩ. Moreover, the enhancement ofI_ONand electron field effect mobility of single layer MoS₂field effect transistors is nearly double forn‐branch operation. This enhanced contact behavior resulting from the two‐step treatment is likely due to the removal of oxidation defects, which can be unintentionally introduced during synthesis or fabrication processes. The removal of oxygen defects is confirmed by scanning tunneling microscopy and X‐ray photoelectron spectroscopy. This two‐step (NH₄)₂S(aq) chemical functionalization process provides a facile pathway to controlling the defect states in transition metal dichalcogenides (TMDs), to enhance the metal‐contact behavior of TMDs.

    

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