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Publisher: London : Nature Publishing Group × Subject: Transition metals ×

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Now showing 1 - 3 of 3
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    Reactive plasma cleaning and restoration of transition metal dichalcogenide monolayers
    (London : Nature Publishing Group, 2021) Marinov, Daniil; de Marneffe, Jean-François; Smets, Quentin; Arutchelvan, Goutham; Bal, Kristof M.; Voronina, Ekaterina; Rakhimova, Tatyana; Mankelevich, Yuri; El Kazzi, Salim; Nalin Mehta, Ankit; Wyndaele, Pieter-Jan; Heyne, Markus Hartmut; Zhang, Jianran; With, Patrick C.; Banerjee, Sreetama; Neyts, Erik C.; Asselberghs, Inge; Lin, Dennis; De Gendt, Stefan
    The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future devices, leveraging their unique physical, optical, and chemical properties. Part of these emerging 2D materials are transition metal dichalcogenides (TMDs). So far there is limited understanding of the cleaning of “monolayer” TMD materials. In this study, we report on the use of downstream H2 plasma to clean the surface of monolayer WS2 grown by MOCVD. We demonstrate that high-temperature processing is essential, allowing to maximize the removal rate of polymers and to mitigate damage caused to the WS2 in the form of sulfur vacancies. We show that low temperature in situ carbonyl sulfide (OCS) soak is an efficient way to resulfurize the material, besides high-temperature H2S annealing. The cleaning processes and mechanisms elucidated in this work are tested on back-gated field-effect transistors, confirming that transport properties of WS2 devices can be maintained by the combination of H2 plasma cleaning and OCS restoration. The low-damage plasma cleaning based on H2 and OCS is very reproducible, fast (completed in a few minutes) and uses a 300 mm industrial plasma etch system qualified for standard semiconductor pilot production. This process is, therefore, expected to enable the industrial scale-up of 2D-based devices, co-integrated with silicon technology.
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    Giant persistent photoconductivity in monolayer MoS2 field-effect transistors
    (London : Nature Publishing Group, 2021) George, A.; Fistul, M.V.; Gruenewald, M.; Kaiser, D.; Lehnert, T.; Mupparapu, R.; Neumann, C.; Hübner, U.; Schaal, M.; Masurkar, N.; Arava, L.M.R.; Staude, I.; Kaiser, U.; Fritz, T.; Turchanin, A.
    Monolayer transition metal dichalcogenides (TMD) have numerous potential applications in ultrathin electronics and photonics. The exposure of TMD-based devices to light generates photo-carriers resulting in an enhanced conductivity, which can be effectively used, e.g., in photodetectors. If the photo-enhanced conductivity persists after removal of the irradiation, the effect is known as persistent photoconductivity (PPC). Here we show that ultraviolet light (λ = 365 nm) exposure induces an extremely long-living giant PPC (GPPC) in monolayer MoS2 (ML-MoS2) field-effect transistors (FET) with a time constant of ~30 days. Furthermore, this effect leads to a large enhancement of the conductivity up to a factor of 107. In contrast to previous studies in which the origin of the PPC was attributed to extrinsic reasons such as trapped charges in the substrate or adsorbates, we show that the GPPC arises mainly from the intrinsic properties of ML-MoS2 such as lattice defects that induce a large number of localized states in the forbidden gap. This finding is supported by a detailed experimental and theoretical study of the electric transport in TMD based FETs as well as by characterization of ML-MoS2 with scanning tunneling spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. The obtained results provide a basis for the defect-based engineering of the electronic and optical properties of TMDs for device applications.
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    Nonlocal dielectric function and nested dark excitons in MoS2
    (London : Nature Publishing Group, 2019) Koitzsch, A.; Pawlik, A.-S.; Habenicht, C.; Klaproth, T.; Schuster, R.; Büchner, B.; Knupfer, M.
    Their exceptional optical properties are a driving force for the persistent interest in atomically thin transition metal dichalcogenides such as MoS2. The optical response is dominated by excitons. Apart from the bright excitons, which directly couple to light, it has been realized that dark excitons, where photon absorption or emission is inhibited by the spin state or momentum mismatch, are decisive for many optical properties. However, in particular the momentum dependence is difficult to assess experimentally and often remains elusive or is investigated by indirect means. Here we study the momentum dependent electronic structure experimentally and theoretically. We use angle-resolved photoemission as a one-particle probe of the occupied valence band structure and electron energy loss spectroscopy as a two-particle probe of electronic transitions across the gap to benchmark a single-particle model of the dielectric function ϵ(q, ω) against momentum dependent experimental measurements. This ansatz captures key aspects of the data surprisingly well. In particular, the energy region where substantial nesting occurs, which is at the origin of the strong light–matter interaction of thin transition metal dichalcogenides and crucial for the prominent C-exciton, is described well and spans a more complex exciton landscape than previously anticipated. Its local maxima in (q≠0,ω) space can be considered as dark excitons and might be relevant for higher order optical processes. Our study may lead to a more complete understanding of the optical properties of atomically thin transition metal dichalcogenides.