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End date: 2023 × Start date: 2020 × DDC: 660 × DDC: 670 × Has files: Yes × Subject: Defects ×

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    Spatially controlled epitaxial growth of 2D heterostructures via defect engineering using a focused He ion beam
    (London : Nature Publishing Group, 2021) Heilmann, Martin; Deinhart, Victor; Tahraoui, Abbes; Höflich, Katja; Lopes, J. Marcelo J.
    The combination of two-dimensional (2D) materials into heterostructures enables the formation of atomically thin devices with designed properties. To achieve a high-density, bottom-up integration, the growth of these 2D heterostructures via van der Waals epitaxy (vdWE) is an attractive alternative to the currently mostly employed mechanical transfer, which is problematic in terms of scaling and reproducibility. Controlling the location of the nuclei formation remains a key challenge in vdWE. Here, a focused He ion beam is used to deterministically place defects in graphene substrates, which serve as preferential nucleation sites for the growth of insulating, 2D hexagonal boron nitride (h-BN). Therewith a mask-free, selective-area vdWE (SAvdWE) is demonstrated, in which nucleation yield and crystal quality of h-BN are controlled by the ion beam parameters used for defect formation. Moreover, h-BN grown via SAvdWE is shown to exhibit electron tunneling characteristics comparable to those of mechanically transferred layers, thereby lying the foundation for a reliable, high-density array fabrication of 2D heterostructures for device integration via defect engineering in 2D substrates.
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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.