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Advanced beyond-silicon electronic technology requires both channel materials and also ultralow-resistance contacts to be discovered.sup.1,2. Atomically thin two-dimensional semiconductors have great potential for realizing high-performance electronic devices.sup.1,3. However, owing to metal-induced gap states (MIGS).sup.4-7, energy barriers at the metal-semiconductor interface--which fundamentally lead to high contact resistance and poor current-delivery capability--have constrained the improvement of two-dimensional semiconductor transistors so far.sup.2,8,9. Here we report ohmic contact between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where the MIGS are sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a contact resistance of 123 ohm micrometres and an on-state current density of 1,135 microamps per micrometre on monolayer MoS.sub.2; these two values are, to the best of our knowledge, the lowest and highest yet recorded, respectively. We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS.sub.2, WS.sub.2 and WSe.sub.2. Our reported contact resistances are a substantial improvement for two-dimensional semiconductors, and approach the quantum limit. This technology unveils the potential of high-performance monolayer transistors that are on par with state-of-the-art three-dimensional semiconductors, enabling further device downscaling and extending Moore's law. Electric contacts of semimetallic bismuth on monolayer semiconductors are shown to suppress metal-induced gap states and thus have very low contact resistance and a zero Schottky barrier height., Author(s): Pin-Chun Shen [sup.1] , Cong Su [sup.2] [sup.3] [sup.4] [sup.5] , Yuxuan Lin [sup.1] [sup.6] , Ang-Sheng Chou [sup.7] [sup.8] , Chao-Ching Cheng [sup.7] , Ji-Hoon Park [sup.1] , [...]

In this work, we report a simple method for solution-processed organic light emitting devices (OLEDs), where single-layer graphene acts as the anode and the hexa-peri-hexabenzocoronene exfoliating agent (HBC-6ImBr) provides surface modification. In SEM images, the PEDOT:PSS solution fully covered the graphene electrode after coating with HBC-6ImBr. The fabricated solution-processed OLEDs with a single-layer graphene anode showed outstanding brightness at 3182 cd/m² and current efficiency up to 6 cd/A which is comparable to that of indium tin oxide films, and the OLED device brightness performance increases six times compared to tri-layer graphene treated with UV-Ozone at the same driving voltage. This method can be used in a wide variety of solution-processed organic optoelectronics on surface-modified graphene anodes. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) semiconducting monolayers such as transition metal dichalcogenides (TMDs) are promising channel materials to extend Moore's Law in advanced electronics. Synthetic TMD layers from chemical vapor deposition (CVD) are scalable for fabrication but notorious for their high defect densities. Therefore, innovative endeavors on growth reaction to enhance their quality are urgently needed. Here, we report that the hydroxide W species, an extremely pure vapor phase metal precursor form, is very efficient for sulfurization, leading to about one order of magnitude lower defect density compared to those from conventional CVD methods. The field-effect transistor (FET) devices based on the proposed growth reach a peak electron mobility ~200 cm2/Vs (~800 cm2/Vs) at room temperature (15 K), comparable to those from exfoliated flakes. The FET device with a channel length of 100 nm displays a high on-state current of ~400 µA/µm, encouraging the industrialization of 2D materials. Chemical vapor deposition enables the scalable production of 2D semiconductors, but the grown materials are usually affected by high defect densities. Here, the authors report a hydroxide vapour phase deposition method to synthesize wafer-scale monolayer WS2 with reduced defect density and electrical properties comparable to those of exfoliated flakes. [ABSTRACT FROM AUTHOR]

Proving the device performance and process feasibility is imperative for the realization of two-dimensional (2D) semiconductor electronics. In this work, we have successfully adopted Tin (Sn) as the Ohmic contact metal to monolayer molybdenum disulfide (MoS2) grown by chemical vapor deposition (CVD) and demonstrated superior short channel n-type field effect transistor (nFET) performance reaching an ON-current of $480~\mu \text{A}/\mu \text{m}$ and keeping the OFF-current below 0.1 nA/ $\mu \text{m}$ at $\text{V}_{DS} = 1$ V. These efforts are close to the low power specification of Si transistors in the metrics of International Roadmap for Devices and Systems (IRDS). The performance improvement could be attributed to the re-melting behavior of Sn metal. We suggest that the Sn deposited at lower temperatures could reduce the formation of interfacial defects caused by heat, and high-melting-point capping metal also could assist the re-melting phenomenon of underlying Sn contact layer. These process modifications are helpful to form smooth Sn coverage on MoS2, thereby reducing the contact resistance to 0.84 $\text{k}\Omega \cdot \mu \text{m}$. This work provides a practical pathway to form low-resistance metal contact on 2D semiconductors for performance improvement. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, bismuth iodide (BiI3), a layered metal halide semiconducting light absorber with a wide bandgap of ≈1.8 eV and strong optical absorption in the visible region, has received greater attention for photovoltaic applications. In this study, ultrasensitive visible‐light photodetectors with graphene/BiI3 vertical heterostructures are achieved by van der Waals epitaxies. The BiI3 films deposited on graphene show flatter morphologies and significantly better crystallinities than that of BiI3 films on SiO2 substrates, mainly due to weak van der Waals interactions at the graphene/BiI3 interface. Hybrid photodetectors with highly crystalline graphene/BiI3 heterostructures demonstrate an ultrahigh responsivity of 6 × 106 A W−1, shot‐noise‐limited detectivity of 7 × 1014 Jones, and a relatively short response time of ≈8 ms. Compared to most previously reported graphene‐based hybrid photodetectors, these devices have comparable photosensitivities but a faster response speed and lower operation voltage, which is quite promising for ultralow intensity visible‐light sensors. Moreover, the electronic structure and interfacial chemistry at the graphene/BiI3 heterojunctions are investigated using photoemission spectroscopy. The results give clear evidence that no chemical interactions occur between graphene and BiI3, resulting in the van der Waals epitaxial growth, and the measured band bending consistently illustrates that a photoinduced charge transfer occurs at the graphene/BiI3 interface. [ABSTRACT FROM AUTHOR]