High contact resistance is one of the primary concerns for electronic device applications of two-dimensional (2D) layered semiconductors. Here, we explore the enhanced carrier transport through metal-semiconductor interfaces in WS2 field effect transistors (FETs) by introducing a typical transition metal, Cu, with two different doping strategies: (i) a "generalized" Cu doping by using randomly distributed Cu atoms along the channel and (ii) a "localized" Cu doping by adapting an ultrathin Cu layer at the metal-semiconductor interface. Compared to the pristine WS2 FETs, both the generalized Cu atomic dopant and localized Cu contact decoration can provide a Schottky-to-Ohmic contact transition owing to the reduced contact resistances by 1 -3 orders of magnitude, and consequently elevate electron mobilities by 5 -7 times higher. Our work demonstrates that the introduction of transition metal can be an efficient and reliable technique to enhance the carrier transport and device performance in 2D TMD FETs. IntroductionTungsten disulfide (WS2) with a semiconducting 2H phase is one of two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibiting a series of unique properties, such as strong spin-orbit coupling, band splitting, and high nonlinear susceptibility 1-3 . Especially for future nanoelectronic applications, WS2 stands out as a promising channel material compared to other 2D semiconductors. For example, WS2 has a direct bandgap of 1.4 -2.0 eV 4-7 for the monolayer and an indirect bandgap of 1.2 -1.3 eV 4-6 for the bulk crystals. The carrier mobility of WS2 has been theoretically predicated up to ~5,300 cm 2 /Vs at 77 K 8 and ~700 -1,100 cm 2 /Vs at room temperature 8,9 , which exceeds most of the commonly used semiconducting TMDs such as MoS2 (340 cm 2 /Vs), MoSe2 (240 cm 2 /Vs), WSe2 (705 cm 2 /Vs), owing to the relatively small effective mass (0.34m0 for electrons and 0.46m0 for holes, where m0 is the free electron mass) 7 . Although the experimentally demonstrated electron mobilities, limited by Coulomb impurities, charge traps, surface defects and roughness, are much lower than the theoretical predication, new techniques have been developed to practically improve the mobility, for example, by exploiting h-BN 10 or high-k 11 dielectrics. For the application of field-effect transistors (FETs), monolayer WS2 FETs are predicated to outperform other TMD FETs in terms of the on-state current density (JD,on) for both p-and n-type transistors (~2,800 μA/μm for the monolayer WS2 versus 2,200 -2,400 μA/μm for the monolayer MoS2, MoSe2, and MoTe2 FETs) 11 . In addition to the carrier mobility, the pristine hysteresis width of WS2 during reliability tests is the lowest compared to MoS2, MoSe2and MoTe2 FETs 12 . The current on/off ratio at room temperature has been experimentally demonstrated up to ~10 6 for the monolayer WS2 FETs 13,14 and to ~10 8 for the multilayer WS2 FETs 15 . A nearly ideal subthreshold swing (SS) of 70 mV/decade at room temperature has been demonstrated in a simple back-gated WS2 FET th...
Two-dimensional (2D) materials may play an important role in future photodetectors due to their natural atom-thin body thickness, unique quantum confinement, and excellent electronic and photoelectric properties. Semimetallic graphene, semiconductor black phosphorus, and transition metal dichalcogenides possess flexible and adjustable bandgaps, which correspond to a wide interaction spectrum ranging from ultraviolet to terahertz. Nevertheless, their absorbance is relatively low, and it is difficult for a single material to cover a wide spectrum. Therefore, the combination of phototransistors based on 2D hybrid structures with other material platforms, such as quantum dots, organic materials, or plasma nanostructures, exhibit ultra-sensitive and broadband optical detection capabilities that cannot be ascribed to the individual constituents of the assembly. This article provides a comprehensive and systematic review of the recent research progress of 2D material photodetectors. First, the fundamental detection mechanism and key metrics of the 2D material photodetectors are introduced. Then, the latest developments in 2D material photodetectors are reviewed based on the strategies of photocurrent enhancement. Finally, a design and implementation principle for high-performance 2D material photodetectors is provided, together with the current challenges and future outlooks.
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