Two dimensional semiconducting materials for ultimately scaled transistors.

Two dimensional (2D) semiconductors have been established as promising candidates to break through the short channel effect that existed in Si metal-oxide-semiconductor field-effect-transistor (MOSFET), owing to their unique atomically layered structure and dangling-bond-free surface. The last decade has witnessed the significant progress in the size scaling of 2D transistors by various approaches, in which the physical gate length of the transistors has shrank from micrometer to sub-one nanometer with superior performance, illustrating their potential as a replacement technology for Si MOSFETs. Here, we review state-of-the-art techniques to achieve ultra-scaled 2D transistors with novel configurations through the scaling of channel, gate, and contact length. We provide comprehensive views of the merits and drawbacks of the ultra-scaled 2D transistors by summarizing the relevant fabrication processes with the corresponding critical parameters achieved. Finally, we identify the key opportunities and challenges for integrating ultra-scaled 2D transistors in the next-generation heterogeneous circuitry.

Mid-Infrared Photodetection of Type-II Dirac Semimetal 1T-PtTe2 Grown by Molecular Beam Epitaxy.

Mid-infrared (MIR) photodetection is of significance in civil and military applications because it shows superiority in absorbing the vibration of various molecules and covering atmospheric transmission windows. Recently, the PtTe2, a typical type-II Dirac semimetal, has come under the spotlight due to its unique photodetection sensibility in the MIR region and robust stability in the atmosphere. Here, the high-quality and large-scale 1T-PtTe2 thin films with air stability were grown by molecular beam epitaxy. Broadband photoresponse of the photodetectors of PtTe2 from 420 nm to 10.7 µm shows high responsivity and detectivity of 0.2 mA W-1 and 2.6 × 107 Jones at 10.7 µm and 1.6 mA W-1 and 2.2 × 108 Jones at 4.7 µm under the atmosphere, respectively. Moreover, the photodetectors exhibit high sensitivity in visible and near-infrared regions (8.2 mA W-1 at 650 nm and 15.6 mA W-1 at 960 nm). The power- and polarization-dependent photoresponse measurements reveal the linear relationship of power photoresponse and obvious anisotropic photoresponse (the ratio of anisotropy ellipse is 8.3 at 10.7 µm), respectively. These results suggest that the PtTe2 could be expected to be an advanced photodetection material for polarization angle-sensitive detection, infrared imaging, and photodetection from the visible to MIR range.

Electrostatically Enhanced Electron-Phonon Interaction in Monolayer 2H-MoSe2 Grown by Molecular Beam Epitaxy.

The enhancement of electron-phonon interaction provides a reasonable explanation for gate-tunable phonon properties in some semiconductors where multiple inequivalent valleys are simultaneously occupied upon charge doping, especially in few-layer transition metal dichalcogenides (TMDs). In this work, we report var der Waals epitaxy of 2H-MoSe2 by molecular beam epitaxy (MBE) and gate-tunable phonon properties in monolayer and bilayer MoSe2. In monolayer MoSe2, we find that out-of-plane phonon mode A1g exhibits a strong softening and shifting toward lower wavenumbers at a high electron doping level, while in-plane phonon mode E2g1 remains unchanged. The softening and shifting of the out-of-plane phonon mode could be attributed to the increase of electron-phonon interaction and the simultaneous occupation of electrons in multiple inequivalent valleys. In bilayer MoSe2, no corresponding changes of phonon modes are detected at the same doping level, which could originate from the occupation of electrons only in single valleys upon high electron doping. This study demonstrates electrostatically enhanced electron-phonon interaction in monolayer MoSe2 and clarifies the relevance between occupation of multiple valleys and phonon properties by comparing Raman spectra of monolayer and bilayer MoSe2 at different doping levels.