The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (R C). Here we present a systematic study of scaling MoS2 devices and contacts with varying ...electrode metals and controlled deposition conditions, over a wide range of temperatures (80 to 500 K), carrier densities (1012 to 1013 cm–2), and contact dimensions (20 to 500 nm). We uncover that Au deposited in ultra-high vacuum (∼10–9 Torr) yields three times lower R C than under normal conditions, reaching 740 Ω·μm and specific contact resistivity 3 × 10–7 Ω·cm2, stable for over four months. Modeling reveals separate R C contributions from the Schottky barrier and the series access resistance, providing key insights on how to further improve scaling of MoS2 contacts and transistor dimensions. The contact transfer length is ∼35 nm at 300 K, which is verified experimentally using devices with 20 nm contacts and 70 nm contact pitch (CP), equivalent to the “14 nm” technology node.
Electron-beam evaporation is commonly used to form metal contacts on two-dimensional (2D) materials. Many evaporated metals contain high levels of stress, but the effect of this stress on 2D device ...performance has yet to be determined. Here, we investigate the impact of tensile-stressed nickel evaporated onto gold contacts of monolayer MoS2 transistors. Optical measurements reveal a distribution of tensile strain along the MoS2 channel between stressed contacts, up to ~0.8% near the contact edges. Further, we show that stressed contacts can substantially influence device performance, leading to negative threshold voltage shifts and increased transconductance. In the limit of short (50 nm) channels with large (<inline-formula> <tex-math notation="LaTeX">2~\mu </tex-math></inline-formula> m) contact stressors, we find that this can cause an on-state current increase up to 2.5x. These results show that contact-induced strain must be closely examined in emerging technologies, and this approach could be used to improve future device performance.
Layered two-dimensional (2D) materials have entered the spotlight as promising channel materials for future optoelectronic devices owing to their excellent electrical and optoelectronic properties. ...However, their limited photodetection range caused by their wide bandgap remains a principal challenge in 2D layered materials-based phototransistors. Here, we developed a germanium (Ge)-gated MoS2 phototransistor that can detect light in the region from visible to infrared (λ = 520–1550 nm) using a detection mechanism based on band bending modulation. In addition, the Ge-gated MoS2 phototransistor is proposed as a multilevel optic-neural synaptic device, which performs both optical-sensing and synaptic functions on one device and is operated in different current ranges according to the light conditions: dark, visible, and infrared. This study is expected to contribute to the development of 2D material-based phototransistors and synaptic devices in next-generation optoelectronics.
Semiconducting transition metal dichalcogenides (TMDs) are promising for flexible high-specific-power photovoltaics due to their ultrahigh optical absorption coefficients, desirable band gaps and ...self-passivated surfaces. However, challenges such as Fermi-level pinning at the metal contact-TMD interface and the inapplicability of traditional doping schemes have prevented most TMD solar cells from exceeding 2% power conversion efficiency (PCE). In addition, fabrication on flexible substrates tends to contaminate or damage TMD interfaces, further reducing performance. Here, we address these fundamental issues by employing: (1) transparent graphene contacts to mitigate Fermi-level pinning, (2) MoO
capping for doping, passivation and anti-reflection, and (3) a clean, non-damaging direct transfer method to realize devices on lightweight flexible polyimide substrates. These lead to record PCE of 5.1% and record specific power of 4.4 W g
for flexible TMD (WSe
) solar cells, the latter on par with prevailing thin-film solar technologies cadmium telluride, copper indium gallium selenide, amorphous silicon and III-Vs. We further project that TMD solar cells could achieve specific power up to 46 W g
, creating unprecedented opportunities in a broad range of industries from aerospace to wearable and implantable electronics.
Real-time thermal sensing on flexible substrates could enable a plethora of new applications. However, achieving fast, sub-millisecond response times even in a single sensor is difficult, due to the ...thermal mass of the sensor and encapsulation. Here, we fabricate flexible monolayer molybdenum disulfide (MoS2) temperature sensors and arrays, which can detect temperature changes within a few microseconds, over 100× faster than flexible thin-film metal sensors. Thermal simulations indicate the sensors’ response time is only limited by the MoS2 interfaces and encapsulation. The sensors also have high temperature coefficient of resistance, ∼1–2%/K and stable operation upon cycling and long-term measurement when they are encapsulated with alumina. These results, together with their biocompatibility, make these devices excellent candidates for biomedical sensor arrays and many other Internet of Things applications.
Using quantum transport simulations of metal-semiconductor junctions, we assess the viability of barrier thinning with dopants and barrier lowering with interfacial layers as solutions for contact ...resistivity in nanoscale transistors. Our atomistic simulations show that the discreteness of dopants leads to increasing variability in contact resistance as dimensions scale below 10 nm. We find that the use of interlayers can counteract low doping caused by atomistic variation, but the interlayer must have band edge Fermi level pinning to provide a net reduction in contact resistivity. For materials with low doping limits, such as n-type germanium, we find that interlayer contacts still have difficulty meeting resistivity targets.
We theoretically study and experimentally demonstrate a pseudomorphic Ge/Ge0.92Sn0.08/Ge quantum-well microdisk resonator on Ge/Si (001) as a route toward a compact GeSn-based laser on silicon. The ...structure theoretically exhibits many electronic and optical advantages in laser design, and microdisk resonators using these structures can be precisely fabricated away from highly defective regions in the Ge buffer using a novel etch-stop process. Photoluminescence measurements on 2.7 μm diameter microdisks reveal sharp whispering-gallery-mode resonances (Q > 340) with strong luminescence.
Bandgap and stress engineering using group IV materials-Si, Ge, and Sn, and their alloys are employed to design a FinFET-based CMOS solution for the 7-nm technology node and beyond. A detailed ...simulation study evaluating the performance of the proposed design is presented. Through the use of a common strain-relaxed buffer layer for p- and n-channel MOSFETs and a careful selection of source/drain stressor materials, the CMOS design is shown to achieve performance benefits over strained Si, meet the I OFF requirements, and provide a path for continued technology scaling.
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