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Metal-semiconductor interface is a bottleneck for efficient transport of charge carriers through Transition Metal Dichalcogenide (TMD) based field-effect transistors (FETs). Injection of charge carriers across such interfaces is mostly limited by Schottky barrier at the contacts which must be reduced to achieve highly efficient contacts for carrier injection into the channel. Here we introduce a universal approach involving dry chemistry to enhance atomic orbital interaction between various TMDs (MoS 2 , WS 2 , MoSe 2 and WSe 2 ) & metal contacts has been experimentally demonstrated. Quantum chemistry between TMDs, Chalcogens and metals has been explored using detailed atomistic (DFT & NEGF) simulations, which is then verified using Raman, PL and XPS investigations. Atomistic investigations revealed lower contact resistance due to enhanced orbital interaction and unique physics of charge sharing between constituent atoms in TMDs with introduced Chalcogen atoms which is subsequently validated through experiments. Besides contact engineering, which lowered contact resistance by 72, 86, 1.8, 13 times in MoS 2 , WS 2 , MoSe 2 and WSe 2 respectively, a novel approach to cure / passivate dangling bonds present at the 2D TMD channel surface has been demonstrated. While the contact engineering improved the ON-state performance (I ON , g m ,  R ON ) of 2D TMD FETs by orders of magnitude, Chalcogen based channel passivation was found to improve gate control (I OFF , SS, & V TH ) significantly. This resulted in an overall performance boost. The engineered TMD FETs were shown to have performance on par with best reported till date.I. Introduction Growth of semiconductor industry is driven by Moore's law 1 which intends to improve the efficiency of electronic gadgets in terms of speed and compactness by 2×, every 1.5 years. This is achieved by aggressive channel length scaling of Silicon MOSFETs. On the other hand, channel length scaling leads to short channel effects (SCE) like drain induced source barrier lowering and threshold voltage roll-off due to compromised gate control over channel. This results into higher source-to-drain leakage current, higher subthreshold slope and lower noise margins, which eventually increases the static power loss across the VLSI system. To mitigate SCE, devices like FinFETs 3, 4, 5 Multi-gate FET 2, 3, 6, 7 , Ultra-thin body (UTB) FETs 6, 8 and Tunnel FETs (TFETs) 9 have been proposed, which offer improved gate control and better SCE immunity. The key in most of the ultrascaled FET concepts is to reduce the channel thickness as the channel length is scaled down. However, scaling channel thickness beyond 5nm leads to mobility degradation and threshold voltage instability due to quantum confinement and surface dangling bonds, which leads to performance roll off. Atomically thin layers of 2D semiconductors like Transition Metal Dichalcogenides (TMDs) 10-15 on the other hand offer better gate control due to lack of dangling bonds perpendicular to their basal plane, as well as missing quantum i...

Assessing experimentally the main optical parameters of graphene (e.g. complex refractive index, carrier density, mobility) in the far-infrared (0.1-10 THz frequencies) can open intriguing perspective in quantum science, due to the possibility to quantum engineer and devise miniaturized devices (frequency comb, random lasers), components (optical switches, spatial light modulators, metamaterial mirrors and modulators) or photonic circuits in which graphene can be integrated with existing well consolidated semiconductor technologies to manipulate their optical properties and induce novel functionalities, potential for their application in quantum science. Here, by exploiting a combination of time domain terahertz (THz) spectroscopy and Fourier transform infrared spectroscopy, we extract the complex refractive index of large area single layer graphene on thin polymeric suspended substrates, flexible and transparent films, and high reflectivity silicon substrates in the 0.4 – 1.8 THz range, and we model our data to extract the relevant optical (refractive index, absorption coefficient, penetration length) electronic (Fermi velocity) and electrical (carrier density, mobility) properties of the different graphene samples in the terahertz.

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Electrical performance of a graphene FET is drastically affected by electron-phonon inelastic scattering. At high electric fields, the out-of-equilibrium population of optical phonons equilibrates by emitting acoustic phonons, which dissipate the energy to heat sinks. The equilibration time of the process is governed by thermal diffusion time, which is few nano-seconds for a typical graphene FET. The nano-second time-scale of the process keeps it elusive to conventional steady-state or DC measurement systems. Here, we employ a time-domain reflectometry-based technique to electrically probe the device for few nano-seconds and investigate the non-equilibrium state. For the first time, the transient nature of electrical transport through graphene FET is revealed. A maximum change of 35% in current and 50% in contact resistance is recorded over a time span of 8 ns, while operating graphene FET at a current density of 1 mA/μm. The study highlights the role of intrinsic heating (scattering) in deciding metal-graphene contact resistance and transport through the graphene channel.