Impact of Contact on the Operation and Performance of Back-Gated Monolayer MoS₂ Field-Effect-Transistors

Metal contacts to atomically thin two-dimensional (2D) crystal based FETs play a decisive role in determining their operation and performance. However, the effects of contacts on the switching behavior, field-effect mobility, and current saturation of monolayer MoS₂ FETs have not been well explored and, hence, is the focus of this work. The dependence of contact resistance on the drain current is revealed by four-terminal-measurements. Without high-κ dielectric boosting, an electron mobility of 44 cm²/(V·s) has been achieved in a monolayer MoS₂ FET on SiO₂ substrate at room temperature. Velocity saturation is identified as the main mechanism responsible for the current saturation in back-gated monolayer MoS₂ FETs at relatively higher carrier densities. Furthermore, for the first time, electron saturation velocity of monolayer MoS₂ is extracted at high-field condition

Role of Metal Contacts in Designing High-Performance Monolayer n‑Type WSe₂ Field Effect Transistors

This work presents a systematic study toward the design and first demonstration of high-performance n-type monolayer tungsten diselenide (WSe₂) field effect transistors (FET) by selecting the contact metal based on understanding the physics of contact between metal and monolayer WSe₂. Device measurements supported by ab initio density functional theory (DFT) calculations indicate that the d-orbitals of the contact metal play a key role in forming low resistance ohmic contacts with monolayer WSe₂. On the basis of this understanding, indium (In) leads to small ohmic contact resistance with WSe₂ and consequently, back-gated In–WSe₂ FETs attained a record ON-current of 210 μA/μm, which is the highest value achieved in any monolayer transition-metal dichalcogenide- (TMD) based FET to date. An electron mobility of 142 cm²/V·s (with an ON/OFF current ratio exceeding 10⁶) is also achieved with In–WSe₂ FETs at room temperature. This is the highest electron mobility reported for any back gated monolayer TMD material till date. The performance of n-type monolayer WSe₂ FET was further improved by Al₂O₃ deposition on top of WSe₂ to suppress the Coulomb scattering. Under the high-κ dielectric environment, electron mobility of Ag–WSe₂ FET reached ∼202 cm²/V·s with an ON/OFF ratio of over 10⁶ and a high ON-current of 205 μA/μm. In tandem with a recent report of p-type monolayer WSe₂ FET (Fang, H. et al. Nano Lett. 2012, 12, (7), 3788−3792), this demonstration of a high-performance n-type monolayer WSe₂ FET corroborates the superb potential of WSe₂ for complementary digital logic applications

Controllable and Rapid Synthesis of High-Quality and Large-Area Bernal Stacked Bilayer Graphene Using Chemical Vapor Deposition

Bilayer graphene has attracted wide attention due to its unique band structure and bandgap tunability under specific (Bernal or AB) stacking order. However, it remains challenging to tailor the stacking order and to simultaneously produce large-scale and high-quality bilayer graphene. This work introduces a fast and reliable method of growing high-quality Bernal stacked large-area (>3 in. × 3 in.) bilayer graphene film or trilayer graphene domains (30 μm × 30 μm) using chemical vapor deposition (CVD) on engineered Cu–Ni alloy catalyst films. The AB stacking order is evaluated by Raman spectra, electron diffraction pattern, and dual gate field-effect-transistor (FET) measurements, and a near-perfect AB stacked bilayer graphene coverage (>98%) is obtained. The synthesized bilayer and trilayer graphene with Bernal stacking exhibit electron mobility as high as 3450 cm²/(V·s) and 1500 cm²/(V·s), respectively, indicating comparable quality with respect to exfoliated bilayer and trilayer graphene. The record high (for CVD bilayer graphene) ON to OFF current ratios (up to 15) obtained for a large number (>50) of dual-gated FETs fabricated at random across the large-area bilayer graphene film also corroborates the success of our synthesis technique. Moreover, through catalyst engineering, growth optimization, and element analysis of catalyst, it is shown that achieving surface catalytic graphene growth mode and precise control of surface carbon concentration are key factors determining the growth of high quality and large area Bernal stacked bilayer graphene on Cu–Ni alloy. This discovery can not only open up new vistas for large-scale electronic and photonic device applications of graphene but also facilitate exploration of novel heterostructures formed with emerging beyond graphene two-dimensional atomic crystals

MoS₂ Field-Effect Transistor for Next-Generation Label-Free Biosensors

Biosensors based on field-effect transistors (FETs) have attracted much attention, as they offer rapid, inexpensive, and label-free detection. While the low sensitivity of FET biosensors based on bulk 3D structures has been overcome by using 1D structures (nanotubes/nanowires), the latter face severe fabrication challenges, impairing their practical applications. In this paper, we introduce and demonstrate FET biosensors based on molybdenum disulfide (MoS₂), which provides extremely high sensitivity and at the same time offers easy patternability and device fabrication, due to its 2D atomically layered structure. A MoS₂-based pH sensor achieving sensitivity as high as 713 for a pH change by 1 unit along with efficient operation over a wide pH range (3–9) is demonstrated. Ultrasensitive and specific protein sensing is also achieved with a sensitivity of 196 even at 100 femtomolar concentration. While graphene is also a 2D material, we show here that it cannot compete with a MoS₂-based FET biosensor, which surpasses the sensitivity of that based on graphene by more than 74-fold. Moreover, we establish through theoretical analysis that MoS₂ is greatly advantageous for biosensor device scaling without compromising its sensitivity, which is beneficial for single molecular detection. Furthermore, MoS₂, with its highly flexible and transparent nature, can offer new opportunities in advanced diagnostics and medical prostheses. This unique fusion of desirable properties makes MoS₂ a highly potential candidate for next-generation low-cost biosensors

Low-Frequency Noise in Bilayer MoS₂ Transistor

Low-frequency noise is a significant limitation on the performance of nanoscale electronic devices. This limitation is especially important for devices based on two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs), which have atomically thin bodies and, hence, are severely affected by surface contaminants. Here, we investigate the low-frequency noise of transistors based on molybdenum disulfide (MoS₂), which is a typical example of TMD. The noise measurements performed on bilayer MoS₂ channel transistors show a <i>noise peak</i> in the gate-voltage dependence data, which has also been reported for graphene. To understand the peak, a trap decay-time based model is developed by revisiting the carrier number fluctuation model. Our analysis reveals that the peak originates from the fact that the decay time of the traps for a 2D device channel is governed by the van der Waals bonds between the 2D material and the surroundings. Our model is generic to all 2D materials and can be applied to explain the V, M and Λ shaped dependence of noise on the gate voltage in graphene transistors, as well as the noise shape dependency on the number of atomic layers of other 2D materials. Since the van der Waals bonding between the surface traps and 2D materials is weak, in accordance with the developed physical model, an annealing process is shown to significantly reduce the trap density, thereby reducing the low-frequency noise

Intercalation Doped Multilayer-Graphene-Nanoribbons for Next-Generation Interconnects

Copper-based interconnects employed in a wide range of integrated circuit (IC) products are fast approaching a dead-end due to their increasing resistivity and diminishing current carrying capacity with scaling, which severely degrades both performance and reliability. Here we demonstrate chemical vapor deposition-synthesized and intercalation-doped multilayer-graphene-nanoribbons (ML-GNRs) with better performance (more than 20% improvement in estimated delay per unit length), 25%/72% energy efficiency improvement at local/global level, and superior reliability w.r.t. Cu for the first time, for dimensions (down to 20 nm width and thickness of 12 nm) suitable for IC interconnects. This is achieved through a combination of GNR interconnect design optimization, high-quality ML-GNR synthesis with precisely controlled number of layers, and effective FeCl₃ intercalation doping. We also demonstrate that our intercalation doping is stable at room temperature and that the doped ML-GNRs exhibit a unique width-dependent doping effect due to increasingly efficient FeCl₃ diffusion in scaled ML-GNRs, thereby indicating that our doped ML-GNRs will outperform Cu even for sub-20 nm widths. Finally, reliability assessment conducted under accelerated stress conditions (temperature and current density) established that highly scaled intercalated ML-GNRs can carry over 2 × 10⁸ A/cm² of current densities, whereas Cu interconnects suffer from immediate breakdown under the same stress conditions and thereby addresses the key criterion of current carrying capacity necessary for an alternative interconnect material. Our comprehensive demonstration of highly reliable intercalation-doped ML-GNRs paves the way for graphene as the next-generation interconnect material for a variety of semiconductor technologies and applications

Functionalization of Transition Metal Dichalcogenides with Metallic Nanoparticles: Implications for Doping and Gas-Sensing

Transition metal dichalcogenides (TMDs), belonging to the class of two-dimensional (2D) layered materials, have instigated a lot of interest in diverse application fields due to their unique electrical, mechanical, magnetic, and optical properties. Tuning the electrical properties of TMDs through charge transfer or doping is necessary for various optoelectronic applications. This paper presents the experimental investigation of the doping effect on TMDs, mainly focusing on molybdenum disulfide (MoS₂), by metallic nanoparticles (NPs), exploring noble metals such as silver (Ag), palladium (Pd), and platinum (Pt) as well as the low workfunction metals such as scandium (Sc) and yttrium (Y) for the first time. The dependence of the doping behavior of MoS₂ on the metal workfunction is demonstrated and it is shown that Pt nanoparticles can lead to as large as 137 V shift in threshold voltage of a back-gated monolayered MoS₂ FET. Variation of the MoS₂ FET transfer curves with the increase in the dose of NPs as well as the effect of the number of MoS₂ layers on the doping characteristics are also discussed for the first time. Moreover, the doping effect on WSe₂ is studied with the first demonstration of p-type doping using Pt NPs. Apart from doping, the use of metallic NP functionalized TMDs for gas sensing application is also demonstrated