Deblina Sarkar

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 cm(2)/V·s (with an ON/OFF current ratio exceeding 10(6)) 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 cm(2)/V·s with an ON/OFF ratio of over 10(6) 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.

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.

A subthermionic tunnel field-effect transistor with an atomically thin channel.

The fast growth of information technology has been sustained by continuous scaling down of the silicon-based metal–oxide field-effect transistor. However, such technology faces two major challenges to further scaling. First, the device electrostatics (the ability of the transistor’s gate electrode to control its channel potential) are degraded when the channel length is decreased, using conventional bulk materials such as silicon as the channel. Recently, two-dimensional semiconducting materials have emerged as promising candidates to replace silicon, as they can maintain excellent device electrostatics even at much reduced channel lengths. The second, more severe, challenge is that the supply voltage can no longer be scaled down by the same factor as the transistor dimensions because of the fundamental thermionic limitation of the steepness of turn-on characteristics, or subthreshold swing. To enable scaling to continue without a power penalty, a different transistor mechanism is required to obtain subthermionic subthreshold swing, such as band-to-band tunnelling. Here we demonstrate band-to-band tunnel field-effect transistors (tunnel-FETs), based on a two-dimensional semiconductor, that exhibit steep turn-on; subthreshold swing is a minimum of 3.9 millivolts per decade and an average of 31.1 millivolts per decade for four decades of drain current at room temperature. By using highly doped germanium as the source and atomically thin molybdenum disulfide as the channel, a vertical heterostructure is built with excellent electrostatics, a strain-free heterointerface, a low tunnelling barrier, and a large tunnelling area. Our atomically thin and layered semiconducting-channel tunnel-FET (ATLAS-TFET) is the only planar architecture tunnel-FET to achieve subthermionic subthreshold swing over four decades of drain current, as recommended in ref. 17, and is also the only tunnel-FET (in any architecture) to achieve this at a low power-supply voltage of 0.1 volts. Our device is at present the thinnest-channel subthermionic transistor, and has the potential to open up new avenues for ultra-dense and low-power integrated circuits, as well as for ultra-sensitive biosensors and gas sensors.

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.

Proposal for tunnel-field-effect-transistor as ultra-sensitive and label-free biosensors

Tunnel field-effect-transistor (TFET) based biosensor is proposed, and it is shown that they can surpass by several orders, the performance of those based on conventional FET (CFET) and hence, can potentially revolutionize the biosensing applications. Analytical formula is derived for the sensitivity and response time to provide physical insights in terms of material bandgap and operation regime of the TFET biosensor for achieving optimal results. At the same time, rigorous numerical simulations have been performed in order to obtain accurate values of sensitivity for both biomolecule and pH sensing operations. The time dependent response of the biosensors has also been discussed through analytical and numerical solutions. It is shown that while the CFET biosensors suffer from fundamental limitations on the maximum sensitivity and minimum detection time achievable, TFET biosensors, with their fundamentally different current injection mechanism in the form of band-to-band tunneling, can overcome such limit...

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 noise peak 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.

2D Semiconductor FETs—Projections and Design for Sub-10 nm VLSI

Two-dimensional (2D) crystal semiconductors, such as the well-known molybdenum disulfide (MoS2), are witnessing an explosion in research activities due to their apparent potential for various electronic and optoelectronic applications. In this paper, dissipative quantum transport simulations using nonequilibrium Greens function formalism are performed to rigorously evaluate the scalability and performance of monolayer/multilayer 2D semiconductor-based FETs for sub-10 nm gate length very large-scale integration (VLSI) technologies. Device design considerations in terms of the choice of prospective 2D material/structure/technology to fulfill sub-10 nm International Technology Roadmap for Semiconductors (ITRS) requirements are analyzed. First, it is found that MoS2 FETs can meet high-performance (HP) requirement up to 6.6 nm gate length using bilayer MoS2 as the channel material, while low-standby-power (LSTP) requirements present significant challenges for all sub-10 nm gate lengths. Second, by studying the effects of underlap (UL) structures, scattering strength, and carrier effective mass, it is found that the high mobility and suitably low effective mass of tungsten diselenide (WSe2), aided by the UL, enable 2D FETs for both HP and LSTP applications at the smallest foreseeable (5.9 nm) gate length. Finally, possible solutions for sub-5 nm gate lengths, specifically anisotropic 2D semiconductor materials for HP and sub-kT/q switch (2D tunnel FET) for LSTP, are also proposed based on the effects of critical material parameters on the device performance.

High-Frequency Behavior of Graphene-Based Interconnects—Part I: Impedance Modeling

This paper presents the first detailed methodology for the accurate evaluation of high-frequency impedance of graphene-based structures relevant to on-chip interconnect and inductor applications. Going beyond the simplifying assumptions of Ohms law, the effects of electric-field variation within a mean free path and current dependency on the nonlocal electric-field are taken into account to accurately capture the high-frequency behavior of graphene ribbons (GRs). At the same time, a simplified approach that may be adopted at lower frequencies is also explained. Starting from the basic Boltzmann equation and combining with the unique dispersion relation for graphene in its hexagonal Brillouin zone, the current density across the GR structure is derived. First, a semi-infinite slab of GR is analyzed using the theory of Fourier integrals, which is followed by the development of a rigorous methodology for practical finite structures based on a self-consistent numerical calculation of the derived current density using the Greens function approach.

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 cm(2)/(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.

High-Frequency Behavior of Graphene-Based Interconnects—Part II: Impedance Analysis and Implications for Inductor Design

This paper provides the first detailed insights into the ultrahigh-frequency behavior of graphene ribbons (GRs) and analyzes their consequences in designing interconnects and low-loss on-chip inductors. In the companion paper (part I), an accurate impedance modeling methodology has been developed based on the Boltzmann equation with the magnetic vector potential Greens function approach incorporating the dependency of current on the nonlocal electric field. Based on the developed methodology, this paper for the first time embarks on the rigorous investigation of the intricate processes occurring at high frequencies in GRs, such as anomalous skin effect (ASE), high-frequency resistance and inductance saturation, intercoupled relation between edge specularity and ASE, and the influence of the linear dimensions on impedance. A comparative study of the high-frequency response of GRs with that of carbon nanotubes (CNTs) and Cu is made to highlight the potential of GR interconnects for high-frequency applications. Subsequently, the high-frequency performance of GR inductors is analyzed, and it is shown that they can achieve 32% and 50% improvements in maximum Q-factor compared to Cu and single-walled CNT inductors with 1/3 metallic fraction, respectively.