Jiahao Kang

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.

Electrical contacts to two-dimensional semiconductors

The performance of electronic and optoelectronic devices based on two-dimensional layered crystals, including graphene, semiconductors of the transition metal dichalcogenide family such as molybdenum disulphide (MoS2) and tungsten diselenide (WSe2), as well as other emerging two-dimensional semiconductors such as atomically thin black phosphorus, is significantly affected by the electrical contacts that connect these materials with external circuitry. Here, we present a comprehensive treatment of the physics of such interfaces at the contact region and discuss recent progress towards realizing optimal contacts for two-dimensional materials. We also discuss the requirements that must be fulfilled to realize efficient spin injection in transition metal dichalcogenides.

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.

High-performance MoS2 transistors with low-resistance molybdenum contacts

In this Letter, molybdenum (Mo) is introduced and evaluated as an alternative contact metal to atomically-thin molybdenum disulphide (MoS2), and high-performance field-effect transistors are experimentally demonstrated. In order to understand the physical nature of the interface and highlight the role of the various factors contributing to the Mo-MoS2 contacts, density functional theory (DFT) simulations are employed, which reveal that Mo can form high quality contact interface with monolayer MoS2 with zero tunnel barrier and zero Schottky barrier under source/drain contact, as well as an ultra-low Schottky barrier (0.1 eV) at source/drain-channel junction due to strong Fermi level pinning. In agreement with the DFT simulations, high mobility, high ON-current, and low contact resistance are experimentally demonstrated on both monolayer and multilayer MoS2 transistors using Mo contacts. The results obtained not only reveal the advantages of using Mo as a contact metal for MoS2 but also highlight the fact that the properties of contacts with 2-dimensional materials cannot be intuitively predicted by solely considering work function values and Schottky theory.

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.

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.

Surface functionalization of two-dimensional metal chalcogenides by Lewis acid–base chemistry

Precise control of the electronic surface states of two-dimensional (2D) materials could improve their versatility and widen their applicability in electronics and sensing. To this end, chemical surface functionalization has been used to adjust the electronic properties of 2D materials. So far, however, chemical functionalization has relied on lattice defects and physisorption methods that inevitably modify the topological characteristics of the atomic layers. Here we make use of the lone pair electrons found in most of 2D metal chalcogenides and report a functionalization method via a Lewis acid-base reaction that does not alter the host structure. Atomic layers of n-type InSe react with Ti(4+) to form planar p-type [Ti(4+)n(InSe)] coordination complexes. Using this strategy, we fabricate planar p-n junctions on 2D InSe with improved rectification and photovoltaic properties, without requiring heterostructure growth procedures or device fabrication processes. We also show that this functionalization approach works with other Lewis acids (such as B(3+), Al(3+) and Sn(4+)) and can be applied to other 2D materials (for example MoS2, MoSe2). Finally, we show that it is possible to use Lewis acid-base chemistry as a bridge to connect molecules to 2D atomic layers and fabricate a proof-of-principle dye-sensitized photosensing device.

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.

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.

Proposal for all-graphene monolithic logic circuits

Since the very inception of integrated circuits, dissimilar materials have been used for fabricating devices and interconnects. Typically, semiconductors are used for devices and metals are used for interconnecting them. This, however, leads to a “contact resistance” between them that degrades device and circuit performance, especially for nanoscale technologies. This letter introduces and explores an “all-graphene” device-interconnect co-design scheme, where a single 2-dimensional sheet of monolayer graphene is proposed to be monolithically patterned to form both active devices (graphene nanoribbon tunnel-field-effect-transistors) as well as interconnects in a seamless manner. Thereby, the use of external contacts is alleviated, resulting in substantial reduction in contact parasitics. Calculations based on tight-binding theory and Non-Equilibrium Greens Function (NEGF) formalism solved self-consistently with the Poissons equation are used to analyze the intricate properties of the proposed structure. ...