Metal-based electronics is attractive for fast and radiation-hard electronic circuits and remains one of the longstanding goals for researchers. The emergence of 1T-TaS$_2$, a layered material exhibiting strong charge density wave (CDW) driven resistivity switching that can be controlled by an external stimulus such as electric field and optical pulses, has triggered a renewed interest in metal-electronics. Here we demonstrate a negative differential resistor (NDR) using electrically driven CDW phase transition in an asymmetrically designed T-junction made up of 1T-TaS$_2$/2H-MoS$_2$ van der Waals heterostructure. The principle of operation of the proposed device is governed by majority carrier transport and is distinct from usual NDR devices employing tunneling of carriers, thus avoids the bottleneck of weak tunneling efficiency in van der Waals heterojunctions. Consequently, we achieve a peak current density in excess of $10^5$ nA$μ$m$^{-2}$, which is about two orders of magnitude higher than that obtained in typical layered material based NDR implementations. The peak current density can be effectively tuned by an external gate voltage as well as photo-gating. The device is robust against ambiance-induced degradation and the characteristics repeat in multiple measurements over a period of more than a month. The findings are attractive for the implementation of active metal-based functional circuits.
Interlayer misorientation in transition metal dichalcogenides alters the interlayer distance, the electronic band structure, and the vibrational modes, but, its effect on the interlayer resistance is not known. This work analyzes the coherent interlayer resistance of misoriented 2H-MoS2 for low energy electrons and holes as a function of the misorientation angle. The electronic interlayer resistance monotonically increases with the supercell lattice constant by several orders of magnitude similar to that of misoriented bilayer graphene. The large hole coupling gives low interlayer hole resista…
We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ~ 0.3 k ohm.um, high on/off ratios up to > 109, and high drive currents exceeding 320 uA um-1. These favorable characteristics are combined with a two-terminal field-effect hole mobility ~ 2x102 cm2 V-…
In this work we present a study on the impact of various intrinsic deformations like ripples, twist, wrap on the electronic properties of ultra-short monolayer MoS2 channels. The effect of deformation (3-7o twist or wrap and 0.3-0.7 buckling amplitude) on a 3.5 nm planar monolayer MoS2 MOSFET is evaluated by the density functional theory and the non-equilibrium Green`s function (DFT-NEGF) approach. We study the channel density of states, transmission spectra and the ID-VD characteristics under the varying conditions, with focus on the negative differential resistance (NDR) behavior. Our result…
We report an approach to achieve low-resistance contacts to MoS2 transistors with the intrinsic performance of the MoS2 channel preserved. Through a dry transfer technique and a metal-catalyzed graphene treatment process, nickel-etched-graphene electrodes were fabricated on MoS2 that yield contact resistance as low as 200 ohm-um. The substantial contact enhancement (~2 orders of magnitude) as compared to pure nickel electrodes, is attributed to the much smaller work function of nickel-graphene electrodes, together with the fact that presence of zigzag edges in the treated graphene surface enha…
In this work we present a study on the negative differential resistance behavior and the impact of various deformations {like ripples, twist, wrap} and defects like vacancies and edge roughness on the electronic properties of short-channel MoS2 armchair nanoribbon MOSFETs. The effect of deformation {3 to 7 degree twist or wrap and 0.3 to 0.7 Ang rippling amplitude} and defects on a 10 nm MoS2 ANR FET is evaluated by the density functional tight binding theory and the non-equilibrium Green`s function approach. We study the channel density of states, transmission spectra and the ID VD characteri…
We have studied the finite bias transport properties of a 2H-1T' MoS$_2$ lateral metal-semiconductor (M-S) junction by non-equilibrium Green's functions calculations, aimed at contacting the 2D channel in a field effect transistor. Our results indicate that (a) despite the fundamentally different electrostatics of line and planar dipoles, the Schottky barrier heights respond similarly to changes in doping and applied bias in 2D and 3D M-S junctions, (b) 2H-1T' MoS$_2$ lateral junctions are free from Fermi level pinning, (c) armchair interfaces have superior contacting properties vs.\ zigzag in…
In this paper, we report a novel chemical doping technique to reduce the contact resistance (Rc) of transition metal dichalcogenides (TMDs) - eliminating two major roadblocks (namely, doping and high Rc) towards demonstration of high-performance TMDs field-effect transistors (FETs). By using 1,2 dichloroethane (DCE) as the doping reagent, we demonstrate an active n-type doping density > 2*1019 cm-3 in a few-layer MoS2 film. This enabled us to reduce the Rc value to a record low number of 0.5 kohm*um, which is ~10x lower than the control sample without doping. The corresponding specific contact…
For the first time, n-type few-layer MoS2 field-effect transistors with graphene/Ti as the hetero-contacts have been fabricated, showing more than 160 mA/mm drain current at 1 μm gate length with an on-off current ratio of 107. The enhanced electrical characteristic is confirmed in a nearly 2.1 times improvement in on-resistance and a 3.3 times improvement in contact resistance with hetero-contacts compared to the MoS2 FETs without graphene contact layer. Temperature dependent study on MoS2/graphene hetero-contacts has been also performed, still unveiling its Schottky contact nature. Transfer …
1T-TaS2 is a layered material that exhibits charge density wave (CDW) induced distinct electrical resistivity phases and has attracted a lot of attention for interesting device applications. However, such resistivity switching effects are often weak, and cannot be modulated by an external gate voltage - limiting their widespread usage. Using a back-gated 1T-TaS2/2H-MoS2 heterojunction, here we show that the usual resistivity switching in TaS2 due to different phase transitions is accompanied with a surprisingly strong modulation in the Schottky barrier height (SBH) at the TaS2/MoS2 interface -…
\ce{MoS_2} is a two dimensional material with a band gap depending on the number of layers and tunable by an external electric field. The experimentally observed intralayer band-to-band tunneling and interlayer band-to-band tunneling in this material present an opportunity for new electronic applications in tunnel field effect transistors. However, such a widely accepted concept has never been supported up by theoretical investigations based on first principles. In this work, using density functional theory, in conjunction with non-equilibrilibrium Green's function techniques and our electric …
Correlation enhanced resistance hysteresis near half filling in MoS2/WSe2 heterobilayer
Department of Physics and State Key Laboratory of Optical Quantum Materials, the Hong Kong University of Science and Technology（香港科技大学物理系及光学量子材料国家重点实验室）；Department of Materials Science and Engineering, Southern University of Science and Technology（南方科技大学材料科学与工程系）
We studied the weak field Hall voltage in 2D electron layers in Si-MOS structures with different mobilities, through the metal-insulator transition. In the vicinity of the critical density on the metallic side of the transition, we have found weak deviations (about 6-20 %) of the Hall voltage from its classical value. The deviations do not correlate with the strong temperature dependence of the diagonal resistivity rho_{xx}(T). The smallest deviation in R_{xy} was found in the highest mobility sample exhibiting the largest variation in the diagonal resistivity ρ_{xx} with temperature (by a fac…
Although monolayer black phosphorus (BP) or phosphorene has been successfully exfoliated and its optical properties have been explored, most of electrical performance of the devices is demonstrated on few-layer phosphorene and ultra-thin BP films. In this paper, we study the channel length scaling of ultra-thin BP field-effect transistors (FETs), and discuss a scheme for using various contact metals to change transistor characteristics. Through studying transistor behaviors with various channel lengths, the contact resistance can be extracted from the transfer length method (TLM). With differe…
We have observed a metal-insulator transition of a quasi-two dimensional electronic system in transition metal dichalcogenide $2H$-TaSe$_2$ caused by doping iron. The sheet resistance of $2H$-Fe$_x$TaSe$_2$ ($0 \leq x \leq 0.120$) single crystals rises about $10^6$ times with the increasing of $x$ at the lowest temperature. We investigated the temperature dependence of the resistance and found a metal-insulator transition with a critical sheet resistance $11.7 \pm 5.4$ k$\rmΩ$. The critical exponent of the localization length $ν$ is estimated $0.31 \pm 0.18$. The values of the critical sheet r…
Progress in high-performance computing demands significant advances in memory technology. Among novel memory technologies that promise efficient device operation on a sub-ns timescale, resistance switching between charge ordered phases of the 1T-TaS$_2$ has shown to be potentially useful for the development of high-speed, energy efficient non-volatile memory device. While ultrafast switching was previously reported with optical pulses, determination of the intrinsic speed limits of actual devices that are triggered by electrical pulses is technically challenging and hitherto still largely unex…
Electrical contact resistance to two-dimensional (2D) semiconductors such as monolayer MoS_{2} is a key bottleneck in scaling the 2D field effect transistors (FETs). The 2D semiconductor in contact with three-dimensional metal creates unique current crowding that leads to increased contact resistance. We developed a model to separate the contribution of the current crowding from the intrinsic contact resistivity. We show that current crowding can be alleviated by doping and contact patterning. Using Landauer-Büttiker formalism, we show that van der Waals (vdW) gap at the interface will ultimat…
Edge contact geometries are thought to yield ultralow contact resistances in most non-ferromagnetic metal-graphene interfaces owing to their large metal-graphene coupling strengths. Here, we examine the contact resistance of edge- versus surface-contacted ferromagnetic metal-graphene interfaces (i.e. nickel- and cobalt-graphene interfaces) using both single-layer and few-layer graphene. Good qualitative agreement is obtained between theory and experiment. In particular, in both theory and experiment, we observe that the contact resistance of edge-contacted ferromagnetic metal-graphene interfac…
Continued progress in high speed computing depends on breakthroughs in both materials synthesis and device architectures. The performance of logic and memory can be enhanced significantly by introducing a memristor, a two terminal device with internal resistance that depends on the history of the external bias voltage. State of the art memristors, based on metal insulator metal (MIM) structures with insulating oxides, such as TiO2, are limited by a lack of control over the filament formation and external control of the switching voltage. Here, we report a class of memristors based on grain bou…
Charge-density-wave dynamics is studied on a submicron length scale in NbSe_3 and o-TaS_3. Regions of negative absolute resistance are observed in the CDW sliding regime at sufficiently low temperatures. The origin of the negative resistance is attributed to the different forces that the deformed CDW and quasi-particles feel: the force on the CDW is merely caused by a difference of the electric potentials, while the quasi-particle current is governed by a difference of the electrochemical potentials.
The extremely high carrier mobility and the unique band structure, make graphene very useful for field-effect transistor applications. According to several works, the primary limitation to graphene based transistor performance is not related to the material quality, but to extrinsic factors that affect the electronic transport properties. One of the most important parasitic element is the contact resistance appearing between graphene and the metal electrodes functioning as the source and the drain. Ohmic contacts to graphene, with low contact resistances, are necessary for injection and extrac…
The resistance R of the 2DEG on the vicinal Si surface shows an unusual behaviour, which is very different from that in the (100) Si MOSFET where an unconventional metal to insulator transition has been reported. The crossover from the insulator with dR/dT<0 to the metal with dR/dT>0 occurs at a low resistance of R_{\Box}^c \sim 0.04h/e^2. At the low-temperature transition, which we attribute to the existence of a narrow impurity band at the interface, a distinct hysteresis in the resistance is detected. At higher temperatures, another change in the sign of dR/dT is seen and related to the cro…
The transversal and longitudinal resistance in the quantum Hall effect regime was measured in a Si MOSFET sample in which a slot-gate allows one to vary the electron density and filling factor in different parts of the sample. In case of unequal gate voltages, the longitudinal resistances on the opposite sides of the sample differ from each other because the originated Hall voltage difference is added to the longitudinal voltage only on one side depending on the gradient of the gate voltages and the direction of the external magnetic field. After subtracting the Hall voltage difference, the in…
We investigate the nature of electron transport through monolayer molybdenum dichalcogenides (MoX$_2$, X=S, Se) suspended between Au and Ti metallic contacts. The monolayer is placed ontop of the close-packed surfaces of the metal electrodes and we focus on the role of the metal-MoX$_2$ binding distance and the contact area. Based on \emph{ab initio} transport calculations we identify two different scattering mechanisms which depend differently on the metal-MoX$_2$ binding distance: (i) An interface resistance between the metal and the supported part of MoX$_2$ which decreases with decreasing …
Using ab-initio methods we investigate the possibility of three-terminal graphene "T-junction" devices and show that these all-graphene edge contacts are energetically feasible when the 1D interface itself is free from foreign atoms. We examine the energetics of various junction structures as a function of the atomic scale geometry. Three-terminal equilibrium Green's functions are used to determine the transmission spectrum and contact resistance of the system. We find that the most symmetric structures have a significant binding energy, and we determine the contact resistances in the junction…
Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. Till date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance (Rc), particularly in the monolayer limit. In this work, we use a combination of theory and experiments in Au and Ni contacted monolayer MoS2 device to conclude the following…
The two-dimensional metal forming at the interface between an oxide insulator and SrTiO3 provides new opportunities for oxide electronics. However, the quantum Hall effect, one of the most fascinating effects of electrons confined in two dimensions, remains underexplored at these complex oxide heterointerfaces. Here, we report the experimental observation of quantized Hall resistance in a SrTiO3 heterointerface based on the modulation-doped amorphous-LaAlO$_{3}$/SrTiO$_{3}$ heterostructure, which exhibits both high electron mobility exceeding 10000 cm$^{2}$/Vs and low carrier density on the or…
Layered two-dimensional (2D) materials have highly anisotropic thermal properties between the in-plane and cross-plane directions. In general, it is thought that cross-plane thermal conductivities ($κ_z$) are low, and therefore c-axis phonon mean free paths (MFPs) are small. Here, we measure $κ_z$ across MoS$_2$ films of varying thickness (20 to 240 nm) and uncover evidence of very long c-axis phonon MFPs at room temperature in these layered semiconductors. Experimental data obtained using time-domain thermoreflectance (TDTR) are in good agreement with first-principles density functional theor…
Variability and lack of control in the nature of contacts between metal/MoS2 interface is a major bottleneck in the realisation of high-performance devices based on layered materials for several applications. In this letter, we report on the reduction in Schottky barrier height at metal/MoS2 interface by engineering the surface states through sulphur treatment. Electrical characteristics for back-gated MoS2 field effect transistor structures were investigated for two high work-function metal contacts Ni and Pd. Contacts on MoS2 treated with sulphur exhibited significant improvements in Ohmic n…
One of the most interesting predicted applications of graphene monolayer based devices is as high quality sensors. In this letter we show, through systematic experiments, a chemical vapor sensor based on the measurement of low frequency resistance fluctuations of single layer graphene field-effect-transistor (SLG-FET) devices. The sensor has extremely high sensitivity, very high specificity, high fidelity and fast response times. The performance of the device using this scheme of measurement (which uses resistance fluctuations as the detection parameter) is more than two orders of magnitude be…
We have studied temperature- and electric-field dependent carrier transport in single flakes of MoS$_{2}$ treated with n-butyllithium. The temperature dependence of the four-terminal resistance follows the Efros-Shklovskii variable range hopping conduction mechanism. From measurements in the Ohmic and non-Ohmic regime, we estimate the localization length and the average hopping length of the carriers, as well as the effective dielectric constant. Furthermore, comparison between two- and four-probe measurements yield a contact resistance that increases significantly with decreasing temperature.
Radiofrequency vortex spin-transfer oscillators based on magnetic tunnel junctions with very low resistance area product were investigated. A high power of excitations has been obtained characterized by a power spectral density containing a very sharp peak at the fundamental frequency and a series of harmonics. The observed behaviour is ascribed to the combined effect of spin transfer torque and Oersted-Ampère field generated by the large applied dc-current. We furthermore show that the synchronization of a vortex oscillation by applying a ac bias current is mostly efficient when the external …
A common issue in low temperature measurements of enhancement-mode metal-oxide-semiconductor (MOS) field-effect transistors (FETs) in the low electron density regime is the high contact resistance dominating the device impedance. In that case a voltage bias applied across the source and drain contact of a Hall bar MOSFET will mostly fall across the contacts (and not across the channel) and therefore magneto-transport measurements become challenging. However, from a physical point of view, the study of MOSFET nanostructures in the low electron density regime is very interesting (impurity limite…
Four-terminal resistances, both longitudinal and diagonal, of a locally gated graphene device are measured in the quantum-Hall (QH) regime. In sharp distinction from previous two-terminal studies [J. R. Williams \textit{et al.}, Science {\bf 317}, 638 (2007); B. Özyilmaz \textit{et al.}, Phys. Rev. Lett. {\bf 99}, 166804 (2007)], asymmetric QH resistances are observed, which provide information on reflection as well as transmission of the QH edge states. Most quantized values of resistances are well analyzed by the assumption that all edge states are equally populated. Contrary to the expectat…
We demonstrate that the radiation induced "zero-resistance state" observed in a two-dimensional electron gas is a result of the non-trivial structure of the density of states of the systems and the photon assisted transport. A toy model of a structureless quantum tunneling junction where the system has oscillatory density of states catches most of the important features of the experiments. We present a generalized Kubo-Greenwood conductivity formula for the photon assisted transport in a general system, and show essentially the same nature of the transport anomaly in a uniform system.
Optical Voltage Profiling of 2D Semiconductors via Proximal Exciton Sensing
University of California, Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Kavli Energy NanoScience Institute（卡弗里纳米科学能源研究所）；National Institute for Materials Science（材料科学国立研究所）
The exploitation of the excellent intrinsic electronic properties of graphene for device applications is hampered by a large contact resistance between the metal and graphene. The formation of edge contacts rather than top contacts is one of the most promising solutions for realizing low ohmic contacts. In this paper the fabrication and characterization of edge contacts to large area CVD-grown monolayer graphene by means of optical lithography using CMOS compatible metals, i.e. Nickel and Aluminum is reported. Extraction of the contact resistance by Transfer Line Method (TLM) as well as the di…
Contact resistance of semiconducting transition metal dichalcogenides has been shown to decrease in lateral heterojunctions formed with their metallic phases but its origins remain elusive. Here we combine first principles and quantum transport calculations to rationalize the contact resistance of these structures in terms of phase, composition (WTe2, MoTe2, WSe2, and MoSe2), and length of the channel. We find that charge injection in metallic 1T'-WTe2/1T'-MoTe2 junctions is nearly ideal as electrode Bloch states remain delocalized through the channel. Mixtures of 1T' selenides and tellurides …
The use of multiple current terminals on millimeter-scale graphene $p-n$ junction devices fabricated with Corbino geometries, or quantum Hall resistance dartboards, have enabled the measurement of several fractional multiples of the quantized Hall resistance at the $ν=2$ plateau ($R_H\approx 12906 Ω$). Experimentally obtained values agreed with corresponding numerical simulations performed with the LTspice circuit simulator. More complicated designs of the quantum Hall resistance dartboard were simulated to establish the potential parameter space within which these Corbino-type devices could o…
Since the advent of graphene, a variety of studies have been performed to elucidate its fundamental physics, or to explore its practical applications. Gate-tunable resistance is one of the most important properties of graphene and has been studied in 1-3 layer graphene in a number of efforts to control the band gap to obtain a large on-off ratio. On the other hand, the transport property of multilayer graphene with more than three layers is less well understood. Here we show a new aspect of multilayer graphene. We found that four-layer graphene shows intrinsic peak structures in the gate volta…
Most studies on moiré superlattices formed from a stack of h-BN and graphene have focused on single layer graphene; graphene with multiple layers is less understood. Here, we show that a moiré superlattice of multilayer graphene shows new features arising from the anisotropic Fermi surface affected by the superlattice structure. The moiré superlattice of a h-BN/AB-stacked tetralayer graphene heterostructure exhibited resistivity peaks showing a complicated dependence on the perpendicular electric field. The peaks were not due to secondary Dirac cones forming, but rather opening of the energy g…
Crossed Andreev reflection (CAR) in metallic nanostructures, a possible basis for solid-state electron entangler devices, is usually investigated by detecting non-local voltages in multi-terminal superconductor/normal metal devices. This task is difficult because other subgap processes may mask the effects of CAR. One of these processes is the generation of charge imbalance (CI) and the diffusion of non-equilibrium quasi-particles in the superconductor. Here we demonstrate a characteristic dependence of non-local CI on a magnetic field applied parallel to the superconducting wire, which can be…
Using resistance fluctuation spectroscopy, we observe current-induced narrow-band noise (NBN) in the magnetic skyrmion-lattice phase of micrometer-sized MnSi. The NBN appears only when electric-current density exceeds a threshold value, indicating that the current-driven motion of the skyrmion lattice triggers the NBN. The observed NBN frequency is 10-10$^4$ Hz at $\sim$10$^{9}$ A/m$^{2}$, implying a skyrmion steady flow velocity of 1-100 $μ$m/s, 3-5 orders of magnitude slower than previously reported. The temperature evolution of the NBN frequency suggests that the steady flow entails thermal…
The goal of this paper is to highlight several issues which are most crucial for the understanding of the ``metal-insulator transition'' in two dimensions. We discuss some common problems in interpreting experimental results on high mobility Si MOSFETs. We analyze concepts and methods used to determine the critical density of electrons at the metal-insulator transition. In particular, we discuss the origin of the temperature dependence of the resistivity and reasons for this dependence to flatten out at some electron density in the vicinity of the metal-insulator transition. This flattening ha…
