Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides

van der Waals Stacking Induced Transition from Schottky to Ohmic Contacts: 2D Metals on Multilayer InSe

Incorporation of two-dimensional (2D) materials in electronic devices inevitably involves contact with metals, and the nature of this contact (Ohmic and/or Schottky) can dramatically affect the electronic properties of the assembly. Controlling these properties to reliably form low-resistance Ohmic contact remains a great challenge due to the strong Fermi level pinning (FLP) effect at the interface. Herein, we employ density functional theory calculations to show that van der Waals stacking can significantly modulate Schottky barrier heights in the contact formed between multilayer InSe and 2D metals by suppressing the FLP effect. Importantly, the increase of InSe layer number induces a transition from Schottky to Ohmic contact, which is attributed to the decrease of the conduction band minimum and rise of the valence band maximum of InSe. Based on the computed tunneling and Schottky barriers, Cd₃C₂ is the most compatible electrode for 2D InSe among the materials studied. This work illustrates a straightforward method for developing more effective InSe-based 2D electronic nanodevices.

van der Waals Epitaxy of High-Mobility Polymorphic Structure of Mo6Te6 Nanoplates/MoTe2 Atomic Layers with Low Schottky Barrier Height

High contact resistance between two-dimensional (2D) transition metal dichalcogenides (TMDs) and metal electrodes is a practical barrier for applications of 2D TMDs to conventional devices. A promising solution to this is polymorphic integration of 1T'-phase semimetallic and 2H-phase semiconducting TMD crystals, which can lower the Schottky barrier of the TMDs. Here, we demonstrate the van der Waals epitaxy of density-controlled single isolated 1T'-Mo₆Te₆ nanoplates on 2H-MoTe₂ atomic layers by using metal-organic chemical vapor deposition. Importantly, in situ grown 1T'-Mo₆Te₆ nanoplates significantly reduce the contact resistance of the 2H-MoTe₂ atomic layers, providing a record high mobility of 1139 cm²/V·s for Pd/1T'-Mo₆Te₆/2H-MoTe₂ back-gated field-effect transistors, along with a low Schottky barrier height ( qϕ_b) of 8.7 meV. These results lead to the possibility of ameliorating the high contact resistance faced by other TMDs and, furthermore, offer polymorphic structures for realizing higher-mobility TMD devices.

Tuning electronic structure of monolayer InP3 in contact with graphene or Ni: effect of a buffer layer and intrinsic In and P-vacancy

Ohmic contact in m-InP₃ and G or Ni interface is achieved by introducing intrinsic defects and inserting a buffer layer.

Improved contacts to p-type MoS2 transistors by charge-transfer doping and contact engineering

MoS₂ is known to show stubborn n-type behavior due to its intrinsic band structure and Fermi level pinning. Here, we investigate the combined effects of molecular doping and contact engineering on the transport and contact properties of monolayer (ML) MoS₂ devices. Significant p-type (hole-transport) behavior was only observed for chemically doped MoS₂ devices with high work function palladium (Pd) contacts, while MoS₂ devices with low work function metal contacts made from titanium showed ambipolar behavior with electron transport favored even after prolonged p-doping treatment. ML MoS₂ transistors with Pd contacts exhibit effective hole mobilities of (2.3 ± 0.7) cm² V^-1 S^-1 and an on/off ratio exceeding 10⁶. We also show that p-doping can help to improve electrical contacts in p-type field-effect transistors: relatively low contact resistances of (482 ± 40) kΩ μm and a Schottky barrier height of ≈156 meV were obtained for ML MoS₂ transistors. To demonstrate the potential application of 2D-based complementary electronic devices, a MoS₂ inverter based on pristine (n-type) and p-doped monolayer MoS₂ was fabricated. This work presents a simple and effective route for contact engineering, which enables the exploration and development of high-efficiency 2D-based semiconductor devices.

Formation of an MoTe2 based Schottky junction employing ultra-low and high resistive metal contacts

Schottky-barrier diodes have great importance in power management and mobile communication because of their informal device technology, fast response and small capacitance. In this research, a p-type molybdenum ditelluride (p-MoTe₂) based Schottky barrier diode was fabricated using asymmetric metal contacts. The MoTe₂ nano-flakes were mechanically exfoliated using adhesive tape and with the help of dry transfer techniques, the flakes were transferred onto silicon/silicon dioxide (Si/SiO₂) substrates to form the device. The Schottky-barrier was formed as a result of using ultra-low palladium/gold (Pd/Au) and high resistive chromium/gold (Cr/Au) metal electrodes. The Schottky diode exhibited a clear rectifying behavior with an on/off ratio of ∼10³ and an ideality factor of ∼1.4 at zero gate voltage. In order to check the photovoltaic response, a green laser light was illuminated, which resulted in a responsivity of ∼3.8 × 10³ A W⁻¹. These values are higher than the previously reported results that were obtained using conventional semiconducting materials. Furthermore, the barrier heights for Pd and Cr with a MoTe₂ junction were calculated to be 90 meV and 300 meV, respectively. In addition, the device was used for rectification purposes revealing a stable rectifying behavior.

Schottky-barrier diodes have great importance in power management and mobile communication because of their informal device technology, fast response and small capacitance.

Self-passivated ultra-thin SnS layers via mechanical exfoliation and post-oxidation

Remarkable optical/electrical features are expected in two-dimensional group-IV monochalcogenides (MXs; M = Sn/Ge and X = S/Se) with a uniquely distorted layered structure. The lone pair electrons in the group-IV atoms are the origin of this structural distortion, while they also cause a strong interlayer force and high chemical reactivity. The fabrication of chemically stable few-to-monolayer MX has been a significant challenge. We have observed that, once the SnS surface is oxidized, the SnO_x top layer works as a passivation layer for the SnS layer underneath. In this work, the SnO_x/SnS hetero-structure is studied structurally, optically, and electrically. When tape-exfoliated bulk SnS is oxygen-annealed under a reduced pressure at 10 Pa, surface oxidation and SnS sublimation proceed simultaneously, resulting in a monolayer-thick SnS layer with the SnO_x passivation layer. The field-effect transistor of nine-layer SnS prepared via mechanical exfoliation exhibits a p-type characteristic because of intrinsic Sn vacancies, whereas ambipolar behavior is observed for the monolayer-thick SnS obtained via oxygen annealing probably owing to the additional n-type doping by S vacancies. This work on monolayer-thick SnS fabrication can be applied to other unstable lone pair analogues and can facilitate future research on MXs.

Impact of Synthesized MoS2 Wafer-Scale Quality on Fermi Level Pinning in Vertical Schottky-Barrier Heterostructures

Transition metal dichalcogenide (TMD)-based vertical Schottky heterostructures have recently shown promise as a next generation device for a variety of applications. In order for these devices to operate effectively, the interface between the TMD and metal contacts must be well-understood and optimized. In this work, the interface between synthesized MoS₂ and gold or platinum metal contacts is explored as a function of MoS₂ film quality to understand Fermi level pinning effects. Raman, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy are used to physically characterize both MoS₂ and MoS₂/metal interface. Metal/MoS₂/metal purely vertical heterostructure cross-point devices were fabricated to explore the injection behavior across the Schottky barrier formed between MoS₂ and the metal. The temperature dependence of the device behavior is used to understand injection mechanisms, and modeling is performed to verify the injection mechanisms across the interface barrier. By combining both physical characterization with electrical results and modeling, Fermi level pinning is investigated as a function of macroscopic MoS₂ quality. Low-quality MoS₂ was found to exhibit much stronger pinning than high-quality films, which is consistent with an observed increase in covalency of the metal/MoS₂ interface. Additionally, MoS₂ was found to pin gold much more strongly than platinum, which is consistent with an increased covalent interaction between MoS₂ and gold. These results show that the synthesis temperature and, therefore, the quality of MoS₂ dramatically impacts Fermi level pinning and the resultant current-voltage characteristics of Schottky barrier-mediated devices.

Nanoscale Investigation of Defects and Oxidation of HfSe2

HfSe₂ is a very good candidate for a transition metal dichalcogenide-based field-effect transistor owing to its moderate band gap of about 1 eV and its high-κ dielectric native oxide. Unfortunately, the experimentally determined charge carrier mobility is about 3 orders of magnitude lower than the theoretically predicted value. This strong deviation calls for a detailed investigation of the physical and electronic properties of HfSe₂. Here, we have studied the structure, density, and density of states of several types of defects that are abundant on the HfSe₂ surface using scanning tunneling microscopy and spectroscopy. Compared to MoS₂ and WSe₂, HfSe₂ exhibits similar type of defects, albeit with a substantially higher density of 9 × 10¹¹ cm^-2. The most abundant defect is a subsurface defect, which shows up as a dim feature in scanning tunneling microscopy images. These dim dark defects have a substantially larger band gap (1.25 eV) than the pristine surface (1 eV), suggesting a substitution of the Hf atom by another atom. The high density of defects on the HfSe₂ surface leads to very low Schottky barrier heights. Conductive atomic force microscopy measurements reveal a very small dependence of the Schottky barrier height on the work function of the metals, suggesting a strong Fermi-level pinning. We attribute the observed Fermi-level pinning (pinning factor ∼0.1) to surface distortions and Se/Hf defects. In addition, we have also studied the HfSe₂ surface after the exposure to air by scanning tunneling microscopy and conductive atomic force microscopy. Partly oxidized layers with band gaps of 2 eV and Schottky barrier heights of ∼0.6 eV were readily found on the surface. Our experiments reveal that HfSe₂ is very air-sensitive, implying that capping or encapsulating of HfSe₂, in order to protect it against oxidation, is a necessity for technological applications.

High-Performance Wafer-Scale MoS2 Transistors toward Practical Application

Atomic thin transition-metal dichalcogenides (TMDs) are considered as an emerging platform to build next-generation semiconductor devices. However, to date most devices are still based on exfoliated TMD sheets on a micrometer scale. Here, a novel chemical vapor deposition synthesis strategy by introducing multilayer (ML) MoS₂ islands to improve device performance is proposed. A four-probe method is applied to confirm that the contact resistance decreases by one order of magnitude, which can be attributed to a conformal contact by the extra amount of exposed edges from the ML-MoS₂ islands. Based on such continuous MoS₂ films synthesized on a 2 in. insulating substrate, a top-gated field effect transistor (FET) array is fabricated to explore key metrics such as threshold voltage (V _T ) and field effect mobility (μ_FE ) for hundreds of MoS₂ FETs. The statistical results exhibit a surprisingly low variability of these parameters. An average effective μ_FE of 70 cm² V^-1 s^-1 and subthreshold swing of about 150 mV dec^-1 are extracted from these MoS₂ FETs, which are comparable to the best top-gated MoS₂ FETs achieved by mechanical exfoliation. The result is a key step toward scaling 2D-TMDs into functional systems and paves the way for the future development of 2D-TMDs integrated circuits.

Charge carrier injection and transport engineering in two-dimensional transition metal dichalcogenides

Ever since two dimensional-transition (2D) metal dichalcogenides (TMDs) were discovered, their fascinating electronic properties have attracted a great deal of attention for harnessing them as critical components in novel electronic devices. 2D-TMDs endowed with an atomically thin structure, dangling bond-free nature, electrostatic integrity, and tunable wide band gaps enable low power consumption, low leakage, ambipolar transport, high mobility, superconductivity, robustness against short channel effects and tunneling in highly scaled devices. However, the progress of 2D-TMDs has been hampered by severe charge transport issues arising from undesired phenomena occurring at the surfaces and interfaces. Therefore, this review provides three distinct engineering strategies embodied with distinct innovative approaches to optimize both carrier injection and transport. First, contact engineering involves 2D-metal contacts and tunneling interlayers to overcome metal-induced interface states and the Fermi level pinning effect caused by low vacancy energy formation. Second, dielectric engineering covers high-k dielectrics, ionic liquids or 2D-insulators to screen scattering centers caused by carrier traps, imperfections and rough substrates, to finely tune the Fermi level across the band gap, and to provide dangling bond-free media. Third, material engineering focuses on charge transfer via substitutional, chemical and plasma doping to precisely modulate the carrier concentration and to passivate defects while preserving material integrity. Finally, we provide an outlook of the conceptual and technical achievements in 2D-TMDs to give a prospective view of the future development of highly scaled nanoelectronic devices.

High performance self-gating graphene/MoS2 diode enabled by asymmetric contacts

A graphene-MoS₂ (GM) heterostructure based diode is fabricated using asymmetric contacts to MoS₂, as well as an asymmetric top gate (ATG). The GM diode exhibits a rectification ratio of 5 from asymmetric contacts, which is improved to 10⁵ after the incorporation of an ATG. This improvement is attributed to the asymmetric modulation of carrier concentration and effective Schottky barrier height (SBH) by the ATG during forward and reverse bias. This is further confirmed from the temperature dependent measurement, where a difference of 0.22 eV is observed between the effective SBH for forward and reverse bias. Moreover, the rectification ratio also depends on carrier concentration in MoS₂ and can be varied with the change in temperature as well as back gate voltage. Under laser light illumination, the device demonstrates strong opto-electric response with 100 times improvement in the relative photo current, as well as a responsivity of 1.9 A W^-1 and a specific detectivity of 2.4 × 10¹⁰ Jones. These devices can also be implemented using other two dimensional (2D) materials and suggest a promising approach to incorporate diverse 2D materials for future nano-electronics and optoelectronics applications.

Comparison of Electrical and Photoelectrical Properties of ReS2 Field-Effect Transistors on Different Dielectric Substrates

As one of the newly discovered transition-metal dichalcogenides (TMDs), rhenium disulfide (ReS₂) has been investigated mostly because of its unique characteristics such as the direct band gap nature even in bulk form, which is not prominent in other TMDs (e.g., MoS₂, WSe₂, etc.). However, this material possesses a low mobility and an on/off ratio, which restrict its usage in high-speed and fast switching applications. Low mobilities or on/off ratios can also be caused by substrate scattering as well as environmental effects. In this study, we used few-layer ReS₂ (FL-ReS₂) as a channel material to investigate the substrate-dependent mobility, current on/off ratio, Schottky barrier height (SBH), and trap density of states of different dielectric substrates. The hexagonal boron nitride (h-BN)/FL-ReS₂/h-BN structure was observed to exhibit a high mobility of 45 cm² V^-1 s^-1, current on/off ratio of about 10⁷, the lowest SBH of about 12 mV at a zero back-gate voltage ( V_bg), and a low trap density of states of about 5 × 10¹³ cm^-3. These quantities are reasonably superior compared to the FL-ReS₂ devices on SiO₂ substrates. We also observed a nearly 5-fold improvement in the photoresponsivity and external quantum efficiency values for the FL-ReS₂ devices on h-BN substrates. We believe that the photonic characteristics of TMDs can be improved by using h-BN as the substrate and capping layer.

MoS2 Memristors Exhibiting Variable Switching Characteristics toward Biorealistic Synaptic Emulation

Memristors based on 2D layered materials could provide biorealistic ionic interactions and potentially enable construction of energy-efficient artificial neural networks capable of faithfully emulating neuronal interconnections in human brains. To build reliable 2D-material-based memristors suitable for constructing working neural networks, the memristive switching mechanisms in such memristors need to be systematically analyzed. Here, we present a study on the switching characteristics of the few-layer MoS₂ memristors made by mechanical printing. First, two types of dc-programmed switching characteristics, termed rectification-mediated and conductance-mediated behaviors, are observed among different MoS₂ memristors, which are attributed to the modulation of MoS₂/metal Schottky barriers and redistribution of vacancies, respectively. We also found that an as-fabricated MoS₂ memristor initially exhibits an analog pulse-programmed switching behavior, but it can be converted to a quasi-binary memristor with an abrupt switching behavior through an electrical stress process. Such a transition of switching characteristics is attributed to field-induced agglomeration of vacancies at MoS₂/metal interfaces. The additional Kelvin probe force microscopy, Auger electron spectroscopy analysis, and electronic characterization results support this hypothesis. Finally, we fabricated a testing device consisting of two adjacent MoS₂ memristors and demonstrated that these two memristors can be ionically coupled to each other. This device interconnection scheme could be exploited to build neural networks for emulating ionic interactions among neurons. This work advances the device physics for understanding the memristive properties of 2D-material-based memristors and serves as a critical foundation for building biorealistic neuromorphic computing systems based on such memristors.

Conversion of Multi-layered MoTe2 Transistor Between P-Type and N-Type and Their Use in Inverter

Both p-type and n-type MoTe₂ transistors are needed to fabricate complementary electronic and optoelectronic devices. In this study, we fabricate air-stable p-type multi-layered MoTe₂ transistors using Au as electrode and achieve the conversion of p-type transistor to n-type by annealing it in vacuum. Temperature-dependent in situ measurements assisted by the results given by first-principle simulations indicate that n-type conductance is an intrinsic property, which is attributed to tellurium vacancies in MoTe₂, while the device in air experiences a charge transfer which is caused by oxygen/water redox couple and is converted to air-stable p-type transistor. Based on p-type and n-type multi-layered MoTe₂ transistors, we demonstrate a complementary inverter with gain values as high as 9 at V_DD = 5 V.

Impact ionization by hot carriers in a black phosphorus field effect transistor

The strong Coulombic interactions in miniaturized structures can lead to efficient carrier multiplication, which is essential for many-body physics and design of efficient photonic devices beyond thermodynamic conversion limits. However, carrier multiplication has rarely been realized in layered semiconducting materials despite strong electronic interactions. Here, we report the experimental observation of unusual carrier multiplication in a multilayer black phosphorus device. Electric field-dependent Hall measurements confirm a substantial increase of carrier density in multilayer black phosphorus channel, which is attributed to the impact ionization by energetic carriers. This mechanism relies on the generation of self-heating induced charge carriers under the large electric field due to competition between electron–electron and electron–phonon interactions in the direct and narrow band gap (0.3 eV) of the multilayer black phosphorus. These findings point the way toward utilization of carrier multiplication to enhance the performance of electronics and optoelectronics devices based on two-dimensional materials.

Carrier multiplication processes based on new electron-hole pair generation is instrumental to realizing ultrafast and efficient optoelectronic devices. Here, the authors demonstrate multilayered black phosphorous-based transistors that show enhanced performance due to carrier multiplication.

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight.

Controllable, Wide-Ranging n-Doping and p-Doping of Monolayer Group 6 Transition-Metal Disulfides and Diselenides

Developing processes to controllably dope transition-metal dichalcogenides (TMDs) is critical for optical and electrical applications. Here, molecular reductants and oxidants are introduced onto monolayer TMDs, specifically MoS2 , WS2 , MoSe2 , and WSe2 . Doping is achieved by exposing the TMD surface to solutions of pentamethylrhodocene dimer as the reductant (n-dopant) and "Magic Blue," [N(C6 H4 -p-Br)3 ]SbCl6 , as the oxidant (p-dopant). Current-voltage characteristics of field-effect transistors show that, regardless of their initial transport behavior, all four TMDs can be used in either p- or n-channel devices when appropriately doped. The extent of doping can be controlled by varying the concentration of dopant solutions and treatment time, and, in some cases, both nondegenerate and degenerate regimes are accessible. For all four TMD materials, the photoluminescence intensity; for all four materials the PL intensity is enhanced with p-doping but reduced with n-doping. Raman and X-ray photoelectron spectroscopy (XPS) also provide insight into the underlying physical mechanism by which the molecular dopants react with the monolayer. Estimates of changes of carrier density from electrical, PL, and XPS results are compared. Overall a simple and effective route to tailor the electrical and optical properties of TMDs is demonstrated.

Tunable Electron and Hole Injection Enabled by Atomically Thin Tunneling Layer for Improved Contact Resistance and Dual Channel Transport in MoS2/WSe2 van der Waals Heterostructure

Two-dimensional (2D) material-based heterostructures provide a unique platform where interactions between stacked 2D layers can enhance the electrical and opto-electrical properties as well as give rise to interesting new phenomena. Here, the operation of a van der Waals heterostructure device comprising of vertically stacked bilayer MoS₂ and few layered WSe₂ has been demonstrated in which an atomically thin MoS₂ layer has been employed as a tunneling layer to the underlying WSe₂ layer. In this way, simultaneous contacts to both MoS₂ and WSe₂ 2D layers have been established by forming a direct metal-semiconductor to MoS₂ and a tunneling-based metal-insulator-semiconductor contacts to WSe₂, respectively. The use of MoS₂ as a dielectric tunneling layer results in an improved contact resistance (80 kΩ μm) for WSe₂ contact, which is attributed to reduction in the effective Schottky barrier height and is also confirmed from the temperature-dependent measurement. Furthermore, this unique contact engineering and type-II band alignment between MoS₂ and WSe₂ enables a selective and independent carrier transport across the respective layers. This contact engineered dual channel heterostructure exhibits an excellent gate control and both channel current and carrier types can be modulated by the vertical electric field of the gate electrode, which is also reflected in the on/off ratio of 10⁴ for both electron (MoS₂) and hole (WSe₂) channels. Moreover, the charge transfer at the heterointerface is studied quantitatively from the shift in the threshold voltage of the pristine MoS₂ and the heterostructure device, which agrees with the carrier recombination-induced optical quenching as observed in the Raman spectra of the pristine and heterostructure layers. This observation of dual channel ambipolar transport enabled by the hybrid tunneling contacts and strong interlayer coupling can be utilized for high-performance opto-electrical devices and applications.

Multi-Valued Logic Circuits Based on Organic Anti-ambipolar Transistors

Multivalued logic circuits, which can handle more information than conventional binary logic circuits, have attracted much attention as a promising way to improve the data-processing capabilities of integrated circuits. In this study, we developed a ternary inverter based on organic field-effect transistors (OFET) as a potential component of high-performance and flexible integrated circuits. Key elements are anti-ambipolar and n-type OFETs connected in series. First, we demonstrate an organic ternary inverter that exhibits three distinct logic states. Second, the operating voltage was greatly reduced by taking advantage of an Al₂O₃ gate dielectric. Finally, the operating voltage was finely tuned by the designing of the device geometry. These results are achievable owing to the flexible controllability of the device configuration, suggesting that the organic ternary inverter plays an important role with regard to high-performance organic integrated circuits.

Parameter control for enhanced peak-to-valley current ratio in a MoS2/MoTe2 van der Waals heterostructure

The Ids-Vds properties of a van der Waals cross-junction of few layered MoS2/MoTe2 were investigated, and the physical device parameters were altered in order to transform the conduction mechanism from thermionic emission to interband tunneling. The pristine heterostructure demonstrated rectification behavior of typical p-n junction diodes, because of the p-type and n-type nature of MoTe2 and MoS2, respectively. Lowering the contact resistance between the metal and channel materials, by changing the electrode metals from Au to Pd and Ti, alone did not give rise to carrier conduction through the hetero-interband tunneling between MoTe2 and MoS2. In addition to the reduction in contact resistance, the chemical doping of MoS2 using Benzyl Viologen (BV) achieves hetero-interband tunneling between MoTe2 and MoS2, which probably narrows the depletion layer by degenerating MoS2. The peak-to-valley ratio of the tunneling current of the BV-doped heterostructure of MoS2/MoTe2 is about 4.8, which is comparable to that of the commercially available Si tunneling diode.

FeIn2S4 Nanocrystals: A Ternary Metal Chalcogenide Material for Ambipolar Field-Effect Transistors

An ambipolar channel layer material is required to realize the potential benefits of ambipolar complementary metal-oxide-semiconductor field-effect transistors, namely their compact and efficient nature, reduced reverse power dissipation, and possible applicability to highly integrated circuits. Here, a ternary metal chalcogenide nanocrystal material, FeIn2S4, is introduced as a solution-processable ambipolar channel material for field-effect transistors (FETs). The highest occupied molecular orbital and the lowest unoccupied molecular orbital of the FeIn2S4 nanocrystals are determined to be -5.2 and -3.75 eV, respectively, based upon cyclic voltammetry, X-ray photoelectron spectroscopy, and diffraction reflectance spectroscopy analyses. An ambipolar FeIn2S4 FET is successfully fabricated with Au electrodes (EF = -5.1 eV), showing both electron mobility (14.96 cm2 V-1 s-1) and hole mobility (9.15 cm2 V-1 s-1) in a single channel layer, with an on/off current ratio of 105. This suggests that FeIn2S4 nanocrystals may be a promising alternative semiconducting material for next-generation integrated circuit development.

Schottky Barrier Height Engineering for Electrical Contacts of Multilayered MoS2 Transistors with Reduction of Metal-Induced Gap States

The difficulty in Schottky barrier height (SBH) control arising from Fermi-level pinning (FLP) at electrical contacts is a bottleneck in designing high-performance nanoscale electronics and optoelectronics based on molybdenum disulfide (MoS₂). For electrical contacts of multilayered MoS₂, the Fermi level on the metal side is strongly pinned near the conduction-band edge of MoS₂, which makes most MoS₂-channel field-effect transistors (MoS₂ FETs) exhibit n-type transfer characteristics regardless of their source/drain (S/D) contact metals. In this work, SBH engineering is conducted to control the SBH of electrical top contacts of multilayered MoS₂ by introducing a metal-interlayer-semiconductor (MIS) structure which induces the Fermi-level unpinning by a reduction of metal-induced gap states (MIGS). An ultrathin titanium dioxide (TiO₂) interlayer is inserted between the metal contact and the multilayered MoS₂ to alleviate FLP and tune the SBH at the S/D contacts of multilayered MoS₂ FETs. A significant alleviation of FLP is demonstrated as MIS structures with 1 nm thick TiO₂ interlayers are introduced into the S/D contacts. Consequently, the pinning factor ( S) increases from 0.02 for metal-semiconductor (MS) contacts to 0.24 for MIS contacts, and the controllable SBH range is widened from 37 meV (50-87 meV) to 344 meV (107-451 meV). Furthermore, the Fermi-level unpinning effect is reinforced as the interlayer becomes thicker. This work widens the scope for modifying electrical characteristics of contacts by providing a platform to control the SBH through a simple process as well as understanding of the FLP at the electrical top contacts of multilayered MoS₂.

High-Performance Solid-State Thermionic Energy Conversion Based on 2D van der Waals Heterostructures: A First-Principles Study

Two-dimensional (2D) van der Waals heterostructures (vdWHs) have shown multiple functionalities with great potential in electronics and photovoltaics. Here, we show their potential for solid-state thermionic energy conversion and demonstrate a designing strategy towards high-performance devices. We propose two promising thermionic devices, namely, the p-type Pt-G-WSe₂-G-Pt and n-type Sc-WSe₂-MoSe₂-WSe₂-Sc. We characterize the thermionic energy conversion performance of the latter using first-principles GW calculations combined with real space Green’s function (GF) formalism. The optimal barrier height and high thermal resistance lead to an excellent performance. The proposed device is found to have a room temperature equivalent figure of merit of 1.2 which increases to 3 above 600 K. A high performance with cooling efficiency over 30% of the Carnot efficiency above 450 K is achieved. Our designing and characterization method can be used to pursue other potential thermionic devices based on vdWHs.

Intrinsic Transport in 2D Heterostructures Mediated through h-BN Tunneling Contacts

Understanding the electronic transport of monolayer transition metal dichalcogenides (TMDs) and their heterostructures is complicated by the difficulty in achieving electrical contacts that do not perturb the material. Typically, metal deposition on monolayer TMDs leads to hybridization between the TMD and the metal, which produces Schottky barriers at the metal/semiconductor interface. In this work, we apply the recently reported hexagonal boron nitride (h-BN) tunnel contact scheme to probe the junction characteristics of a lateral TMD heterostructure grown via chemical vapor deposition. We first measure the electronic properties across the junction before elucidating optoelectronic generation mechanisms via scanning photocurrent microscopy. We find that the rectification ratio measured using the encapsulated, tunnel contact scheme is almost 2 orders of magnitude smaller than that observed via conventional metal contact geometry, which implies that the metal/semiconductor Schottky barriers play large roles in this aspect. Furthermore, we find that both the photovoltaic as well as hot carrier generation effects are dominant mechanisms driving photoresponse, depending on the external biasing conditions. This work is the first time that this encapsulation scheme has been applied to lateral heterostructures and serves as a reference for future electronic measurements on this material. It also simultaneously serves as a framework to more accurately assess the electronic transport characteristics of 2D heterostructures and better inform future device architectures.

Reconfigurable Diodes Based on Vertical WSe2 Transistors with van der Waals Bonded Contacts

New device concepts can increase the functionality of scaled electronic devices, with reconfigurable diodes allowing the design of more compact logic gates being one of the examples. In recent years, there has been significant interest in creating reconfigurable diodes based on ultrathin transition metal dichalcogenide crystals due to their unique combination of gate-tunable charge carriers, high mobility, and sizeable band gap. Thanks to their large surface areas, these devices are constructed under planar geometry and the device characteristics are controlled by electrostatic gating through rather complex two independent local gates or ionic-liquid gating. In this work, similar reconfigurable diode action is demonstrated in a WSe₂ transistor by only utilizing van der Waals bonded graphene and Co/h-BN contacts. Toward this, first the charge injection efficiencies into WSe₂ by graphene and Co/h-BN contacts are characterized. While Co/h-BN contact results in nearly Schottky-barrier-free charge injection, graphene/WSe₂ interface has an average barrier height of ≈80 meV. By taking the advantage of the electrostatic transparency of graphene and the different work-function values of graphene and Co/h-BN, vertical devices are constructed where different gate-tunable diode actions are demonstrated. This architecture reveals the opportunities for exploring new device concepts.

Flexible and High-Performance All-2D Photodetector for Wearable Devices

Emerging novel applications at the forefront of innovation horizon raise new requirements including good flexibility and unprecedented properties for the photoelectronic industry. On account of diversity in transport and photoelectric properties, 2D layered materials have proven as competent building blocks toward next-generation photodetectors. Herein, an all-2D Bi₂ Te₃ -SnS-Bi₂ Te₃ photodetector is fabricated with pulsed-laser deposition. It is sensitive to broadband wavelength from ultraviolet (370 nm) to near-infrared (808 nm). In addition, it exhibits great durability to bend, with intact photoresponse after 100 bend cycles. Upon 370 nm illumination, it achieves a high responsivity of 115 A W^-1 , a large external quantum efficiency of 3.9 × 10⁴ %, and a superior detectivity of 4.1 × 10¹¹ Jones. They are among the best figures-of-merit of state-of-the-art 2D photodetectors. The synergistic effect of SnS's strong light-matter interaction, efficient carrier separation of Bi₂ Te₃ -SnS interface, expedite carrier injection across Bi₂ Te₃ -SnS interface, and excellent carrier collection of Bi₂ Te₃ topological insulator electrodes accounts for the superior photodetection properties. In summary, this work depicts a facile all-in-one fabrication strategy toward a Bi₂ Te₃ -SnS-Bi₂ Te₃ photodetector. More importantly, it reveals a novel all-2D concept for construction of flexible, broadband, and high-performance photoelectronic devices by integrating 2D layered metallic electrodes and 2D layered semiconducting channels.

Local Conduction in Mo xW1- xSe2: The Role of Stacking Faults, Defects, and Alloying

Here, we report on the surface conductivity of WSe₂ and Mo _xW_{1- x}Se₂ (0 ≤ x ≤ 1) crystals investigated with conductive atomic force microscopy. We found that stacking faults, defects, and chemical heterogeneities form distinct two-dimensional and one-dimensional conduction paths on the transition metal dichalcogenide surface. In the case of WSe₂, in addition to step edges, we find a significant amount of stacking faults (formed during the cleaving process) that strongly influence the surface conductivity. These regions are attributed to the alternation of the 2H and 3R polytypism. The stacking faults form regular 2D patterns by alternation of the underlying stacking order, with a periodicity that varies significantly between different regions and samples. In the case of Mo _xW_{1- x}Se₂, its conductivity has a localized nature, which depends on the underlying chemical composition and the Mo/W ratio. Segregation to W-rich and Mo-rich regions during the growth process leads to nonuniform conduction paths on the surface of the alloy. We found a gradual change of the conductivity moving from one region to the other, reminiscent of lateral band bending. Our results demonstrate the use of C-AFM as a nanoscopic tool to probe the electrical properties of largely inhomogeneous samples and show the complicated nature of the surface conductivity of TMDC alloys.

Determination of Carrier Polarity in Fowler-Nordheim Tunneling and Evidence of Fermi Level Pinning at the Hexagonal Boron Nitride/Metal Interface

Hexagonal boron nitride (h-BN) is an important insulating substrate for two-dimensional (2D) heterostructure devices and possesses high dielectric strength comparable to SiO₂. Here, we report two clear differences in their physical properties. The first one is the occurrence of Fermi level pinning at the metal/h-BN interface, unlike that at the metal/SiO₂ interface. The second one is that the carrier of Fowler-Nordheim (F-N) tunneling through h-BN is a hole, which is opposite to an electron in the case of SiO₂. These unique characteristics are verified by I- V measurements in the graphene/h-BN/metal heterostructure device with the aid of a numerical simulation, where the barrier height of graphene can be modulated by a back gate voltage owing to its low density of states. Furthermore, from a systematic investigation using a variety of metals, it is confirmed that the hole F-N tunneling current is a general characteristic because the Fermi levels of metals are pinned in the small energy range around ∼3.5 eV from the top of the conduction band of h-BN, with a pinning factor of 0.30. The accurate energy band alignment at the h-BN/metal interface provides practical knowledge for 2D heterostructure devices.

High Detectivity and Transparent Few-Layer MoS2 /Glassy-Graphene Heterostructure Photodetectors

Layered van der Waals heterostructures have attracted considerable attention recently, due to their unique properties both inherited from individual two-dimensional (2D) components and imparted from their interactions. Here, a novel few-layer MoS₂ /glassy-graphene heterostructure, synthesized by a layer-by-layer transfer technique, and its application as transparent photodetectors are reported for the first time. Instead of a traditional Schottky junction, coherent ohmic contact is formed at the interface between the MoS₂ and the glassy-graphene nanosheets. The device exhibits pronounced wavelength selectivity as illuminated by monochromatic lights. A responsivity of 12.3 mA W^-1 and detectivity of 1.8 × 10¹⁰ Jones are obtained from the photodetector under 532 nm light illumination. Density functional theory calculations reveal the impact of specific carbon atomic arrangement in the glassy-graphene on the electronic band structure. It is demonstrated that the band alignment of the layered heterostructures can be manipulated by lattice engineering of 2D nanosheets to enhance optoelectronic performance.

First-Principles Study on Layered C2N-Metal Interfaces

Using first-principles calculations, we perform a comprehensive study of representative metal (Al, Sc, Pd, Ag, Pt, and Au) contacts with monolayer (ML) and bilayer (BL) C₂N, which is a low-cost and easily synthesized two-dimensional metal-free semiconductor. Through analyzing the geometries, electronic structures, and Fermi level pinning effects of C₂N-metal interfaces, we find metals Al and Sc top contact with ML C₂N are Ohmic, which can be ascribed to the strong interactions and large orbital overlaps. Besides, owing to weak van der Waals interactions at interfaces and low work functions of metallic materials, Ohmic contacts can also be realized in ML/BL C₂N-Ag and BL C₂N-Sc systems. Furthermore, it was also predicted that C₂N-Sc and C₂N-Ag systems still maintain Ohmic features along the edge contacts. Given the lower resistance of the Ag electrode, the C₂N-Ag electrode should be a more attractive electrode in practical applications. These predictions not only provide insights into the fundamental properties of the layered C₂N-metal interfaces but also pave way to design high-performance devices using low-cost layered C₂N.

Charge-Transfer-Induced p-Type Channel in MoS2 Flake Field Effect Transistors

The two-dimensional transition-metal dichalcogenide semiconductor MoS₂ has received extensive attention for decades because of its outstanding electrical and mechanical properties for next-generation devices. One weakness of MoS₂, however, is that it shows only n-type conduction, revealing its limitations for homogeneous PN diodes and complementary inverters. Here, we introduce a charge-transfer method to modify the conduction property of MoS₂ from n- to p-type. We initially deposited an n-type InGaZnO (IGZO) film on top of the MoS₂ flake so that electron charges might be transferred from MoS₂ to IGZO during air ambient annealing. As a result, electron charges were depleted in MoS₂. Such charge depletion lowered the MoS₂ Fermi level, which makes hole conduction favorable in MoS₂ when optimum source/drain electrodes with a high work function are selected. Our IGZO-supported MoS₂ flake field effect transistors (FETs) clearly display channel-type conversion from n- to p-channel in this way. Under short- and long-annealing conditions, n- and p-channel MoS₂ FETs are achieved, respectively, and a low-voltage complementary inverter is demonstrated using both channels in a single MoS₂ flake.

Interface Engineering for Controlling Device Properties of Organic Antiambipolar Transistors

The main purpose of this study is to establish a guideline for controlling the device properties of organic antiambipolar transistors. Our key strategy is to use interface engineering to promote carrier injection at channel/electrode interfaces and carrier accumulation at a channel/dielectric interface. The effective use of carrier injection interlayers and an insulator layer with a high dielectric constant (high-k) enabled the fine tuning of device parameters and, in particular, the onset (V_on) and offset (V_off) voltages. A well-matched combination of the interlayers and a high-k dielectric layer achieved a low peak voltage (0.25 V) and a narrow on-state bias range (2.2 V), indicating that organic antiambipolar transistors have high potential as negative differential resistance devices for multivalued logic circuits.

Tunable Polarity Behavior and High-Performance Photosensitive Characteristics in Schottky-Barrier Field-Effect Transistors Based on Multilayer WS2

Schottky-barrier field-effect transistors (SBFETs) based on multilayer WS₂ with Au as drain/source contacts are fabricated in this paper. Interestingly, the novel polarity behavior of the WS₂ SBFETs can be modulated by drain bias, ranging from p-type to ambipolar and finally to n-type conductivity, due to the transition of band structures and Schottky-barrier heights under different drain and gate biases. The electron mobility and the on/off ratio of electron current can reach as high as 23.4 cm²/(V s) and 8.5 × 10⁷, respectively. Moreover, the WS₂ SBFET possesses high-performance photosensitive characteristics with response time of 40 ms, photoresponsivity of 12.4 A/W, external quantum efficiency of 2420%, and photodetectivity as high as 9.28 × 10¹¹ cm Hz^1/2/W. In conclusion, the excellent performance of the WS₂ SBFETs may pave the way for next-generation electronic and photoelectronic devices.

Large-area niobium disulfide thin films as transparent electrodes for devices based on two-dimensional materials

Direct contacts of a metal with atomically thin two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors have been found to suppress device performance by producing a high contact resistance. NbS₂ is a 2D TMDC and a conductor. It is expected to form ohmic contacts with 2D semiconductors because of its high work function and the van der Waals interface it forms with the semiconductor, with such an interface resulting in weak Fermi level pinning. Despite the usefulness of NbS₂ as an electrode, previous synthesis methods could not control the thickness, uniformity, and shape of the NbS₂ film and hence could not find practical applications in electronics. Here, we report a patternable method for carrying out the synthesis of NbS₂ films in which the number of NbS₂ layers formed over a large area was successfully controlled, which is necessary for the production of customized electrodes. The synthesized NbS₂ films were shown to be highly transparent and uniform in thickness and conductivity over the large area. Furthermore, the synthesized NbS₂ showed half the contact resistance than did the molybdenum metal in MoS₂ field effect transistors (FETs) on a large transparent quartz substrate. The MoS₂ device with NbS₂ showed an electron mobility as high as 12.7 cm² V^-1 s^-1, which was three times higher than that found for the corresponding molybdenum-contacted MoS₂ device. This result showed the high potential of the NbS₂ thin film as a transparent electrode for 2D transition metal dichalcogenide (TMDC) semiconductors with low contact resistance.

Van der Waals interfaces in epitaxial vertical metal/2D/3D semiconductor heterojunctions of monolayer MoS2 and GaN

A promising approach for high speed and high power electronics is to integrate two-dimensional (2D) materials with conventional electronic components such as bulk (3D) semiconductors and metals. In this study we explore a basic integration step of inserting a single monolayer MoS 2 (1L-MoS 2 ) inside a Au / p -GaN junction and elucidate how it impacts the structural and electrical properties of the junction. Epitaxial 1L-MoS 2 in the form of 1-2 μm triangle domains are grown by powder vaporization on a p -doped GaN substrate, and the Au capping layer is deposited by evaporation. Transmission electron microscopy (TEM) of the van der Waals interface indicates that 1L-MoS 2 remained distinct and intact between the Au and GaN and that the Au is epitaxial to GaN only when the 1L-MoS 2 is present. Quantitative TEM analyses of the van der Waals interfaces are performed and yielded the atomic plane spacings in the heterojunction. Electrical characterization of the all-epitaxial, vertical Au / 1L-MoS 2 /p -GaN heterojunctions enables the derivations of Schottky barrier heights (SBH) and drawing of the band alignment diagram. Notably, 1L-MoS 2 appears to be electronically semi-transparent, and thus can be considered as a modifier to the Au contact rather than an independent semiconductor component forming a pn -junction. The I - V analysis and our first principles calculation indicated Fermi level pinning and substantial band bending in GaN at the interface. Lastly, we illustrate how the depletion regions are formed in a bipolar junction with an ultrathin monolayer component using the calculated distribution of the charge density across the Au / 1L-MoS 2 /GaN junction.

Modified MXene: promising electrode materials for constructing Ohmic contacts with MoS2for electronic device applications

Promising electrode materials for constructing Ohmic contact with MoS₂for electronic device application.

Control of polarity in multilayer MoTe2 field-effect transistors by channel thickness

In this study, electronic properties of field-effect transistors (FETs) fabricated from exfoliated MoTe₂ single crystals are investigated as a function of channel thickness. The conductivity type in FETs gradually changes from n-type for thick MoTe₂ layers (above ≈ 65 nm) to ambipolar behavior for intermediate MoTe₂ thickness (between ≈ 60 and 15 nm) to p- type for thin layers (below ≈ 10 nm). The n-type behavior in quasi-bulk MoTe₂ is attributed to doping with chlorine atoms from the TeCl₄ transport agent used for the chemical vapor transport (CVT) growth of MoTe₂. The change in polarity sign with decreasing channel thickness may be associated with increasing role of surface states in ultra-thin layers, which in turn influence carrier concentration and dynamics in the channel due to modulation of Schottky barrier height and band-bending at the metal/semiconductor interface.

Probing defect dynamics in monolayer MoS2 via noise nanospectroscopy

Monolayer molybdenum disulfide (MoS₂) has received intense interest as a strong candidate for next-generation electronics. However, the observed electrical properties of monolayer MoS₂ exhibit several anomalies: samples universally exhibit unexpectedly low mobilities, n-type characteristics, and large contact resistances regardless of contact metal work function. These anomalies have been attributed to the presence of defects, but the mechanism behind this link has been elusive. Here we report the ionization dynamics of sulfur monovacancy defects in monolayer MoS₂ probed via noise nanospectroscopy, realized by combining noise-current analysis with atomic force microscopy. Due to the nanoscale dimension of the in situ channel defined by the tip size, we probe a few monovacancy defects at a time. Monovacancy defects exhibit switching between three distinct ionization configurations, corresponding to charge states 0, -1, and -2. The most probable charge configurations are 0 and -1, providing a plausible mechanism to explain the observed anomalies of MoS₂ monolayers.

Pronounced Photovoltaic Response from Multi-layered MoTe2 Phototransistor with Asymmetric Contact Form

In this study, we fabricate air-stable p-type multi-layered MoTe₂ phototransistor using Au as electrodes, which shows pronounced photovoltaic response in off-state with asymmetric contact form. By analyzing the spatially resolved photoresponse using scanning photocurrent microscopy, we found that the potential steps are formed in the vicinity of the electrodes/MoTe₂ interface due to the doping of the MoTe₂ by the metal contacts. The potential step dominates the separation of photoexcited electron-hole pairs in short-circuit condition or with small V _sd biased. Based on these findings, we infer that the asymmetric contact cross-section between MoTe₂-source and MoTe₂-drain electrodes is the reason to form non-zero net current and photovoltaic response. Furthermore, MoTe₂ phototransistor shows a faster response in short-circuit condition than that with higher biased V _sd within sub-millisecond, and its spectral range can be extended to the infrared end of 1550 nm.

Promising Approach for High-Performance MoS2 Nanodevice: Doping the BN Buffer Layer to Eliminate the Schottky Barriers

Reducing the Schottky barrier height (SBH) of metal-MoS₂ interface with no deteriorating the intrinsic properties of MoS₂ channel layer is crucial to realize the high-performance MoS₂ nanodevice. To realize this expectation, a promising approach is present in this study by doping the boron nitride (BN) buffer layer between metal electrode and MoS₂ channel layer. Results demonstrate that no matter the types of concentrations and dopants the intrinsic electronic structure, low electron effective mass of MoS₂ channel layer, and the weak Fermi level pinning effects of metal/BN-MoS₂ interfaces are preserved and not deteriorated. More importantly, the n- and p-type SBHs of metal/BN-MoS₂ interfaces are significantly reduced by the electron-poor and -rich dopants, respectively, when the doped BN buffer layer spreads all over the nanodevice, which is in contrast to the traditional doping rule. Moreover, both the n- and p-type SBHs are further decreased and even eliminated when the concentrations of dopants increase. The n-type SBH of doped Au/B_xN-MoS₂ interface and the p-type SBH of doped Pt/BN_x-MoS₂ interface can be reduced to -0.21 and -0.61 eV by doping with high concentrations of Li and O, respectively. This theoretical work provides an effective and promising method to realize high-performance MoS₂ nanodevices with negligible SBHs.

Gate-controlled reversible rectifying behaviour in tunnel contacted atomically-thin MoS2 transistor

Atomically thin two-dimensional semiconducting materials integrated into van der Waals heterostructures have enabled architectures that hold great promise for next generation nanoelectronics. However, challenges still remain to enable their applications as compliant materials for integration in logic devices. Here, we devise a reverted stacking technique to intercalate a wrinkle-free boron nitride tunnel layer between MoS₂ channel and source drain electrodes. Vertical tunnelling of electrons therefore makes it possible to suppress the Schottky barriers and Fermi level pinning, leading to homogeneous gate-control of the channel chemical potential across the bandgap edges. The observed features of ambipolar pn to np diode, which can be reversibly gate tuned, paves the way for future logic applications and high performance switches based on atomically thin semiconducting channel.

Van der Waals heterostructures of atomically thin materials hold promise for nanoelectronics. Here, the authors demonstrate a reverted stacking fabrication method for heterostructures and devise a vertical tunnel-contacted MoS₂ transistor, enabling gate tunable rectification and reversible pn to np diode behaviour.

Contact Engineering of Molybdenum Ditelluride Field Effect Transistors through Rapid Thermal Annealing

Understanding and engineering the interface between metal and two-dimensional materials are of great importance to the research and development of nanoelectronics. In many cases the interface of metal and 2D materials can dominate the transport behavior of the devices. In this study, we focus on the metal contacts of MoTe₂ (molybdenum ditelluride) FETs (field effect transistors) and demonstrate how to use post-annealing treatment to modulate their transport behaviors in a controlled manner. We have also carried out low temperature and transmission electron microscopy studies to understand the mechanisms behind the prominent effect of the annealing process. Changes in transport properties are presumably due to anti-site defects formed at the metal-MoTe₂ interface under elevated temperature. The study provides more insights into MoTe₂ field effect devices and suggests guidelines for future optimizations.

Electrical Contacts in Monolayer Arsenene Devices

Arsenene, arsenic analogue of graphene, as an emerging member of two-dimensional semiconductors (2DSCs), is quite promising in next-generation electronic and optoelectronic applications. The metal electrical contacts play a vital role in the charge transport and photoresponse processes of nanoscale 2DSC devices and even can mask the intrinsic properties of 2DSCs. Here, we present a first comprehensive study of the electrical contact properties of monolayer (ML) arsenene with different electrodes by using ab initio electronic calculations and quantum transport simulations. Schottky barrier is always formed with bulk metal contacts owing to the Fermi level pinning (pinning factor S = 0.33), with electron Schottky barrier height (SBH) of 0.12, 0.21, 0.25, 0.35, and 0.50 eV for Sc, Ti, Ag, Cu, and Au contacts and hole SBH of 0.75 and 0.78 eV for Pd and Pt contacts, respectively. However, by contact with 2D graphene, the Fermi level pinning effect can be reduced due to the suppression of metal-induced gap states. Remarkably, a barrier free hole injection is realized in ML arsenene device with graphene-Pt hybrid electrode, suggestive of a high device performance in such a ML arsenene device. Our study provides a theoretical foundation for the selection of favorable electrodes in future ML arsenene devices.

Impact of Metal Contacts on the Performance of Multilayer HfS2 Field-Effect Transistors

HfS₂ is one of the emerging transition metal dichalcogenides and is very promising for low-power nanoelectronics and high-sensitivity optoelectronic device applications. We studied the band structures of 1T-HfS₂ with different thicknesses by first principles simulation, and the impact of different metal contacts to the HfS₂ device performance has been experimentally studied. Back-gate and top-gate HfS₂ field-effect transistors (FETs) were fabricated, and better electrical characteristics have been achieved with the FETs with the Ti/Au contact as compared with the Pt-contacted FETs. Thin layers of Pt and Ti/Au films were deposited on HfS₂ flakes to investigate the metal/HfS₂ interface by using scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. A smoother Ti/Au film was formed on HfS₂, resulting in higher carrier injection and transport efficiency. The phonon behavior being dominated by the interface chemical bonding at the Ti/Au contact region has been confirmed with the more sensitive A_1g phonon mode from the bilayer HfS₂.

Defect Dominated Charge Transport and Fermi Level Pinning in MoS2/Metal Contacts

Understanding the electronic contact between molybdenum disulfide (MoS₂) and metal electrodes is vital for the realization of future MoS₂-based electronic devices. Natural MoS₂ has the drawback of a high density of both metal and sulfur defects and impurities. We present evidence that subsurface metal-like defects with a density of ∼10¹¹ cm^-2 induce negative ionization of the outermost S atom complex. We investigate with high-spatial-resolution surface characterization techniques the effect of these defects on the local conductance of MoS₂. Using metal nanocontacts (contact area < 6 nm²), we find that subsurface metal-like defects (and not S-vacancies) drastically decrease the metal/MoS₂ Schottky barrier height as compared to that in the pristine regions. The magnitude of this decrease depends on the contact metal. The decrease of the Schottky barrier height is attributed to strong Fermi level pinning at the defects. Indeed, this is demonstrated in the measured pinning factor, which is equal to ∼0.1 at defect locations and ∼0.3 at pristine regions. Our findings are in good agreement with the theoretically predicted values. These defects provide low-resistance conduction paths in MoS₂-based nanodevices and will play a prominent role as the device junction contact area decreases in size.

A new insight for ohmic contacts to MoS2: by tuning MoS2 affinity energies but not metal work-functions

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have recently attracted tremendous interest for fundamental studies and applications.