Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides

A tunable floating-base bipolar transistor based on a 2D material homojunction realized using a solid ionic dielectric material

Floating-base bipolar transistors are widely used semiconductor devices because they could both amplify signal current and suppress noise. Employing two-dimensional (2D) materials of ultrahigh photoelectric properties could further improve the device performance. Due to the difficulty in doping, homojunctions are usually not realizable for many 2D materials. Instead, a heterojunction of various 2D materials of different Fermi levels is usually needed. However, the material interface of a heterojunction deteriorates device performance and makes the fabrication process difficult. Here, the doping difficulties have been solved by utilizing a solid ionic dielectric material (LiTaO3) and a floating-base bipolar transistor based on a 2D material (monolayer MoS2 here) homojunction is realized. The transistor shows tunable ambipolar transport characteristics. Particularly, under illumination, the amplification coefficient of a phototransistor can be optimized by changing the gate voltage. The optimized photoresponsivity of the device could reach up to 7.9 A W-1 with an ultrahigh detectivity of 3.39 × 1011 Jones. The overall fabrication processing is compatible to conventional processing. This design can effectively extend the application of 2D materials.

Edge Contact for Carrier Injection and Transport in MoS2 Field-Effect Transistors

The contact properties of van der Waals layered semiconducting materials are not adequately understood, particularly for edge contact. Edge contact is extremely helpful in the case of graphene, for producing efficient contacts to vertical heterostructures, and for improving the contact resistance through strong covalent bonding. Herein, we report on edge contacts to MoS₂ of various thicknesses. The carrier-type conversion is robustly controlled by changing the flake thickness and metal work functions. Regarding the ambipolar behavior, we suggest that the carrier injection is segregated in a relatively thick MoS₂ channel; that is, electrons are in the uppermost layers, and holes are in the inner layers. Calculations reveal that the strength of the Fermi-level pinning (FLP) varies layer-by-layer, owing to the inhomogeneous carrier concentration, and particularly, there is negligible FLP in the inner layer, supporting the hole injection. The contact resistance is large despite the significantly reduced contact resistivity normalized by the contact area, which is attributed to the current-crowding effect arising from the narrow contact area.

Highly efficient broadband photodetectors based on lithography-free Au/Bi2O2Se/Au heterostructures

As one of the bismuth-based oxychalcogenide materials, Bi₂O₂Se ultrathin films have received intense research interest due to their high carrier mobility, narrow bandgaps, ultrafast intrinsic photoresponse and long-term ambient stability; they exhibit great potential in electronic and optoelectronic applications. However, the device performance of photodetectors based on metal/Bi₂O₂Se/metal structures has degraded due to the undesirable defects or contaminants from the electrode deposition or the sample transfer processes. In this work, highly efficient photodetectors based on Au/Bi₂O₂Se junctions were achieved with Au electrodes transferred under the assistance of a probe tip to avoid contaminants from traditional lighography methods. Furthermore, to improve the charge transfer efficiency, specifically by increasing the intensity of the electrical field at the Au/Bi₂O₂Se interface and along the Bi₂O₂Se channels, the device annealing temperature was optimized to narrow the van der Waals gap at the Au/Bi₂O₂Se interface and the device channel length was shortened to improve the overall device performance. Among all the devices, the maximum device photoresponsivity was 9.1 A W^-1, and the device response time could approach 36 μs; moreover, the photodetectors featured broadband spectral responses from 360 nm to 1090 nm.

Monolayer MoS2 field effect transistor with low Schottky barrier height with ferromagnetic metal contacts

Two-dimensional MoS₂ has emerged as promising material for nanoelectronics and spintronics due to its exotic properties. However, high contact resistance at metal semiconductor MoS₂ interface still remains an open issue. Here, we report electronic properties of field effect transistor devices using monolayer MoS₂ channels and permalloy (Py) as ferromagnetic (FM) metal contacts. Monolayer MoS₂ channels were directly grown on SiO₂/Si substrate via chemical vapor deposition technique. The increase in current with back gate voltage (V_g) shows the tunability of FET characteristics. The Schottky barrier height (SBH) estimated for Py/MoS₂ contacts is found to be +28.8 meV (at V_g = 0V), which is the smallest value reported so-far for any direct metal (magnetic or non-magnetic)/monolayer MoS₂ contact. With the application of positive gate voltage, SBH shows a reduction, which reveals ohmic behavior of Py/MoS₂ contacts. Low SBH with controlled ohmic nature of FM contacts is a primary requirement for MoS₂ based spintronics and therefore using directly grown MoS₂ channels in the present study can pave a path towards high performance devices for large scale applications.

The Critical Role of Electrolyte Gating on the Hydrogen Evolution Performance of Monolayer MoS2

According to density functional theory, monolayer (ML) MoS₂ is predicted to possess electrocatalytic activity for the hydrogen evolution reaction (HER) that approaches that of platinum. However, its observed HER activity is much lower, which is widely believed to result from a large Schottky barrier between ML MoS₂ and its electrical contact. In order to better understand the role of contact resistance in limiting the performance of ML MoS₂ HER electrocatalysts, this study has employed well-defined test platforms that allow for the simultaneous measurement of contact resistance and electrocatalytic activity toward the HER during electrochemical testing. At open circuit potential, these measurements reveal that a 0.5 M H₂SO₄ electrolyte can act as a strong p-dopant that depletes free electrons in MoS₂ and leads to extremely high contact resistance, even if the contact resistance of the as-made device in air is originally very low. However, under applied negative potentials this doping is mitigated by a strong electrolyte-mediated gating effect which can reduce the contact and sheet resistances of properly configured ML MoS₂ electrocatalysts by more than 5 orders of magnitude. At potentials relevant to HER, the contact resistance becomes negligible and the performance of MoS₂ electrodes is limited by HER kinetics. These findings have important implications for the design of low-dimensional semiconducting electrocatalysts and photocatalysts.

Improved Sensitivity in Schottky Contacted Two-Dimensional MoS2 Gas Sensor

Two-dimensional (2D) transition-metal dichalcogenides have attracted significant attention as gas-sensing materials owing to their superior responsivity at room temperature and their possible application as flexible electronic devices. Especially, reliable responsivity and selectivity for various environmentally harmful gases are the main requirements for the future chemiresistive-type gas sensor applications. In this study, we demonstrate improved sensitivity of a 2D MoS₂-based gas sensor by controlling the Schottky barrier height. Chemical vapor deposition process was performed at low temperature to obtain layer-controlled 2D MoS₂, and the NO₂ gas responsivity was confirmed by the fabricated gas sensor. Then, the number of MoS₂ layers was fixed and the types of electrode materials were varied for controlling the Schottky barrier height. As the Schottky barrier height increased, the NO₂ responsivity increased, and it was found to be effective for CO and CO₂ gases, which had little reactivity in 2D MoS₂-based gas sensors.

Effects of Annealing Temperature and Ambient on Metal/PtSe2 Contact Alloy Formation

Forming gas annealing is a common process step used to improve the performance of devices based on transition-metal dichalcogenides (TMDs). Here, the impact of forming gas anneal is investigated for PtSe₂-based devices. A range of annealing temperatures (150, 250, and 350 °C) were used both in inert (0/100% H₂/N₂) and forming gas (5/95% H₂/N₂) environments to separate the contribution of temperature and ambient. The samples are electrically characterized by circular transfer length method structures, from which contact resistance and sheet resistance are analyzed. Ti and Ni are used as metal contacts. Ti does not react with PtSe₂ at any given annealing step. In contrast to this, Ni reacts with PtSe₂, resulting in a contact alloy formation. The results are supported by a combination of X-ray photoelectron spectroscopy, Raman spectroscopy, energy-dispersive X-ray spectroscopy, and cross-sectional transmission electron microscopy. The work sheds light on the impact of forming gas annealing on TMD-metal interfaces, and on the TMD film itself, which could be of great interest to improve the contact resistance of TMD-based devices.

Approaching ohmic contact to two-dimensional semiconductors

Two-dimensional semiconductors have attracted immense research interests owing to their intriguing properties and promising applications in electronic and optoelectronic devices. However, the performance of these devices is drastically hindered by the large Schottky barrier at the electric contact interface, which is hardly tunable due to the Fermi level pinning effect. In this review, we will analyze the root causes of the contact problems for the two-dimensional semiconductor devices and summarize the strategies on the basis of different contact geometries, aiming to lift out the Fermi level pinning effect and achieve the ohmic contact. Moreover, the remarkable improvement of the device performance thanks to these optimized contacts will be emphasized. At the end, the merits and limitations of these strategies will be discussed as well, which potentially gives a guideline for handling the electric contact issues in two-dimensional semiconductors devices.

Intimate Ohmic contact to two-dimensional WSe2 via thermal alloying

The most important interface in semiconductor devices is the interface between the semiconductor and the first layer of the metal contact. However, the van der Waals (vdWs) gap in two-dimensional (2D) materials hindered the formation of an intimate contact between the 2D material and the metal electrode, limiting the device performances. We demonstrated a gapless Ohmic contact to 2D WSe₂ by forming a Pt-W-Se alloy, which significantly improved the device performances (contact resistance, current on/off ratio, output current density, field-effect mobility, and hysteresis) of the 2D WSe₂ field-effect transistor. The contact resistance to 2D WSe₂ was reduced by more than seven orders of magnitude after thermal alloying. The disappearance of the vdW gap confirmed by scanning transmission electron microscopy enhanced the hole conduction and quenched the electron conduction. Our strategy of metallurgical alloying is effective to form a low-resistance stable Ohmic contact to WSe₂, which paves the way for utilization of the full potential of 2D materials.

Metallic contact induced van der Waals gap in a MoS2 FET

Electrical metal contacts formed with 2D materials strongly affect device performance. Here, we used scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS) to characterize the interfacial structure formed and physical damage induced between MoS2 and the most commonly used metals, Ti, Cr, Au, and Pd. We further correlated the electrical performance with physical defects observed at the 2D interfacial structure. The contact resistances were higher in the order of Ti, Au, Pd, and Cr contacts, but all 4-point probe mobilities measured with metals in contact with identical quadrilayer MoS2 were ∼65 cm2 V-1 s-1, confirming the reliability of the devices. According to the STEM and EDS analyses, the Ti contact gave rise to a van der Waals gap between the clean quadrilayer MoS2 and the Ti contact. By contrast, Cr migrated into MoS2 while Mo and S counter-migrated into the SiO2 substrate. Au and Pd formed glassy layers that resulted in the migration of Mo and S into the Au and Pd electrodes. These interfacial structures between MoS2 and contact metals strongly correlated with the electrical performance of 2D MoS2 FETs, providing practical guidelines to form van der Waals contacts.

Barrier Formation at the Contacts of Vanadium Dioxide and Transition-Metal Dichalcogenides

Phase-transition field-effect transistors (FETs) are a class of steep-slope devices that show abrupt on/off switching owing to the metal-insulator transition (MIT) induced in the contacting materials. An important avenue to develop phase-transition FETs is to understand the charge injection mechanism at the junction of the contacting MIT materials and semiconductor channels. Here, toward the realization of high-performance phase-transition FETs, we investigate the contact properties of heterojunctions between semiconducting transition-metal dichalcogenides (TMDCs) and vanadium dioxide (VO₂) that undergoes a MIT at a critical temperature (T_c) of approximately 340 K. We fabricated transistors based on molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂) in contact with the VO₂ source/drain electrodes. The VO₂-contacted MoS₂ transistor exhibited n-type transport both below and above T_c. Across the MIT, the on-current was observed to increase only by a factor of 5, in contrast to the order-of-magnitude change in the resistance of the VO₂ electrodes, suggesting the existence of high contact resistance. The Arrhenius analyses of the gate-dependent drain current confirmed the formation of the interfacial barrier at the VO₂/MoS₂ contacts, irrespective of the phase state of VO₂. The VO₂-contacted WSe₂ transistor showed ambipolar transport, indicating that the Fermi level lies near the mid gap of WSe₂. These observations provide insights into the contact properties of phase-transition FETs based on VO₂ and TMDCs and suggest the need for contact engineering for high-performance operations.

Light-Induced Surface Potential Modification in MoS2 Monolayers on Au Nanostripe Arrays

In this work, the surface potential (V_S) of exfoliated MoS₂ monolayers on Au nanostripe arrays with period of 500 nm was investigated using Kelvin probe force microscopy. The surface morphology showed that the suspended MoS₂ region between neighboring Au stripes underwent tensile-strain. In the dark, the V_S of the MoS₂ region on the Au stripe (V_S-Au) was larger than that of the suspended MoS₂ region (V_S-S). However, under green light illumination, V_S-Au became smaller than V_S-S. To explain the V_S modification, band diagrams have been constructed taking into consideration not only the local strain but also the electronic interaction at the MoS₂/Au interface. The results of this work provide a basis for understanding the electrical properties of MoS₂-metal contacts and improving the performance of MoS₂-based optoelectronic devices.

Graphene surface contacts of tin disulfide transistors for switching performance improvement and contact resistance reduction

We investigated the performance improvement of tin disulfide channel transistors by graphene contact configurations. From its two-dimensional nature, graphene can make electric contacts only at the outermost layers of the channel. For intralayer current flow, two graphene flakes are contacted at the channel's top or bottom layer. For interlayer current flow, one flake is contacted at the top and bottom of each layer. We compared the transistor performance in terms of current magnitude, mobility, and subthreshold swing between the configurations. From such observations, we deduced that device characteristics depend on resistivity or doping level of individual graphene flakes. We also found that interlayer flow excels in the on-current magnitude and the mobility, and that top-contact configuration excels in the subthreshold swing.

Scaled-up Direct-Current Generation in MoS2 Multilayer-Based Moving Heterojunctions

Techniques for scaling-up the direct-current (dc) triboelectricity generation in MoS₂ multilayer-based Schottky nanocontacts are vital for exploiting the nanoscale phenomenon for real-world applications of energy harvesting and sensing. Here, we show that scaling-up the dc output can be realized by using various MoS₂ multilayer-based heterojunctions including metal/semiconductor (MS), metal/insulator (tens of nanometers)/semiconductor (MIS), and semiconductor/insulator (a few nanometers)/semiconductor (SIS) moving structures. It is shown that the tribo-excited energetic charge carriers can overcome the interfacial potential barrier by different mechanisms, such as thermionic emission, defect conduction, and quantum tunneling in the case of MS, MIS, and SIS moving structures. By tailoring the interface structure, it is possible to trigger electrical conduction resulting in optimized power output. We also show that the band bending in the surface-charged region of MoS₂ determines the direction of the dc power output. Our experimental results show that engineering the interface structure opens up new avenues for developing next-generation semiconductor-based mechanical energy conversion with high performance.

Ultralow Schottky Barrier Height Achieved by Using Molybdenum Disulfide/Dielectric Stack for Source/Drain Contact

Energy barrier formed at a metal/semiconductor interface is a critical factor determining the performance of nanoelectronic devices. Although diverse methods for reducing the Schottky barrier height (SBH) via interface engineering have been developed, it is still difficult to achieve both an ultralow SBH and a low dependence on the contact metals. In this study, a novel structure, namely, a metal/transition-metal dichalcogenide (TMD) interlayer (IL)/dielectric IL/semiconductor (MTDS) structure, was developed to overcome these issues. Molybdenum disulfide (MoS₂) is a promising TMD IL material owing to its interface characteristics, which yields a low SBH and reduces the reliance on contact metals. Moreover, an ultralow SBH is achieved via the insertion of an ultrathin ZnO layer between MoS₂ and a semiconductor, thereby inducing an n-type doping effect on the MoS₂ IL and forming an interface dipole in the favorable direction at the ZnO IL/semiconductor interfaces. Consequently, the lowest SBH (0.07 eV) and a remarkable improvement in the reverse current density (by a factor of approximately 5400) are achieved, with a wide room for contact-metal dependence. This study experimentally and theoretically validates the effect of the proposed MTDS structure, which can be a key technique for next-generation nanoelectronics.

Contact Engineering High-Performance n-Type MoTe2 Transistors

Semiconducting MoTe₂ is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains underexplored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe₂ transistors with the highest performance to date, including high saturation current (>400 μA/μm at 80 K and >200 μA/μm at 300 K) and relatively low contact resistance (1.2 to 2 kΩ·μm from 80 to 300 K), achieved with Ag contacts and AlO_x encapsulation. We also investigate other contact metals (Sc, Ti, Cr, Au, Ni, Pt), extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer hexagonal boron nitride between MoTe₂ and Sc. These metal-insulator-semiconductor (MIS) contacts partly depin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.

A Horizontal-Gate Monolayer MoS2 Transistor Based on Image Force Barrier Reduction

Transition metal dichalcogenides (TMDCs) have received wide attention as a new generation of semiconductor materials. However, there are still many problems to be solved, such as low carrier mobility, contact characteristics between metal and two-dimensional materials, and complicated fabrication processes. In order to overcome these problems, a large amount of research has been carried out so that the performance of the device has been greatly improved. However, most of these studies are based on complicated fabrication processes which are not conducive to the improvement of integration. In view of this problem, a horizontal-gate monolayer MoS₂ transistor based on image force barrier reduction is proposed, in which the gate is in the same plane as the source and drain and comparable to back-gated transistors on-off ratios up to 1 × 10⁴ have been obtained. Subsequently, by combining the Y-Function method (YFM) and the proposed diode equivalent model, it is verified that Schottky barrier height reduction is the main reason giving rise to the observed source-drain current variations. The proposed structure of the device not only provides a new idea for the high integration of two-dimensional devices, but also provides some help for the study of contact characteristics between two-dimensional materials and metals.

Versatile Electronic Devices Based on WSe2/SnSe2 Vertical van der Waals Heterostructures

Van der Waals heterostructures formed by stacking of various two-dimensional materials are promising in electronic applications. However, the performances of most reported electronic devices based on van der Waals heterostructures are far away from those of existing (Si, Ge, and III-V bulk material based) technologies. Here, we report high-performance heterostructure devices based on vertically stacked tungsten diselenide and tin diselenide. Due to the unique band alignment and the atomic thickness of the material, both charge carrier transport and energy barrier can be effectively modulated by the applied electrical field. As a result, the heterostructure devices show superb characteristics, with a high current on/off ratio of ∼3 × 10⁸, an average subthreshold slope of 126 mV/dec over 5 dec of current change due to band-to-band tunneling, an ultrahigh rectification ratio of ∼3 × 10⁸, and a current density of more than 10⁴ A/cm². Furthermore, a small signal half-wave rectifier circuit based on a majority-carrier-transport-dominated diode is successfully demonstrated, showing great potential in future high-speed electronic applications.

Reconfigurable two-dimensional optoelectronic devices enabled by local ferroelectric polarization

Ferroelectric engineered pn doping in two-dimensional (2D) semiconductors hold essential promise in realizing customized functional devices in a reconfigurable manner. Here, we report the successful pn doping in molybdenum disulfide (MoS2) optoelectronic device by local patterned ferroelectric polarization, and its configuration into lateral diode and npn bipolar phototransistors for photodetection from such a versatile playground. The lateral pn diode formed in this way manifests efficient self-powered detection by separating ~12% photo-generated electrons and holes. When polarized as bipolar phototransistor, the device is customized with a gain ~1000 by its transistor action, reaching the responsivity ~12 A W-1 and detectivity over 1013 Jones while keeping a fast response speed within 20 μs. A promising pathway toward high performance optoelectronics is thus opened up based on local ferroelectric polarization coupled 2D semiconductors.

MoTe2 Lateral Homojunction Field-Effect Transistors Fabricated using Flux-Controlled Phase Engineering

The coexistence of metallic and semiconducting polymorphs in transition-metal dichalcogenides (TMDCs) can be utilized to solve the large contact resistance issue in TMDC-based field effect transistors (FETs). A semiconducting hexagonal (2H) molybdenum ditelluride (MoTe₂) phase, metallic monoclinic (1T') MoTe₂ phase, and their lateral homojunctions can be selectively synthesized in situ by chemical vapor deposition due to the small free energy difference between the two phases. Here, we have investigated, in detail, the structural and electrical properties of in situ-grown lateral 2H/1T' MoTe₂ homojunctions grown using flux-controlled phase engineering. Using atomic-resolution plan-view and cross-sectional transmission electron microscopy analyses, we show that the round regions of near-single-crystalline 2H-MoTe₂ grow out of a polycrystalline 1T'-MoTe₂ matrix. We further demonstrate the operation of MoTe₂ FETs made on these in situ-grown lateral homojunctions with 1T' contacts. The use of a 1T' phase as electrodes in MoTe₂ FETs effectively improves the device performance by substantially decreasing the contact resistance. The contact resistance of 1T' electrodes extracted from transfer length method measurements is 470 ± 30 Ω·μm. Temperature- and gate-voltage-dependent transport characteristics reveal a flat-band barrier height of ∼30 ± 10 meV at the lateral 2H/1T' interface that is several times smaller and shows a stronger gate modulation, compared to the metal/2H Schottky barrier height. The information learned from this analysis will be critical to understanding the properties of MoTe₂ homojunction FETs for use in memory and logic circuity applications.

Short channel monolayer MoS2 field-effect transistors defined by SiO x nanofins down to 20 nm

Layered semiconductors such as transition metal dichalcogenides (TMDs) with proper bandgaps complement the zero-bandgap drawback of graphene, demonstrating great potential for post-silicon complementary metal-oxide-semiconductor technology. Among the TMD family, molybdenum disulfide (MoS₂) is highly attractive for its atomically thin body, large bandgap and decent mechanical and chemical stability. However, current nanofabrication techniques hardly satisfy the requirements of short channel and convenient preparation simultaneously. Here, we demonstrate a simple and effective approach to fabricate short channel chemical vapor deposition (CVD) monolayer MoS₂ field-effect transistors (FET) with channel length down to 20 nm. Electron-beam lithography based on high-resolution negative-tone hydrogen silsesquioxane electron resists were applied to create 20 nm wide SiO _x lines, defining the short channel length. The 20 nm MoS₂ FET displays ON-sate current in excess of 100 μA μm^-1. The corresponding current ON/OFF ratio at room temperature reaches 10⁵. We carefully studied the short channel effect of as-fabricated MoS₂ FETs. Combining with the large-scale growth of CVD method, our results will pave a way for short channel device applications based on atomically thin two-dimensional semiconductors.

All-2D ReS2 transistors with split gates for logic circuitry

Two-dimensional (2D) semiconductors, such as transition metal dichalcogenides (TMDs) and black phosphorus, are the most promising channel materials for future electronics because of their unique electrical properties. Even though a number of 2D-materials-based logic devices have been demonstrated to date, most of them are a combination of more than two unit devices. If logic devices can be realized in a single channel, it would be advantageous for higher integration and functionality. In this study we report high-performance van der Waals heterostructure (vdW) ReS₂ transistors with graphene electrodes on atomically flat hBN, and demonstrate a NAND gate comprising a single ReS₂ transistor with split gates. Highly sensitive electrostatic doping of ReS₂ enables fabrication of gate-tunable NAND logic gates, which cannot be achieved in bulk semiconductor materials because of the absence of gate tunability. The vdW heterostructure NAND gate comprising a single transistor paves a novel way to realize “all-2D” circuitry for flexible and transparent electronic applications.

Controllable P- and N-Type Conversion of MoTe2 via Oxide Interfacial Layer for Logic Circuits

To realize basic electronic units such as complementary metal-oxide-semiconductor (CMOS) inverters and other logic circuits, the selective and controllable fabrication of p- and n-type transistors with a low Schottky barrier height is highly desirable. Herein, an efficient and nondestructive technique of electron-charge transfer doping by depositing a thin Al₂ O₃ layer on chemical vapor deposition (CVD)-grown 2H-MoTe₂ is utilized to tune the doping from p- to n-type. Moreover, a type-controllable MoTe₂ transistor with a low Schottky barrier height is prepared. The selectively converted n-type MoTe₂ transistor from the p-channel exhibits a maximum on-state current of 10 µA, with a higher electron mobility of 8.9 cm² V^-1 s^-1 at a drain voltage (V_ds ) of 1 V with a low Schottky barrier height of 28.4 meV. To validate the aforementioned approach, a prototype homogeneous CMOS inverter is fabricated on a CVD-grown 2H-MoTe₂ single crystal. The proposed inverter exhibits a high DC voltage gain of 9.2 with good dynamic behavior up to a modulation frequency of 1 kHz. The proposed approach may have potential for realizing future 2D transition metal dichalcogenide-based efficient and ultrafast electronic units with high-density circuit components under a low-dimensional regime.

Electron Transport through Metal/MoS2 Interfaces: Edge- or Area-Dependent Process?

In ultrathin two-dimensional (2-D) materials, the formation of ohmic contacts with top metallic layers is a challenging task that involves different processes than in bulk-like structures. Besides the Schottky barrier height, the transfer length of electrons between metals and 2-D monolayers is a highly relevant parameter. For MoS₂, both short (≤30 nm) and long (≥0.5 μm) values have been reported, corresponding to either an abrupt carrier injection at the contact edge or a more gradual transfer of electrons over a large contact area. Here we use ab initio quantum transport simulations to demonstrate that the presence of an oxide layer between a metallic contact and a MoS₂ monolayer, for example, TiO₂ in the case of titanium electrodes, favors an area-dependent process with a long transfer length, while a perfectly clean metal-semiconductor interface would lead to an edge process. These findings reconcile several theories that have been postulated about the physics of metal/MoS₂ interfaces and provide a framework to design future devices with lower contact resistances.

Controlling Injection Barriers for Ambipolar 2D Semiconductors via Quasi-van der Waals Contacts

Barriers that charge carriers experience while injecting into channels play a crucial role on determining the device properties of van der Waals semiconductors (vdWS). Among various strategies to control these barriers, inserting a graphene layer underneath bulk metal may be a promising choice, which is still lacking experimental verification. Here, it is demonstrated that graphene/metal hybrid structures can form quasi-van der Waals contacts (q-vdWC) to ambipolar vdWS, combining the advantages of individual metal and graphene contacts together. A new analysis model is adopted to define the barriers and to extract the barrier heights in ambipolar vdWS. The devices with q-vdWC show significantly reduced Schottky barrier heights and thermionic field emission activation energies, ability of screening the influence from substrate, and Fermi level unpinning effect. Furthermore, phototransistors with these special contacts exhibit enhanced performances. The proposed graphene/metal q-vdWC may be an effective strategy to approach the Schottky-Mott limit for vdWS.

Tuning the Polarity of MoTe2 FETs by Varying the Channel Thickness for Gas-Sensing Applications

In this study, electrical characteristics of MoTe2 field-effect transistors (FETs) are investigated as a function of channel thickness. The conductivity type in FETs, fabricated from exfoliated MoTe2 crystals, switched from p-type to ambipolar to n-type conduction with increasing MoTe2 channel thickness from 10.6 nm to 56.7 nm. This change in flake-thickness-dependent conducting behavior of MoTe2 FETs can be attributed to modulation of the Schottky barrier height and related bandgap alignment. Change in polarity as a function of channel thickness variation is also used for ammonia (NH3) sensing, which confirms the p- and n-type behavior of MoTe2 devices.

A Fermi-Level-Pinning-Free 1D Electrical Contact at the Intrinsic 2D MoS2 -Metal Junction

Currently 2D crystals are being studied intensively for use in future nanoelectronics, as conventional semiconductor devices face challenges in high power consumption and short channel effects when scaled to the quantum limit. Toward this end, achieving barrier-free contact to 2D semiconductors has emerged as a major roadblock. In conventional contacts to bulk metals, the 2D semiconductor Fermi levels become pinned inside the bandgap, deviating from the ideal Schottky-Mott rule and resulting in significant suppression of carrier transport in the device. Here, MoS₂ polarity control is realized without extrinsic doping by employing a 1D elemental metal contact scheme. The use of high-work-function palladium (Pd) or gold (Au) enables a high-quality p-type dominant contact to intrinsic MoS₂ , realizing Fermi level depinning. Field-effect transistors (FETs) with Pd edge contact and Au edge contact show high performance with the highest hole mobility reaching 330 and 432 cm² V^-1 s^-1 at 300 K, respectively. The ideal Fermi level alignment allows creation of p- and n-type FETs on the same intrinsic MoS₂ flake using Pd and low-work-function molybdenum (Mo) contacts, respectively. This device acts as an efficient inverter, a basic building block for semiconductor integrated circuits, with gain reaching 15 at V_D = 5 V.

Environmentally Controlled Charge Carrier Injection Mechanisms of Metal/WS2 Junctions

Here we report on a novel, noninvasive route for operando tailoring of the charge transport properties of metal/WS₂ contacts without the negative impacts to two-dimensional materials arising from conventional doping methods. The doping level of thin WS₂ flakes supported on insulating mica is susceptible to local charge variations induced by the presence of a hydration layer between mica and WS₂. We demonstrate, via the use of several complementary scanning probe techniques, that the direct control of the state and thickness of this intercalated water film controls the charge injection properties of Pt/WS₂ nanocontacts. A switch from unipolar to ambipolar transport was achieved by environmentally controlling the thickness of the intercalated water. We show that the effect persists even for multilayer flakes and that it is completely reversible, opening a new route toward the realization of novel electronics with environmentally controllable functionalities.

MoTe2 van der Waals homojunction p-n diode with low resistance metal contacts

Although many studies have focused on transition metal dichalcogenide heterojunction p-n diodes, homojunction p-n diodes still require more extensive study. We present a van der Waals p-MoTe2/n-MoTe2 homojunction p-n diode with low resistance metal contacts. Such two-dimensional homojunction devices with low contact resistance can be used in various applications in the electronics industry. The device structure consists of stacked nanoflakes of p-MoTe2 and n-MoTe2. In this investigation, we implement a deep ultraviolet light-driven doping technique in a N2 gas environment to modulate the carrier concentration in a multilayered p-MoTe2 flake, which is consequently inverted to n-MoTe2. The deep ultraviolet light-driven doping provides environmental stability in the treated devices. We use ohmic metal contacts for the homojunction p-n diode and achieve excellent gate-dependent rectifying behavior with a rectification ratio of up to 104. Contrary to heterojunctions, the ideality factor is found to be 1.05 for the zero gate bias, indicative of good interface quality at the p-MoTe2/n-MoTe2 junction, owing to low charge trapping sites at the homojunction interface. In addition, low-temperature measurements are performed to determine the barrier height for different gate biases. This study contributes to research on van der Waals homojunction p-n diodes, which show much potential for nanoelectronic and optoelectronic devices.

Dynamically controllable polarity modulation of MoTe2 field-effect transistors through ultraviolet light and electrostatic activation

Energy band engineering is of fundamental importance in nanoelectronics. Compared to chemical approaches such as doping and surface functionalization, electrical and optical methods provide greater flexibility that enables continuous, reversible, and in situ band tuning on electronic devices of various kinds. In this report, we demonstrate highly effective band modulation of MoTe₂ field-effect transistors through the combination of electrostatic gating and ultraviolet light illumination. The scheme can achieve reversible doping modulation from deep n-type to deep p-type with ultrafast switching speed. The treatment also enables noticeable improvement in field-effect mobility by roughly 30 and 2 times for holes and electrons, respectively. The doping scheme also provides good spatial selectivity and allows the building of a photo diode on a single MoTe₂ flake with excellent photo detection and photovoltaic performances. The findings provide an effective and generic doping approach for a wide variety of 2D materials.

Modulating the Functions of MoS2/MoTe2 van der Waals Heterostructure via Thickness Variation

Various functional devices including p-n forward, backward, and Zener diodes are realized with a van der Waals heterostructure that are composed of molybdenum disulfide (MoS₂) and molybdenum ditelluride (MoTe₂) by changing the thickness of the MoTe₂ layer and common gate bias. In addition, the available negative differential transconductance of the heterostructure is utilized to fabricate a many-valued logic device that exhibits three different logic states ( i.e., a ternary inverter). Furthermore, the multivalued logic device can be transformed into a binary inverter using laser irradiation. This work provides a comprehensive understanding of the device fabrication and electronic-device design utilizing thickness control.

Monolayer MoS2 Nanoribbon Transistors Fabricated by Scanning Probe Lithography

Monolayer MoS₂ is a promising material for nanoelectronics; however, the lack of nanofabrication tools and processes has made it very challenging to realize nanometer-scale electronic devices from monolayer MoS₂. Here, we demonstrate the fabrication of monolayer MoS₂ nanoribbon field-effect transistors as narrow as 30 nm using scanning probe lithography (SPL). The SPL process uses a heated nanometer-scale tip to deposit narrow nanoribbon polymer structures onto monolayer MoS₂. The polymer serves as an etch mask during a XeF₂ vapor etch, which defines the channel of a field-effect transistor (FET). We fabricated seven devices with a channel width ranging from 30 to 370 nm, and the fabrication process was carefully studied by electronic measurements made at each process step. The nanoribbon devices have a current on/off ratio > 10⁴ and an extrinsic field-effect mobility up to 8.53 cm²/(V s). By comparing a 30 nm wide device with a 60 nm wide device that was fabricated on the same MoS₂ flake, we found the narrower device had a smaller mobility, a lower on/off ratio, and a larger subthreshold swing. To our knowledge, this is the first published work that describes a working transistor device from monolayer MoS₂ with a channel width smaller than 100 nm.

Universal Fermi-Level Pinning in Transition-Metal Dichalcogenides

Understanding the electron transport through transition-metal dichalcogenide (TMDC)-based semiconductor/metal junctions is vital for the realization of future TMDC-based (opto-)electronic devices. Despite the bonding in TMDCs being largely constrained within the layers, strong Fermi-level pinning (FLP) was observed in TMDC-based devices, reducing the tunability of the Schottky barrier height. We present evidence that metal-induced gap states (MIGS) are the origin for the large FLP similar to conventional semiconductors. A variety of TMDCs (MoSe₂, WSe₂, WS₂, and MoTe₂) were investigated using high-spatial-resolution surface characterization techniques, permitting us to distinguish between defected and pristine regions. The Schottky barrier heights on the pristine regions can be explained by MIGS, inducing partial FLP. The FLP strength is further enhanced by disorder-induced gap states induced by transition-metal vacancies or substitutionals at the defected regions. Our findings emphasize the importance of defects on the electron transport properties in TMDC-based devices and confirm the origin of FLP in TMDC-based metal/semiconductor junctions.

Schottky Barrier Height Modulation Using Interface Characteristics of MoS2 Interlayer for Contact Structure

Schottky barrier height (SBH) engineering of contact structures is a primary challenge to achieve high performance in nanoelectronic and optoelectronic applications. Although SBH can be lowered through various Fermi-level (FL) unpinning techniques, such as a metal/interlayer/semiconductor (MIS) structure, the room for contact metal adoption is too narrow because the work function of contact metals should be near the conduction band edge (CBE) of the semiconductor to achieve low SBH. Here, we propose a novel structure, the metal/transition metal dichalcogenide/semiconductor structure, as a contact structure that can effectively lower the SBH with wide room for contact metal adoption. A perpendicularly integrated molybdenum disulfide (MoS₂) interlayer effectively alleviates FL pinning by reducing metal-induced gap states at the MoS₂/semiconductor interface. Additionally, it can induce strong FL pinning of contact metals near its CBE at the metal/MoS₂ interface. The technique using FL pinning and unpinning at metal/MoS₂/semiconductor interfaces is first introduced in the MIS scheme to allow the use of various contact metals. Consequently, significant reductions of the SBH from 0.48 to 0.12 eV for GaAs and from 0.56 to 0.10 eV for Ge are achieved with several different contact metals. This work significantly reduces the dependence on contact metals with lowest SBH and proposes a new way of overcoming current severe contact issues for future nanoelectronic and optoelectronic applications.

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.