Metal-semiconductor Interface Research Articles

Improvement of the Bias Stress Stability in 2D MoS2 and WS2 Transistors with a TiO2 Interfacial Layer.

The fermi-level pinning phenomenon, which occurs at the metal–semiconductor interface, not only obstructs the achievement of high-performance field effect transistors (FETs) but also results in poor long-term stability. This paper reports on the improvement in gate-bias stress stability in two-dimensional (2D) transition metal dichalcogenide (TMD) FETs with a titanium dioxide (TiO2) interfacial layer inserted between the 2D TMDs (MoS2 or WS2) and metal electrodes. Compared to the control MoS2, the device without the TiO2 layer, the TiO2 interfacial layer deposited on 2D TMDs could lead to more effective carrier modulation by simply changing the contact metal, thereby improving the performance of the Schottky-barrier-modulated FET device. The TiO2 layer could also suppress the Fermi-level pinning phenomenon usually fixed to the metal–semiconductor interface, resulting in an improvement in transistor performance. Especially, the introduction of the TiO2 layer contributed to achieving stable device performance. Threshold voltage variation of MoS2 and WS2 FETs with the TiO2 interfacial layer was ~2 V and ~3.6 V, respectively. The theoretical result of the density function theory validated that mid-gap energy states created within the bandgap of 2D MoS2 can cause a doping effect. The simple approach of introducing a thin interfacial oxide layer offers a promising way toward the implementation of high-performance 2D TMD-based logic circuits.

Electron directed migration cooperated with thermodynamic regulation over bimetallic NiFeP/g-C3N4 for enhanced photocatalytic hydrogen evolution

Bimetallic phosphides have attracted considerable attention in photocatalytic hydrogen evolution reaction (HER), owing to its thermodynamic feasibility. Most reports attributed the HER activity to the reduction of free energy of protonation (ΔGH*), however, few revealed the whereabouts of the photo-induced electrons, which makes the mechanism of HER not very clear. Herein, nickel-iron bimetallic phosphide (NiFeP) is designed onto graphitic carbon nitride (NiFeP/g-C3N4) for the photocatalytic HER, which exhibits a high hydrogen production of 3.549 mmol g−1 h−1. It is found that the co-catalyst of NiFeP can not only reduce the ΔGH*, but also make the photogenerated electrons transfer directionally, which play a synergistic role in enhancing the activity of HER. The reduction of ΔGH2O and ΔGH* over NiFeP in HER can be well demonstrated by the density functional theory (DFT) calculations. Furthermore, the localized distribution of electrons on the surface of NiFeP is verified by measuring the lifetime of photo-generated carriers combined with the REDOX experiment. The shortened lifetime τ, decreased τ1 and τ2 of NiFeP/g-C3N4 indicate that the photoexcited electrons of g-C3N4 are directly transferred to the co-catalyst surface, contributing to the worse delocalization capacity of electrons on the ca-catalyst and its easier reactions with protons trapped on the co-catalyst surface. Furthermore, the degradation with peroxymonosulfate (PMS) confirms the photogenerated holes over 5NiFeP/g-C3N4 are mainly concentrated on the surface of g-C3N4 to decompose H2O for the generation of OH, while the photogenerated electrons are directed to the NiFeP to react with the neighboring PMS or protons to form SO4− or H2. Besides, the density of state intensity (DOS) also confirms the photogenerated electron migration pathway at the metal-semiconductor (NiFeP-g-C3N4) interface. Our research clarifies the mechanism of bimetallic phosphides co-catalytic HER system, which will provide theoretical guidance for the design and preparation of multi-metallic center photocatalytic system with a higher HER activity in the future.

Barrier height inhomogeneities on Pd/n-4H-SiC Schottky diodes in a wide temperature range

Barrier height inhomogeneities on Pd/n-type 4H-SiC Schottky barrier diodes in the 300–800 K temperature range have been investigated. Palladium is known to form silicide above 673 K. Temperature dependent current-voltage (I-V) characteristics were analyzed. Barrier height (BH) and ideality factor (n) were found to be strongly temperature dependent. Barrier height increased, whilst ideality factor decreased with increasing in temperature and the Richardson plot showed some deviation from linearity. This was attributed to barrier inhomogeneities at the metal-semiconductor interface which resulted in a distribution of barrier heights. From the modified Richardson plot, the modified Richardson constant, A** was found to be 155 Acm−2K−2 and 87 Acm-2 K−2 in the 300–525 K and the 550–800 K temperature ranges respectively.

Characterization of inhomogeneous Ni/GaN Schottky diode with a modified log-normal distribution of barrier heights

The current versus voltage (I-V) characteristics of a Ni/GaN Schottky diode are measured from 50 to 400 K and the temperature dependence of the extracted barrier heights and ideality factors is described as a consequence of lateral inhomogeneity at the metal-semiconductor (M-S) interface. It is shown that by invoking a modified log-normal distribution of barrier heights at the M-S interface, the extracted barrier height temperature dependence can be well explained. Further, it is shown that this approach can describe the voltage dependence of the lateral barrier distribution revealing that for effective barrier height values calculated at increasingly higher voltages, the distribution begins to converge on a single value of 0.77 eV. This value is in good agreement with the flat-band barrier height of 0.77 ± 0.02 eV extracted from capacitance-voltage (C-V) measurements on the same device. The same procedure is used to describe the parallel conduction path apparent at low temperatures, revealing its behavior is indicative of an additional Schottky region with an increased density of low barriers which are more heavily perturbed by external bias. Finally, the model is successfully applied to previously published work on various Schottky diodes structures.

Synthesis of Ag/AgCl-modified TiO2/MgAl-layered double hydroxide nanocomposite with enhanced photocatalytic activity

A novel composite photocatalyst Ag/AgCl-modified TiO2/MgAl-layered double hydroxide (denominated as Ag/AgCl@TiO2-LDH) was synthesized via a combination of deposition-precipitation process and photoreduction method by using layered double hydroxide (LDH) platelets as substrate materials. The structure, morphology, and property of as-synthesized Ag/AgCl@TiO2-LDH were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller method (BET), UV–vis diffuse reflectance spectroscopy, etc. The photocatalytic property of Ag/AgCl@TiO2-LDH was tested for degradation of gaseous toluene under simulated sunlight irradiation. The composite catalyst exhibited an enhanced photodegradation activity with 92% of gaseous toluene being degraded. Through kinetics analysis, all degradation processes of gaseous toluene could be properly described by the pseudo-first-order kinetic model. The reaction rate constant of 40 mg Ag/AgCl@TiO2-LDH for the degradation of 172.05 mg m−3 gaseous toluene is 0.026 min−1. Furthermore, the catalyst could be reused for five cycles without obvious reduction in activity. The enhanced photocatalytic activity of Ag/AgCl@TiO2-LDH is attributed to sufficient hydroxide ions in LDH, the Shottky barrier at the metal-semiconductor interface, and surface plasma resonance of Ag/AgCl. The cooperative effect among components separate photogenerated electrons and holes effectively and thus improve the photodegradation efficiency.

Titanate Nanowires as One-Dimensional Hot Spot Generators for Broadband Au-TiO2 Photocatalysis.

Metal–semiconductor nanocomposites have become interesting materials for the development of new photocatalytic hybrids. Along these lines, plasmonic nanoparticles have proven to be particularly efficient photosensitizers due to their ability to transfer plasmonic hot electrons onto large bandgap semiconductors such as TiO2, thus extending the activity of the latter into a broader range of the electromagnetic spectrum. The extent of this photosensitization process can be substantially enhanced in those geometries in which high electromagnetic fields are created at the metal–semiconductor interface. In this manner, the formation of plasmonic hot spots can be used as a versatile tool to engineer the photosensitization process in this family of hybrid materials. Herein, we introduce the use of titanate nanowires as ideal substrates for the assembly of Au nanorods and TiO2 nanoparticles, leading to the formation of robust hybrids with improved photocatalytic properties. Our approach shows that the correct choice of the individual units together with their rational assembly are of paramount importance in the development of complex nanostructures with advanced functionalities.

Novel Electrodes and Engineered Interfaces for Halide-Semiconductor Radiation Detectors

A new class of inorganic halide semiconductors are emerging as high-efficiency low-cost candidates for spectroscopic radiation detection. We report on solving one of the major challenges of these halide radiation detectors. At room temperature halide semiconductor detectors polarize under applied electric field, which not only degrades the charge collection efficiency of the detectors, but also promotes chemical reaction of the metal electrodes with the halide ions. This increases the metal-semiconductor interface noise and early failure of the spectroscopic detection capabilities of the device. We report on a solution to this challenge by application of novel electrodes on Thallium Bromide (TlBr) radiation detectors with virtually defect-free electrode-semiconductor interfaces, showing low noise and high detection stability for an extended period of time under accelerated ageing conditions. A number of TlBr detectors fabricated by this technique have demonstrated continuous stable detection performance (e.g. ±1% change in 662 keV gamma channel) for more than 4000 hours at room temperature. This report also shows continuously recorded 137Cs gamma radiation response of a unidirectionally-biased pixelated TlBr detector over more than 2 months (a total of 2880 data sets), which exhibit excellent stability. The developed approach has resulted in unprecedented low-noise stable performance of halide semiconductor detectors at room temperature, overcoming one of the major obstacles to the full consideration of TlBr (and other halide semiconductors) as a potentially low-cost replacement for Cadmium Zinc Telluride (CZT).

Accumulated charge measurement using a substrate with a restricted-bottom-electrode structure

Accumulated charge measurement (ACM) is a new experimental technique for organic semiconductors to evaluate the charge injection barrier at the semiconductor–metal interface directly using a metal–insulator–semiconductor–metal (MISM) capacitor. In this technique, the precise estimation of the electrostatic capacity of the insulator layer (CI) is required for the analysis. The information of this parameter is, in principle, included in the ACM data; however, it is not directly evaluated because of the error resulting from the charge-spreading effect in an organic MISM capacitor with an unrestricted electrode structure. Therefore, the CI in previous ACM experiments has been independently estimated from the area of the electrode. In this study, a novel design of a substrate with a restricted-bottom-electrode structure is reported. Using the newly designed substrate, it was possible to suppress the charge-spreading effect and successfully estimate precise values of CI directly from the ACM data. Subsequently, it was possible to evaluate the injection barriers at the metal-free phthalocyanine (H2Pc)–Ag and pentacene–Au interfaces, which were 0.4 and 0.15 eV, respectively. The built-in potentials in the semiconductor layer were also determined for the samples used in the measurement.

Direct growth of flexible carbon nanotube interconnects during polymer-ceramic conversion reaction for release of thermal stress

Polymer-derived ceramics, which are a class of emerging ultrahigh-temperature semiconductor materials, face challenges when it comes to the semiconductor–metal interconnects. High thermal stress produced at the semiconductor–metal interface during ultrahigh-temperature applications (>1000 °C), owing to the mismatch in the coefficients of thermal expansion of the two materials, can readily lead to the mechanical failure of the semiconductor–metal interconnection. Herein, flexible carbon nanotube (CNT) interconnects were formed from Ni alloy (Ni95AlSiMn) wires to the SiCN semiconductor through the direct growth of CNT on the Ni alloy wires during the synthesis of SiCN itself by a polymer-ceramic conversion reaction. The catalytic Ni nanoparticles and the gaseous carbon sources needed for the growth of CNT were generated during the polymer-ceramic conversion reaction. Flexible CNT interconnects exhibited outstanding electrical characteristics and excellent interfacial strength. Further, the temperature dependence of the resistance of SiCN could be measured at temperatures of up to 1000 °C without significant mechanical failure or a decrease in the interfacial strength because the thermal stress was readily released by the flexible CNT interconnects. Thus, the fabricated CNT interconnects with excellent electrical, thermal, and thermo mechanical properties are highly suited for various ultrahigh-temperature applications.

Charge-Induced Lattice Compression in Monolayer MoS2

Electron beam or photonic excitation of localized surface plasmon resonances (LSPRs) in gold nanoparticles and their subsequent nonradiative relaxation create a nonuniform charge distribution near the metal–semiconductor interface via hot-electron injection from gold nanoparticles to the monolayer MoS2 conduction band states. These charge domains induce lattice compression in the MoS2 lattice because of the inverse piezoelectric response arising due to the noncentrosymmetric nature of the crystal lattice. Our high-resolution transmission electron microscopy studies reveal that as much as 9–10% biaxial compressive strain is generated in free-standing monolayer MoS2 in the vicinity of gold nanoparticles, which relaxes rapidly within 50 nm from the edge of the gold nanoparticle. The metal interface-induced hot-electron injection via photonic excitation of LSPRs and the resultant lattice distortion also manifest in shifts as well as broadening of A1g(Γ) and E2g1(Γ) Raman modes of the monolayer MoS2 in presenc...

Facet-Engineered Surfaces of Two-Dimensional Layered BiOI and Au–BiOI Substrates for Tuning the Surface-Enhanced Raman Scattering and Visible Light Photodetector Response

A simple and facile solution phase synthesis of two-dimensional (2D) layered BiOI single-crystalline square sheets at tunable reaction conditions yielded dominant exposed (001) facets. The bismuth oxyiodide (BiOI) structures analyzed by scanning electron microscopy and atomic force microscopy (AFM) reveal screw-dislocation-driven growth of spiral pyramid-like layer-by-layer (LBL) stacking of ultrathin BiOI sheet of about 2.5 nm. Vertically aligned, oriented Au nanotriangles (Au NTs) allow more analyte molecules to access their sharp tips to form hot-spots thereby enabling development of ultrasensitive plasmonic sensors. Injection of plasmonic hot carriers into the semiconductor at the plasmonic metal–semiconductor interface (Au–BiOI) fabricated under optimal conditions with Au NTs form tunable surface enhanced Raman scattering substrate with maximum enhancement factor on the order of 106 and 27 nanomol as a limit of detection. Electrical transport measurements on BiOI and Au–BiOI LBL 2D square sheet materials under applied pressure using conducting-AFM established the use of these nanosheets as a nanoscale pressure sensor. Photodetector studies show good photoresposnsivity, reproducibility, and fast photoresponse time indicating that the materials is a promising candidate for visible light-photodetector using 454 and 535 nm led lights.

Electron transport through NiSi2–Si contacts and their role in reconfigurable field-effect transistors

A model is presented which describes reconfigurable field-effect transistors (RFETs) with metal contacts, whose switching is controlled by manipulating the Schottky barriers at the contacts. The proposed modeling approach is able to bridge the gap between quantum effects on the atomic scale and the transistor switching. We apply the model to transistors with a silicon channel and NiSi2 contacts. All relevant crystal orientations are compared, focusing on the differences between electron and hole current, which can be as large as four orders of magnitude. Best symmetry is found for the orientation, which makes this orientation most advantageous for RFETs. The observed differences are analyzed in terms of the Schottky barrier height at the interface. Our study indicates that the precise orientation of the interface relative to a given transport direction, perpendicular or tilted, is an important technology parameter, which has been underestimated during the previous development of RFETs. Most of the conclusions regarding the studied metal-semiconductor interface are also valid for other device architectures.

In vitro antimicrobial and anticancer properties of TiO2 blow-spun nanofibers containing silver nanoparticles

Nanosilver immobilized on TiO2 nanometric fibers (Ag/TiO2) was produced by solution blow spinning and characterized using scanning electron microscopy, transmission electron microscopy, N2 adsorption/desorption, X-ray diffraction, X-ray photoelectron spectroscopy, water contact angle, and inductively coupled plasma emission spectroscopy analyses. The in vitro antimicrobial and anticancer activities of the produced nanofibers was also investigated. Ag/TiO2 nanofibers revealed a crystalline structure compatible with the rutile crystalline phase, as well as a mesoporous and superhydrophilic nature. XPS profiles showed Ti4+ and Ag0, indicating a strong interaction between the Ag nanoparticles and TiO2. The Ag/TiO2 nanofibers presented antimicrobial activity against Staphylococcus aureus, Enterococcus faecalis, and Escherichia coli. The release of silver ions from 5 mg∙mL-1 and 50 mg∙mL-1 of Ag/TiO2 nanofibers was approximately 0.08 μg∙mL-1 and 0.18 μg∙mL-1, respectively. The nanofiber cytotoxicity in both macrophages (ATCC RAW 264.7) and cancer cells (murine AT-84 oral squamous carcinoma cells) was dose-dependent. A concentration of 5 mg∙mL-1 induced partial suppression growth and migration of cancer cells, while a concentration of 50 mg∙mL-1 resulted in complete inhibition of proliferation and migration of murine AT-84 cells. The overall results indicate that Ag/TiO2 nanofibers can selectively inhibit the cellular mechanism of AT-84 by apoptosis with DNA damage and cell death. The antimicrobial and anticancer performance of Ag/TiO2 nanofibers is probably the result of its nanometric dimension, high surface reactivity, and the interaction between TiO2 and Ag. Electron transfer at the metal-semiconductor interface and reactive oxygen species production, in addition to the biological activity of released silver ions, confirm the potential for use as an agent in antimicrobial and anticancer therapy.

DNA-Mediated Self-Assembly and Metallization of Semiconductor Nanorods for the Fabrication of Nanoelectronic Interfaces.

DNA nanostructures provide a powerful platform for the programmable assembly of nanomaterials. Here, this approach is extended to semiconductor nanorods that possess interesting electrical properties and could be utilized for the bottom-up fabrication of nanoelectronic building blocks. The assembly scheme is based on an efficient DNA functionalization of the nanorods. A complete coverage of the rod surface with DNA ensures a high colloidal stability while maintaining the rod size and shape. It furthermore supports the assembly of the nanorods at defined docking positions of a DNA origami platform with binding efficiencies of up to 90 % as well as the formation of nanorod dimers with defined relative orientations. By incorporating orthogonal binding sites for gold nanoparticles, defined metal-semiconductor heterostructures can be fabricated. Subsequent application of a seeded growth procedure onto the gold nanoparticles (AuNPs) allows for to establish a direct metal-semiconductor interface as a crucial basis for the integration of semiconductors in self-assembled nanoelectronic devices.

Frequency dependent dielectric properties of atomic layer deposition grown zinc-oxide based MIS structure

Frequency dependent dielectric properties of zinc oxide (ZnO) based metal-insulator-semiconductor (MIS) structure were analyzed in a wide frequency range. The ZnO thin layer on p-type silicon (100) was prepared by using atomic layer deposition (ALD). Dielectric parameters such as dielectric constant (ε′), dielectric loss (ε''), dielectric loss tangent (tanδ), electrical conductivity (σac) and complex electrical modulus (M∗ = M'+iM″) of this structure have been studied in depth by using spectroscopic technique in the different applied bias voltages and frequencies under the room temperature condition. According to experimental results, ε′, ε'', tanδ, σac, M′ and M″ values were strongly depended on the applied voltage and the frequency in accumulation and depletion region. While the values of ε′ reduce with rising frequency for each applied voltage, ε'' values reduce with rising frequency and decreasing voltage. The σac is observed to weak increase with rising frequency. M′ values go up by rising frequency. M″ values give a peak and the peak location moves to higher frequency by rising applied voltage. The experimental values confirmed that interfacial polarization can be readily ingenerated at low frequency and interface states located in the metal-semiconductor interface have an influence on the dielectric characteristics of Al/ZnO/p-Si MIS structures.

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.

Hydrogen Evolution Reaction Catalyzed By Plasmoinic Photo-Electrodes Under Visible Light Illumination

From the environmental point of view, the efficient control of the visible light driven hydrogen evolution reaction is one of the promising issue. For that, the semiconductor electrode can be regarded as the sufficient material but various semiconductor electrodes cannot be used for the visible light driven reactions due to their wide band gap energy. Recently, plasmoic photoelectric conversion system, which is the introduction of the plasmon active metal nano-structures onto the surface of the semiconductor substrate, has been received much attention due to its wide range photo response abilities. In this system, the electrons are excited and injected into the conduction band of the n-type semiconductor under visible light illuminations. Up to now, various plasmon-induced chemical reactions, even the water splitting, have been established. However, although well-established systems have been reported, the direct control of the reduction reaction at the metal-semiconductor interface using the excited electrons has not been well examined because the almost plasmonic photoconversion systems consist of the n-type semiconductor electrode and used the generated holes for the reactions. In this study, we have tried to establish the plasmon-induced hydrogen evolution system by using p-type semiconductor substrate and plasmon active nano-structures. By the fabrication of the plasmonic Ag dimer structures, we have observed the generation of the photo-current derived from the hydrogen evolution reactions under visible light illumination. Through the photocurrent measurements and the incident photon to current conversion efficiency obtained under different pH conditions, we have confirmed the pH dependence of the photo current value and the photo response characters. In addition to these, as the very interesting point for the current study, we have found the very unique catalytic property for the plasmonic hydrogen evolution especially under specific pH condition. Through the various photoelectrochemical measurements including isotropic selectively, we are sure that our plasmonic hydrogen evolution system could be one of the good candidate for future light energy conversion system.

SERS study of Ag/FeS/4-MBA interface based on the SPR effect.

In this work, an ordered metal-semiconductor molecular system was introduced, and 4-mercaptobenzoic acid (4-MBA) was employed to study the charge transfer (CT) at the metal-semiconductor interface based on surface-enhanced Raman scattering (SERS) spectra. The thickness of the sputtered FeS was controlled so that the surface plasmon resonance (SPR) of Ag underwent a displacement change, and the contribution of the SPR to the CT was studied through surface plasmon (SP) absorption. Furthermore, SERS spectra obtained at different excitation wavelengths were used to calculate the degree of CT in the layer-by-layer sputtering system. When Ag was irradiated with incident light, the strong SPR of Ag was excited, generating an increased electromagnetic field (EM). This amplified EM generated hot electrons at the interface between the FeS and Ag, and then the hot electrons were rearranged. Therefore, we established a simple and effective method for studying the impact of SPR on interfacial CT and analyzed the SERS spectra in accordance with Lombardi's basic theory and the physical effects associated with SPR. This theory is in good agreement with the experimental results. On this basis, we also proposed a mechanism by which SPR impacts the CT, which is beneficial for studying interfacial CT and obtaining an in-depth understanding of the CT mechanism in SERS. This work also enables the expansion of the applications of the SERS technique in the field of nanomaterials.

Threshold Voltage Modulation of Enhancement-Mode InGaAs Schottky-Gate Fin-HEMTs

In this letter, we demonstrate an efficient way to modulate the threshold voltage ( ${V} _{\text T}$ ) of InGaAs high-electron-mobility transistors (HEMTs) through forming the fin-shaped channel. The gate metal is directly deposited on this fin-shaped channel and thus can be regarded as a “Schottky-gate Fin-HEMT.” The device exhibits great fabrication stability and electrical characteristics. The 2- $\mu \text{m}$ gate-length device exhibits positive ${V} _{\text T}$ shift of 2.98 V from planar to fin device with a fin width ( ${W}_{\text {FIN}}$ ) of 54 nm, and an enhancement-mode operation is achieved when ${W}_{\text {FIN}}$ is less than 74 nm. Performances are enhanced both for ON-and OFF-state with ${W}_{\text {FIN}}$ scaling down to the regime of about 100 nm. The modulation mechanism can be attributed to the band bending at metal–semiconductor interface of the side gate, which is verified by the TCAD simulation, and thus depletes the carriers channel much more efficiently than top gate control.

Van der Waals contacts between three-dimensional metals and two-dimensional semiconductors.

As the dimensions of the semiconducting channels in field-effect transistors decrease, the contact resistance of the metal-semiconductor interface at the source and drain electrodes increases, dominating the performance of devices1-3. Two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS2) have been demonstrated to be excellent semiconductors for ultrathin field-effect transistors4,5. However, unusually high contact resistance has been observed across the interface between the metal and the 2D transition-metal dichalcogenide3,5-9. Recent studies have shown that van der Waals contacts formed by transferred graphene10,11 and metals12 on few-layered transition-metal dichalcogenides produce good contact properties. However, van der Waals contacts between a three-dimensional metal and a monolayer 2D transition-metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10-nanometre-thick indium metal capped with 100-nanometre-thick gold electrodes and monolayer MoS2. Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid solution after annealing at 200 degrees Celsius and that the interface between the gold-capped indium and the MoS2 is atomically sharp with no detectable chemical interaction between the metal and the 2D transition-metal dichalcogenide, suggesting van-der-Waals-type bonding between the gold-capped indium and monolayer MoS2. The contact resistance of the indium/gold electrodes is 3,000±300 ohm micrometres for monolayer MoS2 and 800±200 ohm micrometres for few-layered MoS2. These values are among the lowest observed for three-dimensional metal electrodes evaporated onto MoS2, enabling high-performance field-effect transistors with a mobility of 167±20 square centimetres per volt per second. We also demonstrate a low contact resistance of 220±50 ohm micrometres on ultrathin niobium disulfide (NbS2) and near-ideal band offsets, indicative of defect-free interfaces, in tungsten disulfide (WS2) and tungsten diselenide (WSe2) contacted with indiumalloy. Our work provides a simple method of making ultraclean van der Waals contacts using standard laboratory technology on monolayer 2D semiconductors.