Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides

Deterministic Light-to-Voltage Conversion with a Tunable Two-Dimensional Diode

Heterojunctions accompanied by energy barriers are of significant importance in two-dimensional materials-based electronics and optoelectronics. They provide more functional device performance, compared with their counterparts with uniform channels. Multimodal optoelectronic devices could be accomplished by elaborately designing band diagrams and architectures of the two-dimensional junctions. Here, we demonstrate deterministic light-to-voltage conversion based on strong dielectric screening effect in a tunable two-dimensional Schottky diode based on semiconductor/metal heterostructure, where the resultant photovoltage is dependent on the intensity of light input but independent of gate voltage. The converted photovoltage across the diode is independent of gate voltage under both monochromatic laser and white light illumination. In addition, the Fermi level of two-dimensional semiconductor area on dielectric SiO₂ is highly gate-dependent, leading to the tunable rectifying effect of this heterostructure, which corporates a vertical Schottky junction and a lateral homojunction. As a result, a constant open-circuit voltage of ∼0.44 V and a hybrid "photovoltaic + photoconduction" photoresponse behavior are observed under 1 μW illumination of 403 nm laser, in addition to an electrical rectification ratio up to nearly 10⁴. The scanning photocurrent mappings under different bias voltages indicate that the switchable operation mode (photovoltaic, photoconduction, or hybrid) depends on the bias-dependent effective energy barrier at the two-dimensional semiconductor-metal interface. This approach provides a facile and reliable solution for deterministic on-chip light-to-voltage conversion and optical-to-electrical interconnects.

WSe2 as Transparent Top Gate for Infrared Near-Field Microscopy

Independent control of carrier density and out-of-plane displacement field is essential for accessing novel phenomena in two-dimensional (2D) material heterostructures. While this is achieved with independent top and bottom metallic gate electrodes in transport experiments, it remains a challenge for near-field optical studies as the top electrode interferes with the optical path. Here, we characterize the requirements for a material to be used as the top-gate electrode and demonstrate experimentally that few-layer WSe₂ can be used as a transparent, ambipolar top-gate electrode in infrared near-field microscopy. We carry out nanoimaging of plasmons in a bilayer graphene heterostructure tuning the plasmon wavelength using a trilayer WSe₂ gate, achieving a density modulation amplitude exceeding 2 × 10¹² cm^-2. The observed ambipolar gate-voltage response allows us to extract the energy gap of WSe₂, yielding a value of 1.05 eV. Our results provide an additional tuning knob to cryogenic near-field experiments on emerging phenomena in 2D materials and moiré heterostructures.

P-type electrical contacts for two-dimensional transition metal dichalcogenides

Digital logic circuits are based on complementary pairs of n- and p-type field effect transistors (FETs) via complementary metal oxide semiconductor (CMOS) technology. In three dimensional (3D or bulk) semiconductors, substitutional doping of acceptor or donor impurities is used to achieve p- and n-type FETs. However, the controllable p-type doping of low-dimensional semiconductors such as two-dimensional transition metal dichalcogenides (2D TMDs) has proved to be challenging. Although it is possible to achieve high quality, low resistance n-type van der Waals (vdW) contacts on 2D TMDs^1-5, obtaining p-type devices from evaporating high work function metals onto 2D TMDs has not been realised so far. Here we report high-performance p-type devices on single and few-layered molybdenum disulphide (MoS₂) and tungsten diselenide (WSe₂) based on industry-compatible electron beam evaporation of high work function metals such as Pd and Pt. Using atomic resolution imaging and spectroscopy, we demonstrate near ideal vdW interfaces without chemical interactions between the 2D TMDs and 3D metals. Electronic transport measurements reveal that the Fermi level is unpinned and p-type FETs based on vdW contacts exhibit low contact resistance of 3.3 kΩ·µm, high mobility values of ~ 190 cm²-V^-1s^-1 at room temperature with saturation currents in excess of > 10^-5 Amperes per micron (A-μm^-1) and on/off ratio of 10⁷. We also demonstrate an ultra-thin photovoltaic cell based on n- and p-type vdW contacts with an open circuit voltage of 0.6 V and power conversion efficiency of 0.82%.

Nondestructive and local mapping photoresponse of WSe2 by electrostatic force microscopy

We report a local mapping photoresponse of WSe₂ using a second-harmonic (2w) channel based on nondestructive electrostatic force microscopy (EFM). The 2w signals resulting from interaction between WSe₂ and EFM tip are intrinsically related to the electrical conductivity of WSe₂. The photoresponse images and rise/decay time constants of WSe₂ are obtained by local mapping 2w signals under illumination. We observe that the local photoresponse signals of WSe₂ increase with the positive tip gate voltage while the WSe₂ shows a p-type behavior in dark conditions We find that the reduced mobility of the photogenerated charge carriers resulting from the enhanced carrier scattering in the accumulation regime of WSe₂ is responsible for the gate-dependent photoresponse behavior. Our results provide a deep understanding the intrinsic optoelectrical properties of WSe₂ and contribute to the developments in the optoelectronic devices based on van der Waals layered materials.

The non-volatile electrostatic doping effect in MoTe2 field-effect transistors controlled by hexagonal boron nitride and a metal gate

The electrical and optical properties of transition metal dichalcogenides (TMDs) can be effectively modulated by tuning their Fermi levels. To develop a carrier-selectable optoelectronic device, we investigated intrinsically p-type MoTe₂, which can be changed to n-type by charging a hexagonal boron nitride (h-BN) substrate through the application of a writing voltage using a metal gate under deep ultraviolet light. The n-type part of MoTe₂ can be obtained locally using the metal gate pattern, whereas the other parts remain p-type. Furthermore, we can control the transition rate to n-type by applying a different writing voltage (i.e., − 2 to − 10 V), where the n-type characteristics become saturated beyond a certain writing voltage. Thus, MoTe₂ was electrostatically doped by a charged h-BN substrate, and it was found that a thicker h-BN substrate was more efficiently photocharged than a thinner one. We also fabricated a p–n diode using a 0.8 nm-thick MoTe₂ flake on a 167 nm-thick h-BN substrate, which showed a high rectification ratio of ~ 10⁻⁴. Our observations pave the way for expanding the application of TMD-based FETs to diode rectification devices, along with optoelectronic applications.

Graphene BioFET sensors for SARS-CoV-2 detection: a multiscale simulation approach

Biological Field-Effect Transistors (BioFETs) have already demonstrated enormous potential for detecting minute amounts of ions and molecules. The use of two-dimensional (2D) materials has been shown to boost their performance and to enable the design of new applications. This combination deserves special interest in the current pandemic caused by the SARS-CoV-2 virus which demands fast, reliable and cheap detection methods. However, in spite of the experimental advances, there is a lack of a comprehensive and in-depth computational approach to capture the mechanisms underlying the sensor behaviour. Here, we present a multiscale platform that combines detailed atomic models of the molecules with mesoscopic device-level simulations. The fine-level description exploited in this approach accounts for the charge distribution of the receptor, its reconfiguration when the target binds to it, and the consequences in terms of sensitivity on the transduction mechanism. The results encourage the further exploration of improved sensor designs and 2D materials combined with diverse receptors selected to achieve the desired specificity.

Fabrication of near-invisible solar cell with monolayer WS2

Herein, we developed a near-invisible solar cell through a precise control of the contact barrier between an indium tin oxide (ITO) electrode and a monolayer tungsten disulfide (WS₂), grown by chemical vapor deposition (CVD). The contact barrier between WS₂ and ITO was controlled by coating various thin metals on top of ITO (M_x/ITO) and inserting a thin layer of WO₃ between M_x/ITO and the monolayer WS₂, which resulted in a drastic increase in the Schottky barrier height (up to 220 meV); this could increase the efficiency of the charge carrier separation in our Schottky-type solar cell. The power conversion efficiency (PCE) of the solar cell with the optimized electrode (WO₃/M_x/ITO) was more than 1000 times that of a device using a normal ITO electrode. Large-scale fabrication of the solar cell was also investigated, which revealed that a simple size expansion with large WS₂ crystals and parallel long electrodes could not improve the total power (P_T) obtained from the complete device even with an increase in the device area; this can be explained by the percolation theory. This problem was addressed by reducing the aspect ratio (width/channel length) of the unit device structure to a value lower than a critical threshold. By repeating the experiments on this optimized unit device with an appropriate number of series and parallel connections, P_T could be increased up to 420 pW from a 1-cm² solar cell with a very high value (79%) of average visible transmission (AVT).

Electron-Irradiation-Induced Degradation of Transfer Characteristics in Super-Junction VDMOSFET

High electric-field stress is an effective solution to the recovery of irradiated devices. In this paper, the dependence of the recovery level on the magnitude of gate voltage and duration is investigated. Compared with the scheme of high gate-bias voltage with a short stress time, the transfer characteristics are significantly recovered by applying a low electric field with a long duration. When the electric field and stress time are up to a certain value, the threshold voltage almost approaches the limitation, which is less than that before irradiation. Meanwhile, the effect of temperature on the recovery of the irradiated devices is also demonstrated. The result indicates that a high temperature of 175 °C used for the irradiated devices’ annealing does not play a role in promoting the recovery of transfer characteristics. In addition, to obtain a deep-level understanding of threshold degradation, the first-principles calculations of three Si/SiO2 interfaces are performed. It is found that new electronic states can be clearly observed in the conduction bans and valence bands after the Si-H/-OH bonds are broken by electron irradiation. However, their distribution depends on the selection of the passivation scheme. Ultimately, it can be observed that the threshold voltage linearly decreases with the increase in interface charge density. These results can provide helpful guidance in the deep interpretation of threshold degradation and the recovery of the irradiated super-junction devices.

High-Efficiency WSe2 Photovoltaic Devices with Electron-Selective Contacts

A rapid surge in global energy consumption has led to a greater demand for renewable energy to overcome energy resource limitations and environmental problems. Recently, a number of van der Waals materials have been highlighted as efficient absorbers for very thin and highly efficient photovoltaic (PV) devices. Despite the predicted potential, achieving power conversion efficiencies (PCEs) above 5% in PV devices based on van der Waals materials has been challenging. Here, we demonstrate a vertical WSe₂ PV device with a high PCE of 5.44% under one-sun AM1.5G illumination. We reveal the multifunctional nature of a tungsten oxide layer, which promotes a stronger internal electric field by overcoming limitations imposed by the Fermi-level pinning at WSe₂ interfaces and acts as an electron-selective contact in combination with monolayer graphene. Together with the developed bottom contact scheme, this simple yet effective contact engineering method improves the PCE by more than five times.

Oxidation of tantalum disulfide (TaS2) films for gate dielectric and process design of two-dimensional field-effect device

Ta-based high-κdielectrics can be synthesized via the oxidation of TaS₂films. In this study, we investigated the wet and dry oxidation of TaS₂films via thermal annealing and plasma irradiation, respectively. The specific vibration observed via Raman spectroscopy, the bonding states observed via x-ray photoelectron spectroscopy, and capacitance measurements confirmed the oxidation of TaS₂films with a dielectric constant of ∼14.9. Moreover, the electrical transport of the TaS₂films along the in-plane direction indicated a change in conductivity before and after the oxidation. The thickness of the oxidized film was estimated. Accordingly, the layer-by-layer oxidation was limited to approximately 50 nm via plasma irradiation, whereas the TaS₂films within 150 nm were fully oxidized via thermal annealing in ambient air. Therefore, a Ta-oxide/TaS₂structure was fabricated as a stack material of insulator and metal when the thickness of the pristine film was greater than 50 nm. In addition, Ta-oxide films were integrated into bottom-gated two-dimensional (2D) field-effect transistors (FETs) using the dry transfer method. 2D FETs with multilayer MoTe₂and MoS₂films asp-type andn-type channels, respectively, were successfully fabricated. In particular, the Ta-oxide film synthesized via dry oxidation was used as a gate dielectric, and the device process could be simplified because the Ta-oxide/TaS₂heterostructure can function as a stack material for gate insulators and gate electrodes. An anti-ambipolar transistor consisting of an MoTe₂/MoS₂heterojunction was also fabricated. For the transfer characteristics, a relatively sharp on-state bias range below 10 V and sufficiently high peak-to-valley ratio of 10⁶atV_DS = 3 V were obtained using the high-κ gate dielectric of Ta-oxide despite the presence of the multilayer channels (∼20 nm).

Reconfigurable carrier type and photodetection of MoTe2 of various thicknesses by deep ultraviolet light illumination

Tuning of the Fermi level in transition metal dichalcogenides (TMDCs) leads to devices with excellent electrical and optical properties. In this study, we controlled the Fermi level of MoTe₂ by deep ultraviolet (DUV) light illumination in different gaseous environments. Specifically, we investigated the reconfigurable carrier type of an intrinsic p-MoTe₂ flake that gradually transformed into n-MoTe₂ after illumination with DUV light for 30, 60, 90, 120, 160, 250, 500, 900, and 1200 s in a nitrogen (N₂) gas environment. Subsequently, we illuminated this n-MoTe₂ sample with DUV light in oxygen (O₂) gas and reversed its carrier polarity toward p-MoTe₂. However, using this doping scheme to reveal the effect of DUV light on various layers (3-30 nm) of MoTe₂ is challenging. The DUV + N₂ treatment significantly altered the polarity of MoTe₂ of different thicknesses from p-type to n-type under the DUV + N₂ treatment, but the DUV + O₂ treatment did not completely alter the polarity of thicker n-MoTe₂ flakes to p-type. In addition, we investigated the photoresponse of MoTe₂ after DUV light treatment in N₂ and O₂ gas environments. From the time-resolved photoresponsivity at different polarity states of MoTe₂, we have shown that the response time of the DUV + O₂ treated p-MoTe₂ is faster than that of the pristine and doped n-MoTe₂ films. These carrier polarity modulations and photoresponse paves the way for wider applications of MoTe₂ in optoelectronic devices.

Ultrafast Operation of 2D Heterostructured Nonvolatile Memory Devices Provided by the Strong Short-Time Dielectric Breakdown Strength of h-BN

Recently, the ultrafast operation (∼20 ns) of a two-dimensional (2D) heterostructured nonvolatile memory (NVM) device was demonstrated, attracting considerable attention. However, there is no consensus on its physical origin. In this study, various 2D NVM device structures are compared. First, we reveal that the hole injection at the metal/MoS₂ interface is the speed-limiting path in the NVM device with the access region. Therefore, MoS₂ NVM devices with a direct tunneling path between source/drain electrodes and the floating gate are fabricated by removing the access region. Indeed, a 50 ns program/erase operation is successfully achieved for devices with metal source/drain electrodes as well as graphite source/drain electrodes. This controlled experiment proves that an atomically sharp interface is not necessary for ultrafast operation, which is contrary to the previous literature. Finally, the dielectric breakdown strength (E_BD) of h-BN under short voltage pulses is examined. Since a high dielectric breakdown strength allows a large tunneling current, ultrafast operations can be achieved. Surprisingly, an E_BD = 26.1 MV/cm for h-BN is realized under short voltage pulses, largely exceeding the E_BD = ∼12 MV/cm from the direct current (DC) measurement. This suggests that the high E_BD of h-BN can be the physical origin of the ultrafast operation.

Large-Area, High-Specific-Power Schottky-Junction Photovoltaics from CVD-Grown Monolayer MoS2

The deployment of two-dimensional (2D) materials for solar energy conversion requires scalable large-area devices. Here, we present the design, modeling, fabrication, and characterization of monolayer MoS₂-based lateral Schottky-junction photovoltaic (PV) devices grown by using chemical vapor deposition (CVD). The device design consists of asymmetric Ti and Pt metal contacts with a work function offset to enable charge separation. These early stage devices show repeatable performance under 1 sun illumination, with V_OC of 160 mV, J_SC of 0.01 mA/cm², power conversion efficiency of 0.0005%, and specific power of 1.58 kW/kg. An optoelectronic model for this device is developed and validated with experimental results. This model is used to understand loss mechanisms and project optimized device designs. The model predicts that a 2D PV device with ∼70 kW/kg of specific power can be achieved with minimum optimization to the current devices. By increasing the thickness of the absorber layer, we can achieve even higher performance devices. Finally, a 25 mm² area solar cell made with a 0.65 nm thick MoS₂ monolayer is demonstrated, showing V_OC of 210 mV under 1 sun illumination. This is the first demonstration of a large-area PV device made with CVD-grown scalable 2D materials.

Layer-by-layer epitaxy of multi-layer MoS2 wafers

The 2D semiconductor of MoS₂ has great potential for advanced electronics technologies beyond silicon. So far, high-quality monolayer MoS₂ wafers have been available and various demonstrations from individual transistors to integrated circuits have also been shown. In addition to the monolayer, multilayers have narrower band gaps but improved carrier mobilities and current capacities over the monolayer. However, achieving high-quality multi-layer MoS₂ wafers remains a challenge. Here we report the growth of high-quality multi-layer MoS₂ 4-inch wafers via the layer-by-layer epitaxy process. The epitaxy leads to well-defined stacking orders between adjacent epitaxial layers and offers a delicate control of layer numbers up to six. Systematic evaluations on the atomic structures and electronic properties were carried out for achieved wafers with different layer numbers. Significant improvements in device performances were found in thicker-layer field-effect transistors (FETs), as expected. For example, the average field-effect mobility (μ_FE) at room temperature (RT) can increase from ∼80 cm²·V^–1·s^–1 for monolayers to ∼110/145 cm²·V^–1·s^–1 for bilayer/trilayer devices. The highest RT μ_FE of 234.7 cm²·V^–1·s^–1 and record-high on-current densities of 1.70 mA·μm^–1 at V_ds = 2 V were also achieved in trilayer MoS₂ FETs with a high on/off ratio of >10⁷. Our work hence moves a step closer to practical applications of 2D MoS₂ in electronics.

Highly Tunable Carrier Tunneling in Vertical Graphene-WS2-Graphene van der Waals Heterostructures

Owing to the fascinating properties, the emergence of two-dimensional (2D) materials brings various important applications of electronic and optoelectronic devices from field-effect transistors (FETs) to photodetectors. As a zero-band-gap material, graphene has excellent electric conductivity and ultrahigh carrier mobility, while the ON/OFF ratio of the graphene FET is severely low. Semiconducting 2D transition metal chalcogenides (TMDCs) exhibit an appropriate band gap, realizing FETs with high ON/OFF ratio and compensating for the disadvantages of graphene transistors. However, a Schottky barrier often forms at the interface between the TMDC and metallic contact, which limits the on-state current of the devices. Here, we lift the two limits of the 2D-FET by demonstrating highly tunable field-effect tunneling transistors based on vertical graphene-WS₂-graphene van der Waals heterostructures. Our devices show a low off-state current below 1 pA and a high ON/OFF ratio exceeding 10⁶ at room temperature. Moreover, the carrier transport polarity of the device can be effectively tuned from n-type under small bias voltage to bipolar under large bias by controlling the crossover from a direct tunneling region to the Fowler-Nordheim tunneling region. Further, we find that the effective barrier height can be controlled by an external gate voltage. The temperature dependence of carrier transport demonstrates that both tunneling and thermionic emission contribute to the operation current at elevated temperature, which significantly enhances the on-state current of the tunneling transistors.

Bridging the gap between atomically thin semiconductors and metal leads

Electrically interfacing atomically thin transition metal dichalcogenide semiconductors (TMDSCs) with metal leads is challenging because of undesired interface barriers, which have drastically constrained the electrical performance of TMDSC devices for exploring their unconventional physical properties and realizing potential electronic applications. Here we demonstrate a strategy to achieve nearly barrier-free electrical contacts with few-layer TMDSCs by engineering interfacial bonding distortion. The carrier-injection efficiency of such electrical junction is substantially increased with robust ohmic behaviors from room to cryogenic temperatures. The performance enhancements of TMDSC field-effect transistors are well reflected by the low contact resistance (down to 90 Ωµm in MoS₂, towards the quantum limit), the high field-effect mobility (up to 358,000 cm²V^-1s^-1 in WSe₂), and the prominent transport characteristics at cryogenic temperatures. This method also offers possibilities of the local manipulation of atomic structures and electronic properties for TMDSC device design.

Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects

Motivated by the high expectation for efficient electrostatic modulation of charge transport at very low voltages, atomically thin 2D materials with a range of bandgaps are investigated extensively for use in future semiconductor devices. However, researchers face formidable challenges in 2D device processing mainly originated from the out-of-plane van der Waals (vdW) structure of ultrathin 2D materials. As major challenges, untunable Schottky barrier height and the corresponding strong Fermi level pinning (FLP) at metal interfaces are observed unexpectedly with 2D vdW materials, giving rise to unmodulated semiconductor polarity, high contact resistance, and lowered device mobility. Here, FLP observed from recently developed 2D semiconductor devices is addressed differently from those observed from conventional semiconductor devices. It is understood that the observed FLP is attributed to inefficient doping into 2D materials, vdW gap present at the metal interface, and hybridized compounds formed under contacting metals. To provide readers with practical guidelines for the design of 2D devices, the impact of FLP occurring in 2D semiconductor devices is further reviewed by exploring various origins responsible for the FLP, effects of FLP on 2D device performances, and methods for improving metallic contact to 2D materials.

Reduced Fermi Level Pinning at Physisorptive Sites of Moire-MoS2/Metal Schottky Barriers

Weaker Fermi level pinning (FLP) at the Schottky barriers of 2D semiconductors is electrically desirable as this would allow a minimizing of contact resistances, which presently limit device performances. Existing contacts on MoS₂ have a strong FLP with a small pinning factor of only ∼0.1. Here, we show that Moire interfaces can stabilize physisorptive sites at the Schottky barriers with a much weaker interaction without significantly lengthening the bonds. This increases the pinning factor up to ∼0.37 and greatly reduces the n-type Schottky barrier height to ∼0.2 eV for certain metals such as In and Ag, which can have physisorptive sites. This then accounts for the low contact resistance of these metals as seen experimentally. Such physisorptive interfaces can be extended to similar systems to better control SBHs in highly scaled 2D devices.

Analysis of Schottky barrier heights and reduced Fermi-level pinning in monolayer CVD-grown MoS2field-effect-transistors

Chemical vapor deposition (CVD)-grown monolayer (ML) molybdenum disulfide (MoS₂) is a promising material for next-generation integrated electronic systems due to its capability of high-throughput synthesis and compatibility with wafer-scale fabrication. Several studies have described the importance of Schottky barriers in analyzing the transport properties and electrical characteristics of MoS₂field-effect-transistors (FETs) with metal contacts. However, the analysis is typically limited to single devices constructed from exfoliated flakes and should be verified for large-area fabrication methods. In this paper, CVD-grown ML MoS₂was utilized to fabricate large-area (1 cm × 1 cm) FET arrays. Two different types of metal contacts (i.e. Cr/Au and Ti/Au) were used to analyze the temperature-dependent electrical characteristics of ML MoS₂FETs and their corresponding Schottky barrier characteristics. Statistical analysis provides new insight about the properties of metal contacts on CVD-grown MoS₂compared to exfoliated samples. Reduced Schottky barrier heights (SBH) are obtained compared to exfoliated flakes, attributed to a defect-induced enhancement in metallization of CVD-grown samples. Moreover, the dependence of SBH on metal work function indicates a reduction in Fermi level pinning compared to exfoliated flakes, moving towards the Schottky-Mott limit. Optical characterization reveals higher defect concentrations in CVD-grown samples supporting a defect-induced metallization enhancement effect consistent with the electrical SBH experiments.

Asymmetric Chemical Functionalization of Top-Contact Electrodes: Tuning the Charge Injection for High-Performance MoS2 Field-Effect Transistors and Schottky Diodes

The fabrication of high-performance (opto-)electronic devices based on 2D channel materials requires the optimization of the charge injection at electrode-semiconductor interfaces. While chemical functionalization with chemisorbed self-assembled monolayers has been extensively exploited to adjust the work function of metallic electrodes in bottom-contact devices, such a strategy has not been demonstrated for the top-contact configuration, despite the latter being known to offer enhanced charge-injection characteristics. Here, a novel contact engineering method is developed to functionalize gold electrodes in top-contact field-effect transistors (FETs) via the transfer of chemically pre-modified electrodes. The source and drain Au electrodes of the molybdenum disulfide (MoS₂ ) FETs are functionalized with thiolated molecules possessing different dipole moments. While the modification of the electrodes with electron-donating molecules yields a marked improvement of device performance, the asymmetric functionalization of the source and drain electrodes with different molecules with opposed dipole moment enables the fabrication of a high-performance Schottky diode with a rectification ratio of ≈10³ . This unprecedented strategy to tune the charge injection in top-contact MoS₂ FETs is of general applicability for the fabrication of high-performance (opto-)electronic devices, in which asymmetric charge injection is required, enabling tailoring of the device characteristics on demand.

Interface effects of Schottky devices built from MoS2and high work function metals

Schottky junctions, formed by high work function metals and semiconductors, are important devices in electronics and optoelectronics. The metal deposition in traditional Schottky interfaces usually damages the semiconductor surface and causes defect states, which reduces the Schottky barrier height and device performance. This can be avoided in the atomically smooth interface formed by two-dimensional (2D) metals and semiconductors. For better interface tailoring engineering, it is particularly important to understand various interface effects in such 2D Schottky devices under critical or boundary conditions. Here we report the fabrication and testing of three types of MoS₂devices, i.e., using PtTe₂, Cr and Au as contact materials. While the Cr/MoS₂contact is an ohmic contact, the other two are Schottky contacts. The van-der-Waals interface of PtTe₂-MoS₂results in a well-defined OFF state and a significant rectification ratio of 10⁴. This parameter, together with an ideality factor 2.1, outperforms the device based on evaporated Au. Moreover, a device in the intermediate condition is also presented. An abrupt increase in the reverse current is observed and understood based on the enhanced tunneling current. Our work manifests the essential role of doping concentration and provides another example for 2D Schottky interface design.

Modulation of electrical properties in MoTe2 by XeF2-mediated surface oxidation

Transition metal dichalcogenides (TMDs) are promising candidates for the semiconductor industry owing to their superior electrical properties. Their surface oxidation is of interest because their electrical properties can be easily modulated by an oxidized layer on top of them. Here, we demonstrate the XeF₂-mediated surface oxidation of 2H-MoTe₂ (alpha phase MoTe₂). MoTe₂ exposed to XeF₂ gas forms a thin and uniform oxidized layer (∼2.5 nm-thick MoO _x ) on MoTe₂ regardless of the exposure time (within ∼120 s) due to the passivation effect and simultaneous etching. We used the oxidized layer for contacts between the metal and MoTe₂, which help reduce the contact resistance by overcoming the Fermi level pinning effect by the direct metal deposition process. The MoTe₂ field-effect transistors (FETs) with a MoO _x interlayer exhibited two orders of magnitude higher field-effect hole mobility of 6.31 cm² V^-1 s^-1 with a high on/off current ratio of ∼10⁵ than that of the MoTe₂ device with conventional metal contacts (0.07 cm² V^-1 s^-1). Our work shows a straightforward and effective method for forming a thin oxide layer for MoTe₂ devices, applicable for 2D electronics.

Persistence of Monoclinic Crystal Structure in 3D Second-Order Topological Insulator Candidate 1T'-MoTe2 Thin Flake Without Structural Phase Transition

A van der Waals material, MoTe₂ with a monoclinic 1T' crystal structure is a candidate for 3D second-order topological insulators (SOTIs) hosting gapless hinge states and insulating surface states. However, due to the temperature-induced structural phase transition, the monoclinic 1T' structure of MoTe₂ is transformed into the orthorhombic T_d structure as the temperature is lowered, which hinders the experimental verification and electronic applications of the predicted SOTI state at low temperatures. Here, systematic Raman spectroscopy studies of the exfoliated MoTe₂ thin flakes with variable thicknesses at different temperatures, are presented. As a spectroscopic signature of the orthorhombic T_d structure of MoTe₂ , the out-of-plane vibration mode D at ≈ 125 cm^-1 is always visible below a certain temperature in the multilayer flakes thicker than ≈ 27.7 nm, but vanishes in the temperature range from 80 to 320 K when the flake thickness becomes lower than ≈ 19.5 nm. The absence of the out-of-plane vibration mode D in the Raman spectra here demonstrates not only the disappearance of the monoclinic-to-orthorhombic phase transition but also the persistence of the monoclinic 1T' structure in the MoTe₂ thin flakes thinner than ≈ 19.5 nm at low temperatures down to 80 K, which may be caused by the high enough density of the holes introduced during the gold-enhanced exfoliation process and exposure to air. The MoTe₂ thin flakes with the low-temperature monoclinic 1T' structure provide a material platform for realizing SOTI states in van der Waals materials at low temperatures, which paves the way for developing a new generation of electronic devices based on SOTIs.

Unexpected Electron Transport Suppression in a Heterostructured Graphene-MoS2 Multiple Field-Effect Transistor Architecture

We demonstrate a graphene-MoS2 architecture integrating multiple field-effect transistors (FETs), and we independently probe and correlate the conducting properties of van der Waals coupled graphene-MoS2 contacts with those of the MoS2 channels. Devices are fabricated starting from high-quality single-crystal monolayers grown by chemical vapor deposition. The heterojunction was investigated by scanning Raman and photoluminescence spectroscopies. Moreover, transconductance curves of MoS2 are compared with the current-voltage characteristics of graphene contact stripes, revealing a significant suppression of transport on the n-side of the transconductance curve. On the basis of ab initio modeling, the effect is understood in terms of trapping by sulfur vacancies, which counterintuitively depends on the field effect, even though the graphene contact layer is positioned between the backgate and the MoS2 channel.

An application-specific image processing array based on WSe2 transistors with electrically switchable logic functions

With the rapid development of artificial intelligence, parallel image processing is becoming an increasingly important ability of computing hardware. To meet the requirements of various image processing tasks, the basic pixel processing unit contains multiple functional logic gates and a multiplexer, which leads to notable circuit redundancy. The pixel processing unit retains a large optimizing space to solve the area redundancy issues in parallel computing. Here, we demonstrate a pixel processing unit based on a single WSe₂ transistor that has multiple logic functions (AND and XNOR) that are electrically switchable. We further integrate these pixel processing units into a low transistor-consumption image processing array, where both image intersection and image comparison tasks can be performed. Owing to the same image processing power, the consumption of transistors in our image processing unit is less than 16% of traditional circuits.

Reducing circuit redundancy represents a priority for the scalability of parallel computing hardware. Here, the authors report the realization of pixel processing units consisting of single 2D WSe₂ transistors implementing electrically-switchable logic functions. This strategy enables the fabrication of an image processing array with ~16% transistor consumption compared to traditional circuits.

Intriguing interfacial characteristics of the CS contact with MX2 (M = Mo, W; X = S, Se, Te) and MXY ((X ≠ Y) = S, Se, Te) monolayers

Using (hybrid) first principles calculations, the electronic band structure, type of Schottky contact and Schottky barrier height established at the interface of the most stable stacking patterns of the CS–MX₂ (M = Mo, W; X = S, Se, Te) and CS–MXY ((X ≠ Y) = S, Se, Te) MS vdWH are investigated. The electronic band structures of CS–MX₂ and CS–MXY MS vdWH seem to be simple sum of CS, MX₂ and MXY monolayers. The projected electronic properties of the CS, MX₂ and MXY layers are well preserved in CS–MX₂ and CS–MXY MS vdWH. Their smaller effective mass (higher carrier mobility) render promising prospects of CS–WS₂ and CS–MoSeTe as compared to other MS vdWH in nanoelectronic and optoelectronic devices, such as a high efficiency solar cell. In addition, we found that the effective mass of holes is higher than that of electrons, suggesting that these heterostructures can be utilized for hole/electron separation. Interestingly, the MS contact led to the formation of a Schottky contact or ohmic contact, therefore we have used the Schottky Mott rule to calculate the Schottky barrier height (SBH) of CS–MX₂ (M = Mo, W; X = S, Se, Te) and CS–MXY ((X ≠ Y) = S, Se, Te) MS vdWH. It was found that CS–MX₂ (M = Mo, W; X = S, Se, Te) and CS–MXY ((X ≠ Y) = S, Se, Te) (in both model-I and -II) MS vdWH form p-type Schottky contacts. These p-type Schottky contacts can be considered a promising building block for high-performance photoresponsive optoelectronic devices, p-type electronics, CS-based contacts, and for high-performance electronic devices.

Electronic band structure, type of Schottky contact and Schottky barrier height established at the interface of the CS–MX₂ (M = Mo, W; X = S, Se, Te) and CS–MXY ((X ≠ Y) = S, Se, Te) MS vdWH.

Multi-scale modeling of 2D GaSe FETs with strained channels

Electronic devices based on bidimensional materials (2DMs) are the subject of an intense experimental research, that demands a tantamount theoretical activity. The latter must be hold up by a varied set of tools able to rationalize, explain and predict the operation principles of the devices. However, in the broad context of multi-scale computational nanoelectronics, there is currently a lack of simulation tools connecting atomistic descriptions with semi-classical mesoscopic device-level simulations and able to properly explain the performance of many state-of-the-art devices. To contribute to filling this gap we present a multi-scale approach that combines fine-level material calculations with a semi-classical drift-diffusion transport model. Its use is exemplified by assessing 2DM field effect transistors with strained channels, showing excellent capabilities to capture the changes in the crystal structure and their impact into the device performance. Interestingly, we verify the capacity of strain in monolayer GaSe to enhance the conduction of one type of carrier, enabling the possibility to mimic the effect of chemical doping on 2D materials. These results illustrate the great potential of the proposed approach to bridge levels of abstraction rarely connected before and thus contribute to the theoretical modeling of state-of-the-art 2DM-based devices.

Hole doping effect of MoS2 via electron capture of He+ ion irradiation

Beyond the general purpose of noble gas ion sputtering, which is to achieve functional defect engineering of two-dimensional (2D) materials, we herein report another positive effect of low-energy (100 eV) He⁺ ion irradiation: converting n-type MoS₂ to p-type by electron capture through the migration of the topmost S atoms. The electron capture ability via He⁺ ion irradiation is valid for supported bilayer MoS₂; however, it is limited at supported monolayer MoS₂ because the charges on the underlying substrates transfer into the monolayer under the current condition for He⁺ ion irradiation. Our technique provides a stable and universal method for converting n-type 2D transition metal dichalcogenides (TMDs) into p-type semiconductors in a controlled fashion using low-energy He⁺ ion irradiation.

Ultralow Power Optical Synapses Based on MoS2 Layers by Indium-Induced Surface Charge Doping for Biomimetic Eyes

Biomimetic eyes, with their excellent imaging functions such as large fields of view and low aberrations, have shown great potentials in the fields of visual prostheses and robotics. However, high power consumption and difficulties in device integration severely restrict their rapid development. In this study, an artificial synaptic device consisting of a molybdenum disulfide (MoS₂ ) film coated with an electron injection enhanced indium (In) layer is proposed to increase the channel conductivity and reduce the power consumption. This artificial synaptic device achieves an ultralow power consumption of 68.9 aJ per spike, which is several hundred times lower than those of the optical artificial synapses reported in literature. Furthermore, the multilayer and polycrystalline MoS₂ film shows persistent photoconductivity performance, effectively resulting in short-term plasticity, long-term plasticity, and their transitions between each other. A 5 × 5 In/MoS₂ synaptic device array is constructed into a hemispherical electronic retina, demonstrating its impressive image sensing and learning functions. This research provides a new methodology for effective control of artificial synaptic devices, which have great opportunities used in bionic retinas, robots, and visual prostheses.

Status and prospects of Ohmic contacts on two-dimensional semiconductors

In recent years, two-dimensional materials have received more and more attention in the development of semiconductor devices, and their practical applications in optoelectronic devices have also developed rapidly. However, there are still some factors that limit the performance of two-dimensional semiconductor material devices, and one of the most important is Ohmic contact. Here, we elaborate on a variety of approaches to achieve Ohmic contacts on two-dimensional materials and reveal their physical mechanisms. For the work function mismatch problem, we summarize the comparison of barrier heights between different metals and 2D semiconductors. We also examine different methods to solve the problem of Fermi level pinning. For the novel 2D metal-semiconductor contact methods, we analyse their effects on reducing contact resistance from two different perspectives: homojunction and heterojunction. Finally, the challenges of 2D semiconductors in achieving Ohmic contacts are outlined.

Molecule-Upgraded van der Waals Contacts for Schottky-Barrier-Free Electronics

The applications of any ultrathin semiconductor device are inseparable from high-quality metal-semiconductor contacts with designed Schottky barriers. Building van der Waals (vdWs) contacts of 2D semiconductors represents an advanced strategy of lowering the Schottky barrier height by reducing interface states, but will finally fail at the theoretical minimum barrier due to the inevitable energy difference between the semiconductor electron affinity and the metal work function. Here, an effective molecule optimization strategy is reported to upgrade the general vdWs contacts, achieving near-zero Schottky barriers and creating high-performance electronic devices. The molecule treatment can induce the defect healing effect in p-type semiconductors and further enhance the hole density, leading to an effectively thinned Schottky barrier width and improved carrier interface transmission efficiency. With an ultrathin Schottky barrier width of ≈2.17 nm and outstanding contact resistance of ≈9 kΩ µm in the optimized Au/WSe₂ contacts, an ultrahigh field-effect mobility of ≈148 cm²  V^-1 s^-1 in chemical vapor deposition grown WSe₂ flakes is achieved. Unlike conventional chemical treatments, this molecule upgradation strategy leaves no residue and displays a high-temperature stability at >200 °C. Furthermore, the Schottky barrier optimization is generalized to other metal-semiconductor contacts, including 1T-PtSe₂ /WSe₂ , 1T'-MoTe₂ /WSe₂ , 2H-NbS₂ /WSe₂ , and Au/PdSe₂ , defining a simple, universal, and scalable method to minimize contact resistance.

High performance sub-bandgap photodetection via internal photoemission based on ideal metal/2D-material van der Waals Schottky interface

Two-dimensional (2D) materials have been demonstrated to be promising candidates to design high performance photodetectors owing to their strong light-matter interaction. However, the performance of 2D material photodetectors is still unsatisfactory, such as slow response speed due to defects and vulnerable contact interface, which impede their rapid development in the field of optoelectronics. In this paper, we obtained the ideal and large photosensitive van der Waals Schottky interface by the laminating-flipping method. Hence, a fast response speed (<1 ms) and high detectivity (>10¹² Jones) are observed on the van der Waals Schottky junction photodiode. More importantly, benefiting from the flat Schottky interface (the roughness ∼0.6 nm), a sub-bandgap light response modulated by the Schottky barrier height (cut-off edge at 1050 nm) has been detected based on the large Au/MoSe₂ sensitive Schottky interface internal photoemission. As a result, a universal strategy for the sub-bandgap near-infrared van der Waals Schottky junction detector of 2D materials was obtained.

Local Interactions of Atmospheric Oxygen with MoS2 Crystals

Thin and single MoS2 flakes are envisioned to contribute to the flexible nanoelectronics, particularly in sensing, optoelectronics and energy harvesting. Thus, it is important to study their stability and local surface reactivity. Their most straightforward surface reactions in this context pertain to thermally induced interactions with atmospheric oxygen. This review focuses on local and thermally induced interactions of MoS2 crystals and single MoS2 flakes. First, experimentally observed data for oxygen-mediated thermally induced morphological and chemical changes of the MoS2 crystals and single MoS2 flakes are presented. Second, state-of-the-art mechanistic insight from computer simulations and arising open questions are discussed. Finally, the properties and fate of the Mo oxides arising from thermal oxidation are reviewed, and future directions into the research of the local MoS2/MoOx interface are provided.

The Schottky-Mott Rule Expanded for Two-Dimensional Semiconductors: Influence of Substrate Dielectric Screening

A comprehensive understanding of the energy level alignment mechanisms between two-dimensional (2D) semiconductors and electrodes is currently lacking, but it is a prerequisite for tailoring the interface electronic properties to the requirements of device applications. Here, we use angle-resolved direct and inverse photoelectron spectroscopy to unravel the key factors that determine the level alignment at interfaces between a monolayer of the prototypical 2D semiconductor MoS₂ and conductor, semiconductor, and insulator substrates. For substrate work function (Φ_sub) values below 4.5 eV we find that Fermi level pinning occurs, involving electron transfer to native MoS₂ gap states below the conduction band. For Φ_sub above 4.5 eV, vacuum level alignment prevails but the charge injection barriers do not strictly follow the changes of Φ_sub as expected from the Schottky-Mott rule. Notably, even the trends of the injection barriers for holes and electrons are different. This is caused by the band gap renormalization of monolayer MoS₂ by dielectric screening, which depends on the dielectric constant (ε_r) of the substrate. Based on these observations, we introduce an expanded Schottky-Mott rule that accounts for band gap renormalization by ε_r -dependent screening and show that it can accurately predict charge injection barriers for monolayer MoS₂. It is proposed that the formalism of the expanded Schottky-Mott rule should be universally applicable for 2D semiconductors, provided that material-specific experimental benchmark data are available.

Tuning the Optical Band Gap of Two-Dimensional WS2 Integrated with Gold Nanocubes by Introducing Palladium Nanostructures

The two characteristic absorption peaks of semiconducting two-dimensional tungsten disulfide (WS₂) are red-shifted after integrating with gold nanocube (AuNC) arrays. The amount of the red shift is reduced when the AuNCs are coated with a high concentration of Pd. A negligible shift was observed in the absorption peaks of WS₂ when smaller amounts of Pd are introduced to the surface of AuNCs. Conversely, the photoluminescence (PL) of WS₂ is blue-shifted when measured on top of AuNCs and AuNCs coated with different amounts of Pd. AuNC-Pd Janus nanoparticles are prepared by depositing Pd atoms asymmetrically on AuNCs assembled into 2-D arrays on the surface of a glass substrate by the chemical reduction of Pd ions. Due to the large AuNC or AuNC-Pd/WS₂ Schottky barrier, the plasmon-induced hot electron transfer (PHET) from AuNCs and AuNCs coated with a high concentration of Pd is responsible for the red shift of the absorption spectrum of WS₂. Introducing a lower concentration of Pd to AuNCs increases the Schottky barrier further due to the formation of the Au-Pd equilibrium Fermi level of lower energy, reducing the efficiency of PHET. The effect of Pd on the Fermi level of AuNCs vanishes at high Pd deposition. Pauli blocking and phase-space filling are responsible for the blue shift of PL of WS₂ on top of AuNCs and AuNCs coated with Pd. The Pauli blocking effect is directly proportional to the PHET efficiency. This explains the significant blue shift of PL of WS₂ after integrating with AuNCs and AuNCs coated with a high concentration of Pd. Additionally, depositing Pd onto AuNCs elongates the lifetime of the hot electrons and enhances the PHET efficiency.

Ta2 Ni3 Se8 : 1D van der Waals Material with Ambipolar Behavior

In this study, high-purity and centimeter-scale bulk Ta₂ Ni₃ Se₈ crystals are obtained by controlling the growth temperature and stoichiometric ratio between tantalum, nickel, and selenium. It is demonstrated that the bulk Ta₂ Ni₃ Se₈ crystals could be effectively exfoliated into a few chain-scale nanowires through simple mechanical exfoliation and liquid-phase exfoliation. Also, the calculation of electronic band structures confirms that Ta₂ Ni₃ Se₈ is a semiconducting material with a small bandgap. A field-effect transistor is successfully fabricated on the mechanically exfoliated Ta₂ Ni₃ Se₈ nanowires. Transport measurements at room temperature reveal that Ta₂ Ni₃ Se₈ nanowires exhibit ambipolar semiconducting behavior with maximum mobilities of 20.3 and 3.52 cm² V^-1 s^-1 for electrons and holes, respectively. The temperature-dependent transport measurement (from 90 to 295 K) confirms the carrier transport mechanism of Ta₂ Ni₃ Se₈ nanowires. Based on these characteristics, the obtained 1D vdW material is expected to be a potential candidate for additional 1D materials as channel materials.

On the Contact Optimization of ALD-Based MoS2 FETs: Correlation of Processing Conditions and Interface Chemistry with Device Electrical Performance

Despite the extensive ongoing research on MoS₂ field effect transistors (FETs), the key role of device processing conditions in the chemistry involved at the metal-to-MoS₂ interface and their influence on the electrical performance are often overlooked. In addition, the majority of reports on MoS₂ contacts are based on exfoliated MoS₂, whereas synthetic films are even more susceptible to the changes made in device processing conditions. In this paper, working FETs with atomic layer deposition (ALD)-based MoS₂ films and Ti/Au contacts are demonstrated, using current-voltage (I-V) characterization. In pursuit of optimizing the contacts, high-vacuum thermal annealing as well as O₂/Ar plasma cleaning treatments are introduced, and their influence on the electrical performance is studied. The electrical findings are linked to the interface chemistry through X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) analyses. XPS evaluation reveals that the concentration of organic residues on the MoS₂ surface, as a result of resist usage during the device processing, is significant. Removal of these contaminations with O₂/Ar plasma changes the MoS₂ chemical state and enhances the MoS₂ electrical properties. Based on the STEM analysis, the observed progress in the device electrical characteristics could also be associated with the formation of a continuous TiS _x layer at the Ti-to-MoS₂ interface. Scaling down the Ti interlayer thickness and replacing it with Cr is found to be beneficial as well, leading to further device performance advancements. Our findings are of value for attaining optimal contacts to synthetic MoS₂ films.

Fabrication of Large-Area Molybdenum Disulfide Device Arrays Using Graphene/Ti Contacts

Two-dimensional (2D) molybdenum disulfide (MoS₂) is the most mature material in 2D material fields owing to its relatively high mobility and scalability. Such noticeable properties enable it to realize practical electronic and optoelectronic applications. However, contact engineering for large-area MoS₂ films has not yet been established, although contact property is directly associated to the device performance. Herein, we introduce graphene-interlayered Ti contacts (graphene/Ti) into large-area MoS₂ device arrays using a wet-transfer method. We achieve MoS₂ devices with superior electrical and photoelectrical properties using graphene/Ti contacts, with a field-effect mobility of 18.3 cm²/V∙s, on/off current ratio of 3 × 10⁷, responsivity of 850 A/W, and detectivity of 2 × 10¹² Jones. This outstanding performance is attributable to a reduction in the Schottky barrier height of the resultant devices, which arises from the decreased work function of graphene induced by the charge transfer from Ti. Our research offers a direction toward large-scale electronic and optoelectronic applications based on 2D materials.

Modification of interface and electronic transport in van der Waals heterojunctions by UV/O3

Two-dimensional (2D) van der Waals heterojunctions have many unique properties, and energy band modulation is central to applying these properties to electronic devices. Taking the 2D graphene/MoS₂heterojunction as a model system, we demonstrate that the band structure can be finely tuned by changing the graphene structure of the 2D heterojunction via ultraviolet/ozone (UV/O₃). With increasing UV/O₃exposure time, graphene in the heterojunction has more defect structures. The varied defect levels in graphene modulate the interfacial charge transfer, accordingly the band structure of the heterojunction. And the corresponding performance change of the graphene/MoS₂field effect transistor indicates the shift of the Schottky barrier height after UV/O₃treatment. The result further proves the effective band structure modulation of the graphene/MoS₂heterojunction by UV/O₃. This work will be beneficial to both fundamental research and practical applications of 2D van der Waals heterojunction in electronic devices.

Mobility Extraction in 2D Transition Metal Dichalcogenide Devices-Avoiding Contact Resistance Implicated Overestimation

Schottky barrier (SB) transistors operate distinctly different from conventional metal-oxide semiconductor field-effect transistors, in a unique way that the gate impacts the carrier injection from the metal source/drain contacts into the channel region. While it has been long recognized that this can have severe implications for device characteristics in the subthreshold region, impacts of contact gating of SB in the on-state of the devices, which affects evaluation of intrinsic channel properties, have been yet comprehensively studied. Due to the fact that contact resistance (R_C ) is always gate-dependent in a typical back-gated device structure, the traditional approach of deriving field-effect mobility from the maximum transconductance (g_m ) is in principle not correct and can even overestimate the mobility. In addition, an exhibition of two different threshold voltages for the channel and the contact region leads to another layer of complexity in determining the true carrier concentration calculated from Q = C_OX * (V_G -V_TH ). Through a detailed experimental analysis, the effect of different effective oxide thicknesses, distinct SB heights, and doping-induced reductions in the SB width are carefully evaluated to gain a better understanding of their impact on important device metrics.

Energy-Level Alignment at Interfaces between Transition-Metal Dichalcogenide Monolayers and Metal Electrodes Studied with Kelvin Probe Force Microscopy

We studied the energy-level alignment at interfaces between various transition-metal dichalcogenide (TMD) monolayers, MoS₂, MoSe₂, WS₂, and WSe₂, and metal electrodes with different work functions (WFs). TMDs were deposited on SiO₂/silicon wafers by chemical vapor deposition and transferred to Al and Au substrates, with significantly different WFs to identify the metal-semiconductor junction behavior: oxide-terminated Al (natural oxidation) and Au (UV-ozone oxidation) with a WF difference of 0.8 eV. Kelvin probe force microscopy was employed for this study, based on which electronic band diagrams for each case were determined. We observed the Fermi-level pinning for MoS₂, while WSe₂/metal junctions behaved according to the Schottky-Mott limit. WS₂ and MoSe₂ exhibited intermediate behavior.

Large-Area, Two-Dimensional MoS2 Exfoliated on Gold: Direct Experimental Access to the Metal-Semiconductor Interface

Two-dimensional semiconductors such as MoS₂ are promising for future electrical devices. The interface to metals is a crucial and critical aspect for these devices because undesirably high resistances due to Fermi level pinning are present, resulting in unwanted energy losses. To date, experimental information on such junctions has been obtained mainly indirectly by evaluating transistor characteristics. The fact that the metal-semiconductor interface is typically embedded, further complicates the investigation of the underlying physical mechanisms at the interface. Here, we present a method to provide access to a realistic metal-semiconductor interface by large-area exfoliation of single-layer MoS₂ on clean polycrystalline gold surfaces. This approach allows us to measure the relative charge neutrality level at the MoS₂-gold interface and its spatial variation almost directly using Kelvin probe force microscopy even under ambient conditions. By bringing together hitherto unconnected findings about the MoS₂-gold interface, we can explain the anomalous Raman signature of MoS₂ in contact to metals [ACS Nano. 7, 2013, 11350] which has been the subject of intense recent discussions. In detail, we identify the unusual Raman mode as the A_1g mode with a reduced Raman shift (397 cm^-1) due to the weakening of the Mo-S bond. Combined with our X-ray photoelectron spectroscopy data and the measured charge neutrality level, this is in good agreement with a previously predicted mechanism for Fermi level pinning at the MoS₂-gold interface [Nano Lett. 14, 2014, 1714]. As a consequence, the strength of the MoS₂-gold contact can be determined from the intensity ratio between the reduced A_1greduced mode and the unperturbed A_1g mode.

Highly Efficient Experimental Approach to Evaluate Metal to 2D Semiconductor Interfaces in Vertical Diodes with Asymmetric Metal Contacts

The energy band alignments and associated material properties at the contacts between metal and two-dimensional (2D) semiconducting transition metal dichalcogenide (SCTMD) films determine the important traits in 2D SCTMD-based electronic and optical device applications. In this work, we realize 2D vertical diodes with asymmetric metal-SCTMD contact areas where currents are dominated by the contact-limited charge flows in the transport regimes of Fowler-Nordheim tunneling and Schottky emission. With straightforward current-voltage characteristics, we can accurately evaluate the interface parameters such as Schottky barrier heights and the vertical effective masses of tunneling charges. In particular, the differing contact areas and resultant current rectifications allow us to address specific Schottky barrier locations with respect to the conduction and valence band edges of 2D semiconducting WSe₂, WS₂, MoSe₂, and MoS₂, thereby determining whether p-type holes or n-type electrons become the majority charge carriers in the SCTMD devices. We demonstrate that our experimental and analytical approaches can be utilized as a simple but powerful material metrology to qualitatively and quantitatively evaluate various metal-SCTMD contacts.

Tunable Doping of Rhenium and Vanadium into Transition Metal Dichalcogenides for Two-Dimensional Electronics

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) with unique electrical properties are fascinating materials used for future electronics. However, the strong Fermi level pinning effect at the interface of TMDCs and metal electrodes always leads to high contact resistance, which seriously hinders their application in 2D electronics. One effective way to overcome this is to use metallic TMDCs or transferred metal electrodes as van der Waals (vdW) contacts. Alternatively, using highly conductive doped TMDCs will have a profound impact on the contact engineering of 2D electronics. Here, a novel chemical vapor deposition (CVD) using mixed molten salts is established for vapor-liquid-solid growth of high-quality rhenium (Re) and vanadium (V) doped TMDC monolayers with high controllability and reproducibility. A tunable semiconductor to metal transition is observed in the Re- and V-doped TMDCs. Electrical conductivity increases up to a factor of 108 in the degenerate V-doped WS2 and WSe2 . Using V-doped WSe2 as vdW contact, the on-state current and on/off ratio of WSe2 -based field-effect transistors have been substantially improved (from ≈10-8 to 10-5 A; ≈104 to 108 ), compared to metal contacts. Future studies on lateral contacts and interconnects using doped TMDCs will pave the way for 2D integrated circuits and flexible electronics.

Ultrathin Three-Monolayer Tunneling Memory Selectors

High-density memory arrays require selector devices, which enable selection of a specific memory cell within a memory array by suppressing leakage current through unselected cells. Such selector devices must have highly nonlinear current-voltage characteristics and excellent endurance; thus selectors based on a tunneling mechanism present advantages over those based on the physical motion of atoms or ions. Here, we use two-dimensional (2D) materials to build an ultrathin (three-monolayer-thick) tunneling-based memory selector. Using a sandwich of h-BN, MoS₂, and h-BN monolayers leads to an "H-shaped" energy barrier in the middle of the heterojunction, which nonlinearly modulates the tunneling current when the external voltage is varied. We experimentally demonstrate that tuning the MoS₂ Fermi level can improve the device nonlinearity from 10 to 25. These results provide a fundamental understanding of the tunneling process through atomically thin 2D heterojunctions and lay the foundation for developing high endurance selectors with 2D heterojunctions, potentially enabling high-density non-volatile memory systems.

Electrical contacts to few-layer MoS2 with phase-engineering and metal intercalation for tuning the contact performance

Due to Fermi-level pinning in metal-two-dimensional MoS₂ junctions, improving the performance of MoS₂-based electrical devices is still under extensive study. The device performance of few-layer MoS₂ depends strongly on the number of layers. In this work, via density-functional theory calculations, a comprehensive understanding from the atomistic view was reached for the interlayer interaction between metal and few-layer MoS₂ with phase-engineering and intercalation doping, which are helpful for improving the contact performance. These two methods are probed to tune the performance of few-layer MoS₂-based field-effect transistors, and both of them can tune the Schottky barrier height. Phase-engineering, which means that the MoS₂ layer in contact with metal is converted to the T phase, can transform the Schottky barrier from n- to p-type. Intercalation doping, which takes advantage of annealing and results in metal atom interaction in between MoS₂ layers, makes the MoS₂ layers become quasi-freestanding and converts the indirect bandgap into direct bandgap. Our atomistic insights help improve the performance of few-layer MoS₂-based electronic devices.

A comparative study of electrical and opto-electrical properties of a few-layer p-WSe2/n-WS2 heterojunction diode on SiO2 and h-BN substrates

Since the innovation of van der Waals heterostructures of 2D materials, the p–n junction diode, a building block of electronics and opto-electronics has been studied in various ways. To date most of them have been studied on SiO₂ or other oxide substrates, although the oxide substrates cause significant degradation of the 2D material's intrinsic properties and device performances. Whereas using hexagonal boron nitride (h-BN) as an underlying layer to the 2D materials is known to preserve their properties. Here we have carefully analyzed the electrical and opto-electrical properties of a p-WSe₂/n-WS₂ van der Waals heterojunction diode on SiO₂ and the h-BN substrates. Besides the usual enhancement of the field-effect mobility of WSe₂ and WS₂, we have achieved a significant enhancement of the diode rectification ratio and excellent photovoltaic characteristics on the h-BN substrate. We have obtained more than an order-of-magnitude enhancement of the diode rectification ratio and about two-fold increments in the overall opto-electronics behavior on the h-BN substrate compared with those on the SiO₂ substrate. The values of self-powered photo responsivity and external quantum efficiency are 3 A/W and 588% respectively on the h-BN substrate at 10 mW cm⁻² photo-power density and 633 nm wavelength, whereas they reduce to about one-half on the SiO₂ substrate.

A few-layer WSe₂/WS₂ heterojunction diode on an h-BN substrate shows improved electronic and optoelectronic characteristics with a robust diode rectification ratio and photo responsivity compared to that on a SiO₂ substrate.

Ultralow contact resistance between semimetal and monolayer semiconductors

Advanced beyond-silicon electronic technology requires both channel materials and also ultralow-resistance contacts to be discovered^1,2. Atomically thin two-dimensional semiconductors have great potential for realizing high-performance electronic devices^1,3. However, owing to metal-induced gap states (MIGS)^4-7, energy barriers at the metal-semiconductor interface-which fundamentally lead to high contact resistance and poor current-delivery capability-have constrained the improvement of two-dimensional semiconductor transistors so far^2,8,9. Here we report ohmic contact between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where the MIGS are sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a contact resistance of 123 ohm micrometres and an on-state current density of 1,135 microamps per micrometre on monolayer MoS₂; these two values are, to the best of our knowledge, the lowest and highest yet recorded, respectively. We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS₂, WS₂ and WSe₂. Our reported contact resistances are a substantial improvement for two-dimensional semiconductors, and approach the quantum limit. This technology unveils the potential of high-performance monolayer transistors that are on par with state-of-the-art three-dimensional semiconductors, enabling further device downscaling and extending Moore's law.