Conduction Mechanisms in CVD-Grown Monolayer MoS2 Transistors: From Variable-Range Hopping to Velocity Saturation

Study of an MoS 2 phototransistor using a compact numerical method enabling detailed analysis of 2D material phototransistors

Research on two-dimensional material-based phototransistors has recently become a topic of great interest. However, the high number of design features, which impact the performance of these devices, and the multi-physical nature of the device operation make the accurate analysis of these devices a challenge. Here, we present a simple yet effective numerical framework to overcome this challenge. The one-dimensional framework is constructed on the drift-diffusion equations, Poisson's equation, and wave propagation in multi-layered medium formalism. We apply this framework to study phototransistors made from monolayer molybdenum disulfide ( MoS 2 ) placed on top of a back-gated silicon-oxide-coated silicon substrate. Numerical results, which show good agreement with the experimental results found in the literature, emphasize the necessity of including the inhomogeneous background for accurately calculating device metrics such as quantum efficiency and bandwidth. For the first time in literature, we calculate the phase noise of these phototransistors, which is a crucial performance metric for many applications where precise timing and synchronization are critical. We determine that applying a low drain-to-source voltage is the key requirement for low phase noise.

Physical vapor deposition-free scalable high-efficiency electrical contacts to MoS2

Fermi-level pinning caused by the kinetic damage during metallization has been recognized as one of the major reasons for the non-ideal behavior of electrical contacts, forbidding reaching the Schottky-Mott limit. In this manuscript, we present a scalable technique wherein Indium, a low-work-function metal, is diffused to contact a few-layered MoS2flake. The technique exploits a smooth outflow of Indium over gold electrodes to make edge contacts to pre-transferred MoS2flakes. We compare the performance of three pairs of contacts made onto the same MoS2flake, the bottom-gold, top-gold, and Indium contacts, and find that the Indium contacts are superior to other contacts. The Indium contacts maintain linearI-Vcharacteristics down to cryogenic temperatures with an extracted Schottky barrier height of ∼2.1 meV. First-principle calculations show the induced in-gap states close to the Fermi level, and the damage-free contact interface could be the reason for the nearly Ohmic behavior of the Indium/MoS2interface.

CVD Synthesis of MoS2 Using a Direct MoO2 Precursor: A Study on the Effects of Growth Temperature on Precursor Diffusion and Morphology Evolutions

In this study, the influence of growth temperature variation on the synthesis of MoS2 using a direct MoO2 precursor was investigated. The research showed that the growth temperature had a strong impact on the resulting morphologies. Below 650 °C, no nucleation or growth of MoS2 occurred. The optimal growth temperature for producing continuous MoS2 films without intermediate-state formation was approximately 760 °C. However, when the growth temperatures exceeded 800 °C, a transition from pure MoS2 to predominantly intermediate states was observed. This was attributed to enhanced diffusion of the precursor at higher temperatures, which reduced the local S:Mo ratio. The diffusion equation was analyzed, showing how the diffusion coefficient, diffusion length, and concentration gradients varied with temperature, consistent with the experimental observations. This study also investigated the impact of increasing the MoO2 precursor amount, resulting in the formation of multilayer MoS2 domains at the outermost growth zones. These findings provide valuable insights into the growth criteria for the effective synthesis of clean and large-area MoS2, thereby facilitating its application in semiconductors and related industries.

Physics-based bias-dependent compact modeling of 1/f noise in single- to few-layer 2D-FETs

1/f noise is a critical figure of merit for the performance of transistors and circuits. For two-dimensional devices (2D-FETs), and especially for applications in the GHz range where short-channel FETs are required, the velocity saturation (VS) effect can result in the reduction of 1/f noise at high longitudinal electric fields. A new physics-based compact model has been for the first time introduced for single- to few-layer 2D-FETs in this study, precisely validating 1/f noise experiments for various types of devices. The proposed model mainly accounts for the measured 1/f noise bias dependence as the latter is defined by different physical mechanisms. Thus, analytical expressions are derived, valid in all regions of operation in contrast to conventional approaches available in the literature so far, accounting for carrier number fluctuation (ΔN), mobility fluctuation (Δμ) and contact resistance (ΔR) effects based on the underlying physics that rules these devices. The ΔN mechanism due to trapping/detrapping together with an intense Coulomb scattering effect dominates the 1/f noise from the medium to the strong accumulation region while Δμ has also been demonstrated to modestly contribute in the subthreshold region. ΔR can also be significant in a very high carrier density. The VS induced reduction of 1/f noise measurements at high electric fields was also remarkably captured by the model. The physical validity of the model can also assist in extracting credible conclusions when conducting comparisons between experimental data from devices with different materials or dielectrics.

Interface contact and modulated electronic properties by in-plain strains in a graphene-MoS2 heterostructure

Designing a specific heterojunction by assembling suitable two-dimensional (2D) semiconductors has shown significant potential in next-generation micro-nano electronic devices. In this paper, we study the structural and electronic properties of graphene-MoS2 (Gr-MoS2) heterostructures with in-plain biaxial strain using density functional theory. It is found that the interaction between graphene and monolayer MoS2 is characterized by a weak van der Waals interlayer coupling with the stable layer spacing of 3.39 Å and binding energy of 0.35 J m-2. In the presence of MoS2, the linear bands on the Dirac cone of graphene are slightly split. A tiny band gap about 1.2 meV opens in the Gr-MoS2 heterojunction due to the breaking of sublattice symmetry, and it could be effectively modulated by strain. Furthermore, an n-type Schottky contact is formed at the Gr-MoS2 interface with a Schottky barrier height of 0.33 eV, which can be effectively modulated by in-plane strain. Especially, an n-type ohmic contact is obtained when 6% tensile strain is imposed. The appearance of the non-zero band gap in graphene has opened up new possibilities for its application and the ohmic contact predicts the Gr-MoS2 van der Waals heterojunction nanocomposite as a competitive candidate in next-generation optoelectronics and Schottky devices.

Temperature dependent studies on centimeter-scale MoS2and vdW heterostructures

Transition metal dichalcogenides is an emerging 2D semiconducting material group which has excellent physical properties in the ultimately scaled thickness dimension. Specifically, van der Waals heterostructures hold the great promise in further advancing both the fundamental scientific knowledge and practical technological applications of 2D materials. Although 2D materials have been extensively studied for various sensing applications, temperature sensing still remains relatively unexplored. In this work, we experimentally study the temperature-dependent Raman spectroscopy and electrical conductivity of molybdenum disulfide (MoS₂) and its heterostructures with platinum dichalcogenides (PtSe₂and PtTe₂) to explore their potential to become the next-generation temperature sensor. It is found that the MoS₂-PtX₂heterostructure shows the great promise as the high-sensitivity temperature sensor.

CVD Synthesis of Intermediate State-Free, Large-Area and Continuous MoS2 via Single-Step Vapor-Phase Sulfurization of MoO2 Precursor

The low evaporation temperature and carcinogen classification of commonly used molybdenum trioxide (MoO3) precursor render it unsuitable for the safe and practical synthesis of molybdenum disulfide (MoS2). Furthermore, as evidenced by several experimental findings, the associated reaction constitutes a multistep process prone to the formation of uncontrolled amounts of intermediate MoS2-yOy phase mixed with the MoS2 crystals. Here, molybdenum dioxide (MoO2), a chemically more stable and safer oxide than MoO3, was utilized to successfully grow cm-scale continuous films of monolayer MoS2. A high-resolution optical image stitching approach and Raman line mapping were used to confirm the composition and homogeneity of the material grown across the substrate. A detailed examination of the surface morphology of the continuous film revealed that, as the gas flow rate increased by an order of magnitude, the grain-boundary separation dramatically reduced, implying a transition from a kinetically to thermodynamically controlled growth. Importantly, the single-step vapor-phase sulfurization (VPS) reaction of MoO2 was shown to suppress intermediate state formations for a wide range of experimental parameters investigated and is completely absent, provided that the global S:Mo loading ratio is set higher than the stoichiometric ratio of 3:1 required by the VPS reaction.

Enhancement of Photodetective Properties on Multilayered MoS2 Thin Film Transistors via Self-Assembled Poly-L-Lysine Treatment and Their Potential Application in Optical Sensors

Photodetectors and display backplane transistors based on molybdenum disulfide (MoS2) have been regarded as promising topics. However, most studies have focused on the improvement in the performances of the MoS2 photodetector itself or emerging applications. In this study, to suggest a better insight into the photodetector performances of MoS2 thin film transistors (TFTs), as photosensors for possible integrated system, we performed a comparative study on the photoresponse of MoS2 and hydrogenated amorphous silicon (a-Si:H) TFTs. As a result, in the various wavelengths and optical power ranges, MoS2 TFTs exhibit 2~4 orders larger photo responsivities and detectivities. The overall quantitative comparison of photoresponse in single device and inverters confirms a much better performance by the MoS2 photodetectors. Furthermore, as a strategy to improve the field effect mobility and photoresponse of the MoS2 TFTs, molecular doping via poly-L-lysine (PLL) treatment was applied to the MoS2 TFTs. Transfer and output characteristics of the MoS2 TFTs clearly show improved photocurrent generation under a wide range of illuminations (740~365 nm). These results provide useful insights for considering MoS2 as a next-generation photodetector in flat panel displays and makes it more attractive due to the fact of its potential as a high-performance photodetector enabled by a novel doping technique.

Maskless Device Fabrication and Laser-Induced Doping in MoS2 Field Effect Transistors Using a Thermally Activated Cyclic Polyphthalaldehyde Resist

We present a novel maskless device fabrication technique for rapid prototyping of two-dimensional (2D)-based electronic materials. The technique is based on a thermally activated and self-developed cyclic polyphthalaldehyde (c-PPA) resist using a commercial Raman system and 532 nm laser illumination. Following the successful customization of electrodes to form field effect transistors based on MoS₂ monolayers, the laser-induced electronic doping of areas beneath the metal contacts that were exposed during lithography was investigated using both surface potential mapping and device characterization. An effective change in the doping level was introduced depending on the laser intensity, i.e., low laser powers resulted in p-doping, while high laser powers resulted in n-doping. Fabricated devices present a low contact resistance down to 10 kΩ·μm at a back-gate voltage of V_G = 80 V, which is attributed to the laser-induced n-type doping at the metal contact regions.

Optoelectronic synapse using monolayer MoS2 field effect transistors

Optical data sensing, processing and visual memory are fundamental requirements for artificial intelligence and robotics with autonomous navigation. Traditionally, imaging has been kept separate from the pattern recognition circuitry. Optoelectronic synapses hold the special potential of integrating these two fields into a single layer, where a single device can record optical data, convert it into a conductance state and store it for learning and pattern recognition, similar to the optic nerve in human eye. In this work, the trapping and de-trapping of photogenerated carriers in the MoS₂/SiO₂ interface of a n-channel MoS₂ transistor was employed to emulate the optoelectronic synapse characteristics. The monolayer MoS₂ field effect transistor (FET) exhibits photo-induced short-term and long-term potentiation, electrically driven long-term depression, paired pulse facilitation (PPF), spike time dependent plasticity, which are necessary synaptic characteristics. Moreover, the device’s ability to retain its conductance state can be modulated by the gate voltage, making the device behave as a photodetector for positive gate voltages and an optoelectronic synapse at negative gate voltages.

Impact of thermal annealing in forming gas on the optical and electrical properties of MoS2monolayer

Technological applications involving 2D MoS2require transfer of chemical vapor deposition (CVD) grown material from its original substrate and subsequent lithographic processes. Inevitably, those steps contaminate the surface of the 2D material with polymeric residues affecting the electronic and optical properties of the MoS2. Annealing in forming gas is considered an efficient treatment to partially remove such residues. However, hydrogen also interacts with MoS2creating or saturating sulfur vacancies. Sulfur vacancies are known to be at the origin of n-doping evident in the majority of as-grown MoS2samples. In this context, investigating the impact of thermal annealing in forming gas on the electronic and optical properties of MoS2monolayer is technologically important. In order to address this topic, we have systematically studied the evolution of CVD grown MoS2monolayer using Raman spectroscopy, photoluminescence, x-ray photoelectron spectroscopy and transport measurements through a series of thermal annealing in forming gas at temperatures up to 500 °C. Efficient removal of the polymeric residues is demonstrated at temperatures as low as 200 °C. Above this value, carrier density modulation is identified by photoluminescence, x-ray photoelectron spectroscopy and electrical characterization and is correlated to the creation of sulfur vacancies. Finally, the degradation of the MoS2single layer is verified with annealing at or above 350 °C through Raman and photocurrent measurements.

High-mobility junction field-effect transistor via graphene/MoS2 heterointerface

Monolayer molybdenum disulfide (MoS₂) possesses a desirable direct bandgap with moderate carrier mobility, whereas graphene (Gr) exhibits a zero bandgap and excellent carrier mobility. Numerous approaches have been suggested for concomitantly realizing high on/off current ratio and high carrier mobility in field-effect transistors, but little is known to date about the effect of two-dimensional layered materials. Herein, we propose a Gr/MoS₂ heterojunction platform, i.e., junction field-effect transistor (JFET), that enhances the carrier mobility by a factor of ~ 10 (~ 100 cm² V⁻¹ s⁻¹) compared to that of monolayer MoS₂, while retaining a high on/off current ratio of ~ 10⁸ at room temperature. The Fermi level of Gr can be tuned by the wide back-gate bias (V_BG) to modulate the effective Schottky barrier height (SBH) at the Gr/MoS₂ heterointerface from 528 meV (n-MoS₂/p-Gr) to 116 meV (n-MoS₂/n-Gr), consequently enhancing the carrier mobility. The double humps in the transconductance derivative profile clearly reveal the carrier transport mechanism of Gr/MoS₂, where the barrier height is controlled by electrostatic doping.

Phonon-Assisted Hopping through Defect States in MoS2: A Multiscale Simulation

Understanding the carrier transport mechanism in transition metal dichalcogenides (TMDs) is essential for their device application. Experiments demonstrated that at low carrier density and room temperature, the conductivity in TMDs is dominant by activation hopping transport through localized S-vacancy states. In this work, a multiscale model combining ab initio calculation and the Marcus theory is applied to study such transport. We identify phonon-assisted hopping (PAH) as the most possible mechanism for the activation hopping. It is found that the macroscopic conductivity is mainly contributed by a few microscopic percolation paths. Analysis on the hopping distance indicates nearest-neighbor hopping behavior. The dependence of PAH mobility on defect concentration, temperature, and energy mismatch between defect sites is discussed. It is shown that all these factors can strongly affect the mobility. We further proposed that alloying can be an efficient way to tune the mobility due to increased energy mismatch effect.

MoS2-enabled dual-mode optoelectronic biosensor using a water soluble variant of μ-opioid receptor for opioid peptide detection

Owing to their unique electrical and optical properties, two-dimensional transition metal dichalcogenides have been extensively studied for their potential applications in biosensing. However, simultaneous utilization of both optical and electrical properties has been overlooked, yet it can offer enhanced accuracy and detection versitility. Here, we demonstrate a dual-mode optoelectronic biosensor based on monolayer molybdenum disulfide (MoS₂) capable of producing simultaneous electrical and optical readouts of biomolecular signals. On a single platform, the biosensor exhibits a tunable photonic Fano-type optical resonance while also functioning as a field-effect transistor (FET) based on a optically transparent gate electrode. Furthermore, chemical vapor deposition grown MoS₂ provides a clean surface for direct immobilization of a water-soluble variant of the μ-opioid receptor (wsMOR), via a nickel ion-mediated linker chemistry. We utilize a synthetic opioid peptide to show the operation of the electronic and optical sensing modes. The responses of both modes exhibit a similar trend with dynamic ranges of four orders of magnitude and detection limits of <1 nM. Our work explores the potential of a versatile multimodal sensing platform enabled by monolayer MoS₂, since the integration of electrical and optical sensors on the same chip can offer flexibility in read-out and improve the accuracy in detection of low concentration targets.

High-Mobility InSe Transistors: The Nature of Charge Transport

InSe is a high-mobility layered semiconductor with mobility being highly sensitive to any surrounding media that could act as a source of extrinsic scattering. However, little effort has been made to understand electronic transport in thin InSe layers with native surface oxide formed spontaneously upon exposure to an ambient environment. Here, we explore the influence of InO_x/InSe interfacial trap states on electronic transport in thin InSe layers. We show that wet oxidation (processed in an ambient environment) causes massive deep-lying band-tail states, through which electrons conduct via 2D variable-range hopping with a short localization length of 1-3 nm. In contrast, a high-quality InO_x/InSe interface can be formed in dry oxidation (processed in pure oxygen), with a low trap density of 10¹² eV^-1 cm^-2. Metal-insulator transition can be thus observed in the gate sweep of the field-effect transistors (FETs), indicative of band transport predominated by extended states above the mobility edge. A room-temperature band mobility of 10³ cm²/V s is obtained. The profound difference in the transport behavior between the wet and dry InSe FETs suggests that fluctuating Coulomb potential arising from trapped charges at the InO_x/InSe interface is the dominant source of disorders in thin InSe channels.

Vertically Stacked CVD-Grown 2D Heterostructure for Wafer-Scale Electronics

This paper demonstrates, for the first time, wafer-scale graphene/MoS₂ heterostructures prepared by chemical vapor deposition (CVD) and their application in vertical transistors and logic gates. A CVD-grown bulk MoS₂ layer is utilized as the vertical channel, whereas CVD-grown monolayer graphene is used as the tunable work-function electrode. The short vertical channel of the transistor is formed by sandwiching bulk MoS₂ between the bottom indium tin oxide (ITO, drain electrode) and the top graphene (source electrode). The electron injection barriers at the graphene-MoS₂ junction and ITO-MoS₂ junction are modulated effectively through variation of the Schottky barrier height and its effective barrier width, respectively, because of the work-function tunability of the graphene electrode. The resulting vertical transistor with the CVD-grown MoS₂/graphene heterostructure exhibits a current density exceeding 7 A/cm², a subthreshold swing of 410 mV/dec, and an on-off current ratio exceeding 10³. The large-area synthesis, transfer, and patterning processes of both semiconducting MoS₂ and metallic graphene facilitate construction of a wafer-scale array of transistors and logic gates such as NOT, NAND, and NOR.

Impact of H-Doping on n-Type TMD Channels for Low-Temperature Band-Like Transport

Band-like transport behavior of H-doped transition metal dichalcogenide (TMD) channels in field effect transistors (FET) is studied by conducting low-temperature electrical measurements, where MoTe₂ , WSe₂ , and MoS₂ are chosen for channels. Doped with H atoms through atomic layer deposition, those channels show strong n-type conduction and their mobility increases without losing on-state current as the measurement temperature decreases. In contrast, the mobility of unintentionally (naturally) doped TMD FETs always drops at low temperatures whether they are p- or n-type. Density functional theory calculations show that H-doped MoTe₂ , WSe₂ , and MoS₂ have Fermi levels above conduction band edge. It is thus concluded that the charge transport behavior in H-doped TMD channels is metallic showing band-like transport rather than thermal hopping. These results indicate that H-doped TMD FETs are practically useful even at low-temperature ranges.

Pinch-Off Formation in Monolayer and Multilayers MoS2 Field-Effect Transistors

The discovery of layered materials, including transition metal dichalcogenides (TMD), gives rise to a variety of novel nanoelectronic devices, including fast switching field-effect transistors (FET), assembled heterostructures, flexible electronics, etc. Molybdenum disulfide (MoS₂), a transition metal dichalcogenides semiconductor, is considered an auspicious candidate for the post-silicon era due to its outstanding chemical and thermal stability. We present a Kelvin probe force microscopy (KPFM) study of a MoS₂ FET device, showing direct evidence for pinch-off formation in the channel by in situ monitoring of the electrostatic potential distribution along the conducting channel of the transistor. In addition, we present a systematic comparison between a monolayer MoS₂ FET and a few-layer MoS₂ FET regarding gating effects, electric field distribution, depletion region, and pinch-off formation in such devices.

Selectively Metallized 2D Materials for Simple Logic Devices

We herein demonstrate, for the first time, transparent, flexible, and large-area monolithic MoS₂ transistors and logic gates. Each single transistor consists of only two components: a monolithic chemical vapor deposition-grown MoS₂ and an ion gel. Additional electrode materials are not required. The uniqueness of the device configuration is attributed to two factors. One is that a MoS₂ layer is a semiconductor, but it can be doped degenerately; monolithic MoS₂ can thus serve as both the electrodes and the channel of a transistor via selective doping of the material at certain positions. The other is the use of an electrolyte gate dielectric that permits effective gating (<3 V) even from an electrode coplanar with the channel. The resulting monolithic MoS₂ transistors yield excellent device performance, including a maximum mobility of 1.5 cm²/V s, an on-off ratio of 10⁵, and a turn-on voltage of -0.69 V. This unique transistor architecture was successfully applied to various semiconductors such as ReS₂ and indium-gallium-zinc oxide. Furthermore, the presented devices exhibit excellent mechanical, operational, and environmental stabilities. Fabrication of complex logic circuits (NOT, NAND, and NOR gates) by integration of the monolithic MoS₂ transistors is demonstrated. Finally, the monolithic MoS₂ transistor was connected to drive red, green, and blue light-emitting diode pixels, which yielded high luminance at a low voltage (<3 V). We believe that the unique architecture of the devices provides a facile way for low-cost, flexible, and high-performance two-dimensional electronics.

Enhanced Performance of a CVD MoS2 Photodetector by Chemical in Situ n-Type Doping

Transition metal dichalcogenides (TMDs) are a category of promising two-dimensional (2D) materials for the optoelectronic devices, and their unique characteristics include tunable band gap, nondangling bonds as well as compatibility to large-scale fabrication, for instance, chemical vapor deposition (CVD). MoS₂ is one of the first TMDs that is well studied in the photodetection area widely. However, the low photoresponse restricts its applications in photodetectors unless the device is applied with ultrahigh source-drain voltage ( V_DS) and gate voltage ( V_GS). In this work, the photoresponse of a MoS₂ photodetector was improved by a chemical in situ doping method using gold chloride hydrate. The responsivity and specific detectivity were increased to 99.9 A/W and 9.4 × 10¹² Jones under low V_DS (0.1 V) and V_GS (0 V), which are 14.6 times and 4.8 times higher than those of a pristine photodetector, respectively. The photoresponse enhancement results from chlorine n-type doping in CVD MoS₂ which reduces the trapping of photoinduced electrons and promotes the photogating effect. This novel doping strategy leads to great applications of high-performance MoS₂ photodetectors potentially and opens a new avenue to enhance photoresponse for other 2D materials.

Monolayer MoS2 Nanoribbon Transistors Fabricated by Scanning Probe Lithography

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

Gate-Controlled Metal-Insulator Transition in TiS3 Nanowire Field-Effect Transistors

We explore the electrical characteristics of TiS₃ nanowire field-effect transistor (FETs), over the wide temperature range from 3 to 350 K. These nanomaterials have a quasi-one-dimensional (1D) crystal structure and exhibit a gate-controlled metal-insulator transition (MIT) in their transfer curves. Their room-temperature mobility is ∼20-30 cm²/(V s), 2 orders of magnitude smaller than predicted previously, a result that we explain quantitatively in terms of the influence of polar-optical phonon scattering in these materials. In the insulating state (<∼220 K), the transfer curves exhibit unusual mesoscopic fluctuations and a current suppression near zero bias that is common to charge-density wave (CDW) systems. The fluctuations have a nonmonotonic temperature dependence and wash out at a temperature close to that of the bulk MIT, suggesting they may be a feature of quantum interference in the CDW state. Overall, our results demonstrate that quasi-1D TiS₃ nanostructures represent a viable candidate for FET realization and that their functionality is influenced by complex phenomena.

Exploring Multifunctional Applications of Hexagonal Boron Arsenide Sheet: A DFT Study

The electronic, transport, optical, thermoelectric, and thermodynamic properties of the two-dimensional (2D) stable monolayer of boron arsenide (BAs) sheets have been investigated using state-of-art theoretical calculations. The energetic, dynamic, thermal, and mechanical stability of the BAs sheet have also been studied to unravel the feasibility of experimental realization of this material. Interestingly, the band gap of this semiconducting sheet changes from direct into indirect band gap material by the application of biaxial strain of 10% and it becomes metallic at 14% of biaxial strain. Furthermore, positive phonon vibrational modes have been observed for all applied biaxial strains, which ensure the stability of the sheet under strain. The semiconducting property is preserved when cutting 2D sheet into one-dimensional nanoribbons, and the band gap is size dependent. In addition, the calculated optical properties exhibit strong anisotropy. BAs nanomaterial has strong adsorption in the UV-visible region. The calculated Seebeck coefficient and power factor values show that BAs sheet is an ideal candidate for thermal management and thermoelectric applications. Finally, the thermodynamic properties have been calculated on the basis of the phonon frequencies. These rich properties of BAs nanosheets endow the system with promising applications in nanoelectronics and photovoltaics.

High carrier mobility in monolayer CVD-grown MoS2 through phonon suppression

Mobility engineering is one of the most important challenges that determine the optoelectronic performance of two-dimensional (2D) materials. So far, charged-impurity scattering and electrical-contact barriers have been suppressed through high-κ dielectrics and seamless contact engineering, giving rise to carrier-mobility improvement in exfoliated 2D semiconducting MoS2. Here we demonstrate a facile and scalable technique to effectively suppress both Coulomb scattering and electron-phonon scattering via the HfO2 overlayer, resulting in a large mobility improvement in CVD-grown monolayer MoS2, in excess of 60 cm2 V-1 s-1. Surface passivation and suppression of Coulomb scattering can partially contribute to the mobility increase. Interestingly, we correlate the mobility increase with phonon quenching through Raman and temperature-dependent mobility measurements. The experimental method is facile, industrially scalable, and renders phonon engineering an additional leverage towards further improvements in 2D semiconductor mobility and device performance.

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

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

High-Field Transport and Velocity Saturation in Synthetic Monolayer MoS2

Two-dimensional semiconductors such as monolayer MoS₂ are of interest for future applications including flexible electronics and end-of-roadmap technologies. Most research to date has focused on low-field mobility, but the peak current-driving ability of transistors is limited by the high-field saturation drift velocity, v_sat. Here, we measure high-field transport as a function of temperature for the first time in high-quality synthetic monolayer MoS₂. We find that in typical device geometries (e.g. on SiO₂ substrates) self-heating can significantly reduce current drive during high-field operation. However, with measurements at varying ambient temperature (from 100 to 300 K), we extract electron v_sat = (3.4 ± 0.4) × 10⁶ cm/s at room temperature in this three-atom-thick semiconductor, which we benchmark against other bulk and layered materials. With these results, we estimate that the saturation current in monolayer MoS₂ could exceed 1 mA/μm at room temperature, in digital circuits with near-ideal thermal management.

Van der Waals Coupled Organic Molecules with Monolayer MoS2 for Fast Response Photodetectors with Gate-Tunable Responsivity

As a direct-band-gap transition metal dichalcogenide (TMD), atomic thin MoS₂ has attracted extensive attention in photodetection, whereas the hitherto unsolved persistent photoconductance (PPC) from the ungoverned charge trapping in devices has severely hindered their employment. Herein, we demonstrate the realization of ultrafast photoresponse dynamics in monolayer MoS₂ by exploiting a charge transfer interface based on surface-assembled zinc phthalocyanine (ZnPc) molecules. The formed MoS₂/ZnPc van der Waals interface is found to favorably suppress the PPC phenomenon in MoS₂ by instantly separating photogenerated holes toward the ZnPc molecules, away from the traps in MoS₂ and the dielectric interface. The derived MoS₂ detector then exhibits significantly improved photoresponse speed by more than 3 orders (from over 20 s to less than 8 ms for the decay) and a high responsivity of 430 A/W after Al₂O₃ passivation. It is also demonstrated that the device could be further tailored to be 2-10-fold more sensitive without severely sacrificing the ultrafast response dynamics using gate modulation. The strategy presented here based on surface-assembled organic molecules may thus pave the way for realizing high-performance TMD-based photodetection with ultrafast speed and high sensitivity.

Synergetic photoluminescence enhancement of monolayer MoS2via surface plasmon resonance and defect repair

The weak light-absorption and low quantum yield (QY) in monolayer MoS₂ are great challenges for the applications of this material in practical optoelectronic devices. Here, we report on a synergistic strategy to obtain highly enhanced photoluminescence (PL) of monolayer MoS₂ by simultaneously improving the intensity of the electromagnetic field around MoS₂ and the QY of MoS₂. Self-assembled sub-monolayer Au nanoparticles underneath the monolayer MoS₂ and bis(trifluoromethane)sulfonimide (TFSI) treatment to the MoS₂ surface are used to boost the excitation field and the QY, respectively. An enhancement factor of the PL intensity as high as 280 is achieved. The enhancement mechanisms are analyzed by inspecting the contribution of the PL spectra from A excitons and A⁻ trions under different conditions. Our study takes a further step to developing high-performance optoelectronic devices based on monolayer MoS₂.

A synergistic strategy is reported to obtain a highly enhanced photoluminescence (PL) of monolayer MoS₂ by simultaneously improving the intensity of the electromagnetic field around MoS₂ and the QY of MoS₂.

Field Effect Modulation of Heterogeneous Charge Transfer Kinetics at Back-Gated Two-Dimensional MoS2 Electrodes

The ability to improve and to modulate the heterogeneous charge transfer kinetics of two-dimensional (2D) semiconductors, such as MoS₂, is a major challenge for electrochemical and photoelectrochemical applications of these materials. Here we report a continuous and reversible physical method for modulating the heterogeneous charge transfer kinetics at a monolayer MoS₂ working electrode supported on a SiO₂/p-Si substrate. The heavily doped p-Si substrate serves as a back gate electrode; application of a gate voltage (V_BG) to p-Si tunes the electron occupation in the MoS₂ conduction band and shifts the conduction band edge position relative to redox species dissolved in electrolyte in contact with the front side of the MoS₂. The gate modulation of both charge density and energy band alignment impacts charge transfer kinetics as measured by cyclic voltammetry (CV). Specifically, cyclic voltammograms combined with numerical simulations suggest that the standard heterogeneous charge transfer rate constant (k⁰) for MoS₂ in contact with the ferrocene/ferrocenium (Fc^0/+) redox couple can be modulated by over 2 orders of magnitude from 4 × 10^-6 to 1 × 10^-3 cm/s, by varying V_BG. In general, the field effect offers the potential to tune the electrochemical properties of 2D semiconductors, opening up new possibilities for fundamental studies of the relationship between charge transfer kinetics and independently controlled electronic band alignment and band occupation.

Grains in Selectively Grown MoS2 Thin Films

Transition metal dichalcogenides (TMDCs) have recently been studied using various synthesis methods, such as chemical vapor deposition for large-scale production. Despite the realization of large-scale production with high material quality, a range of approaches have been made to solve the patterning issue of TMDCs focusing on the application of integrated devices; however, patterning is still under study to accurately represent nanoscale-sized patterns, as well as the desired positions and shapes. Here, an insulating substrate is treated selectively with O₂ plasma, and MoS₂ growth is induced in the superhydrophilic area. Selectively well-grown MoS₂ patterns are confirmed by atomic force microscopy and Raman and photoluminescence spectroscopy. In addition, the grain size, according to the growth size, and grain boundary are analyzed by annual dark field transmission electron microscopy (TEM) and spherical aberration-corrected scanning TEM to confirm the selective growth. An analysis of the device performance and the optical properties reveals an enhancement with increasing grain size. This method presents the path of the growth technique for patterning, as well as the direction that can be applied to devices and integrated circuits.

Effect of Carrier Localization on Electrical Transport and Noise at Individual Grain Boundaries in Monolayer MoS2

Despite its importance in the large-scale synthesis of transition metal dichalcogenides (TMDC) molecular layers, the generic quantum effects on electrical transport across individual grain boundaries (GBs) in TMDC monolayers remain unclear. Here we demonstrate that strong carrier localization due to the increased density of defects determines both temperature dependence of electrical transport and low-frequency noise at the GBs of chemical vapor deposition (CVD)-grown MoS₂ layers. Using field effect devices designed to explore transport across individual GBs, we show that the localization length of electrons in the GB region is ∼30-70% lower than that within the grain, even though the room temperature conductance across the GB, oriented perpendicular to the overall flow of current, may be lower or higher than the intragrain region. Remarkably, we find that the stronger localization is accompanied by nearly 5 orders of magnitude enhancement in the low-frequency noise at the GB region, which increases exponentially when the temperature is reduced. The microscopic framework of electrical transport and noise developed in this paper may be readily extended to other strongly localized two-dimensional systems, including other members of the TMDC family.

Negative Differential Conductance & Hot-Carrier Avalanching in Monolayer WS2 FETs

The high field phenomena of inter-valley transfer and avalanching breakdown have long been exploited in devices based on conventional semiconductors. In this Article, we demonstrate the manifestation of these effects in atomically-thin WS2 field-effect transistors. The negative differential conductance exhibits all of the features familiar from discussions of this phenomenon in bulk semiconductors, including hysteresis in the transistor characteristics and increased noise that is indicative of travelling high-field domains. It is also found to be sensitive to thermal annealing, a result that we attribute to the influence of strain on the energy separation of the different valleys involved in hot-electron transfer. This idea is supported by the results of ensemble Monte Carlo simulations, which highlight the sensitivity of the negative differential conductance to the equilibrium populations of the different valleys. At high drain currents (>10 μA/μm) avalanching breakdown is also observed, and is attributed to trap-assisted inverse Auger scattering. This mechanism is not normally relevant in conventional semiconductors, but is possible in WS2 due to the narrow width of its energy bands. The various results presented here suggest that WS2 exhibits strong potential for use in hot-electron devices, including compact high-frequency sources and photonic detectors.

Electronic Transport in Bilayer MoS2 Encapsulated in HfO2

The exact nature of the interface between a two-dimensional crystal and its environment can have a significant impact on the electronic transport within the crystal, and can place fundamental limitations on transistor performance and long-term functionality. Two-dimensional transition-metal dichalcogenides are a new class of transistor channel material with electronic properties that can be tailored through dielectric engineering of the material/environmental interface. Here, we report electrical transport measurements carried out in the insulating regime of bilayer molybdenum disulfide, which has been encapsulated within a high-κ hafnium oxide dielectric. Temperature- and carrier-density-dependent measurements show that for T < 130 K the transport is governed by resonant tunneling, and at T = 4.2 K the tunneling peak lineshape is well-fitted by a Lorentzian with an amplitude less than e²/h. Estimates of tunneling time give τ ∼ 1.2 ps corresponding to a frequency f ∼ 0.84 THz. The tunneling processes are observable up to T ∼ 190 K (more than a factor of 6 higher than that previously reported for MoS₂ on SiO₂) despite the onset of variable range hopping at T ∼ 130 K, demonstrating the coexistence of the two transport processes within the same temperature range. At constant temperature, varying the Fermi energy allows experimental access to each transport process. The results are interpreted in terms of an increase in charge carrier screening length and a decrease in electron-phonon coupling induced by the hafnium oxide. Our results represent the first demonstration of the intermediate tunneling-hopping transport regime in a two-dimensional material. The results suggest that interface engineering may be a macroscopic tool for controlling quantum transport within such materials as well as for increasing the operating temperatures for resonant-tunneling devices derived from such materials, with applications in high-frequency electronics and logic devices.

Ambient effects on electrical characteristics of CVD-grown monolayer MoS2 field-effect transistors

Monolayer materials are sensitive to their environment because all of the atoms are at their surface. We investigate how exposure to the environment affects the electrical properties of CVD-grown monolayer MoS₂ by monitoring electrical parameters of MoS₂ field-effect transistors as their environment is changed from atmosphere to high vacuum. The mobility increases and contact resistance decreases simultaneously as either the pressure is reduced or the sample is annealed in vacuum. We see a previously unobserved, non-monotonic change in threshold voltage with decreasing pressure. This result could be explained by charge transfer on the MoS₂ channel and Schottky contact formation due to adsorbates at the interface between the gold contacts and MoS₂. Additionally, from our electrical measurements it is plausible to infer that at room temperature and pressure water and oxygen molecules adsorbed on the surface act as interface traps and scattering centers with a density of several 10¹² cm^-2 eV^-1, degrading the electrical properties of monolayer MoS₂.

Horizontally and vertically aligned growth of strained MoS2 layers with dissimilar wetting and catalytic behaviors

We report the chemical vapor deposition (CVD) based growth of aligned MoS₂ two-dimensional (2D) nanostructures and their wetting and catalytic behaviors.

Trap-mediated electronic transport properties of gate-tunable pentacene/MoS2 p-n heterojunction diodes

We investigated the trap-mediated electronic transport properties of pentacene/molybdenum disulphide (MoS₂) p-n heterojunction devices. We observed that the hybrid p-n heterojunctions were gate-tunable and were strongly affected by trap-assisted tunnelling through the van der Waals gap at the heterojunction interfaces between MoS₂ and pentacene. The pentacene/MoS₂ p-n heterojunction diodes had gate-tunable high ideality factor, which resulted from trap-mediated conduction nature of devices. From the temperature-variable current-voltage measurement, a space-charge-limited conduction and a variable range hopping conduction at a low temperature were suggested as the gate-tunable charge transport characteristics of these hybrid p-n heterojunctions. Our study provides a better understanding of the trap-mediated electronic transport properties in organic/2-dimensional material hybrid heterojunction devices.

Nanoscale-Barrier Formation Induced by Low-Dose Electron-Beam Exposure in Ultrathin MoS2 Transistors

Utilizing an innovative combination of scanning-probe and spectroscopic techniques, supported by first-principles calculations, we demonstrate how electron-beam exposure of field-effect transistors, implemented from ultrathin molybdenum disulfide (MoS₂), may cause nanoscale structural modifications that in turn significantly modify the electrical operation of these devices. Quite surprisingly, these modifications are induced by even the relatively low electron doses used in conventional electron-beam lithography, which are found to induce compressive strain in the atomically thin MoS₂. Likely arising from sulfur-vacancy formation in the exposed regions, the strain gives rise to a local widening of the MoS₂ bandgap, an idea that is supported both by our experiment and by the results of first-principles calculations. A nanoscale potential barrier develops at the boundary between exposed and unexposed regions and may cause extrinsic variations in the resulting electrical characteristics exhibited by the transistor. The widespread use of electron-beam lithography in nanofabrication implies that the presence of such strain must be carefully considered when seeking to harness the potential of atomically thin transistors. At the same time, this work also promises the possibility of exploiting the strain as a means to achieve "bandstructure engineering" in such devices.

Transport Dynamics of Neutral Excitons and Trions in Monolayer WS2

Understanding the spatial motion of excitons is of both fundamental interest and central importance for optoelectrical applications. Here, we have investigated the temperature (T) dependence of the transport dynamics of neutral excitons and charged excitons (trions) in atomically thin two-dimensional crystals of the transition-metal dichalcogenide WS₂. The transport dynamics of neutral excitons can be divided into three temperature ranges, where the diffusion of neutral excitons is governed by thermal activation (≤∼75 K), ionized impurity scattering (∼75 K ≤ T ≤∼200 K), and LO phonon scattering (≥∼200 K). The trions have a diffusion length that is over 20 times longer than that of neutral excitons at very low temperatures (≤∼10 K), which may be related to theoretically predicted Pauli-blocking effects during the excitation process.

Thermally Assisted Nonvolatile Memory in Monolayer MoS2 Transistors

We demonstrate a novel form of thermally-assisted hysteresis in the transfer curves of monolayer MoS₂ FETs, characterized by the appearance of a large gate-voltage window and distinct current levels that differ by a factor of ∼10². The hysteresis emerges for temperatures in excess of 400 K and, from studies in which the gate-voltage sweep parameters are varied, appears to be related to charge injection into the SiO₂ gate dielectric. The thermally-assisted memory is strongly suppressed in equivalent measurements performed on bilayer transistors, suggesting that weak screening in the monolayer system plays a vital role in generating its strongly sensitive response to the charge-injection process. By exploiting the full features of the hysteretic transfer curves, programmable memory operation is demonstrated. The essential principles demonstrated here point the way to a new class of thermally assisted memories based on atomically thin two-dimensional semiconductors.

Influence of Gas Adsorption and Gold Nanoparticles on the Electrical Properties of CVD-Grown MoS2 Thin Films

Molybdenum disulfide (MoS2) has increasingly attracted attention from researchers and is now one of the most intensively explored atomic-layered two-dimensional semiconductors. Control of the carrier concentration and doping type of MoS2 is crucial for its application in electronic and optoelectronic devices. Because the MoS2 layers are atomically thin, their transport characteristics may be very sensitive to ambient gas adsorption and the resulting charge transfer. We investigated the influence of the ambient gas (N2, H2/N2, and O2) choice on the resistance (R) and surface work function (WF) of trilayer MoS2 thin films grown via chemical vapor deposition. We also studied the electrical properties of gold (Au)-nanoparticle (NP)-coated MoS2 thin films; their R value was found to be 2 orders of magnitude smaller than that for bare samples. While the WF largely varied for each gas, R was almost invariant for both the bare and Au-NP-coated samples regardless of which gas was used. Temperature-dependent transport suggests that variable range hopping is the dominant mechanism for electrical conduction for bare and Au-NP-coated MoS2 thin films. The charges transferred from the gas adsorbates might be insufficient to induce measurable R change and/or be trapped in the defect states. The smaller WF and larger localization length of the Au-NP-coated sample, compared with the bare sample, suggest that more carriers and less defects enhanced conduction in MoS2.

Theoretical analysis of the combined effects of sulfur vacancies and analyte adsorption on the electronic properties of single-layer MoS2

We report a first-principles theoretical investigation on the electronic structure and electron transport of defective single-layer (SL) MoS2, as well as of corresponding structures adsorbed with benzyl viologen (BV), which was shown to provide improved performance of a field effect transistor. O2 adsorption was included to gain an understanding of the response upon air-exposure. Following analysis of the structure and stability of sulfur single vacancy and line defects in SL MoS2, we investigated the local transport at the adsorbed sites via a transport model that mimics a scanning tunneling spectroscopy experiment. Distinct current-voltage characteristics were indicated for adsorbed oxygen species at a sulfur vacancy. The electronic structures of defective MoS2 indicated the emergence of impurity states in the bandgap due to sulfur defects and oxygen adsorption. Electron transport calculations for the MoS2 surface with an extended defect in a device setting demonstrated that physisorption of BV enhances the output current, while facile chemisorption by O2 upon air-exposure causes degradation of electron transport.

Large area chemical vapor deposition of monolayer transition metal dichalcogenides and their temperature dependent Raman spectroscopy studies

We investigate the growth mechanism and temperature dependent Raman spectroscopy of chemical vapor deposited large area monolayer of MoS2, MoSe2, WS2 and WSe2 nanosheets up to 70 μm in lateral size. Further, our temperature dependent Raman spectroscopy investigation shows that softening of Raman modes as temperature increases from 80 K to 593 K is due to the negative temperature coefficient and anharmonicity. The temperature dependent softening modes of chemical vapor deposited monolayers of all TMDCs were explained on the basis of a double resonance phonon process which is more active in an atomically thin sample. This process can also be fundamentally pertinent in other emerging two-dimensional layered and heterostructured materials.