Monolayer MoS2 Field-effect Transistors Research Articles

Salt-assisted growth of monolayer MoS2 for high-performance hysteresis-free field-effect transistor

Atomically thin layered materials such as MoS2 have future versatile applications in low power electronics. Here, we demonstrate the growth of a salt-assisted large scale, high-quality monolayer MoS2 toward the realization of a high-performance hysteresis-free field-effect transistor (FET). Density functional theory calculations are implemented to monitor the effects of the Schottky barrier and metal-induced gap states between our metal electrodes and MoS2 for achieving high carrier transport. The role of absorbed molecules and oxide traps on the hysteresis are studied in detail. For the first time, a hysteresis-free intrinsic transistor behavior is obtained by an amplitude sweep pulse I–V measurement with varying pulse widths. Under this condition, a significant enhancement of the field-effect mobility up to 30 cm2 V−1 s−1 is achieved. Moreover, to correlate these results, a single-pulse time-domain drain current analysis is carried out to unleash the fast and slow transient charge trapping phenomena. Our findings on the hysteresis-free transfer characteristic and high intrinsic field-effect mobility in salt-assisted monolayer MoS2 FETs will be beneficial for future device applications in complex memory, logic, and sensor systems.

Two-Dimensional Cold Electron Transport for Steep-Slope Transistors.

Room-temperature Fermi-Dirac electron thermal excitation in conventional three-dimensional (3D) or two-dimensional (2D) semiconductors generates hot electrons with a relatively long thermal tail in energy distribution. These hot electrons set a fundamental obstacle known as the "Boltzmann tyranny" that limits the subthreshold swing (SS) and therefore the minimum power consumption of 3D and 2D field-effect transistors (FETs). Here, we investigated a graphene (Gr)-enabled cold electron injection where the Gr acts as the Dirac source to provide the cold electrons with a localized electron density distribution and a short thermal tail at room temperature. These cold electrons correspond to an electronic refrigeration effect with an effective electron temperature of ∼145 K in the monolayer MoS2, which enables the transport factor lowering and thus the steep-slope switching (across for three decades with a minimum SS of 29 mV/decade at room temperature) for a monolayer MoS2 FET. Especially, a record-high sub-60-mV/decade current density (over 1 μA/μm) can be achieved compared to conventional steep-slope technologies such as tunneling FETs or negative capacitance FETs using 2D or 3D channel materials. Our work demonstrates the potential of a 2D Dirac-source cold electron transistor as a steep-slope transistor concept for future energy-efficient nanoelectronics.

Surface functionalization toward top-gated monolayer MoS2 field-effect transistors with ZrO2/Al2O3 as composite dielectrics

Growth of high-k dielectrics on two-dimensional (2D) materials is vital for the fabrication of electronic and optoelectronic devices. In this work, the uniform high-k ZrO2 dielectric thin films were successfully grown by atomic layer deposition (ALD) on monolayer MoS2 with Al2O3 buffer layers. No significant variations appear for MoS2 after deposition of Al and ZrO2 thin films. The properties of composite dielectrics on MoS2 films have been studied by Cr-Au/ZrO2/Al2O3/MoS2 Metal-oxide-semiconductor (MOS) devices, in which H2O and O3 were used as the oxidants for ZrO2 growth, respectively. Moreover, the top-gated monolayer MoS2 field-effect transistors (FETs) with ZrO2/Al2O3 as composite dielectrics have been successfully fabricated. Importantly, the top-gated structure gives the transistors with a high mobility of 11.14 cm2 V−1 s−1, a high on/off-state current ratio of 107 at VDS = 0.1 V, a small subthreshold swing of 0.93 V dec−1, and a proper threshold voltage of 2.29 V. Hence, the integration of the high-k dielectrics on MoS2 develops the possibilities to prepare high-performance top-gated FETs based on 2D materials.

Approaching barrier-free contacts to monolayer MoS2 employing [Co/Pt] multilayer electrodes

The broken inversion symmetry and time-reversal symmetry along with the large spin–orbit interactions in monolayer MoS2 make it an ideal candidate for novel valleytronic applications. However, the realization of efficient spin-valley-controlled devices demands the integration of perpendicular magnetic anisotropy (PMA) electrodes with negligible Schottky barriers. Here, as the first demonstration, we fabricated a monolayer MoS2 field-effect transistor with PMA electrodes: Pt/[Co/Pt]3 and [Co/Pt]2. The I–V curves of PMA/MoS2 contacts show symmetric and linear behavior reflecting Ohmic nature. The flat-band Schottky barrier heights (SBHs) extracted using the temperature and gate voltage dependence of the I–V curves were found to be 10.2 and 9.6 meV. The observed SBHs are record low values reported thus far for any metal/monolayer MoS2 contact. High-quality PMA electrodes with almost zero SBH play a paramount role in the future development of novel spintronic/valleytronic devices; hence, our results can open a new route toward the realization of novel technological devices employing two-dimensional materials.

Tip-Based Cleaning and Smoothing Improves Performance in Monolayer MoS2 Devices.

Two-dimensional (2D) materials and heterostructures are promising candidates for nanoelectronics. However, the quality of material interfaces often limits the performance of electronic devices made from atomically thick 2D materials and heterostructures. Atomic force microscopy (AFM) tip-based cleaning is a reliable technique to remove interface contaminants and flatten heterostructures. Here, we demonstrate AFM tip-based cleaning applied to hBN-encapsulated monolayer MoS2 transistors, which results in electrical performance improvements of the devices. To investigate the impact of cleaning on device performance, we compared the characteristics of as-transferred heterostructures and transistors before and after tip-based cleaning using photoluminescence (PL) and electronic measurements. The PL linewidth of monolayer MoS2 decreased from 84 meV before cleaning to 71 meV after cleaning. The extrinsic mobility of monolayer MoS2 field-effect transistors increased from 21 cm2/Vs before cleaning to 38 cm2/Vs after cleaning. Using the results from AFM topography, photoluminescence, and back-gated field-effect measurements, we infer that tip-based cleaning enhances the mobility of hBN-encapsulated monolayer MoS2 by reducing interface disorder. Finally, we fabricate a MoS2 field-effect transistor (FET) from a tip-cleaned heterostructure and achieved a device mobility of 73 cm2/Vs. The results of this work could be used to improve the electrical performance of heterostructure devices and other types of mechanically assembled van der Waals heterostructures.

Benchmarking monolayer MoS2 and WS2 field-effect transistors

Here we benchmark device-to-device variation in field-effect transistors (FETs) based on monolayer MoS2 and WS2 films grown using metal-organic chemical vapor deposition process. Our study involves 230 MoS2 FETs and 160 WS2 FETs with channel lengths ranging from 5 μm down to 100 nm. We use statistical measures to evaluate key FET performance indicators for benchmarking these two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers against existing literature as well as ultra-thin body Si FETs. Our results show consistent performance of 2D FETs across 1 × 1 cm2 chips owing to high quality and uniform growth of these TMDs followed by clean transfer onto device substrates. We are able to demonstrate record high carrier mobility of 33 cm2 V−1 s−1 in WS2 FETs, which is a 1.5X improvement compared to the best reported in the literature. Our experimental demonstrations confirm the technological viability of 2D FETs in future integrated circuits.

Giant persistent photoconductivity in monolayer MoS2 field-effect transistors

Monolayer transition metal dichalcogenides (TMD) have numerous potential applications in ultrathin electronics and photonics. The exposure of TMD-based devices to light generates photo-carriers resulting in an enhanced conductivity, which can be effectively used, e.g., in photodetectors. If the photo-enhanced conductivity persists after removal of the irradiation, the effect is known as persistent photoconductivity (PPC). Here we show that ultraviolet light (λ = 365 nm) exposure induces an extremely long-living giant PPC (GPPC) in monolayer MoS2 (ML-MoS2) field-effect transistors (FET) with a time constant of ~30 days. Furthermore, this effect leads to a large enhancement of the conductivity up to a factor of 107. In contrast to previous studies in which the origin of the PPC was attributed to extrinsic reasons such as trapped charges in the substrate or adsorbates, we show that the GPPC arises mainly from the intrinsic properties of ML-MoS2 such as lattice defects that induce a large number of localized states in the forbidden gap. This finding is supported by a detailed experimental and theoretical study of the electric transport in TMD based FETs as well as by characterization of ML-MoS2 with scanning tunneling spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. The obtained results provide a basis for the defect-based engineering of the electronic and optical properties of TMDs for device applications.

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.

Schottky Barrier Height and Image Force Lowering in Monolayer MoS2 Field Effect Transistors.

Understanding the nature of the barrier height in a two-dimensional semiconductor/metal interface is an important step for embedding layered materials in future electronic devices. We present direct measurement of the Schottky barrier height and its lowering in the transition metal dichalcogenide (TMD)/metal interface of a field effect transistor. It is found that the barrier height at the gold/ single-layer molybdenum disulfide (MoS2) interfaces decreases with increasing drain voltage, and this lowering reaches 0.5–1 V We also show that increase of the gate voltage induces additional barrier lowering.

Effect of localized helium ion irradiation on the performance of synthetic monolayer MoS2 field-effect transistors.

Helium ion irradiation is a known method of tuning the electrical conductivity and charge carrier mobility of novel two-dimensional semiconductors. Here, we report a systematic study of the electrical performance of chemically synthesized monolayer molybdenum disulfide (MoS2) field-effect transistors irradiated with a focused helium ion beam as a function of increasing areal irradiation coverage. We determine an optimal coverage range of approx. 10%, which allows for the improvement of both the carrier mobility in the transistor channel and the electrical conductance of the MoS2, due to doping with ion beam-created sulfur vacancies. Larger areal irradiations introduce a higher concentration of scattering centers, hampering the electrical performance of the device. In addition, we find that irradiating the electrode–channel interface has a deleterious impact on charge transport when contrasted with irradiations confined only to the transistor channel.

Hydrogen Plasma Exposure of Monolayer MoS2 Field-Effect Transistors and Prevention of Desulfurization by Monolayer Graphene.

Atomic vacancies related to structural disorder and doping variation influence carrier transport in monolayer transition-metal dichalcogenide devices. Here, we investigate the effect of hydrogen plasma exposure (HPE) on monolayer MoS2 field-effect transistors (FETs). We observe that a 1% increase in sulfur vacancy after HPE results in incremental 0.06 eV of the Schottky barrier. Short-range scattering from the sulfur vacancies reduces the carrier mobility of monolayer MoS2 by 2 orders of magnitude. Despite the defects and grain boundaries formed during the chemical vapor deposition and transferring process, the surface desulfurization induced by the proton exposure and thermally accelerated oxidation can be blocked by monolayer graphene cladding with a van der Waals contact distance of 2.5 Å. The material-level study indicates a promising route for a low-cost and robust fabrication of smart sensor circuits on a monolithic MoS2 wafer, where the bare MoS2 FETs can serve as proton sensors, with their electronic readout processed by a logic circuit of graphene-protected pristine FETs with a high on/off ratio.

Laser annealing towards high-performance monolayer MoS2 and WSe2 field effect transistors

The transition metal dichalcogenides (TMDCs) have been intensively investigated as promising nanoelectronic and optoelectronic materials. However, the pervasive adsorbates on the surfaces of monolayer TMDCs, including oxygen and water molecules from the ambient environment, tend to degrade the device performance, thus hindering specific applications. In this work, we report the effect of laser irradiation on the transport and photoresponse of monolayer MoS2 and WSe2 devices, and this laser annealing process is demonstrated as a straightforward approach to remove physically adsorbed contaminants. Compared to vacuum pumping and in situ thermal annealing treatments, the field-effect transistors after laser annealing show a more than one order of magnitude higher on-state current, and no apparent degradation of device performance at low temperatures. The mobility of the monolayer WSe2 devices can be enhanced by three to four times, and for single-layered MoS2 devices with the commonly used SiO2 as the back-gate, the mobility increases by 20 times, reaching . The efficient cleaning effect of laser annealing is also supported by the reduction of channel and contact resistance revealed by a transmission line experiment. Further, an enhanced photocurrent, by a factor of ten, has been obtained in the laser annealed device. These findings pave the way for high-performance monolayer TMDC-based electronic and optoelectronic devices with a clean surface and intrinsic properties.

Direct observation of the hysteretic Fermi level modulation in monolayer MoS2 field effect transistors

The hysteresis in the transfer curve of MoS2 has significant impact on the device performance. However, the hysteresis mechanism is still not clear. Here, we investigate the hysteresis of the monolayer MoS2 by probing the local Fermi level variations as a function of the back gate voltage in different atmosphere using the Kelvin probe microscopy. While the Fermi level of the MoS2 in air is much lower than that in vacuum, both the MoS2 devices in vacuum and air show large Fermi level hysteresis. The Fermi level hysteresis direction is clock-wise, identical to that observed in the transfer curves. Both the hysteresis in Fermi level and transfer curve can be explained consistently by taking into account the charge trapping. Our findings confirm that carrier density modulation in MoS2 plays a vital role in the hysteresis, and provide insight into the hysteresis mechanism for the optimization of the device performance.

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

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

Impact of Heat Treatment on a Hetero-Stacked MoS2/${h}$ -BN Field-Effect Transistor

We investigated device properties before and after heat treatment for a hetero-stacked two-dimensional field-effect transistor (FET). In dual-gated monolayer MoS2 FET using a top h-BN layer and bottom SiO2 substrate, careful but harsh heat treatment is implemented in high vacuum at 200 °C for 18 h. Under the top-gate bias sweep, the field-effect mobility increases by ~9 times, and the channel carrier density doubles after the treatment. The heat treatment effect is more noticeable in the top-transferred h-BN than in the bottom SiO2 layer, because it leads to homogeneous adhesion between the layers by diminishing the adverse effects of interfacial bubbles or adsorbates. A top-gate dielectric capacitance for h-BN of 55 fF is increased to ~70 fF after the treatment, which is comparable to the theoretical value. This indicates that strong capacitive coupling for the top gate is formed, as confirmed by the capacitance-voltage measurement.

High-performance monolayer MoS2 field-effect transistor with large-scale nitrogen-doped graphene electrodes for Ohmic contact

A finite Schottky barrier and large contact resistance between monolayer MoS2 and electrodes are the major bottlenecks in developing high-performance field-effect transistors (FETs) that hinder the study of intrinsic quantum behaviors such as valley-spin transport at low temperature. A gate-tunable graphene electrode platform has been developed to improve the performance of MoS2 FETs. However, intrinsic misalignment between the work function of pristine graphene and the conduction band of MoS2 results in a large threshold voltage for the FETs, because of which Ohmic contact behaviors are observed only at very high gate voltages and carrier concentrations (∼1013 cm−2). Here, we present high-performance monolayer MoS2 FETs with Ohmic contact at a modest gate voltage by using a chemical-vapor-deposited (CVD) nitrogen-doped graphene with a high intrinsic electron carrier density. The CVD nitrogen-doped graphene and monolayer MoS2 hybrid FETs platform exhibited a large negative shifted threshold voltage of −54.2 V and barrier-free Ohmic contact under zero gate voltage. Transparent contact by nitrogen-doped graphene led to a 214% enhancement in the on-current and a fourfold improvement in the field-effect carrier mobility of monolayer MoS2 FETs compared with those of a pristine graphene electrode platform. The transport measurements, as well as Raman and X-ray photoelectron spectroscopy analyses before and after thermal annealing, reveal that the atomic C-N bonding in the CVD nitrogen-doped graphene is responsible for the dominant effects of electron doping. Large-scale nitrogen-doped graphene electrodes provide a promising device platform for the development of high-performance devices and the study of unique quantum behaviors.

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.

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

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

Sub-10 nm Monolayer MoS2 Transistors Using Single-Walled Carbon Nanotubes as an Evaporating Mask.

Transition-metal dichalcogenides are promising challengers to conventional semiconductors owing to their remarkable electrical performance and suppression of short-channel effects (SCEs). In particular, monolayer molybdenum disulfide has exhibited superior suppression of SCEs owing to its atomic thickness, high effective carrier mass, and low dielectric constant. However, difficulties still remain in large-scale stable fabrication of nanometer-scale channels. Herein, a method to fabricate electrodes with sub-10 nm gaps was demonstrated using horizontally aligned single-walled carbon nanotubes as an evaporation mask. The widths of the nanogaps exhibit robust stability to various process parameters according to the statistical results. Based on these nanogaps, ultrashort-channel length monolayer MoS2 field-effect transistors were produced. Monolayer MoS2 devices with a 7.5 nm channel length and a 10 nm thick HfO2 dielectric layer exhibited excellent performances with an ON/OFF ratio up to 107, a mobility of 17.4 cm2/V·s, a subthreshold swing of about 120 mV/dec, and a drain-induced barrier lowering of about 140 mV/V, all of which suggest a superior suppression of SCEs. This work provides a universal and stable method for large-scale fabrication of ultrashort-channel 2D-material transistors.

Monolayer MoS2 field-effect transistors patterned by photolithography for active matrix pixels in organic light-emitting diodes

Two-dimensional molybdenum disulfide (MoS2) has substantial potential as a semiconducting material for devices. However, it is commonly prepared by mechanical exfoliation, which limits flake size to only a few micrometers, which is not sufficient for processes such as photolithography and circuit patterning. Chemical vapor deposition (CVD) has thus become a mainstream fabrication technique to achieve large-area MoS2. However, reports of conventional photolithographic patterning of large-area 2D MoS2-based devices with high mobilities and low switching voltages are rare. Here we fabricate CVD-grown large-area MoS2 field-effect transistors (FETs) by photolithography and demonstrate their potential as switching and driving FETs for pixels in analog organic light-emitting diode (OLED) displays. We spin-coat an ultrathin hydrophobic polystyrene layer on an Al2O3 dielectric, so that the uniformity of threshold voltage (Vth) of the FETs might be improved. Our MoS2 FETs show a high linear mobility of approximately 10 cm2 V−1 s−1, due to a large grain size around 60 μm, and a high ON/OFF current ratio of 108. Dynamic switching of blue and green OLED pixels is shown at ~5 V, demonstrating their application potential.