MoS2 Field-effect Transistors Research Articles

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.

Short-channel robustness from negative capacitance in 2D NC-FETs

To date, the robustness of performance, including tolerance to channel-length scaling effects, in scaled transistors has become increasingly important. Negative capacitance (NC) field-effect transistors (FETs) have drawn considerable attention and many studies have revealed that the NC effect is beneficial for device scaling. However, there is a lack of experimental evidence of short-channel behavior in NC-FETs with two-dimensional (2D) semiconducting channels and theoretical studies are limited. Here, we experimentally study 2D MoS2-based NC-FETs using MoS2 with CMOS-compatible hafnium zirconium oxide (HfZrO2 or HZO) as the ferroelectric (FE) and demonstrate remarkable short-channel behavior compared to similar 2D MoS2 FETs. It was observed that the subthreshold switching improvement becomes increasingly significant at shorter channel lengths, down to 20 nm. From analysis of the capacitive network, we show that the NC effect is impacted by a larger magnitude of polarization in the ferroelectric, which enhances gate control and is beneficial to channel-length scaling.

Dual-gate MoS2 phototransistor with atomic-layer-deposited HfO2 as top-gate dielectric for ultrahigh photoresponsivity

An asymmetric dual-gate (DG) MoS2 field-effect transistor (FET) with ultrahigh electrical performance and optical responsivity using atomic-layer-deposited HfO2 as a top-gate (TG) dielectric was fabricated and investigated. The effective DG modulation of the MoS2 FET exhibited an outstanding electrical performance with a high on/off current ratio of 6 × 108. Furthermore, a large threshold voltage modulation could be obtained from −20.5 to −39.3 V as a function of the TG voltage in a DG MoS2 phototransistor. Meanwhile, the optical properties were systematically explored under a series of gate biases and illuminated optical power under 550 nm laser illumination. An ultrahigh photoresponsivity of 2.04 × 105 AW−1 has been demonstrated with the structure of a DG MoS2 phototransistor because the electric field formed by the DG can separate photogenerated electrons and holes efficiently. Thus, the DG design for 2D materials with ultrahigh photoresponsivity provides a promising opportunity for the application of optoelectronic devices.

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.

DNA-based functionalization of two-dimensional MoS2 FET biosensor for ultrasensitive detection of PSA

Molybdenum disulfide (MoS2) field-effect transistor (FET) biosensors are being increasingly studied due to their highly sensitive and label-free biodetection, system-level integration and easy fabrication. However, the stability and detection ability of a pristine device deteriorate due to the absorption of H2O and CO and usually exhibit high false signals. Herein, a novel ecofriendly technology is employed to functionalize MoS2 FETs using the conjugation of a DNA tetrahedron and biotin-streptavidin (B-SA). The functionlized devices demonstrate ultrahigh sensitivity of the target protein (prostate-specific antigen, PSA) with a limit of detection of 1fg/mL and a wide linear range of 1fg/mL to 100ng/mL in a phosphate-buffered saline, which are 100 and 10,000 times higher than the reported highest values, respectively; these results are attributed to the stable immobilization provided by the DNA tetrahedron as well as the multilevel amplification and anchor system (B-SA system), which improves the antibody-antigen (Ab-Ag) binding. Real-time measurement results indicate that our proposed biosensor can also detect trace PSA (1fg/mL) in a clinically relevant sample of healthy human serum.

Electrically Tunable Room Temperature Hysteresis Crossover in Underlap MoS2 Field-Effect Transistors.

Clockwise to anticlockwise hysteresis crossover in current-voltage transfer characteristics of field-effect transistors (FETs) with graphene and MoS2 channels holds significant promise for nonvolatile memory applications. However, such crossovers have been shown to manifest only at high temperature. In this work, for the first time, we demonstrate room temperature hysteresis crossover in few-layer MoS2 FETs using a gate-drain underlap design to induce a differential response from traps near the MoS2-HfO2 channel-gate dielectric interface, also referred to as border traps, to applied gate bias. The appearance of trap-driven anticlockwise hysteresis at high gate voltages in underlap FETs can be unambiguously attributed to the presence of an underlap since transistors with and without the underlap region were fabricated on the same MoS2 channel flake. The underlap design also enables room temperature tuning of the anticlockwise hysteresis window (by 140×) as well as the crossover gate voltage (by 2.6×) with applied drain bias and underlap length. Comprehensive measurements of the transfer curves in ambient and vacuum conditions at varying sweep rates and temperatures (RT, 45 °C, and 65 °C) help segregate the quantitative contributions of adsorbates, interface traps, and bulk HfO2 traps to the clockwise and anticlockwise hysteresis.

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.

High Current Nb-Doped P-Channel MoS₂ Field-Effect Transistor Using Pt Contact

This letter demonstrates unipolar p-type MoS2 field-effect transistors (p-FETs). The p-FETs are fabricated using high work function Pt as the contact electrode and p-type MoS 2 film as the active channel. The p-FETs, with a channel length of 1- $\mu $ m, show an output current of $-10~\mu $ A/ $\mu $ m with a drain voltage of −1 V. The Pt electrode, formed by slow electron-beam evaporation, shows a contact barrier height of 0.13 eV. In comparison, a faster deposition rate results in a larger resistance and a higher contact barrier of the Pt electrode. Raman characterization provides certain support for the improved contact interface of the slowly deposited Pt electrode, which may be an essential factor in improving current drive.

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.

Enhanced Electrical Performance of Van der Waals Heterostructure

AbstractIn this study, a novel van der Waals (vdW) heterostructure field‐effect transistor (FET) using vdW stacking as next‐generation device architecture is demonstrated. The MoS2/WS2 heterostructure FET exhibits a large improvement in the drain current and field‐effect mobility (from 43.3 to 62.4 cm2 V−1 s−1) compared with single MoS2 FET. Such significant enhancement is mainly due to the charge‐transfer effect. The vdW self‐encapsulation effect in the device channel also helps to reduce the Schottky barrier height from 120 to 52 meV at the contact interface. Finally, the WS2/MoS2/WS2 double heterostructure FETs further boosts the mobility up to 102.5 cm2 V−1 s−1 at room temperature (25 °C) and 169.7 cm2 V−1 s−1 at 30 K. The much higher mobility exhibited at the lower temperature indicates the suppression of Coulomb scattering. Thus the electrical performance of the double‐heterostructure FETs can be further enhanced.

Layer thickness influenced irradiation effects of proton beam on MoS2 field effect transistors

We investigated the influence of the flake thickness for molybdenum disulfide (MoS2) field effect transistors on the effect of a 150 keV high-energy proton beam applied on these devices. The electrical characteristics of the devices with channel thicknesses ranging from monolayer to bulk were measured before and after proton irradiation with a proton fluence of 5 × 1014 cm−2. The subthreshold swing (SS), threshold voltage shift and electron mobility were extracted with the Y-function method after proton irradiation and significant degradation were observed. It is found that, with the increase of layer thickness, mobility degradation and threshold voltage shift both eased, but the SS degradation was insensitive to the MoS2 flake thickness increase. We also demonstrate that the threshold voltage shift is dominated by oxide charges; however, the mobility and SS degradations are mainly affected by the interface traps. Our study will enhance the understanding of the influence of high-energy particles on MoS2-based nano-electronic devices. By increasing the MoS2 flake thickness to a certain extent, one can hopefully find a balance between effectively resisting shift and achieving high mobility and small SS degradation.

Understanding ambipolar transport in MoS2 field effect transistors: the substrate is the key

2D materials offer a pathway for further scaling of CMOS technology. However, for this to become a reality, both n-MOS and p-MOS should be realized, ideally with the same (standard) material. In the specific case of MoS2 field effect transistors (FETs), ambipolar transport is seldom reported, primarily due to the phenomenon of Fermi level pinning (FLP). In this study we identify the possible sources of FLP in MoS2 FETs and resolve them individually. A novel contact transfer technique is used to transfer contacts on top of MoS2 flake devices that results in a significant increase in the hole branch of the transfer characteristics as compared to conventionally fabricated contacts. We hypothesize that the pinning not only comes from the contact-MoS2 interface, but also from the MoS2-substrate interface. We confirm this by shifting to an hBN substrate which leads to a 10 fold increase in the hole current compared to the SiO2 substrate. Furthermore, we analyse MoS2 FETs of different channel thickness on three different substrates, SiO2, hBN and Al2O3, by correlating the p-branch I ON/I OFF to the position of oxide defect band in these substrates. FLP from the oxide is reduced in the case of Al2O3 which enables us to observe ambipolar transport in a bilayer MoS2 FET. These results highlight that MoS2 is indeed an ambipolar material, and the absence of ambipolar transport in MoS2 FETs is strongly correlated to its dielectric environment and processing conditions.

Sensor behavior of MoS2 field-effect transistor with light injection toward chemical recognition.

The application of field-effect transistor (FET) devices with atomically thin channels as sensors has attracted significant attention, where the adsorption of atoms/molecules on the channels can be detected by the change in the properties of FET. Thus, to further enhance the chemical sensitivity of FETs, we developed a method to distinguish the chemical properties of adsorbates from the electric behavior of FET devices. Herein, we explored the variation in the FET properties of an MoS2-FET upon visible light injection and the effect of molecule adsorption for chemical recognition. By injecting light, the drain current (Id) increased from the light-off state, which is defined as (ΔId)ph. We examined this effect using CuPc molecules deposited on the channel. The (ΔId)phvs. wavelength continuous spectrum in the visible region showed a peak at the energy for the excitation from the highest occupied orbital (HOMO) to the molecule-induced state (MIS). The energy position and the intensity of this feature showed a sensitive variation with the adsorption of the CuPc molecule and are in good agreement with previously reported photo-absorption spectroscopy data, indicating that this technique can be employed for chemical recognition.

Air-stable and efficient electron doping of monolayer MoS2 by salt-crown ether treatment.

To maximize the potential of transition-metal dichalcogenides (TMDCs) in device applications, the development of a sophisticated technique for stable and highly efficient carrier doping is critical. Here, we report the efficient n-type doping of monolayer MoS2 using KOH/benzo-18-crown-6, resulting in a doped TMDC that is air-stable. MoS2 field-effect transistors show an increase in on-current of three orders of magnitude and degenerate the n-type behaviour with high air-stability for ∼1 month as the dopant concentration increases. Transport measurements indicate a high electron density of 3.4 × 1013 cm-2 and metallic-type temperature dependence for highly doped MoS2. First-principles calculations support electron doping via surface charge transfer from the K/benzo-18-crown-6 complex to monolayer MoS2. Patterned doping is demonstrated to improve the contact resistance in MoS2-based devices.

Field-effect-transistor-based ion sensors: ultrasensitive mercury(II) detection via healing MoS2 defects.

The contamination of water with heavy metal ions represents a harsh environmental problem resulting from societal development. Among various hazardous compounds, mercury ions (Hg2+) surely belong to the most poisonous ones. Their accumulation in the human body results in health deterioration, affecting vital organs and eventually leading to chronic diseases, and, in the worst-case scenario, early death. High selectivity and sensitivity for the analyte of choice can be achieved in chemical sensing using suitable active materials capable of interacting at the supramolecular level with the chosen species. Among them, 2D transition metal dichalcogenides (TMDCs) have attracted great attention as sensory materials because of their unique physical and chemical properties, which are highly susceptible to environmental changes. In this work, we have fabricated MoS2-based field-effect transistors (FETs) and exploited them as platforms for Hg2+ sensing, relying on the affinity of heavy metal ions for both point defects in TMDCs and sulphur atoms in the MoS2 lattice. X-ray photoelectron spectroscopy characterization showed both a significant reduction of the defectiveness of MoS2 when exposed to Hg2+ with increasing concentration and a shift in the binding energy of 0.2 eV suggesting p-type doping of the 2D semiconductor. The efficient defect healing has been confirmed also by low-temperature photoluminescence measurements by monitoring the attenuation of defect-related bands after Hg2+ exposure. Transfer characteristics in MoS2 FETs provided further evidence that Hg2+ acts as a p-dopant of MoS2. Interestingly, we observed a strict correlation of doping with the concentration of Hg2+, following a semi-log trend. Hg2+ concentrations as low as 1 pM can be detected, being way below the limits imposed by health regulations. Electrical characterization also revealed that our sensor can be efficiently washed and used multiple times. Moreover, the developed devices displayed a markedly high selectivity for Hg2+ against other metal ions as ruled by soft/soft interaction among chemical systems with appropriate redox potentials, being a generally applicable approach to develop chemical sensing devices combining high sensitivity, selectivity and reversibility, to meet technological needs.

A New Low Cost Current Measurement Circuit with Temperature Compensation

The electrical characterization of semiconductors devices, when submitted to ionizing radiation should be done in a large range of currents; however, the instrumentation with this ability is very expensive. This work proposes a low-cost circuit using commercial off-the-shelf components (COTS) that enables the measurement of electrical currents in the order of pA range. The circuit presents an output current that is an amplified version of the current to be measured, using the exponential relationship between currents and voltages in Bipolar Junction Transistors (BJTs) and Metal Oxide Silicon Field Effect Transistors (MOSFETs) when operating in the weak inversion region. Furthermore, a block was introduced in order to compensate the gain’s temperature dependence. The results showed that the operating range for the current that will be measured was more than seven decades using BJTs and five decades by using MOSFETs with a high linearity. The circuit version using MOSFETs was able to measure currents as low as 100 fA. The current gain has also good linearity for over five decades. This circuit has a stable behavior for the range of 20 °C to 40 °C, because of the temperature compensation block.

MoS2 FET fabrication using adhesion lithography and their application to chemical sensors

Molybdenum disulfide (MoS2) field effect transistors (FETs) are investigated for chemical sensor applications. However, the formation of electrodes after a MoS2 transistor is transferred to the substrate results in process damage. In this work, MoS2 FETs are fabricated by implementing a gate, source, and drain pre-formation, and then by transferring MoS2 using polydimethylsiloxane. The fabricated FETs are characterized after their exposure to ethanol vapor as a case study for chemical sensor applications. A sub-threshold swing of 72 mV/dec can be observed for a fabricated FET with a field effect mobility of 5.05 cm2 V−1 s−1. The ON/OFF ratio is approximately 104. No significant change in the FET’s properties due to contact resistance is observed. Next, V th is shifted to a 1.7 V-positive value upon ethanol vapor exposure. By removing the ethanol vapor, a 1.4 V-negative shift in the threshold voltage value is observed compared with that before the ethanol vapor removal.

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.