MoS2 Field-effect Transistors Research Articles

In situ study of sensor behavior of MoS2 field effect transistor for methyl orange molecule in ultra high vacuum condition

We investigate the sensor behavior of the MoS2 field effect transistor (FET) device with the deposition of methyl orange (MO) molecule which is widely used as a chemical probe. The channel of the FET is made of the single layer of MoS2 which makes it highly sensitive to the molecule adsorption, but at the same time the behavior depends much on the surface conditions of the MoS2 channel. In order to make the channel-surface conditions more defined, we prepare an in situ experimental system in which the molecule deposition and the surface- and electrical-characterization of the MoS2 FET are executed in a single ultra-high vacuum chamber. This system makes it possible to examine the change of the FET properties with precise control of the molecule coverage in the sub-monolayer region without the effect of the atmosphere. We detected the shift of the I d–V g curve of the MoS2-FET device with the increase of the molecule coverage (θ) of the MO molecule, which is quantitatively analyzed by plotting the threshold voltage (V th) of the I d–V g curve as a function of θ. The V th shifts towards the negative direction and the initial change with θ can be expressed with an exponential function of θ, which can be accounted for with the Langmuir type adsorption of the molecule for the first layer and the charge transfer from the molecule to the substrate. The V th versus θ curve shows a kink at a certain θ, which is conserved as the starting of the second layer growth. We detected the adsorption of MO far less than monolayer and the phase change from the first layer to the second layer growth, which is realized by the benefit of the in situ UHV experimental condition.

ALD-Based Synthesis of Wafer-Scale Transition Metal Dichalcogenides Towards Nanoelectronics and Optoelectronics Applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS2 and WS2 have been widely studied due to their unique physical and electronic properties even at one atomic layer thickness. Since the first report on the MoS2 field-effect transistor (FET) device [1], great efforts have been made in the past decade to extend the intriguing features of TMDs to practical applications. However, such integration has been severely bottlenecked by the lack of effective approaches to achieve wafer-scale, uniform, crystalline and stoichiometric TMD films.Till now, it is still prevalent to fabricate TMD-based electronic devices using mechanical exfoliation methods. Although single-crystal flakes with least defects can be obtained, the size, thickness and location of the exfoliated TMD flakes are largely uncontrollable, making it unsuitable for large-scale device integration and massive production. So far, various synthetic approaches have been proposed to grow large-area TMD thin films. Chemical vapor deposition (CVD)-based method is one of the major routines to synthesize high-quality monolayer and few-layer TMD flakes [2-4]. For example, in typical CVD processes to grow MoS2, the films can be achieved either by the reduction reaction between the S and MoO3 source powders or by sulfurizing the Mo (or MoOx) films pre-deposited by using physical vapor deposition. However, the nucleation sites in the CVD process are uncontrollable which may lead to the vertical stacking of the MoS2 flakes in triangle shape deteriorating the crystalline properties of the film. On the other hand, the pre-deposited Mo or MoOx film by using e-beam evaporation or sputtering usually suffer from rough surface morphology and unsatisfactory film uniformity which further limit the homogeneity of the large-scale film and device integration.Atomic layer deposition (ALD) is a surface-controlled film fabrication and can provide a possible route towards the synthesis of TMD thin films since the ALD approach follows the layer-by-layer deposition mechanism [5,6]. As compared to the conventional CVD method, ALD can enable precise thickness control on atomic scale and excellent film uniformity as well as good stoichiometry and crystallinity with proper annealing steps. In this work, we employed the ALD-based synthesis method to grow wafer-scale MoS2 and WS2 thin films. Non-toxic MoCl5, WCl5 and hexamethyldisilathiane (HMDST) precursors have been used which are different and superior to those used in previous work. Film characterizations have confirmed the wafer-level uniformity, crystallinity and stoichiometry of the synthesized films. Homogeneous electrical behaviors have also been obtained from the fabricated FET device arrays. In addition, optoelectronic devices such as photodetectors and simple logic gates such as n-type inverter arrays have been demonstrated using the wafer-scale MoS2 FET arrays. Furthermore, we have used the substitutional doping in the ALD process to achieve controllable p-type doping of MoS2 films, which can provide solid basis for the complimentary metal-oxide-semiconductor (CMOS) integrated circuit applications. This work shows that the ALD-based synthesis of the 2D TMD thin films is an attractive approach and has provided a promising platform to further push forward the circuit and system level applications of the TMD materials.

Transfer printing of gate dielectric and carrier doping with poly(vinyl-alcohol) coating to fabricate top-gate molybdenum disulfide field-effect transistors

This study reposts the fabrication of top-gate molybdenum disulfide (MoS2) field-effect transistor (FET) by the transfer printing of a gate dielectric in conjunction with a poly(vinyl-alcohol) (PVA) coating for carrier doping. The spin-coated PVA film increases the carrier concentration in MoS2, while the back-gate MoS2 FET cannot be turned off. The transferred top-gate structure with the PVA coating makes it possible to turn off the fabricated device without permanent damage to MoS2. The results of this study suggest interesting directions for the research and development of two-dimensional material-based functional devices.

MoS2 doping by atomic layer deposition of high-k dielectrics using alcohol as process oxidants

We developed doping technique of transition metal dichalcogenides based on atomic layer deposition (ALD) of oxide thin films. In this study, we deposited ALD Al2O3 overlayer using various oxidant including iso-propyl alcohol (IPA) and ethanol and investigated the doping effects depending on the choice of oxidant for ALD process. The doping effects were investigated by the change in the performance of bottom-gated MoS2 field effect transistors. Experimental results indicate that Al2O3 overlayer deposited using IPA as ALD reactant produces more efficient n-type doping effect compared to ethanol. Moreover, doping was effectively controlled by modulating atomic layer deposition process. ALD process using effective oxidant material is expected to be practically used for realizing the fabrication of threshold voltage-controllable transistors or further applications such as diodes.

High-performance Te-doped p-type MoS2 transistor with high-K insulators

Molybdenum disulfide (MoS2) films possess intrinsic n-type conductivity, and thus development of p-type MoS2 films for realizing practical next-generation complementary metal oxide semiconductor devices is extremely challenging. The use of dopants is a well-known conventional approach for engineering intrinsic conductivity with improved device performance. Herein, we demonstrate the n-type conductivity suppressing property of the tellurium (Te)-doped multilayer MoS2, grown by metal-organic chemical vapor deposition. The back-gated as-grown Te-doped multilayer MoS2 field-effect transistor (FET) is observed to exhibit p-type behavior with a maximum mobility of 0.036 cm2/V·s, ON/OFF current ratio of 7.8×103, and a Schottky barrier height of 32 meV. A quantitative Schottky barrier height of a p-type multilayer MoS2-based FET with Au electrode has been obtained by analyzing its low-temperature transport characteristics. Enhanced device performance has been achieved by doping the p-type MoS2 transistors in a back-gate structure and high-K dielectric gate insulators with Te. pFET devices containing Al2O3 dielectric insulators exhibit extremely high performance, including a maximum mobility of ∼182 cm2/V·s, maximum ON/OFF current ratio of ∼105, and a low subthreshold swing of ∼215 mV/dec. These improvements have been attributed to the charge screening effect associated with the high-K dielectrics and a low Schottky barrier height.

Polarization-sensitive photodetectors based on three-dimensional molybdenum disulfide (MoS2) field-effect transistors

Abstract The molybdenum disulfide (MoS2)-based photodetectors are facing two challenges: the insensitivity to polarized light and the low photoresponsivity. Herein, three-dimensional (3D) field-effect transistors (FETs) based on monolayer MoS2 were fabricated by applying a self–rolled-up technique. The unique microtubular structure makes 3D MoS2 FETs become polarization sensitive. Moreover, the microtubular structure not only offers a natural resonant microcavity to enhance the optical field inside but also increases the light-MoS2 interaction area, resulting in a higher photoresponsivity. Photoresponsivities as high as 23.8 and 2.9 A/W at 395 and 660 nm, respectively, and a comparable polarization ratio of 1.64 were obtained. The fabrication technique of the 3D MoS2 FET could be transferred to other two-dimensional materials, which is very promising for high-performance polarization-sensitive optical and optoelectronic applications.

In Situ Study of Molecular Doping of Chlorine on MoS2 Field Effect Transistor Device in Ultrahigh Vacuum Conditions.

We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS2 channel when the channel part is exposed to Cl2 gas. The gas exposure and the electrical measurement of the MoS2 FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS2 FET can provide and understand the surface properties. With the Cl2 gas exposure to the channel, the plot of the drain current versus the gate voltage (Id–Vg curve) shifts monotonically toward the positive direction of Vg, suggesting that the adsorbate acts as an electron acceptor. The Id–Vg shifts are numerically estimated by measuring the onset of Id (threshold voltage, Vth) and the mobility as a function of the dosing amounts of the Cl2 gas. The behaviors of both the Vth shift and the mobility with the Cl2 dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS2 surface up to the monolayer coverage. The charge transfer makes the Vth shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS2 channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl2 molecule. The monotonic shift of Vth with the coverage suggests that the MoS2 device sensor has a good sensitivity to detect 10–3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s.

Graphene based Van der Waals contacts on MoS2 field effect transistors

Device performance of two dimensional (2D) material based field effect transistors is severely limited by the relatively high contact resistance encountered at the contact-channel interface. Metal-graphene hybrid contacts have been previously used to improve the contact resistance of devices based on thick exfoliated materials. Here we report a novel 2D FET fabrication process entailing the transfer of metal-graphene hybrid contacts on top of 3 monolayer-thick chemical vapor deposition (CVD) MoS2, enabling a lithography free contacting strategy, with respect to MoS2. Three different metal-graphene stacks consisting of Ni, Pd and Ru, have been fabricated, transferred onto MoS2 and characterized extensively using electrical and physical characterization techniques. We find strong correlation between the measured electrical characteristics and physical characterization of the contact interface. From Raman spectra measurement, maximum charge transfer of 1.7 × 1013 cm−2 is observed between graphene and Ru, leading to an improved contact resistance for MoS2 devices with Ru-Gr contacts. Ru-Gr contact shows the lowest contact resistance of 9.34 kΩ · µm among the three metal-graphene contact stacks reported in this article. This contact resistance is also the best among reported CVD grown graphene contacted MoS2 devices. Using more than 400 devices, we study the impact of the different metal-graphene contacts on other electrical parameters such as hysteresis, sub-threshold swing and threshold voltage. The metal-graphene contact stack transfer technique represents a technologically relevant contacting approach which can be further up-scaled to larger wafer areas.

Gate Stack Engineering in MoS2 Field‐Effect Transistor for Reduced Channel Doping and Hysteresis Effect

Abstract2D transition metal dichalcogenides (TMDs) are promising semiconductive films for applications in future devices due to their prosperous and tunable band structures. However, most TMD‐based top gate transistors suffer from a significant doping effect in the channel due to the subsequent deposition high‐k dielectric layer and metal gate, which limits their practical applications. In this work, the channel doping effect caused by various processing steps based on mechanical exfoliated MoS2 sheets is systematically investigated. This work illustrates a clear correlation among these steps and provides a simple and efficient methodology to realize high‐performance enhancement mode MoS2 field effect transistors, which can be extended to other 2D materials.

A physics-based compact model for MoS2 field-effect transistors considering the band-tail effect and contact resistance

In this paper, we present a compact surface-potential-based drain current model in molybdenum disulfide (MoS2) field-effect transistors (FETs). Considering variable range hopping (VRH) transport via band-tail states in MoS2 transistors, an explicit solution for surface potential has been derived and it provides a good description over different regions of operation by comparisons with numerical data. Based on the charge-sheet model (CSM), which applies to drift-diffusion transport, the current expression including contact resistance and velocity saturation effect is developed. Furthermore, the presented model is validated and shows a good agreement with the experimental data for MoS2 FETs.

Scratching lithography for wafer-scale MoS2 monolayers

Monolayer MoS2 is an emerging two-dimensional (2D) semiconductor with promise on novel electronics and optoelectronics. Standard micro-fabrication techniques such as lithography and etching are usually involved to pattern such materials for devices but usually face great challenges on yielding clean structures without edge, surface and interface contaminations induced during the fabrication process. Here a direct writing patterning approach for wafer-scale MoS2 monolayers is reported. By controllable scratching by a tip, wafer-scale monolayer MoS2 films on various substrates are patterned in an ultra-clean manner. MoS2 field effect transistors fabricated from this scratching lithography show excellent performances, evidenced from a room-temperature on–off ratio exceeding 1010 and a high field-effect mobility of 50.7 cm2 V−1 s−1, due to the cleanness of as-fabricated devices. Such scratching approach can be also applied to other 2D materials, thus providing an alternate patterning strategy to 2D-materials based devices.

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.

Experimental and modeling study of 1/f noise in multilayer MoS2 and MoSe2 field-effect transistors

In field-effect transistors (FETs) with two-dimensional (2D) transition metal dichalcogenide channels, the dependence of field-effect mobility on atomic layer thickness has been studied and interpreted in terms of interface scattering and interlayer coupling resistance (Rint). However, a model for 1/f noise, such as in MoS2 and in MoSe2 FETs, for various contact metals and layer number thicknesses has not been reported. In this work, we have experimentally studied current–voltage and 1/f noise on MoS2 and MoSe2 FETs with source and drain contacts of high and low work function metals to understand both the mobility and the noise behavior. We have developed a noise model incorporating layer number dependent Hooge parameters and Rint. The noise and mobility models utilize screening lengths for charge, mobility, and Hooge parameter to describe the variation of these quantities with a layer number. Using our single model topology with appropriate fitting parameters for each material and each contact metal, the model captures the experimentally observed layer thickness dependence of the Hooge parameter. Our noise analysis is fully comprehensive and, hence, could be applied to any 2D layered systems.

Efficient and Versatile Modeling of Mono- and Multi-Layer MoS2 Field Effect Transistor

Two-dimensional (2D) materials with intrinsic atomic-level thicknesses are strong candidates for the development of deeply scaled field-effect transistors (FETs) and novel device architectures. In particular, transition-metal dichalcogenides (TMDCs), of which molybdenum disulfide (MoS2) is the most widely studied, are especially attractive because of their non-zero bandgap, mechanical flexibility, and optical transparency. In this contribution, we present an efficient full-wave model of MoS2-FETs that is based on (1) defining the constitutive relations of the MoS2 active channel, and (2) simulating the 3D geometry. The former is achieved by using atomistic simulations of the material crystal structure, the latter is obtained by using the solver COMSOL Multiphysics. We show examples of FET simulations and compare, when possible, the theoretical results to the experimental from the literature. The comparison highlights a very good agreement.

Enhanced mobility of MoS2 field-effect transistors by combining defect passivation with dielectric-screening effect* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61774064, 61974048, and 61851406).

A facile method of combining the defect engineering with the dielectric-screening effect is proposed to improve the electrical performance of MoS2 transistors. It is found that the carrier mobility of the transistor after the sulfur treatment on the MoS2 channel is greatly enhanced due to the reduction of the sulfur vacancies during vulcanization of MoS2. Furthermore, as compared to those transistors with HfO2 and SiO2 as the gate dielectric, the Al2O3-gate dielectric MoS2 FET shows a better electrical performance after the sulfur treatment, with a lowered subthreshold swing of 179.4 mV/dec, an increased on/off ratio of 2.11×106, and an enhanced carrier mobility of 64.74 cm2/V⋅s (about twice increase relative to the non-treated MoS2 transistor with SiO2 as the gate dielectric). These are mainly attributed to the fact that a suitable k-value gate dielectric can produce a dominant dielectric-screening effect overwhelming the phonon scattering, increasing the carrier mobility, while a larger k-value gate dielectric will enhance the phonon scattering to counteract the dielectric-screening effect, reducing the carrier mobility.

Hf₀.₅Zr₀.₅O₂ Ferroelectric Embedded Dual-Gate MoS₂ Field Effect Transistors for Memory Merged Logic Applications

In this letter, a combination of multi-gate field effect transistor with ferroelectric is proposed for a new concept of memory merged logic device. For the first time, dual-gate MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> field effect transistor (FET) with a Hf <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (HZO) back gate insulator is fabricated. Because of the manipulation of charge density in the channel by both electric field from the top/back gates and the polarization field form the HZO, the ferroelectric embedded dual-gate (FEDG) MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FET can work as a normal n-type top-gate MOSFET or a ferroelectric memory with 1 V memory window and retention time up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> s, separately or simultaneously. In memory merged logic operation, the output current of the devices depends on the top-gate voltage, back-gate voltage and the polarization of the HZO, which greatly extend the possible function that one transistor can implement. The FEDG FET has the benefits of diminishing the power dissipation of inter connection between logic and memory arrays in the integrated circuit, and reducing the number of transistors in circuits comparing to standard MOSFET configurations, which shows a great potential in the ultra-low power consumption in memory computing (IMC) and neuromorphic computing (NC) applications.

Real-time effect of electron beam on MoS2 field-effect transistors

Irradiation of MoS2 field-effect transistors (FETs) fabricated on Si/SiO2 substrates with electron beams (e-beams) below 30 keV creates electron–hole pairs (EHP) in the SiO2, which increase the interface trap density (Nit) and change the current path in the channel, resulting in performance changes. In situ measurements of the electrical characteristics of the FET performed using a nano-probe system mounted inside a scanning electron microscope show that e-beam irradiation enables both multilayer and monolayer MoS2 channels act as conductors. The e-beams mostly penetrate the channel owing to their large kinetic energy, while the EHPs formed in the SiO2 layer can contribute to the conductance by flowing into the MoS2 channel or inducing the gate bias effect. The analysis of the device parameters in the initial state and the vent-evacuation state after e-beam irradiation can clarify the effect of the interplay between the e-beam-induced EHPs and ambient adsorbates on the carrier behavior, which depends on the thickness of the MoS2 layer. DC and low frequency noise analysis reveals that the e-beam-induced EHPs increase Nit from 109–1010 to 1011 cm−2 eV−1 in both monolayer and multilayer devices, while the interfacial Coulomb scattering parameter αSC increases by three times in the monolayer and decreases to one-tenth of its original value in the multilayer. In other words, an MoS2 layer with a thickness of ∼30 nm is less sensitive to adsorbates by surface screening. Thus, the carrier mobility in the monolayer device decreases from 45.7 to 40 cm2 V−1 s−1, while in the 30 nm-thick multilayer device, it increases from 4.9 to 5.6 cm2 V−1 s−1. This is further evidenced by simulations of the distribution of interface traps and channel carriers in the MoS2 FET before and after e-beam irradiation, demonstrating that Coulomb scattering decreases as the effective channel moves away from the interface.

Understanding and Mapping Sensitivity in MoS2 Field-Effect-Transistor-Based Sensors.

Sensors based on two-dimensional (2D) field-effect transistors (FETs) are extremely sensitive and can detect charged analytes with attomolar limits of detection (LOD). Despite some impressive LODs, the operating mechanisms and factors that determine the signal-to-noise ratio in 2D FET-based sensors remain poorly understood. These uncertainties, coupled with an expansive design space for sensor layout and analyte positioning, result in a field with many reported highlights but limited collective progress. Here, we provide insight into sensing mechanisms of 2D molybdenum disulfide (MoS2) FETs by realizing precise control over the position and charge of an analyte using a customized atomic force microscope (AFM), with the AFM tip acting as an analyte. The sensitivity of the MoS2 FET channel is revealed to be nonuniform, manifesting sensitive hotspots with locations that are stable over time. When the charge of the analyte is varied, an asymmetry is observed in the device drain-current response, with analytes acting to turn the device off leading to a 2.5× increase in the signal-to-noise ratio (SNR). We developed a numerical model, applicable to all FET-based charge-detection sensors, that confirms our experimental observation and suggests an underlying mechanism. Further, extensive characterization of a set of different MoS2 FETs under various analyte conditions, coupled with the numerical model, led to the identification of three distinct SNRs that peak with dependence on the layout and operating conditions used for a sensor. These findings reveal the important role of analyte position and coverage in determining the optimal operating bias conditions for maximal sensitivity in 2D FET-based sensors, which provides key insights for future sensor design and control.

A novel contact engineering method for transistors based on two-dimensional materials

Contact engineering is of critical importance for two-dimensional (2D) transition metal dichalcogenide (TMD)-based devices. However, there are only a few solutions to overcome this obstacle because of the complexity of the TMD-contact interface. In this work, we propose a novel method using a soft plasma treatment followed by the seamless deposition of a metal electrode to reduce the contact resistance of MoS2 field effect transistors (FETs). The treated FETs exhibit three times higher mobility than the control FETs without plasma treatment. The soft plasma treatment can remove the facial sulfur atoms and expose the middle Mo atoms so that they come into direct contact with the metal electrode, thus greatly improving the contact behavior. First-principles calculation is also performed to support the experimental results. Our potentially scalable strategy can be extended to the whole family of TMD based FETs to provide a possible route of device processsing technology for 2D device application.

Single digit parts-per-billion NOx detection using MoS2/hBN transistors

2D materials offer excellent possibilities for high performance gas detection due to their high surface-to-volume ratio, high surface activities, tunable electronic properties and dramatic change in resistivity upon molecular adsorption. This paper demonstrates a simple field effect transistor (FET) of molybdenum disulphide (MoS2) fabricated on a hexagonal boron nitride (hBN) substrate that can detect NOx down to concentrations of 6 ppb and possibly far below at room temperature (RT) with a systematic optimization of the device design and fabrication parameters as well as the device operating conditions. The effects of the substrate, number of MoS2 layers, channel layout and biasing conditions on the response of MoS2 FETs to NOx were investigated, providing directions for maximizing the sensitivity. This work also sheds light the issues of recovery and stability and present a methodology for calibration of the sensors which is critical for repeatable and reliable measurements.