Molybdenum Disulfide Field-effect Transistors Research Articles

Research on photoresponse characteristics of bilayer molybdenum disulfide field effect transistors

The field-effect transistors (FETs) have attracted much attention because of the unique properties of the layered MoS2 nanosheet as an ideal channel material. The phototransistors based on the MoS2 FET have advantages of high photoresponsivity, rapid and stable response. This paper is concerned with the fabrication of bilayer MoS2 based FET and the research on the photoelectric characteristics of the devices. The effect of the optical power and gate voltage on the photoelectric properties of the device was researched through the contrast experiments. It has been achieved that the photocurrent increases with the increase of optical power at the same gate voltage and drain voltage. The photoresponsivity also been enhanced with the gate voltage increased. And the photoresponsivity of bilayer MoS2 FET was enhanced more than 100 folds by using MoS2/MoO3 as the channel material. The results indicated that the MoS2/MoO3 nanosheets can be a promising heterostructure material to offer a compelling case for application in enhancing the photoresponse characteristics of MoS2 transistors. These unique properties make the bilayer MoS2 based FET to be a great potential candidate of the next generation optoelectronic devices, which have the trend of miniaturization and integration.

A New Velocity Saturation Model of MoS2 Field-Effect Transistors

In this letter, we present a new velocity saturation model for molybdenum disulfide (MoS2) field-effect transistors (FETs) based on the surface potential. Unlike former models, in addition to modulating the field-effect mobility, our model adjusts the upper limit of current integration to its saturation value. Moreover, the expression of charge density at the drain under the velocity saturation effect is provided. As an example, the model is used to simulate the electrical characteristics of a MoS2 FET under different trap densities, temperatures, and channel lengths. A short channel device is fabricated and evaluated. The excellent agreement between the experimental data and the simulation results validates our model.

Defect-Free Copolymer Gate Dielectrics for Gating MoS2 Transistors

In this study, the poly(2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane-co-cyclohexyl methacrylate) [p(V4D4-co-CHMA)] copolymer was developed for use as a gate dielectric in molybdenum disulfide (MoS2) field-effect transistors (FETs). The p(V4D4-co-CHMA) copolymer was synthesized via the initiated chemical vapor deposition (iCVD) of two types of monomers: 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4D4) and cyclohexyl methacrylate (CHMA). Four vinyl groups of V4D4 monomers and cyclohexyl groups of CHMA monomers were introduced to enhance the electrical strength of gate dielectrics through the formation of a highly crosslinked network and to reduce the charge trap densities at the MoS2–dielectric interface, respectively. The iCVD-grown p(V4D4-co-CHMA) copolymer films yielded a dielectric constant of 2.3 and a leakage current of 3.8 × 10–11 A/cm2 at 1 MV/cm. The resulting MoS2 FETs with p(V4D4-co-CHMA) gate dielectrics exhibited excellent electrical properties, including an electron ...

Monolayer Molybdenum Disulfide Transistors with Single-Atom-Thick Gates.

Two-dimensional transition-metal dichalcogenides (TMDs) are unique candidates for the development of next-generation electronic devices. However, the large contact resistance between metal and the monolayer TMDs have significantly limited the devices' performance. Also, the integration of ultrathin high- k dielectric layers with TMDs remains difficult due to the lack of dangling bonds on the surface of TMDs. We present monolayer molybdenum disulfide field-effect transistors with bottom local gates consisting of monolayer graphene. The atomic-level thickness and surface roughness of graphene facilitate the growth of high-quality ultrathin HfO2 and suppress gate leakage. Strong displacement fields above 8 V/nm can be applied using a single graphene gate to electrostatically dope the MoS2, which reduces the contact resistances between Ni and monolayer MoS2 to 2.3 kΩ·μm at low gate voltages. The devices exhibit excellent switching characteristics including a near-ideal subthreshold slope of 64 millivolts per decade, low threshold voltage (∼0.5 V), high channel conductance (>100 μS/μm), and low hysteresis. Scaled devices with 50 and 14 nm channels as well as ultrathin (5 nm) gate dielectrics show effective immunity to short-channel effects. The device fabricated on flexible polymeric substrate also exhibits high performance and has a fully transparent channel region that is desirable in optical-related studies and practical applications.

Molybdenum disulfide field-effect transistor biosensor for ultrasensitive detection of DNA by employing morpholino as probe

This work reports on a molybdenum disulfide (MoS2) based field-effect transistor (FET) biosensor for ultrasensitive label-free detection of DNA via phosphorodiamidate morpholino oligos (PMO)-DNA hybridization. After the chip was fabricated and the sensing channel was modified with positive charges, the negatively charged MoS2 nanosheet was drop-casted onto the channel, enabling MoS2 to tightly bind to the sensing surface via electrostatic interactions. Meanwhile, DNA analogue, PMO, was immobilized on the MoS2 surface, and detection of PMO-DNA hybridization was conducted by the fabricated MoS2 FET biosensor. Due to the neutral character and high affinity of PMO, a limit of detection (LOD) down to 6 fM was obtained, which is lower than that of the previously reported MoS2 FET DNA biosensor based on DNA-DNA hybridization. In addition, the MoS2 FET biosensor also showed high sequence specificity capable of distinguishing the complementary DNA from one-base mismatched DNA, three-base mismatched DNA and noncomplementary DNA. Moreover, the unique FET biosensor was able to detect DNA in complex sample like serum, making the method potential in disease diagnostics.

A new surface-potential-based compact model for the MoS2 field effect transistors in active matrix display applications

We present a continuous surface-potential-based compact model for molybdenum disulfide (MoS2) field effect transistors based on the multiple trapping release theory and the variable-range hopping theory. We also built contact resistance and velocity saturation models based on the analytical surface potential. This model is verified with experimental data and is able to accurately predict the temperature dependent behavior of the MoS2 field effect transistor. Our compact model is coded in Verilog-A, which can be implemented in a computer-aided design environment. Finally, we carried out an active matrix display simulation, which suggested that the proposed model can be successfully applied to circuit design.

All-dry transferred single- and few-layer MoS2 field effect transistor with enhanced performance by thermal annealing

We report on the experimental demonstration of all-dry stamp transferred single- and few-layer (1L to 3L) molybdenum disulfide (MoS2) field effect transistors (FETs), with a significant enhancement of device performance by employing thermal annealing in moderate vacuum. Three orders of magnitude reduction in both contact and channel resistances have been attained via thermal annealing. We obtain a low contact resistance of 22 kΩ μm after thermal annealing of 1L MoS2 FETs stamp-transferred onto gold (Au) contact electrodes. Furthermore, nearly two orders of magnitude enhancement of field effect mobility are also observed after thermal annealing. Finally, we employ Raman and photoluminescence measurements to reveal the phenomena of alloying or hybridization between 1L MoS2 and its contacting electrodes during annealing, which is responsible for attaining the low contact resistance.

An ultrasensitive detection of miRNA-155 in breast cancer via direct hybridization assay using two-dimensional molybdenum disulfide field-effect transistor biosensor

MicroRNAs (miRNAs), critical biomarkers of acute and chronic diseases, play key regulatory roles in many biological processes. As a result, robust assay platforms to enable an accurate and efficient detection of low-level miRNAs in complex biological samples are of great significance. In this work, a label-free and direct hybridization assay using molybdenum disulfide (MoS2) field-effect transistor (FET) biosensor has been developed for ultrasensitive detection of miRNA-155 as a breast cancer biomarker in human serum and cell-line samples. MoS2, the novel 2D layered material with excellent physical and chemical properties, was prepared through sequential solvent exchange method and was used as an active channel material. MoS2 was comprehensively characterized by spectroscopic and microscopic methods and it was applied for fabrication of FET device by drop-casting MoS2 flacks suspension onto the FET surface. MoS2 FET device showed a relatively low subthreshold swing of 48.10mV/decade and a high mobility of 1.98 × 103cm2V−1s−1. Subsequently, probe miRNA-155 strands were immobilized on the surface of the MoS2 FET device. Under optimized conditions detection limit of 0.03fM and concentration range 0.1fM to 10nM were achieved. The developed biosensor not only was capable to identification of fully matched versus one-base mismatch miRNA-155 sequence, but also it could detect target miRNA-155 in spiked real human serum and extracts from human breast cancer cell-line samples. This approach paves a way for label-free, early detection of miRNA as a biomarker in cancer diagnostics with very high sensitivity and good specificity, thus offering a significant potential for clinical application.

Effect of substrate and temperature on the electronic properties of monolayer molybdenum disulfide field-effect transistors

The use of two-dimensional nanostructured molybdenum disulfide (MoS2) films in field-effect transistors (FETs) in place of graphene was investigated. Monolayer MoS2 films were fabricated by chemical vapor deposition. The output and transfer curves of supported and suspended MoS2 FETs were measured. The mobility of the suspended device reached 364.2 cm2 V−1 s−1 at 150 °C. The hysteresis of the supported device in transfer curves was much larger than that of the suspended device, and it increased at higher temperatures. These results indicate that the device mobility was limited by Coulomb scattering at ambient temperature, and surface/interface phonon scattering at 150 °C, and the injection of electrons, via quantum tunneling through the Schottky barrier at the contact, was enhanced at higher temperatures and led to the increase of the hysteresis. The suspended MoS2 films show potential for application as a channel material in electronic devices, and further understanding the causes of hysteresis in a material is important for its use in technologies, such as memory devices and sensing cells.

Label-Free and Recalibrated Multilayer MoS2 Biosensor for Point-of-Care Diagnostics.

Molybdenum disulfide (MoS2) field-effect transistor (FET)-based biosensors have attracted significant attention as promising candidates for highly sensitive, label-free biomolecule detection devices. In this paper, toward practical applications of biosensors, we demonstrate reliable and quantitative detection of a prostate cancer biomarker using the MoS2 FET biosensor in a nonaqueous environment by reducing nonspecific molecular binding events and realizing uniform chemisorption of anti-PSA onto the MoS2 surface. A systematic and statistical study on the capability of the proposed device is presented, and the biological binding events are directly confirmed and characterized through intensive structural and electrical analysis. Our proposed biosensor can reliably detect various PSA concentrations with a limit of 100 fg/mL. Moreover, rigorous theoretical simulations provide a comprehensive understanding of the operating mechanism of the MoS2 FET biosensors, and further suggests the enhancement of the sensitivity through engineering device design parameters.

Fabrication of hybrid self-assembled monolayer/hafnium oxide gate dielectric by radical oxidation for molybdenum disulfide field-effect transistors

In this study, radical oxidation is applied to the fabrication of a hybrid self-assembled monolayer (SAM)/hafnium oxide (HfOx) gate dielectric in molybdenum disulfide (MoS2) field-effect transistors. The fabrication process involves radical oxidation to form HfOx at the surface of metallic HfN, SAM formation by immersion, and the deterministic transfer of MoS2 flakes. A subthreshold slope of 75 mV/dec and small hysteresis were demonstrated, indicating superior interfacial properties. Cross-sectional transmission electron microscopy revealed the uniform formation of the HfOx layer at the surface of HfN. The SAM is indispensable for the superior interfacial properties in MoS2 field-effect transistors. The radical oxidation is not restricted to the oxidation of silicon and germanium substrates and was also found to be applicable to the fabrication of a high-k gate dielectric. This study opens up interesting possibilities of radical oxidation for research on functional electronic devices.

Induction heating effect on the performance of flexible MoS2 field-effect transistors

We investigated the induction heating effect on device characteristics of flexible molybdenum disulfide (MoS2) field-effect transistors (FETs). A polyimide film was employed as a flexible substrate, and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate was coated on the flexible substrate as a bottom gate. After the annealing process on the flexible MoS2 FETs by induction heating, the field effect mobility was enhanced from 2.13 to 5.58 cm2/V·s with the slight increase of the on-off ratio from 5.17 × 102 to 1.98 × 103. Moreover, the low field mobility was almost unchanged from 7.75 to 7.33 cm2/V·s, indicating that the induction heating mainly contributed to the enhancement of the device performances by contact improvement between electrodes and MoS2. With the simple model of the diode and resistor connected in series, it was confirmed that the Schottky diode disappeared with contact enhancement. Our findings can contribute to the contact improvement with minimum damage when low dimensional nanomaterials are used as channel materials on flexible substrates.

Junctionless Diode Enabled by Self-Bias Effect of Ion Gel in Single-Layer MoS2 Device.

The self-biasing effects of ion gel from source and drain electrodes on electrical characteristics of single layer and few layer molybdenum disulfide (MoS2) field-effect transistor (FET) have been studied. The self-biasing effect of ion gel is tested for two different configurations, covered and open, where ion gel is in contact with either one or both, source and drain electrodes, respectively. In open configuration, the linear output characteristics of the pristine device becomes nonlinear and on-off ratio drops by 3 orders of magnitude due to the increase in "off" current for both single and few layer MoS2 FETs. However, the covered configuration results in a highly asymmetric output characteristics with a rectification of around 103 and an ideality factor of 1.9. This diode like behavior has been attributed to the reduction of Schottky barrier width by the electric field of self-biased ion gel, which enables an efficient injection of electrons by tunneling at metal-MoS2 interface. Finally, finite element method based simulations are carried out and the simulated results matches well in principle with the experimental analysis. These self-biased diodes can perform a crucial role in the development of high-frequency optoelectronic and valleytronic devices.

Characterization of intrinsic subgap density-of-states in exfoliated MoS2 FETs using a multi-frequency capacitance-conductance technique

A multi-frequency capacitance-conductance technique is proposed for characterizing the intrinsic density-of-states (DOS: gint(E)) inside an energy bandgap range (EV < E < EC) by de-embedding the structure-dependent parameters such as parasitic capacitance and resistance in a fabricated exfoliated molybdenum disulfide (MoS2) field effect transistor (EM-FET). The proposed technique uses the measured frequency-dispersive capacitance (Cm) and conductance (Gm=1/Rm=ωCmDm) data with the measured dissipation factor Dm(=Gm/ωCm) at a frequency range of 0.3 kHz to 10 kHz. To extract gint(E), an equivalent circuit model of the MoS2 FET converted from a two-element model for the parallel-mode (Cm-Dm) measurement was developed with this technique.

Effect of ultrathin Fe dusting layer on electrical transport properties of few-layer MoS2 field-effect transistors

Transition-metal-doped MoS2 has been predicted to be a potential candidate for a two dimensional dilute magnetic semiconductor, while the effect of transition metal dopants on the electrical properties of MoS2-based devices has received relatively less attention so far. Here, the authors report on a systematic electrical transport study of Fe-dusted few-layer molybdenum disulfide (MoS2) field-effect transistors via repeated in situ Fe deposition (total nominal thickness ≤ 2 nm) and electrical measurements in ultrahigh vacuum. It is found that an ultrathin Fe adsorption layer (≪0.5 nm) n-dopes MoS2 without noticeably affecting the electrical characteristics of the device. In contrast, a thicker Fe layer (0.5–2 nm) results in the loss of the carrier tunability and a nonlinear current-voltage characteristic with the differential conductance approximately linear to drain voltage. The authors show through result analysis and qualitative modeling that Fermi level pinning in MoS2 directly underneath Fe nanoclusters plays an important role in causing these degradations. The pinning effect can be partially removed by in situ oxidizing the Fe nanoclusters for a short duration of 16 min. The mechanism of Fermi level pinning is discussed.

Use of self-assembled monolayers for selective metal removal and ultrathin gate dielectrics in MoS2 field-effect transistors

We demonstrate a simple and rapid method for the fabrication of multilayer molybdenum disulfide (MoS2) field-effect transistors (FETs) by exploiting various functions of an octadecylphosphonic acid (ODPA)-based self-assembled monolayer (SAM). The SAM is used both for selective metal removal in patterning to simplify the fabrication process and as an ultrathin gate dielectrics in the device. The low voltage operation of the MoS2 FETs was observed with a low gate leakage current and small hysteresis. In addition, selective metal removal can effectively narrow the spacing between the gate and contact electrodes, resulting in improved device characteristics. This approach also makes it possible to transfer thinner and smaller MoS2 flakes. This study suggests intriguing applications of the SAM for the fabrication of functional electronic devices.

Control of threshold voltage by gate metal electrode in molybdenum disulfide field-effect transistors

This study reports the control of threshold voltage (Vth) by engineering a gate metal electrode in molybdenum disulfide (MoS2) field-effect transistors (FETs). The fabrication process for gate stacks involves the deposition of aluminum oxides (AlOx) on a high-work-function metal and the subsequent formation of a self-assembled monolayer (SAM) by an immersion method. A positive Vth of 0.15 V was demonstrated using a platinum (Pt) metal as a gate electrode accompanied by a low density of traps at the interface owing to the close-packed SAM. Raman spectroscopy revealed that the transferred MoS2 has a multi-layer structure. The Pt gate electrode exhibits a Vth shift of about 1 V in the positive direction compared with the aluminum (Al) gate electrode. This Vth shift is consistent with the difference in the work function of Pt and Al gate metal electrodes. The low–temperature process employed in this study makes it possible to assemble dissimilar materials including the Pt metal, deposited AlOx, and organic SAM molecules to manipulate the Vth in MoS2 FETs. This work is expected to open up intriguing directions for research on the applications and developments of functional FETs based on two-dimensional materials.

Effects of plasma treatment on the electrical reliability of multilayer MoS2 field-effect transistors

Electric stability and recovery properties of multilayer molybdenum disulfide (MoS2) field-effect transistors (FETs) were investigated by plasma treatment via inductively coupled plasma and polymethyl methacrylate (PMMA) passivation. Plasma treatment is proposed as an efficient method for cleaning the surface of MoS2. The results indicate that performance degradation of MoS2 field-effect transistors owing to oxygen absorption is largely restored by the plasma treatment. It is also shown that PMMA passivation (7% chlorobenzene) degrades the performance of the MoS2 FET. The performance degradation of the device can be restored by the plasma treatment. It is suggested that high-density plasma treatment with minimized damage to the surface can be an efficient method of surface cleaning. These findings are especially important for the nanolithography and nanodevice fabrication of two-dimensional devices made of MoS2, graphene, etc., using PMMA resistors.

Strain Gated Bilayer Molybdenum Disulfide Field Effect Transistor with Edge Contacts

Silicon nitride stress capping layer is an industry proven technique for increasing electron mobility and drive currents in n-channel silicon MOSFETs. Herein, the strain induced by silicon nitride is firstly characterized through the changes in photoluminescence and Raman spectra of a bare bilayer MoS2 (Molybdenum disulfide). To make an analogy of the strain-gated silicon MOSFET, strain is exerted to a bilayer MoS2 field effect transistor (FET) through deposition of a silicon nitride stress liner that warps both the gate and the source-drain area. Helium plasma etched MoS2 layers for edge contacts. Current on/off ratio and other performance metrics are measured and compared as the FETs evolve from back-gated, to top-gated and finally, to strain-gated configurations. While the indirect band gap of bilayer MoS2 at 0% strain is 1.25 eV, the band gap decreases as the tensile strain increases on an average of ~100 meV per 1% tensile strain, and the decrease in band gap is mainly due to lowering the conduction band at K point. Comparing top- and strain-gated structures, we find a 58% increase in electron mobility and 46% increase in on-current magnitude, signalling a benign effect of tensile strain on the carrier transport properties of MoS2.

Self-Assembled Monolayer-Based Gate Dielectrics for MoS2 FETs

In this study, self-assembled monolayer (SAM)-based gate dielectrics is applied to the fabrication of molybdenum disulfide (MoS2) field-effect transistors (FETs). A fabrication process of MoS2 FETs involves the selective formation of a SAM on aluminum oxides and the manual transfer of MoS2 flakes. A subthreshold slope (SS) of 69 mV/dec and no hysteresis were attained with the ultrathin SAM-based gate dielectrics. These results indicate the superior interfacial properties of the MoS2/SAM structure. Cross-sectional transmission electron microscopy revealed a flat and abrupt interface of the MoS2/SAM structure. It is found that the SAM-based gate dielectrics is applicable to the fabrication of low-voltage MoS2 FETs. This study opens up intriguing possibilities of SAM-based gate dielectrics in functional electronic devices.