Molybdenum Disulfide Field-effect Transistors Research Articles

Enhanced Electrical Transport Properties of Molybdenum Disulfide Field-Effect Transistors by Using Alkali Metal Fluorides as Dielectric Capping Layers.

The electronic properties of 2D materials are highly influenced by the molecular activity at their interfaces. A method was proposed to address this issue by employing passivation techniques using monolayer MoS2 field-effect transistors (FETs) while preserving high performance. Herein, we have used alkali metal fluorides as dielectric capping layers, including lithium fluoride (LiF), sodium fluoride (NaF), and potassium fluoride (KF) dielectric capping layers, to mitigate the environmental impact of oxygen and water exposure. Among them, the LiF dielectric capping layer significantly improved the transistor performance, specifically in terms of enhanced field effect mobility from 74 to 137 cm2/V·s, increased current density from 17 μA/μm to 32.13 μA/μm at a drain voltage of Vd of 1 V, and decreased subthreshold swing to 0.8 V/dec The results have been analytically verified by X-ray photoelectron spectroscopy (XPS) and Raman, and photoluminescence (PL) spectroscopy, and the demonstrated technique can be extended to other transition metal dichalcogenide (TMD)-based FETs, which can become a prospect for cutting-edge electronic applications. These findings highlight certain important trade-offs and provide insight into the significance of interface control and passivation material choice on the electrical stability, performance, and enhancement of the MoS2 FET.

Drain-Induced Multifunctional Ambipolar Electronics Based on Junctionless MoS2.

Applying a drain bias to a strongly gate-coupled semiconductor influences the carrier density of the channel. However, practical applications of this drain-bias-induced effect in the advancement of switching electronics have remained elusive due to the limited capabilities of its current modulation known to date. Here, we show strategies to largely control the current by utilizing drain-bias-induced carrier type switching in an ambipolar molybdenum disulfide (MoS2) field-effect transistor with Pt bottom contacts. Our CMOS-compatible device architecture, incorporating a partially gate-coupled p-n junction, achieves multifunctionality. The ambipolar MoS2 device operates as an ambipolar transistor (on/off ratios exceeding 107 for both NMOS and PMOS), a rectifier (rectification ratio of ∼3 × 106), a reversible negative breakdown diode with an adjustable breakdown voltage (on/off ratio exceeding 109 with a maximum current as high as 10-4 A), and a photodetector. Finally, we demonstrate a complementary inverter (gain of ∼24 at Vdd = 1.5 V), which is highly facile to fabricate without the need for complex heterostructures and doping processes. Our study provides strategies to achieve high-performance ambipolar MoS2 devices and to effectively utilize drain bias for electrical switching.

Ultrathin Van der Waals Lanthanum Oxychloride Dielectric for 2D Field-Effect Transistors.

Downsizing silicon-based transistors can result in lower power consumption, faster speeds, and greater computational capacity, although it is accompanied by the appearance of short-channel effects. The integration of high-mobility 2D semiconductor channels with ultrathin high dielectric constant (high-κ) dielectric in transistors is expected to suppress the effect. Nevertheless, the absence of a high-κ dielectric layer featuring an atomically smooth surface devoid of dangling bonds poses a significant obstacle in the advancement of 2D electronics. Here, ultrathin van der Waals (vdW) lanthanum oxychloride (LaOCl) dielectrics are successfully synthesized by precisely controlling the growth kinetics. These dielectrics demonstrate an impressive high-κ value of 10.8 and exhibit a remarkable breakdown field strength (Ebd ) exceeding 10 MV cm-1 . Remarkably, the conventional molybdenum disulfide (MoS2 ) field-effect transistor (FET) featuring a dielectric made of LaOCl showcases an almost negligible hysteresis when compared to FETs employing alternative gate dielectrics. This can be attributed to the flawlessly formed vdW interface and excellent compatibility established between LaOCl and MoS2 . These findings will motivate the further exploration of rare-earth oxychlorides and the development of more-than-Moore nanoelectronic devices.

A Peripheral-Free True Random Number Generator Based on Integrated Circuits Enabled by Atomically Thin Two-Dimensional Materials.

A true random number generator (TRNG) is essential to ensure information security for Internet of Things (IoT) edge devices. While pseudorandom number generators (PRNGs) have been instrumental, their deterministic nature limits their application in security-sensitive scenarios. In contrast, hardware-based TRNGs derived from physically unpredictable processes offer greater reliability. This study demonstrates a peripheral-free TRNG utilizing two cascaded three-stage inverters (TSIs) in conjunction with an XOR gate composed of monolayer molybdenum disulfide (MoS2) field-effect transistors (FETs) by exploiting the stochastic charge trapping and detrapping phenomena at and/or near the MoS2/dielectric interface. The entropy source passes the NIST SP800-90B tests with a minimum normalized entropy of 0.8780, while the generated bits pass the NIST SP800-22 randomness tests without any postprocessing. Moreover, the keys generated using these random bits are uncorrelated with near-ideal entropy, bit uniformity, and Hamming distances, exhibiting resilience against machine learning (ML) attacks, temperature variations, and supply bias fluctuations with a frugal energy expenditure of 30 pJ/bit. This approach offers an advantageous alternative to conventional silicon, memristive, and nanomaterial-based TRNGs as it obviates the need for extensive peripherals while harnessing the potential of atomically thin 2D materials in developing low-power TRNGs.

Design of a novel aptamer/molecularly imprinted polymer hybrid modified Ag–Au@Insulin nanoclusters/Au-gate-based MoS2 nanosheet field-effect transistor for attomolar detection of BRCA1 gene

Early detection of breast cancer, the first main cause of death in women, with robust assay platforms using appropriate biomarkers is of great importance for diagnosis and follow-up of the disease progression. This paper introduces an extra selective and sensitive label-free aptasensor for the screening of BRCA1 gene biomarker by taking advantage of a gate modified with aptamer and molecularly imprinted polymer hybrid (MIP) as a new synthetic receptor film coupled with an electrolyte-gated molybdenum disulfide (MoS2) field-effect transistor (FET). The Au gate surface of FET was modified with insulin stabilized bimetallic Ag–Au@nanoclusters (Ag–Au@InsNCs), after which, the immobilization of the hybridized aptamer and o-phenylenediamine was electropolymerized to form an aptamer-MIP hybrid receptor. The output characteristics of Apta-MIP hybrid modified Au gate MoS2 FET device were followed as a result of change in electrical double layer capacitance of electrolye-gate interface. The magnitude of decrease in the drain current showed a linear response over a wide concentration range of 10 aM to 1 nM of BRCA1 ssDNA with a sensitivity as high as 0.4851 μA/decade of concentration and a limit of detection (LOD) of 3.0 aM while very low responses observed for non-imprinted polymer. The devised aptasensor not only was capable to the discrimination of the complementary versus one-base mismatch BRCA1 ssDNA sequence, but also it could detect the complementary BRCA1 ssDNA in spiked human serum samples over a wide concentration range of 10 aM to 1.0 nM with a low LOD of 6.4 aM and a high sensitivity 0.3718 μA/decade.

Molybdenum disulfide field-effect transistors with enhanced charge-injection by inserting ultrathin pentacene layer under source/drain electrodes

We report a method to improve charge-injection properties of molybdenum disulfide (MoS2)-based field-effect transistors (FETs) by introducing ultrathin pentacene film as the buffer layer under the source/drain electrodes. Compared with conventional devices with metal-only electrodes, the electrical characteristics of MoS2 FETs with the organic buffer layer exhibit a ten-fold enhancement in on-state current and a four-fold increase in field-effect mobility. These improvements are attributed to the introduction of organic buffer layer with hole-type that result in optimized contact properties. The Schottky barrier height is lowered by approximately 0.3 eV for the electron-injection in MoS2 FETs. Furthermore, electrons in metal and MoS2 could be effectively attracted to the surfaces by holes in pentacene film resulting in a tunnelling probability, which is responsible for the improved charge-injection. Current studies will be helpful in the development of high performance MoS2 optoelectrical devices.

Highly Stretchable MoS2‐Based Transistors with Opto‐Synaptic Functionalities

AbstractStretchable devices can form intimate interfaces with the attached objects, giving birth to widespread applications in wearable electronics, bioelectronics, and artificial bionics. The emerging 2D materials are considered to be ideal candidates for stretchable electronics due to their ultra‐thin nature and excellent mechanical properties. However, stretchable 2D semiconductor devices previously demonstrated usually work at insufficient strain range with poor device performances mostly due to a mechanical failure. Here, the fabrication of buckled monolayer molybdenum disulfide (MoS2) field effect transistors (FETs) on elastomeric substrates is reported. These stretchable MoS2 FETs show stable performances with mobility of ≈30 cm2 V−1 s−1, on/off ratio of ≈108, and subthreshold swing (SS) of ≈180 mV dec−1 after many cycled stretching‐release processes under more than 10% strain. In particular, the feasibility of applying these stretchable MoS2 transistors in optoelectronic synapse and neural network simulation in recognition tasks has been demonstrated.

Tailoring the Electrical Characteristics of MoS2 FETs through Controllable Surface Charge Transfer Doping Using Selective Inkjet Printing.

Surface charge transfer doping (SCTD) has been regarded as an effective approach to tailor the electrical characteristics of atomically thin transition metal dichalcogenides (TMDs) in a nondestructive manner due to their two-dimensional nature. However, the difficulty of achieving rationally controlled SCTD on TMDs via conventional doping methods, such as solution immersion and dopant vaporization, has impeded the realization of practical optoelectronic and electronic devices. Here, we demonstrate controllable SCTD of molybdenum disulfide (MoS2) field-effect transistors using inkjet-printed benzyl viologen (BV) as an n-type dopant. By adjusting the BV concentration and the areal coverage of inkjet-printed BV dopants, controllable SCTD results in BV-doped MoS2 FETs with elaborately tailored electrical performance. Specifically, the suggested solvent system creates well-defined droplets of BV ink having a volume of ∼2 pL, which allows the high spatial selectivity of SCTD onto the MoS2 channels by depositing the BV dopant on demand. Our inkjet-printed SCTD method provides a feasible solution for achieving controllable doping to modulate the electrical and optical performances of TMD-based devices.

Interface optimization of La-based gate dielectric for molybdenum disulfide field-effect transistors

Two-dimensional (2D) layered materials such as graphene, transition metal dichalcogenides are considered as the promising candidates for new generation electronics and optoelectronics, owing to their unique physical properties and abundant design freedom. However, the problem of heterogeneous integration of two-dimensional materials with gate dielectric materials has not been solved, especially the problem of interface qualities, which limits the development of two-dimensional electronics. Herein, we demonstrate the feasibility of interface optimization through post-deposition annealing process. The proposed La-based oxide MoS2 transistor offers high mobility of 60 cm2/V·s, increased by 136% up to ∼ 50uA/um and low interface trap density after post-deposition annealing treatment. The detailed research results show that N2 atmosphere annealing treatment can not only promote the composition of LaAlO3, but also suppress the Coulomb scattering generated from interface state charge. All these results provide an enhanced fabrication scheme for heterogeneous integration with the optimized interface and offer reference for high-reliability manufacturing of low-power two-dimensional electronic devices.

A wafer-scale van der Waals dielectric made from an inorganic molecular crystal film

Van der Waals dielectrics, such as hexagonal boron nitride, are widely used to preserve the intrinsic properties of two-dimensional semiconductors in electronic devices. However, fabricating these materials on the wafer scale and integrating them with two-dimensional semiconductors is challenging because their synthesis typically requires mechanical exfoliation or vapour deposition processes. Here we show that a high-κ van der Waals dielectric can be created on wafer scales using an inorganic molecular crystal film of antimony trioxide (Sb2O3) fabricated via thermal evaporation deposition. Monolayer molybdenum disulfide (MoS2) field-effect transistors supported by this dielectric substrate exhibit enhanced electron mobility—from 26 cm2 V−1 s−1 to 145 cm2 V−1 s−1—and reduced transfer-curve hysteresis compared with when using SiO2 substrate. MoS2 transistors directly gated by the Sb2O3 film can operate with a supply voltage of 0.8 V, on/off ratio of 108 and subthreshold swing of 64 mV dec−1 at 300 K.

MoS2 transistors gated by ferroelectric HfZrO2 with MoS2/mica heterojunction interface

Two-dimensional (2D) molybdenum disulfide (MoS2) field-effect transistor (FET) gated by negative capacitance (NC) is a promising architecture to overcome the thermionic limit and thus reduce device consumption. Here, top-gated MoS2 NCFETs have been prepared by transferring a mica flake on MoS2 channel to form a van der Waals heterojunction interface, together with a ferroelectric HfZrO2 (HZO) deposited on mica. Stable NC effects are demonstrated for MoS2 NCFETs. The MoS2 NCFETs integrated with mica/HZO gate stack provide competitive electrical characteristics when they are applied with a gate voltage sweep-width in the range of 1−3 V and a sweep-rate from 0.01 to 0.2 V s−1, including steep-slope of sub 60 mV dec−1 in four orders of magnitude of drain current, on/off current ratio over 106, and small hysteresis-width less than 50 mV. Outstanding performance should be ascribed to damage-free properties of mica/MoS2 van der Waals interface and capacitance matching between the HZO ferroelectric and mica dielectric. The results confirm the promising nature of mica/HZO gate stack and potential applications for future electronics.

Modulation of 1 MeV electron irradiation on ultraviolet response in MoS2 FET

The material, electrical and ultraviolet optoelectronic properties of few layers bottom molybdenum disulfide (MoS2) field effect transistors (FETs) device was investigated before and after 1 MeV electron irradiation. Due to the participation of SiO2 in conduction, we discovered novel photoelectric properties and a relatively long photogenerated carrier lifetime (several tens of seconds). Electron irradiation causes lattice distortion, the decrease of carrier mobility, and the increase of interface state. It leads to the degradation of output characteristics, transfer characteristics and photocurrent of the MoS2 FET.

Impact of the channel length on molybdenum disulfide field effect transistors with hafnia-based high-k dielectric gate

Field effect transistors (FETs) using two-dimensional molybdenum disulfide (MoS2) as the channel material has been considered one of the most potential candidates for future complementary metal-oxide-semiconductor technology with low power consumption. However, the understanding of the correlation between the device performance and material properties, particularly for devices with scaling-down channel lengths, is still insufficient. We report in this paper back-gate FETs with chemical-vapor-deposition grown and transferred MoS2 and Zr doped HfO2 ((Hf,Zr)O2, HZO) high-k dielectric gates with channel lengths ranging from 10 to 30 µm with a step of 5 µm. It has been demonstrated that channels with the length to width ratio of 0.2 lead to the most superior performance of the FETs. The MoS2/HZO hybrid FETs show a stable threshold voltage of ∼1.5 V, current on/off ratio of >104, and field effect mobility in excess of 0.38 cm2 V−1 s−1. The impact of the channel lengths on FET performance is analyzed and discussed in depth. A hysteresis loop has been observed in the Ids − Vgs characteristics of the hybrid FETs, which has been further studied and attributed to the charge effect at the interfaces. The HZO films show a relatively weak ferroelectric orthorhombic phase and thus serve mainly as the high-k dielectric gate. Charge trapping in the HZO layer that might induce hysteresis has been discussed. Our results show that MoS2/HZO hybrid FETs possess great potential in future low power and high-speed integrated circuits, and future work will focus on further improvement of the transistor performances using ferroelectric HZO films and the study of devices with even shorter MoS2 channels.

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.

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.

Unscrambling for Subgap Density-of-States in Multilayered MoS2Field Effect Transistors under DC Bias Stress via Optical Charge-Pumping Capacitance-Voltage Spectroscopy

Herein, we quantitatively analyze the evolution of the subgap density of states (DOSs) for multilayered molybdenum disulfide (m-MoS2) field effect transistors (FETs) with bilayered SiNx/SiOx gate dielectrics under positive bias stress (PBS) and negative bias stress (NBS) by using optical charge-pumping capacitance-voltage spectroscopy. To decouple external gas ambient effects on device instability, hydrophobic fluoropolymers (cyclized transparent optical polymer; CYTOP) are employed for m-MoS2 FETs followed by the evaluation of subgap-DOSs in the devices for the respective PBS (or NBS). Through extraction of subgap-DOSs and their deconvolution with an analytical model of acceptor (or donor)-like states, it is shown that the device instability is closely correlated with state transitions of DOSs, corresponding to shallow (or midgap) levels for monosulfur vacancy (VS) (or disulfur vacancy ( $\text{V}_{\mathrm {S2}}$ )). Moreover, after PBS, the initial states of VS(0) (or $\text{H}_{\mathrm {S2}}$ (0)) transit toward VS (−1) (or $\text{H}_{\mathrm {S2}}(-1$ )) via electron trapping, whereas the transition toward VS (0) and $\text{H}_{\mathrm {S2}}$ (+1) during NBS is assessed from the initial $\text{V}_{\mathrm {S}}(-1$ ) and $\text{H}_{\mathrm {S2}}$ (0) states. Furthermore, technology computer-aided design (TCAD) simulation based on the extracted DOSs properly replicates the measured I-V characteristics of m-MoS2 FETs with (and without) CYTOP encapsulation. In this study, subgap-DOS characterization via optical charge pumping and validation of the quantitative evolution of subgap-DOSs suggest that this platform can be potentially beneficial for an in-depth understanding of the origins of device instability for transition metal dichalcogenide (MX2)-based FETs, where M is a transition metal and X is a chalcogenide.

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.

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.

Field-effect transistor bioassay for ultrasensitive detection of folate receptor 1 by ligand-protein interaction.

A miniaturized and integrated bioassay was developed based on molybdenum disulfide (MoS2) field-effect transistor (FET) functionalized with bovine serum albumin-folic acid (BSA-FA) for monitoring FOLR1. We performed the electrical test of FOLR1 within therange 100fg/mL to 10ng/mL, and the limit of detection was 0.057pg/mL. The ultrahigh sensitivity of the bioassay was realized by ligand-protein interaction between FA and FOLR1, with a ligand-protein binding ratio of 3:1. The formation of FA-FOLR1 was confirmed with ELISA. Thebinding affinity dissociation constant KD was12 ± 6pg/mL. This device can work well for FOLR1 detection in human serum, which presents its promising application in point-of-care diagnosis. This study supports the future applications of such ligand-protein-based bioassays in the clinical practices. Graphical abstract MoS2-based FET device for detecting folate receptor 1 (FOLR1) was fabricated. The molecular folic acid as a probe can specifically bound to FOLR1 with a high affinity.

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.