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.

Temperature-Dependent Conduction and Photoresponse in Few-Layer ReS2.

The electrical behavior and the photoresponse of rhenium disulfide field-effect transistors (FETs) have been widely studied; however, only a few works have investigated the photocurrent as a function of temperature. In this paper, we perform the electrical characterization of few-layer ReS2-based FETs with Cr-Au contacts over a wide temperature range. We exploit the temperature-dependent transfer and output characteristics to estimate the effective Schottky barrier at the Cr-Au/ReS2 interface and to investigate the temperature behavior of parameters, such as the threshold voltage, carrier concentration, mobility, and subthreshold swing. Through time-resolved photocurrent measurements, we show that the photocurrent increases with temperature and exhibits a linear dependence on the incident light power at both low and room temperatures and a longer rise/decay time at higher temperatures. We surmise that the photocurrent is affected by the photobolometric effect and light-induced desorption of adsorbates which are facilitated by the high temperature and the low pressure.

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.

Electrical and Electronic Properties of Magnesium/Molybdenum Disulfide Heterojunction Field Effect Transistors: A Theoretical Study

The present article designs two-dimensional heterojunction duplex material FETs based on binary monolayer material, Mg and molybdenum disulfide. Despite having a hexagonal crystal structure, the monolayer Mg and molybdenum disulfide have good lattice matching ability, with a mismatch degree of approximately 5%. The electrostatic characteristics of Mg/molybdenum disulfide field effect transistors (FETs) are well suited for compact fabrication. Electronic structure of first-principles investigations, optical, mechanical, and electrochemical properties of MODES field-effect transistors based on density functional theory are mastered in order to master the electrostatic doping associated features of FETs. Based on the Silvaco TCAD platform, this simulation study was performed. There is theoretical value in engineering practice, both in terms of design and application.

Tapping the Performance of a Tungsten Disulfide Field-Effect Transistor with Deep Structure Optimization and Theoretical Simulation

As a representative material for two-dimensional transition metal dichalcogenides (2D TMDCs), tungsten disulfide (WS2) has a theoretical carrier mobility of up to 103 cm2·V–1·s–1 at room temperature (298 K) and is expected to be an excellent channel material for transistors. Meanwhile, the thickness of WS2 and the substrate thickness have an important impact on transistor performance. However, until now, despite extensive research on WS2-based field-effect transistors (WS2-FETs), WS2-FET performance has not been fully explored due to the large random thickness of WS2 nanosheets prepared using mechanical peeling methods and the lack of systematic studies on the preparation and testing of FET conditions and theoretical analysis, thus considerably limiting the application and development of WS2-FETs. In this study, we prepared FETs based on high-quality WS2 nanosheets via micromechanical exfoliation and investigated the factors influencing their performance. The optimum electrical properties of the WS2-FETs were obtained by regulating the thickness of the WS2 nanosheet and dielectric layer substrate and the annealing treatment, combined with theoretical simulation analysis. The resulting 9-layer WS2 nanosheets exhibited the best electrical properties, with a carrier mobility at 298 K of 531.41 cm2·V–1·s–1. The results of this study provide a basis for further research and optimization of high-performance logic devices based on 2D TMDCs.

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.

Enhanced Selectivity in Volatile Organic Compound Gas Sensors Based on ReS2-FETs under Light-Assisted and Gate-Bias Tunable Operation.

Using a single-device two-dimensional (2D) rhenium disulfide (ReS2) field-effect transistor (FET) with enhanced gas species selectivity by light illumination, we reported a selective and sensitive detection of volatile organic compound (VOC) gases. 2D materials have the advantage of a high surface-area-to-volume ratio for high sensitivity to molecules attached to the surface and tunable carrier concentration through field-effect control from the back-gate of the channel, while keeping the top surface open to the air for chemical sensing. In addition to these advantages, ReS2 has a direct band gap also in multilayer cases, which sets it apart from other transition-metal dichalcogenides (TMDCs). We take advantage of the effective response of ReS2 to light illumination to improve the selectivity and gas-sensing efficiency of a ReS2-FET device. We found that light illumination modulates the drain current response in a ReS2-FET to adsorbed molecules, and the sensing activity differs depending on the gas species used, such as acetone, ethanol, and methanol. Furthermore, wavelength and carrier density rely on certain variations in light-modulated sensing behaviors for each chemical. The device will distinguish the gas concentration in a mixture of VOCs using the differences induced by light illumination, enhancing the selectivity of the sensor device. Our results shed new light on the sensing technologies for realizing a large-scale sensor network in the Internet-of-Things era.

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.