MoS2 Channel Research Articles

Electrode‐Induced Self‐Healed Monolayer MoS2 for High Performance Transistors and Phototransistors (Adv. Mater. 41/2021)

Self-Healing Devices An innovative concept of a self-healed electrode-channel system employing ultrathin metallic copper monosulfide (CuS) and monolayered molybdenum disulfide (MoS2) is demonstrated by John Hong, SeungNam Cha, and co-workers in article number 2102091 for the design of defect-curable transistors and phototransistors. Excess sulfur adatoms from the CuS electrode spontaneously heal the defect sites in the MoS2 channel for record-high transistor and phototransistor performance.

Free-Standing Multilayer Molybdenum Disulfide Memristor for Brain-Inspired Neuromorphic Applications.

Recently, atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDs) have attracted great interest in electronic and opto-electronic devices for high-integration-density applications such as data storage due to their small vertical dimension and high data storage capability. Here, we report a memristor based on free-standing multilayer molybdenum disulfide (MoS2) with a high current on/off ratio of ∼103 and a stable retention for at least 3000 s. Through light modulation of the carrier density in the suspended MoS2 channel, the on/off ratio can be further increased to ∼105. Moreover, the essential photosynaptic functions with short- and long-term memory (STM and LTM) behaviors are successfully mimicked by such devices. These results also indicate that STM can be transferred to LTM by increasing the light stimuli power, pulse duration, and number of pulses. The electrical measurements performed under vacuum and ambient air conditions propose that the observed resistive switching is due to adsorbed oxygen and water molecules on both sides of the MoS2 channel. Thus, our free-standing 2D multilayer MoS2-based memristors propose a simple approach for fabrication of a low-power-consumption and reliable resistive switching device for neuromorphic applications.

Large difference of doping behavior of tetracyanoquinodimethane (TCNQ) and 2,3,5,6-Tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) on field effect transistor with channel of atomic layer MoS2

We examined the mechanism of the large difference of the doping effect of two molecules of tetracyanoquinodimethane(TCNQ) and 2,3,5,6-Tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) on the field effect transistor (FET) property, which is composed of the atomically thin MoS2 channel. The electron-acceptor behaviors of the two molecules are detected by examining the threshold voltage shift (ΔVth) of the drain current vs gate voltage curve plot. It shows only a mild shift of ∼4 V for the TCNQ but it shows a large value of ∼33 V for the F4-TCNQ molecule. The similar rapid change with small amount of F4-TCNQ deposition was detected as the Fermi level shift in the ultra violet photoemission spectroscopy (UPS). The DFT calculation shows the flat lying configuration for both molecules after the structural optimization, and the difference of the charge transfer to the substrate is not large enough to explain the difference of theΔVth. Instead, we want to propose a model in which the bonding of the F atom of the F4-TCNQ molecule at the S-defect contribute to the initial rapid variation of theΔVth.

Demonstration of intrinsic STDP learning capability in all-2D multi-state MoS2 memory and its application in modelling neuromorphic speech recognition

The human brain can be characterized by its large number of adaptive synapses, connecting billions of neurons capable of both learning and perceiving the environment. Neuromorphic computing, based on brain-inspired principles, is a promising technology, to build low-power, distributed, fault-tolerant intelligent systems mainly for perception tasks. Here, we demonstrate the intrinsic capability of floating gate (FG) MoS2 device (MoS2 FG-FET) to model the spike time dependent plasticity (STDP) learning rule that is based on the transient response of the MoS2 channel to spikes applied to the source and gate leads. We implemented the STDP learning protocol in a neuromorphic speech recognition system (NSRS), inspired by the human auditory pathway, for various auditory recognition tasks. Our proposed NSRS consists of a cochlea model, an unsupervised feature learning stage, and a simple linear classifier. The unsupervised learning stage uses the biologically plausible STDP learning in novel two-dimensional MoS2 FG-FET memory which circumvents the requirement of any other learning circuitry. Demonstration of STDP modelling in two-dimensional (2D) MoS2 is an important step towards incorporating 2D architectures for reduced device footprints in neuromorphic learning circuits.

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.

Junction Field‐Effect Transistors Based on PdSe2/MoS2 Heterostructures for Photodetectors Showing High Responsivity and Detectivity

Abstract2D materials have shown great promise for next‐generation high‐performance photodetectors. However, the performance of photodetectors based on 2D materials is generally limited by the tradeoff between photoresponsivity and photodetectivity. Here, a novel junction field‐effect transistor (JFET) photodetector consisting of a PdSe2 gate and MoS2 channel is constructed to realize high responsivity and high detectivity through effective modulation of top junction gate and back gate. The JFET exhibits high carrier mobility of 213 cm2 V−1 s−1. What is more, the high responsivity of 6 × 102 A W−1, as well as the high detectivity of 1011 Jones, are achieved simultaneously through the dual‐gate modulation. The high performance is attributed to the modulation of the depletion region by the dual‐gate, which can effectively suppress the dark current and enhance the photocurrent, thereby realizing high detectivity and responsivity. The JFET photodetector provides a new approach to realize photodetectors with high responsivity and detectivity.

Ultrasensitive Photodetection in MoS2 Avalanche Phototransistors.

Recently, there have been numerous studies on utilizing surface treatments or photosensitizing layers to improve photodetectors based on 2D materials. Meanwhile, avalanche breakdown phenomenon has provided an ultimate high‐gain route toward photodetection in the form of single‐photon detectors. Here, the authors report ultrasensitive avalanche phototransistors based on monolayer MoS2 synthesized by chemical vapor deposition. A lower critical field for the electrical breakdown under illumination shows strong evidence for avalanche breakdown initiated by photogenerated carriers in MoS2 channel. By utilizing the photo‐initiated carrier multiplication, their avalanche photodetectors exhibit the maximum responsivity of ≈3.4 × 107 A W−1 and the detectivity of ≈4.3 × 1016 Jones under a low dark current, which are a few orders of magnitudes higher than the highest values reported previously, despite the absence of any additional chemical treatments or photosensitizing layers. The realization of both the ultrahigh photoresponsivity and detectivity is attributed to the interplay between the carrier multiplication by avalanche breakdown and carrier injection across a Schottky barrier between the channel and metal electrodes. This work presents a simple and powerful method to enhance the performance of photodetectors based on carrier multiplication phenomena in 2D materials and provides the underlying physics of atomically thin avalanche photodetectors.

Modulation of the contact barrier at VS2/MoS2 interface: A first principles study

Two-dimensional metallic VS2 is explored as promising electrode material for MoS2 based device by theoretically evaluating of the contact barrier at VS2/MoS2 interface. The electronic band structure analysis indicates that p-type Ohmic contact is formed at the VS2/MoS2 interface. The tunneling barrier can be avoided by selecting proper stacking order of VS2/MoS2 heterojunction. The atom intercalation strategy is valid, which can not only change the contact nature from p-type to n-type, but also weaken the tunneling barrier. Further quantum transport simulations reveal that n-type Ohmic contact between the electrode and MoS2 channel can be achieved by Na and Mg intercalation. Our work may benefit for design of MoS2 based device.

Ultrathin Multibridge Channel Transistor Enabled by van der Waals Assembly.

Multibridge channel field-effect transistors (MBCFETs) enable improved gate control and flow of a large drive current and they are regarded as promising candidates for next-generation transistor architecture. However, in achieving a larger drive current with a thinner channel, limitations arise from the decrease in mobility when the thickness of the Si nanosheet is less than 5nm. In addition, an increase in the leakage current is unavoidable when a large number of channels are stacked. Here, a 2D ultrathin MBCFET is demonstrate, constructed based on 2nm/2nm MoS2 channels. The normalized drive current (23.11 µA*µmµm-1 ) in each level channel of this MBCFET exceeds that of the latest seven-level-stacked Si MBCFET, while the leakage current is only 0.4% of this value, with the subthreshold swing reaching 60mVdec-1 and an on/off ratio reaching up to 4×108 at room temperature. Furthermore, the drive current of this 2D ultrathin MBCFET can be further increased by regulating the polarity of the operation voltage to reduce the injection barrier. The combination of 2D materials and an MBC structure has the potential for use in high-performance and low-power-consumption electronics.

An ultrasensitive molybdenum-based double-heterojunction phototransistor

Two-dimensional (2D) materials are promising for next-generation photo detection because of their exceptional properties such as a strong interaction with light, electronic and optical properties that depend on the number of layers, and the ability to form hybrid structures. However, the intrinsic detection ability of 2D material-based photodetectors is low due to their atomic thickness. Photogating is widely used to improve the responsivity of devices, which usually generates large noise current, resulting in limited detectivity. Here, we report a molybdenum-based phototransistor with MoS2 channel and α-MoO3-x contact electrodes. The device works in a photo-induced barrier-lowering (PIBL) mechanism and its double heterojunctions between the channel and the electrodes can provide positive feedback to each other. As a result, a detectivity of 9.8 × 1016 cm Hz1/2 W−1 has been achieved. The proposed double heterojunction PIBL mechanism adds to the techniques available for the fabrication of 2D material-based phototransistors with an ultrahigh photosensitivity.

Hybrid Gate Dielectric of MoS2 Transistors for Enhanced Photo‐Electronic Stability

AbstractMoS2 thin‐film transistors (TFTs) have been widely studied for use as driving TFTs of active‐matrix displays considering their outstanding electrical advantages such as high mobility and high on/off current ratio. However, due to the atomically thin nature of MoS2, the device performance of MoS2 TFTs suffers from trap sites at the interface. In this study, a hybrid gate dielectric based on an interface engineering strategy using poly(1,3,5‐trivinyl‐1,3,5‐trimethyl cyclotrisiloxane) (pV3D3) via initiated chemical vapor deposition is investigated to enhance the negative bias illumination stress (NBIS) stability of MoS2 TFTs. Compared to a single oxide dielectric layer (Al2O3), a hybrid dielectric layer (pV3D3/Al2O3) exhibits decreased threshold voltage shift under NBIS by reducing functional groups, such as hydroxyl (OH−) group, which act as charge trapping sites at the interface between the MoS2 channel and the gate dielectric. It is confirmed by quantitative analysis using the stretched‐exponential model. Tau (τ), one of the modeling parameters in the stretched‐exponential model, decreases from 210 to 120 s, indicating the improvement in stability. Furthermore, in a low‐frequency noise (1/f) measurement, hybrid‐dielectric‐based TFTs show an order of magnitude lower noise power spectral density (SID/ID2) than single‐oxide‐dielectric‐based TFTs.

Electric bias-induced edge degradation of few-layer MoS2 devices

In this work, we experimentally investigate the effects of electric bias on the degradation of few-layer MoS2 back-gated field-effect transistors in ambient air. The devices were fabricated using mechanically exfoliated MoS2 flakes, which were transferred to a Si/SiO2 substrate by a PDMS-based transfer. We report an accelerated electric bias-induced degradation of the devices under investigation and used optical and scanning electron microscopy (SEM) to monitor changes of the morphology of the MoS2 channel. In particular, we found a linear dependency of the degradation on the electric field between the Ti/Au source and drain contacts. In addition, we identify four regions in which morphological changes occur, of which the edges of the MoS2 channel are most affected.

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.

Ultrafast, Kinetically Limited, Ambient Synthesis of Vanadium Dioxides through Laser Direct Writing on Ultrathin Chalcogenide Matrix

Vanadium dioxide (VO2) is a strongly correlated electronic material and has attracted significant attention due to its metal-to-insulator transition and diverse smart applications. Traditional synthesis of VO2 usually requires minutes or hours of global heating and low oxygen partial pressure to achieve thermodynamic control of the valence state. Further patterning of VO2 through a series of lithography and etching processes may inevitably change its surface valence, which poses a great challenge for the assembly of micro- and nanoscale VO2-based heterojunction devices. Herein, we report an ultrafast method to simultaneously synthesize and pattern VO2 on the time scale of seconds under ambient conditions through laser direct writing on a V5S8 "canvas". The successful ambient synthesis of VO2 is attributed to the ultrafast local heating and cooling process, resulting in controlled freezing of the intermediate oxidation phase during the relatively long kinetic reaction. A Mott memristor based on a V5S8-VO2-V5S8 lateral heterostructure can be fabricated and integrated with a MoS2 channel, delivering a transistor with abrupt switching transfer characteristics. The other device with a VSxOy channel exhibits a large negative temperature coefficient of approximately 4.5%/K, which is highly desirable for microbolometers. The proposed approach enables fast and efficient integration of VO2-based heterojunction devices and is applicable to other intriguing intermediate phases of oxides.

Multifunctional molybdenum disulfide flash memory using a PEDOT:PSS floating gate

Two-dimensional transition metal dichalcogenide materials (TMDs), such as molybdenum disulfide (MoS2), have been considered promising candidates for future electronic applications owing to their electrical, mechanical, and optical properties. Here, we present a new concept for multifunctional MoS2 flash memory by combining a MoS2 channel with a PEDOT:PSS floating layer. The proposed MoS2 memory devices exhibit a switching ratio as high as 2.3 × 107, a large memory window (54.6 ± 7.80 V), and high endurance (>1,000 cycles). As the PEDOT:PSS film enables a low-temperature solution-coating process and mechanical flexibility, the proposed P-memory can be embedded on a polyimide substrate over a rigid silicon substrate, offering high mechanical endurance (over 1,000 cycle bending test). Furthermore, both MoS2 and PEDOT:PSS have a bandgap that is desirable in optoelectronic memory operation, where charge carriers are stored differently in the floating gate depending on light illumination. As a new application that combines photodiodes and memory functions, we demonstrate multilevel memory programming based on light intensity and color.

Confinement effects on the solvation structure of solvated alkali [correction] metal cations in a single-digit 1T-MoS2 nanochannel: A first-principles study.

Confinement plays an important role in determining ion transport in porous materials, which, in turn, may influence the performance of many energy storage and desalination devices. In this work, we combined density functional theory (DFT) with an implicit solvation model and ab initio molecular dynamics (AIMD) to investigate the effects of nanoconfinement on several solvated alkaline metal cations in a single-digit 1T-MoS2 nanochannel. Our DFT calculations with a solvation model indicated that cations with stronger hydration energy introduce a higher number of co-intercalated water molecules into the channel, consistent with early experimental observation obtained for MXene (2D transition metal carbide) channels. The predicted optimal water numbers for the cations were then used for AIMD simulations that explicitly include the effects of the solvent. When compared with the cations in bulk solution, our simulations showed that the hydration structure and coordination number (CN) of the solvated cations confined in the MoS2 channel can be significantly altered. We found that larger cations with weaker hydration energy (K+, Rb+, and Cs+) exhibited a distinctive CN decrease under confinement, while smaller cations (Li+ and Na+) retained a similar hydration shell as in the bulk solution. More specifically, the hydration shell of large cations (K+, Rb+, and Cs+) in MoS2 showed similar features of the coordination angle to the bulk, which suggests the partially broken hydration shell with no geometry change under confinement. Our simulations provided insights into the change of the hydration structure of alkaline metal cations under confinement, which may have important implications on their transport in the 1T-MoS2 channel.

Improved low-frequency noise in CVD bilayer MoS2 field-effect transistors

In MoS2 field-effect transistors, the current or voltage fluctuations related to either mobility- or number-dependent relationships are characterized by low-frequency noise. This noise can typically be used to evaluate the application limits of MoS2-based electronic devices. In this work, the low-frequency noise characteristics of single-crystal bilayer MoS2 grown by chemical vapor deposition (CVD) are systematically investigated and found to offer significant performance improvements compared with those based on the monolayer MoS2 channel. At f = 100 Hz, the normalized drain current power spectral density (SI/Id2) is 2.4 × 10−10 Hz−1 and 3.1 × 10−9 Hz−1 for bilayer and monolayer MoS2 transistors, respectively. The 1/f noise behavior can be accurately described by McWhorter's carrier number fluctuation model for both transistor types, suggesting that carrier trapping and de-trapping by dielectric defects is the dominant mechanism of 1/f noise in CVD MoS2 transistors. Furthermore, a minimal WLSI/Id2 of 3.1 × 10−10 μm2/Hz (where W is the gate width and L is the gate length) is achieved at Vbg = 3 V by effectively reducing the contact resistance of bilayer MoS2 transistors using a back-gate voltage. These results demonstrate that CVD bilayer MoS2 is a promising candidate for future large-scale 2D-semiconductor-based electronic applications with improved noise performance.

2D MoS2 Charge Injection Memory Transistors Utilizing Hetero‐Stack SiO2/HfO2 Dielectrics and Oxide Interface Traps

AbstractAmong advanced devices with 2D semiconductors, charge injection memory field effect transistors (CIM FETs) may be one of the most important and practical ones. Reported CIM FETs utilize three layers (for tunneling, trapping, and bulk dielectric) in general, resulting in high switching voltages over 10 V. Here, nonvolatile CIM FETs are fabricated with MoS2 channel and hetero‐stack bilayer oxide dielectrics adopting 5 nm‐thin SiO2 and 25 nm‐thick HfO2, where the charge traps are expected at the SiO2/HfO2 oxide interface. It is nicely observed from the device that a low pulse gate voltage below ±7 V is enough to obtain program and erase states, which would originate from the tunneling electrons trapped at the hetero‐stack oxide interface. For comparison, other CIM FET devices are also fabricated but with tri‐layer dielectric of 5 nm polystyrene‐brush/5 nm HfO2/25 nm SiO2. Expectedly, the latter with tri‐layer requires at least ±10 V for memory operations. The former with a hetero‐stack oxide bilayer is now determined as an optimum device because of low operating voltages and less process complexity, and it is extended to a circuit application for a long‐term memory switching of an organic light‐emitting diode (OLED) pixel.

Material and Device Structure Designs for 2D Memory Devices Based on the Floating Gate Voltage Trajectory.

Two-dimensional heterostructures have been extensively investigated as next-generation nonvolatile memory (NVM) devices. In the past decade, drastic performance improvements and further advanced functionalities have been demonstrated. However, this progress is not sufficiently supported by the understanding of their operations, obscuring the material and device structure design policy. Here, detailed operation mechanisms are elucidated by exploiting the floating gate (FG) voltage measurements. Systematic comparisons of MoTe2, WSe2, and MoS2 channel devices revealed that the tunneling behavior between the channel and FG is controlled by three kinds of current-limiting paths, i.e., tunneling barrier, 2D/metal contact, and p-n junction in the channel. Furthermore, the control experiment indicated that the access region in the device structure is required to achieve 2D channel/FG tunneling by preventing electrode/FG tunneling. The present understanding suggests that the ambipolar 2D-based FG-type NVM device with the access region is suitable for further realizing potentially high electrical reliability.

Improved subthreshold swing of MoS2 negative-capacitance transistor by fluorine-plasma treatment on ferroelectric Hf0.5Zr0.5O2 gate dielectric

In this work, the ferroelectricity of hafnium zirconium oxide (Hf0.5Zr0.5O2, HZO) is enhanced by fluorine (F)-plasma treatment, which is used to fabricate MoS2 negative-capacitance field-effect transistor. Measurements show that the subthreshold swing of the transistor is significantly reduced to 17.8 mV dec−1 over almost four orders of output current, as compared to its counterpart without the F-plasma treatment (37.4 mV dec−1). The involved mechanism is that during the F-plasma treatment, F atoms can be incorporated into the HZO bulk to passive its oxygen vacancies and interface traps, thus forming robust Zr–F and Hf–F bonds. Therefore, the F-plasma-treated HZO film exhibits much less oxygen vacancies than the untreated HZO film, which is beneficial to enhancing the amplification effect on the surface potential of the MoS2 channel during the NC operation.