MoS2 Field-effect Transistors Research Articles

Reliability of high-performance monolayer MoS2 transistors on scaled high-κ HfO2

The successful integration of ultrathin high-κ insulators is essential for the advancement of ultra-scaled field-effect transistors (FETs) based on two-dimensional (2D) semiconductors in future technology nodes. However, defects within the high-κ stack or at the interfaces can significantly degrade the performance of these “interface-only” devices, raising questions regarding their long-term reliability. Here, we study the reliability of monolayer MoS2 FETs on ultra-thin high-κ HfO2. Interestingly, we observe a two-stage threshold voltage shift (ΔVTH) under positive bias temperature stress (PBTS) and hot carrier degradation (HCD). This two-stage ΔVTH is absent in devices fabricated on exfoliated hBN, suggesting that the donor state generation (negative ΔVTH) is induced by atomic-layer-deposition (ALD) processes in HfO2-based devices. Elastic Recoil Detection Analysis (ERDA) indicates that hydrogen, likely from the ALD precursor, is a probable cause, highlighting the need for ALD process refinement to improve 2D FET stability for CMOS compatibility.

Controllable synthesis of nonlayered high-κ Mn3O4 single-crystal thin films for 2D electronics

Two-dimensional (2D) materials have been identified as promising candidates for future electronic devices. However, high dielectric constant (κ) materials, which can be integrated with 2D semiconductors, are still rare. Here, we report a hydrate-assisted thinning chemical vapor deposition (CVD) technique to grow manganese oxide (Mn3O4) single crystal nanosheets, enabled by a strategy to minimize the substrate lattice mismatch and control the growth kinetics. The material demonstrated a dielectric constant up to 135, an equivalent oxide thickness (EOT) as low as 0.8 nm, and a breakdown field strength (Ebd) exceeding 10 MV/cm. MoS2 field-effect transistors (FETs) integrated with Mn3O4 thin films through mechanical stacking method operate under low voltages (<1 V), achieving a near 108 Ion/Ioff ratio and a subthreshold swing (SS) as low as 84 mV/dec. The MoS2 FET exhibit nearly zero hysteresis (<2 mV/MV cm⁻¹) and a low drain-induced barrier lowering (~20 mV/V). This work further expands the family of 2D high-κ dielectric materials and provides a feasible exploration for the epitaxial growth of single-crystal thin films of non-layered materials.

Enhanced Carrier Dynamics and Excitation of Optically Stimulated Artificial Synapse Using van der Waals Passivation Layers.

Advances in the semiconductor industry have been limited owing to the constraints imposed by silicon-based CMOS technology; hence, innovative device design approaches are necessary. This study focuses on "more than Moore" approaches, specifically in neuromorphic computing. Although MoS2 devices have attracted attention as neuromorphic computing candidates, their performances have been limited due to environment-induced perturbations to carrier dynamics and the formation of defect states. This study explores the integration of hydrocarbon (HC) layers onto active MoS2 channels to enhance neuromorphic computing characteristics. HC layers were employed in the proposed MoS2 field-effect transistor to facilitate stable optoelectrical control over the MoS2 channel under high-power stimulation. The improved electrical performance, stability, and synaptic behaviors of the HC-capped MoS2 devices compared to uncapped counterparts were experimentally demonstrated. The combination of optical and electrical tuning allowed for in-sensor computing applications that mimic human sensory behaviors. The impact of HC passivation on device performance was evaluated, and its potential for applications in neuromorphic computing with high stability was demonstrated across wide-ranging environmental conditions. The unique capabilities of HC-capped MoS2 devices were demonstrated by examining the spike duration-dependent plasticity and spiking timing-dependent plasticity. Thus, the proposed approach offers a promising avenue for advancing neuromorphic computing technologies.

Post-Treatment of Monolayer MoS2 Field-Effect Transistors with H2O Vapor: Alleviation of Remote Channel Doping.

Atomic layer deposition (ALD) of high-k dielectric films on MoS2 channels can lead to inadvertent remote electron doping of channels owing to nonequilibrium ALD conditions, such as the low temperatures and short purge times required for pinhole-free coating, as well as the weak physical adsorption of ALD precursors on MoS2. In this study, we propose the application of a simple and effective H2O vapor post-treatment (H2O PT) at 100 °C immediately after complete integration of bottom- and top-gate monolayer MoS2 field-effect transistors (FETs), to address the inadvertent channel doping effect. When H2O PT was applied to bottom-gate monolayer MoS2 FETs with an ALD-Al2O3 passivation layer, the mitigation of channel doping was confirmed through electrical and optical measurements. Chemical analyses indicated that H2O PT alleviated the remote doping effect by reducing the number of C/H impurities and possible oxygen defects near the ALD-Al2O3/MoS2 interface. These impurities and defects were associated with the incomplete reaction of the ALD precursor due to the nonequilibrium ALD used for the complete coating of Al2O3 on MoS2. Applying H2O PT to top-gate monolayer MoS2 FET arrays significantly narrowed the distributions of the threshold voltage and subthreshold swing. Moreover, it reduced the gate dielectric leakage current induced by the dielectric damage incurred during physical vapor deposition of the gate electrode. Finally, the time-dependent stability of top-gate monolayer MoS2 FETs subjected to H2O PT was confirmed for at least two months in air.

Emission of THz waves in MoS2 field-effect transistors

Terahertz (THz) waves are excited by the instability of plasma waves propagating along the field-effect transistor (FET) channel. This study investigates the instability and emission of THz plasmas waves in the MoS2 FET channel. A self-consistent quantum hydrodynamic model is built, and the dispersion relation depicting the THz plasma wave propagation in the MoS2 FETs is obtained. Furthermore, the evolution of linear THz plasma waves under weak magnetic fields and quantum effects under asymmetric boundary conditions is numerically analyzed. The results show that the external magnetic field, wave vector in y direction, quantum effects, thickness of the dielectric layer between the gate and channel, temperature, electron viscosity and collision damping influence the instability increment and radiation frequency of THz waves in MoS2 FETs. Moreover, the radiation power of THz waves in MoS2 FETs is stronger than that in graphene FETs. This study provides a new method for developing THz radiation sources.

High-Performance Edge-Contact Monolayer Molybdenum Disulfide Transistors.

Edge contact is essential for achieving the ultimate device pitch scaling of stacked nanosheet transistors with monolayer 2-dimensional (2D) channels. However, due to large edge-contact resistance between 2D channels and contact metal, there is currently a lack of high-performance edge-contact device technology for 2D material channels. Here, we report high-performance edge-contact monolayer molybdenum disulfide (MoS2) field-effect transistors (FETs) utilizing well-controlled plasma etching techniques. Plasma etching with pure argon improves the edge dangling bonds and thus improves the edge-contact quality. Edge-contact monolayer MoS2 FET shows good ohmic contact even at cryogenic temperatures (20 K), achieving a record-low contact resistance (R c) of 1.25 kΩ·μm among all edge-contact MoS2 devices. The record-high on-state current of 436 μA/μm and transconductance of 123 μS/μm at V ds = 1 V are achieved on an edge-contact monolayer MoS2 FET with L ch = 120 nm. This work highlights the great potential of edge contacts for high-performance monolayer transition metal dichalcogenide (TMD) material electronics.

Percolative phase transition in few-layered MoSe2 field-effect transistors using Co and Cr contacts.

The metal-to-insulator phase transition (MIT) in two-dimensional (2D) materials under the influence of a gating electric field has revealed interesting electronic behavior and the need for a deeper fundamental understanding of electron transport processes, while attracting much interest in the development of next-generation electronic and optoelectronic devices. Although the mechanism of the MIT in 2D semiconductors is a topic under debate in condensed matter physics, our work demonstrates the tunable percolative phase transition in few-layered MoSe2 field-effect transistors (FETs) using different metallic contact materials. Here, we attempted to understand the MIT through temperature-dependent electronic transport measurements by tuning the carrier density in a MoSe2 channel under the influence of an applied gate voltage. In particular, we have examined this phenomenon using the conventional chromium (Cr) and ferromagnetic cobalt (Co) as two metal contacts. For both Cr and Co, our devices demonstrated n-type behavior with a room-temperature field-effect mobility of 16 cm2 V-1 s-1 for the device with Cr-contacts and 92 cm2 V-1 s-1 for the device with Co-contacts, respectively. With low temperature measurements at 50 K, the mobilities increased significantly to 65 cm2 V-1 s-1 for the device with Cr and 394 cm2 V-1 s-1 for the device with Co-contacts. By fitting our experimental data to the percolative phase transition theory, the temperature-dependent conductivity data show a transition from an insulating-to-metallic behavior at a bias of ∼28 V for Cr-contacts and ∼20 V for Co-contacts. This cross-over of the conductivity can be attributed to an increase in carrier density as a function of the gate bias in temperature-dependent transfer characteristics. By extracting the critical exponents, we find that the transport behavior in the device with Co-contacts aligns closely with the 2D percolation theory. In contrast, the devices with Cr-contacts deviate significantly from the 2D limit at low temperatures.

Performance Assessment of Ultrascaled Vacuum Gate Dielectric MoS2 Field-Effect Transistors: Avoiding Oxide Instabilities in Radiation Environments.

Gate dielectrics are essential components in nanoscale field-effect transistors (FETs), but they often face significant instabilities when exposed to harsh environments, such as radioactive conditions, leading to unreliable device performance. In this paper, we evaluate the performance of ultrascaled transition metal dichalcogenide (TMD) FETs equipped with vacuum gate dielectric (VGD) as a means to circumvent oxide-related instabilities. The nanodevice is computationally assessed using a quantum simulation approach based on the self-consistent solutions of the Poisson equation and the quantum transport equation under the ballistic transport regime. The performance evaluation includes analysis of the transfer characteristics, subthreshold swing, on-state and off-state currents, current ratio, and scaling limits. Simulation results demonstrate that the investigated VGD TMD FET, featuring a gate-all-around (GAA) configuration, a TMD-based channel, and a thin vacuum gate dielectric, collectively compensates for the low dielectric constant of the VGD, enabling exceptional electrostatic control. This combination ensures superior switching performance in the ultrascaled regime, achieving a high current ratio and steep subthreshold characteristics. These findings position the GAA-VGD TMD FET as a promising candidate for advanced radiation-hardened nanoelectronics.

Highly Efficient Electrode of Dirac Semimetal PtTe2 for MoS2-Based Field Effect Transistors.

Two-dimensional van der Waals (vdW) layered materials not only are an intriguing fundamental scientific research platform but also provide various applications to multifunctional quantum devices in the field-effect transistors (FET) thanks to their excellent physical properties. However, a metal-semiconductor (MS) interface with a large Schottky barrier causes serious problems for unleashing their intrinsic potentials toward the advancements in high-performance devices. Here, we show that exfoliated vdW Dirac semimetallic PtTe2 can be an excellent electrode for electrons in MoS2 FETs. High-performance FET characteristics reaching the FET mobility of 85 cm2 V-1 s-1 are observed with a negligibly small Schottky barrier height and large on-off current ratio over 108, which is among the highest performances in reported electrodes for MoS2. Discussions are had on the reason that exfoliated PtTe2 with Dirac states shows highly efficient electrode performances based on the comparisons with various other metal electrodes. The orbital hybrid effect between Dirac-semimetal PtTe2 and MoS2 was evidenced by the relative shift of Raman spectra peaks in the heterojunction, resulting in good ohmic contacts. These findings provide an important route to find high-performance electrodes for layered vdW materials and their related quantum devices.

Stabilizing Electron Transport of 2D Materials.

2D materials are promising candidates for beyond-Si electronic devices. However, their stability is a key bottleneck in their industrial applications. The instability of 2D materials is mainly attributed to their intrinsic susceptibility to O2 and H2O-particularly to reactive oxygen species (ROS), which have strong oxidizing properties. Inspired by the antioxidant effect of vitamin C (VC) in organisms, a strategy based on the use of VC to stabilize electron transport in 2D materials is developed, which significantly improves the performance and stability of these materials and devices. The mobility is increased by more than an order of magnitude, and excellent performance of the device is maintained in air for >327 days, which is the best reported stability for MoS2 field-effect transistors to date. VC scavenges existing ROS via oxidation reactions and inhibits the generation of ROS by shielding excitons from oxygen quenching, which provides 2D materials lasting protection from electron trapping and oxidative damage, stabilizing electron transport. This approach, which is based on the simple utilization of readily available VC, has considerable potential for large-scale applications in the 2D material electronics industry.

Reconfiguration of Intrinsic Depletion-Mode Characteristics of MoS2 Field-Effect Transistors to High-Performance Enhancement-Mode Operation Using an Argon Plasma-Induced p-Type Doping Technique.

The intrinsic n-type behavior and unavailability of the appropriate p-type doping method for MoS2 allows only n-type conduction with depletion mode (D-mode) characteristics and forbids the implementation of p-type field-effect transistors (FETs). The D-mode characteristic results in a high off-current (IOFF) at zero gate bias, which limits the usage of MoS2 FETs for industry-scale (n-channel metal-oxide semiconductor) NMOS/(complementary metal-oxide semiconductor) CMOS-logic-based applications due to significant power dissipation. Both these issues, i.e., i) missing technique for p-type doping and ii) D-mode operation are addressed here through the application of argon (Ar) plasma and subsequent O2 bath. Here, Ar plasma results in the physical removal of sulfur (S) atoms from the MoS2 surface, introducing sulfur vacancies, and the O2 bath results in the chemical bonding of O2 molecules with molybdenum (Mo) atoms at the introduced S vacancy sites. This leads to the formation of shallow acceptor states near the valance band (VB) of MoS2, resulting in p-type doping and enhancement mode (E-mode) characteristics of MoS2 FETs. Moreover, using Ar plasma results in the reduction of contact resistance (RC) of E-mode MoS2 FETs and hence facilitates achieving high-performance top-gated E-mode MoS2 FETs with IOFF (at zero gate bias) in tens of picoamperes and ION/IOFF in seven orders.

One‐Step Van der Waals Integration of Multi‐Threshold 2D Functional Circuits

AbstractTransition‐metal dichalcogenides (TMDs), as atomically thin semiconductors, have shown immense promise for next‐generation electronics due to their high mobility and potential for creating van der Waals heterostructures. However, precise control of the threshold voltage (Vth) in field‐effect transistors (FETs) remains a significant challenge, impeding the development of 2D material circuits with tailored electronic functions. Herein, a graphene‐assisted one‐step van der Waals integration technique is presented for fabricating multi‐threshold 2D functional circuits. Graphene's unique property of having a surface without dangling bonds is utilized as an intermediary layer, allowing for the transfer of electrodes with various work functions and high‐κ dielectric layers onto 2D channel materials in a single step. This technique has successfully produced Al‐gated MoS2 FETs with a Vth of −0.2 V and Au‐gated MoS2 FETs with a Vth of 1.9 V. This precision enables the development of functional devices such as rail‐to‐rail inverters with large noise margins. Furthermore, more complex multi‐threshold 2D circuits are achieved, including basic logic gates (NOT, NAND, NOR), six‐transistor static random‐access memory (6T‐SRAM), and ring oscillators (RO). This work showcases a scalable and effective strategy for threshold voltage engineering in 2D material‐based circuits, paving the way for sophisticated electronic and optoelectronic applications.

(Invited) Bilayer Molybdenum Disulfide Strain Controlled Field Effect Transistor

Silicon nitride stress capping layers are pivotal in the semiconductor industry for their role in enhancing electron mobility and driving currents in n-channel silicon MOSFETs. This research delves into the effects of silicon nitride-induced strain on the electronic properties of bilayer Molybdenum Disulfide (MoS2), a promising two-dimensional semiconductor. We first examine the modifications in the photoluminescence and Raman spectra of bare bilayer MoS2 under strain. By depositing a silicon nitride stress liner on a bilayer MoS2 field effect transistor (FET), which impacts both the gate and the source-drain regions, we replicate the stress conditions akin to those experienced in silicon MOSFETs. This methodical approach enables us to comprehensively study the evolution from back-gated to top-gated, and ultimately, to strain-gated FET configurations. Our findings indicate that tensile strain crucially modifies the electronic structure of MoS2, primarily reducing the indirect band gap by lowering the conduction band at the K point. Performance evaluations of the FETs demonstrate a marked increase in electron mobility and on-current in the strain-gated configurations compared to top-gated setups, highlighting the positive effects of tensile strain on carrier transport within MoS2. This study not only furthers our understanding of strain effects on MoS2 but also underscores the potential of strain engineering in optimizing semiconductor devices.

Improved Thermal Dissipation in a MoS2 Field-Effect Transistor by Hybrid High-k Dielectric Layers.

Transition metal dichalcogenides like MoS2 have been considered as crucial channel materials beyond silicon to continuously advance transistor scaling down owing to their two-dimensional structure and exceptional electrical properties. However, the undesirable interface morphology and vibrational phonon frequency mismatch between MoS2 and the dielectric layer induce low thermal boundary conductance, resulting in overheating issues and impeding electrical performance improvement in the MoS2 field-effect transistors. Here, we employed hybrid high-k dielectric layers of Al2O3/HfO2 to simultaneously reduce the interfacial thermal resistance and improve device electrical performance. The enhanced contact, greater vibrational phonon overlapping region, and stronger interfacial bonding force between the top Al2O3 layer and MoS2 promote the heat removal efficiency across the interface to the substrate. Under the same input power density, the temperature profile of the MoS2 transistor on the Al2O3/HfO2 has been largely reduced compared to that of the device on HfO2, with a maximum reduction of 49.5 °C. In addition, the field-effect mobility and current of MoS2 devices on the Al2O3/HfO2 high-k dielectric layers have been significantly improved, attributed to the depressed electron scattering and trap states at the interface. The design of the hybrid high-k dielectric layers provides an efficient solution to simultaneously improve the thermal and electrical performance of the two-dimensional devices.

Device simulation study of multilayer MoS2 Schottky barrier field-effect transistors

Molybdenum disulfide (MoS2) is a representative two-dimensional layered transition-metal dichalcogenide semiconductor. Layer-number-dependent electronic properties are attractive in the development of nanomaterial-based electronics for a wide range of applications including sensors, switches, and amplifiers. MoS2field-effect transistors (FETs) have been studied as promising future nanoelectronic devices with desirable features of atomic-level thickness and high electrical properties. When a naturallyn-doped MoS2is contacted with metals, a strong Fermi-level pinning effect adjusts a Schottky barrier and influences its electronic characteristics significantly. In this study, we investigate multilayer MoS2Schottky barrier FETs (SBFETs), emphasizing the metal-contact impact on device performance via computational device modeling. We find thatp-type MoS2SBFETs may be built with appropriate metals and gate voltage control. Furthermore, we propose ambipolar multilayer MoS2SBFETs with asymmetric metal electrodes, which exhibit gate-voltage dependent ambipolar transport behavior through optimizing metal contacts in MoS2device. Introducing a dual-split gate geometry, the MoS2SBFETs can further operate in four distinct configurations:p - p,n - n,p - n, andn - p. Electrical characteristics are calculated, and improved performance of a high rectification ratio can be feasible as an attractive feature for efficient electrical and photonic devices.

Low-Symmetry Van der Waals Dielectric GaInS3 Triggered 2D MoS2 Giant Anisotropy via Symmetry Engineering.

Low-symmetry structures in van der Waals materials have facilitated the advancement of anisotropic electronic and optoelectronic devices. However, the intrinsic low symmetry structure exhibits a small adjustable anisotropy ratio (1-10), which hinders its further assembly and processing into high-performance devices. Here, a novel 2D anisotropic dielectric, GaInS3 (GIS), which induces isotropic MoS2 to exhibit significant anisotropic optical and electrical responses is demonstrated. With the excellent gate modulation ability of 2D GIS (dielectric constant k ∼12), MoS2 field effect transistor (FET) shows an adjustable conductance ratio from isotropic to anisotropic under dual-gate modulation, up to 106. Theoretical calculations indicate that anisotropy originates from lattice mismatch-induced charge density deformation at the interface. Moreover, the MoS2/GIS photodetector demonstrates high responsivity (≈4750 A W-1) and a large dichroic ratio (≈167). The anisotropic van der Waals dielectric GIS paves the way for the development of 2D transition metal dichalcogenides (TMDCs) in the fields of anisotropic photonics, electronics, and optoelectronics.

Back-side stress to ease p-MOSFET degradation on e-MRAM chips

Abstract The magnetoresistive random access memory process makes a great contribution to threshold voltage deterioration of metal–oxide–silicon field-effect transistors, especially on p-type devices. Herein, a method was proposed to reduce the threshold voltage degradation by utilizing back-side stress. Through the deposition of tensile material on the back side, positive charges generated by silicon–hydrogen bond breakage were inhibited, resulting in a potential reduction in threshold voltage shift by up to 20%. In addition, it was found that the method could only relieve silicon–hydrogen bond breakage physically, thus failing to provide a complete cure. However, it holds significant potential for applications where additional thermal budget is undesired. Furthermore, it was also concluded that the method used in this work is irreversible, with its effect sustained to the chip package phase, and it ensures competitive reliability of the resulting magnetic tunnel junction devices.

Characterization of bulk trap density using fully I–V-based optoelectronic differential ideality factor in multi-layer MoS2 FETs

Abstract Molybdenum disulfide (MoS2) has excellent optoelectronic properties, chemical stability, and a two-dimensional (2D) structure, making MoS2 a very versatile field-effect device material. Herein, we characterize MoS2 and utilize a photo-responsive I–V technique for extracting the energy distribution of the bulk traps in multi-layer MoS2 field effect transistors (FET). This method uses the differential ideality factor in both dark and light conditions. The differential ideality factor enables the efficient quantitative extraction of the device trap density by considering the nonlinear characteristics of the subthreshold region (V ON &lt; V GS &lt; V T). To accurately differentiate between the sub-bandgap traps and the interface traps near the conduction band, near-infrared light (λ= 1530 nm) optical illumination was used for the light state characterization. The bulk trap densities under dark state and light state conditions were derived for multi-layer (7-layer and 9-layer) MoS2 FET channels, and the influence of light illumination and overall multi-layer thickness on the bulk trap density was confirmed. The accurate extraction of the trap density enables the design of MoS2 FETs with long-term stability and high optoelectronic performance.

Enhancing the Performance of MoS2 Field-Effect Transistors Using Self-Assembled Monolayers: A Promising Strategy to Alleviate Dielectric Layer Scattering and Improve Device Performance.

Field-effect transistors (FETs) based on two-dimensional molybdenum disulfide (2D-MoS2) have great potential in electronic and optoelectronic applications, but the performances of these devices still face challenges such as scattering at the contact interface, which results in reduced mobility. In this work, we fabricated high-performance MoS2-FETs by inserting self-assembling monolayers (SAMs) between MoS2 and a SiO2 dielectric layer. The interface properties of MoS2/SiO2 were studied after the inductions of three different SAM structures including (perfluorophenyl)methyl phosphonic acid (PFPA), (4-aminobutyl) phosphonic acid (ABPA), and octadecylphosphonic acid (ODPA). The SiO2/ABPA/MoS2-FET exhibited significantly improved performances with the highest mobility of 528.7 cm2 V-1 s-1, which is 7.5 times that of SiO2/MoS2-FET, and an on/off ratio of ~106. Additionally, we investigated the effects of SAM molecular dipole vectors on device performances using density functional theory (DFT). Moreover, the first-principle calculations showed that ABPA SAMs reduced the frequencies of acoustic and optical phonons in the SiO2 dielectric layer, thereby suppressing the phonon scattering to the MoS2 channel and further improving the device's performance. This work provided a strategy for high-performance MoS2-FET fabrication by improving interface properties.

Tunable Doping Strategy for Few-Layer MoS2 Field-Effect Transistors via NH3 Plasma Treatment.

Molybdenum disulfide (MoS2) is a promising candidate for next-generation transistor channel materials, boasting outstanding electrical properties and ultrathin structure. Conventional ion implantation processes are unsuitable for atomically thin two-dimensional (2D) materials, necessitating nondestructive doping methods. We proposed a novel approach: tunable n-type doping through sulfur vacancies (VS) and p-type doping by nitrogen substitution in MoS2, controlled by the duration of NH3 plasma treatment. Our results reveal that NH3 plasma exposure of 20 s increases the 2D sheet carrier density (n2D) in MoS2 field-effect transistors (FETs) by +4.92 × 1011 cm-2 at a gate bias of 0 V, attributable to sulfur vacancy generation. Conversely, treatment of 40 s reduces n2D by -3.71 × 1011 cm-2 due to increased nitrogen doping. X-ray photoelectron spectroscopy, Raman spectroscopy, and photoluminescence analyses corroborate these electrical characterization results, indicating successful n- and p-type doping. Temperature-dependent measurements show that the Schottky barrier height at the metal-semiconductor contact decreases by -31 meV under n-type conditions and increases by +37 meV for p-type doping. This study highlights NH3 plasma treatment as a viable doping method for 2D materials in electronic and optoelectronic device engineering.