Gate-controlled reversible rectifying behaviour in tunnel contacted atomically-thin MoS2 transistor

Probing Electronic Band Structure of Monolayer MoS2 in Gate-Controlled Resonant Tunneling Diodes

Experimental determination of band structures of monolayer transition metal dichalcogenides (TMDCs) is crucially important in the design and tailoring of the properties of TMDCs. Resonant tunneling spectroscopy (RTS) is an effective technique to probe the band structures of low-dimensional systems by measuring the density of states (DOS) and energy dispersions. Here, we report the investigation of the band structure of monolayer MoS2 (ML-MoS2) in a gate-controlled resonant tunneling diode. Three distinct resonant tunneling kinks are observed in the characteristic current-voltage curves at 0.47, 0.70, and 0.81 V, respectively, which correspond to the conduction band local minimum of ML-MoS2 at K, Q1, and Q2 points. When applying a large positive gate voltage to enhance ML-MoS2 conductivity, the three resonant kinks shift to lower bias at 0.10, 0.32, and 0.39 V, respectively, which is in excellent agreement with the theoretical calculations. Our work offers an effective and more precise way to explore the electronic band structures of TMDCs using RTS.

Giant tunnel electroresistance through a Van der Waals junction by external ferroelectric polarization

The burgeoning interest in two-dimensional semiconductors stems from their potential as ultrathin platforms for next-generation transistors. Nonetheless, there persist formidable challenges in fully obtaining high-performance complementary logic components and the underlying mechanisms for the polarity modulation of transistors are not yet fully understood. Here, we exploit both ferroelectric domain-based nonvolatile modulation of Fermi level in transitional metal dichalcogenides (MoS2) and quantum tunneling through nanoscale hexagonal boron nitride (h-BN). Our prototype devices, termed as vertical tunneling ferroelectric field-effect transistor, utilizes a Van der Waals MoS2/h-BN/metal tunnel junction as the channel. The Fermi level of MoS2 is bipolarly tuned by ferroelectric domains and sensitively detected by the direct quantum tunneling strength across the junction, demonstrating an impressive electroresistance ratio of up to 109 in the vertical tunneling ferroelectric field-effect transistor. It consumes only 0.16 fJ of energy to open a ratio window exceeding 104. This work not only validates the effectiveness of tailored tunnel barriers in manipulating electronic flow but also highlights a new avenue for the design flexibility and functional versatility of advanced ferroelectric memory technology.

Reliable wafer-scale integration of two-dimensional materials and metal electrodes with van der Waals contacts

Since the first report on single-layer MoS2 based transistor, rapid progress has been achieved in two-dimensional (2D) material-based atomically thin electronics, providing an alternative approach to solve the bottleneck in silicon device miniaturization. In this scenario, reliable contact between the metal electrodes and the subnanometer-thick 2D materials becomes crucial in determining the device performance. Here, utilizing the quasi-van der Waals (vdW) epitaxy of metals on fluorophlogopite mica, we demonstrate an all-stacking method for the fabrication of 2D devices with high-quality vdW contacts by mechanically transferring pre-deposited metal electrodes. This technique is applicable for complex device integration with sizes up to the wafer scale and is also capable of tuning the electric characteristics of the interfacial junctions by transferring selective metals. Our results provide an efficient, scalable, and low-cost technique for 2D electronics, allowing high-density device integration as well as a handy tool for fundamental research in vdW materials.

Adjustment methods of Schottky barrier height in one- and two-dimensional semiconductor devices

The Schottky contact which is a crucial interface between semiconductors and metals is becoming increasingly significant in nano-semiconductor devices. A Schottky barrier, also known as the energy barrier, controls the depletion width and carrier transport across the metal-semiconductor interface. Controlling or adjusting Schottky barrier height (SBH) has always been a vital issue in the successful operation of any semiconductor device. This review provides a comprehensive overview of the static and dynamic adjustment methods of SBH, with a particular focus on the recent advancements in nano-semiconductor devices. These methods encompass the work function of the metals, interface gap states, surface modification, image-lowering effect, external electric field, light illumination, and piezotronic effect. We also discuss strategies to overcome the Fermi-level pinning effect caused by interface gap states, including van der Waals contact and 1D edge metal contact. Finally, this review concludes with future perspectives in this field.

Low Ohmic contact resistance and high on/off ratio in transition metal dichalcogenides field-effect transistors via residue-free transfer

Beyond-silicon technology demands ultrahigh performance field-effect transistors. Transition metal dichalcogenides provide an ideal material platform, but the device performances such as the contact resistance, on/off ratio and mobility are often limited by the presence of interfacial residues caused by transfer procedures. Here, we show an ideal residue-free transfer approach using polypropylene carbonate with a negligible residue coverage of ~0.08% for monolayer MoS2 at the centimetre scale. By incorporating a bismuth semimetal contact with an atomically clean monolayer MoS2 field-effect transistor on hexagonal boron nitride substrate, we obtain an ultralow Ohmic contact resistance of ~78 Ω µm, approaching the quantum limit, and a record-high on/off ratio of ~1011 at 15 K. Such an ultra-clean fabrication approach could be the ideal platform for high-performance electrical devices using large-area semiconducting transition metal dichalcogenides.

Contact-engineered reconfigurable two-dimensional Schottky junction field-effect transistor with low leakage currents

Two-dimensional (2D) materials have been considered promising candidates for future low power-dissipation and reconfigurable integrated circuit applications. However, 2D transistors with intrinsic ambipolar transport polarity are usually affected by large off-state leakage currents and small on/off ratios. Here, we report the realization of a reconfigurable Schottky junction field-effect transistor (SJFET) in an asymmetric van der Waals contact geometry, showing a balanced and switchable n- and p-unipolarity with the I_ds on/off ratio kept >10⁶. Meanwhile, the static leakage power consumption was suppressed to 10^-5 nW. The SJFET worked as a reversible Schottky rectifier with an ideality factor of ~1.0 and a tuned rectifying ratio from 3 × 10⁶ to 2.5 × 10^-6. This empowered the SJFET with a reconfigurable photovoltaic performance in which the sign of the open-circuit voltage and photo-responsivity were substantially switched. This polarity-reversible SJFET paves an alternative way to develop reconfigurable 2D devices for low-power-consumption photovoltaic logic circuits.

The Trend of 2D Transistors toward Integrated Circuits: Scaling Down and New Mechanisms

2D transition metal chalcogenide (TMDC) materials, such as MoS₂ , have recently attracted considerable research interest in the context of their use in ultrascaled devices owing to their excellent electronic properties. Microprocessors and neural network circuits based on MoS₂ have been developed at a large scale but still do not have an advantage over silicon in terms of their integrated density. In this study, the current structures, contact engineering, and doping methods for 2D TMDC materials for the scaling-down process and performance optimization are reviewed. Devices are introduced according to a new mechanism to provide the comprehensive prospects for the use of MoS₂ beyond the traditional complementary-metal-oxide semiconductor in order to summarize obstacles to the goal of developing high-density and low-power integrated circuits (ICs). Finally, prospects for the use of MoS₂ in large-scale ICs from the perspectives of the material, system performance, and application to nonlogic functionalities such as sensor circuits and analogous circuits, are briefly analyzed. The latter issue is along the direction of "more than Moore" research.

Challenges for Nanoscale CMOS Logic Based on Two-Dimensional Materials

For ultra-scaled technology nodes at channel lengths below 12 nm, two-dimensional (2D) materials are a potential replacement for silicon since even atomically thin 2D semiconductors can maintain sizable mobilities and provide enhanced gate control in a stacked channel nanosheet transistor geometry. While theoretical projections and available experimental prototypes indicate great potential for 2D field effect transistors (FETs), several major challenges must be solved to realize CMOS logic circuits based on 2D materials at the wafer scale. This review discusses the most critical issues and benchmarks against the targets outlined for the 0.7 nm node in the International Roadmap for Devices and Systems scheduled for 2034. These issues are grouped into four areas; device scaling, the formation of low-resistive contacts to 2D semiconductors, gate stack design, and wafer-scale process integration. Here, we summarize recent developments in these areas and identify the most important future research questions which will have to be solved to allow for industrial adaptation of the 2D technology.

Approaching the External Quantum Efficiency Limit in 2D Photovoltaic Devices

2D transition metal dichalcogenides (TMDs) are promising candidates for realizing ultrathin and high-performance photovoltaic devices. However, the external quantum efficiency (EQE) and power conversion efficiency (PCE) of most 2D photovoltaic devices face great challenges in exceeding 50% and 3%, respectively, due to the low efficiency of photocarrier separation and collection. Here, this study demonstrates photovoltaic devices with defect-free interface and recombination-free channel based on 2D WS₂ , showing high EQE of 92% approaching the theoretical limit and high PCE of 5.0%. The high performances are attributed to the van der Waals metal contact without interface defects and Fermi-level pinning, and the fully depleted channel without photocarrier recombination, leading to intrinsic photocarrier separation and collection with high efficiency. Furthermore, this study demonstrates that the strategy can be extended to other TMDs such as MoSe₂ and WSe₂ with EQE of 92% and 94%, respectively. This work proposes a universal strategy for building high-performance 2D photovoltaic devices. The nearly ideal EQE provides great potential for PCE approaching the Shockley-Queisser limit.

Imaging tunable quantum Hall broken-symmetry orders in graphene

ABSTARCT

When electrons populate a flat band their kinetic energy becomes negligible, forcing them to organize in exotic many-body states to minimize their Coulomb energy^1-5. The zeroth Landau level of graphene under a magnetic field is a particularly interesting strongly interacting flat band because interelectron interactions are predicted to induce a rich variety of broken-symmetry states with distinct topological and lattice-scale orders^6-11. Evidence for these states stems mostly from indirect transport experiments that suggest that broken-symmetry states are tunable by boosting the Zeeman energy¹² or by dielectric screening of the Coulomb interaction¹³. However, confirming the existence of these ground states requires a direct visualization of their lattice-scale orders¹⁴. Here we image three distinct broken-symmetry phases in graphene using scanning tunnelling spectroscopy. We explore the phase diagram by tuning the screening of the Coulomb interaction by a low- or high-dielectric-constant environment, and with a magnetic field. In the unscreened case, we find a Kekulé bond order, consistent with observations of an insulating state undergoing a magnetic-field driven Kosterlitz-Thouless transition^15,16. Under dielectric screening, a sublattice-unpolarized ground state¹³ emerges at low magnetic fields, and transits to a charge-density-wave order with partial sublattice polarization at higher magnetic fields. The Kekulé and charge-density-wave orders furthermore coexist with additional, secondary lattice-scale orders that enrich the phase diagram beyond current theory predictions^6-10. This screening-induced tunability of broken-symmetry orders may prove valuable to uncover correlated phases of matter in other quantum materials.

Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects

Motivated by the high expectation for efficient electrostatic modulation of charge transport at very low voltages, atomically thin 2D materials with a range of bandgaps are investigated extensively for use in future semiconductor devices. However, researchers face formidable challenges in 2D device processing mainly originated from the out-of-plane van der Waals (vdW) structure of ultrathin 2D materials. As major challenges, untunable Schottky barrier height and the corresponding strong Fermi level pinning (FLP) at metal interfaces are observed unexpectedly with 2D vdW materials, giving rise to unmodulated semiconductor polarity, high contact resistance, and lowered device mobility. Here, FLP observed from recently developed 2D semiconductor devices is addressed differently from those observed from conventional semiconductor devices. It is understood that the observed FLP is attributed to inefficient doping into 2D materials, vdW gap present at the metal interface, and hybridized compounds formed under contacting metals. To provide readers with practical guidelines for the design of 2D devices, the impact of FLP occurring in 2D semiconductor devices is further reviewed by exploring various origins responsible for the FLP, effects of FLP on 2D device performances, and methods for improving metallic contact to 2D materials.

Rectifying optoelectronic memory based on WSe2/graphene heterostructures

van der Waals heterostructures composed of two-dimensional materials vertically stacked have been extensively studied to develop various multifunctional devices. Here, we report WSe₂/graphene heterostructure devices with a top floating gate that can serve as multifunctional devices. They exhibit gate-controlled rectification inversion, rectified nonvolatile memory effects, and multilevel optoelectronic memory effects. Depending on the polarity of the gate voltage pulses (V _Gp), electrons or holes can be trapped in the floating gate, resulting in rectified nonvolatile memory properties. Furthermore, upon repeated illumination with laser pulses, positive or negative staircase photoconductivity is observed depending on the history of V _Gp, which is ascribed to the tunneling of electrons or holes between the WSe₂ channel and the floating gate. These multifunctional devices can be used to emulate excitatory and inhibitory synapses that have different neurotransmitters. Various synaptic functions, such as potentiation/depression curves and spike-timing-dependent plasticity, have been also implemented using these devices. In particular, 128 optoelectronic memory states with nonlinearity less than 1 can be achieved by controlling applied laser pulses and V _Gp, suggesting that the WSe₂/graphene heterostructure devices with a top floating gate can be applied to optoelectronic synapse devices.

Reversible Half Wave Rectifier Based on 2D InSe/GeSe Heterostructure with Near-Broken Band Alignment

2D van der Waals heterostructures (vdWHs) offer tremendous opportunities in designing multifunctional electronic devices. Due to the ultrathin nature of 2D materials, the gate-induced change in charge density makes amplitude control possible, creating a new programmable unilateral rectifier. The study of 2D vdWHs-based reversible unilateral rectifier is lacking, although it can give rise to a new degree of freedom for modulating the output state. Here, a InSe/GeSe vdWH-FET is constructed as a gate-controllable half wave rectifier. The device exhibits stepless adjustment from forward to backward rectifying performance, leading to multiple operation states of output level. Near-broken band alignment in the InSe/GeSe vdWH-FET is a crucial feature for high-performance reversible rectifier, which is shown to have backward and forward rectification ratio of 1:38 and 963:1, respectively. Being further explored as a new bridge rectifier, the InSe/GeSe device has great potential in future gate-controllable alternating current/direct current convertor. These results indicate that 2D vdWHs with near-broken band alignment can offer a pathway to simplify the commutating circuit and regulating speed circuit.

Ambipolar 2D Semiconductors and Emerging Device Applications

With the rise of 2D materials, new physics and new processing techniques have emerged, triggering possibilities for the innovation of electronic and optoelectronic devices. Among them, ambipolar 2D semiconductors are of excellent gate-controlled capability and distinctive physical characteristic that the major charge carriers can be dynamically, reversibly and rapidly tuned between holes and electrons by electrostatic field. Based on such properties, novel devices, like ambipolar field-effect transistors, light-emitting transistors, electrostatic-field-charging PN diodes, are developed and show great advantages in logic and reconfigurable circuits, integrated optoelectronic circuits, and artificial neural network image sensors, enriching the functions of conventional devices and bringing breakthroughs to build new architectures. This review first focuses on the basic knowledge including fundamental principle of ambipolar semiconductors, basic material preparation techniques, and how to obtain the ambipolar behavior through electrical contact engineering. Then, the current ambipolar 2D semiconductors and their preparation approaches and main properties are summarized. Finally, the emerging new device structures are overviewed in detail, along with their novel electronic and optoelectronic applications. It is expected to shed light on the future development of ambipolar 2D semiconductors, exploring more new devices with novel functions and promoting the applications of 2D materials.

Asymmetric Schottky Contacts in van der Waals Metal-Semiconductor-Metal Structures Based on Two-Dimensional Janus Materials

Physical and electronic asymmetry plays a crucial role in rectifiers and other devices with a directionally variant current-voltage (I-V) ratio. Several strategies for practically creating asymmetry in nanoscale components have been demonstrated, but complex fabrication procedures, high cost, and incomplete mechanistic understanding have significantly limited large-scale applications of these components. In this work, we present density functional theory calculations which demonstrate asymmetric electronic properties in a metal-semiconductor-metal (MSM) interface composed of stacked van der Waals (vdW) heterostructures. Janus MoSSe has an intrinsic dipole due to its asymmetric structure and, consequently, can act as either an n-type or p-type diode depending on the face at the interior of the stacked structure (SeMoS-SMoS vs. SMoSe-SMoS). In each configuration, vdW forces dominate the interfacial interactions, and thus, Fermi level pinning is largely suppressed. Our transport calculations show that not only does the intrinsic dipole cause asymmetric I-V characteristics in the MSM structure but also that different transmission mechanisms are involved across the S-S (direct tunneling) and S-Se interface (thermionic excitation). This work illustrates a simple and practical method to introduce asymmetric Schottky barriers into an MSM structure and provides a conceptual framework which can be extended to other 2D Janus semiconductors.

Bismuth sulfide bridged hierarchical Bi2S3/BiOCl@ZnIn2S4 for efficient photocatalytic Cr(VI) reduction

Delicate construction based on 2D epitaxial heterostructure can be an effective route to adequately excavate and utilize its superiorities. Here, a core-shell Bi₂S₃/BiOCl@ZnIn₂S₄ hierarchical heterostructure is rationally designed and built by Bi₂S₃ epitaxial growth on two-dimensional template-like BiOCl and ZnIn₂S₄ nanosheets in-situ growth. The epitaxial growth of Bi₂S₃ on BiOCl endows the tight contact between them. More importantly, Bi₂S₃ as the interlayer could offer an extra intimate junction to ZnIn₂S₄ due to the chemical interaction of S^2- between Bi₂S₃ and ZnIn₂S₄. Such a Bi₂S₃/BiOCl@ZnIn₂S₄ composite was explored for visible-light-driven reduction of Cr(VI), and much satisfactory performance was achieved, which is about 3.3 and 24.1-fold increase compared to that of ZnIn₂S₄ and Bi₂S₃/BiOCl respectively. Efficient generation, separation and transfer of photo-generated charge carriers inherited from this ternary hierarchical composite made significant contributions to the highly elevated photocatalytic activity. This work may stimulate the construction of multiple hierarchical composites based on 2D epitaxial heterostructure material for efficient photocatalysis or other optoelectronics.

Introducing Electrode Contact by Controlled Micro-Alloying in Few-Layered GaTe Field Effect Transistors

Recently, gallium telluride (GaTe) has triggered much attention for its unique properties and offers excellent opportunities for nanoelectronics. Yet it is a challenge to bridge the semiconducting few-layered GaTe crystals with metallic electrodes for device applications. Here, we report a method on fabricating electrode contacts to few-layered GaTe field effect transistors (FETs) by controlled micro-alloying. The devices show linear I-V curves and on/off ratio of ∼10 4 on HfO 2 substrates. Kelvin probe force microscope (KPFM) and energy dispersion spectrum (EDS) are performed to characterize the electrode contacts, suggesting that the lowered Schottky barrier by the diffusion of Pd element into the GaTe conduction channel may play an important role. Our findings provide a strategy for the engineering of electrode contact for future device applications based on 2DLMs.

Defect-Engineered Atomically Thin MoS2 Homogeneous Electronics for Logic Inverters

Ultrathin molybdenum disulfide (MoS₂ ) presents ideal properties for building next-generation atomically thin circuitry. However, it is difficult to construct logic units of MoS₂ monolayer using traditional silicon-based doping schemes, such as atomic substitution and ion implantation, as they cause lattice disruption and doping instability. An accurate and feasible electronic structure modulation strategy from defect engineering is proposed to construct homogeneous electronics for MoS₂ monolayer logic inverters. By utilizing the energy-matched electron induction of the solution process, numerous pure and lattice-stable monosulfur vacancies (V_monos ) are introduced to modulate the electronic structure of monolayer MoS₂ via a shallow trapping effect. The resulting modulation effectively reduces the electronic concentration of MoS₂ and improves the work function by 100 meV. Under modulation of V_monos , an atomically thin homogenous monolayer MoS₂ logic inverter with a voltage gain of 4 is successfully constructed. A brand-new and practical design route of defect modulation for 2D-based circuit development is provided.

Multimode Signal Processor Unit Based on the Ambipolar WSe2-Cr Schottky Junction

A Schottky barrier is a double-edged sword in electronic and optoelectronic devices, especially devices based on two-dimensional materials. It may restrict the carrier transport in devices, but it can also realize multifunctional devices by architecture design. We designed a simple but novel device structure based on theWSe₂-Cr Schottky junction with an asymmetric Schottky contact area of the source and drain. A significant rectification ratio over 10⁵ and multiple rectifying states (e.g., full pass, forward pass, off, and backward pass) were achieved in the single Schottky junction tuned by gate voltage. Furthermore, switching characteristic, rectification characteristic, and amplitude of a sin wave can be effectively modulated by the electrical field or light illumination in a signal process circuit based on the WSe₂-Cr Schottky junction. The highly tunable Schottky junction working as a multimode signal processor unit has great potential in future optoelectronic-integrated chips.

Metallic contact induced van der Waals gap in a MoS2 FET

Electrical metal contacts formed with 2D materials strongly affect device performance. Here, we used scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS) to characterize the interfacial structure formed and physical damage induced between MoS2 and the most commonly used metals, Ti, Cr, Au, and Pd. We further correlated the electrical performance with physical defects observed at the 2D interfacial structure. The contact resistances were higher in the order of Ti, Au, Pd, and Cr contacts, but all 4-point probe mobilities measured with metals in contact with identical quadrilayer MoS2 were ∼65 cm2 V-1 s-1, confirming the reliability of the devices. According to the STEM and EDS analyses, the Ti contact gave rise to a van der Waals gap between the clean quadrilayer MoS2 and the Ti contact. By contrast, Cr migrated into MoS2 while Mo and S counter-migrated into the SiO2 substrate. Au and Pd formed glassy layers that resulted in the migration of Mo and S into the Au and Pd electrodes. These interfacial structures between MoS2 and contact metals strongly correlated with the electrical performance of 2D MoS2 FETs, providing practical guidelines to form van der Waals contacts.

Tuning electronic structure of monolayer InP3 in contact with graphene or Ni: effect of a buffer layer and intrinsic In and P-vacancy

Ohmic contact in m-InP₃ and G or Ni interface is achieved by introducing intrinsic defects and inserting a buffer layer.

Electronic structure of two-dimensional In and Bi metal on BN nanosheets

The electronic structures of two-dimensional (2D) indium (In) and bismuth (Bi) metal on BN nanosheets are systematically studied using hybrid density functional theory (DFT). We found that 2D In and Bi metal effectively modulate the band gap of a BN nanosheet. We also found that the indirect band gap of the 2D In and Bi metal electronic structures are 0.70 and 0.09 eV, respectively. This modulation originates from the charge transfer between the 2D metal and BN nanosheet interfaces, as well as from the electron redistribution of the In/BN and Bi/BN heterojunctions of the s and p orbitals. Our results provide an insight into 2D In/BN and Bi/BN heterojunctions, which should be useful in the design of 2D In and Bi metal–semiconductor-based devices.

The electronic structures of two-dimensional (2D) indium (In) and bismuth (Bi) metal on BN nanosheets are systematically studied using hybrid density functional theory (DFT).

Uncovering the Conduction Behavior of van der Waals Ambipolar Semiconductors

A long-standing puzzle about van der Waals semiconductors (vdWS) is regarding the origin(s) of the conduction behavior they exhibit. Of particular interest are those with ambipolar conduction, which may provide an alternative choice for practical applications when considering the difficulties of doping the ultrathin bodies of vdWS. Here, the conduction behavior of ambipolar vdWS is analytically and theoretically studied. Using numerical simulation, it is shown that ambipolar vdWS can be fully captured by a Schottky-barrier FET model. Based on this, it is found that the widely observed conduction polarity transition while changing the body thickness mainly comes from the tuning of band alignment at the metal/vdWS interfaces. This transition can be suppressed/inversed by introducing an inert hBN layer between the vdWS and the substrate. Through first-principles calculations, it is demonstrated that metal/vdWS/substrate interactions play a crucial role in tuning the Schottky-barrier heights, which finally determines the conduction behavior that ambipolar vdWS exhibit.

Charge carrier injection and transport engineering in two-dimensional transition metal dichalcogenides

Ever since two dimensional-transition (2D) metal dichalcogenides (TMDs) were discovered, their fascinating electronic properties have attracted a great deal of attention for harnessing them as critical components in novel electronic devices. 2D-TMDs endowed with an atomically thin structure, dangling bond-free nature, electrostatic integrity, and tunable wide band gaps enable low power consumption, low leakage, ambipolar transport, high mobility, superconductivity, robustness against short channel effects and tunneling in highly scaled devices. However, the progress of 2D-TMDs has been hampered by severe charge transport issues arising from undesired phenomena occurring at the surfaces and interfaces. Therefore, this review provides three distinct engineering strategies embodied with distinct innovative approaches to optimize both carrier injection and transport. First, contact engineering involves 2D-metal contacts and tunneling interlayers to overcome metal-induced interface states and the Fermi level pinning effect caused by low vacancy energy formation. Second, dielectric engineering covers high-k dielectrics, ionic liquids or 2D-insulators to screen scattering centers caused by carrier traps, imperfections and rough substrates, to finely tune the Fermi level across the band gap, and to provide dangling bond-free media. Third, material engineering focuses on charge transfer via substitutional, chemical and plasma doping to precisely modulate the carrier concentration and to passivate defects while preserving material integrity. Finally, we provide an outlook of the conceptual and technical achievements in 2D-TMDs to give a prospective view of the future development of highly scaled nanoelectronic devices.

Doping-Free Complementary Logic Gates Enabled by Two-Dimensional Polarity-Controllable Transistors

Atomically thin two-dimensional (2D) materials belonging to transition metal dichalcogenides, due to their physical and electrical properties, are an exceptional vector for the exploration of next-generation semiconductor devices. Among them, due to the possibility of ambipolar conduction, tungsten diselenide (WSe₂) provides a platform for the efficient implementation of polarity-controllable transistors. These transistors use an additional gate, named polarity gate, that, due to the electrostatic doping of the Schottky junctions, provides a device-level dynamic control of their polarity, that is, n- or p-type. Here, we experimentally demonstrate a complete doping-free standard cell library realized on WSe₂ without the use of either chemical or physical doping. We show a functionally complete family of complementary logic gates (INV, NAND, NOR, 2-input XOR, 3-input XOR, and MAJ) and, due to the reconfigurable capabilities of the single devices, achieve the realization of highly expressive logic gates, such as exclusive-OR (XOR) and majority (MAJ), with fewer transistors than possible in conventional complementary metal-oxide-semiconductor logic. Our work shows a path to enable doping-free low-power electronics on 2D semiconductors, going beyond the concept of unipolar physically doped devices, while suggesting a road to achieve higher computational densities in two-dimensional electronics.

A comparative study on top-gated and bottom-gated multilayer MoS2 transistors with gate stacked dielectric of Al2O3/HfO2

Top-gated and bottom-gated transistors with multilayer MoS₂ channel fully encapsulated by stacked Al₂O₃/HfO₂ (9 nm/6 nm) were fabricated and comparatively studied. Excellent electrical properties are demonstrated for the TG transistors with high on-off current ratio of 10⁸, high field-effect mobility of 10² cm² V^-1 s^-1, and low subthreshold swing of 93 mV dec^-1. Also, enhanced reliability has been achieved for the TG transistors with threshold voltage shift of 10^-3-10^-2 V MV^-1 cm^-1 after 6 MV cm^-1 gate-biased stressing. All improvement for the TG device can be ascribed to the formed device structure and dielectric environment. Degradation of the performance for the BG transistors should be attributed to reduced gate capacitance density and deteriorated interface properties related to vdW gap with a thickness about 0.4 nm. So, the TG transistor with MoS₂ channel fully encapsulated by stacked Al₂O₃/HfO₂ is a promising way to fabricate high-performance ML MoS₂ field-effect transistors for practical electron device applications.

Tuning Electronic Structure of Single Layer MoS2 through Defect and Interface Engineering

Transition-metal dichalcogenides (TMDs) have emerged in recent years as a special group of two-dimensional materials and have attracted tremendous attention. Among these TMD materials, molybdenum disulfide (MoS₂) has shown promising applications in electronics, photonics, energy, and electrochemistry. In particular, the defects in MoS₂ play an essential role in altering the electronic, magnetic, optical, and catalytic properties of MoS₂, presenting a useful way to engineer the performance of MoS₂. The mechanisms by which lattice defects affect the MoS₂ properties are unsettled. In this work, we reveal systematically how lattice defects and substrate interface affect MoS₂ electronic structure. We fabricated single-layer MoS₂ by chemical vapor deposition and then transferred onto Au, single-layer graphene, hexagonal boron nitride, and CeO₂ as substrates and created defects in MoS₂ by ion irradiation. We assessed how these defects and substrates affect the electronic structure of MoS₂ by performing X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, and scanning tunneling microscopy/spectroscopy measurements. Molecular dynamics and first-principles based simulations allowed us to conclude the predominant lattice defects upon ion irradiation and associate those with the experimentally obtained electronic structure. We found that the substrates can tune the electronic energy levels in MoS₂ due to charge transfer at the interface. Furthermore, the reduction state of CeO₂ as an oxide substrate affects the interface charge transfer with MoS₂. The irradiated MoS₂ had a faster hydrogen evolution kinetics compared to the as-prepared MoS₂, demonstrating the concept of defect controlled reactivity in this phase. Our findings provide effective probes for energy band and defects in MoS₂ and show the importance of defect engineering in tuning the functionalities of MoS₂ and other TMDs in electronics, optoelectronics, and electrochemistry.

Chemical Vapor Transport Deposition of Molybdenum Disulfide Layers Using H2O Vapor as the Transport Agent

Molybdenum disulfide (MoS2) layers show excellent optical and electrical properties and have many potential applications. However, the growth of high-quality MoS2 layers is a major bottleneck in the development of MoS2-based devices. In this paper, we report a chemical vapor transport deposition method to investigate the growth behavior of monolayer/multi-layer MoS2 using water (H2O) as the transport agent. It was shown that the introduction of H2O vapor promoted the growth of MoS2 by increasing the nucleation density and continuous monolayer growth. Moreover, the growth mechanism is discussed.