Multilayer Graphene-WSe2 Heterostructures for WSe2 Transistors

Tuning Polarity in WSe2/AlScN FeFETs via Contact Engineering

Recent advancements in ferroelectric field-effect transistors (FeFETs) using two-dimensional (2D) semiconductor channels and ferroelectric Al0.68Sc0.32N (AlScN) allow high-performance nonvolatile devices with exceptional ON-state currents, large ON/OFF current ratios, and large memory windows (MW). However, previous studies have solely focused on n-type FeFETs, leaving a crucial gap in the development of p-type and ambipolar FeFETs, which are essential for expanding their applicability to a wide range of circuit-level applications. Here, we present a comprehensive demonstration of n-type, p-type, and ambipolar FeFETs on an array scale using AlScN and multilayer/monolayer WSe2. The dominant injected carrier type is modulated through contact engineering at the metal-semiconductor junction, resulting in the realization of all three types of FeFETs. The effect of contact engineering on the carrier injection is further investigated through technology-computer-aided design simulations. Moreover, our 2D WSe2/AlScN FeFETs achieve high electron and hole current densities of ∼20 and ∼10 μA/μm, respectively, with a high ON/OFF ratio surpassing ∼107 and a large MW of >6 V (0.14 V/nm).

Atomically thin MXene/WSe2Schottky heterojunction towards enhanced photogenerated charge carrier

Two-dimensional materials garner increasing interest in next-generation electronics and optoelectronic devices due to their atomic-thin nature and distinctive physical properties. Building on these advances, we present the successful synthesis of a heterostructure composed of the semi-metallic Ti3C2-MXene and the semiconducting WSe2, in which the atomic layers are vertically aligned. The wet impregnation method effectively synthesizes an atomically thin Ti3C2-MXene/WSe2heterostructure characterized by atomic force microscopy, Raman and time-resolved photoluminescence (TRPL) analysis. In addition, the current-voltage characteristics at the heterostructure reveal the Schottky junction probed by the scanning tunnelling microscopy and the conductive atomic force microscopy tip. The Schottky heterojunction also exhibits enhanced photocatalytic properties by improving the photogenerated charge carriers and inhibiting recombination. This work demonstrates the unique 2D-2D Ti3C2-MXene/WSe2vertical heterojunction possesses superior photon trapping ability and can efficiently transport photogenerated charge carriers to the reaction sites to enhance photocatalysis performance.

Fabrication of p-type 2D single-crystalline transistor arrays with Fermi-level-tuned van der Waals semimetal electrodes

High-performance p-type two-dimensional (2D) transistors are fundamental for 2D nanoelectronics. However, the lack of a reliable method for creating high-quality, large-scale p-type 2D semiconductors and a suitable metallization process represents important challenges that need to be addressed for future developments of the field. Here, we report the fabrication of scalable p-type 2D single-crystalline 2H-MoTe2 transistor arrays with Fermi-level-tuned 1T'-phase semimetal contact electrodes. By transforming polycrystalline 1T'-MoTe2 to 2H polymorph via abnormal grain growth, we fabricated 4-inch 2H-MoTe2 wafers with ultra-large single-crystalline domains and spatially-controlled single-crystalline arrays at a low temperature (~500 °C). Furthermore, we demonstrate on-chip transistors by lithographic patterning and layer-by-layer integration of 1T' semimetals and 2H semiconductors. Work function modulation of 1T'-MoTe2 electrodes was achieved by depositing 3D metal (Au) pads, resulting in minimal contact resistance (~0.7 kΩ·μm) and near-zero Schottky barrier height (~14 meV) of the junction interface, and leading to high on-state current (~7.8 μA/μm) and on/off current ratio (~105) in the 2H-MoTe2 transistors.

Role of Bilayer Graphene Microstructure on the Nucleation of WSe2 Overlayers

Over the past few years, graphene grown by chemical vapor deposition (CVD) has gained prominence as a template to grow transition metal dichalcogenide (TMD) overlayers. The resulting two-dimensional (2D) TMD/graphene vertical heterostructures are attractive for optoelectronic and energy applications. However, the effects of the microstructural heterogeneities of graphene grown by CVD on the growth of the TMD overlayers are relatively unknown. Here, we present a detailed investigation of how the stacking order and twist angle of CVD graphene influence the nucleation of WSe₂ triangular crystals. Through the combination of experiments and theory, we correlate the presence of interlayer dislocations in bilayer graphene with how WSe₂ nucleates, in agreement with the observation of a higher nucleation density of WSe₂ on top of Bernal-stacked bilayer graphene versus twisted bilayer graphene. Scanning/transmission electron microscopy (S/TEM) data show that interlayer dislocations are present only in Bernal-stacked bilayer graphene but not in twisted bilayer graphene. Atomistic ReaxFF reactive force field molecular dynamics simulations reveal that strain relaxation promotes the formation of these interlayer dislocations with localized buckling in Bernal-stacked bilayer graphene, whereas the strain becomes distributed in twisted bilayer graphene. Furthermore, these localized buckles in graphene are predicted to serve as thermodynamically favorable sites for binding WSe_x molecules, leading to the higher nucleation density of WSe₂ on Bernal-stacked graphene. Overall, this study explores synthesis-structure correlations in the WSe₂/graphene vertical heterostructure system toward the site-selective synthesis of TMDs by controlling the structural attributes of the graphene substrate.

Homogeneous in-plane WSe2 P-N junctions for advanced optoelectronic devices

Conventional doping schemes of silicon (Si) microelectronics are incompatible with atomically thick two-dimensional (2D) transition metal dichalcogenides (TMDCs), which makes it challenging to construct high-quality 2D homogeneous p-n junctions. Herein, we adopt a simple yet effective plasma-treated doping method to seamlessly construct a lateral 2D WSe₂ p-n homojunction. WSe₂ with ambipolar transport properties was exposed to O₂ plasma to form WO_x on the surface in a self-limiting process that induces hole doping in the underlying WSe₂via electron transfer. Different electrical behaviors were observed between the as-exfoliated (ambipolar) region and the O₂ plasma-treated (p-doped) region under electrostatic modulation of the back-gate bias (V_BG), which produces a p-n in-plane homojunction. More importantly, a small contact resistance of 710 Ω μm with a p-doped region transistor mobility of ∼157 cm² V^-1 s^-1 was achieved due to the transformation of Schottky contact into Ohmic contact after plasma treatment. This effectively avoids Fermi-level pinning and significantly improves the performance of photodetectors. The resultant WSe₂ p-n junction device thus exhibits a high photoresponsivity of ∼7.1 × 10⁴ mA W^-1 and a superior external quantum efficiency of ∼228%. Also, the physical mechanism of charge transfer in the WSe₂ p-n homojunction was analyzed. Our proposed strategy offers a powerful route to realize low contact resistance and high photoresponsivity in 2D TMDC-based optoelectronic devices, paving the way for next-generation atomic-thickness optoelectronics.

Steep-slope transistors enabled with 2D quantum coupling stacks

As down scaling of transistors continues, there is a growing interest in developing steep-slope transistors with reduced subthreshold slope (SS) below the Boltzmann limit. In this work, we successfully fabricated steep-slope MoS₂transistors by incorporating a graphene layer, inserted in the gate stack. For our comprehensive study, we have applied density functional theory to simulate and calculate the change of SS effected by different 2D quantum materials, including graphene, germanene and 2D topological insulators, inserted within the gate dielectric. This theoretical study showed that graphene/MoS₂devices had steep SS (27.2 mV/decade), validating our experimental approach (49.2 mV/decade). Furthermore, the simulations demonstrated very steep SS (8.6 mV/decade) in WTe₂/MoS₂devices. We conclude that appropriate combination of various 2D quantum materials for the gate-channel stacks, leads to steep SS and is an effective method to extend the scaling of transistors with exceptional performance.

Site-selective growth of two-dimensional materials: strategies and applications

Over the years, there have been major advances in two-dimensional (2D) materials on account of their excellent and unique properties. Among the various strategies for 2D material fabrication, chemical vapor deposition (CVD) is considered as the most promising method to achieve large-area and high-quality 2D film growth. Furthermore, to realize the potential applications of 2D materials in different fields, the integration of 2D materials into functional devices is essential. However, the materials made by common CVD are randomly distributed on substrates, which is disadvantageous for fabricating arrays of devices. To solve this problem, a site-selective growth method was developed to meet the requirement of batch production for practical applications because it achieves control over the locations of products and benefits the subsequent direct integration. Herein, state-of-the-art methods for site-selective synthesis, including seeded growth and patterned growth, are reviewed. Then, the electronic and optoelectronic applications of the as-grown 2D materials are also reviewed. Finally, the remaining challenges and future prospects regarding site-selective methods and applications are discussed.

Improving performance of monolayer arsenene tunnel field-effect transistors by defects

We systematically investigate the transport properties of monolayer arsenene tunneling field-effect transistors (TFETs) along the armchair and zigzag directions using first-principles calculations based on density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) approach. We introduce five types of defects at the source-channel interface and study their influences on the device performance. The pristine arsenene TFETs along the armchair direction have large ON-state currents due to the small effective masses, but still cannot meet the International Technology Roadmaps of Semiconductor 2022 (ITRS 2022) requirements for high performance (HP) devices. The adsorption of one and two H atoms can significantly enhance the ON-state currents to above 1412 μA μm^-1 and reduce subthreshold swing (SS) to below 60 mV decade^-1 for both n- and p-type devices, satisfying the ITRS 2022 requirements for HP devices. Besides, the p-type As and the n-type Li adatoms can improve the performance of p-type and n-type devices, respectively. The pristine arsenene TFETs along the zigzag direction with low ON-state currents already meet the ITRS 2022 requirements for low-power (LP) devices. The performance of the p-type TFETs as LP devices can be improved by p-type SV and the As adatom by increasing the ON-state currents and/or reducing the SS. On the other hand, the adsorption of one H adatom can remarkably increase the ON-state current of the p-type TFET to 1563 μA μm^-1 and reduce SS to 34 mV decade^-1, allowing the device to work as an HP device. We further confirm that the enhancement of the ON-state currents is due to the shortening of the band-to-band tunneling path through the defect induced gap states. Our calculations provide a theoretical guide to improve the performance of TFETs based on arsenene or other monolayer materials by suitable defects.

Surface Modification of 2D Photocatalysts for Solar Energy Conversion

2D materials show many particular properties, such as high surface-to-volume ratio, high anisotropic degree, and adjustable chemical functionality. These unique properties in 2D materials have sparked immense interest due to their applications in photocatalytic systems, resulting in significantly enhanced light capture, charge-transfer kinetics, and surface reaction. Herein, the research progress in 2D photocatalysts based on varied compositions and functions, followed by specific surface modification strategies, is introduced. Fundamental principles focusing on light harvesting, charge separation, and molecular adsorption/activation in the 2D-material-based photocatalytic system are systemically explored. The examples described here detail the use of 2D materials in various photocatalytic energy-conversion systems, including water splitting, carbon dioxide reduction, nitrogen fixation, hydrogen peroxide production, and organic synthesis. Finally, by elaborating the challenges and possible solutions for developing these 2D materials, the review is expected to provide some inspiration for the future research of 2D materials used on efficient photocatalytic energy conversions.

CVD-Assisted Synthesis of 2D Layered MoSe2 on Mo Foil and Low Frequency Raman Scattering of Its Exfoliated Few-Layer Nanosheets on CaF2 Substrates

Transition-metal dichalcogenides (TMDCs) are unique layered materials with exotic properties. So, examining their structures holds tremendous importance. 2H-MoSe₂ (analogous to MoS₂; Gr. 6 TMDC) is a crucial optoelectronic material studied extensively using Raman spectroscopy. In this regard, low-frequency Raman (LFR) spectroscopy can probe this material's structure as it reveals distinct vibration modes. Here, we focus on understanding the microstructural evolution of different 2H-MoSe₂ morphologies and their layers using LFR scattering. We grew phase-pure 2H-MoSe₂ (with variable microstructures) directly on a Mo foil using a two-furnace ambient-pressure chemical vapor deposition (CVD) system by carefully controlling the process parameters. We analyzed the layers of exfoliated flakes after ultrasonication and drop-cast 2H-MoSe₂ of different layer thicknesses by choosing different concentrations of 2H-MoSe₂ solutions. Further detailed analyses of the respective LFR regions confirm the presence of newly identified Raman signals for the 2H-MoSe₂ nanosheets drop-cast on Raman-grade CaF₂. Our results show that CaF₂ is an appropriate Raman-enhancing substrate compared to Si/SiO₂ as it presents new LFR modes of 2H-MoSe₂. Therefore, CaF₂ substrates are a promising medium to characterize in detail other TMDCs using LFR spectroscopy.

Influence of the interface structure and strain on the rectification performance of lateral MoS2/graphene heterostructure devices

We systematically study the influence of interface configuration and strain on the electronic and transport properties of lateral MoS₂/graphene heterostructures by first-principles calculations and quantum transport simulations. We first identify the favorable heterostructure configurations with C-S and/or C-Mo bonds at the interfaces. Strain can be applied to graphene or MoS₂ and would not change the relative stabilities of different heterostructures. Band alignment calculations show that all the lateral heterostructures have n-type Schottky contacts. The current-voltage characteristics of the lateral MoS₂/graphene heterostructure diodes exhibit good rectification performance. Too strong and too weak interface interactions do not benefit electronic transport. The MoS₂/graphene heterostructures with moderate C-S bonds at the interface have larger currents through the junctions than those with C-Mo bonds at the interface. The maximal rectification ratio of the lateral diode with strain applied to MoS₂ can reach up to 10⁵. With strain applied to graphene, the currents through the heterostructures can increase by 1-2 orders of magnitude due to the reduced Schottky barrier heights at the interface, but the rectification ratio is reduced with a maximal value of 10⁴. Our calculations can serve as a theoretical guide to design rectifier and diode devices based on two-dimensional lateral heterostructures.

Gradual Edge Contact between Mo and MoS2 Formed by Graphene-Masked Sulfurization for High-Performance Field-Effect Transistors

Two-dimensional materials have attracted great attention for their outstanding electronic properties. In particular, molybdenum disulfide (MoS₂) shows great potential as a next-generation semiconductor due to its tunable direct bandgap with a high on-off ratio and extraordinary stability. However, the performance of MoS₂ synthesized by physical vapor deposition has been limited by contact resistance between an electrode and MoS₂, which determines overall device characteristics. Here, in order to reduce the contact resistance, we use in situ sulfurization of Mo by H₂S gas treatment masked by a patterned graphene gas barrier, so that the Mo channel area can be selectively formed, resulting in a gradual edge contact between Mo and MoS₂. Compared with field-effect transistors with a top contact between the Au/Ti electrode and the MoS₂ channel, a gradual edge contact between the Mo electrode and the MoS₂ channel provides a considerably enhanced electrical performance.

High-Crystalline Monolayer Transition Metal Dichalcogenides Films for Wafer-Scale Electronics

Chemical vapor deposition (CVD) using liquid-phase precursors has emerged as a viable technique for synthesizing uniform large-area transition metal dichalcogenide (TMD) thin films. However, the liquid-phase precursor-assisted growth process typically suffers from small-sized grains and unreacted transition metal precursor remainders, resulting in lower-quality TMDs. Moreover, synthesizing large-area TMD films with a monolayer thickness is also quite challenging. Herein, we successfully synthesized high-quality large-area monolayer molybdenum diselenide (MoSe₂) with good uniformity via promoter-assisted liquid-phase CVD process using the transition metal-containing precursor homogeneously modified with an alkali metal halide. The formation of a reactive transition metal oxyhalide and reduction of the energy barrier of chalcogenization by the alkali metal promoted the growth rate of the TMDs along the in-plane direction, enabling the full coverage of the monolayer MoSe₂ film with negligible few-layer regions. Note that the fully selenized monolayer MoSe₂ with high crystallinity exhibited superior electrical transport characteristics compared with those reported in previous works using liquid-phase precursors. We further synthesized various other monolayer TMD films, including molybdenum disulfide, tungsten disulfide, and tungsten diselenide, to demonstrate the broad applicability of the proposed approach.

Laser Writable Multifunctional van der Waals Heterostructures

Achieving multifunctional van der Waals nanoelectronic devices on one structure is essential for the integration of 2D materials; however, it involves complex architectural designs and manufacturing processes. Herein, a facile, fast, and versatile laser direct write micro/nanoprocessing to fabricate diode, NPN (PNP) bipolar junction transistor (BJT) simultaneously based on a pre-fabricated black phosphorus/molybdenum disulfide heterostructure is demonstrated. The PN junctions exhibit good diode rectification behavior. Due to different carrier concentrations of BP and MoS₂ , the NPN BJT, with a narrower base width, renders better performance than the PNP BJT. Furthermore, the current gain can be modulated efficiently through laser writing tunable base width W_B , which is consistent with the theoretical results. The maximum gain for NPN and PNP is found to be ≈41 (@W_B ≈600 nm) and ≈12 (@W_B ≈600 nm), respectively. In addition, this laser write processing technique also can be utilized to realize multifunctional WSe₂ /MoS₂ heterostructure device. The current work demonstrates a novel, cost-effective, and universal method to fabricate multifunctional nanoelectronic devices. The proposed approach exhibits promise for large-scale integrated circuits based on 2D heterostructures.

Energy-Efficient Tunneling Field-Effect Transistors for Low-Power Device Applications: Challenges and Opportunities

Conventional field-effect transistors (FETs) have long been considered a fundamental electronic component for a diverse range of devices. However, nanoelectronic circuits based on FETs are not energy efficient because they require a large supply voltage for switching applications. To reduce the supply voltage in standard FETs, which is hampered by the 60 mV/decade limit established by the subthreshold swing (SS), a new class of FETs have been designed, tunnel FETs (TFETs). A TFET utilizes charge-carrier transportation in device channels using quantum mechanical based band-to-band tunneling despite of conventional thermal injection. The TFETs fabricated with thin semiconducting film or nanowires can attain a 100-fold power drop compared to complementary metal-oxide-semiconductor (CMOS) transistors. As a result, the use of TFETs and CMOS technology together could ameliorate integrated circuits for low-power devices. The discovery of two-dimensional (2D) materials with a diverse range of electronic properties has also opened new gateways for condensed matter physics, nanotechnology, and material science, thus potentially improving TFET-based devices in terms of device design and performance. In this review, state-of-art TFET devices exhibiting different semiconducting channels and geometries are comprehensively reviewed followed by a brief discussion of the challenges that remain for the development of high-performance devices. Lastly, future prospects are presented for the improvement of device design and the working efficiency of TFETs.

Tunable thermal expansion coefficient of transition-metal dichalcogenide lateral heterostructures

The thermal expansion effect plays an important role in governing the thermal stability or the stable configuration of quasi-two-dimensional atomic layers, where the difference between the thermal expansion coefficient of different kinds of atomic layer in lateral heterostructure may cause strong thermal rippling of the atomic layer. We investigate the thermal expansion phenomenon in the WSe₂-MoS₂ lateral heterostructure. We find that the thermal expansion coefficient can be enhanced by more than a factor of two via varying the ratio between the WSe₂ and MoS₂ components in the heterostructure. The underlying mechanism is disclosed to be the buckling of the WSe₂ region that is induced by the misfit strain at the coherent interface between WSe₂ and MoS₂. These findings shall be helpful in handling the thermal stability of functional devices based on the transition-metal dichalcogenide lateral heterostructures and other similar quasi-two-dimensional lateral heterostructures.

Reduction of the ambient effect in multilayer InSe transistors and a strategy toward stable 2D-based optoelectronic applications

Indium selenide (InSe) photodetection devices attract significant research interest. However, InSe is unstable and degrades rapidly in ambient conditions, thus it is still a challenge to fabricate stable optoelectronic devices. In this work, multilayer InSe FETs are fabricated, and their photoresponse properties are investigated. Both positive and negative photoconductivities are observed for the first time in the same InSe FET in a wide spectral range from 450 nm to 660 nm, which can be tuned through changing either the gate bias or the source-drain bias. A physical mechanism is proposed to explain the dual-photoresponse phenomenon in our devices. Based on the proposed physical mechanism, as a proof of concept, a facile and simple approach is used to eliminate the negative photoconductivity of the InSe FET. Our results will offer valuable strategies for stable multilayer InSe optoelectronic device design, and a practical scheme for improving the performance of other transition metal dichalcogenide devices as well.

P-N conversion of charge carrier types and high photoresponsive performance of composition modulated ternary alloy W(SxSe1-x)2 field-effect transistors

Transition metal dichalcogenides (TMDs) have emerged as a new class of two-dimensional (2D) materials, which are promising for diverse applications in nanoelectronics, optoelectronics, and photonics. To satisfy the requirements of the building blocks of functional devices, systematic modulation of the band gap and carrier type of TMDs materials becomes a significant challenge. Here, we report a salt-assisted chemical vapor deposition (CVD) approach for the simultaneous growth of alloy W(SxSe1-x)2 nanosheets with variable alloy compositions. Electrical transport studies based on the as-fabricated W(SxSe1-x)2 nanosheet field-effect transistors (FETs) demonstrate that charge carrier types of alloy nanosheet transistors can be systematically tuned by adjusting the alloy composition. Temperature-dependent current measurement shows that the main scattering mechanism is the charged impurity scattering. The effective Schottky barrier heights of bipolar W(SxSe1-x)2 transistors are initially increased and then decreased with increasing positive (or negative) gate voltage, which is tunable by varying the alloy composition. In addition, the tunability of these W(SxSe1-x)2-based ambipolar transistors is suitable for logic and analog applications and represents a critical step toward future fundamental studies as well as for the rational design of new 2D electronics with tailored spectral responses, and simpler and higher integration densities. Finally, the high photoresponsivity (up to 914 mA W-1) and detectivity (4.57 × 1010 Jones) of ultrathin W(SxSe1-x)2 phototransistors imply their potential applications in flexible light-detection and light-harvesting devices. These band gap engineered 2D structures could open up an exciting opportunity and contribute to finding diverse applications in future functional electronic/optoelectronic devices.

Tungsten Diselenide Top-gate Transistors with Multilayer Antimonene Electrodes: Gate Stacks and Epitaxially Grown 2D Material Heterostructures

We have demonstrated that with e-beam deposition of a thin Al₂O₃ layer before atomic layer deposition, a uniform Al₂O₃ film can be obtained on WSe₂/sapphire samples. Device performances are observed for WSe₂ top-gate transistors by using oxide stacks as the gate dielectric. By using thermal evaporation, epitaxially grown multilayer antimonene can be prepared on both MoS₂ and WSe₂ surfaces. With multilayer antimonene as the contact metal, a significant increase in drain currents and ON/OFF ratios is observed for the device, which indicates that high contact resistance between metal/2D material interfaces is a critical issue for 2D devices. The observation of multilayer antimonene grown on different 2D material surfaces has demonstrated less dependence on the substrate lattice constant of the unique van der Waals epitaxy for 2D materials. The results have also demonstrated that stacking 2D materials with different materials plays an important role in the practical applications of 2D devices.

Selective Growth of WSe2 with Graphene Contacts

Nanoelectronics of two-dimensional (2D) materials and related applications are hindered with critical contact issues with the semiconducting monolayers. To solve these issues, a fundamental challenge is selective and controllable fabrication of p-type or ambipolar transistors with a low Schottky barrier. Most p-type transistors are demonstrated with tungsten selenides (WSe₂) but a high growth temperature is required. Here, we utilize seeding promoter and low pressure CVD process to enhance sequential WSe₂ growth with a reduced growth temperature of 800 °C for reduced compositional fluctuations and high hetero-interface quality. Growth behavior of the sequential WSe₂ growth at the edge of patterned graphene is discussed. With optimized growth conditions, high-quality interface of the laterally stitched WSe₂-graphene is achieved and characterized with transmission electron microscopy (TEM). Device fabrication and electronic performances of the laterally stitched WSe₂-graphene are presented.

Spatially controlled lateral heterostructures of graphene and transition metal dichalcogenides toward atomically thin and multi-functional electronics

Edge contacts between two-dimensional (2D) materials in the in-plane direction can achieve minimal contact area and low contact resistance, producing atomically thin devices with improved performance. Particularly, lateral heterojunctions of metallic graphene and semiconducting transition metal dichalcogenides (TMDs) exhibit small Schottky barrier heights due to graphene's low work-function. However, issues exist with the fabrication of highly transparent and flexible multi-functional devices utilizing lateral heterostructures (HSs) of graphene and TMDs via spatially controlled growth. This review demonstrates the growth and electronic applications of lateral HSs of graphene and TMDs, highlighting key technologies controlling the wafer-scale growth of continuous films for practical applications. It deepens the understanding of the spatially controlled growth of lateral HSs using chemical vapor deposition methods, and also contributes to the applications that depend on the scale-up of all-2D electronics with ultra-high electrical performance.

Intimate Ohmic contact to two-dimensional WSe2 via thermal alloying

The most important interface in semiconductor devices is the interface between the semiconductor and the first layer of the metal contact. However, the van der Waals (vdWs) gap in two-dimensional (2D) materials hindered the formation of an intimate contact between the 2D material and the metal electrode, limiting the device performances. We demonstrated a gapless Ohmic contact to 2D WSe₂ by forming a Pt-W-Se alloy, which significantly improved the device performances (contact resistance, current on/off ratio, output current density, field-effect mobility, and hysteresis) of the 2D WSe₂ field-effect transistor. The contact resistance to 2D WSe₂ was reduced by more than seven orders of magnitude after thermal alloying. The disappearance of the vdW gap confirmed by scanning transmission electron microscopy enhanced the hole conduction and quenched the electron conduction. Our strategy of metallurgical alloying is effective to form a low-resistance stable Ohmic contact to WSe₂, which paves the way for utilization of the full potential of 2D materials.

Scaling-up Atomically Thin Coplanar Semiconductor-Metal Circuitry via Phase Engineered Chemical Assembly

Two-dimensional (2D) layered semiconductors, with their ultimate atomic thickness, have shown promise to scale down transistors for modern integrated circuitry. However, the electrical contacts that connect these materials with external bulky metals are usually unsatisfactory, which limits the transistor performance. Recently, contacting 2D semiconductors using coplanar 2D conductors has shown promise in reducing the problematic high contact resistance. However, many of these methods are not ideal for scaled production. Here, we report on the large-scale, spatially controlled chemical assembly of the integrated 2H-MoTe₂ field-effect transistors (FETs) with coplanar metallic 1T'-MoTe₂ contacts via phase engineered approaches. We demonstrate that the heterophase FETs exhibit ohmic contact behavior with low contact resistance, resulting from the coplanar seamless contact between 2H and 1T'-MoTe₂ confirmed by transmission electron microscopy characterizations. The average mobility of the heterophase FETs was measured to be as high as 23 cm² V^-1 s^-1 (comparable with those of exfoliated single crystals), due to the large 2H-MoTe₂ single-crystalline domain size (486 ± 187 μm). By developing a patterned growth method, we realize the 1T'-MoTe₂ gated heterophase FET array whose components of the channel, gate, and contacts are all 2D materials. Finally, we transfer the heterophase device array onto a flexible substrate and demonstrate the near-infrared photoresponse with high photoresponsivity (∼1.02 A/W). Our study provides a basis for the large-scale application of phase-engineered coplanar MoTe₂ semiconductor-metal structure in advanced electronics and optoelectronics.

High-Mobility InSe Transistors: The Nature of Charge Transport

InSe is a high-mobility layered semiconductor with mobility being highly sensitive to any surrounding media that could act as a source of extrinsic scattering. However, little effort has been made to understand electronic transport in thin InSe layers with native surface oxide formed spontaneously upon exposure to an ambient environment. Here, we explore the influence of InO_x/InSe interfacial trap states on electronic transport in thin InSe layers. We show that wet oxidation (processed in an ambient environment) causes massive deep-lying band-tail states, through which electrons conduct via 2D variable-range hopping with a short localization length of 1-3 nm. In contrast, a high-quality InO_x/InSe interface can be formed in dry oxidation (processed in pure oxygen), with a low trap density of 10¹² eV^-1 cm^-2. Metal-insulator transition can be thus observed in the gate sweep of the field-effect transistors (FETs), indicative of band transport predominated by extended states above the mobility edge. A room-temperature band mobility of 10³ cm²/V s is obtained. The profound difference in the transport behavior between the wet and dry InSe FETs suggests that fluctuating Coulomb potential arising from trapped charges at the InO_x/InSe interface is the dominant source of disorders in thin InSe channels.

Growth of Complex 2D Material-Based Structures with Naturally Formed Contacts

The difficulty of processing two-dimensional (2D) transition metal dichalcogenide (TMD) materials into working devices with any scalability is one of the largest impediments to capitalizing on their industrial promise. Here, we describe a versatile, simple, and scalable technique to directly grow self-contacted thin-film materials over a range of TMDs (MoS₂, MoSe₂, WS₂, and WSe₂), where predeposited bulk metallic contacts serve as the nucleation site for the TMD material to grow, forming naturally contacted device structures in a single step. The conditions for growth as well as optical and physical properties are reported. Because the material grows controllably around the lithographically defined patterns, wafer scale circuits and complex device geometries can be envisioned, including lateral heterostructures of different TMD materials.

Metal-Guided Selective Growth of 2D Materials: Demonstration of a Bottom-Up CMOS Inverter

2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large-area electronics and circuits strongly relies on wafer-scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal-guided selective growth (MGSG), is reported. The success of control over the transition-metal-precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p- and n-type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom-up complementary metal-oxide-semiconductor inverter based on p-type WSe₂ and n-type MoSe₂ is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.

Graphene and Carbon Nanotube Heterojunction Transistors with Individual Gate Control

Heterogeneously integrated nanomaterial devices show interesting characteristics for transistors and sensors due to their band diagram or steep material junctions. If these junctions and band alignments can be tuned by an electrical input bias, the device platform not only could be expanded but also could be used to explore fundamental characteristics. However, most reports on hetero-nanomaterial junctions use a global back-gate voltage, which makes it difficult to control band alignment at an interface. To explore device junctions, this study reports a laterally integrated heterojunction of graphene and a carbon nanotube (CNT) network film with individual gate electrodes to tune the band alignment corresponding to the Fermi level shift of graphene in contact with the semiconducting CNT network film. By developing the fabrication process, multiple gate structures are designed to apply a gate bias to CNTs and graphene separately. The threshold voltage shift of the CNT transistor depends on the gate voltage of graphene. Based on the thermionic emission theory, the barrier height between graphene and CNTs for both the conduction and valence band sides varies from 70 to 85 meV, with a linear change as a function of the applied gate voltage to graphene. Although the current Fermi level shift is small, this device platform may realize the exploration of fundamental properties and device concepts.

Neutral and defect-induced exciton annihilation in defective monolayer WS2

As defects and exciton-exciton annihilation (EEA) frequently govern the properties of nanoscale optoelectronic devices based on monolayer transition metal dichalcogenides (TMDCs), understanding the interaction between defects and EEA is of fundamental importance. Here we perform a systematic investigation of the effect of defects on EEA of neutral excitons and defect-bound excitons in monolayer WS2, using fluorescence lifetime imaging technology. Scanning transmission electron microscopy confirms the creation of atomic-scale defects introduced by argon plasma treatment in defective WS2. Defects can bind neutral excitons or trions to form defect-bound excitons. And defects have a slight effect on the lifetime of neutral excitons. However, owing to the impeded exciton diffusion caused by defects, the EEA rate of neutral excitons reduces from 0.26 cm2 s-1 in the pristine monolayer to 0.16 cm2 s-1 in the defective monolayer. For defect-bound excitons, the EEA rate of 0.068 cm2 s-1 is obtained, which results from the localized nature of defect-bound excitons and suppressed exciton diffusion. Our results reveal the important role of defect-EEA interactions in tailoring the properties of monolayer TMDCs.

Control of the metal/WS2 contact properties using 2-dimensional buffer layers

Transition metal dichalcogenides (TMDC) have recently attracted much attention as a promising platform for the realization of 2-dimensional (2D) electronic devices. One of the major challenges for their wide-scale application is the control of the potential barrier at the metal/TMDC junction. Using conductive atomic force microscopy (c-AFM) we have investigated modifications of the Schottky barrier height (SBH) across a Pt/WS2 junction by the introduction of thin buffer layers of graphene and MoSe2. While graphene greatly reduces the contact resistance in both bias directions, thin layers of MoSe2 lower the Schottky barrier and leave the rectifying properties of the junction intact. We have studied the dependence of the transport properties on the thickness of the graphene and MoSe2 buffer layers. In both cases, the charge transport characteristics can be tailored by varying the buffer layer thickness. The edge of single layer graphene is observed to form an ohmic contact to the underlying WSe2 substrate. This study demonstrates that the introduction of atomically thin MoSe2 and graphene buffer layers is a feasible and elegant method to control the Schottky barrier when contacting TMDCs. The results are important for the fabrication of devices utilizing 2D materials.

High Photoresponsivity in Ultrathin 2D Lateral Graphene:WS2:Graphene Photodetectors Using Direct CVD Growth

We show that reducing the degree of van der Waals overlapping in all 2D ultrathin lateral devices composed of graphene:WS₂:graphene leads to significant increase in photodetector responsivity. This is achieved by directly growing WS₂ using chemical vapor deposition (CVD) in prepatterned graphene gaps to create epitaxial interfaces. Direct-CVD-grown graphene:WS₂:graphene lateral photodetecting transistors exhibit high photoresponsivities reaching 121 A/W under 2.7 × 10⁵ mW/cm² 532 nm illumination, which is around 2 orders of magnitude higher than similar devices made by the layer-by-layer transfer method. The photoresponsivity of our direct-CVD-grown device shows negative correlation with illumination power under different gate voltages, which is different from similar devices made by the transfer method. We show that the high photoresponsivity is due to the lowering of effective Schottky barrier height by improving the contact between graphene and WS₂. Furthermore, the direct CVD growth reduces overlapping sections of WS₂:Gr and leads to more uniform lateral systems. This approach provides insights into scalable manufacturing of high-quality 2D lateral electronic and optoelectronic devices.

Electrically and Optically Tunable Responses in Graphene/Transition-Metal-Dichalcogenide Heterostructures

Heterostructures involving layered two-dimensional (2D) transition metal dichalcogenides (TMDCs) are not only fundamentally interesting to explore emerging properties at atomically thin limit, but also technically important to achieve novel optoelectronic devices. However, achieving tunable optoelectronic properties and clarifying interlayer processes (charge transfer, energy transfer) in 2D heterostructures have remained part of the key challenges so far. Here, by fabricating heterostructures of graphene and monolayer TMDCs (n-type MoS₂ and p-type WSe₂), we demonstrate both electrically and optically tunable responses of the heterostructures, revealing the critical interface processes between graphene and TMDCs. In MoS₂/graphene heterostructures, electron transfer from MoS₂ to graphene is observed, and gate-tunable interface relaxation induces the electrically controlled photoluminescence (PL), whereas in WSe₂/graphene heterostructures, electron transfer from graphene to WSe₂ is observed, and the PL is tuned by carrier density, which can be controlled by the gate voltage. The interlayer process can also be modulated by laser intensity, which enables photoinduced doping on graphene and optically tunable electrical characteristics of graphene. Combining the tunable Fermi level of graphene and strong light-matter interaction of monolayer TMDCs, our demonstrations are important for the design of multifunctional and efficient optoelectronic devices with TMDC/graphene heterostructures.

Efficient Energy Transfer across Organic-2D Inorganic Heterointerfaces

Combining organic and inorganic semiconductors enables us to integrate complementary advantages of each material system into a single hybrid material platform. Here, we report a study on the energy transport across a hybrid interface consisting of j-aggregates of organic dye and monolayer molybdenum disulfide (MoS₂). The excellent overlap between the photoluminescence spectra of j-aggregates and the absorption of MoS₂ B-exciton enables the material system to be used to study Förster resonance energy transfer (FRET) across the hybrid interface. We report a short Förster radius of 1.88 nm for the hybrid system. We achieve this by fabricating photodetectors based on the hybrid organic-inorganic interface that combine the high absorption of organics with the high-charge mobility of inorganics. Concomitantly, the hybrid photodetectors show nearly 93 ± 5% enhancement of photoresponsivity in the excitonic spectral overlap regime due to efficient energy transfer (ET) from j-aggregate to MoS₂. This work not only provides valuable insight into the ET mechanism across such hybrid organic-inorganic interfaces but also demonstrates the feasibility of the platform for designing efficient energy conversion and optoelectronic devices.

High Mobilities in Layered InSe Transistors with Indium-Encapsulation-Induced Surface Charge Doping

Tunability and stability in the electrical properties of 2D semiconductors pave the way for their practical applications in logic devices. A robust layered indium selenide (InSe) field-effect transistor (FET) with superior controlled stability is demonstrated by depositing an indium (In) doping layer. The optimized InSe FETs deliver an unprecedented high electron mobility up to 3700 cm² V^-1 s^-1 at room temperature, which can be retained with 60% after 1 month. Further insight into the evolution of the position of the Fermi level and the microscopic device structure with different In thicknesses demonstrates an enhanced electron-doping behavior at the In/InSe interface. Furthermore, the contact resistance is also improved through the In insertion between InSe and Au electrodes, which coincides with the analysis of the low-frequency noise. The carrier fluctuation is attributed to the dominance of the phonon scattering events, which agrees with the observation of the temperature-dependent mobility. Finally, the flexible functionalities of the logic-circuit applications, for instance, inverter and not-and (NAND)/not-or (NOR) gates, are determined with these surface-doping InSe FETs, which establish a paradigm for 2D-based materials to overcome the bottleneck in the development of electronic devices.

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.