Tunability of short-channel effects in MoS2 field-effect devices

Ultrafast Non-Volatile Floating-Gate Memory Based on All-2D Materials

The explosive growth of massive-data storage and the demand for ultrafast data processing require innovative memory devices with exceptional performance. 2D materials and their van der Waal heterostructures with atomically sharp interfaces hold great promise for innovations in memory devices. Here, this work presents non-volatile, floating-gate memory devices with all functional layers made of 2D materials, achieving ultrafast programming/erasing speeds (20 ns), high extinction ratios (up to 108), and multi-bit storage capability. These devices also exhibit long-term data retention exceeding 10 years, facilitated by a high gate-coupling ratio (GCR) and atomically sharp interfaces between functional layers. Additionally, this work demonstrates the realization of an "OR" logic gate on a single-device unit by synergistic electrical and optical operations. The present results provide a solid foundation for next-generation ultrahigh-speed, ultralong lifespan, non-volatile memory devices, with a potential for scale-up manufacturing and flexible electronics applications.

Broadband Photodetector Based on FePS3/WS2 van der Waals Type II Heterostructure

In order to understand broadband photodetectors from ultraviolet-visible (UV-vis) to the near-infrared range, one needs to find novel two-dimensional (2D) van der Waals (vdW) materials with broadband optoelectronic performance. Transition metal phosphorus sulfides (TMPSs) have been reported as a new type of vdW material with generally broadband and p-type conductivity. Here, we report a high-performance and broadband photodetector consist of p-type FePS3 and n-type WS2 with a working range of 405-785 nm. The maximum values of responsivity and specific detectivity are 32.5 mA/W and 1.73 × 1012 jones at 405 nm and 2 V bias, which are better than those of its individual constituents and many other 2D vdW heterostructures. The high performance of the FePS3/WS2 photodetector is attributed to the built-in electric field in the FePS3/WS2 p-n heterostructure and type II band alignment. Present study demonstrates that the material family of TMPSs could be a promising platform for broadband photodetector applications.

A critical review of fabrication challenges and reliability issues in top/bottom gated MoS2field-effect transistors

The voyage of semiconductor industry to decrease the size of transistors to achieve superior device performance seems to near its physical dimensional limitations. The quest is on to explore emerging material systems that offer dimensional scaling to match the silicon- based technologies. The discovery of atomic flat two-dimensional materials has opened up a completely new avenue to fabricate transistors at sub-10 nanometer level which has the potential to compete with modern silicon-based semiconductor devices. Molybdenum disulfide (MoS₂) is a two-dimensional layered material with novel semiconducting properties at atomic level seems like a promising candidate that can possibly meet the expectation of Moore's law. This review discusses the various 'fabrication challenges' in making MoS₂based electronic devices from start to finish. The review outlines the intricate challenges of substrate selection and various synthesis methods of mono layer and few-layer MoS₂. The review focuses on the various techniques and methods to minimize interface defect density at substrate/MoS₂interface for optimum MoS₂-based device performance. The tunable band-gap of MoS₂with varying thickness presents a unique opportunity for contact engineering to mitigate the contact resistance issue using different elemental metals. In this work, we present a comprehensive overview of different types of contact materials with myriad geometries that show a profound impact on device performance. The choice of different insulating/dielectric gate oxides on MoS₂in co-planar and vertical geometry is critically reviewed and the physical feasibility of the same is discussed. The experimental constraints of different encapsulation techniques on MoS₂and its effect on structural and electronic properties are extensively discussed.

Van der Waals Layer Transfer of 2D Materials for Monolithic 3D Electronic System Integration: Review and Outlook

Two-dimensional materials (2DMs) have attracted a great deal of interest due to their immense potential for scientific breakthroughs and technological innovations. While some 2D transition metal dichalcogenides (TMDC) such as MoS₂ and WS₂ are considered as the ultimate channel materials in unltrascaled transistors as replacements for Si, there has also been increasing interest in the monolithic 3D integration of 2DMs on the Si CMOS platform or in flexible electronics as back-end-of-line transistors, memory devices/selectors, and sensors, taking advantage of 2DM properties such as a high current driving capability with low leakage current, nonvolatile switching characteristics, a large surface-to-volume ratio, and a tunable bandgap. However, the realization of both of these scenarios critically depends on the development of manufacturing-viable high-yield 2DM layers transfer from the growth substrate to the Si, since the growth of high-quality 2DM layers often requires a high-temperature growth process on template substrates. Motivated by this, extensive efforts have been made by the 2DM research community to develop various 2DM layer transfer methods, leveraging the van der Waals transfer capability of the layer-structured 2DMs. These efforts have led to a number of successful demonstrations of wafer-scale 2D TMDC layer transfer, while 2DM-enabled template growth/transfer of some functional bulk materials such as III-V, Ge, and AlN has also been demonstrated. This review surveys and compares different 2DM transfer methods developed recently, with a focus on large-area 2D TMDC film transfer along with an introduction of 2DM template-assisted van der Waals growth/transfer of non-2D thin films. We will also briefly present an outlook of our envisioned multifunctionalities in 3D integrated electronic systems enabled by monolithic 3D integration of 2DMs and III-V via van der Waals transfer and discuss possible technology options for overcoming remaining challenges.

Multiwavelength High-Detectivity MoS2 Photodetectors with Schottky Contacts

Photodetection is one of the vital functions for the multifunctional "More than Moore" (MtM) microchips urgently required by Internet of Things (IoT) and artificial intelligence (AI) applications. The further improvement of the performance of photodetectors faces various challenges, including materials, fabrication processes, and device structures. We demonstrate in this work MoS₂ photodetectors with a nanoscale channel length and a back-gate device structure. With the mechanically exfoliated six-monolayer-thick MoS₂, a Schottky contact between source/drain electrodes and MoS₂, a high responsivity of 4.1 × 10³ A W^-1, and a detectivity of 1.34 × 10¹³ cm Hz^1/2 W^-1 at 650 nm were achieved. The devices are also sensitive to multiwavelength lights, including 520 and 405 nm. The electrical and optoelectronic properties of the MoS₂ photodetectors were studied in depth, and the working mechanism of the devices was analyzed. The photoinduced Schottky barrier lowering (PIBL) was found to be important for the high performance of the phototransistor.

Two dimensional semiconducting materials for ultimately scaled transistors

Two dimensional (2D) semiconductors have been established as promising candidates to break through the short channel effect that existed in Si metal-oxide-semiconductor field-effect-transistor (MOSFET), owing to their unique atomically layered structure and dangling-bond-free surface. The last decade has witnessed the significant progress in the size scaling of 2D transistors by various approaches, in which the physical gate length of the transistors has shrank from micrometer to sub-one nanometer with superior performance, illustrating their potential as a replacement technology for Si MOSFETs. Here, we review state-of-the-art techniques to achieve ultra-scaled 2D transistors with novel configurations through the scaling of channel, gate, and contact length. We provide comprehensive views of the merits and drawbacks of the ultra-scaled 2D transistors by summarizing the relevant fabrication processes with the corresponding critical parameters achieved. Finally, we identify the key opportunities and challenges for integrating ultra-scaled 2D transistors in the next-generation heterogeneous circuitry.

Selective Electron Beam Patterning of Oxygen-Doped WSe2 for Seamless Lateral Junction Transistors

Surface charge transfer doping (SCTD) using oxygen plasma to form a p-type dopant oxide layer on transition metal dichalcogenide (TMDs) is a promising doping technique for 2D TMDs field-effect transistors (FETs). However, patternability of SCTD is a key challenge to effectively switch FETs. Herein, a simple method to selectively pattern degenerately p-type (p+ )-doped WSe2 FETs via electron beam (e-beam) irradiation is reported. The effect of the selective e-beam irradiation is confirmed by the gate-tunable optical responses of seamless lateral p+ -p diodes. The OFF state of the devices by inducing trapped charges via selective e-beam irradiation onto a desired channel area in p+ -doped WSe2 , which is in sharp contrast to globally p+ -doped WSe2 FETs, is realized. Selective e-beam irradiation of the PMMA-passivated p+ -WSe2 enables accurate control of the threshold voltage (Vth ) of WSe2 devices by varying the pattern size and e-beam dose, while preserving the low contact resistance. By utilizing hBN as the gate dielectric, high-performance WSe2 p-FETs with a saturation current of -280 µA µm-1 and on/off ratio of 109 are achieved. This study's technique demonstrates a facile approach to obtain high-performance TMD p-FETs by e-beam irradiation, enabling efficient switching and patternability toward various junction devices.

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.

Area-Selective Chemical Doping on Solution-Processed MoS2 Thin-Film for Multi-Valued Logic Gates

Multi-valued logic gates are demonstrated on solution-processed molybdenum disulfide (MoS₂) thin films. A simple chemical doping process is added to the conventional transistor fabrication procedure to locally increase the work function of MoS₂ by decreasing sulfur vacancies. The resulting device exhibits pseudo-heterojunctions comprising as-processed MoS₂ and chemically treated MoS₂ (c-MoS₂). The energy-band misalignment of MoS₂ and c-MoS₂ results in a sequential activation of the MoS₂ and c-MoS₂ channel areas under a gate voltage sweep, which generates a stable intermediate state for ternary operation. Current levels and turn-on voltages for each state can be tuned by modulating the device geometries, including the channel thickness and length. The optimized ternary transistors are incorporated to demonstrate various ternary logic gates, including the inverter, NMIN, and NMAX gates.

Structure and electronic properties of MoSe2/PtS2 van der Waals heterostructure

Structure and electronic properties of MoSe2/PtS2 van der Waals heterostructure and their dependence on the interlayer coupling, biaxial strain and external electric field are systematically investigated by using the first-principles...

Computational discovery of PtS2/GaSe van der Waals heterostructure for solar energy applications

2D van der Waals (vdW) heterostructures as potential materials for solar energy-related applications have been brought to the forefront for researchers. Here, by employing first-principles calculations, we proposed that the PtS₂/GaSe vdW heterostructure is a distinguished candidate for photocatalytic water splitting and solar cells. It is shown that the PtS₂/GaSe heterostructure exhibits high thermal stability with an indirect band gap of 1.81 eV. We further highlighted the strain induced type-V to type-II band alignment transitions and band gap variations in PtS₂/GaSe heterostructures. More importantly, the outstanding absorption coefficients in the visible light region and high carrier mobility further guarantee the photo energy conversion efficiency of PtS₂/GaSe heterostructures. Interestingly, the natural type-V band alignments of PtS₂/GaSe heterostructures are appropriate for the redox potential of water. On the other hand, the power conversion efficiency of ZnO/(PtS₂/GaSe heterostructure)/CIGS (copper indium gallium diselenide) solar cells can achieve ∼17.4%, which can be further optimized up to ∼18.5% by increasing the CIGS thickness. Our present study paves the way for facilitating the potential application of vdW heterostructures as a promising photocatalyst for water splitting as well as the buffer layer for solar cells.

Enhanced Electrical Performance of Monolayer MoS2 with Rare Earth Element Sm Doping

Rare earth (RE) element-doped two-dimensional (2D) transition metal dichalcogenides (TMDCs) with applications in luminescence and magnetics have received considerable attention in recent years. To date, the effect of RE element doping on the electronic properties of monolayer 2D-TMDCs remains unanswered due to challenges including the difficulty of achieving valid monolayer doping and introducing RE elements with distinct valence and atomic configurations. Herein, we report a unique strategy to grow the Sm-doped monolayer MoS₂ film by using an atmospheric pressure chemical vapor deposition method with the substrate face down on top of the growth source. A stable monolayer triangular Sm-doped MoS₂ was achieved. The threshold voltage of an Sm-doped MoS₂-based field effect transistor (FET) moved from -12 to 0 V due to the p-type character impurity state introduced by Sm ions in monolayer MoS₂. Additionally, the electrical performance of the monolayer MoS₂-based FET was improved by RE element Sm doping, including a 500% increase of the on/off current ratio and a 40% increase of the FET's mobility. The electronic property enhancement resulted from Sm doping MoS₂, which led internal lattice strain and changes in Fermi energy levels. These findings provide a general approach to synthesize RE element-doped monolayer 2D-TMDCs and to enrich their applications in electrical devices.

High-Throughput Growth of Wafer-Scale Monolayer Transition Metal Dichalcogenide via Vertical Ostwald Ripening

For practical device applications, monolayer transition metal dichalcogenide (TMD) films must meet key industry needs for batch processing, including the high-throughput, large-scale production of high-quality, spatially uniform materials, and reliable integration into devices. Here, high-throughput growth, completed in 12 min, of 6-inch wafer-scale monolayer MoS₂ and WS₂ is reported, which is directly compatible with scalable batch processing and device integration. Specifically, a pulsed metal-organic chemical vapor deposition process is developed, where periodic interruption of the precursor supply drives vertical Ostwald ripening, which prevents secondary nucleation despite high precursor concentrations. The as-grown TMD films show excellent spatial homogeneity and well-stitched grain boundaries, enabling facile transfer to various target substrates without degradation. Using these films, batch fabrication of high-performance field-effect transistor (FET) arrays in wafer-scale is demonstrated, and the FETs show remarkable uniformity. The high-throughput production and wafer-scale automatable transfer will facilitate the integration of TMDs into Si-complementary metal-oxide-semiconductor platforms.

Few-Layered MoS2 Field-Effect Transistors with a Vertical Channel of Sub-10 nm

Few-layered molybdenum disulfide (MoS₂) has demonstrated promising advantages for the integration of next-generation electronic devices. A vertical short-channel MoS₂ transistor with a channel length of sub-10 nm can be realized using mica as the insulated mesa and MoS₂ flake dry-transferred onto the mica as the channel. A near-perfect symmetrical and fully saturated output characteristic can be obtained for the positive or negative drain-source voltage. This result is attributed to an effective transformation of the drain-source electrode contact from Schottky contact to Ohmic contact via forming gas annealing. The vertical-channel MoS₂ transistor with a channel length of 8.7 nm exhibits excellent electrical characteristics, for example, a negligible hysteresis voltage of 60 mV, an extraordinarily small subthreshold swing of 73 mV/dec, a considerably weakened drain-induced barrier-lowering effect (100 mV/V), and the first-reported intrinsic delay time of 2.85 ps. Moreover, a logic inverter can be realized using the two vertical-channel MoS₂ transistors, with a high voltage gain of 33. Experimental results indicate that the developed method is a potential approach for fabricating MoS₂ transistors with an ultrashort channel and high performance, and consequently, manufacturing MoS₂-based integrated circuits.

Staircase-like transfer characteristics in multilayer MoS2 field-effect transistors

Layered semiconductors, such as MoS2, have attracted interest as channel materials for post-silicon and beyond-CMOS electronics. Much attention has been devoted to the monolayer limit, but the monolayer channel is not necessarily advantageous in terms of the performance of field-effect transistors (FETs). Therefore, it is important to investigate the characteristics of FETs that have multilayer channels. Here, we report the staircase-like transfer characteristics of FETs with exfoliated multilayer MoS2 flakes. Atomic force microscope characterizations reveal that the presence of thinner terraces at the edges of the flakes accompanies the staircase-like characteristics. The anomalous staircase-like characteristics are ascribable to a difference in threshold-voltage shift by charge transfer from surface adsorbates between the channel center and the thinner terrace at the edge. This study reveals the importance of the uniformity of channel thickness.

Schottky Barrier Variable Graphene/Multilayer-MoS2 Heterojunction Transistor Used to Overcome Short Channel Effects

A single-layer MoS₂ achieves excellent gate controllability within the nanoscale channel length of a field-effect transistor (FET) owing to an ultra-short screening length. However, multilayer MoS₂ (ML-MoS₂) is more vulnerable to short channel effects (SCEs) owing to its thickness and long screening length. We eliminated the SCEs in an ML-MoS₂ FET (thickness of 4-13 nm) at a channel length of sub-30 nm using a Schottky barrier (SB) variable graphene/ML-MoS₂ heterojunction. Although the band modulation in the ML-MoS₂ channel worsens with a decrease in the channel length, which is similar to the SCEs occurring in conventional FETs, the variable Fermi level (E_F) of a graphene electrode along the gate voltage allows control of the SB at the graphene/MoS₂ junction and backs up the current modulation through a variable SB. Electrical measurements and a theoretical band simulation demonstrate the efficient SB modulation of our graphene nanogap (GrNG) ML-MoS₂ FET with three distinct carrier transports along V_gs: a thermionic emission at a low SB, Fowler-Nordheim tunneling at a moderate SB, and direct tunneling at a high SB. Our GrNG FET shows an extremely high on-off current ratio of ∼10⁸, which is approximately three-orders of magnitude better than a previously reported metal nanogap (MeNG) FET and a self-aligned metal/graphene nanogap FET with a similar MoS₂ thickness. Our GrNG FET also exhibits a 100,000-times higher on-off ratio, 100-times lower subthreshold swing, and 10-times lower drain induced barrier.

Probing the Field-Effect Transistor with Monolayer MoS2 Prepared by APCVD

The two-dimensional materials can be used as the channel material of transistor, which can further decrease the size of transistor. In this paper, the molybdenum disulfide (MoS₂) is grown on the SiO₂/Si substrate by atmospheric pressure chemical vapor deposition (APCVD), and the MoS₂ is systematically characterized by the high-resolution optical microscopy, Raman spectroscopy, photoluminescence spectroscopy, and the field emission scanning electron microscopy, which can confirm that the MoS₂ is a monolayer. Then, the monolayer MoS₂ is selected as the channel material to complete the fabrication process of the back-gate field effect transistor (FET). Finally, the electrical characteristics of the monolayer MoS₂-based FET are tested to obtain the electrical performance. The switching ratio is 10³, the field effect mobility is about 0.86 cm²/Vs, the saturation current is 2.75 × 10^-7 A/μm, and the lowest gate leakage current is 10^-12 A. Besides, the monolayer MoS₂ can form the ohmic contact with the Ti/Au metal electrode. Therefore, the electrical performances of monolayer MoS₂-based FET are relatively poor, which requires the further optimization of the monolayer MoS₂ growth process. Meanwhile, it can provide the guidance for the application of monolayer MoS₂-based FETs in the future low-power optoelectronic integrated circuits.

Short channel monolayer MoS2 field-effect transistors defined by SiO x nanofins down to 20 nm

Layered semiconductors such as transition metal dichalcogenides (TMDs) with proper bandgaps complement the zero-bandgap drawback of graphene, demonstrating great potential for post-silicon complementary metal-oxide-semiconductor technology. Among the TMD family, molybdenum disulfide (MoS₂) is highly attractive for its atomically thin body, large bandgap and decent mechanical and chemical stability. However, current nanofabrication techniques hardly satisfy the requirements of short channel and convenient preparation simultaneously. Here, we demonstrate a simple and effective approach to fabricate short channel chemical vapor deposition (CVD) monolayer MoS₂ field-effect transistors (FET) with channel length down to 20 nm. Electron-beam lithography based on high-resolution negative-tone hydrogen silsesquioxane electron resists were applied to create 20 nm wide SiO _x lines, defining the short channel length. The 20 nm MoS₂ FET displays ON-sate current in excess of 100 μA μm^-1. The corresponding current ON/OFF ratio at room temperature reaches 10⁵. We carefully studied the short channel effect of as-fabricated MoS₂ FETs. Combining with the large-scale growth of CVD method, our results will pave a way for short channel device applications based on atomically thin two-dimensional semiconductors.

Employing a Bifunctional Molybdate Precursor To Grow the Highly Crystalline MoS2 for High-Performance Field-Effect Transistors

Growth of the large-sized and high-quality MoS₂ single crystals for high-performance low-power electronic applications is an important step to pursue. Despite the significant improvement made in minimizing extrinsic MoS₂ contact resistance based on interfacial engineering of the devices, the electron mobility of field-effect transistors (FETs) made of a synthetic monolayer MoS₂ is yet far below the expected theoretical values, implying that the MoS₂ crystal quality needs to be further improved. Here, we demonstrate the high-performance two-terminal MoS₂ FETs with room-temperature electron mobility up to ∼90 cm² V^-1 s^-1 based on the sulfurization growth of the bifunctional precursor, sodium molybdate dihydrate. This unique transition-metal precursor, serving as both the crystalline Mo source and seed promotor (sodium), could facilitate the lateral growth of the highly crystalline monolayer MoS₂ crystals (edge length up to ∼260 μm). Substrate surface treatment with oxygen plasma prior to the deposition of the Mo precursor is fundamental to increase the wettability between the Mo source and the substrate, promoting the thinning and coalescence of the source clusters during the growth of large-sized MoS₂ single crystals. The control of growth temperature is also an essential step to grow a strictly monolayer MoS₂ crystal. A proof-of-concept for thermoelectric device integration utilizing monolayer MoS₂ sheds light on its potential in low-voltage and self-powered electronics.

Ultimate limit in size and performance of WSe2 vertical diodes

Precise doping-profile engineering in van der Waals heterostructures is a key element to promote optimal device performance in various electrical and optical applications with two-dimensional layered materials. Here, we report tungsten diselenide- (WSe₂) based pure vertical diodes with atomically defined p-, i- and n-channel regions. Externally modulated p- and n-doped layers are respectively formed on the bottom and the top facets of WSe₂ single crystals by direct evaporations of high and low work-function metals platinum and gadolinium, thus forming atomically sharp p-i-n heterojunctions in the homogeneous WSe₂ layers. As the number of layers increases, charge transport through the vertical WSe₂ p-i-n heterojunctions is characterized by a series of quantum tunneling events; direct tunneling, Fowler-Nordheim tunneling, and Schottky emission tunneling. With optimally selected WSe₂ thickness, our vertical heterojunctions show superb diode characteristics of an unprecedentedly high current density and low turn-on voltages while maintaining good current rectification.

MoS2 Negative-Capacitance Field-Effect Transistors with Subthreshold Swing below the Physics Limit

The Boltzmann distribution of electrons induced fundamental barrier prevents subthreshold swing (SS) from less than 60 mV dec^-1 at room temperature, leading to high energy consumption of MOSFETs. Herein, it is demonstrated that an aggressive introduction of the negative capacitance (NC) effect of ferroelectrics can decisively break the fundamental limit governed by the "Boltzmann tyranny". Such MoS₂ negative-capacitance field-effect transistors (NC-FETs) with self-aligned top-gated geometry demonstrated here pull down the SS value to 42.5 mV dec^-1 , and simultaneously achieve superior performance of a transconductance of 45.5 μS μm and an on/off ratio of 4 × 10⁶ with channel length less than 100 nm. Furthermore, the inserted HfO₂ layer not only realizes a stable NC gate stack structure, but also prevents the ferroelectric P(VDF-TrFE) from fatigue with robust stability. Notably, the fabricated MoS₂ NC-FETs are distinctly different from traditional MOSFETs. The on-state current increases as the temperature decreases even down to 20 K, and the SS values exhibit nonlinear dependence with temperature due to the implementation of the ferroelectric gate stack. The NC-FETs enable fundamental applications through overcoming the Boltzmann limit in nanoelectronics and open up an avenue to low-power transistors needed for many exciting long-endurance portable consumer products.

Bias-switchable negative and positive photoconductivity in 2D FePS3 ultraviolet photodetectors

Metal-phosphorus-trichalcogenides (MPTs), represented by NiPS₃, FePS₃, etc, are newly developed 2D wide-bandgap semiconductors and have been proposed as excellent candidates for ultraviolet (UV) optoelectronics. In spite of having superior advantages for solar-blind UV photodetectors, including those free of surface trap states, being highly compatible with versatile integrations as well as having an appropriate band gap, to date relevant study is rare. In this work, the photoresponse characteristic of UV detectors based on few-layer FePS₃ has been comprehensively investigated. The responsivity of the photodetector, which is observed to be determined by bias gate voltage, may achieve as high as 171.6 mAW^-1 under the illumination of 254 nm weak light, which is comparable to most commercial UV detectors. Notably, both negative and positive photoconductivities exist in the FePS₃ photodetectors and can be controllably switched with bias voltage. The eminent and novel photoresponse property paves the way for the further development and practical use of 2D MPTs in high-performance UV photodetections.

Understanding contact gating in Schottky barrier transistors from 2D channels

In this article, a novel two-path model is proposed to quantitatively explain sub-threshold characteristics of back-gated Schottky barrier FETs (SB-FETs) from 2D channel materials. The model integrates the "conventional" model for SB-FETs with the phenomenon of contact gating - an effect that significantly affects the carrier injection from the source electrode in back-gated field effect transistors. The two-path model is validated by a careful comparison with experimental characteristics obtained from a large number of back-gated WSe₂ devices with various channel thicknesses. Our findings are believed to be of critical importance for the quantitative analysis of many three-terminal devices with ultrathin body channels.

Progress on Electronic and Optoelectronic Devices of 2D Layered Semiconducting Materials

2D layered semiconducting materials (2DLSMs) represent the thinnest semiconductors, holding many novel properties, such as the absence of surface dangling bonds, sizable band gaps, high flexibility, and ability of artificial assembly. With the prospect of bringing revolutionary opportunities for electronic and optoelectronic applications, 2DLSMs have prospered over the past twelve years. From materials preparation and property exploration to device applications, 2DLSMs have been extensively investigated and have achieved great progress. However, there are still great challenges for high-performance devices. In this review, we provide a brief overview on the recent breakthroughs in device optimization based on 2DLSMs, particularly focussing on three aspects: device configurations, basic properties of channel materials, and heterostructures. The effects from device configurations, i.e., electrical contacts, dielectric layers, channel length, and substrates, are discussed. After that, the affect of the basic properties of 2DLSMs on device performance is summarized, including crystal defects, crystal symmetry, doping, and thickness. Finally, we focus on heterostructures based on 2DLSMs. Through this review, we try to provide a guide to improve electronic and optoelectronic devices of 2DLSMs for achieving practical device applications in the future.

Thermal Light Emission from Monolayer MoS2

Layered transition metal dichalcogenide semiconductors, such as MoS₂ and WSe₂ , exhibit a range of fascinating properties and are being currently explored for a variety of electronic and optoelectronic devices. These properties include a low thermal conductivity and a large Seebeck coefficient, which make them promising for thermoelectric applications. Moreover, transition metal dichalcogenides undergo an indirect-to-direct bandgap transition when thinned down in thickness, leading to strong excitonic photo- and electroluminescence in monolayers. Here, it is demonstrated that a MoS₂ monolayer sheet, freely suspended in vacuum over a distance of 150 nm, emits visible light as a result of Joule heating. Due to the poor transfer of heat to the contact electrodes, as well as the suppressed heat dissipation through the underlying substrate, the electron temperature can reach ≈1500-1600 K. The resulting narrow-band light emission from thermally populated exciton states is spatially located to an only ≈50 nm wide region in the center of the device and goes along with a negative differential electrical conductance of the channel.

A two-dimensional semiconductor transistor with boosted gate control and sensing ability

Transistors with exfoliated two-dimensional (2D) materials on a SiO₂/Si substrate have been applied and have been proven effective in a wide range of applications, such as circuits, memory, photodetectors, gas sensors, optical modulators, valleytronics, and spintronics. However, these devices usually suffer from limited gate control because of the thick SiO₂ gate dielectric and the lack of reliable transfer method. We introduce a new back-gate transistor scheme fabricated on a novel Al₂O₃/ITO (indium tin oxide)/SiO₂/Si "stack" substrate, which was engineered with distinguishable optical identification of exfoliated 2D materials. High-quality exfoliated 2D materials could be easily obtained and recognized on this stack. Two typical 2D materials, MoS₂ and ReS₂, were implemented to demonstrate the enhancement of gate controllability. Both transistors show excellent electrical characteristics, including steep subthreshold swing (62 mV dec^-1 for MoS₂ and 83 mV dec^-1 for ReS₂), high mobility (61.79 cm² V^-1 s^-1 for MoS₂ and 7.32 cm² V^-1 s^-1 for ReS₂), large on/off ratio (~10⁷), and reasonable working gate bias (below 3 V). Moreover, MoS₂ and ReS₂ photodetectors fabricated on the basis of the scheme have impressively leading photoresponsivities of 4000 and 760 A W^-1 in the depletion area, respectively, and both have exceeded 10⁶ A W^-1 in the accumulation area, which is the best ever obtained. This opens up a suite of applications of this novel platform in 2D materials research with increasing needs of enhanced gate control.

Layer Dependence and Light Tuning Surface Potential of 2D MoS2 on Various Substrates

Here surface potential of chemical vapor deposition (CVD) grown 2D MoS₂ with various layers is reported, and the effect of adherent substrate and light illumination on surface potential of monolayer MoS₂ are investigated. The surface potential of MoS₂ on Si/SiO₂ substrate decreases from 4.93 to 4.84 eV with the increase in the number of layer from 1 to 4 or more. Especially, the surface potentials of monolayer MoS₂ are strongly dependent on its adherent substrate, which are determined to be 4.55, 4.88, 4.93, 5.10, and 5.50 eV on Ag, graphene, Si/SiO₂ , Au, and Pt substrates, respectively. Light irradiation is introduced to tuning the surface potential of monolayer MoS₂ , with the increase in light intensity, the surface potential of MoS₂ on Si/SiO₂ substrate decreases from 4.93 to 4.74 eV, while increases from 5.50 to 5.56 eV on Pt substrate. The I-V curves on vertical of monolayer MoS₂ /Pt heterojunction show the decrease in current with the increase of light intensity, and Schottky barrier height at MoS₂ /Pt junctions increases from 0.302 to 0.342 eV. The changed surface potential can be explained by trapped charges on surface, photoinduced carriers, charge transfer, and local electric field.

Sub-10 nm Nanopattern Architecture for 2D Material Field-Effect Transistors

Two-dimensional materials (2DMs) are competitive candidates in replacing or supplementing conventional semiconductors owing to their atomically uniform thickness. However, current conventional micro/nanofabrication technologies realize hardly ultrashort channel and integration, especially for sub-10 nm. Meanwhile, experimental device performance associated with the scaling of dimension needs to be investigated, due to the short channel effects. Here, we show a novel and universal technological method to fabricate sub-10 nm gaps with sharp edges and steep sidewalls. The realization of sub-10 nm gaps derives from a corrosion crack along the cleavage plane of Bi₂O₃. By this method, ultrathin body field-effect transistors (FETs), consisting of 8.2 nm channel length, 6 nm high-k dielectric, and 0.7 nm monolayer MoS₂, exhibit no obvious short channel effects. The corresponding current on/off ratio and subthreshold swing reaches to 10⁶ and 140 mV/dec, respectively. Moreover, integrated circuits with sub-10 nm channel are capable of operating as digital inverters with high voltage gain. The results suggest our technological method can be used to fabricate the ultrashort channel nanopatterns, build the experimental groundwork for 2DMs FETs with sub-10 nm channel length and 2DMs integrated circuits, and offer new potential opportunities for large-scale device constructions and applications.

Effect of Al2O3 Deposition on Performance of Top-Gated Monolayer MoS2-Based Field Effect Transistor

Deposition of high-k dielectrics on two-dimensional MoS₂ is an important process for successful application of the transition-metal dichalcogenides in electronic devices. Here, we show the effect of H₂O reactant exposure on monolayer (1L) MoS₂ during atomic layer deposition (ALD) of Al₂O₃. The results showed that the ALD-Al₂O₃ caused degradation of the performance of 1L MoS₂ field effect transistors (FETs) owing to the formation of Mo-O bonding and trapping of H₂O molecules at the Al₂O₃/MoS₂ interface. Furthermore, we demonstrated that reduced duration of exposure to H₂O reactant and postdeposition annealing were essential to the enhancement of the performance of top-gated 1L MoS₂ FETs. The mobility and on/off current ratios were increased by factors of approximately 40 and 10³, respectively, with reduced duration of exposure to H₂O reactant and with postdeposition annealing.

Covalent Nitrogen Doping and Compressive Strain in MoS2 by Remote N2 Plasma Exposure

Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homojunctions. Herein, we propose an effective strategy for doping of MoS2 with nitrogen through a remote N2 plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2 plasma exposure using in situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS2, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism. Furthermore, the electrical characterization demonstrates that p-type doping of MoS2 is achieved by nitrogen doping, which is in agreement with theoretical predictions. Notably, we found that the presence of nitrogen can induce compressive strain in the MoS2 structure, which represents the first evidence of strain induced by substitutional doping in a transition metal dichalcogenide material. Finally, our first principle calculations support the experimental demonstration of such strain, and a correlation between nitrogen doping concentration and compressive strain in MoS2 is elucidated.

Carrier Polarity Control in α-MoTe2 Schottky Junctions Based on Weak Fermi-Level Pinning

The polarity of the charge carriers injected through Schottky junctions of α-phase molybdenum ditelluride (α-MoTe2) and various metals was characterized. We found that the Fermi-level pinning in the metal/α-MoTe2 Schottky junction is so weak that the polarity of the carriers (electron or hole) injected from the junction can be controlled by the work function of the metals, in contrast to other transition metal dichalcogenides such as MoS2. From the estimation of the Schottky barrier heights, we obtained p-type carrier (hole) injection from a Pt/α-MoTe2 junction with a Schottky barrier height of 40 meV at the valence band edge. n-Type carrier (electron) injection from Ti/α-MoTe2 and Ni/α-MoTe2 junctions was also observed with Schottky barrier heights of 50 and 100 meV, respectively, at the conduction band edge. In addition, enhanced ambipolarity was demonstrated in a Pt-Ti hybrid contact with a unique structure specially designed for polarity-reversible transistors, in which Pt and Ti electrodes were placed in parallel for injecting both electrons and holes.

Toward High-Performance Top-Gate Ultrathin HfS2 Field-Effect Transistors by Interface Engineering

Top-gate HfS2 field-effect transistors (FETs) with 5 nm HfO2 as dielectrics are successfully demonstrated, with on/off ratio of 10(5) and subthreshold swing of 95 mV dec(-1) . Moreover, due to the self-functionalization of HfS2 , uniform and ultrathin HfO2 film free of pinhole-like defects could be deposited on HfS2 , which is dramatically different from other transition metal dichalcogenide FETs.

Analysing black phosphorus transistors using an analytic Schottky barrier MOSFET model

Owing to the difficulties associated with substitutional doping of low-dimensional nanomaterials, most field-effect transistors built from carbon nanotubes, two-dimensional crystals and other low-dimensional channels are Schottky barrier MOSFETs (metal-oxide-semiconductor field-effect transistors). The transmission through a Schottky barrier-MOSFET is dominated by the gate-dependent transmission through the Schottky barriers at the metal-to-channel interfaces. This makes the use of conventional transistor models highly inappropriate and has lead researchers in the past frequently to extract incorrect intrinsic properties, for example, mobility, for many novel nanomaterials. Here we propose a simple modelling approach to quantitatively describe the transfer characteristics of Schottky barrier-MOSFETs from ultra-thin body materials accurately in the device off-state. In particular, after validating the model through the analysis of a set of ultra-thin silicon field-effect transistor data, we have successfully applied our approach to extract Schottky barrier heights for electrons and holes in black phosphorus devices for a large range of body thicknesses.

Conventional models of transistors are not applicable to devices made from nanomaterials because their operation is dominated by gate-dependent transmission through a Schottkybarrier. Here, the authors develop an analytical model and compare it to data taken from ultrathin silicon field-effect transistors.

Conduction Mechanisms in CVD-Grown Monolayer MoS2 Transistors: From Variable-Range Hopping to Velocity Saturation

We fabricate transistors from chemical vapor deposition-grown monolayer MoS2 crystals and demonstrate excellent current saturation at large drain voltages (Vd). The low-field characteristics of these devices indicate that the electron mobility is likely limited by scattering from charged impurities. The current-voltage characteristics exhibit variable range hopping at low Vd and evidence of velocity saturation at higher Vd. This work confirms the excellent potential of MoS2 as a possible channel-replacement material and highlights the role of multiple transport phenomena in governing its transistor action.

Synthesis, properties and applications of 2D non-graphene materials

As an emerging class of new materials, two-dimensional (2D) non-graphene materials, including layered and non-layered, and their heterostructures are currently attracting increasing interest due to their promising applications in electronics, optoelectronics and clean energy. In contrast to traditional semiconductors, such as Si, Ge and III-V group materials, 2D materials show significant merits of ultrathin thickness, very high surface-to-volume ratio, and high compatibility with flexible devices. Owing to these unique properties, while scaling down to ultrathin thickness, devices based on these materials as well as artificially synthetic heterostructures exhibit novel and surprising functions and performances. In this review, we aim to provide a summary on the state-of-the-art research activities on 2D non-graphene materials. The scope of the review will cover the preparation of layered and non-layered 2D materials, construction of 2D vertical van der Waals and lateral ultrathin heterostructures, and especially focus on the applications in electronics, optoelectronics and clean energy. Moreover, the review is concluded with some perspectives on the future developments in this field.