Ambipolar MoS2 Transistors by Nanoscale Tailoring of Schottky Barrier Using Oxygen Plasma Functionalization

Defects and Defect Engineering of Two-Dimensional Transition Metal Dichalcogenide (2D TMDC) Materials

As layered materials, transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) materials. Interestingly, the characteristics of these materials are transformed from bulk to monolayer. The atomically thin TMDC materials can be a good alternative to group III-V and graphene because of their emerging tunable electrical, optical, and magnetic properties. Although 2D monolayers from natural TMDC materials exhibit the purest form, they have intrinsic defects that limit their application. However, the synthesis of TMDC materials using the existing fabrication tools and techniques is also not immune to defects. Additionally, it is difficult to synthesize wafer-scale TMDC materials for a multitude of factors influencing grain growth mechanisms. While defect engineering techniques may reduce the percentage of defects, the available methods have constraints for healing defects at the desired level. Thus, this holistic review of 2D TMDC materials encapsulates the fundamental structure of TMDC materials, including different types of defects, named zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D). Moreover, the existing defect engineering methods that relate to both formation of and reduction in defects have been discussed. Finally, an attempt has been made to correlate the impact of defects and the properties of these TMDC materials.

Recent progress in plasma modification of 2D metal chalcogenides for electronic devices and optoelectronic devices

Two-dimensional metal chalcogenides (2D MCs) present a great opportunity for overcoming the size limitation of traditional silicon-based complementary metal-oxide-semiconductor (CMOS) devices. Controllable modulation compatible with CMOS processes is essential for the improvement of performance and the large-scale applications of 2D MCs. In this review, we summarize the recent progress in plasma modification of 2D MCs, including substitutional doping, defect engineering, surface charge transfer, interlayer coupling modulation, thickness control, and nano-array pattern etching in the fields of electronic devices and optoelectronic devices. Finally, challenges and outlooks for plasma modulation of 2D MCs are presented to offer valuable references for future studies.

Analysis of p-Type Doping in Graphene Induced by Monolayer-Oxidized TMDs

Doping is one of the most difficult technological challenges for realizing reliable two-dimensional (2D) material-based semiconductor devices, arising from their ultrathinness. Here, we systematically investigate the impact of different types of nonstoichiometric solid MOx (M are W or Mo) dopants obtained by oxidizing transition metal dichalcogenides (TMDs: WSe2 or MoS2) formed on graphene FETs, which results in p-type doping along with disorders. From the results obtained in this study, we were able to suggest an analytical technique to optimize the optimal UV-ozone (UVO) treatment to achieve high p-type doping concentration in graphene FETs (∼2.5 × 1013 cm-2 in this study) without generating defects, mainly by analyzing the time dependency of D and D' peaks measured by Raman spectroscopy. Furthermore, an analysis of the structure of graphene sheets using TEM indicates that WOx plays a better protective role in graphene, compared to MoOx, suggesting that WOx is more effective for preventing the degradation of graphene during UVO treatment. To enhance the practical application aspect of our work, we have fabricated a graphene photodetector by selectively doping the graphene through oxidized TMDs, creating a p-n junction, which resulted in improved photoresponsivity compared to the intrinsic graphene device. Our results offer a practical guideline for the utilization of surface charge transfer doping of graphene toward CMOS applications.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

Plasma Treatment for Achieving Oxygen Substitution in Layered MoS2 and the Room-Temperature Mid-Infrared (10 μm) Photoresponse

Highly sensitive photodetectors in the mid-infrared (MIR, 3-15 μm) are highly desired in a growing number of applications. However, only a handful of narrow-band-gap semiconductors are suitable for this purpose, most of which require cryogenic cooling to increase the signal-to-noise ratio. The realization of high-performance MIR photodetectors operating at room temperature remains a challenge. Herein, we report on plasma-treated few-layer MoS2 for room-temperature MIR (10 μm) photodetection. Oxygen plasma treatment, which is a mature microfabrication process, is employed. The ion kinetic energy of oxygen plasma is adjusted to 70-130 eV. A photoresponsivity of 0.042 mA/W and a detectivity of 1.57 × 107 Jones are obtained under MIR light (10 μm) illumination with an average power density of 114.6 mW/cm2. The photoresponse is attributed to the introduction of electronic states in the band gap of MoS2 through oxygen substitution. A graphene/plasma-treated MoS2/graphene device is further demonstrated to shorten the active channel while maintaining the illumination area. The photoresponsivity and detectivity are largely boosted to 1.8 A/W and 2.64 × 109 Jones, respectively. The excellent detective performance of the graphene/plasma-treated MoS2/graphene device is further demonstrated in single-detector MIR (10 μm) scanning imaging. This work offers a facile approach to constructing integrated MoS2-based MIR photodetectors.

Isomer Discrimination via Defect Engineering in Monolayer MoS2

The all-surface nature of two-dimensional (2D) materials renders them highly sensitive to environmental changes, enabling the on-demand tailoring of their physical properties. Transition metal dichalcogenides, such as 2H molybdenum disulfide (MoS2), can be used as a sensory material capable of discriminating molecules possessing a similar structure with a high sensitivity. Among them, the identification of isomers represents an unexplored and challenging case. Here, we demonstrate that chemical functionalization of defect-engineered monolayer MoS2 enables isomer discrimination via a field-effect transistor readout. A multiscale characterization comprising X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, and electrical measurement corroborated by theoretical calculations revealed that monolayer MoS2 exhibits exceptional sensitivity to the differences in the dipolar nature of molecules arising from their chemical structure such as the one in difluorobenzenethiol isomers, allowing their precise recognition. Our findings underscore the potential of 2D materials for molecular discrimination purposes, in particular for the identification of complex isomers.

AFM Study of Roughness Development during ToF-SIMS Depth Profiling of Multilayers with a Cs+ Ion Beam in a H2 Atmosphere

The influence of H₂ flooding on the development of surface roughness during time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling was studied to evaluate the different aspects of a H₂ atmosphere in comparison to an ultrahigh vacuum (UHV) environment. Multilayer samples, consisting of different combinations of metal, metal oxide, and alloy layers of different elements, were bombarded with 1 and 2 keV Cs⁺ ion beams in UHV and a H₂ atmosphere of 7 × 10^-7 mbar. The surface roughness S_a was measured with atomic force microscopy (AFM) on the initial surface and in the craters formed while sputtering, either in the middle of the layers or at the interfaces. We found that the roughness after Cs⁺ sputtering depends on the chemical composition/structure of the individual layers, and it increases with the sputtering depth. However, the increase in the roughness was, in specific cases, approximately a few tens of percent lower when sputtering in the H₂ atmosphere compared to the UHV. In the other cases, the average surface roughness was generally still lower when H₂ flooding was applied, but the differences were statistically insignificant. Additionally, we observed that for the initially rough surfaces with an S_a of about 5 nm, sputtering with the 1 keV Cs⁺ beam might have a smoothing effect, thereby reducing the initial roughness. Our observations also indicate that Cs⁺ sputtering with ion energies of 1 and 2 keV has a similar effect on roughness development, except for the cases with initially very smooth samples. The results show the beneficial effect of H₂ flooding on surface roughness development during the ToF-SIMS depth profiling in addition to a reduction of the matrix effect and an improved identification of thin layers.

Real-time numerical system convertor via two-dimensional WS2-based memristive device

The intriguing properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs) enable the exploration of new electronic device architectures, particularly the emerging memristive devices for in-memory computing applications. Implementation of arithmetic logic operations taking advantage of the non-linear characteristics of memristor can significantly improve the energy efficiency and simplify the complexity of peripheral circuits. Herein, we demonstrate an arithmetic logic unit function using a lateral volatile memristor based on layered 2D tungsten disulfide (WS₂) materials and some combinational logic circuits. Removable oxygen ions were introduced into WS₂ materials through oxygen plasma treatment process. The resistive switching of the memristive device caused by the thermophoresis-assisted oxygen ions migration has also been revealed. Based on the characteristics of excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), and spike rate dependent plasticity (SRDP), a real-time numerical system convertor was successfully accomplished, which is a significant computing function of arithmetic logic unit. This work paves a new way for developing 2D memristive devices for future arithmetic logic applications.

Fermi-Level Pinning-Free WSe2 Transistors via 2D Van der Waals Metal Contacts and Their Circuits

Precise control over the polarity of transistors is a key necessity for the construction of complementary metal-oxide-semiconductor circuits. However, the polarity control of 2D transistors remains a challenge because of the lack of a high-work-function electrode that completely eliminates Fermi-level pinning at metal-semiconductor interfaces. Here, a creation of clean van der Waals contacts is demonstrated, wherein a metallic 2D material, chlorine-doped SnSe₂ (Cl-SnSe₂ ), is used as the high-work-function contact, providing an interface that is free of defects and Fermi-level pinning. Such clean contacts made from Cl-SnSe₂ can pose nearly ideal Schottky barrier heights, following the Schottky-Mott limit and thus permitting polarity-controllable transistors. With the integration of Cl-SnSe₂ as contacts, WSe₂ transistors exhibit pronounced p-type characteristics, which are distinctly different from those of the devices with evaporated metal contacts, where n-type transport is observed. Finally, this ability to control the polarity enables the fabrication of functional logic gates and circuits, including inverter, NAND, and NOR.

Asymmetric Chemical Functionalization of Top-Contact Electrodes: Tuning the Charge Injection for High-Performance MoS2 Field-Effect Transistors and Schottky Diodes

The fabrication of high-performance (opto-)electronic devices based on 2D channel materials requires the optimization of the charge injection at electrode-semiconductor interfaces. While chemical functionalization with chemisorbed self-assembled monolayers has been extensively exploited to adjust the work function of metallic electrodes in bottom-contact devices, such a strategy has not been demonstrated for the top-contact configuration, despite the latter being known to offer enhanced charge-injection characteristics. Here, a novel contact engineering method is developed to functionalize gold electrodes in top-contact field-effect transistors (FETs) via the transfer of chemically pre-modified electrodes. The source and drain Au electrodes of the molybdenum disulfide (MoS₂ ) FETs are functionalized with thiolated molecules possessing different dipole moments. While the modification of the electrodes with electron-donating molecules yields a marked improvement of device performance, the asymmetric functionalization of the source and drain electrodes with different molecules with opposed dipole moment enables the fabrication of a high-performance Schottky diode with a rectification ratio of ≈10³ . This unprecedented strategy to tune the charge injection in top-contact MoS₂ FETs is of general applicability for the fabrication of high-performance (opto-)electronic devices, in which asymmetric charge injection is required, enabling tailoring of the device characteristics on demand.

Multiscale Investigation of the Structural, Electrical and Photoluminescence Properties of MoS2 Obtained by MoO3 Sulfurization

In this paper, we report a multiscale investigation of the compositional, morphological, structural, electrical, and optical emission properties of 2H-MoS2 obtained by sulfurization at 800 °C of very thin MoO3 films (with thickness ranging from ~2.8 nm to ~4.2 nm) on a SiO2/Si substrate. XPS analyses confirmed that the sulfurization was very effective in the reduction of the oxide to MoS2, with only a small percentage of residual MoO3 present in the final film. High-resolution TEM/STEM analyses revealed the formation of few (i.e., 2-3 layers) of MoS2 nearly aligned with the SiO2 surface in the case of the thinnest (~2.8 nm) MoO3 film, whereas multilayers of MoS2 partially standing up with respect to the substrate were observed for the ~4.2 nm one. Such different configurations indicate the prevalence of different mechanisms (i.e., vapour-solid surface reaction or S diffusion within the film) as a function of the thickness. The uniform thickness distribution of the few-layer and multilayer MoS2 was confirmed by Raman mapping. Furthermore, the correlative plot of the characteristic A1g-E2g Raman modes revealed a compressive strain (ε ≈ -0.78 ± 0.18%) and the coexistence of n- and p-type doped areas in the few-layer MoS2 on SiO2, where the p-type doping is probably due to the presence of residual MoO3. Nanoscale resolution current mapping by C-AFM showed local inhomogeneities in the conductivity of the few-layer MoS2, which are well correlated to the lateral changes in the strain detected by Raman. Finally, characteristic spectroscopic signatures of the defects/disorder in MoS2 films produced by sulfurization were identified by a comparative analysis of Raman and photoluminescence (PL) spectra with CVD grown MoS2 flakes.

Bidirectional doping of two-dimensional thin-layer transition metal dichalcogenides using soft ammonia plasma

Because of suitable band gap and high mobility, two-dimensional transition metal dichalcogenide (TMD) materials are promising in future microelectronic devices. However, controllable p-type and n-type doping of TMDs is still a challenge. Herein, we develop a soft plasma doping concept and demonstrate both n-type and p-type doping for TMDs including MoS₂ and WS₂ through adjusting the plasma working parameters. In particular, p-type doping of MoS₂ can be realized when the radio frequency (RF) power is relatively small and the processing time is short: the off-state current increases from ∼10^-10 A to ∼10^-8 A, the threshold voltage is positively shifted from -26.2 V to 8.3 V, and the mobility increases from 7.05 cm² V^-1 s^-1 to 16.52 cm² V^-1 s^-1. Under a relatively large RF power and long processing time, n-type doping was realized for MoS₂: the threshold voltage was negatively shifted from 6.8 V to -13.3 V and the mobility is reduced from 10.32 cm² V^-1 s^-1 to 3.2 cm² V^-1 s^-1. For the former, suitable plasma treatment can promote the substitution of N elements for S vacancies and lead to p-type doping, thus reducing the defect density and increasing the mobility value. For the latter, due to excessive plasma treatment, more S vacancies will be produced, leading to heavier n-type doping as well as a decrease in mobility. We confirm the results by systematically analyzing the optical, compositional, thickness and structural characteristics of the samples before and after such soft plasma treatments via Raman, photoluminescence (PL), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. Due to its nondestructive and expandable nature and compatibility with the current microelectronics industry, this potentially generic method may be used as a reliable technology for the development of diverse and functional TMD-based devices.

Two-Dimensional Lateral Heterostructures Made by Selective Reaction on a Patterned Monolayer MoS2 Matrix

Two-dimensional (2D) heterostructures have attracted widespread attention for their promising prospects in the fields of electronics and optoelectronics. However, in order to truly realize 2D-material-based integrated circuits, precisely controllable fabrication of 2D heterostructures is crucial and urgently needed. Here, we demonstrate an ex situ growth method of MoSe₂/MoS₂ lateral heterostructures by selective selenization of a laser-scanned, ultrathin oxidized region (MoO_x) on a monolayer MoS₂ matrix. In our method, monolayer MoS₂ is scanned by a laser with a pre-designed pattern, where the laser-scanned MoS₂ is totally oxidized into MoO_x. The oxidized region is then selenized in a furnace, while the unoxidized MoS₂ region remains unchanged, delivering a MoSe₂/MoS₂ heterostructure. Unlike in situ laser direct growth methods, our method separates the laser-scanned process from the selenization process, which avoids the long time of point-by-point selenization of MoS₂ by laser, making the efficiency of the synthesis greatly improved. The formation process of the heterostructure is studied by Raman spectroscopy and Auger electron spectroscopy. This simple and controllable approach to lateral heterostructures with desired patterns paves the way for fast and mass integration of devices based on 2D heterostructures.

Thickness Trends of Electron and Hole Conduction and Contact Carrier Injection in Surface Charge Transfer Doped 2D Field Effect Transistors

One of the main limiting factors in the performance of devices based on two-dimensional (2D) materials is Fermi level pinning at the contacts, which creates Schottky barriers (SBs) that increase contact resistance and, for most transition metal dichalcogenides (TMDs), limit hole conduction. A promising method to mitigate these problems is surface charge transfer doping (SCTD), which places fixed charge at the surface of the material and thins the SBs by locally shifting the energy bands. We use a mild O₂ plasma to convert the top few layers of a given TMD into a substoichiometric oxide that serves as a p-type SCTD layer. A comprehensive experimental study, backed by TCAD simulations, involving MoS₂, MoSe₂, MoTe₂, WS₂, and WSe₂ flakes of various thicknesses exposed to different plasma times is used to investigate the underlying mechanisms responsible for SCTD. The surface charge at the top of the channel and the gate-modulated surface potential at the bottom are found to have competing effects on the channel potential, which results in a decrease in the doping-induced threshold shift and an increase in minimum OFF state current with increasing thickness. Additionally, an undoped channel region is shown to mitigate carrier injection issues in sufficiently thin flakes. Notably, the band movements underlying the SCTD effects are independent of the particular semiconductor material, SCTD strategy, and doping polarity. Consequently, our findings provide critical insights for the design of high-performance transistors for a wide range of materials and SCTD mechanisms including TMD devices with strong hole conduction.

Tunable Optical Transition in 2H-MoS2 via Direct Electrochemical Engineering of Vacancy Defects and Surface S-C Bonds

Although surface engineering has been regarded to be a great approach to modulate the optical and electrical properties of nanomaterials, the spontaneous covalent functionalization on semiconducting 2H-MoS₂ is a notoriously difficult process, while several reactions have been performed on metallic 1T-MoS₂. This limitation in functionalization is attributed to the difficulty of electron transfer from 2H-TMD to the reacting molecules due to its semiconducting property and neutral charge state. Unfortunately, this is an all too important prerequisite step toward creating chemically reactive radical species for surface functionalization reactions. Herein, an electrochemical approach was developed for facilitating direct surface functionalization of 2H-MoS₂ with 4-bromobenzene diazonium tetraborate (4-BBDT). Successful functionalization was characterized using various microscopic and spectroscopic analyses. During the course of investigating the change of optical transition seen for modified 2H-MoS₂ using photoluminescence measurement combined with theoretical calculations, our study uncovered that the controlling S-C bond and sulfur vacancy generation could tune the electronic structure of functionalized 2H-MoS₂.

Ohmic contact engineering in few-layer black phosphorus: approaching the quantum limit

Achieving good quality Ohmic contacts to van der Waals materials is a challenge, since at the interface between metal and van der Waals material different conditions can occur, ranging from the presence of a large energy barrier between the two materials to the metallization of the layered material below the contacts. In black phosphorus (bP), a further challenge is its high reactivity to oxygen and moisture, since the presence of uncontrolled oxidation can substantially change the behavior of the contacts. Here we study three of the most commonly used metals as contacts to bP, chromium, titanium, and nickel, and investigate their influence on contact resistance against the variability between different flakes and different samples. We investigate the gate dependence of the current-voltage characteristics of field-effect transistors fabricated with these metals on bP, observing good linearity in the accumulation regime for all metals investigated. Using the transfer length method, from an analysis of ten devices, both at room temperature and at low temperature, Ni results to provide the lowest contact resistance to bP and minimum scattering between different devices. Moreover, we observe that our best devices approach the quantum limit for contact resistance both for Ni and for Ti contacts.

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.

Intrinsic defects of GaSe

GaSe is a layered semiconductor with an optical band gap tunable by the number of layers in a thin film. This is promising for application in micro/optoelectronics and photovoltaics. However, for that, knowledge about the intrinsic defects are needed, since they may influence device behavior. Here we present a comprehensive study of intrinsic point defects in both bulk and monolayer (ML) GaSe, using an optimized hybrid functional which reproduces the band gap and is Koopmans' compliant. Formation energies and charge transition levels are calculated, the latter in good agreement with available experimental data. We find that the only intrinsic donor is the interlayer gallium interstitial, which is absent in the case of the ML. The vacancies are acceptors, the selenium interstitial is electrically inactive, and small intrinsic defect complexes have formation energies too high to play a role in the electronic properties of samples grown under quasi-equilibrium conditions. Bulk GaSe is well compensated by the intrinsic defects, and is an ideal substrate. The ML is intrinsically p-type, and p-type doping cannot be compensated either. The opening of the band gap changes the defect physics considerably with respect to the bulk.

Conductive Atomic Force Microscopy of Semiconducting Transition Metal Dichalcogenides and Heterostructures

Semiconducting transition metal dichalcogenides (TMDs) are promising materials for future electronic and optoelectronic applications. However, their electronic properties are strongly affected by peculiar nanoscale defects/inhomogeneities (point or complex defects, thickness fluctuations, grain boundaries, etc.), which are intrinsic of these materials or introduced during device fabrication processes. This paper reviews recent applications of conductive atomic force microscopy (C-AFM) to the investigation of nanoscale transport properties in TMDs, discussing the implications of the local phenomena in the overall behavior of TMD-based devices. Nanoscale resolution current spectroscopy and mapping by C-AFM provided information on the Schottky barrier uniformity and shed light on the mechanisms responsible for the Fermi level pinning commonly observed at metal/TMD interfaces. Methods for nanoscale tailoring of the Schottky barrier in MoS₂ for the realization of ambipolar transistors are also illustrated. Experiments on local conductivity mapping in monolayer MoS₂ grown by chemical vapor deposition (CVD) on SiO₂ substrates are discussed, providing a direct evidence of the resistance associated to the grain boundaries (GBs) between MoS₂ domains. Finally, C-AFM provided an insight into the current transport phenomena in TMD-based heterostructures, including lateral heterojunctions observed within Mo_xW_1-xSe₂ alloys, and vertical heterostructures made by van der Waals stacking of different TMDs (e.g., MoS₂/WSe₂) or by CVD growth of TMDs on bulk semiconductors.

Low-damaged p-type doping of MoS2 using direct nitrogen plasma modulated by toroidal-magnetic-field

Low damaged doping of two-dimensional (2D) materials proves to be a significant obstacle in realizing fundamental devices such as p-n junction diodes and transistors due to its atom layer thickness. In this work, the defect formation energy and p-type conduction behavior of nitrogen plasma doping are investigated by first principle calculation. Low damaged substitutional p-type doping in MoS₂ using low energy nitrogen plasma composed of N⁺ and N₂ ⁺ is achieved by a novel toroidal magnetic field (TMF). The TMF helps to raise the concentration of N₂ ⁺ ions at low RF power condition. The electrical characteristics of double-layer MoS₂ field-effect transistors (FETs) clearly show an efficient p-type doping behavior. Atomic force microscope is applied to verify the slight damage in MoS₂. X-ray photoelectron spectroscopy, photoluminescence and Raman spectroscopy confirm the effective p-type doping characteristic with weak damage. These findings provide a low damage technology for efficient carrier modulation of MoS₂ and other homogeneous TMDC materials, which overcomes barriers in developing 2D electronic and optoelectronic devices.

A tunable floating-base bipolar transistor based on a 2D material homojunction realized using a solid ionic dielectric material

Floating-base bipolar transistors are widely used semiconductor devices because they could both amplify signal current and suppress noise. Employing two-dimensional (2D) materials of ultrahigh photoelectric properties could further improve the device performance. Due to the difficulty in doping, homojunctions are usually not realizable for many 2D materials. Instead, a heterojunction of various 2D materials of different Fermi levels is usually needed. However, the material interface of a heterojunction deteriorates device performance and makes the fabrication process difficult. Here, the doping difficulties have been solved by utilizing a solid ionic dielectric material (LiTaO3) and a floating-base bipolar transistor based on a 2D material (monolayer MoS2 here) homojunction is realized. The transistor shows tunable ambipolar transport characteristics. Particularly, under illumination, the amplification coefficient of a phototransistor can be optimized by changing the gate voltage. The optimized photoresponsivity of the device could reach up to 7.9 A W-1 with an ultrahigh detectivity of 3.39 × 1011 Jones. The overall fabrication processing is compatible to conventional processing. This design can effectively extend the application of 2D materials.

Edge Contact for Carrier Injection and Transport in MoS2 Field-Effect Transistors

The contact properties of van der Waals layered semiconducting materials are not adequately understood, particularly for edge contact. Edge contact is extremely helpful in the case of graphene, for producing efficient contacts to vertical heterostructures, and for improving the contact resistance through strong covalent bonding. Herein, we report on edge contacts to MoS₂ of various thicknesses. The carrier-type conversion is robustly controlled by changing the flake thickness and metal work functions. Regarding the ambipolar behavior, we suggest that the carrier injection is segregated in a relatively thick MoS₂ channel; that is, electrons are in the uppermost layers, and holes are in the inner layers. Calculations reveal that the strength of the Fermi-level pinning (FLP) varies layer-by-layer, owing to the inhomogeneous carrier concentration, and particularly, there is negligible FLP in the inner layer, supporting the hole injection. The contact resistance is large despite the significantly reduced contact resistivity normalized by the contact area, which is attributed to the current-crowding effect arising from the narrow contact area.

Improved Sensitivity in Schottky Contacted Two-Dimensional MoS2 Gas Sensor

Two-dimensional (2D) transition-metal dichalcogenides have attracted significant attention as gas-sensing materials owing to their superior responsivity at room temperature and their possible application as flexible electronic devices. Especially, reliable responsivity and selectivity for various environmentally harmful gases are the main requirements for the future chemiresistive-type gas sensor applications. In this study, we demonstrate improved sensitivity of a 2D MoS₂-based gas sensor by controlling the Schottky barrier height. Chemical vapor deposition process was performed at low temperature to obtain layer-controlled 2D MoS₂, and the NO₂ gas responsivity was confirmed by the fabricated gas sensor. Then, the number of MoS₂ layers was fixed and the types of electrode materials were varied for controlling the Schottky barrier height. As the Schottky barrier height increased, the NO₂ responsivity increased, and it was found to be effective for CO and CO₂ gases, which had little reactivity in 2D MoS₂-based gas sensors.

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.

Universal Fermi-Level Pinning in Transition-Metal Dichalcogenides

Understanding the electron transport through transition-metal dichalcogenide (TMDC)-based semiconductor/metal junctions is vital for the realization of future TMDC-based (opto-)electronic devices. Despite the bonding in TMDCs being largely constrained within the layers, strong Fermi-level pinning (FLP) was observed in TMDC-based devices, reducing the tunability of the Schottky barrier height. We present evidence that metal-induced gap states (MIGS) are the origin for the large FLP similar to conventional semiconductors. A variety of TMDCs (MoSe₂, WSe₂, WS₂, and MoTe₂) were investigated using high-spatial-resolution surface characterization techniques, permitting us to distinguish between defected and pristine regions. The Schottky barrier heights on the pristine regions can be explained by MIGS, inducing partial FLP. The FLP strength is further enhanced by disorder-induced gap states induced by transition-metal vacancies or substitutionals at the defected regions. Our findings emphasize the importance of defects on the electron transport properties in TMDC-based devices and confirm the origin of FLP in TMDC-based metal/semiconductor junctions.

A WSe2 vertical field emission transistor

We report the first observation of a gate-controlled field emission current from a tungsten diselenide (WSe2) monolayer, synthesized by chemical-vapour deposition on a SiO2/Si substrate. Ni contacted WSe2 monolayer back-gated transistors, under high vacuum, exhibit n-type conduction and drain-bias dependent transfer characteristics, which are attributed to oxygen/water desorption and drain induced Schottky barrier lowering, respectively. The gate-tuned n-type conduction enables field emission, i.e. the extraction of electrons by quantum tunnelling, even from the flat part of the WSe2 monolayers. Electron emission occurs under an electric field ∼100 V μm-1 and exhibits good time stability. Remarkably, the field emission current can be modulated by the back-gate voltage. The first field-emission vertical transistor based on the WSe2 monolayer is thus demonstrated and can pave the way to further optimize new WSe2 based devices for use in vacuum electronics.

Defect Engineering in 2D Materials: Precise Manipulation and Improved Functionalities

Two-dimensional (2D) materials have attracted increasing interests in the last decade. The ultrathin feature of 2D materials makes them promising building blocks for next-generation electronic and optoelectronic devices. With reducing dimensionality from 3D to 2D, the inevitable defects will play more important roles in determining the properties of materials. In order to maximize the functionality of 2D materials, deep understanding and precise manipulation of the defects are indispensable. In the recent years, increasing research efforts have been made on the observation, understanding, manipulation, and control of defects in 2D materials. Here, we summarize the recent research progress of defect engineering on 2D materials. The defect engineering triggered by electron beam (e-beam), plasma, chemical treatment, and so forth is comprehensively reviewed. Firstly, e-beam irradiation-induced defect evolution, structural transformation, and novel structure fabrication are introduced. With the assistance of a high-resolution electron microscope, the dynamics of defect engineering can be visualized in situ. Subsequently, defect engineering employed to improve the performance of 2D devices by means of other methods of plasma, chemical, and ozone treatments is reviewed. At last, the challenges and opportunities of defect engineering on promoting the development of 2D materials are discussed. Through this review, we aim to build a correlation between defects and properties of 2D materials to support the design and optimization of high-performance electronic and optoelectronic devices.

Uncovering the Conduction Behavior of van der Waals Ambipolar Semiconductors

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

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

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

Atmospheric and Long-term Aging Effects on the Electrical Properties of Variable Thickness WSe2 Transistors

Atmospheric and long-term aging effects on electrical properties of WSe₂ transistors with various thicknesses are examined. Although countless published studies report electrical properties of transition-metal dichalcogenide materials, many are not attentive to testing environment or to age of samples, which we have found significantly impacts results. Our as-fabricated exfoliated WSe₂ pristine devices are predominantly n-type, which is attributed to selenium vacancies. Transfer characteristics of as-fabricated devices measured in air then vacuum reveal physisorbed atmospheric molecules significantly reduced n-type conduction in air. First-principles calculations suggest this short-term reversible atmospheric effect can be attributed primarily to physisorbed H₂O on pristine WSe₂, which is easily removed from the pristine surface in vacuum due to the low adsorption energy. Devices aged in air for over 300 h demonstrate irreversibly increased p-type conduction and decreased n-type conduction. Additionally, they develop an extended time constant for recovery of the atmospheric adsorbents effect. Short-term atmospheric aging (up to approximately 900 h) is attributed to O₂ and H₂O molecules physisorbed to selenium vacancies where electron transfer from the bulk and adsorbed binding energies are higher than the H₂O-pristine WSe₂. The residual/permanent aging component is attributed to electron trapping molecular O₂ and isoelectronic O chemisorption at selenium vacancies, which also passivates the near-conduction band gap state, p-doping the material, with very high binding energy. All effects demonstrated have the expected thickness dependence, namely, thinner devices are more sensitive to atmospheric and long-term aging effects.

Ultraviolet Light-Induced Persistent and Degenerated Doping in MoS2 for Potential Photocontrollable Electronics Applications

Efficient modulation of carrier concentration is fundamentally important for tailoring the electronic and photoelectronic properties of semiconducting materials. Photoinduced doping is potentially a promising way to realize such a goal for atomically thin nanomaterials in a rapid and defect-free manner. However, the wide applications of photoinduced doping in nanomaterials are severely constrained by the low doping concentration and poor stability that can be reached. Here, we propose a novel photoinduced doping mechanism based on the external photoelectric effect of metal coating on nanomaterials to significantly enhance the achievable doping concentration and stability. This approach is preliminarily demonstrated by an MX₂ (M is Mo or Re; X is S or Se) nanoflake modified through a simple process of sequentially depositing and annealing an Au layer on the surface of the flake. Under ultraviolet (UV) light illumination, the modified MX₂ achieves degenerated n-type doping density of 10¹⁴ cm^-2 rapidly according to the experimentally observed >10⁴ times increment in the channel current. The doping level persists after the removal of UV illumination with a nonobservable decrease over 1 day in vacuum (less than 23% over 7 days under an ambient environment). This photoinduced doping approach may contribute a major leap to the development of photocontrollable nanoelectronics.

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight.

Tunable Electron and Hole Injection Enabled by Atomically Thin Tunneling Layer for Improved Contact Resistance and Dual Channel Transport in MoS2/WSe2 van der Waals Heterostructure

Two-dimensional (2D) material-based heterostructures provide a unique platform where interactions between stacked 2D layers can enhance the electrical and opto-electrical properties as well as give rise to interesting new phenomena. Here, the operation of a van der Waals heterostructure device comprising of vertically stacked bilayer MoS₂ and few layered WSe₂ has been demonstrated in which an atomically thin MoS₂ layer has been employed as a tunneling layer to the underlying WSe₂ layer. In this way, simultaneous contacts to both MoS₂ and WSe₂ 2D layers have been established by forming a direct metal-semiconductor to MoS₂ and a tunneling-based metal-insulator-semiconductor contacts to WSe₂, respectively. The use of MoS₂ as a dielectric tunneling layer results in an improved contact resistance (80 kΩ μm) for WSe₂ contact, which is attributed to reduction in the effective Schottky barrier height and is also confirmed from the temperature-dependent measurement. Furthermore, this unique contact engineering and type-II band alignment between MoS₂ and WSe₂ enables a selective and independent carrier transport across the respective layers. This contact engineered dual channel heterostructure exhibits an excellent gate control and both channel current and carrier types can be modulated by the vertical electric field of the gate electrode, which is also reflected in the on/off ratio of 10⁴ for both electron (MoS₂) and hole (WSe₂) channels. Moreover, the charge transfer at the heterointerface is studied quantitatively from the shift in the threshold voltage of the pristine MoS₂ and the heterostructure device, which agrees with the carrier recombination-induced optical quenching as observed in the Raman spectra of the pristine and heterostructure layers. This observation of dual channel ambipolar transport enabled by the hybrid tunneling contacts and strong interlayer coupling can be utilized for high-performance opto-electrical devices and applications.

Oxide-mediated recovery of field-effect mobility in plasma-treated MoS2

Precise tunability of electronic properties of two-dimensional (2D) nanomaterials is a key goal of current research in this field of materials science. Chemical modification of layered transition metal dichalcogenides leads to the creation of heterostructures of low-dimensional variants of these materials. In particular, the effect of oxygen-containing plasma treatment on molybdenum disulfide (MoS₂) has long been thought to be detrimental to the electrical performance of the material. We show that the mobility and conductivity of MoS₂ can be precisely controlled and improved by systematic exposure to oxygen/argon plasma and characterize the material using advanced spectroscopy and microscopy. Through complementary theoretical modeling, which confirms conductivity enhancement, we infer the role of a transient 2D substoichiometric phase of molybdenum trioxide (2D-MoO _x ) in modulating the electronic behavior of the material. Deduction of the beneficial role of MoO _x will serve to open the field to new approaches with regard to the tunability of 2D semiconductors by their low-dimensional oxides in nano-modified heterostructures.

Enhancing ambipolar carrier transport of black phosphorus field-effect transistors with Ni–P alloy contacts

High-performance ambipolar black phosphorus field-effect transistors with low-resistance ohmic contacts were achieved via low-temperature vacuum annealing.

Anisotropic carrier mobility in buckled two-dimensional GaN

Converting FGaNH to HGaNH can significantly suppress hole mobility (even close to zero) and result in a transition from p-type-like semiconductor (FGaNH) to n-type-like semiconductor (HGaNH).