WS2 Field-effect Transistors Research Articles

WS2 Nanosheet-Based Ultrascaled Field-Effect Transistor for Hydrogen Gas Sensing: Addressing the Sensitivity-Downscaling Trade-Off.

In this paper, we propose an ultrascaled WS2 field-effect transistor equipped with a Pd/Pt sensitive gate for high-performance and low-power hydrogen gas sensing applications. The proposed nanosensor is simulated by self-consistently solving a quantum transport equation with electrostatics at the ballistic limit. The gas sensing principle is based on the gas-induced change in the metal gate work function. The hydrogen gas nanosensor leverages the high sensitivity of two-dimensional WS2 to its sur-rounding electrostatic environment. The computational investigation encompasses the nanosensor's behavior in terms of potential profile, charge density, current spectrum, local density of states (LDOS), transfer characteristics, and sensitivity. Additionally, the downscaling-sensitivity trade-off is analyzed by considering the impact of drain-to-source voltage and the electrostatics parameters on subthreshold performance. The simulation results indicate that the downscaling-sensitivity trade-off can be optimized through enhancements in electrostatics, such as utilizing high-k dielectrics and reducing oxide thickness, as well as applying a low drain-to-source voltage, which also contributes to improved energy efficiency. The proposed nanodevice meets the prerequisites for cutting-edge gas nanosensors, offering high sensing performance, improved scaling capability, low power consumption, and complementary metal-oxide-semiconductor compatibility, making it a compelling candidate for the next generation of ultrascaled FET-based gas nanosensors.

Realization of Extremely High-Gain and Low-Power in nMOS Inverter Based on Monolayer WS2 Transistor Operating in Subthreshold Regime.

In this work, we report an n-type metal-oxide-semiconductor (nMOS) inverter using chemical vapor deposition (CVD)-grown monolayer WS2 field-effect transistors (FETs). Our large-area CVD-grown monolayer WS2 FETs exhibit outstanding electrical properties including a high on/off ratio, small subthreshold swing, and excellent drain-induced barrier lowering. These are achieved by n-type doping using AlOx/Al2O3 and a double-gate structure employing high-k dielectric HfO2. Due to the superior subthreshold characteristics, monolayer WS2 FETs show high transconductance and high output resistance in the subthreshold regime, resulting in significantly higher intrinsic gain compared to conventional Si MOSFETs. Therefore, we successfully realize subthreshold operating monolayer WS2 nMOS inverters with extremely high gains of 564 and 2056 at supply voltage (VDD) of 1 and 2 V, respectively, and low power consumption of ∼2.3 pW·μm-1 at VDD = 1 V. In addition, the monolayer WS2 nMOS inverter is further expanded to the demonstration of logic circuits such as AND, OR, NAND, NOR logic gates, and SRAM. These findings suggest the potential of monolayer WS2 for high-gain and low-power logic circuits and validate the practical application in large areas.

Area-selective chalcogenization of transition metals through graphene mask

Area-selection reactions have been extensively investigated to control or change physicochemical properties of substances with micro- or nanoscale precision. Several polymeric materials called photoresists have been used to mask and pattern the specific region, which can block chemical reactions or deposition. However, they are not suitable for certain chemical reaction since they are vulnerable to high temperature. Here, we report the graphene mask to achieve area-selective chalcogenization, which is performed at high temperature by chemical vapor deposition method. Due to its physicochemical properties, graphene does not allow chalcogen precursor gases to penetrate into metal films. Several characterizations are performed to prove the successful sulfurization and selenization of molybdenum and tungsten films. As an application, WS2 field-effect transistors with graphene mask are fabricated, and they show the typical characteristics of transistors successfully. Therefore, we expect that graphene-assisted area-selective reaction can be utilized for various fields such as semiconductors, sensors, and etc.

Quantum Transport Properties of Monolayer MoS2, WS2, and Black Phosphorus: A Comparative Study

A comparative study of the performance analysis of dual-gate ballistic monolayer Molybdenum disulfide (MoS2), tungsten disulfide (WS2), and black phosphorus (BP) field-effect transistors (FETs) is presented. A thorough investigation of output and transfer characteristics infers that WS2 FET exhibits better performance as compared to MoS2 and BP. Furthermore, among all three FETs (MoS2, WS2, and BP), the WS2 based FET has a higher carrier velocity. However, variation of gate capacitance (CG) with gate voltage (VG) reflects a very good electrostatic gate control of MoS2 FET due to higher surface charge accumulation. Except for CG, the overall performance of WS2 based FET is better than MoS2 and BP.

Interdigitated electrodes enhanced photosensitive monolayer WS2 field effect transistor on glass substrate

Recently, 2D-material-based optoelectronic devices are integrated on optical-fiber as a substrate. However, silicon substrate, usually for optoelectronic device integration, impedes fiber-optic integration due to its much larger refractive index. Here, we fabricated a highly transparent photosensitive field effect transistor (FET) using monolayer WS2 on a fiber-compatible glass substrate with interdigitated electrodes. Experiments shows that the current switching ratio for the FET reach up to 105. Additionally, the interdigitated electrodes could improve photocurrent collection capability, leading to a relative high responsivity of 1.48 mA/W. Meanwhile, the difference in the photocurrent induced by light illumination from the front and back to the device is only 1.25%. This indicates light beam can pass through the device, facilitating online monitoring of optical power. Moreover, it is also shown that the transmitted light beam could be reflected to retransmit thought the device, thus increasing photocurrent by 6.8%. Such glass-substrate provide a simple and feasible strategy for future incorporation of optical-fiber with on-chip device.

Modulated wafer-scale WS2 films based on atomic-layer-deposition for various device applications.

Tungsten disulfide (WS2) is promising for potential applications in transistors and gas sensors due to its high mobility and high adsorption of gas molecules onto edge sites. This work comprehensively studied the deposition temperature, growth mechanism, annealing conditions, and Nb doping of WS2 to prepare high-quality wafer-scale N- and P-type WS2 films by atomic layer deposition (ALD). It shows that the deposition and annealing temperature greatly influence the electronic properties and crystallinity of WS2, and insufficient annealing will seriously reduce the switch ratio and on-state current of the field effect transistors (FETs). Besides, the morphologies and carrier types of WS2 films can be controlled by adjusting the processes of ALD. The obtained WS2 films and the films with vertical structures were used to fabricate FETs and gas sensors, respectively. Among them, the Ion/Ioff ratio of N- and P-type WS2 FETs is 105 and 102, respectively, and the response of N- and P-type gas sensors is 14% and 42% under 50 ppm NH3 at room temperature, respectively. We have successfully demonstrated a controllable ALD process to modify the morphology and doping behavior of WS2 films with various device functionalities based on acquisitive characteristics.

Statistical Assessment of High-Performance Scaled Double-Gate Transistors from Monolayer WS2.

Scaling of monolayer transition metal dichalcogenide (TMD) field-effect transistors (FETs) is an important step toward evaluating the application space of TMD materials. Although some work on ultrashort channel monolayer (ML) TMD FETs has been published, there exist no comprehensive studies that assess their performance in a statistically relevant manner, providing critical insights into the impact of the device geometry. Part of the reason for the absence of such a study is the substantial variability of TMD devices when processes are not carefully controlled. In this work, we show a statistical study of ultrashort channel double-gated ML WS2 FETs exhibiting excellent device performance and limited device-to-device variations. From a detailed analysis of cross-sectional scanning transmission electron microscopy (STEM) images and careful technology computer aided design (TCAD) simulations, we evaluated, in particular, an unexpected deterioration of the subthreshold characteristics for our shortest devices. Two potential candidates for the observed behavior were identified, i.e., buckling of the TMD on the substrate and loss of gate control due to the source geometry and the high-k dielectric between the metal gate and the metal source electrode.

Comparative analysis of Schottky barriers for heterogeneous defect domains in monolayer WS2 field-effect transistors

A major bottleneck in the applications of two-dimensional transition metal dichalcogenides (TMDs) is the realization of ohmic contacts by overcoming the Schottky barrier (SB) at metal–semiconductor interfaces. Although the physical properties of point defects in TMDs are the key to determining the SB characteristics, the correlation between the defect types and SBs is yet to be systematically explored. Here, we investigated the SB characteristics depending on the S vacancy (SV) and W vacancy (WV) in hexagonal WS2 field-effect transistors (FETs). This was implemented by fabricating the SV and WV FETs in each domain on a single-flake monolayer. Compared to WV FET, the SV FET exhibited a 10 times higher doping concentration and 4 times higher Coulomb scattering coefficient (α) resulting in higher low-frequency noise. The comparative SB characteristics of both FETs were investigated by Arrhenius plot for the on-current at high temperatures. Below 360 K, the activation energies (Ea) of the SV and WV FETs exhibited different values of 0.071 and 0.115 eV, respectively, whereas those values are similar above 360 K because sufficient thermal energy allows for tunneling at SBs. These results help us understand the correlation between the defect type, doping level, and SB in TMD-based electronic devices.

Control of the Schottky barrier height in monolayer WS2 FETs using molecular doping

Developing controllable doping processes for two-dimensional (2D) semiconductors is critical to developing next-generation electronic and optoelectronic devices. Understanding the nature of the contacts is an essential step in realizing efficient charge injection in transition metal dichalcogenides. In this study, post-growth n-doping of chemical vapor deposition grown monolayer (1 L) WS2 is achieved through molecular reductant solution treatment. The doping level can be effectively controlled by the treatment time and dopant solution concentrations. The doped WS2 field-effect transistors showed profound threshold voltage shifts and tunable channel currents. This molecular n-doping technique is beneficial for the selective area doping needed for electrical contacts and reduces the contact resistance (Rc) in 1 L WS2 by more than two orders of magnitude. The significant reduction of Rc is attributed to the high electron-doping density achieved in WS2, which leads to a significant reduction of the Schottky barrier height. The dependence of mobility on temperature indicates clear evidence of the strong suppression of charge-impurity scattering after doping. High levels of doping allow the observation of a metal–insulator transition in monolayer WS2 due to strong electron–electron interactions. This doping technique provides a viable route for tailoring the electrical properties and improving the contacts in transition metal dichalcogenides, paving the way for high-performance 2D nanoelectronic devices.

Device and Circuit Analysis of Double Gate Field Effect Transistor with Mono-Layer WS2-Channel at Sub-2 nm Technology Node

In this work, WS2 was adopted as a channel material among transition metal dichalcogenides (TMD) materials that have recently been in the spotlight, and the circuit power performance (power consumption, operating frequency) of the monolayer WS2 field-effect transistor with a double gate structure (DG WS2-FET) was analyzed. It was confirmed that the effective capacitance, which is circuit power performance, was greatly changed by the extrinsic capacitance components of DG WS2-FET, and the spacer region length (LSPC) and dielectric constant (KSPC) values of the spacer that could affect the extrinsic capacitance components were analyzed to identify the circuit power performance. As a result, when LSPC is increased by 1.5 nm with the typical spacer material (KSPC = 7.5), increased operating speed (+4.9%) and reduced active power (–6.8%) are expected. In addition, it is expected that the spacer material improvement by developing the low-k spacer from KSPC = 7.5 to KSPC = 2 at typical LSPC = 8 nm can increase the operating speed by 36.8% while maintaining similar active power consumption. Considering back-end-of-line (BEOL), the change in circuit power performance according to wire length was also analyzed. From these results, it can be seen that reducing the capacitance components of the extrinsic region is very important for improving the circuit power performance of the DG WS2-FET.

Epitaxy of NiTe2 on WS2 for the p-Type Schottky Contact and Increased Photoresponse.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have great potential applications in the electronic and optoelectronic devices. Nevertheless, due to the difficulty in the efficient doping of atomic-thickness TMDCs or Fermi level pinning (FLP) effects at the metal/semiconductor interface, most TMDC devices exhibit the n-type conduction polarity, which significantly limits their functional applications based on the p-n junction. Here, 2D semi-metal NiTe2 nanosheets were epitaxially grown on the WS2 monolayer by a two-step chemical vapor deposition route. The microstructure and optical characterizations confirm that the vertically stacked NiTe2/WS2 heterostructures are formed by van der Waals epitaxy. Interestingly, p-type WS2 field-effect transistors can be obtained with the hole mobility of ∼4.22 cm2/V·s, when the epitaxial NiTe2 sheets act as the source/drain electrodes. This is attributed to the decreased FLP effect and hence the low potential barrier for holes at the van der Waals contacts. Furthermore, the photodetectors based on the heterostructures show a 2 orders of magnitude increase in the switch ratio, responsivity, and detectivity and a 1 order of magnitude increase in the rise and decay speeds relative to those based on pristine WS2. This work paves the way to realize the p-type contact for monolayer WS2 with significantly enhanced optoelectronic performance.

Top-gate field-effect transistor based on monolayer WS2 with an ion-gel gate dielectric

A top-gate field-effect transistor (FET), based on monolayer (ML) tungsten disulfide (WS2), and with an ion-gel dielectric was developed. The high electrical contact resistance of the Schottky contacts at the n-type transition metal dichalcogenides/metal electrode interfaces often adversely affects the device performance. We report the contact resistance and Schottky barrier height of an FET with Au electrodes. The FET is based on ML WS2 that was synthesized using chemical vapour deposition and was assessed using the transfer-length method and low-temperature measurements. Raman and photoluminescence spectra were recorded to determine the optical properties of the WS2 layers. The ML WS2 FET with an ion-gel top gate dielectric exhibits n-type behaviour, with a mobility, on/off ratio of 1.97 cm2 V−1·s−1, 1.51 × 105, respectively.

Enhanced Performance of WS2 Field-Effect Transistor through Mono and Bilayer h-BN Tunneling Contacts.

Transition metal dichalcogenides (TMDs) are of great interest owing to their unique properties. However, TMD materials face two major challenges that limit their practical applications: contact resistance and surface contamination. Herein, a strategy to overcome these problems by inserting a monolayer of hexagonal boron nitride (h-BN) at the chromium (Cr) and tungsten disulfide (WS2 ) interface is introduced. Electrical behaviors of direct metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) contacts with mono- and bilayer h-BN in a four-layer WS2 field-effect transistor (FET) are evaluated under vacuum from 77 to 300 K. The performance of the MIS contacts differs based on the metal work function when using Cr and indium (In). The contact resistance is significantly reduced by approximately ten times with MIS contacts compared with that for MS contacts. An electron mobility up to ≈115 cm2 V-1 s-1 at 300 K is achieved with the insertion of monolayer h-BN, which is approximately ten times higher than that with MS contacts. The mobility and contact resistance enhancement are attributed to Schottky barrier reduction when h-BN is introduced between Cr and WS2 . The dependence of the tunneling mechanisms on the h-BN thickness is investigated by extracting the tunneling barrier parameters.

Atomic layer deposited Al2O3 passivation layer for few-layer WS2 field effect transistors

We have investigated the effect of an Al2O3 passivation layer on the performance of few-layer WS2 FETs. While the performance of WS2 FETs is often limited by a substantial decrease in carrier mobility owing to charged impurities and a Schottky barrier between the WS2 and metal electrodes, the introduction of an Al2O3 overlayer by atomic layer deposition (ALD) suppressed the influence of charged impurities by high-κ dielectric screening effect and reduced the effective Schottky barrier height. We argue that n-doping of WS2, induced by positive fixed charges formed at Al2O3/WS2 interface during the ALD process, is responsible for the reduction of the effective Schottky barrier height in the devices. In addition, the Al2O3 passivation layer protected the device from oxidation, and maintained stable electrical performance of the WS2 FETs over 57 d. Thus, the ALD of Al2O3 overlayer provides a facile method to enhance the performance of WS2 FETs and to ensure ambient stability.

Charge-Transfer Effect and Enhanced Photoresponsivity of WS2- and MoSe2-Based Field Effect Transistors with π-Conjugated Polyelectrolyte

The characteristics of field effect transistors (FETs) fabricated using two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be modulated by surface treatment of the active layers. In this study, an ionic π-conjugated polyelectrolyte, poly(9,9-bis(4'-sulfonatobutyl)fluorene-alt-1,4-phenylene) potassium (FPS-K), was used for the surface treatment of MoSe2 and WS2 FETs. The photoluminescence (PL) intensities of monolayer (1L)-MoSe2 and 1L-WS2 clearly decreased, and the PL peaks were red-shifted after FPS-K treatment, suggesting a charge-transfer effect. In addition, the n-channel current of both the MoSe2 and WS2 FETs increased and the threshold voltage (Vth) shifted negatively after FPS-K treatment owing to the charge-transfer effect. The photoresponsivity of the MoSe2 FET under light irradiation (λex = 455 nm) increased considerably, from 5300 A W-1 to approximately 10 000 A W-1, after FPS-K treatment, and similar behavior was observed in the WS2 FET. The results can be explained in terms of the increase in electron concentration due to photogating. The external quantum efficiency and photodetectivity of both FETs were also enhanced by the charge-transfer effect resulting from surface treatment with FPS-K containing mobile cations (K+) and fixed anions (SO3-), as well as by the photogating effect. The variation in charge-carrier density due to the photogating and charge-transfer effects is estimated to be approximately 2 × 1012 cm-2. The results suggest that π-conjugated polyelectrolytes such as FPS-K can be a promising candidate for the passivation of TMDC-based FETs and obtaining enhanced photoresponsivity.

Isoemissive Photoluminescence from a Quaternary System of Valley-Polarized, Defect-Bound Excitons and Trions in Two-Dimensional Transition Metal Dichalcogenides

Interest in the use of local band extrema, known as valleys, as a possible information carrier has in recent years been advanced by the isolation of 2D transition metal dichalcogenides (TMDCs). In some monolayer TMDCs, the structural inversion asymmetry leads to spin-valley locked states that present potential advantages for information storage and manipulation beyond existing charge- and spin-based semiconductor technologies. However, understanding of the role of defects on exciton recombination and valley scattering remains lacking and limits progress toward applications. Here, we report gate voltage-dependent polarized photoluminescence measurements on CVD-grown monolayer WS2 and MoS2 field-effect transistors and observe a quaternary system of valley polarized excitons and trions. Intriguingly, we find the spectra to exhibit isoemissive points, which were previously seen only as evidence of a binary system. We propose a rate model to explain the existence of isoemission in a quaternary system and find that the presence of isoemissive points imply a kinetic relationship between recombination and valley scattering. This relationship sheds light on the inverse relation seen between the polarization degree and photoluminescence intensity. Further consideration of the phenomenon also led to the identification of a possible new mechanism for charge carrier detrapping using the energy released from trion recombination.

Performance improvement in p-Type WS2 field-effect transistors with 1T phase contacts

The non-ideal contact between the metal electrode and semiconducting channel has become one of the major bottlenecks degrading the performance of field-effect transistors (FETs) based on two-dimensional (2D) materials. The formation of the Schottky barrier prohibiting the carrier injection as well as the Fermi level pinning effect have a strong impact on the device performance. In this work, we fabricated a 2D WS2 FET device with engineered metallic 1T phase at the source/drain region by using Li intercalation method. As compared to the device with conventional 2H-WS2 channel, the engineered FET with a 2H-WS2/1T-WS2 junction has exhibited greatly improved performance such as over 10 times higher carrier mobility and steeper subthreshold slope. Such results have demonstrated the boosted carrier injection into the channel over the engineered metal contact as well as the reduced tunnel barrier width between the 2H/1T-WS2 junction. This can be attractive for the large-scale integration of the 2D devices towards high-performance and high-reproducibility nanoelectronics applications.

Efficient control of emission and carrier polarity in WS2 monolayer by indium doping

Substitutional doping of two-dimensional (2D) transition metal dichalcogenides (TMDs) has been recognized as a promising strategy to tune their optoelectronic properties for a wide array of applications. However, controllable doping of TMDs remains a challenging issue due to the natural doping of these materials. Here, we demonstrate the controllable growth of indium-doped p-type WS2 monolayers with various doping concentrations via chemical vapor deposition (CVD) of a host tungsten (W) source and indium (In) dopant. Scanning transmission electron microscopy confirmed that In atoms successfully substitute the W atoms in the WS2 lattice. Intriguingly, the photoluminescence of the doped sample experiences strong intensity modulation by the doping concentration, which first increases remarkably with an enhancement factor up to ~35 and then decreases gradually when further increasing the doping concentration. Such a phenomenon is attributed to the progressive change of the exciton to trion ratio as well as the defect concentration in the doped samples. The assignment was further verified by the electric behavior of the fabricated In-doped WS2 field effect transistors, which changes regularly from n-type to bipolar and finally to p-type behavior with increasing doping concentration. The successful growth of p-type monolayer WS2 and the dual control of its optical and electrical properties by In doping may provide a promising method to engineer the opto-electronic properties of 2D materials.

Benchmarking monolayer MoS2 and WS2 field-effect transistors

Here we benchmark device-to-device variation in field-effect transistors (FETs) based on monolayer MoS2 and WS2 films grown using metal-organic chemical vapor deposition process. Our study involves 230 MoS2 FETs and 160 WS2 FETs with channel lengths ranging from 5 μm down to 100 nm. We use statistical measures to evaluate key FET performance indicators for benchmarking these two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers against existing literature as well as ultra-thin body Si FETs. Our results show consistent performance of 2D FETs across 1 × 1 cm2 chips owing to high quality and uniform growth of these TMDs followed by clean transfer onto device substrates. We are able to demonstrate record high carrier mobility of 33 cm2 V−1 s−1 in WS2 FETs, which is a 1.5X improvement compared to the best reported in the literature. Our experimental demonstrations confirm the technological viability of 2D FETs in future integrated circuits.

Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire.

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS2 monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS2 domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS2 monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS2 field effect transistors exhibited an ION/OFF of ∼107 and mobility of 16 cm2/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.