Enhanced Electrical Transport Properties of Molybdenum Disulfide Field-Effect Transistors by Using Alkali Metal Fluorides as Dielectric Capping Layers

The electronic properties of 2D materials are highly influenced by the molecular activity at their interfaces. A method was proposed to address this issue by employing passivation techniques using monolayer MoS2 field-effect transistors (FETs) while preserving high performance. Herein, we have used alkali metal fluorides as dielectric capping layers, including lithium fluoride (LiF), sodium fluoride (NaF), and potassium fluoride (KF) dielectric capping layers, to mitigate the environmental impact of oxygen and water exposure. Among them, the LiF dielectric capping layer significantly improved the transistor performance, specifically in terms of enhanced field effect mobility from 74 to 137 cm2/V·s, increased current density from 17 μA/μm to 32.13 μA/μm at a drain voltage of Vd of 1 V, and decreased subthreshold swing to 0.8 V/dec The results have been analytically verified by X-ray photoelectron spectroscopy (XPS) and Raman, and photoluminescence (PL) spectroscopy, and the demonstrated technique can be extended to other transition metal dichalcogenide (TMD)-based FETs, which can become a prospect for cutting-edge electronic applications. These findings highlight certain important trade-offs and provide insight into the significance of interface control and passivation material choice on the electrical stability, performance, and enhancement of the MoS2 FET.

Surface Passivation of Layered MoSe2 via van der Waals Stacking of Amorphous Hydrocarbon

Development of efficient surface passivation methods for semiconductor devices is crucial to counter the degradation in their electrical performance owing to scattering or trapping of carriers in the channels induced by molecular adsorption from the ambient environment. However, conventional dielectric deposition involves the formation of additional interfacial defects associated with broken covalent bonds, resulting in accidental electrostatic doping or enhanced hysteretic behavior. In this study, centimeter-scaled van der Waals passivation of transition metal dichalcogenides (TMDCs) is demonstrated by stacking hydrocarbon (HC) dielectrics onto MoSe₂ field-effect transistors (FETs), thereby enhancing the electric performance and stability of the device, accompanied with the suppression of chemical disorder at the HC/TMDCs interface. The stacking of HC onto MoSe₂ FETs enhances the carrier mobility of MoSe₂ FET by over 50% at the n-branch, and a significant decrease in hysteresis, owing to the screening of molecular adsorption. The electron mobility and hysteresis of the HC/MoSe₂ FETs are verified to be nearly intact compared to those of the fabricated HC/MoSe₂ FETs after exposure to ambient environment for 3 months. Consequently, the proposed design can act as a model for developing advanced nanoelectronics applications based on layered materials for mass production.

Capping Layers to Improve the Electrical Stress Stability of MoS2 Transistors

Two-dimensional (2D) materials offer exciting possibilities for numerous applications, including next-generation sensors and field-effect transistors (FETs). With their atomically thin form factor, it is evident that molecular activity at the interfaces of 2D materials can shape their electronic properties. Although much attention has focused on engineering the contact and dielectric interfaces in 2D material-based transistors to boost their drive current, less is understood about how to tune these interfaces to improve the long-term stability of devices. In this work, we evaluated molybdenum disulfide (MoS₂) transistors under continuous electrical stress for periods lasting up to several days. During stress in ambient air, we observed temporary threshold voltage shifts that increased at higher gate voltages or longer stress durations, correlating to changes in interface trap states (ΔN_it) of up to 10¹² cm^-2. By modifying the device to include either SU-8 or Al₂O₃ as an additional dielectric capping layer on top of the MoS₂ channel, we were able to effectively reduce or even eliminate this unstable behavior. However, we found this encapsulating material must be selected carefully, as certain choices actually amplified instability or compromised device yield, as was the case for Al₂O₃, which reduced yield by 20% versus all other capping layers. Further refining these strategies to preserve stability in 2D devices will be crucial for their continued integration into future technologies.

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.

Layered deposition of SnS2 grown by atomic layer deposition and its transport properties

In this work, we report on the layered deposition of few-layer tin disulfide (SnS₂) using atomic layer deposition (ALD). By varying the ALD cycles it was possible to deposit poly-crystalline SnS₂ with small variation in layer numbers. Based on the ALD technique, we developed the process technology growing few-layer crystalline SnS₂ film (3-6 layers) and we investigated their electrical properties by fabricating bottom-gated thin film transistors using the ALD SnS₂ as the transport channel. SnS₂ devices showed typical n-type characteristic with on/off current ratio of ∼8.32 × 10⁶, threshold voltage of ∼2 V, and a subthreshold swing value of 830 mV decade^-1 for the 6 layers SnS₂. The developed SnS₂ ALD technique may aid the realization of two-dimensional SnS₂ based flexible and wearable devices.

Threshold Voltage Control of Multilayered MoS2 Field-Effect Transistors via Octadecyltrichlorosilane and their Applications to Active Matrixed Quantum Dot Displays Driven by Enhancement-Mode Logic Gates

In recent past, for next-generation device opportunities such as sub-10 nm channel field-effect transistors (FETs), tunneling FETs, and high-end display backplanes, tremendous research on multilayered molybdenum disulfide (MoS₂ ) among transition metal dichalcogenides has been actively performed. However, nonavailability on a matured threshold voltage control scheme, like a substitutional doping in Si technology, has been plagued for the prosperity of 2D materials in electronics. Herein, an adjustment scheme for threshold voltage of MoS₂ FETs by using self-assembled monolayer treatment via octadecyltrichlorosilane is proposed and demonstrated to show MoS₂ FETs in an enhancement mode with preservation of electrical parameters such as field-effect mobility, subthreshold swing, and current on-off ratio. Furthermore, the mechanisms for threshold voltage adjustment are systematically studied by using atomic force microscopy, Raman, temperature-dependent electrical characterization, etc. For validation of effects of threshold voltage engineering on MoS₂ FETs, full swing inverters, comprising enhancement mode drivers and depletion mode loads are perfectly demonstrated with a maximum gain of 18.2 and a noise margin of ≈45% of 1/2 V_DD . More impressively, quantum dot light-emitting diodes, driven by enhancement mode MoS₂ FETs, stably demonstrate 120 cd m^-2 at the gate-to-source voltage of 5 V, exhibiting promising opportunities for future display application.

Effects of HfO2 encapsulation on electrical performances of few-layered MoS2 transistor with ALD HfO2 as back-gate dielectric

The carrier mobility of MoS₂ transistors can be greatly improved by the screening role of high-k gate dielectric. In this work, atomic-layer deposited (ALD) HfO₂ annealed in NH₃ is used to replace SiO₂ as the gate dielectric to fabricate back-gated few-layered MoS₂ transistors, and good electrical properties are achieved with field-effect mobility (μ) of 19.1 cm² V^-1 s^-1, subthreshold swing (SS) of 123.6 mV dec^-1 and on/off ratio of 3.76 × 10⁵. Furthermore, enhanced device performance is obtained when the surface of the MoS₂ channel is coated by an ALD HfO₂ layer with different thicknesses (10, 15 and 20 nm), where the transistor with a 15 nm HfO₂ encapsulation layer exhibits the best overall electrical properties: μ = 42.1 cm² V^-1 s^-1, SS = 87.9 mV dec^-1 and on/off ratio of 2.72 × 10⁶. These improvements should be associated with the enhanced screening effect on charged-impurity scattering and protection from absorption of environmental gas molecules by the high-k encapsulation. The capacitance equivalent thickness of the back-gate dielectric (HfO₂) is only 6.58 nm, which is conducive to scaling of the MoS₂ transistors.

2D MoS2 Neuromorphic Devices for Brain-Like Computational Systems

Hardware implementation of artificial synapses/neurons with 2D solid-state devices is of great significance for nanoscale brain-like computational systems. Here, 2D MoS₂ synaptic/neuronal transistors are fabricated by using poly(vinyl alcohol) as the laterally coupled, proton-conducting electrolytes. Fundamental synaptic functions, such as an excitatory postsynaptic current, paired-pulse facilitation, and a dynamic filter for information transmission of biological synapse, are successfully emulated. Most importantly, with multiple input gates and one modulatory gate, spiking-dependent logic operation/modulation, multiplicative neural coding, and neuronal gain modulation are also experimentally demonstrated. The results indicate that the intriguing 2D MoS₂ transistors are also very promising for the next-generation of nanoscale neuromorphic device applications.

Sulfur vacancy-induced reversible doping of transition metal disulfides via hydrazine treatment

Chemical doping of transition metal dichalcogenides (TMDCs) has drawn significant interest because of its applicability to the modification of electrical and optical properties of TMDCs. This is of fundamental and technological importance for high-efficiency electronic and optoelectronic devices. Here, we present a simple and facile route to reversible and controllable modulation of the electrical and optical properties of WS₂ and MoS₂via hydrazine doping and sulfur annealing. Hydrazine treatment of WS₂ improves the field-effect mobilities, on/off current ratios, and photoresponsivities of the devices. This is due to the surface charge transfer doping of WS₂ and the sulfur vacancies formed by its reduction, which result in an n-type doping effect. The changes in the electrical and optical properties are fully recovered when the WS₂ is annealed in an atmosphere of sulfur. This method for reversible modulation can be applied to other transition metal disulfides including MoS₂, which may enable the fabrication of two-dimensional electronic and optoelectronic devices with tunable properties and improved performance.

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.

Tunable electrical properties of multilayer HfSe2 field effect transistors by oxygen plasma treatment

HfSe₂ field effect transistors are systematically studied in order to selectively tune their electrical properties by optimizing layer thickness and oxygen plasma treatment. The optimized plasma-treated HfSe₂ field effect transistors showed a high on/off ratio improvement of four orders of magnitude, from 27 to 10⁵, a field effect mobility increase from 2.16 to 3.04 cm² V^-1 s^-1, a subthreshold swing improvement from 30.6 to 4.8 V dec^-1, and a positive threshold voltage shift between depletion mode and enhancement mode, from -7.02 to 11.5 V. The plasma-treated HfSe₂ photodetector also demonstrates a reasonable photoresponsivity from the visible to the near-infrared region of light.