Influence of water vapor on the electronic property of MoS2 field effect transistors

Enhanced Carrier Dynamics and Excitation of Optically Stimulated Artificial Synapse Using van der Waals Passivation Layers

Advances in the semiconductor industry have been limited owing to the constraints imposed by silicon-based CMOS technology; hence, innovative device design approaches are necessary. This study focuses on "more than Moore" approaches, specifically in neuromorphic computing. Although MoS2 devices have attracted attention as neuromorphic computing candidates, their performances have been limited due to environment-induced perturbations to carrier dynamics and the formation of defect states. This study explores the integration of hydrocarbon (HC) layers onto active MoS2 channels to enhance neuromorphic computing characteristics. HC layers were employed in the proposed MoS2 field-effect transistor to facilitate stable optoelectrical control over the MoS2 channel under high-power stimulation. The improved electrical performance, stability, and synaptic behaviors of the HC-capped MoS2 devices compared to uncapped counterparts were experimentally demonstrated. The combination of optical and electrical tuning allowed for in-sensor computing applications that mimic human sensory behaviors. The impact of HC passivation on device performance was evaluated, and its potential for applications in neuromorphic computing with high stability was demonstrated across wide-ranging environmental conditions. The unique capabilities of HC-capped MoS2 devices were demonstrated by examining the spike duration-dependent plasticity and spiking timing-dependent plasticity. Thus, the proposed approach offers a promising avenue for advancing neuromorphic computing technologies.

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.

UV-SWIR broad range photodetectors made from few-layer α-In2Se3 nanosheets

Photodetectors are very important for many applications. However, inexpensive infrared photodetectors with high performance at room temperature are still rare. Furthermore, it is still a great challenge to realize on-chip wide-spectrum detection by using conventional photodetectors. van der Waals semiconductors are promising for high performance optoelectronic devices. Here, we report broad range photodetectors made from few-layer α-In2Se3 nanosheets. The photodetectors show response in an unexpected broad range from ultraviolet (325 nm) to short-wavelength infrared (1800 nm) at room temperature. The optical response in the long wavelengths beyond the bandgap is attributed to oxygen absorption and oxygen-associated selenium defects in In2Se3, supported by theoretical simulation and controlled experiments. High responses to 700 nm and 1550 nm lasers are demonstrated on the same In2Se3 device. The stability of In2Se3 under the atmosphere at room temperature and the low power consumption of the devices make the In2Se3 photodetectors promising for optoelectronic applications.