Ultraviolet Wavelength-Dependent Optoelectronic Properties in Two-Dimensional NbSe₂–WSe₂ van der Waals Heterojunction-Based Field-Effect Transistors

Atomically thin two-dimensional (2D) van der Waals (vdW) heterostructures are one of the very important research issues for stacked optoelectronic device applications. In this study, using the transferred and stacked NbSe₂–WSe₂ films as electrodes and a channel, we fabricated the field-effect transistor (FET) devices based on 2D–2D vdW metal–semiconductor heterojunctions (HJs) and systematically studied their ultraviolet (UV) wavelength-dependent electrical and photoresponse properties. Upon the exposure to UV light with a wavelength of 365 nm, the NbSe₂–WSe₂ vdW HJFET devices exhibited threshold voltage shift toward positive gate bias direction, a current increase, and a nonlinear photocurrent increase upon applying a gate bias due to the contribution of the photogenerated hole current. In contrast, for the 254 nm UV-irradiated FET devices, the drain current was decreased dramatically and the threshold voltage was negatively shifted. The time-resolved photoresponse properties showed that the device current after turning off the 254 nm UV light was completely and much more rapidly recovered compared with the case of the persistent photocurrent after turning off the 365 nm UV light. Interestingly, we found that the wettability of the WSe₂ surface was changed with increasing irradiation time only after 254 nm UV irradiation. The measured wetting behavior on the WSe₂ surface provided direct evidence that the experimentally observed UV-wavelength-dependent phenomena was attributed to the UV-induced dissociative adsorption of oxygen and water molecules, leading to the modulation of charge trap states on the photogenerated and intrinsic carriers in the p-type WSe₂ channel. This study will help provide an understanding of the influence of environmental and electrical measurement conditions on the electrical and optical properties of 2D–2D vdW HJ devices for a variety of device applications through the stacking of 2D heterostructures

Irradiation Effects of High-Energy Proton Beams on MoS₂ Field Effect Transistors

We investigated the effect of irradiation on molybdenum disulfide (MoS₂) field effect transistors with 10 MeV high-energy proton beams. The electrical characteristics of the devices were measured before and after proton irradiation with fluence conditions of 10¹², 10¹³, and 10¹⁴ cm^–2. For a low proton beam fluence condition of 10¹² cm^–2, the electrical properties of the devices were nearly unchanged in response to proton irradiation. In contrast, for proton beam fluence conditions of 10¹³ or 10¹⁴ cm^–2, the current level and conductance of the devices significantly decreased following proton irradiation. The electrical changes originated from proton-irradiation-induced traps, including positive oxide-charge traps in the SiO₂ layer and trap states at the interface between the MoS₂ channel and the SiO₂ layer. Our study will enhance the understanding of the influence of high-energy particles on MoS₂-based nanoelectronic devices

Electric Stress-Induced Threshold Voltage Instability of Multilayer MoS₂ Field Effect Transistors

We investigated the gate bias stress effects of multilayered MoS₂ field effect transistors (FETs) with a back-gated configuration. The electrical stability of the MoS₂ FETs can be significantly influenced by the electrical stress type, relative sweep rate, and stress time in an ambient environment. Specifically, when a positive gate bias stress was applied to the MoS₂ FET, the current of the device decreased and its threshold shifted in the positive gate bias direction. In contrast, with a negative gate bias stress, the current of the device increased and the threshold shifted in the negative gate bias direction. The gate bias stress effects were enhanced when a gate bias was applied for a longer time or when a slower sweep rate was used. These phenomena can be explained by the charge trapping due to the adsorption or desorption of oxygen and/or water on the MoS₂ surface with a positive or negative gate bias, respectively, under an ambient environment. This study will be helpful in understanding the electrical-stress-induced instability of the MoS₂-based electronic devices and will also give insight into the design of desirable devices for electronics applications

Electrical and Optical Characterization of MoS₂ with Sulfur Vacancy Passivation by Treatment with Alkanethiol Molecules

We investigated the physical properties of molybdenum disulfide (MoS₂) atomic crystals with a sulfur vacancy passivation after treatment with alkanethiol molecules including their electrical, Raman, and photoluminescence (PL) characteristics. MoS₂, one of the transition metal dichalcogenide materials, is a promising two-dimensional semiconductor material with good physical properties. It is known that sulfur vacancies exist in MoS₂, resulting in the n-type behavior of MoS₂. The sulfur vacancies on the MoS₂ surface tend to form covalent bonds with sulfur-containing groups. In this study, we deposited alkanethiol molecules on MoS₂ field effect transistors (FETs) and then characterized the electrical properties of the devices before and after the alkanethiol treatment. We observed that the electrical characteristics of MoS₂ FETs dramatically changed after the alkanethiol treatment. We also observed that the Raman and PL spectra of MoS₂ films changed after the alkanethiol treatment. These effects are attributed to the thiol (−SH) end groups in alkanethiols bonding at sulfur vacancy sites, thus altering the physical properties of the MoS₂. This study will help us better understand the electrical and optical properties of MoS₂ and suggest a way of tailoring the properties of MoS₂ by passivating a sulfur vacancy with thiol molecules

Hydrogen-Induced Morphotropic Phase Transformation of Single-Crystalline Vanadium Dioxide Nanobeams

We report a morphotropic phase transformation in vanadium dioxide (VO₂) nanobeams annealed in a high-pressure hydrogen gas, which leads to the stabilization of metallic phases. Structural analyses show that the annealed VO₂ nanobeams are hexagonal-close-packed structures with roughened surfaces at room temperature, unlike as-grown VO₂ nanobeams with the monoclinic structure and with clean surfaces. Quantitative chemical examination reveals that the hydrogen significantly reduces oxygen in the nanobeams with characteristic nonlinear reduction kinetics which depend on the annealing time. Surprisingly, the work function and the electrical resistance of the reduced nanobeams follow a similar trend to the compositional variation due mainly to the oxygen-deficiency-related defects formed at the roughened surfaces. The electronic transport characteristics indicate that the reduced nanobeams are metallic over a large range of temperatures (room temperature to 383 K). Our results demonstrate the interplay between oxygen deficiency and structural/electronic phase transitions, with implications for engineering electronic properties in vanadium oxide systems