Kyungjune Cho

Oxygen environmental and passivation effects on molybdenum disulfide field effect transistors

We investigated the effects of passivation on the electrical characteristics of molybdenum disulfide (MoS(2)) field effect transistors (FETs) under nitrogen, vacuum, and oxygen environments. When the MoS(2) FETs were exposed to oxygen, the on-current decreased and the threshold voltage shifted in the positive gate bias direction as a result of electrons being trapped by the adsorbed oxygen at the MoS(2) surface. In contrast, the electrical properties of the MoS(2) FETs changed only slightly in the different environments when a passivation layer was created using polymethyl methacrylate (PMMA). Specifically, the carrier concentration of unpassivated devices was reduced to 6.5 × 10(15) cm(-2) in oxygen from 16.3 × 10(15) cm(-2) in nitrogen environment. However, in PMMA-passivated devices, the carrier concentration remained nearly unchanged in the range of 1-3 × 10(15) cm(-2) regardless of the environment. Our study suggests that surface passivation is important for MoS(2)-based electronic 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.

Photoelectron spectroscopic imaging and device applications of large-area patternable single-layer MoS2 synthesized by chemical vapor deposition.

Molybdenum disulfide (MoS2) films, which are only a single atomic layer thick, have been synthesized by chemical vapor deposition (CVD) and have gained significant attention due to their band-gap semiconducting properties. However, in order for them to be useful for the fabrication of practical devices, patterning processes that can be used to form specific MoS2 structures must be integrated with the existing synthetic approaches. Here, we report a method for the synthesis of centimeter-scale, high-quality single-layer MoS2 that can be directly patterned during CVD, so that postpatterning processes can be avoided and device fabrication can be streamlined. Utilizing X-ray photoelectron spectroscopic imaging, we characterize the chemical states of these CVD-synthesized single-layer MoS2 films and demonstrate that the triangular-shaped MoS2 are single-crystalline single-domain monolayers. We also demonstrate the use of these high-quality and directly patterned MoS2 films in electronic device applications by fabricating and characterizing field effect transistors.

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.

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 10(12), 10(13), and 10(14) cm(-2). For a low proton beam fluence condition of 10(12) cm(-2), the electrical properties of the devices were nearly unchanged in response to proton irradiation. In contrast, for proton beam fluence conditions of 10(13) or 10(14) 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.

Cutaneous metastasis to the scalp from squamous cell carcinoma of the cervix

Carcinoma of the cervix is a common neoplasm. Early detection and improved treatment techniques have resulted in better control of cancer and longer survival, however, invasive and metastatic disease still occur. Cervical cancer usually spreads through direct local extension and via the lymphatics, haematogenous metastasis is relatively infrequent and cutaneous metastases are very rare. Metastasis to the scalp is extremely rare and only three cases of scalp metastasis from cervical cancer have been reported in the literature. We now report a patient with cervical cancer who presented with metastasis to this very unusual site.

Gate-bias stress-dependent photoconductive characteristics of multi-layer MoS2 field-effect transistors

We investigated the photoconductive characteristics of molybdenum disulfide (MoS2) field-effect transistors (FETs) that were fabricated with mechanically exfoliated multi-layer MoS2 flakes. Upon exposure to UV light, we observed an increase in the MoS2 FET current because of electron-hole pair generation. The MoS2 FET current decayed after the UV light was turned off. The current decay processes were fitted using exponential functions with different decay characteristics. Specifically, a fast decay was used at the early stages immediately after turning off the light to account for the exciton relaxation, and a slow decay was used at later stages long after turning off the light due to charge trapping at the oxygen-related defect sites on the MoS2 surface. This photocurrent decay phenomenon of the MoS2 FET was influenced by the measurement environment (i.e., vacuum or oxygen environment) and the electrical gate-bias stress conditions (positive or negative gate biases). The results of this study will enhance the understanding of the influence of environmental and measurement conditions on the optical and electrical properties of MoS2 FETs.

Enhancement of photodetection characteristics of MoS2 field effect transistors using surface treatment with copper phthalocyanine

Recently, two-dimensional materials such as molybdenum disulfide (MoS2) have been extensively studied as channel materials for field effect transistors (FETs) because MoS2 has outstanding electrical properties such as a low subthreshold swing value, a high on/off ratio, and good carrier mobility. In this study, we characterized the electrical and photo-responsive properties of MoS2 FET when stacking a p-type organic copper phthalocyanine (CuPc) layer on the MoS2 surface. We observed that the threshold voltage of MoS2 FET could be controlled by stacking the CuPc layers due to a charge transfer phenomenon at the interface. Particularly, we demonstrated that CuPc/MoS2 hybrid devices exhibited high performance as a photodetector compared with the pristine MoS2 FETs, caused by more electron-hole pairs separation at the p-n interface. Furthermore, we found the optimized CuPc thickness (∼2 nm) on the MoS2 surface for the best performance as a photodetector with a photoresponsivity of ∼1.98 A W(-1), a detectivity of ∼6.11 × 10(10) Jones, and an external quantum efficiency of ∼12.57%. Our study suggests that the MoS2 vertical hybrid structure with organic material can be promising as efficient photodetecting devices and optoelectronic circuits.

Carrier lifetimes in dielectric cap disordered GaAs/AlGaAs multiple quantum well with SiN capping layers

Time resolved photoluminescence (PL) characteristics of a SiN cap disordered GaAs/AlGaAs multiple quantum well (MQW) structure exhibit a decrease in carrier lifetime in conjunction with an increase in quantum well disordering (QWD) as the SiN capping layer thickness is increased. The decrease in carrier lifetime is attributed to enhanced carrier trapping due to the defects introduced during dielectric cap quantum well disordering and the relaxation of the momentum conservation during radiative recombination by QWD. Potential applications of these effects on high speed optical devices such as laser diodes (LD’s) and optical modulators are discussed.

Trap-mediated electronic transport properties of gate-tunable pentacene/MoS2 p-n heterojunction diodes

We investigated the trap-mediated electronic transport properties of pentacene/molybdenum disulphide (MoS2) p-n heterojunction devices. We observed that the hybrid p-n heterojunctions were gate-tunable and were strongly affected by trap-assisted tunnelling through the van der Waals gap at the heterojunction interfaces between MoS2 and pentacene. The pentacene/MoS2 p-n heterojunction diodes had gate-tunable high ideality factor, which resulted from trap-mediated conduction nature of devices. From the temperature-variable current-voltage measurement, a space-charge-limited conduction and a variable range hopping conduction at a low temperature were suggested as the gate-tunable charge transport characteristics of these hybrid p-n heterojunctions. Our study provides a better understanding of the trap-mediated electronic transport properties in organic/2-dimensional material hybrid heterojunction devices.