Byungjin Cho

Enhanced Performance of MoS2 Photodetectors by Inserting an ALD-Processed TiO2 Interlayer

2D molybdenum disulfide (MoS2 ) possesses excellent optoelectronic properties that make it a promising candidate for use in high-performance photodetectors. Yet, to meet the growing demand for practical and reliable MoS2 photodetectors, the critical issue of defect introduction to the interface between the exfoliated MoS2 and the electrode metal during fabrication must be addressed, because defects deteriorate the device performance. To achieve this objective, the use of an atomic layer-deposited TiO2 interlayer (between exfoliated MoS2 and electrode) is reported in this work, for the first time, to enhance the performance of MoS2 photodetectors. The TiO2 interlayer is inserted through 20 atomic layer deposition cycles before depositing the electrode metal on MoS2 /SiO2 substrate, leading to significantly enhanced photoresponsivity and response speed. These results pave the way for practical applications and provide a novel direction for optimizing the interlayer material.

Self-Formed Channel Devices Based on Vertically Grown 2D Materials with Large-Surface-Area and Their Potential for Chemical Sensor Applications

2D layered materials with sensitive surfaces are promising materials for use in chemical sensing devices, owing to their extremely large surface-to-volume ratios. However, most chemical sensors based on 2D materials are used in the form of laterally defined active channels, in which the active area is limited to the actual device dimensions. Therefore, a novel approach for fabricating self-formed active-channel devices is proposed based on 2D semiconductor materials with very large surface areas, and their potential gas sensing ability is examined. First, the vertical growth phenomenon of SnS2 nanocrystals is investigated with large surface area via metal-assisted growth using prepatterned metal electrodes, and then self-formed active-channel devices are suggested without additional pattering through the selective synthesis of SnS2 nanosheets on prepatterned metal electrodes. The self-formed active-channel device exhibits extremely high response values (>2000% at 10 ppm) for NO2 along with excellent NO2 selectivity. Moreover, the NO2 gas response of the gas sensing device with vertically self-formed SnS2 nanosheets is more than two orders of magnitude higher than that of a similar exfoliated SnS2 -based device. These results indicate that the facile device fabrication method would be applicable to various systems in which surface area plays an important role.

Low Power Switching Characteristics of CNT Field Effect Transistor Device with Al-Doped ZrHfO2 Gate Dielectric

In this report, we demonstrated a reliable switching effect of carbon nanotube (CNT) field-effect transistor (FET) devices integrated with 99% semiconducting CNT as a channel and high-k oxide as the dielectric. CNT FET devices with high-k oxides of Al-ZrHfO2 and Al2O3 were electrically characterized and compared. There was no considerable hysteresis in the Al2O3-based CNT FET device. The Al-ZrHfO2 with a tetragonal phase-based high dielectric constant (~47), designed by an atomic layer deposition process, showed a reliable switching effect as well as low operation voltage (<±3u2009V). Charge trapping/detrapping process via oxygen vacancy-related defects of Al-ZrHfO2 was proposed as a primary mechanism to explain a current change of a counterclockwise direction and threshold voltage (Vth) shift for transfer properties. The suggested charge trapping model within bulk oxide was experimentally proven since the hysteresis from the adsorption/desorption of gas molecules to CNT surface was negligible. Endurance characteristics of the CNT switching devices remained stable without any serious current fluctuation during a repetitive cycling test. The memory device with reliable switching properties as well as low operation power would pave a road toward next-generation memory components of portable electronic gadgets.

Scalable integration of periodically aligned 2D-MoS2 nanoribbon array

Despite the recent growing interest in 2D-MoS2 nanostructures for versatile platforms of sensor applications, robust and practical 2D-MoS2 nanostructures have rarely been reported to date due to the absence of a facile, scalable, and repeatable nanofabrication method. Herein, we show the fabrication of periodically aligned 2D-MoS2 nanoribbon (MNR) array with an area of 2.25 cm2 via direct metal transfer. The MNR width is scalable from 463 nm to 135 nm at 650 nm pitch, and the smallest width is approximately 80 nm. The robust and stable material characteristics are analyzed via various spectroscopic analyses and application sensor devices.

3D Atomistic Tomography of W-based Alloyed 2D Transition Metal Dichalcogenides

Increased interest in two-dimensional (2D) materials and heterostructures for use as components of electrical devices has led to the use of an atomically mixed phase between semiconducting and metallic transition metal dichalcogenides that exhibited enhanced interfacial characteristics. To understand the lattice structure and properties of 2D materials on the atomic scale, diverse characterization methods such as Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and X-ray photoemission spectroscopy (XPS) have been applied. However, determination of the exact chemical distribution, which is a critical factor for the interfacial layer, was hindered by limitations of these typical methods. In this work, atom-probe tomography (APT) was introduced for the first time to analyze the three-dimensional atomic distribution and composition variation of the atomic-scale multilayered alloy structure W xNb(1- x)Se2. Composition profiles and theoretical calculations for each atom demonstrated the reaction kinetics and stoichiometric inhomogeneity of the W xNb(1- x)Se2 layer. The role of the intermediate layer was investigated by fabrication of a WSe2-based field-effect transistor. Introduction of W xNb(1- x)Se2 between metallic NbSe2 and semiconducting WSe2 layers resulted in improved charge transport with lowering of the contact barrier.

Facile Fabrication of a Two-Dimensional TMD/Si Heterojunction Photodiode by Atmospheric-Pressure Plasma-Enhanced Chemical Vapor Deposition

A growth technique to directly prepare two-dimensional (2D) materials onto conventional semiconductor substrates, enabling low-temperature, high-throughput, and large-area capability, is needed to realize competitive 2D transition-metal dichalcogenide (TMD)/three-dimensional (3D) semiconductor heterojunction devices. Therefore, we herein successfully developed an atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technique, which could grow MoS2 and WS2 multilayers directly onto PET flexible substrate as well as 4-in. Si substrates at temperatures of <200 °C. The as-fabricated MoS2/Si and WS2/Si heterojunctions exhibited large and fast photocurrent responses under illumination of a green light. The measured photocurrent was linearly proportional to the laser power, indicating that trapping and detrapping of the photogenerated carriers at defect states could not significantly suppress the collection of photocarriers. All the results demonstrated that our AP-PECVD method could produce high-quality TMD/Si 2D-3D heterojunctions for optoelectronic applications.