• Applied Microscopy
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• v.48 no.2
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• pp.43-48
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• 2018

Transition metal dichalcogenide (TMD) materials have distinctive structures in comparison with other two-dimensional materials. TMD materials' structure is held together by van der Waals and covalent intralayer interactions; consequently, TMDs exhibit multiple phases and properties depending on their structure. This article reviews some of the research currently being undertaken to control TMD phases to utilize their different properties. This review introduces some trials for changing the phase of TMDs.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.291.2-291.2
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• 2016

3D semiconductor material of silicon that is used throughout the semiconductor industry currently faces a physical limitation of the development of semiconductor process technology. The research into the next generation of nano-semiconductor materials such as semiconductor properties superior to replace silicon in order to overcome the physical limitations, such as the 2-dimensional graphene material in 2D transition-metal dichalcogenide (TMD) has been researched. In particular, 2D TMD doping without severely damage of crystal structure is required different conventional methods such as ion implantation in 3D semiconductor device. Here, we study a p-type doping technique on tungsten diselenide (WSe2) for p-channel 2D transistors by adjusting the concentration of hydrochloric acid through Raman spectroscopy and electrical/optical measurements. Where the performance parameters of WSe2 - based electronic device can be properly designed or optimized. (on currents increasing and threshold voltage positive shift.) We expect that our p-doping method will make it possible to successfully integrate future layered semiconductor devices.

• Proceeding of EDISON Challenge
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• 2016.03a
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• pp.368-372
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• 2016

In this paper, the physics-based analytical model of transition metal dichalcogenide (TMD)-based double-gate (DG) tunneling field-effect transistors (TFETs) is proposed. The proposed model is derived by using the two-dimensional (2-D) Landauer formula and the Wentzel-Kramers-Brillouin (WKB) approximation. For improving the accuracy, nonlinear and continuous lateral energy band profile is applied to the model. 2-D density of states (DOS) and two-band effective Hamiltonian for TMD materials are also used in order to consider the 2-D nature of TMD-based TFETs. The model is validated by using the tight-binding non-equilibrium Green's function (NEGF)-based quantum transport simulation in the case of monolayer molybdenum disulfide ($MoS_2$)-based TFETs.

• Journal of the Korean Electrochemical Society
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• v.23 no.2
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• pp.25-38
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• 2020

Photoelectrochemical water splitting has been considered as the most promising technology for generating hydrogen energy. Transition metal dichalcogenide (TMD) compounds have currently attracted tremendous attention due to their outstanding ability towards the catalytic water-splitting hydrogen evolution reaction (HER). Herein, we report the synthesis method of various transition metal dichalcogenide including MoS₂, MoSe₂, WS₂, and WSe₂ nanosheets as excellent catalysts for solar-driven photoelectrochemical (PEC) hydrogen evolution. Photocathodes were fabricated by growing the nanosheets directly onto Si nanowire (NW) arrays, with a thickness of 20 nm. The metal ion layers were formed by soaking the metal chloride ethanol solution and subsequent sulfurization or selenization produced the transition metal chalcogenide. They all exhibit excellent PEC performance in 0.5 M H₂SO₄; the photocurrent reaches to 20 mA cm^-2 (at 0 V vs. RHE) and the onset potential is 0.2 V under AM1.5 condition. The quantum efficiency of hydrogen generation is avg. 90%. The stability of MoS₂ and MoSe₂ is 90% for 3h, which is higher than that (80%) of WS₂ and WSe₂. Detailed structure analysis using X-ray photoelectron spectroscopy for before/after HER reveals that the Si-WS₂ and Si-WSe₂ experience more oxidation of Si NWs than Si-MoS₂ and Si-MoSe₂. This can be explained by the less protection of Si NW surface by their flake shape morphology. The high catalytic activity of TMDs should be the main cause of this enhanced PEC performance, promising efficient water-splitting Si-based PEC cells.

• Journal of Adhesion and Interface
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• v.21 no.3
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• pp.107-112
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• 2020

Recently, the atomically thin transition-metal dichalcogenide (TMD) semiconductors have attracted much attention owing to their remarkable properties such as tunable bandgap with high carrier mobility, flexibility, transparency, etc. However, because these TMD materials have a significant drawback that they are easily degraded in an ambient environment, various attempts have been made to improve chemical stability. In this research article, I report a method to improve the air stability of WSe₂ one of the TMD materials via surface passivation with an h-BN insulator, and its application to field-effect transistors (FETs). With a modified hot pick-up transfer technique, a vertical heterostructure of h-BN/WSe₂ was successfully made, and then the structure was used to fabricate the top-gate bottom-contact FETs. The fabricated WSe₂-based FET exhibited not only excellent air stability, but also high hole mobility of 150 ㎠/Vs at room temperature, on/off current ratios up to 3×10⁶, and 192 mV/decade of subthreshold swing.

• Vacuum Magazine
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• v.3 no.4
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• pp.8-13
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• 2016

The need of space-resolved x-ray photoelectron spectroscopy (XPS) has developed scanning photoelectron microscopy (SPEM). SPEM provides space-resolved XPS data from a spot of a sample as well as images of specific element, chemical state, valency distribution on the surface of a sample. Based on technical advancement of tight x-ray focusing, sample positioning accuracy, and electron analyzer efficiency, SPEM is now capable of providing ~100 nm space resolution for typical XPS functionality, and SPEM has become actively applied for the investigation of chemical state, valency, and electronic structure on the surface of newly discovered materials, such as graphene layers, dichalcogenide 2D-materials, and heterogenous new functional materials.

• Electronics and Telecommunications Trends
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• v.29 no.6
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• pp.43-52
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• 2014

수 원자층 두께의 전이금속 칼코겐화물 이차원 반도체 소재는 스위칭 소자 등에 활용하기에 적합한 밴드갭 에너지를 가지며, 높은 이동도와 우수한 광반응성으로 인해 최근 큰 관심을 끌고 있다. 특히 이차원 소재이므로 dangling bond가 없다는 점, 구조적 안정성, 실리콘에 뒤지지 않는 고이동도, 직접천이 특성 등으로 인해 차세대 전자소자용, 더 나아가 실리콘 반도체 대체 소재로써의 가능성도 점쳐지고 있다. 본고에서는 전이금속 칼코겐화물 이차원 반도체의 소재 특성과 제조방법, 소자 응용면에서의 기술개발 동향, 시장전망 등에 대해 소개하고, 이 소재가 현재 기대하는 만큼 중요하게 활용되고 기술이 발전하기 위해서 반드시 해결해야 할 숙제 등에 대해 논의하고자 한다.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.575-575
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• 2012

Synthesis of novel two dimensional materials has gained tremendous attention recently as they are considered as alternative materials for replacing graphene that suffers from a lack of bandgap, a property that is essential for many applications. Single layer molybdenum disulfide ($MoS_2$) has a direct bandgap (1.8eV) that is promising for use in next-generation optoelectronics and energy harvesting devices. We have successfully grown high quality single layer $MoS_2$ by a facile vapor-solid transport route. As-grown single layer $MoS_2$ was carefully characterized by using X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy and electrical transport measurement. The results indicate that a high quality single layer $MoS_2$ can be successfully grown on silicon substrate. This may open up great opportunities for the exploration of novel nanoelectronic devices.

• Proceeding of EDISON Challenge
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• 2016.03a
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• pp.295-298
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• 2016

Future logic device over the FinFET generation requires a complete electrostatics and transport characteristic for low-power and high-speed operation as extremely scaled devices. Silicon, Germanium and III-V based nanowire-based MOSFET devices and few-layer TMDC (Transition metal dichalcogenide monolayers) based multi-gate devices have been brought attention from device engineers due to those excellent electrostatic and novel device characteristic. In this study, we simulated ultrascaled Si/Ge/InAs gate-all-around nanowire MOSFET and MoS2 TMDC based DG MOSFET and TFET device by tight-binding NEGF method. As a result, we can find promising candidates of the future logic device of each channel material and device structures.

• Journal of Surface Science and Engineering
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• v.49 no.2
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• pp.186-190
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• 2016

In this study, we fabricated the transparent photovoltaic device using 2-dimensional transition-metal dichalcogenides and investigated the transparency and photovoltaic characteristics. P-n heterojunction was formed by mechanical exfoliation and aligned transfer method on the transparent sheet using n-type $MoS_2$ and p-type $WSe_2$. Our transparent photovoltaic device exhibited the open-circuit voltage of ~ 0.15 V and the short-circuit current of 0.48 nA under illumination of white light.