TaeWan Kim

Wafer-scale production of highly uniform two-dimensional MoS2 by metal-organic chemical vapor deposition

Semiconducting two-dimensional (2D) materials, particularly extremely thin molybdenum disulfide (MoS2) films, are attracting considerable attention from academia and industry owing to their distinctive optical and electrical properties. Here, we present the direct growth of a MoS2 monolayer with unprecedented spatial and structural uniformity across an entire 8 inch SiO2/Si wafer. The influences of growth pressure, ambient gases (Ar, H2), and S/Mo molar flow ratio on the MoS2 layered growth were explored by considering the domain size, nucleation sites, morphology, and impurity incorporation. Monolayer MoS2-based field effect transistors achieve an electron mobility of 0.47 cm2 V-1 s-1 and on/off current ratio of 5.4xa0×xa0104. This work demonstrates the potential for reliable wafer-scale production of 2D MoS2 for practical applications in next-generation electronic and optical devices.

Sensor based on chemical vapour deposition-grown molybdenum disulphide for gas sensing application

Over the past few decades, sensors based on field-effect transistors have drawn much attention. Initially three dimensional materials were used for sensing, which were later replaced by two dimensional materials because of their ease of manufacturing and large specific areas. Amongst the transition metal dichalcogenides, MoS2 has been widely used for the fabrication of sensors owing to its ability to differentiate between a charge donor and an acceptor analyte. In this work, we fabricated sensors using chemical vapour deposition grown-MoS2. MoS2 was grown on a p-Si/SiO2 substrate using Mo(CO)6 as a precursor, the growth was carried out by the sublimation of the precursor under a flow of high purity H2S at high temperature. The aim of this work is to achieve a level of sensitivity that would enable the detection of individual gas analytes upon adsorption to the MoS2 surface. To efficiently detect individual gas analytes upon adsorption to the surface, we used interdigitated electrodes in the device architecture to increase the area of the channels for analyte adsorption. We used CO2 and O2 gases, which acted as charge donors. A trilayer MoS2 film was examined, and the detection sensitivity for O2 was higher in comparison to CO2. The fabricated device showed significant sensitivity up to parts per million detection level.

Development of particle characteristics diagnosis system for nanoparticle analysis in vacuum

A particle characteristics diagnosis system (PCDS) was developed to measure nano-sized particle properties by a combination of particle beam mass spectrometry, scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDS). It allows us to measure the size distributions of nano-sized particles in real time, and the shape and composition can be determined by in situ SEM imaging and EDS scanning. PCDS was calibrated by measuring the size-classified nano-sized NaCl particles generated using an aqueous solution of NaCl by an atomizer. After the calibration, the characteristics of nano-sized particles sampled from the exhaust line of the plasma-enhanced chemical vapor deposition process were determined using PCDS.

Observation of photoluminescence from large-scale layer-controlled 2D ß-Cu2S synthesized by the vapor-phase sulfurization of copper thin films

Two-dimensional (2D) copper chalcogenides (Cu2-x X where Xxa0=xa0S, Se, Te) have had much attention regarding various applications due to their remarkable optical and electrical properties, abundance, and environmentally friendly natures. This work indicates that highly uniform Cu2-x S (where 0xa0<xa0xxa0<xa01) nanosheets can be obtained by the two-step method of Cu deposition by sputtering with precisely controlled and extremely low growth rate followed by vapor-phase sulfurization. The phase transformations of thin Cu2-x S films upon the Cu seed layer thickness are investigated. A unique thickness-constrained synthesis process using vapor-phase sulfurization is employed here, which evolves from a vertical to lateral growth mechanism based on the optimization of the Cu seed layer thickness. Atomically thin 2D β-Cu2S film was successfully synthesized using the thinnest Cu seed film. We have systematically investigated the phase- and thickness-dependent optical properties of Cu2-x S films at room temperature. Micro-photoluminescence (PL) spectroscopy reveals that the 2D β-Cu2S film possesses a direct band gap with an energy of 1.1 eV while the PL intensities are greatly suppressed in the multilayer Cu2-x S (where 0xa0≤xa0xxa0<xa01).

The Study on the Precursor Adsorption using in-situ Nanoparticle-assisted Attenuated Total Reflectance Infrared Spectroscopy

The adsorption behavior of tris (dimethylamino)-cyclopentadienyl-zirconium (Cp-Zr) precursor using an in-situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) was studied. In attempt to improve the detection intensity of an adsorbed precursor, nanoparticles were uniformly distributed on the Ge ATR crystal surface employing the spray method. The absorption characteristics studies were carried out over the Ge crystal temperature in the range of 30∼50℃. Upon increasing the temperature, a reduction of absorption was observed. Based on the peak intensities of ATR-FT-IR spectroscopy, higher-ZrO₂ absorption efficiency occurs when the nano-particles are utilized compared to pure Ge crystal.

Structural defects in a nanomesh of bulk MoS2 using an anodic aluminum oxide template for photoluminescence efficiency enhancement

Two-dimensional (2D) materials beyond graphene have attracted considerable interest because of the zero bandgap drawbacks of graphene. Transition metal dichalcogenides (TMDs), such as MoS2 and WSe2, are the potential candidates for next 2D materials because atomically thin layers of TMDs exhibit unique and versatile electrical and optical properties. Although bulk TMDs materials have an indirect bandgap, an indirect-to-direct bandgap transition is observed in monolayers of TMDs (MoS2, WSe2, and MoSe2). Optical properties of TMD films can be improved by the introduction of structural defects. For example, large-area spatial tuning of the optical transition of bulk MoS2 films is achieved by using an anodic aluminum oxide (AAO) template to induce structural defects such as edge- and terrace-terminated defects in a nanomesh structure. Strong photoluminescence emission peaks with a band gap of 1.81u2009eV are observed, possibly because of radiative transition at the defect sites. This work shows that the AAO template lithography method has potential for the production of homogenous large-scale nanomesh structures for practical semiconductor processing applications in future MoS2-based electronic and optical devices.

Characterization of particle generated during plasma-enhanced chemical vapor deposition on amorphous carbon layer using particle beam mass spectrometer

Nanoscale particles that can affect the process yield were generated using the plasma-enhanced chemical vapor deposition (PECVD) process. A particle beam mass spectrometer is remarkably useful to understand particle behavior in a reactor. The properties of nanoscale particles obtained from the exhaust line of PECVD during amorphous carbon layer (ACL) growth under various growth conditions were investigated. The number concentration and the size of the nanoscale particles generated were quite sensitive to growth parameters such as radio frequency (RF) plasma power, C2H2, and dopant (i.e., NH3 and B2H6) gas flow rate. For an ACL growth at higher RF plasma power, the average size of the nanoscale particles decreased as the number concentration of particles increased while they are opposingly increased in particle size and decreased in concentration for a higher C2H2 flow rate. A reduction in both the average nanoscale particle size and their number concentration occurred when NH3 gas was used for nitrogen-do...

Photoresponse and Field Effect Transport Studies in InAsP–InP Core–Shell Nanowires

A ternary InAsyP1−y alloy is suitable for an application to near-infrared (NIR) optical devices as their direct bandgap energy covers the entire NIR band. A nanowire (NW) system allows an epitaxial integration of InAsyP1−y alloy on any type of substrate since the lattice mismatch strain can be relieved through the NW sidewall. Nevertheless, the very large surface to volume ratio feature of the NWs leads to enormous surface states which are susceptible to surface recombination of free carriers. Here, ternary InAs0.75P0.25 NWs are grown with InP passivation layer (i.e., core–shell structure) to minimize the influence of the surface states, thus increasing their optical and electrical properties. A photoresponse study was achieved through the modeled band structure of the grown NWs. The model and experimental results suggest that 5-nm-thick InP shell efficiently passivates the surface states of the InAs0.75P0.25 NWs. The fabricated core–shell photodetectors and field-effect transistors exhibit improved photoresponse and transport properties compared to its counterpart core-only structure.

Measurement of the Particle Current Changes Associated with the Flatness of Deflector Mesh Surface in Particle Beam Mass Spectrometer System

Advanced Device Technology, University of Science & Technology, 305-350, Daejeon, KoreaReceived March 19, 2016; revised March 31, 2016; accepted March 31, 2016Abstract The surface flatness of metal meshes in a deflector of particle beam mass spectrometer (PBMS) requiredideally flat, and this can specify the particle trajectories which goes through the detector. In this research, chargedparticle current was measured using the different surface roughness deflectors. NaCl particles were generatedmonodispersed in its size by using differential mobility analyzer and the whole processes were followed the waycalibrating PBMS. The results indicate that the mesh surface morphology in the deflector can affect to the particle sizeand the concentration errors, and sensitivity of PBMS.Keywords: PBMS, Deflector, Nickel mesh, Electric field, Flatness

Stability of UV exposed RR-P3BT films by spectroscopic ellipsometry

Stability of regioregular poly(3-butylthiophene) (RR-P3BT) films under irradiation of ultra-violet (UV) light has been studied by spectroscopic ellipsometry at room temperature. Consistent decrease in dielectric function with UV exposure time showed the degree of degradation of polymer. This work suggests that, protective methods are mandatory to use this kind of material in optical devices.