• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.132.2-132.2
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• 2013

Electrons in graphene make ballistic transport with very high mobility (${\sim}2{\times}105 $cm2V-1s-1), which holds promises for applications in fast electronic devices. However, such expectations have been hampered by the semi-metallicity or zero bandgap of graphene, which makes it impossible to completely turn off graphene transistor devices. Here, we report the observations of local bandgap modulations in Moir$\acute{e}$ patterned graphene on metal substrates using scanning tunneling microscopy and spectroscopy. The Moir$\acute{e}$ patterned graphene was made by combinations of self-assembly processes, and they showed additional electronic states that could be interpreted as sub-band states. Our experimental observations could be explained with orbital transitions of carbon atoms from sp2 to sp3, as supported by our density functional theory calculation results. Our findings will add new poweful components for device applications.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2005.10a
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• pp.498-501
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• 2005

Photonic crystals, periodic structure with a high refractive index contrast modulation, have recently become very interesting platform for manipulation of light. The existence of a photonic bandgap, a frequency range in which propagation of light is prevented in all direction, makes photonic crystal very useful in application where spatial localization of light is required for waveguide, beam splitter, and cavity. But fabrication of 3 dimensional photonic crystal is still difficult process. a concept that has recently attracted a lot of attention is a planar photonic crystal based on a dielectric membrane, suspended in the air, and perforated with 2 dimensional lattice of hole. We show that the polymer slabs suspended in air with triangular lattice of air hole can exhibit the in-plane photonic bandgap for TE-like modes. The fabrication of Si master with pillar structure using hot embossing process was investigated for 2 dimensional low-index-contrast photonic crystal waveguide.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.12 no.1
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• pp.137-142
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• 2008

The band-gap reference voltage generator which can be operated by low voltage is proposed in this paper. The proposed BGR circuit can be realized in logic process by using parasitic NPN BJTs because a $Low-V_T$ transistors are not necessary. The proposed BGR circuit is designed and fabricated using $0.18{\mu}m$ triple-well process. The mean voltage of measured VREF is 0.72V and the three sigma$(3{\sigma})$ is 45.69mv.

• Journal of the Microelectronics and Packaging Society
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• v.28 no.1
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• pp.13-20
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• 2021

GaN has shown good potential owing to its better chemical stability than other materials and tunable bandgap with materials such as InN and AlN. Tunable bandgap allows GaN to make the maximum utilization of the solar spectrum, thus improves the solar-to-hydrogen (STH) efficiency. In addition, GaN band gap contains the oxidation and reduction level of water, so it can split water without external voltage. However, STH efficiency using GaN itself is low and has been actively studied recently to improve it. In this thesis, we have summarized the studies related to the use of GaN as a photoelectrode for photoelectrochemical water splitting.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.35 no.3
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• pp.308-310
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• 2022

Ultrawide bandgap gallium oxide (Ga₂O₃) semiconductors are known to have excellent photocatalytic properties due to their high redox potential. In this study, CO₂ reduction is demonstrated using nanostructured Ga₂O₃ photocatalyst under ultraviolet (254 nm) light source conditions. After the CO₂ reduction, C₂H₄ remained as a by-product in this work. Nanostructured Ga₂O₃ photocatalyst also showed an excellent endurance characteristic. Photogenerated electron-hole pairs boosted the CO₂ reduction to C₂H₄ via nanostructured Ga₂O₃ photocatalyst, which is attributed to the ultrawide and almost direct bandgap characteristics of the gallium oxide semiconductor. The findings in this work could expedite the realization of CO₂ reduction and a simultaneous C₂H₄ production using a low cost and high performance photocatalyst.

• Nanomaterials
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• v.11 no.3
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• pp.767-780
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• 2021

Tin sulfide polymorph (π-SnS) nanoparticles exhibit promising optoelectrical characteristics for photovoltaic and hydrogen production performance, mainly because of the possibility of tuning their properties by adjusting the synthesis conditions. This study demonstrates a chemical approach to synthesize π-SnS nanoparticles and the engineering of their properties by altering the Sn precursor concentration (from 0.04 M to 0.20 M). X-ray diffraction and Raman studies confirmed the presence of pure cubic SnS phase nanoparticles with good crystallinity. SEM images indicated the group of cloudy shaped grains, and XPS results confirmed the presence of Sn and S in the synthesized nanoparticles. Optical studies revealed that the estimated energy bandgap values of the as-synthesized π-SnS nanoparticles varied from 1.52 to 1.68 eV. This work highlights the effects of the Sn precursor concentration on the properties of the π-SnS nanoparticles and describes the bandgap engineering process. Optimized π-SnS nanoparticles were used to deposit nanocrystalline π-SnS thin films using the drop-casting technique, and their physical properties were improved by annealing (300 ℃ for 2 h).

• Korean Chemical Engineering Research
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• v.56 no.4
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• pp.585-591
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• 2018

Characteristics of nano-structured $SiO_2:Zn$ hollow powders prepared in the micro drop fluidized reactor process were investigated with respect to bandgap energy and surface activity. The $SiO_2:Zn$ hollow powders were successfully prepared continuously in the one step process with reasonable production efficiency, with varying the amount of THAM (tris(hydroxymethyl)-aminomethane) additive and concentration of $Zn^{2+}$ ions. The doping of $Zn^{2+}$ ions into $SiO_2$ lattice led to the reduction of bandgap energy by forming the acceptor level of $Zn^{2+}$ below the conduction band of $Si^{4+}$ ions. The hollow shape also contributed to reduce the bandgap energy of $SiO_2:Zn$ powders. The doping of $Zn^{2+}$ ions into $SiO_2$ hollow powders could enhance the surface activity by forming SiO-H stretching and oxygen vacancies at the surface of $SiO_2:Zn$ powders.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.16 no.3 s.94
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• pp.300-310
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• 2005

In this paper we propose the compact three-layer EBG structure. The unit cell of the proposed EBG structure is composed of a square patch in the upper layer and a square ring stripline in the lower layer that are connected to the ground plane through conducting vias. Reflection phase analysis method and tangential transmission method were considered to accomplish effective EM simulation and measurement. EM simulation results indicate that bandgap characteristics of the EBG structure using both methods is nearly identical. Parametric studies have been performed with the EM simulator to analyze the properties of the EBG structure by investigating the phase shift of the normally incident plane wave, and the transmission measurements between simple monopole antennas positioned near the EBGstructure have been done. The operating fiefuency bandgap of the proposed EBG structure is about 34 $\%$ lower than the conventional EBG structure with the same size. Measured results show bandgap from 0.930 GHz to 0.945 GHz.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.32 no.4
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• pp.121-127
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• 2022

β-Gallium oxide (Ga₂O₃), an ultra-wide bandgap semiconductor, has attracted great attention due to its promising applications for high voltage power devices. The most stable phase among five different polytypes, β-Ga₂O₃ has the wider bandgap of 4.9 eV and higher breakdown electric field of 8 MV/cm. Furthermore, it can be grown from melt source, implying higher growth rate and lower fabrication cost than other wide bandgap semiconductors such as SiC, GaN and diamond for the power device applications. In this study, β-Ga₂O₃ bulk crystals were grown by the edge-defined film-fed growth (EFG) process. The growth direction and the principal surface were set to be the [010] direction and the (100) plane of the β-Ga₂O₃ crystal, respectively. The spectra measured by Raman an alysis could exhibit the crystal phase an d impurity dopin g in the β-Ga₂O₃ ingot, and the crystallinity quality and crystal direction were analyzed using high-resolution X-ray diffraction (HRXRD). The crystal quality and various properties of as-grown β-Ga₂O₃ ribbon was systematically analyzed in order to investigate the spatial variation in entire crystal grown by EFG method.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.25 no.3A
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• pp.430-434
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• 2000

In this paper, a novel photonic bandgap(PBG) structure is proposed for increasing stropband of lowpass filter without the size increment of circuit for application in microstrip circuits. The proposed structure is connected in parallel two periodic structures which have different center frequency of the stopband. The wide stopband is achieved by two periodic structures of two different stopbands. We also show the performance improvement of microstrip patch antenna by etching of the proposed structure in ground plane.