• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.461-461
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• 2013

Over the last decade, zinc oxide (ZnO) thin films have attracted considerable attention owing to large band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature [1-3]. Recent interest in ZnO related researches has been switched into the fabrication and characterization of low-dimensional nanostructures, such as nano-wires and nano-dots that can be applicable to manufacture the optoelectronic devices such as ultraviolet lasers, light-emitting-diodes and detectors. Since the optical properties of ZnO nano-structures might be distinct from those of bulk materials or thin films, the low-dimensional phenomena should be examined further. In order to utilize such advanced optoelectronic devices, one of the challenges is how to control the surface state related emissions that are drastically increased with increasing the density of the nano-structures and the surface-to-volume ratio. This paper reports the synthesis and characterization of self-assembled ZnO hexagonal nano-disks grown by radio-frequency magnetron sputtering. X-ray diffraction data and scanning electron microscopy data showed that ZnO hexagonal nano-disks were nucleated on top of the flat surfaces as the film thickness reached to 1.56 ${\mu}m$ and then the number of nano-disks increased with increasing the film thickness. The lateral size of hexagonal nano-disks was ~720 nm and height was ~74 nm. The strong photo luminescence spectra obtained at 10 K was also observed, which was assigned to a surface exciton emission at 3.3628 eV arising from the surface sites of hexagonal nano-disks.

• Journal of the Korean Ceramic Society
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• v.36 no.8
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• pp.813-819
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• 1999

Inner crack-like pores with controlled amount of Ca impurity level in the high purity alumina single crystal sapphire had been created by micro-fabrication technique which includes ion implantation photo-lithography Ar ion milling and hot press technique. The morphological change and the healing of crack-like pore in the Ca doped high purity single crystal alumina during high temperature heat treatment in vacuum were observed using optical microscopy. The hexagonal bridging ligaments were developed and the size of hexagonal bridging ligaments had been increased with temperature and Ca amount and had grown to their corner rounded. It appeared that the hexagonal bridging ligaments would have an equilbrium size with temperature and the amount of Ca addition. Three kinds of crack-like pore healing type were observed. Edge regression and ligament growth were observed from relatively low temperature in the crack-like pore. Edge regression were found in almost all of the crack-like pore but the ligament growth were found only in the several crack-like pores accelerating heating very fast. Flow type healing was observed above $1800^{\circ}C$ and It healed the crack-like pore very slowly.

• Journal of the Korean Ceramic Society
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• v.40 no.2
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• pp.206-211
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• 2003

Effects of substrates and buffer layer upon the formation of crystalline phases and ferroelectricity of $YMnO_3$ thin films were investigated. The hexagonal $YMnO_3$ was easily formed on Si(100) while the mixed phases, hexagonal and orthorhombic $YMnO_3$, on $Pt(111)/TiO_2/SiO_2/Si$ substrate. When the $Y_2O_3$ buffer layer of 70 nm thick was inserted between the substrates and the $YMnO_3,$ the c-axis oriented hexagonal single phase formed on both substrates, Si(100) and $Pt(111)/TiO_2/SiO_2/Si$. The leakage current density of the hexagonal $YMnO_3$ thin films was lower than that consisting of mixed phases, hexagonal and orthorhombic. Furthermore the hexagonal $YMnO_3$ with c-axis preferred orientation showed the lowest leakage current density. The remnant polarization from a P-E hysteresis curve for the $YMnO_3$ formed on Si(100) was 0.14 without buffer layer and $0.24_{mu}C/cm^2$ for that with buffer layer. For the $Pt(111)/TiO_3/SiO_3/Si$ substrates, the specimen without $Y_2O_3$buffer layer did not show the hysteresis curve, while the buffer-layered has the remnant polarization of $1.14_{mu}C/cm^2$. It was concluded that the leakage current density and the ferroelectricity for the $YMnO_3$ thin films could be controlled by varying crystalline phases and their preferred orientation which depend on the kind of substrates and whether the $Y_2O_3$buffer layer exist or not.

• Korean Journal of Materials Research
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• v.21 no.2
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• pp.111-114
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• 2011

As a growth-template of ZnO nanorods (NR), a hexagonal $\beta-Ni(OH)_2$ nanosheet (NS) was synthesized with the low temperature hydrothermal process and its microstructure was investigated using a high resolution scanning electron microscope and transmission electron microscope. Zinc nitrate hexahydrate was hydrolyzed by hexamethylenetetramine with the same mole ratio and various temperatures, growth times and total concentrations. The optimum hydrothermal processing condition for the best crystallinity of hexagonal $\beta-Ni(OH)_2$ NS was determined to be with 3.5 mM at $95^{\circ}C$ for 2 h. The prepared $Ni(OH)_2$ NSs were two dimensionally arrayed on a substrate using an air-water interface tapping method, and the quality of the array was evaluated using an X-ray diffractometer. Because of the similarity of the lattice parameter of the (0001) plane between ZnO (wurzite a = 0.325 nm, c = 0.521 nm) and hexagonal $\beta-Ni(OH)_2$ (brucite a = 0.313 nm, c = 0.461 nm) on the synthesized hexagonal $\beta-Ni(OH)_2$ NS, ZnO NRs were successfully grown without seeds. At 35 mM of divalent Zn ion, the entire hexagonal $\beta-Ni(OH)_2$ NSs were covered with ZnO NRs, and this result implies the possibility that ZnO NR can be grown epitaxially on hexagonal $\beta-Ni(OH)_2$ NS by a soluble process. After the thermal annealing process, $\beta-Ni(OH)_2$ changed into NiO, which has the property of a p-type semiconductor, and then ZnO and NiO formed a p-n junction for a large area light emitting diode.

• Journal of the Institute of Electronics and Information Engineers
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• v.50 no.3
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• pp.102-112
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• 2013

In this paper, two step processing method of contour extraction and complement which contain hexagonal shape for low contrast and noisy images is proposed. This method is based on the combination of Laplacian-Gaussian filter and an idea of filters which are dependent on the shape. At the first step, an algorithm which has six masks as its extractors to extract the hexagonal edges especially in the corners is used. Here, two tricorn filters are used to detect the tricorn joints of hexagons and other four masks are used to enhance the line segments of hexagonal edges. As a natural image, a corneal endothelial cell image which usually has regular hexagonal form is selected. The edge extraction of hexagonal shapes in corneal endothelial cell is important for clinical diagnosis. The proposed algorithm and other conventional methods are applied to noisy hexagonal images to evaluate each efficiency. As a result, this proposed algorithm shows a robustness against noises and better detection ability in the aspects of the output signal to noise ratio, the edge coincidence ratio and the extraction accuracy factor as compared with other conventional methods. At the second step, the lacking part of the thinned image by an energy minimum algorithm is complemented, and then the area and distribution of cells which give necessary information for medical diagnosis are computed.

• Bulletin of the Korean Mathematical Society
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• v.54 no.2
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• pp.417-428
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• 2017

We classify the extreme bilinear forms of the unit ball of the space of bilinear forms on ${\mathbb{R}}^2$ with hexagonal norms.

• Journal of the Korean Chemical Society
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• v.38 no.11
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• pp.785-791
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• 1994

The $Sr_1-_xY_xMnO_3$ (x = 0.0∼1.0) system was synthesized using amorphous citrate process. The stability of various structures and the electronic transport properties of this system were investigated. X-ray diffraction study indicated that the $Sr_1-_xY_xMnO_3$ system has three different structures depending on composition, namely, 4L-hexagonal perovskite (when x is less than 0.3), pseudocubic perovskite (when x is 0.3∼0.7), and hexagonal nonperovskite (when x is larger than 0.7) structures. The structural changes and electronic properties were interpreted based on two factors, i.e., the size of cations and the oxidation state of manganese ion. When the concentration of Y substitution exceeds 30%, the Mn-Mn repulsive interaction dominates over intermetallic attraction, and thus structure changes to pseudocubic perovskite. In perovskite phase the unit cell dimensions increases with increasing $Mn^{3+}$ ions due to yttrium substitution. The band gap of $Sr_{0.9}Y_{0.1}MnO_3$ is greater than that of $Sr_{0.5}Y_{0.5}MnO_3$. The greater band gap of $Sr_{0.9}Y_{0.1}MnO_3$ indicates that the 4L-hexagonal structure is more stabilized than cubic perovskite due to the Mn-Mn bond.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.30 no.2
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• pp.41-46
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• 2020

The characteristics of the residual stress on the types of the substrate was investigated with adjusting the V/III ratio during GaN growth via the HVPE method. GaN single crystal layers were grown on a sapphire substrate and a GaN template under the conditions of V/III ratio 5, 10, and 15, respectively. During GaN growth, multiple hexagonal pits in GaN single crystal were differently revealed in accordance with growth condition and substrate type, and their distribution and depth were measured via optical microscopy(OM) and white light interferometry(WLI). As a result, it was confirmed that the distribution area and depth of hexagonal pit tended to increase as the V/III ratio increased. Moreover, it was found that the residual stress in GaN single crystal decreased as the distribution area and depth of the pit increased through measuring micro Raman spectrophotometer. In the case of GaN growth according to substrate type, the GaN on GaN template showed lower residual stress than the GaN grown on sapphire substrate.

• The Journal of the Acoustical Society of Korea
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• v.18 no.3E
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• pp.37-43
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• 1999

In this paper, we present a new technique for the optimal local decomposition of convex structuring elements on a hexagonal grid, which are used as templates for morphological image processing. Each basis structuring element in a local decomposition is a local convex structuring element, which can be contained in hexagonal window centered at the origin. Generally, local decomposition of a structuring element results in great savings in the processing time for computing morphological operations. First, we define a convex structuring element on a hexagonal grid and formulate the necessary and sufficient conditions to decompose a convex structuring element into the set of basis convex structuring elements. Further, a cost function was defined to represent the amount of computation or execution time required for performing dilations on different computing environments and by different implementation methods. Then the decomposition condition and the cost function are applied to find the optimal local decomposition of convex structuring elements, which guarantees the minimal amount of computation for morphological operation. Simulation shows that optimal local decomposition results in great reduction in the amount of computation for morphological operations. Our technique is general and flexible since different cost functions could be used to achieve optimal local decomposition for different computing environments and implementation methods.

• Journal of Institute of Control, Robotics and Systems
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• v.20 no.2
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• pp.163-169
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• 2014

Using the classical cascade voltage control strategy, this paper proposes an analytical solution to an MPC (Model Predictive Control) problem with a hexagonal input constraint set for the inner-loop to regulate the output voltage of the UPS (Uninterruptible Power Supply). Focus is placed on how to deal with the hexagonal input constraint set without any approximation. Following the conventional cascade voltage control strategy, the PI (Proportional-Integral) controller is used in the outer-loop in order to regulate the output voltage. The simulation results illustrate that the capacitor voltage rapidly goes to its reference in a satisfactory manner while keeping other state variables bounded under an unexpected load changes.