• Journal of Surface Science and Engineering
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• v.47 no.6
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• pp.297-302
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• 2014

ZnS nanowires were synthesized in order to investigate $NO_2$ gas sensing properties. They were grown on the sapphire substrate using ZnS powders. SEM (scanning electron microscopy) showed the diameter and length of the ZnS nanowires were approximately in the range of 50 - 100 nm and a few $10s\;{\mu}m$, respectively. They were also found to be composed of Wurtzite- structured single crystals from TEM (transmission electron microscopy) analysis. $NO_2$ gas sensing performance of the ZnS nanowire was measured with electrical resistance changes caused by $NO_2$ gas with a concentration of 1-5ppm. The sensor was UV treated with an intensity of $1.2mW/cm^2$ to facilitate charge carrier transfer. The responses of the ZnS nanowires to the $NO_2$ gas at room temperature, treated with UV of two different wavelengths of 365 nm and 254 nm, are measured to be 124.53 - 206.87 % and 233.97 - 554.83%, respectively. In the current work, the effect of UV treatment on the gas sensing performance of the ZnS nanowires was studied. And the underlying mechanism for the electrical resistance changes of the ZnS nanowires by $NO_2$ gas was also discussed.

• Korean Journal of Materials Research
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• v.4 no.5
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• pp.590-599
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• 1994

A work on the brittle to ductile transition (BDT) in single crystal alumina has been performed to understand and assess the dynamics of dislocation mobility around crack tip of brittle material. The critical stress intensity factor and yield strengths were obtained from bending test using precracked specimens at elevated temperatures. It was found that the BDT temperature was dependent on strain rate and orientation of specimen : for (1120) fracture surface, $1034^{\circ}C$, $1150^{\circ}C$ for $4.2 \times 10^{-6}$, $4.2 \times 10^{-7}s^{-1}$ respectively. Under a 4 point bending test, the moving distance of dislocation generated near crack front in ductile range is determined by an etch pits method. The velocity of dislocation in sapphire obtained from the double etching method was applied to modelling study.

• Korean Journal of Materials Research
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• v.13 no.10
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• pp.673-676
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• 2003

We have investigated $Al_{x}$ $Ga_{l-x}$ N/GaN epilayers (x = 0.08, 0.15) grown by metal organic vapor phase epitaxy on sapphire with photoluminescence(PL), and persistent photoconductivity(PPC) experiments. An anomalous S-shaped shift behavior of temperature dependencies of PL peak energy is observed for the x = 0.15 sample. In PPC measurement, showed that the dark current recovery time of $Al_{x}$$Ga_{l-x}$ N/GaN epilayers mainly depends on the Al content. These behaviors are usually attributed to the presence of carrier localization states. All these phenomena are explained based on the alloy compositional fluctuations in the $Al_{x}$ /$Ga_{l-x}$ N/ epilayers. The photocurrent quenching observed in PPC measurements for $Al_{x}$ $Ga_{l-x}$ N/ epilayers less than 0.2 $\mu\textrm{m}$ thickness indicates that the presence of metastable state in the bandgap of GaN layer, and that the excess holes in the valence band recombine with free electrons.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.32 no.1
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• pp.47-52
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• 2019

Micro-LEDs show lower efficiencies compared to general LEDs having large areas. Simulations were carried out using ray-tracing software to investigate the change in light extraction efficiency and light distribution according to chip-size of blue flip-chip micro-LEDs (FC ${\mu}-LEDs$). After fixing the height of the square FC ${\mu}-LED$ chip at $158{\mu}m$, the length of one side was varied, with dimensions of 2, 5, 10, 30, 50, 100, 300, and $500{\mu}m$. The highest light-extraction efficiency was obtained at $10{\mu}m$, beyond which the efficiency decreased as the chip-size increased. The chip size-dependence of the FC ${\mu}-LEDs$ both without the patterned sapphire substrate, as well as vertical FC ${\mu}-LEDs$, were analyzed.

• Journal of the Korean Physical Society
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• v.73 no.12
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• pp.1834-1839
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• 2018

A tabletop ultrafast soft x-ray has been generated from the laser-produce plasma with a femtosecond pulsed Ti:Sapphire laser. The estimated total flux of Al $K{\alpha}$ is of $2.2{\times}10^9photons/sec$ in $4{\pi}$ radian and the parameters related to the optical performance were obtained. The tungsten/silicon multilayer, flat quartz and bent thallium acid phthalate (TLAP) crystal were used for monochromatization of soft x-ray to refine the aluminum $K{\alpha}$ radiation and compared the respective value of $E/{\Delta}E$. To estimate the size of the x-ray source beam generated by a fs laser, the approximation using the FWHM obtained from the x-ray beam scan near the focal point was discussed, and the size of the diameter was about $9.76{\mu}m$.

• Korean Journal of Materials Research
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• v.31 no.11
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• pp.635-641
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• 2021

Cr thin films with O added are deposited on sapphire substrate by DC sputtering and are nitrided in NH₃ atmosphere between 300 and 900 ℃ for various times. X-ray diffraction results show that nitridation begins at 500 ℃, forming CrN and Cr₂N. Cr oxides of Cr₂O₃ are formed at 600 ℃. And, at temperatures higher than 900 ℃, the intermediate materials of Cr₂N and Cr₂O₃ disappear and CrN is dominant. The atomic concentration ratios of Cr and O are 77% and 23%, respectively, over the entire thickness of as-deposited Cr thin film. In the sample nitrided at 600 ℃, a CrN layer in which O is substituted with N is formed from the surface to 90 nm, and the concentrations of Cr and N in the layer are 60% and 40%, respectively. For this reason, CrN and Cr₂N are distributed in the CrN region, where O is substituted with N by nitridation, and Cr oxynitrides are formed in the region below this. The nitridation process is controlled by inter-diffusion of O and N and the parabolic growth law, with activation energy of 0.69 eV.

• Proceedings of the Materials Research Society of Korea Conference
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• 2003.03a
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• pp.44-44
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• 2003

Growth mechanism of carbon nanotubes(CNTs) synthesized by chemical vapor deposition is abided by two growth modes. These growth modes are classified by the position of activated catalytic metal particle in the CNTs. Growth mode can be also affected by interaction between substrate and catalytic metal and induced energy such as thermal and plasma. We studied the reaction of catalytic metal to the substrate and growth mode of CNTs. Various substrates such as Si(100), graphite plate, coming glass, sapphire and AAO membrane are used to study the relation between catalytic metal and substrate in the synthesis of CNTs. For catalytic metal, thin film was deposited on various substrate via sputtering technique with a thickness of ∼20nm and magnetic fluids with none-sized particles were dispersed on AAO membrane. After laying process on AAO membrane, it was dried at 80$^{\circ}C$ for 8 hour. Synthesizing of CNTs was carried out at 900$^{\circ}C$ in NH3/C2H2 mixture gases flow for 10minutes.

• Journal of the Korean Society for Precision Engineering
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• v.12 no.7
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• pp.46-52
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• 1995

This paper proposes methodologies for the development of radiation thermometer using InSb photo-detector of which spectral sensitivity is excellent over the wave length range of 2 .mu. m .approx. 5 .mu. m. The proposed radiation thermometer has broad measurement range from normal to high, up to more than 1000 .deg. C, with high accuracy, and can measure temperature on the material surface or heat emission noncontactely with high speed. Optical system was consisted of two convex lens with foruslength of 15.2mm for infrared lay focusing, Ge filter to cut the short wave length components and sapphire filter to cut the long wave length components. The cold shielded was installed in the whole surface of the light-absorbing element to remove the error- mometer, calibration using black body furnace which has temperature range of 90 .deg. C .approx. 1100 .deg. C was carried out, and temperature calaibration curve was obtained by exponential function curvefitting. The result shows maximum error less than 0.24%(640K .+-. 1.6K) over the measurement range of 90 .deg. C .approx. 700 .deg. C, and from this result the usefulness of the developed thermometer has been confirmed.

• Journal of the Korean Society for Heat Treatment
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• v.35 no.2
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• pp.51-56
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• 2022

In this study, we controlled the shape of a carbon layer on gallium oxide templates. Gallium oxide layers were deposited on sapphire substrates using mist chemical vapor deposition. Subsequently, carbon layers were formed using radio frequency plasma chemical vapor deposition. Various shapes of carbon structures appeared according to the fraction of methane gas, used as a precursor. As methane gas concentration was adjusted from 1 to 100%, The shapes of carbon structures varied to diamonds, nanowalls, and spheres. The growth of carbon isotope structures on Ga₂O₃ templates will give rise to improving the electrical and thermal properties in the next-generation electronic applications.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.18 no.3
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• pp.97-100
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• 2008

Polar and non-polar GaN was grown by the HVPE on various substrates and influence of polarity has been investigated. The $10\;{\mu}m$ thickness GaN were grown by HVPE is along A-plane ($11{\bar{2}}0$), C-plane (0001) and M-Plane ($10{\bar{1}}0$) sapphire substrate respectively. Surface properties were observed by optical microscope and atomic force microscopy. High resolution X-ray diffraction (HR-XRD) confirms the wurtzite structure. The donor band exciton peak located at ${\sim}3.4\;eV$ and also located yellow luminescence peak at 2.2 eV. The polarity of the GaN film has a strong influence on the morphology and the optical properties.