• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2005.11a
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• pp.309-310
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• 2005

In the fabrication of high performance red organic light emitting diode, 2-TNA TA [4,4',4" -tris (2-naphthylphenyl- phenylamino)-triphenylamine] as hole injection material and N PH [N,N'-bis (1-naphthyl) -N,N' -diphenyl-1, 1'-biphenyl-4,4'- diamine] as hole transport material were deposited on the ITO (indium tin oxide)/glass substrate by vacuum evaporation, And then, red color emission layer was deposited using Alq3 as a host material and Rubrene (5,6,11,12- tetraphenylnaphthacene) and GDI 4234 as dopants. Finally, small molecular weight OLED with the structure of ITO/2-TNATA/ NPB/Alq3+Rubrene+GDI4234/Alq3/LiF/Al was obtained by in-situ deposition of Alq3, LiF and Al as electron transport material, electron injection material and cathode. respectively. Green OLED fabricated in our experiments showed the color coordinate of CIE(0.65,0.35) and the maximum luminescence efficiency of 2.1 lm/W at 7 V with the peak emission wavelength of 632 nm.

• Journal of the Korean institute of surface engineering
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• v.47 no.4
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• pp.192-197
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• 2014

Red-emitting nitride phosphors recently attracted considerable attention because of their high thermal stability and high color rendering index properties. For excellent phosphor of white light-emitting-diode, ternary nitride phosphor of $Sr/SmSi_5N_8:Eu^{2+}$ with different $Eu^{2+}$ ion concentration were synthesized by solid state reaction method. In this work, red-emitting nitride $Sr/SmSi_5N_8:Eu^{2+}$ phosphor was successfully synthesized by using multi-step high frequency induction heat treatment. The effects of molar ratio of component and experimental conditions on luminescence property of prepared phosphors have been investigated. The structure and luminescence properties of prepared $Sr/SmSi_5N_8:Eu^{2+}$ phosphors were investigated by XRD and photoluminescence spectroscopy. The excitation spectra of $Sr/SmSi_5N_8:Eu^{2+}$ phosphors indicated broad excitation wavelength range of 300 - 550 nm, namely from UV to visible area with distinct enhanced emission peaks. With an increase of $Eu^{2+}$ ion concentration, the peak position of emission in spectra was red-shifted from 613 to 671 nm. After via multi-step heat treatment, prepared phosphor showed excellent luminescence properties, such as high emission intensity and low thermal quenching, better than commercial phosphor of $Y_3Al_5O_{12}:Ce^{3+}$. Using $Eu_2O_3$ as a raw material for $Eu^{2+}$ dopant with nitrogen gas flowing instead of using commercial EuN chemical for $Sr/SmSi_5N_8:Eu^{2+}$ synthesis is one of characteristic of this work.

• Journal of Biomedical Engineering Research
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• v.23 no.2
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• pp.165-170
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• 2002

The aim of this study was to analyze the effect of different sensory information on the motor function which is resulted from visual stimuli. Particularly, it was focused on the effect of complementary color stimuli on reaction time. Twenty volunteers(10 men & 10 women), between the age of 20 and 25 yearn participated in this experiment. Experiments were carried out in a light & sound-attenuated chamber, and the overall system consisted of a PC. interface card. LEDs. key board switch, and display panel. Although many measurements of sensory-motor integration has been studied the quantitative analysis of sensory-motor integration has not been developed well. Quantitative analyses were performed to investigate the effect of the different sensory information on the arm motor system in the point of view sensory-motor integration. The result showed that the reaction time for visual stimuli of complementary colors was faster than that under same color environments : and, in same color environments and the reaction speed was varied inversely with respect to the magnitude of the light wavelength.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.145-145
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• 2011

Light-emitting diodes (LEDs) based on III-nitrides compound semiconductors have achieved a high performance device available for display and illumination sector. However, the conventional c-plane oriented LED structures are still showing several problems given by the quantum confined Stark effect (QCSE) due to the effects of strong piezoelectric and spontaneous polarizations. The QCSE results in spatial separation of electron and hole wavefunctions in quantum wells, thereby decreasing the internal quantum efficiency and red-shifting the emission wavelength. Due to demands for improvement of device performance, nonpolar structure has been attracting attentions, since the quantum wells grown on nonpolar templates are free from the QCSE. However, current device performance for nonpolar LEDs is still lower than those for conventional LEDs. In this study, we discuss the potential possibilities of nonpolar LEDs for commercialization. In this study, we characterized current-light output power relation of the a-plane InGaN/GaN LEDs structures with the variation of quantum well structures. On-wafer electroluminescence measurements were performed with short pulse (10 us) and low duty factor (1 %) conditions applied for eliminating thermal effects. The well and barrier widths, and indium compositions in quantum well structures were changed to analyze the efficiency droop phenomenon.

• Korean Journal of Materials Research
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• v.23 no.3
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• pp.194-198
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• 2013

In this paper, we studied a p-type reflector based on indium tin oxide (ITO) for vertical-type ultraviolet light-emitting diodes (UV LEDs). We investigated the reflectance properties with different deposition methods. An ITO layer with a thickness of 50 nm was deposited by two different methods, sputtering and e-beam evaporation. From the measurement of the optical reflection, we obtained 70% reflectance at a wavelength of 382 nm by means of sputtering, while only 30% reflectance resulted when using the e-beam evaporation method. Also, the light output power of a $1mm{\times}1mm$ vertical chip created with the sputtering method recorded a twofold increase over a chip created with e-beam evaporation method. From the measurement of the root mean square (RMS), we obtained a RMS value 1.3 nm for the ITO layer using the sputtering method, while this value was 5.6 nm for the ITO layer when using the e-beam evaporation method. These decreases in the reflectance and light output power when using the e-beam evaporation method are thought to stem from the rough surface morphology of the ITO layer, which leads to diffused reflection and the absorption of light. However, the turn-on voltage and operation voltage of the two samples showed identical results of 2.42 V and 3.5 V, respectively. Given these results, we conclude that the two ITO layers created by different deposition methods showed no differences in the electric properties of the ohmic contact and series resistance.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.03a
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• pp.18-20
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• 2010

The mechanism for suppression of Ag agglomeration in Ag-Mg alloy ohmic contact to p-GaN is investigated. The Ag-Mg alloy ohmic contact shows low contact resistivity of $6.3\;{\times}\;10^{-5}\;{\Omega}cm^2$, high reflectance of 85.5% at 460 nm wavelength after annealing at $400^{\circ}C$ and better thermal stability than Ag contact The formation of Ga vacancies increase the net hole concentration, lowering the contact resistivity. Moreover, the oxidation of Mg atoms in Ag film increase the work function of Ag-Mg alloy contact and prevents Ag oxidation. The inhibition of oxygen diffusion by Mg oxide suppresses the Ag agglomeration, leading to enhance light reflectance and thermal stability.

• Journal of the Korean Vacuum Society
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• v.20 no.2
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• pp.127-134
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• 2011

We investigate the crystallographic orientation and strain states of the Ni/Ag ohmic contacts on p-type GaN. The Ag film in the Ni/Ag contact was severely agglomerated during high temperature annealing in air ambient. As a results, after annealing for 24 h, the Ni/Ag contact shows non-linear I-V curve and low light reflectance of ~21% at 460 nm wavelength. High-resolution X-ray diffraction results show that the interplanar spacing of Ag (111) planes is almost same to that of bilk Ag after annealing for 24 hrs, indicating that the in-plane tensile strain in the Ag film was fully relaxed due to the Ag agglomeration.

• Journal of the Korean Graphic Arts Communication Society
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• v.18 no.2
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• pp.41-54
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• 2000

Photochemical modulation of Bragg-reflection wavelengths based on isomerization of an azobenzene (Azo) and subsequent change in reflectance was investigated in cholesteric liquid crystals (ChLCs) which reflect light in visible wavelength region. Irradiation at 366 nm, which causes an efficient transcis isomerization of Azo, led to change in reflected color of ChLCs toward shorter wavelengths with a concomitant lowering of phase transition. Reversible change in color was induced all-optically by alternate irradiation at effective wavelengths for reversible isomerization of Azo. A considerable variation in reflectance was also observed when the photoinduced change in color was measured by a probe light with the same handedness as the ChLCs. The spectral Position of selective light reflection in the initial states played an important role to produce a normal-mode and a reverse-mode switching in photoinduced modulation of reflectance of the ChLCs with respect to the probe light.

• Korean Journal of Environmental Agriculture
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• v.30 no.4
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• pp.382-389
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• 2011

BACKGROUND: Light-emitting diodes (LEDs) with lower electric cost and the specific wavelength have been considering as a novel light source for plant production in greenhouse conditions as well as in a closed culture system. Supplementary lighting for day-length extension was considered as light intensity, light quality, and/or photoperiod control on plant growth and development. Effects of supplementary blue or red LED radiation with lower light intensity on growth of Ageratum (Ageratum houstonianum Mill., cv. Blue Field), African marigold (Tagetes erecta L., cv. Orange Boy), and Salvia (Salvia splendens F. Sello ex Ruem & Schult., cv. Red Vista) were discussed during sunrise and sunset twilight in the experiment. METHODS AND RESULTS: Supplementary lighting by blue and red LEDs for 30 (Treatment B30; R30) or 60 (Treatment B60; R60) min. per day were established in greenhouse conditions. Photosynthetic photon flux for supplementary radiation was kept at $15{\mu}mol\;m^{-2}\;s^{-1}$ on the culture bed. Natural condition without supplementary light was considered as a control. The highest shoot and root dry weights were shown in African marigold exposed by red light for 60 min. per day. Supplementary blue and red lighting regardless of the radiation time significantly stimulated development of lateral branches in African marigold. Stem growth in Ageratum and Salvia seedlings was significantly promoted by red radiation as well as natural light. CONCLUSIONS: Extending of the radiation time at sunrise and sunset twilight using LEDs stimulated reproductive growth of flowering plant species. Different characteristics on growth under supplementary blue or red lighting conditions were also observed in the seedlings during supplementary radiation.

• Korean Journal of Materials Research
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• v.18 no.2
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• pp.92-97
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• 2008

In spray pyrolysis, the effects of the preparation temperature, flow rate of the carrier gas and concentration of the spray solution on characteristics such as the morphology, size, and emission intensity of $Ca_8Mg(SiO_4)_4Cl_2:Eu^{2+}$ phosphor powders under long-wavelength ultraviolet light were investigated. The phosphor powders obtained post-treatment had a range of micron sizes with regular morphologies. However, the composition, crystal structure and photoluminescence intensity of the phosphor powders were affected by the preparation conditions of the precursor powders. The $Ca_8Mg(SiO_4)_4Cl_2:Eu^{2+}$ phosphor powders prepared at temperatures that were lower and higher than $700^{\circ}C$ had low photoluminescence intensities due to deficiencies related to the of Cl component. The phosphor powders with the deficient Cl component had impurity peaks of $Ca_2SiO_4$. The optimum flow rates of the carrier gas in the preparation of the $Ca_8Mg(SiO_4)_4Cl_2:Eu^{2+}$ phosphor powders with high photoluminescence intensities and regular morphologies were between 40 and 60 l/minute. Phosphor powders prepared from a spray solution above 0.5 M had regular morphologies and high photoluminescence intensities.