• Journal of the Korean Crystal Growth and Crystal Technology
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• v.12 no.1
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• pp.11-20
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• 2002

The world at the end of the $20^{th}$ Century has become "blue" Indeed, this past decade has witnessed a "blue rush" towards the development of violet-blue-green light emitting diodes (LEDs) and laser diodes (LDs) based on wide bandgap III-Nitride semiconductors. And the hard work has culminated with, first, the demonstration of commercial high brightness blue and green LEDs and of commercial violet LDs, at the very end of this decade. Thanks to their extraordinary properties, these semiconductor materials have generated a plethora of activity in semiconductor science and technology. Novel approaches are explored daily to improve the current optoelectronics state-of-the-art. Such improvements will extend the usage and the efficiency of new light sources (e.g. white LEDs), support the rising information technology age (e.g. high density optical data storage), and enhance the environmental awareness capabilities of humans (ultraviolet and visible photon detectors and sensors). Such opportunities and many others will be reviewed in this presentation.

• Journal of Advanced Marine Engineering and Technology
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• v.29 no.7
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• pp.795-800
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• 2005

The optical properties of $ZnS_{y}Se_{1-x-y}:Te_x\;(x\;<\;0.08,\;y\∼0.11$) alloys grown by molecular beam epitaxy (MBE) have been investigated by photoluminescence (PL) and PL-excitation (PLE) spectroscopy. Good optical properties and high crystal quality were established with lattice match condition to GaAs substrate. At room temperature, emission in the visible spectrum region from blue to green was obtained by varying the Te content of the ZnSSe:Te alloy. The efficient blue and green emission were assigned to $Te_{1}$Tel and $Te_{n}$ (n$\geq$2) cluster bound excitons, respectively. Bright blue (462 nm) and green (535 nm) light emitting diodes (LEDs) have been developed using ZnSSe:Te system as an active layer.

• Current Optics and Photonics
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• v.2 no.5
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• pp.468-473
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• 2018

We investigate the temperature dependence of efficiency droop in InGaN/GaN multiple-quantum-well (MQW) blue light-emitting diodes (LEDs) in the temperature range from 20 to $80^{\circ}C$. When the external quantum efficiency (EQE) and the wall-plug efficiency (WPE) of the LED sample were measured as injection current and temperature varied, the droop of EQE and WPE was found to be reduced with increasing temperature. As the temperature increased from 20 to $80^{\circ}C$, the droop ratio of EQE was decreased from 16% to 14%. This reduction in efficiency droop with temperature can be interpreted by a temperature-dependent carrier distribution in the MQWs. When the carrier distribution and radiative recombination rate in MQWs were simulated and compared for different temperatures, the carrier distribution was found to become increasingly homogeneous as the temperature increased, which is believed to partly contribute to the reduction in efficiency droop with increasing temperature.

• 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.

• Journal of the Optical Society of Korea
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• v.18 no.6
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• pp.772-776
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• 2014

We report a numerical analysis of the light-extraction efficiency (LEE) of light-emitting diodes (LEDs) on patterned sapphire substrates (PSSs). We considered various n-sided, regular convex pyramids, where n is an integer and $n{\geq}3$. We then considered four cross sections: extruded, subtracted, truncated-extruded, and truncated-subtracted. Ray-tracing simulations were carried out with these polygonal pyramid patterns, and the dimensions of the patterns were systematically varied. Optimized pattern shapes were determined for large LEE. An extruded circular pyramid with a slant angle of $45^{\circ}$ was found to be the optimal patterned shape.

• Bulletin of the Korean Chemical Society
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• v.27 no.7
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• pp.1043-1047
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• 2006

New electroluminescence polymer, poly(5,10-dihexyl-5,10-dihydroindolo[3,2-b]indole-2,7-diyl) (PININO) was synthesized by Yamamoto conditions with Ni(0) catalyst. The full characterization of structures and properties as well as the performances of the electroluminescence devices of the new polymer are presented. The resulting polymer, which exhibits good solubility in common organic solvents, was used as the electroluminescence layer for the light-emitting diodes (LEDs) (ITO/PEDOT/polymer/Al). PININO shows turn-on voltage of 2.5 V, and electroluminescence (EL) with maximum peak at 490 nm, maximum brightness of 40 cd/$m^{2}$ at 8 V, and efficiency of 0.002 cd/A at 350 mA/$cm^{2}$.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2005.11a
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• pp.51-52
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• 2005

Optical characteristics of excitonic blue and green emission of Te-doped ZnSSe:Te epitaxial layers, grown by molecular beam epitaxy, were investigated by photoluminescence (PL) measurements. The Te-doped ternary specimen shows strong blue or green emission (at 300K) which is assigned to $Te_1$ or $Te_n$ $(n{\geq}2)$ cluster cluster bound exciton, respectively. Bright green and blue light emitting diodes (LEDs) have been developed using ZnSSe:Te system as an active layer. The green LEDs exhibit a fairly long device lifetime (>2000 h) when operated at 3 $A/cm^2$ under CW condition at room temperature.

• Journal of the Optical Society of Korea
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• v.16 no.4
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• pp.349-353
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• 2012

Uniform spreading of injection current in light-emitting diodes (LEDs) is one of the crucial requirements for better device performances. It is reported that non-uniform current spreading leads to low output power, high current crowding, heating, and reliability degradation of the LED device. This paper reports on the effects of different surface and electrode geometries in the LEDs. To increase the output power of LEDs and reduce the series resistance, a rectangular-type LED (RT-LED) with a centered island cathode has been fabricated and investigated by comparison with a conventional LED (CV-LED). The performances of RT-LEDs were prominently enhanced via uniform current spreading and low current crowding. Performances in terms of increased output power and lower forward voltage of simulated RT-LEDs are much superior to those of CV-LEDs. Based on these results, we investigated the correlation between device geometries and optical characteristics through the fabricated CV and RT-LEDs. The measured output power and forward voltage of the RT-LEDs at 100 mA are 64.7% higher and 8% smaller compared with those of the CV-LEDs.

• Research in Plant Disease
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• v.22 no.2
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• pp.100-106
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• 2016

We have studied the mycelia growth of four soil-borne fungal pathogens under light qualities and two lighting types (continuous and intermittent) provided by light-emitting diodes (LEDs). As a result, each mycelia growth on Phytophthora cactorum KACC40166, Athelia rolfsii KACC40170, and Helicobasidium mompa KACC40836 strain showed the similar growth rates within 10% or less difference among treatments compared to dark control, regardless of lighting types. However, the mycelia growth on Rosellinia necatrix KACC40168 strain was significantly suppressed by blue, blue+green and blue+red LED as well as fluorescent lamp compared to a dark control, in common with lighting types. The melanin pigment on R. necatrix KACC40168 strain showed relatively to induce more strongly under green LED and fluorescent lamp, whereas no induction under red LED and a control, regardless of lighting types. Thus, the hypha width on R. necatrix KACC40168 was significantly thinned by blue and blue+green LED compared to a control, in common with lighting types.

• Journal of Environmental Science International
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• v.21 no.6
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• pp.757-761
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• 2012

The purpose of this study was to determine optimum conditions for the cultivation of Tetraselmis suecica (T. suecica) under illumination of four different types of LEDs (i.e., blue, red, white, and mixed). Initial cell concentration was $4{\times}10^4$ cells/mL and temperature of reactor was maintained between 21-$24^{\circ}C$. Specific growth rates were 0.72 $day^{-1}$(white), 0.58 $day^{-1}$(red), 0.49 $day^{-1}$(mixed), and 0.49 $day^{-1}$(blue). Thus, white LEDs was used for the cultivation of T. suecica. Tests with white LEDs under different light intensity, which was conducted to determine optimum light intensity of white LEDs, showed that 9,000 lux of illumination resulted in fastest cell growth and greatest cell concentrations. To avoid shadow effects by dense cell populations, aeration was performed. Cell concentration increased 3.8 times when aeration was used.