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김종규,Kim, Jong-Gyu,Schubert, E. Fred 한국광학회 2010 광학과 기술 Vol.14 No.3
발광 다이오드가 처음 만들어진 것은 100년 전으로 거슬러 올라간다. 이 글은 100년 전 발광 다이오드가 우연히 발견된 이후부터 현재 없어서는 안될 귀중한 고출력/고효율 대체 조명용 광원으로 인식되기까지의 역사를 돌아보고자 한다. 또한 앞으로 기대되는 발광 다이오드의 발전가능성과 응용성에 대해서도 다루고자 한다.
Reduction of efficiency droop in GaInN/GaN light-emitting diodes with thick AlGaN cladding layers
An Mao,조제희,E. Fred Schubert,Joong Kon Son,Cheolsoo Sone,하우진,Sunyong Hwang,김종규 대한금속·재료학회 2012 ELECTRONIC MATERIALS LETTERS Vol.8 No.1
GaInN/GaN multiple-quantum-well light-emitting diodes (LEDs) with 0.4 µm-thick AlGaN cladding layers and two quantum wells (QWs), designed for investigating the origin of efficiency droop, are demonstrated to have a lower efficiency droop than typical GaInN/GaN LEDs with 5 QWs. Considering the much less electron leakage over the active region, and the larger carrier density due to the smaller active volume of the LED with AlGaN cladding layers than those of the typical LED, it is suggested that the dominant mechanism responsible for the efficiency droop is electron leakage rather than the Auger recombination which scales with the cubic power of the carrier density.
Demonstration of optical interference filters utilizing tunable refractive index layers
Poxson, David J.,Mont, Frank W.,Schubert, Martin F.,Kim, Jong Kyu,Cho, Jaehee,Schubert, E. Fred The Optical Society 2010 Optics express Vol.18 No.suppl4
<P>Optical interference filters utilizing tunable refractive index layers are shown to have higher spectral fidelity as compared to conventional filters consisting of non-tunable refractive index layers. To demonstrate this increase in spectral fidelity, we design and compare a variety of optical interference filters employing both tunable and non-tunable refractive index layers. Additionally, a five-layer optical interference filter utilizing tunable refractive index layers is designed and fabricated for use with a Xenon lamp to replicate the Air Mass 0 solar irradiance spectrum and is shown to have excellent spectral fidelity.</P>
Guan-Bo Lin,Dong-Yeong Kim,Qifeng Shan,Jaehee Cho,Schubert, E. Fred,Hyunwook Shim,Cheolsoo Sone,Jong Kyu Kim IEEE 2013 IEEE photonics journal Vol.5 No.4
<P>The dependence of the polarization-induced electric field in GaInN/GaN multiple-quantum-well light-emitting diodes (LEDs) on the GaN quantum barrier (QB) thickness is investigated. Electrostatic arguments and simulations predict that a thin QB thickness reduces the electric field in the quantum wells (QWs) and also improves the LED efficiency. We experimentally demonstrate that the QW electric field decreases with decreasing QB thickness. The lower electric field results in a better overlap of electron and hole wave functions and better carrier confinement in the QWs. A reduced efficiency droop and enhanced internal quantum efficiency is demonstrated for GaInN/GaN LEDs when the QB thickness is reduced from 24.5 to 9.1 nm.</P>
Ma, Ming,Mont, Frank W,Yan, Xing,Cho, Jaehee,Schubert, E Fred,Kim, Gi Bum,Sone, Cheolsoo Optical Society of America 2011 Optics express Vol.19 No.5
<P>We investigate the effects of the refractive index of the encapsulant on the light-extraction efficiency (LEE) of light-emitting diodes (LEDs) for GaN LEDs (n ??? 2.5) and AlGaInP LEDs (n ??? 3.0). For non-absorbing rectangular parallelepiped LED chips, as the refractive index of the encapsulant increases, the LEE first increases quasi-linearly, then increases sub-linearly, and finally a saturation is reached. Furthermore, LEDs with a dual-layer graded-refractive-index (GRIN) encapsulant (n(encapsulant 1) = 1.57 and n(encapsulant 2) = 1.41) is fabricated through a two-step curing process. We demonstrate that such an LED further enhances the LEE by reducing Fresnel reflection loss at the encapsulant/air interface by 35% compared with an LED encapsulated with a single-layer encapsulant (n(encapsulant) = 1.57).</P>
Yan, Xing,Poxson, David J.,Cho, Jaehee,Welser, Roger E.,Sood, Ashok K.,Kim, Jong Kyu,Schubert, E. Fred WILEY‐VCH Verlag 2013 Advanced Functional Materials Vol.23 No.5
<P><B>Abstract</B></P><P>An optimized four‐layer tailored‐ and low‐refractive index anti‐reflection (AR) coating on an inverted metamorphic (IMM) triple‐junction solar cell device is demonstrated. Due to an excellent refractive index matching with the ambient air by using tailored‐ and low‐refractive index nanoporous SiO<SUB>2</SUB> layers and owing to a multiple‐discrete‐layer design of the AR coating optimized by a genetic algorithm, such a four‐layer AR coating shows excellent broadband and omnidirectional AR characteristics and significantly enhances the omnidirectional photovoltaic performance of IMM solar cell devices. Comparing the photovoltaic performance of an IMM solar cell device with the four‐layer AR coating and an IMM solar cell with the conventional SiO<SUB>2</SUB>/TiO<SUB>2</SUB> double layer AR coating, the four‐layer AR coating achieves an angle‐of‐incidence (AOI) averaged short‐circuit current density, <I>J</I><SUB>SC</SUB>, enhancement of 34.4%, whereas the conventional double layer AR coating only achieves an AOI‐averaged <I>J</I><SUB>SC</SUB> enhancement of 25.3%. The measured reflectance reduction and omnidirectional photovoltaic performance enhancement of the four‐layer AR coating are to our knowledge, the largest ever reported in the literature of solar cell devices.</P>