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Ryu Han-Youl 한국물리학회 2020 Current Applied Physics Vol.20 No.12
It is well known that carrier distribution in InGaN multiple quantum wells (MQWs) can be significantly inhomogeneous. However, the conventional ABC recombination model assumes that carriers are uniformly distributed throughout the MQW. In this paper, a modified ABC model that considers the unequal carrier density in the QWs was developed. From the analysis of the developed ABC model, the effective recombination coefficients and modified internal quantum efficiency (IQE) were obtained for an arbitrary carrier distribution in MQWs. The efficiency droop was found to be aggravated as the carrier distribution was increasingly inhomogeneous. However, it was also found that the effect of inhomogeneous carrier distribution alone was not sufficient to explain the IQE droop with the theoretical Auger recombination coefficient based on indirect Auger processes. The developed ABC model is expected to provide insight into the influence of inhomogeneous carrier distributions in MQWs on the efficiency droop in GaN-based light-emitting diodes.
Han-Youl Ryu,Jong-In Shim IEEE 2010 IEEE journal of quantum electronics Vol.46 No.5
<P>Light extraction efficiency (LEE) in thin-film InGaN vertical light-emitting diode (LED) structures with photonic crystal patterns is studied using the three-dimensional finite-difference time-domain simulation. We systematically investigate the dependence of LEE on various structural parameters of photonic crystal vertical LEDs such as the thickness of the p-GaN and n-GaN layers, and air-hole depth and size. It is found that high LEE of > 80% is obtainable from unencapsulated photonic crystal LEDs for a wide range of structural parameters. In particular, higher LEE is observed for the structures with relatively long-period photonic crystal patterns and possible mechanisms for the large enhancement of LEE are discussed.</P>
Ryu, Han-Youl,Jeon, Ki-Seong,Kang, Min-Goo,Choi, Yunho,Lee, Jeong-Soo Optical Society of America 2013 Optics express Vol.21 No.1
<P>We investigate the dependence of various efficiencies in GaN-based vertical blue light-emitting diode (LED) structures on the thickness and doping concentration of the n-GaN layer by using numerical simulations. The electrical efficiency (EE) and the internal quantum efficiency (IQE) are found to increase as the thickness or doping concentration increases due to the improvement of current spreading. On the contrary, the light extraction efficiency (LEE) decreases with increasing doping concentration or n-GaN thickness by the free-carrier absorption. By combining the results of EE, IQE, and LEE, wall-plug efficiency (WPE) of the vertical LED is calculated, and the optimum thickness and doping concentration of the n-GaN layer is found for obtaining the maximum WPE.</P>
Han-Youl Ryu 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.3
The light extraction characteristics in micro-cavity InGaN light-emitting diode (LED) structures with a vertical injection geometry are investigated using a three-dimensional finite-difference time-domain (FDTD) simulation. The micro-cavity vertical LED structures consist of sub-micronthick GaN/InGaN heterostructures placed on a high-reflectance metal reflector. The simulation results show that the light extraction efficiency has a strong dependence on the posiwion of the quantum-well (QW) relative to the bottom mirror and on the thickness of the micro-cavity LED. The extraction efficiency of optimally designed micro-cavity vertical LED structures is found to be enhanced by more than 8 times compared with that of conventional planar InGaN LED structures. The light extraction characteristics in micro-cavity InGaN light-emitting diode (LED) structures with a vertical injection geometry are investigated using a three-dimensional finite-difference time-domain (FDTD) simulation. The micro-cavity vertical LED structures consist of sub-micronthick GaN/InGaN heterostructures placed on a high-reflectance metal reflector. The simulation results show that the light extraction efficiency has a strong dependence on the posiwion of the quantum-well (QW) relative to the bottom mirror and on the thickness of the micro-cavity LED. The extraction efficiency of optimally designed micro-cavity vertical LED structures is found to be enhanced by more than 8 times compared with that of conventional planar InGaN LED structures.
Low-threshold Photonic Crystal Lasers from InGaAsP Free-standing Slab Structures
Ryu, Han-Youl,Kim, Se-Heom,Kwon, Soon-Hong,Park, Hong-Gyu,Lee, Yong-Hee Optical Society of Korea 2002 Current Optics and Photonics Vol.6 No.3
Photonic band gap structures have a high potential for nearly zero-threshold lasers. This paper describes new-types of low-threshold photonic crystal lasers fabricated in InGaAsP slab waveguides free-standing in air. Two-types of photonic crystal lasers are studied. One is a single-cell nano-cavity laser formed in a square array of air holes. This photonic band gap laser operates in the smallest possible whispering gallery mode with a theoretical Q >30000 and exhibits low threshold pump power of 0.8 mW at room temperature. The nther laser does not have any cavity structure and the lasing operation originates from the enhanced optical density of states near photonic band edges. A very low threshold of 35 $\mu$W (incident pump power) is achieved from this laser at 80 K, one of the lowest values ever reported. This low threshold is benefited from low optical losses as well as enhanced material gain at low temperature.
Effect of current spreading on the efficiency droop of InGaN light-emitting diodes.
Ryu, Han-Youl,Shim, Jong-In Optical Society of America 2011 Optics express Vol.19 No.4
<P>We investigate the effects of current spreading on the efficiency droop of InGaN blue light-emitting diodes with lateral injection geometry based on numerical simulation. Current crowding near the mesa edge and the decrease in the current spreading length with current density are shown to cause significant efficiency droop. It is found that the efficiency droop can be reduced considerably as the uniformity of current spreading is improved by increasing the resistivity of the p-type current spreading layer or decreasing the sheet resistance of the n-GaN layer. The droop reduction is well interpreted by the uniformity of carrier distribution in the plane of quantum wells.</P>
Han-Youl Ryu,Jong-In Shim,Cheol-Hoi Kim,Jin Hyoung Choi,Hyun Min Jung,Min-Soo Noh,Jong-Moo Lee,Eun-Soo Nam IEEE 2011 IEEE photonics technology letters Vol.23 No.24
<P>The authors investigate efficiency and electron leakage characteristics in GaN-based light-emitting diodes (LEDs) without AlGaN electron-blocking-layer (EBL) structures. Both simulation and electroluminescence (EL) measurement results show that the internal quantum efficiency decreases rapidly as the thickness of an undoped GaN interlayer between active layers and a p-GaN layer increases, which is caused by electron leakage from active layers to the p-GaN due to inefficient hole injection. However, photoluminescence (PL) measurement results show that the quality of active layers deteriorates as the interlayer thickness decreases. The EL and PL results imply that the optimization of the undoped GaN interlayer thickness is important for achieving high internal quantum efficiency in AlGaN-EBL-free LEDs.</P>
Ryu, Ha-Jung,Jung, Ho-Youl,Park, Jung-Sun,Ryu, Gil-Mi,Heo, Jee Yeon,Kim, Jae-Jung,Moon, Song-Mean,Kim, Hung-Tae,Lee, Jong-Young,Koh, Insong,Kim, Jun-Woo,Rho, Jae Kyun,Han, Bok-Ghee,Kim, Hyungtae,Park, S.Karger 2006 International archives of allergy and immunology Vol.139 No.3
<P><I>Background and Methods:</I> Numerous genetic studies have mapped asthma susceptibility genes to a region on chromosome 5q31-33 in several populations. This region contains a cluster of cytokines and other immune-related genes important in immune response. In the present study, to determine the genetic variations and patterns of linkage disequilibrium (LD), we resequenced all the exons and promoter regions of the 29 asthma candidate genes in the chromosome 5q31-33 region. <I>Results:</I> We identified a total of 314 genetic variants, including 289 single nucleotide polymorphisms (SNPs), 22 insertion/deletion polymorphisms and 3 microsatellites. Standardized variance data for allele frequency revealed substantial differences in SNP allele frequencies among different ethnic groups. Interestingly, significant ethnic differences were observed mainly in intron SNPs. LD block analysis using 174 common SNPs with a frequency of >10% disclosed strong LD within most candidate genes. No significant LD was observed across genes, except for one LD block (CD14-IK block). Gene-based haplotype analyses showed that 1-5 haplotype-tagging SNPs may be used to define the six or fewer common haplotypes with a frequency of >5%, regardless of the number of SNPs. <I>Conclusion:</I> Overall, our results provide useful information for the identification of immune-mediated disease genes in the chromosome 5q31-33 region, as well as valuable evidence for gene-based haplotype analysis in disease association studies.</P><P>Copyright © 2006 S. Karger AG, Basel</P>