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Song, Hooyoung,Kim, Eun Kyu,Han, Il Ki,Lee, Sung-Ho,Hwang, Sung-Min American Scientific Publishers 2011 Journal of Nanoscience and Nanotechnology Vol.11 No.10
<P>We fabricated yellow (575 nm) emitting a-plane InGaN/GaN light emitting diode (LED). Microstructure and stress relaxation of the InGaN well layer were observed from the images of dark field transmission electron microscopy. The LED chip was operated at 3.7 V, 20 mA, and the polarization-free characteristic in nonpolar InGaN layer was confirmed from a small blue-shift of approximaely 1.7 nm with increase of current density. The high photoluminescence (PL) efficiency of 30.4% showed that this non-polar InGaN layer has a potential of application to green-red long wavelength light emitters. The PL polarization ratio at 290 K was 0.25 and the energy difference between two subbands was estimated to be 40.2 meV. The low values of polarization and energy difference were due to the stress relaxation of InGaN well layer.</P>
Studies of defect states of ZnO thin films under different annealing conditions
Song, Hooyoung,Kim, Jae-Hoon,Kim, Eun Kyu Elsevier 2009 Microelectronics journal Vol.40 No.2
<P><B>Abstract</B></P><P>ZnO thin films were grown by the pulsed laser deposition technique on <I>c</I>-plane sapphire substrates at a substrate temperature of 500°C with 1×10<SUP>−4</SUP>Torr ambient gas. After the deposition process, ZnO thin films were annealed at 1000°C for 5min under N<SUB>2</SUB> or O<SUB>2</SUB> ambient gas, respectively. In the X-ray patterns, the (002) peak of the annealed sample was shifted from that of the as-grown sample, which indicates a reduced lattice constant of about 1%. Even though the X-ray diffraction patterns in the samples annealed under O<SUB>2</SUB> and N<SUB>2</SUB> annealing gases were almost the same, photoluminescence spectra showed the generation of a shallow level with a few meV, and deep-level states were generated at <I>E</I><SUB>v</SUB>+0.594eV. In addition, a defect state appeared at <I>E</I><SUB>c</SUB>−0.607eV, which originated from hydrogen plasma irradiation on the ZnO sample.</P>
[분산 데이타베이스] Hadoop 분산 파일 시스템의 효과적인 저장 공간 절약 기법
안후영(HooYoung Ahn),이경하(KyongHa Lee),이수호(SooHo Lee),이윤준(YoonJoon Lee),이상민(SangMin Lee),김영균(YoungKyun Kim) 한국정보과학회 2013 정보과학회 컴퓨팅의 실제 논문지 Vol.19 No.3
분산 파일시스템은 서로 분산된 여러 서버들을 가지고 파일 시스템을 구성함으로써 높은 확장성과 고가용성을 지원한다. 현재의 분산 파일시스템들은 내고장성을 위해 주로 데이터 복제 기법에 의존한다. 그러나 이 방법은 저장 공간을 추가로 필요로 하는 단점을 가진다. 이 논문에서는 이 추가 저장 공간의 문제점을 해결하고자 erasure coding을 이용한 패리티 블록의 작성을 통해 데이터 복제본을 대체함으로써 동일 수준의 내고장성을 유지하면서도 저장 공간을 보다 절약하는 방법을 제안한다. 특히, 동일 노드 상의 서로 다른 파일의 블록들을 하나의 스트라이프로 구성하여 패리티 블록을 생성함으로써 인코딩, 디코딩 시의 I/O 효율을 향상한다. 또한, erasure coding 적용 이후에도 신뢰성이 유지되는 블록 배치 기법을 제안하고 구현하였다. Conventional distributed file systems support high scalability and availability with many machines. Most of the distributed file systems mainly rely on data replication for providing fault-tolerance. However, the replication scheme also increases the storage overhead of the file system. To address the issue, we provide a novel method which substitutes the replica blocks with parity blocks by using erasure coding techniques. Especially, by making a stripe with the blocks which come across different files but are stored in a single node, we reduce I/O cost for encoding and decoding blocks significantly. Furthermore, we also propose and implement a novel block placement policy that supports high reliability as much as when using data replication scheme.