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Correlation between pit formation and phase separation in thick InGaN film on a Si substrate
우현석,조용철,김종민,조상은,노정현,이준호,김형상,한철구,임현식 한국물리학회 2018 Current Applied Physics Vol.18 No.12
We demonstrate improved surface pit and phase separation in thick InGaN grown on a GaN/Si (111) substrate, using plasma-assisted molecular beam epitaxy with an indium modulation technique. The formation of surface pit and compositional inhomogeneity in the InGaN epilayer are investigated using atomic force microscopy, scanning electron microscopy and temperature-dependent photoluminescence. Indium elemental mapping directly reveals that poor compositional homogeneity occurs near the pits. The indium-modulation epitaxy of InGaN minimizes the surface indium segregation, leading to the reduction in pit density and size. The phase separation in InGaN with a higher pit density is significantly suppressed, suggesting that the pit formation and the phase separation are correlated. We propose an indium migration model for the correlation between surface pit and phase separation in InGaN.
우현석,이석형,조형준 한국데이터정보과학회 2013 한국데이터정보과학회지 Vol.24 No.1
금융시장의 규모가 점점 더 커짐에 따라 고객정보 관리 미숙 또는 부실한 의사결정, 즉 신용 리스크 관리 실패로 인한 손실이 막대하게 증가하고 있다. 따라서 신용 리스크 관리가 점차 더 중요해지고, 이런 신용 리스크를 최소화하는 기본적인 도구인 신용 평점 모형이 절실히 요구된다. 신용평점 모형은 주로 이항형 목표변수만 이용하여 개발 연구되었다. 본 논문에서는 순서형 다항 자료 또는 경시적 이항 자료 같은 다른 형태의 목표 변수를 고려한 신용평점 모형구축 방법을 제시한다. 그 개발된 모형을 실제 자료와 랜덤화한 자료에 적용하여 Kolmogorov-Smirnov 통계량으로 비교 분석한다. As the financial market becomes larger, the loss increases due to the failure of the credit risk managements from the poor management of the customer information or poor decision-making. Thus, the credit risk management also becomes more important and it is essential to develop a credit scoring model, which is a fundamental tool used to minimize the credit risk. Credit scoring models have been studied and developed only for binary target variables. In this paper, we consider other types of target variables such as ordinal multinomial data or longitudinal binary data and suggest credit scoring models. We then apply our developed models to real data and random data, and investigate their performance through Kolmogorov-Smirnov statistic.
Phase separation suppression in InxGa1xN on a Si substrate using an indium modulation technique
우현석,조한솔,김종민,조상은,조용철,노정현,이준호,서용곤,박정호,김형상,한철구,임현식 한국물리학회 2017 Current Applied Physics Vol.17 No.8
A high quality, single phase InGaN film is fabricated on a GaN/Si (111) substrate by optimizing the pulse patterned In supply with a plasma-assisted MBE technique. Compositional phase separation in InGaN is considerably suppressed. The optical and structural properties of the single phase InGaN epitaxial film are consistently confirmed by atomic force microscopy, X-ray diffraction and photoluminescence measurements. We propose a growth mechanism for single phase InGaN in terms of optimal incorporation and surface migration of In atoms.
우현석,김종민,조상은,조용철,노정현,김형상,한철구,임현식 한국진공학회 2017 Applied Science and Convergence Technology Vol.26 No.3
An InGaN/GaN multiple quantum well (MQW) structure is grown on a GaN/sapphire template using a plasma-assisted molecular beam epitaxy (PA-MBE). The fluctuation of the quantum well thickness formed from roughly-grown InGaN layer results in a disordered photoluminescence (PL) spectrum. The surface morphologies of the InGaN layers with various In compositions are investigated by reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM). A blurred InGaN/GaN hetero-interface and the non-uniform QW size is confirmed by high resolution transmission electron microscopy (HR-TEM). Inhomogeneity of the quantum confinement results in a degradation of the quantum efficiency even though the InGaN layer has a uniform In composition.
우현석,이종경,조용철,한재석,김종민,김형상,노정현,이준호,박정호,한철구,임현식 한국물리학회 2015 Current Applied Physics Vol.15 No.9
We report on the effect of oxygen annealing for GaN surface on the Schottky barrier configuration and leakage current in Ni-AlGaN/GaN Schottky barrier diodes. After oxygen annealing, their turn-on voltage and reverse-bias leakage current characteristics are significantly improved due to a reduction in the Schottky barrier height (SBH) and suppression in the surface states respectively. Interface state density extracted from the Terman method was reduced by 2 orders of magnitude. X-ray photoelectron spectroscopy measurements show that the oxygen annealing induces Ga2O3 on the GaN surface. The formation of Ga2O3 reduces the interface state density as well as lowers the SBH through the modification of hybridized metal induced gap states.
고등학교 축구선수의 포지션별 기초체력, 등속성 근력 및 무산소성 파워 비교 분석
우현석(Woo, Hyeon-Seok),허유섭(Huh, Yu-Sub) 한국체육과학회 2020 한국체육과학회지 Vol.29 No.4
This study"s purpose is to furnish basic dat a for training programs, injury prevention, and rehabilitation therapy to improve exercise ability and performance in various levels by analyzing basal physical fitness, isokinetic strength and anaerobic power in each position among high school soccer players. For the subjects, 21 high school soccer players were compared and analyzed for basal physical fitness, isokinetic muscular strength of lower limbs and anaerobic power. Mean and standard deviations were estimated using SPSS WIN(Ver. 19.0) for data analysis. One-way ANOVA was conducted for analysis on basal physical fitness, isokinetic muscular strength of lower limbs, and anaerobic power. The study findings are as follows: 1. As a result of comparing basic physical strength in each position among high school soccer players, there were statistically significant differences found among groups in back strength and body reaction(p<.05). 2. As a result of comparing isokinetic muscular force of knee joints in varying positions among subjects, statistically significant differences were found among groups in the right knee extensor muscle at 180° per second(p<.05) and the right knee flexor muscle at 240°/sec(p<.05). 3. As a result of comparing anaerobic power in each position among subjects, peak and mean power didn"t have statistically significant differences(p>.05). In conclusion, taking all the results of basic physical strength, isokinetic muscular strength and anaerobic power among high school soccer male players into consideration, factors and abilities of fundamental basic physical fitness are similar. However, it would be helpful to exercise individually more in order to improve physical strength and skills needed in each position. Research should be continually conducted to develop training programs for each position and objectify them.