• Proceedings of the Korean Vacuum Society Conference
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• 1999.07a
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• pp.113-113
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• 1999

최근의 고품질 III-N 화합물 반도체 박막성장과 더불어 청색계열의 LED 및 LD의 성공적인 실현은 본 연구분야에 대한 새롭고 헌신적인 상업적, 학문적/ 기술적 투자환경을 유도해 나가고 있다. 특히, c-축 배향 단결정 사파이어를 기판재료로 사용하고 얇은 GaN buffer의 사용은 고온에서 그 위에 성장되는 성장박막의 특성을 크게 향상시키는 것으로 알려져 있다. 그러나 절연체를 기판으로 사용함에 따른 소자구조 및 제작공정의 복잡성과 기판과 GaN 박막사이의 큰 격자 부정합에 따른 결함센터 등은 소자의 전기, 광학적, 구조적 특성에 부정적인 영향을 미치고 있다. 이러한 문제점을 해결하고 양질의 박막을 성장하기 위한 GaN 혹은 그 대체 기판의 개발에 많은 연구투자가 이루어지고 있는 현실 속에서 Si을 기판으로 이용한 GaN 성장의 가능성이 조심스럽게 점쳐지고 있다. 현재까지의 연구결과를 참조할 때 대체로 복잡한 interlayer를 사용하여 박막성장이 일부 이루어졌으나 그 재현성이나 성장의 중요인자에 대한 해석은 아직 분명하게 밝혀져 있지 않다. 본 연구에서는 원자적 관점에서 Si의 표면에 일부 변화를 유도하고, MOCVD 방법으로 그 위에 성장되는 GaN 박막의 광학적 및 표면 morphology 등에 미치는 영향을 분석하여 핵심적인 성장인자를 추출하고자 시도하였다. 성장된 GaN/Si 박막의 물성은 SEM(AFM), PL, XRD, Auger depth profile 장비등을 이용하여 조사하였으며 사파이어 기판 위에 성장된 GaN 박막의 특성들과 비교 검토하였다.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.292-292
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• 2014

Wide band gap semiconductors, such as III-nitrides (GaN, AlN, InN, and their alloys), SiC, and diamond are expected to play an important role in the next-generation electronic devices. Specifically, GaN-based high electron mobility transistors (HEMTs) have been targeted for high power, high frequency, and high temperature operation electronic devices for mobile communication systems, radars, and power electronics because of their high critical breakdown fields, high saturation velocities, and high thermal conductivities. For the stable operation, high power, high frequency and high breakdown voltage and high current density, the fabrication methods have to be optimized with considerable attention. In this study, low ohmic contact resistance and smooth surface morphology to AlGaN/GaN on 2 inch c-plane sapphire substrate has been obtained with stepwise annealing at three different temperatures. The metallization was performed under deposition of a composite metal layer of Ti/Al/Ni/Au with thickness. After multi-layer metal stacking, rapid thermal annealing (RTA) process was applied with stepwise annealing temperature program profile. As results, we obtained a minimum specific contact resistance of $1.6{\times}10^{-7}{\Omega}cm2$.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.15 no.5
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• pp.449-454
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• 2002

The use of a thin film of indium between the ITO and the $n^+-lnP$ contact layers for InP/InGaAs HPTs was studied without degrading its excellent optical transmittance properties. $ITO/n^+-lnP$ ohmic contact was successfully achieved by the deposition of indium and annealing. The specific contact resistance of about $6.6{\times}10^{-4}\Omega\textrm{cm}^2$ was measured by use of the transmission line method (TLM). However, as the thermal annealing was just performed to $ITO/n^+-lnP$ contact without the deposition of indium between ITO and $n^+-lnP$, it exhibited Schottky characteristics. In the applications, the DC characteristics of InP/InGaAs HPTs with ITO emitter contacts was compared with those of InP/InGaAs HBTs with the opaque emitter contacts.

• KSBB Journal
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• v.14 no.4
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• pp.495-502
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• 1999

The optimization of fed-batch yeast fermentation process has been performed using genetic algorithm(GA). Three strategies were designed and applied to obtain the optimal feed rate profiles. Genes in the chromosome (input variables for optimization) included feed rates on fixed time intervals (strategy I), or swiching times $t_s1\;and\;t_s2$, and feed rates on singular arc (strategy II), or feed rates and the length of time interval (strategy III). Strategy III showed the best results for all initial conditions due to efficient utilization of genetic information. Simulation results using GA showed similar or better performance compared with previous results by variational caculus and singular control approach.

• Journal of the Institute of Electronics Engineers of Korea SD
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• v.42 no.1
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• pp.25-32
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• 2005

In this paper, it is attempted to provide a unified explanation for typical short channel GaAs MESFET’s and SOI-structured Si JFET's behaviors such as: i) drain voltage-induced threshold voltage roll-off, ii) finite output ac resistance beyond the saturation, and iii) weak dependence of the drain saturation current on the channel length. Replacing the conventional GCA with a new assumption that is suggested in order to include the longitudinal field variation, and taking into account the channel current continuity and the field-dependent mobility, we can derive the two-dimensional potential in both depletion region and undepleted conducting channel. Obtained expressions for the threshold voltage and the drain current will be considerably accurate over the entire operating region. Moreover, in comparison with the conventional channel length shortening models, our model seems to be more reasonable in explaining the Early effect.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.325-325
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• 2012

1차원구속 반도체인 nanowires (NWs)는 전기적, 광학적으로 일반 bulk구조와 다른 특성을 가지고 있어서 현재 많은 연구가 되고 있다. 일반적으로 NWs는 Au 등의 금속 촉매를 이용하여 성장을 하게 되는데 이때 촉매가 오염물로 작용을 해서 결함을 만들어서 bandgap내에 defect level을 형성하게 된다. 본 연구는 Si (111) 기판 위에 GaAs NWs 와 InAs NWs를 촉매를 이용하지 않고 성장 하였다. vapour-liquid-solid (VLS)방법으로 성장하는 GaAs NWs는 Ga의 droplet을 이용하게 되는데 Ga이 Si 기판위에 자연 산화막에 존재하는 핀홀(pinhole)로 이동하여 1차적으로 Ga droplet 형성하고 이후 공급되는 Ga과 As은 SiO2 보다 GaAs와 sticking coefficient 가 좋기 때문에 Ga drolept을 중심으로 빠른 선택적 성장을 하게 되면서 NWs로 성장을 하게 된다. 반면에 InAs NWs를 성장 할 시에 droplet 방법으로 성장을 하게 되면 NWs가 아닌 박막 형태로 성장을 하게 되는데 이것으로 InAs과 GaAs의 $SiO_2$와의 sticking coefficient 의 차이를 추측을 할 수 있다. InAs NWs는 GaAs NWs는 달리 native oxide를 이용하지 않고 InAs 과 Si 사이의 11.5%의 큰 lattice mismatch를 이용한다. 이종의 epitaxy 방법에는 크게 3종류 (Frank-van der Merwe mode, Stranski-Krastanov mode, Volmer-Weber mode)가 있는데 각기 다른 adatom 과 surface의 adhesive force로 나누어지게 된다. 이 중 Volmer-Weber mode epitaxy는 adatom 의 cohesive force가 surface와의 adhesive force보다 큰 경우 성장 되는 방식으로 InAs NWs 는 이 방식을 이용한다. 즉 droplet을 이용하지 않는 vapour-solid (VS) 방법으로 성장을 하였다. 이 때 In 의 migration을 억제하기 위해서 VLS mode 의 GaAs NWs 보다 As의 공급을 10배 이상 하였다. FE-SEM 분석 결과 GaAs NWs는 Ga droplet을 확인 할 수 있었고 InAs NWs는 droplet이 존재하지 않았다. GaAs와 InAs NW는 density와 length가 V/III가 높을수록 증가 하였다.

• ETRI Journal
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• v.16 no.3
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• pp.1-10
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• 1994

Undoped GaAs has been successfully grown by chemical beam epitaxy (CBE) via surface decomposition process using arsine $(AsH_3)$ and trimethylgallium (TMG). Three distinct regions of temperature-dependent growth rates were identified in the range of temperatures from 570 to $690^{\circ}C$. The growth rates were found strongly dependent on the V/III ratio between 5 and 30. The growth rate at low V/III ratio seems to be determined by arsenic produced on the surface, whereas at high V/III ratio it shows dependence on the adsorption of TMG. Hall measurement and photoluminescence (PL) analysis show that the films are all p-type and that carbon impurities are primarily responsible for the background doping. Carbon concentrations have been found to be reduced by two orders of magnitude as compared to those of epilayers grown by CBE which employs TMG and arsenic obtained from precracked $AsH_3$ in a high temperature cell. It was also found that hydrogen atoms dissociated from unprecracked $AsH_3$ play an important role in removing hydrocarbon-containing species resulting in a significant reduction of car-bon impurities.

• Journal of Chest Surgery
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• v.29 no.1
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• pp.7-13
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• 1996

The conventional glutaraldehyde (GA) fixation method of tissue valves is considered to be responsible for accelerated valve degeneration. The release of toxic GA from the valve tissue is believed to limit endothelial cell (EC) ingrowth. Removal of toxic GA by reaction with L-glutamic acid and storage in a Paraben solution may offer good EC growth. To investigate the conditions for endothelialization of tissue valves, the growth properties of ECs on the conventionally and alternatively treated pericardial tissue were compared. Conventional preparation included zero-pressure fixation for 72 hours in phosphated-buffered saline (PBS) solution containing 0.5% GA at 4$^{\circ}C$ and storage into PBS containing 0.2% GA(group I). Alternatively treated pericardial tissues were divided into three postfixation treatment groups : (1) storage in PBS solution containing Paraben(group II), (2) treatment with PBS containing 8$^{\circ}C$ L-glutamic acid(PH 7.35) and storage in PBS solution containing Paraben (g oup III), (3) treatment with L-glutamic acid dissolved in distilled water (PH 3.5) (group IV). Pericardial tissue were transferred into the 24-well plate after storage for 4 weeks. ECs were harvested enzymatically from the bovine pulmonary artery and grown to confluence on culture flask surfaces. Detached ECs by trypsin were incubated into the each well of the 24-well plate including test pericardial tissues. Cells were detached by trypsin, 1, 2, 3, 5, 7 days after incubation and counted on the hemacytometer. Cell viability test was performed by frypan-blue exclusion method. Acute cell death in the group I were found even after prolonged washing. The group II showed prolonged cell survival compared with the group I. Both group III and group IV showed better cell growth than group II. There was no statistically significant difference between group III and group IV method in terms of EC growth. This results suggest that treatment by L-glutamic ac id and storage in a Paraben solution be a promising approach for improvement of durability of GA-treated tissue valves.

• Proceedings of the Materials Research Society of Korea Conference
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• 2009.05a
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• pp.29.1-29.1
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• 2009

Fabrication of good quality P-type GaN remained as a challenge for many years which hindered the III-V nitrides from yielding visible light emitting devices. Firstly Amano et al succeeded in obtaining P-type GaN films using Mg doping and post Low Energy Electron Beam Irradiation (LEEBI) treatment. However only few region of the P-GaN was activated by LEEBI treatment. Later Nakamura et al succeeded in producing good quality P-GaN by thermal annealing method in which the as deposited P-GaN samples were annealed in N2 ambient at temperatures above $600^{\circ}C$. The carrier concentration of N type and P-type GaN differs by one order which have a major effect in AlGaN based deep UV-LED fabrication. So increasing the P-type GaN concentration becomes necessary. In this study we have proposed a novel method of activating P-type GaN by electrochemical potentiostatic method. Hydrogen bond in the Mg-H complexes of the P-type GaN is removed by electrochemical reaction using KOH solution as an electrolyte solution. Full structure LED sample grown by MOCVD serves as anode and platinum electrode serves as cathode. Experiments are performed by varying KOH concentration, process time and applied voltage. Secondary Ion Mass Spectroscopy (SIMS) analysis is performed to determine the hydrogen concentration in the P-GaN sample activated by annealing and electrochemical method. Results suggest that the hydrogen concentration is lesser in P-GaN sample activated by electrochemical method than conventional annealing method. The output power of the LED is also enhanced for full structure samples with electrochemical activated P-GaN. Thus we propose an efficient method for P-GaN activation by electrochemical reaction. 30% improvement in light output is obtained by electrochemical activation method.

• Journal of Information Display
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• v.7 no.4
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• pp.13-16
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• 2006

II-$III_{2}-(S,Se)_{4}$ structured of phosphor have been used at various fields because they have high luminescent efficiency and broad emission band. Among these phosphors, europium doped $BaGa_{2}S_{4}$ was prepared by solid-state method. We investigated the possibility of applying [ ] due to emissive property of UV region. Also, general sulfide phosphors were synthesized by using injurious $H_{2}S$ $CS_{2}$ gas. However, this study prepared $BaGa_{2}S_{4}:Eu^{2+}$ phosphor is addition to excess sulfur under 5% $H_{2}$/95% $N_{2}$ reduction atmosphere. So, this process could involved large scale synthesis because of non-harmfulness and simple process. The photo-luminescence efficiency of the prepared $BaGa_{2}S_{4}:Eu^{2+}$ phosphor increased by 20% compared with commercial $BaGa_{2}S_{4}:Eu^{2+}$ phosphor. From this, we could conclude that the prepared $BaGa_{2}S_{4}:Eu^{2+}$ could be applied to green phosphor for white LED of three wavelengths.