• 한국진공학회:학술대회논문집
• /
• 한국진공학회 2012년도 제43회 하계 정기 학술대회 초록집
• /
• pp.174-175
• /
• 2012

(In, Ga) N-based III-nitride semiconductor materials have been viewed as the most promising materials for the applications of blue and green light emitting devices such as light-emitting diodes (LEDs) and laser diodes. Although the InGaN alloy can have wide range of visible wavelength by changing the In composition, it is very hard to grow high quality epilayers of In-rich InGaN because of the thermal instability as well as the large lattice and thermal mismatches. In order to avoid phase separation of InGaN, various kinds of structures of InGaN have been studied. If high-quality In-rich InGaN/GaN multiple quantum well (MQW) structures are available, it is expected to achieve highly efficient phosphor-free white LEDs. In this study, we proposed a novel InGaN double hetero-structure grown on GaN nano-pyramids to generate broad-band red-color emission with high quantum efficiency. In this work, we systematically studied the optical properties of the InGaN pyramid structures. The nano-sized hexagonal pyramid structures were grown on the n-type GaN template by metalorganic chemical vapor deposition. SiNx mask was formed on the n-type GaN template with uniformly patterned circle pattern by laser holography. GaN pyramid structures were selectively grown on the opening area of mask by lateral over-growth followed by growth of InGaN/GaN double hetero-structure. The bird's eye-view scanning electron microscope (SEM) image shows that uniform hexagonal pyramid structures are well arranged. We showed that the pyramid structures have high crystal quality and the thickness of InGaN is varied along the height of pyramids via transmission electron microscope. Because the InGaN/GaN double hetero-structure was grown on the nano-pyramid GaN and on the planar GaN, simultaneously, we investigated the comparative study of the optical properties. Photoluminescence (PL) spectra of nano-pyramid sample and planar sample measured at 10 K. Although the growth condition were exactly the same for two samples, the nano-pyramid sample have much lower energy emission centered at 615 nm, compared to 438 nm for planar sample. Moreover, nano-pyramid sample shows broad-band spectrum, which is originate from structural properties of nano-pyramid structure. To study thermal activation energy and potential fluctuation, we measured PL with changing temperature from 10 K to 300 K. We also measured PL with changing the excitation power from 48 ${\mu}W$ to 48 mW. We can discriminate the origin of the broad-band spectra from the defect-related yellow luminescence of GaN by carrying out PL excitation experiments. The nano-pyramid structure provided highly efficient broad-band red-color emission for the future applications of phosphor-free white LEDs.

• 한국진공학회지
• /
• 제20권1호
• /
• pp.42-49
• /
• 2011

$SiN_x$ 박막을 $200^{\circ}C$의 저온에서 $SiH_4$ 가스의 흐름 비율을 바꾸어 가며 PECVD 기법으로 성장하였다. 시료의 광 특성을 규명하기 위하여 상온 광 발광 스펙트럼을 측정하였다. 성장 시 $SiH_4$ 가스의 흐름 비율이 증가함에 따라 시료의 발광 최대치 파장이 장파장으로 이동하였으나, $SiH_4$ 가스의 흐름 비율과 무관하게 모든 시료에서 1.8, 1.9, 2.2, 2.4, 그리고 3.1 eV 에너지의 발광 현상을 관찰하였다. $N_2$, $H_2$, 그리고 $O_2$ 가스 분위기에서 후열처리를 거친 후, 발광 스펙트럼의 변화를 조사하였다. 열처리 후의 발광 세기는 증가하였고, 특히, $H_2$ 및 $O_2$가스 열처리로 인하여 발광 최대치 파장이 단파장으로 이동하였으나, 특정한 파장에서 발광효과는 여전히 존재하였다. 발광 메카니즘에 대하여, $SiN_x$ 박막의 에너지 갭 내에 Si와 N 원자의 비결합 결함에 의한 에너지 준위 모델을 설정하였고, 이 에너지 준위의 천이에 의한 발광으로 이해하였다. 그리고 저온에서 성장한 $SiN_x$ 박막의 발광 효과는 앞으로 구부러짐이 가능한 Si 계 광소자 개발 가능성을 보여주고 있다.

• Journal of Ginseng Research
• /
• 제24권3호
• /
• pp.107-112
• /
• 2000

적변삼 표피조직의 적갈색의 침적물은 강력한 산화제에 의해서만 산화(탈색)되는 chelation power가 매우 큰 화학결합을 형성하고 있었다. CLSM으로 관찰한 결과, 적변삼 조직의바깥쪽으로 두껍게 침적물이 형성되어 있음이 확인되었으며, 조직의 형광강도를 이용하여 두께를 측정한 결과 약 120rm의 침적물이 축적되어 있음이 확인되었다. 또한 Spectrum분석 결과 적변삼 표피의 물질은 건강삼의 표피조직과는 다른 functional group을 포함하는 것으로 확인되었다. 건강삼과 적변삼의 표피조직을 강산과 강염으로 가수분해 하여도 조직에 침적되어 있는 적색의 물질은 용출되지 않았다. 적변삼과 건강삼의 표피조직과 뿌리의 각 부위별 총 페놀성화합물의 함량을 측정한 결과, 건강삼의 표피조직에서 가장 높게 나타났다. 이러한 결과는 페놀성 물질이 적변물질의 원인이 된다는 보고와 차이가 있는 것으로 페놀성 물질은 적변현상 유발의 직접적인 원인물질이 아님이 확인되었다. 따라서 인삼의 적변물질은 인삼뿌리의 유기성분(세포벽 분해산물, 세포내용물, azodicarbonamide 등)과 철이온 복합체로 추론되며, 이물질에 대한 계속적인 추적을 진행하고 있다.

• 한국결정성장학회지
• /
• 제30권4호
• /
• pp.131-135
• /
• 2020

단결정상을 가지는 CaZrO₃ : Eu³⁺ 형광체를 스컬용융법으로 합성하였다. 합성된 형광체의 결정구조, 형태 및 광학적 특성은 XRD, SEM, UV 형광반응 및 PL을 분석하였다. 출발 원료는 CaO : ZrO₂ : Eu₂O₃를 0.962 : 1.013 : 0.025 mol%로 하여 냉각도가니에 충진하였다. 냉각도가니는 내부 직경 120 mm, 높이 150 mm이며, 혼합된 파우더 3 kg은 3.4 MHz의 출력 주파수로 1시간 이내에 완전히 용융되어 2시간 동안 유지시킨 후 자연냉각 시켰다. XRD 측정에서는 다른 결정상은 측정되지 않았으며 페로브스카이트 구조의 정방정계로 분석되었다. 합성된 형광체는 UV 광에 의해 여기 될 수 있고 방출 스펙트럼 결과는 615 nm에서 자기 쌍극자 전이 ⁵D₀→⁷F₂로 인해 CaZrO₃ : Eu³⁺의 밝은 적색 발광이 우세하였다.

• 한국전기전자재료학회논문지
• /
• 제33권5호
• /
• pp.393-399
• /
• 2020

A series of phosphors, SrWO₄:5 mol% Dy³⁺, SrWO₄:5 mol% Sm³⁺, and SrWO₄:5 mol% Dy³⁺:x Sm³⁺ (x＝1~15 mol%), were prepared using a facile co-precipitation. The crystal structure, morphology, photoluminescence properties, and application in anti-counterfeiting fields were investigated. The crystalline structures of the prepared phosphors were found to be tetragonal systems with the dominant peak occurring at the (112) plane. The excitation spectra of the Dy³⁺ singly-doped SrWO₄ phosphors were composed of an intense charge-transfer band centered at 246 nm in the range of 210~270 nm and two weak peaks at 351 nm and 387 nm due to the ⁶H_15/2→⁶P_7/2 and ⁶H_15/2→⁴I_13/2 transitions of Dy³⁺ ions, respectively. The wavelength of 246 nm was optimum for exciting the luminescence of Dy³⁺ and Sm³⁺ co-doped SrWO₄ phosphors. The emission spectra consisted of two intense blue and yellow emission bands at 480 nm and 573 nm corresponding to the ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions of Dy³⁺, and two strong emission peaks at 599 nm and 643 nm originating from the ⁴G_5/2→⁶H_7/2 and ⁴G_5/2→⁶H_9/2 transitions of Sm³⁺, respectively. As the concentration of Sm³⁺ ions increased, the emission intensities of Dy³⁺ rapidly decreased, while the emission intensities of Sm³⁺ gradually increased. These results suggest that the color of the emission light can be tuned from yellow to white by changing the concentration of Sm³⁺ ions at a fixed 5 mol% Dy³⁺. Furthermore, the fluorescent security inks were synthesized for use in anti-counterfeiting applications.

• 한국결정성장학회지
• /
• 제8권2호
• /
• pp.211-220
• /
• 1998

수평 전기로에서 $AgGaS_2$ 다결정을 합성하여 HWE 방법으로 $AgGaS_2$ 단결정 박막을 성장하였다. $AgGaS_2$ 단결정 박막을 성잘할 때 증발원과 기판의 온도를 각각 $590^{\circ}C$, $440^{\circ}C$로 성장하였을 때 이중결정 X-선 요동곡선(double crystal X-ray diffraction rocking curve, DCRC)의 반폭치(FWHM)값이 124 arcsec로 가장 작아 최적 성장조건이었다. 상온에서 $AgGaS_2$ 단결정 박막의 광흡수 특성으로부터 에너지 띠간격이 2.61cV였다. Band edge에 해당하는 광전도도 peak의 온도 의존성은 Varshni 관계식으로 설명되었으며, Vaeshni 관계식의 상수값은 Eg(0) = 2.7284eV, $\alpha$= 8.695$\times$10-4 eV/K, $\beta$= 332K 로 주어졌다. 광발광 봉우리는 20K에서 414.3nm(2.9926eV)와 414.1nm(2.7249eV)는 free exciton(Ex)의 upper polariton과 lower polariton인 {{{{{E}`_{x} ^{u} }}}}와 {{{{{E}`_{x} ^{L} }}}}, 423.6nm(2.9269eV)는 bound exciton emission에 의한 I로 관측되었다. 또한 455nm(2.7249eV)의 peak는 donor-acceptor pair(DAP)에 기인하는 광발광 봉우리로 관측되었다.

• 한국결정성장학회지
• /
• 제19권1호
• /
• pp.15-18
• /
• 2009

본 연구에서는 근 자외선 및 가시광 영역에서 우수한 발광 강도를 가지는 적색 형광체를 얻기 위하여 고상 반응법으로 합성하여 $EU^{3+}$와 $Bi^{3+}$가 도핑 된 ${Y_2}{SiO_5}$의 발광 특성을 관찰하였다. 근자외선 영역인 ${\lambda}_{ex}=395nm$ 기준으로 측정하였고, $^5D_0-^7F_2$의 에너지 천이에 의해 612 nm 영역에서 강한 peak가 발생하였다. CST가 $Eu^{3+}-O^{2-}$에 의해 258 nm 영역 대에서 생성되었고, $Bi^{3+}$가 함께 도핑 된 것은 $Eu^{3+}-Bi^{3+}-O^{2-}$에 상호작용에 의해 282 nm 영역대의 장파장 쪽으로 이동하였다. $350\;nm{\sim}480\;nm$ 영역 대에 '$^7F_0{\to}^5L_9$ (364 nm), $^7F_0{\to}^5G_3$(381 nm), $^7F_0{\to}^5L_6$(395nm), $^7F_0{\to}^5D_3$(415 mn) and $^7F_0{\to}^5D_2$(466 nm)는 $Bi^{3+}$와 $EU^{3+}$ f-f 천이에 의해 발생하였다. $Bi^{3+}$의 도핑농도가 증가할수록 발광 강도가 증가함을 보이다가 0.125 mol 일 때 발광강도가 가장 우수하였고, $Bi^{3+}$의 도핑 농도가 0.125 mol 이상 되면 발광강도가 현저히 감소하는 것을 확인하였다.

• 천문학회지
• /
• 제51권1호
• /
• pp.17-25
• /
• 2018

Detecting exoplanets around giant stars sheds light on the later-stage evolution of planetary systems. We observed the M giant HD 18438 and the K giant HD 158996 as part of a Search for Exoplanets around Northern circumpolar Stars (SENS) and obtained 38 and 24 spectra from 2010 to 2017 using the high-resolution Bohyunsan Observatory Echelle Spectrograph (BOES) at the 1.8m telescope of Bohyunsan Optical Astronomy Observatory in Korea. We obtained precise RV measurements from the spectra and found long-period radial velocity (RV) variations with period 719.0 days for HD 18438 and 820.2 days for HD 158996. We checked the chromospheric activities using Ca $\text\tiny{II}$ H and $H{\alpha}$ lines, HIPPARCOS photometry and line bisectors to identify the origin of the observed RV variations. In the case of HD 18438, we conclude that the observed RV variations with period 719.0 days are likely to be caused by the pulsations because the periods of HIPPARCOS photometric and $H{\alpha}$ EW variations for HD 18438 are similar to that of RV variations in Lomb-Scargle periodogram, and there are no correlations between bisectors and RV measurements. In the case of HD 158996, on the other hand, we did not find any similarity in the respective periodograms nor any correlation between RV variations and line bisector variations. In addition, the probability that the real rotational period can be as longer than the RV period for HD 158996 is only about 4.3%. Thus we conclude that observed RV variations with a period of 820.2 days of HD 158996 are caused by a planetary companion, which has the minimum mass of 14.0 $M_{Jup}$, the semi-major axis of 2.1 AU, and eccentricity of 0.13 assuming the stellar mass of $1.8 M_{\odot}$. HD 158996 is so far one of the brightest and largest stars to harbor an exoplanet candidate.

• 한국전기전자재료학회논문지
• /
• 제29권3호
• /
• pp.186-191
• /
• 2016

The effects of $Eu^{3+}$ doping on the structural, morphological, and optical properties of $MgMoO_4:Dy^{3+},Eu^{3+}$ phosphors prepared by solid-state reaction technique were investigated. XRD patterns exhibited that all the synthesized phosphors showed a monoclinic system with a dominant (220) diffraction peak, irrespective of the content of $Eu^{3+}$ ions. The surface morphology of $MgMoO_4:Dy^{3+},Eu^{3+}$ phosphors was studied using scanning electron microscopy and the grains showed a tendency to agglomerate as the content of $Eu^{3+}$ ions increased. The excitation spectra of the phosphor powders were composed of a strong charge transfer band centered at 294 nm in the range of 230~340 nm and two intense peaks at 354 and 389 nm, respectively, arising from the $^6H_{15/2}{\rightarrow}^6P_{7/2}$ and $^6H_{15/2}{\rightarrow}^4M_{21/2}$ transitions of $Dy^{3+}$ ions. The emission spectra of the $Mg_{0.85}MoO_4$:10 mol% $Dy^{3+}$ phosphors without incorporating $Eu^{3+}$ ions revealed a strong yellow band centered at 573 nm resulting from the $^4F_{9/2}{\rightarrow}^6H_{13/2}$ transition of $Dy^{3+}$. As the content of $Eu^{3+}$ was increased, the intensity of the yellow emission was gradually decreased, while that of red emission band located at 614 nm began to appear, approached a maximum value at 10 mol%, and then decreased at 15 mol% of $Eu^{3+}$. These results indicated that white light emission could be achieved by controlling the contents of the $Dy^{3+}$ and $Eu^{3+}$ ions incorporated into the $MgMoO_4$ host crystal.

• 한국전기전자재료학회:학술대회논문집
• /
• 한국전기전자재료학회 2002년도 제4회 영호남학술대회 논문집
• /
• pp.84-91
• /
• 2002

The stochiometric composition of $AgGaS_2$/GaAs polycrystal source materials for the $AgGaS_2$/GaAs epilayer was prepared from horizontal furnace. From the extrapolation method of X-ray diffraction patterns it was found that the polycrystal $AgGaS_2$/GaAs has tetragonal structure of which lattice constant an and Co were 5.756 $\AA$ and 10.305 $\AA$, respectively. $AgGaS_2$/GaAs epilayer was deposited on throughly etched GaAs(100) substrate from mixed crystal $AgGaS_2$/GaAs by the Hot Wall Epitaxy (HWE) system. The source and substrate temperature were $590^{\circ}C$ and $440^{\circ}C$ respectively. The crystallinity of the grown $AgGaS_2$/GaAs epilayer was investigated by the DCRC (double crystal X-ray diffraction rocking curve). The optical energy gaps were found to be 2.61 eV for $AgGaS_2$/GaAs epilayer at room temperature. The temperature dependence of the photocurrent peak energy is well explained by the Varshni equation, then the constants in the Varshni equation are given by $\alpha=8.695{\times}10^{-4}$ eV/K, and $\beta=332K$. From the photocurrent spectra by illumination of polarized light of the $AgGaS_2$/GaAs epilayer, we have found that crystal field splitting ${\Delta}Cr$ was 0.28 eV at 20 K. From the PL spectra at 20 K, the peaks corresponding to free and bound excitons and a broad emission band due to D-A pairs are identified. The binding energy of the free excitons are determined to be 0.2676 eV and 0.2430 eV and the dissociation energy of the bound excitons to be 0.4695 eV.