• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.121-121
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• 2010

ZnO is a promising material for the application of high efficiency light emitting diodes with short wavelength region for its large bandgap energy of 3.37 eV which is similar to GaN (3.39 eV) at room temperature. The large exciton binding energy of 60 meV in ZnO provide provides higher efficiency of emission for optoelectronic device applications. Several ZnO/ZnMgO multiple quantum well (MQW) structures have been grown on various substrates such as sapphire, GaN, Si, and so on. However, the achievement of high quality ZnO/ZnMgO MQW structures has been somehow limited by the use of lattice-mismatched substrates. Therefore, we propose the optical properties of ZnO/ZnMgO multiple quantum well (MQW) structures with different well widths grown on lattice-matched ZnO substrates by molecular beam epitaxy. Photoluminescence (PL) spectra show MQW emissions at 3.387 and 3.369 eV for the ZnO/ZnMgO MQW samples with well widths of 2 and 5 nm, respectively, due to the quantum confinement effect. Time-resolved PL results show an efficient photo-generated carrier transfer from the barrier to the MQWs, which leads to an increased intensity ratio of the well to barrier emissions for the ZnO/ZnMgO MQW sample with the wider width. From the power-dependent PL spectra, we observed no PL peak shift of MQW emission in both samples, indicating a negligible built-in electric field effect in the ZnO/$Zn_{0.9}Mg_{0.1}O$ MQWs grown on lattice-matched ZnO substrates.

• Analytical Science and Technology
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• v.24 no.6
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• pp.451-459
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• 2011

Eight coordinated terbium(III) complexes, tris (2-pyrazinecarboxylato)(phenanthroline) terbium(III) [$Tb(pzc)_3$(phen)], tris (5-methyl-2-pyrazinecarboxylato) (phenanthroline) terbium(III) [$Tb(mpzc)_3$(phen)] and tris(2-picolinato) (phenanthroline) terbium(III) [$Tb(pic)_3$(phen)], have been synthesized and characterized by Fourier transform infrared (FT-IR), UV-Visible and X-ray photoelectron spectroscopy. Photoluminescence (PL) spectroscopy shows that these complexes emitted strong green luminescence. When powder samples of the $Tb^{3+}$ complexes are examined using time-resolved spectroscopic analysis, the luminescence lifetimes are found to be 0.87 ms and 1.0 ms, respectively. Thermogravimetric analysis reveals the terbium complexes to have good thermal stability up to $333-379^{\circ}C$. Cyclic voltammetry shows that HOMO-LUMO energy gap of the $Tb^{3+}$ complexes ranges from 4.26~4.41 eV. These values are similar to those obtained from the UV-visible spectra. Overall, the synthesized $Tb^{3+}$ complexes may be useful advanced materials for green light emitting devices.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.47-47
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• 2011

There have been numerous efforts to enhance the efficiency of light-emitting diodes (LEDs) by using low dimensional structures such as quantum dots (QDs), wire (QWRs), and wells (QWs). We demonstrate QD/QWR/QW hybrid structured LEDs by using nano-scaled pyramid structures of GaN with ~260 nm height. Photoluminescence (PL) showed three multi-peak spectra centered at around 535 nm, 600 nm, 665 nm for QWs, QWRs, and QDs, respectively. The QD emission survived at room temperature due to carrier localization, whereas the QW emission diminished from 10 K to 300 K. We confirmed that hybrid LEDs had zero-, one-, and two-dimensional behavior from a temperature-dependent time-resolved PL study. The radiative lifetime of the QDs was nearly constant over the temperature, while that of the QWs increased with increasing temperature, due to low dimensional behavior. Cathodoluminescence revealed spatial distributions of InGaN QDs, QWRs, and QWs on the vertices, edges, and sidewalls, respectively. We investigated the blue-shifted electroluminescence with increasing current due to the band-filling effect. The hybrid LEDs provided broad-band spectra with high internal quantum efficiency, and color-tunability for visible light-emitting sources.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.9
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• pp.1290-1298
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• 2003

The stirred tank reactor is one of the most commonly used devices in industry for achieving mixing and reaction. Here we report on results obtained from the large eddy simulations of flow inside the tank performed using a spectral multi-domain technique. The computations were driven by specifying the impeller-induced flow at the blade tip radius. Stereoscopic PlY measurements (Hill et al. $^{(1)}$) along with the theoretical model of the impeller-induced flow (Yoon et al. $^{(2)}$) were used in defining the impeller-induced flow as superposition of circumferential, jet and tip vortex pair components. Large eddy simulation of flow in a stirred tank was carried out for the three different Reynolds numbers of 4000, 16000 and 64000. The effect of different Reynolds numbers is well observed in both instantaneous and time averaged flow fields. The instantaneous and mean vortex structures are identified by plotting an isosurfaces of swirling strength for all Reynolds numbers. The Reynolds number dependency of the non-dimensional eddy viscosity, resolved scale and subgrid scale dissipations is clearly shown in this study.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.298-299
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• 2012

현재 반도체 나노구조는 단전자 트랜지스터, 레이저, LED, 적외선 검출기 등과 같은 고효율 광전자 소자에서의 응용을 위해 활발한 연구가 진행 되고 있다. 이러한 응용 분야를 위한 다양한 종류의 나노구조 성장이 광범위하게 시도 되고 있지만 주로 III-V 족 화합물 반도체에 대한 연구가 주를 이룬 반면 II-VI 족 화합물 반도체에 대한 연구는 아직 미흡하다. 하지만 II-VI 족 화합물 반도체는 III-V 족 화합물 반도체와 비교했을 때 더 큰 엑시톤 결합에너지(exciton binding energy)를 가지는 우수한 특성을 보이고 있으며 이러한 성질을 가지는 II-VI 족 화합물 반도체 중에서도 넓은 에너지 갭을 가지는 CdTe 양자점은 녹색 영역대의 광전자 소자로서 활용되고 있다. 본 연구에서는 분자 선속 에피 성장법(molecular beam epitaxy; MBE)과 원자 층 교대 성장법(atomic layer epitaxy; ALE)으로 CdTe 두께에 따른CdTe/ZnTe 나노구조의 광학적 특성을 연구하였다. 광루미네센스(photoluminescence; PL)를 통해 CdTe/ZnTe 나노구조에서 CdTe 두께에 따른 에너지 밴드와 열적 활성화 에너지를 관찰하였다. 또한 시분해 광루미네센스(Time-resolved PL)를 통해 CdTe 두께에 따른 CdTe/ZnTe 나노구조의 운반자 동역학을 조사하였다. 저온 광루미네센스 측정 결과 CdTe 두께가 증가할수록 각 샘플의 피크는 더 낮은 에너지 영역대로 이동하는 것을 관찰할 수 있다. 1.2 에서 2.0 ML로 증가할 때 광 루미네센스의 작은 적색편이를 관찰할 수 있는데, 이는 CdTe 양자우물에서 양자점으로의 구조적인 전이가 일어남에 따라 구속효과가 증가하였기 때문이다. 또한 2.0 에서 3.6 ML까지 CdTe 두께가 증가할 때 측정된 적색편이 현상은 양자점의 사이즈 증가함에 따른 것이다. 마지막으로 3.6 에서 4.4 ML로 CdTe 두께가 증가할 때 큰 적색편이 현상을 볼 수 있는데 이는 CdTe 양자점에서 양자세선으로의 구조적 전이에 따라 구속효과가 증가하였기 때문이다. 온도 의존 광루미네센스(Temperature-dependent PL) 측정 결과 1.2 와 3.0 ML 두께의 CdTe/ZnTe 나노구조에서 구속된 전자의 열적 활성화 에너지가 18 과 35 meV로 관찰되었다. 3.0 ML CdTe/ZnTe 나노구조에서 가장 큰 열적 활성화 에너지를 갖는 것은 양자점의 균일도가 좋아지고 저차원 나노구조로의 구조적 전이가 일어나면서 운반자 구속효과에 다른 쿨롱 상호작용이 증가하였기 때문이다.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.259-259
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• 2010

Periodically polarity inverted (PPI) ZnO structures on (0001) Al2O3 substrates are demonstrated by plasmas assisted molecular beam epitaxy. The patterning and re-growth methods are used to realize the PPI ZnO by employing the polarity controlling method. For the in-situ polarity controlling of ZnO films, Cr-compound buffer layers are used.[1, 2] The region with the CrN intermediate layer and the region with the Cr2O3 and Al2O3 substrate were used to grow the Zn- and O-polar ZnO films, respectively. The growth behaviors with anisotropic properties of PPI ZnO heterostructures are investigated. The periodical polarity inversion is evaluated by contrast images of piezo-response microscopy. Structural and optical interface properties of PPI ZnO are investigated by the transmission electron microcopy (TEM) and micro photoluminescence ($\mu$-PL). The inversion domain boundaries (IDBs) between the Zn and the O-polar ZnO regions were clearly observed by TEM. Moreover, the investigation of spatially resolved local photoluminescence characteristics of PPI ZnO revealed stronger excitonic emission at the interfacial region with the IDBs compared to the Zn-polar or the O-polar ZnO region. The possible mechanisms will be discussed with the consideration of the atomic configuration, carrier life time, and geometrical effects. The successful realization of PPI structures with nanometer scale period indicates the possibility for the application to the photonic band-gap structures or waveguide fabrication. The details of application and results will be discussed.