GaN is most commonly used to make LED elements. But, due to differences of the thermal expansion coefficient and lattice mismatch with sapphire, dislocations have occurred at about 109~1010/cm2. Generally, a low temperature GaN buffer layer is used between the GaN layer and the sapphire substrate in order to reduce the dislocation density and improve the characteristics of the thin film, and thus to increase the efficiency of the LED. Further, patterned sapphire substrate (PSS) are applied to improve the light extraction efficiency. In this experiment, using an AlN buffer layer on PSS in place of the GaN buffer layer that is used mainly to improve the properties of the GaN film, light extraction efficiency and overall properties of the thin film are improved at the same time. The AlN buffer layer was deposited by using a sputter and the AlN buffer layer thickness was determined to be 25 nm through XRD analysis after growing the GaN film at 1070˚C on the AlN buffer CPSS (C-plane Patterned Sapphire Substrate, AlN buffer 25 nm, 100 nm, 200 nm, 300 nm). The GaN film layer formed by applying a 2 step epitaxial lateral overgrowth (ELOG) process, and by changing temperatures (1020~1070˚C) and pressures (85~300 Torr). To confirm the surface morphology, we used SEM, AFM, and optical microscopy. To analyze the properties (dislocation density and crystallinity) of a thin film, we used HR-XRD and Cathodoluminescence.

Nanosphere lithography is an inexpensive, simple, high-throughput nanofabrication process. NSL can be done in different ways, such as drop coating, spin coating or by means of tilted evaporation. Nitride-based light-emitting diodes (LEDs) are applied in different places, such as liquid crystal displays and traffic signals. The characteristics of gallium nitride (GaN)-based LEDs can be enhanced by fabricating nanopatterns on the top surface of the LEDs. In this work, we created differently sized (420, 320 and 140 nm) nanopatterns on the upper surfaces of GaN-based LEDs using a modified nanosphere lithography technique. This technique is quite different from conventional NSL. The characterization of the patterned GaN-based LEDs revealed a dependence on the size of the holes in the pattern created on the LED surface. The depths of the patterns were 80 nm as confirmed by AFM. Both the photoluminescence and electroluminescence intensities of the patterned LEDs were found to increase with an increase in the size of holes in the pattern. The light output power of the 420-nm hole-patterned LED was 1.16 times higher than that of a conventional LED. Moreover, the current-voltage characteristics were improved with the fabrication of differently sized patterns over the LED surface using the proposed nanosphere lithography method.

The ZnO thin films were grown on GaN template substrates by RF magnetron sputtering at different RF powers and n-ZnO/p-GaN heterojunction LEDs were fabricated to investigate the effect of the RF power on the characteristics of the n-ZnO/p-GaN LEDs. For the growth of the ZnO thin films, the substrate temperature was kept constant at 200˚C and the RF power was varied within the range of 200 to 500W at different growth times to deposit films of 100 nm thick. The electrical, optical and structural properties of ZnO thin films were investigated by ellipsometry, X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) and by assessing the Hall effect. The characteristics of the n-ZnO/p-GaN LEDs were evaluated by current-voltage (I-V) and electroluminescence (EL) measurements. ZnO thin films were grown with a preferred c-axis orientation along the (0002) plane. The XRD peaks shifted to low angles and the surface roughness became non-uniform with an increase in the RF power. Also, the PL emission peak was red-shifted. The carrier density and the mobility decreased with the RF power. For the n-ZnO/p-GaN LED, the forward current at 20 V decreased and the threshold voltage increased with the RF power. The EL emission peak was observed at approximately 435 nm and the luminescence intensity decreased. Consequently, the crystallinity of the ZnO thin films grown with RF sputtering powers were improved. However, excess Zn affected the structural, electrical and optical properties of the ZnO thin films when the optimal RF power was exceeded. This excess RF power will degrade the characteristics of light emitting devices.

In this study, GaN powders were synthesized from gallium oxide-hydroxide (GaOOH) through an ammonification process in an NH3 flow with the variation of B2O3 additives within a temperature range of 300-1050˚C. The additive effect of B2O3 on the hexagonal phase GaN powder synthesis route was examined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transformation infrared transmission (FTIR) spectroscopy. With increasing the mol% of B2O3 additive in the GaOOH precursor powder, the transition temperature and the activation energy for GaN powder formation increased while the GaN synthesis limit-time (tc) shortened. The XPS results showed that Boron compounds of B2O3 and BN coexisted in the synthesized GaN powders. From the FTIR spectra, we were able to confirm that the GaN powder consisted of an amorphous or cubic phase B2O3 due to bond formation between B and O and the amorphous phase BN due to B-N bonds. The GaN powder synthesized from GaOOH and B2O3 mixed powder by an ammonification route through β-Ga2O3 intermediate state. During the ammonification process, boron compounds of B2O3 and BN coated β-Ga2O3 and GaN particles limited further nitridation processes.

In this study, we have investigated highly efficient nanoscale surface corrugated light emitting diodes (LEDs) for the enhancement of light extraction efficiency (LEE) of nitride semiconductor LEDs. Nanoscale indium tin oxide (ITO) surface corrugations are fabricated by using the conformal nanoimprint technique; it was possible to observe an enhancement of LEE for the ITO surface corrugated LEDs. By incorporating this novel method, we determined that the total output power of the surface corrugated LEDs were enhanced by 45.6% for patterned sapphire substrate LEDs and by 41.9% for flat c-plane substrate LEDs. The enhancement of LEE through nanoscale surface corrugations was studied using 3-dimensional Finite Different Time Domain (FDTD) calculation. From the FDTD calculations, we were able to separate the light extraction from the top and bottom sides of device. This process revealed that light extraction from the top and bottom sides of a device strongly depends on the substrate and the surface corrugation. We found that enhanced LEE could be understood through the mechanism of enhanced light transmission due to refractive index matching and the increase of light scattering from the corrugated surface. LEE calculations for the encapsulated LEDs devices also revealed that low LEE enhancement is expected after encapsulation due to the reduction of the refractive index contrast.

본 연구에서는 GaN 나노와이어의 인장, 압축, 하중 제거 전산모사를 분자동역학 방법을 통하여 수행하였고, 평형 분자 동역학 방법인 Green-Kubo 방법을 이용하여 각각의 변형된 구조의 나노와이어의 열전도율을 구하였다. 단면의 형상이 육 각형이고, 길이 방향이 [0001] 격자 방향으로 형성된 나노와이어에 인장 하중이 작용하게 되면 나노와이어의 원자 구조는 초기의 wurtzite 구조에서 정방정계 구조로 변형된다. 초기 상태에 압축 하중이 작용하는 경우에는 상변이 현상은 나타나 지 않는다. 압축에서 인장으로 변형률이 증가함에 따라 나노와이어의 열전도율은 감소하는 경향을 나타낸다. 이 같은 열전 도율의 변화는 변형률에 따른 포논의 감쇠시간 감소에 의한 것이다. 인장에 의해 변형된 정방정계 구조의 나노와이어에서 인장 하중을 제거하는 경우에는 초기의 wurtzite 구조로의 역상변이 현상이 나타나고, 이와 같은 역상변이 과정에 wurtzite 구조와 정방정계 구조가 동시에 나타나는 중간 단계가 존재한다. 중간 단계의 열전도율은 같은 변형률에서 wurtzite 구조 일 때보다 낮은 특성을 갖는다. 내부 원자 구조에 따른 열전도율의 차이는 구조적 변형에 의한 포논의 군속도 변화에 따른 것이다.

Ultraviolet (UV) light emitting diodes (LEDs) were grown on a patterned n-type GaN substrate (PNS) with 200 nm silicon-di-oxide (SiO2) nano pattern diameter to improve the light output efficiency of the diodes. Wet etched self assembled indium tin oxide (ITO) nano clusters serve as a dry etching mask for converting the SiO2 layer grown on the n-GaN template into SiO2 nano patterns by inductively coupled plasma etching. PNS is obtained by n-GaN regrowth on the SiO2 nano patterns and UV-LEDs were fabricated using PNS as a template. Two UV-LEDs, a reference LED without PNS and a 200 nm PNS UV-LEDs were fabricated. Scanning Electron microscopy (SEM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), Photoluminescence (PL) and Light output intensity- Input current- Voltage (L-I-V) characteristics were used to evaluate the ITO-SiO2 nanopattern surface morphology, threading dislocation propagation, PNS crystalline property, PNS optical property and UVLED device performance respectively. The light out put intensity was enhanced by 1.6times@100mA for the LED grown on PNS compared to the reference LED with out PNS.

Due to their novel properties, GaN based semiconductors and their nanostructures are promising components in a wide range of nanoscale device applications. In this work, the gallium nitride is deposited on c-axis oriented sapphire and porous SWCNT substrates by molecular beam epitaxy using a novel single source precursor of Me2Ga(N3)NH2C(CH3)3 with ammonia as an additional source of nitrogen. The advantage of using a single molecular precursor is possible deposition at low substrate temperature with good crystal quality. The deposition is carried out in a substrate temperature range of 600-750˚C. The microstructural, structural, and optical properties of the samples were analyzed by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and photoluminescence. The results show that substrate oriented columnar-like morphology is obtained on the sapphire substrate while sword-like GaN nanorods are obtained on porous SWCNT substrates with rough facets. The crystallinity and surface morphology of the deposited GaN were influenced significantly by deposition temperature and the nature of the substrate used. The growth mechanism of GaN on sapphire as well as porous SWCNT substrates is discussed briefly.

This study develops a highly transparent ohmic contact scheme using indium oxide doped ZnO (IZO)as a current spreading layer for p-GaN in order to increase the optical output power of nitride-based light-emitting diodes (LEDs). IZO based contact layers of IZO, Ni/IZO, and NiO/IZO were prepared by e-beamevaporation, followed by a post-deposition annealing. The transmittances of the IZO based contact layers werein excess of 80% throughout the visible region of the spectrum. Specific contact resistances of 3.4×10−4,1.2×10−4, 9.2×0−5, and 3.6×10−5Ω·cm2 for IZO, Ni/Au, Ni/IZO, and NiO/IZO, respectively were obtained. Theforward voltage and the optical output power of GaN LED with a NiO/IZO ohmic contact was 0.15V lower andwas increased by 38.9%, respectively, at a forward current of 20mA compared to that of a standard GaN LEDwith an Ni/Au ohmic contact due to its high transparency, low contact resistance, and uniform current spreading.

This study develops a highly transparent ohmic contact using phosphorus doped ZnO with current spreading for p-GaN to increase the optical output power of nitride-based light-emitting diodes (LEDs). The phosphorus doped ZnO transparentohmic contact layer was prepared by radio frequency magnetron sputtering with post-deposition annealing. The transmittance ofthe phosphorus doped ZnO exceeds 90% in the region of 440nm to 500nm. The specific contact resistance of the phosphorusdoped ZnO on p-GaN was determined to be 7.82×10-3Ω·cm2 after annealing at 700oC. GaN LED chips with dimensions of300×300µm fabricated with the phosphorus doped ZnO transparent ohmic contact were developed and produced a 2.7Vincrease in forward voltage under a nominal forward current of 20mA compared to GaN LED with Ni/Au Ohmic contact.However, the output power increased by 25% at the injection current of 20mA compared to GaN LED with the Ni/Au contactscheme.

Growth behavior of InGaN/GaN self-assembled quantum dots (QDs) was investigated with respect to different growth parameters in low pressure metalorganic chemical vapor deposition. Locally formed examples of three dimensional InGaN islands were confirmed from the surface observation image with increasing indium source ratio and growth time. The InGaN/GaN QDs were formed in Stranski-Krastanow (SK) growth mode by the continuous supply of metalorganic (MO) sources, whereas they were formed in the Volmer-Weber (V-W) growth mode by the periodic interruption of the MO sources. High density InGaN QDs with 1~2nm height and 40~50nm diameter were formed by the S-K growth mode. Dome shape InGaN dots with 200~400nm diameter were formed by the V-W growth mode. InN content in InGaN QDs was estimated to be reduced with the increase of growth temperature. A strong peak between 420-460 nm (2.96-2.70 eV) was observed for the InGaN QDs grown by S-K growth mode in photoluminescence spectrum together with the GaN buffer layer peak at 362.2 nm (3.41 eV).

Mg-doped and In-Mg co-doped p-type GaN epilayers were grown in a low-pressure metal organic chemical vapor deposition technique. The effect of In doping on the p-GaN layer was studied through photoluminescence (PL), persistent photoconductivity (PPC), and transmission electron microscopy (TEM) at room temperature. For the In-doped p-GaN layer, the PL intensity increases significantly and the peak position shifts to 3.2 eV from 2.95 eV of conventional p-GaN. Additionally, In doping greatly reduces the PPC, which was very strong in conventional p-GaN. A reduction in the dislocation density is also evidenced upon In doping in p-GaN according to TEM images. The improved optical properties of the In-doped p-GaN layer are attributed to the high crystalline quality and to the active participation of incorporated Mg atoms.

儒學은 기본적으로 인간의 사회적 존재됨에 주목하고, 그 실현에 진력하는 학문이다. 19세기 한말의 사회는 정치, 경제, 문화 기타 모든 분야에 걸쳐서 미중유의 혼란과 위기에 놓여 있었다. 艮齋 田愚는 우리 역사상 초유의 난세에 처하여 견디기 어려운 고뇌의 삶을 살았던 것으로 보인다. 그러면 그의 출처는 그렇게 비난을 받아야 마땅한 것일까? 그는 끝까지 ‘守道’로써 일관하였다. 그렇기 때문에 비난이 일어났는데, 간재가 외연의 행적과는 달리 후세 사람들로부터 정당하게 평가 받고자 했던 속뜻은 무엇이었을까? 그에 대한 儒學思想史的 자리매김은 이런 논의를 거치지 않고서는 오류를 면하기 어려울 것이다. 이상의 관점으로 간재의 ‘去之守義의 守道意識’과 ‘自靖의 出處觀’을 살펴보았다. 간재의 ‘去之守義의 守道意識’에서는 그는 단순히 난세를 피하여 자기 한 몸 깨끗이 살겠다고 숨어 지내는 것이 아니라, 죽음에 이르더라도 수도하며 살겠다는 의지를 표현하였다. 그는 유학자의 본분은 도의 수호와 실천으로 여겼음을 확인할 수 있었다. ‘自靖의 출처관’에서는 도를 지켜야 한다는 확고한 신념에서 비롯된 것을 확인할 수 있었다. 또한 수도하여 후일을 기약하고, 사회 기강 확립을 위한 근간으로 생각하였음을 확인 하였다. 본고의 이러한 말이 간재의 학문과 의리를 억지로 정당화 한다거나 굳이 옹호하고 과대평가하기 위해서가 아니라, 다만 한 사람의 학행과 인간을 이해하고 평가함에 있어서는 편협하거나 일면적인 것만을 기준으로 해서는 안된다는 것을 전제한다.