• Applied Microscopy
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• 제41권3호
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• pp.215-222
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• 2011

We discuss the experimental procedure for extracting reliable phase information from a defocus series of transmission electron microscopy (TEM) dark-field images using the transport of intensity equation (TIE). Taking InGaN/GaN multi-quantum well light-emitting diode as a model system, various factors affecting the final result of reconstructed phase such as TEM sample preparation, TEM imaging condition, image alignment, the correction of defocus values and the use of high frequency pass filter are evaluated. The obtained phase of wave function was converted to the geometric phase of the corresponding lattice planes, which was then used for the two-dimensional mapping of lattice strain following the dark-field inline holography (DIH) routine. The strain map obtained by DIH after optimized image processing is compared with that obtained by the geometric phase analysis of high resolution TEM (HRTEM) image, manifesting that DIH yields more accurate and reliable strain information than HRTEM-based GPA.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제45회 하계 정기학술대회 초록집
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• pp.99-99
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• 2013

In this talk, I will present two research works in progress, which are: i) mapping of piezoelectric polarization and associated charge density distribution in the heteroepitaxial InGaN/GaN multi-quantum well (MQW) structure of a light emitting diode (LED) by using inline electron holography and ii) in-situ observation of the polarization switching process of an ferroelectric Pb(Zr1-x,Tix)O3 (PZT) thin film capacitor under an applied electric field in transmission electron microscope (TEM). In the first part, I will show that strain as well as total charge density distributions can be mapped quantitatively across all the functional layers constituting a LED, including n-type GaN, InGaN/GaN MQWs, and p-type GaN with sub-nm spatial resolution (~0.8 nm) by using inline electron holography. The experimentally obtained strain maps were verified by comparison with finite element method simulations and confirmed that not only InGaN QWs (2.5 nm in thickness) but also GaN QBs (10 nm in thickness) in the MQW structure are strained complementary to accommodate the lattice misfit strain. Because of this complementary strain of GaN QBs, the strain gradient and also (piezoelectric) polarization gradient across the MQW changes more steeply than expected, resulting in more polarization charge density at the MQW interfaces than the typically expected value from the spontaneous polarization mismatch alone. By quantitative and comparative analysis of the total charge density map with the polarization charge map, we can clarify what extent of the polarization charges are compensated by the electrons supplied from the n-doped GaN QBs. Comparison with the simulated energy band diagrams with various screening parameters show that only 60% of the net polarization charges are compensated by the electrons from the GaN QBs, which results in the internal field of ~2.0 MV cm-1 across each pair of GaN/InGaN of the MQW structure. In the second part of my talk, I will present in-situ observations of the polarization switching process of a planar Ni/PZT/SrRuO3 capacitor using TEM. We observed the preferential, but asymmetric, nucleation and forward growth of switched c-domains at the PZT/electrode interfaces arising from the built-in electric field beneath each interface. The subsequent sideways growth was inhibited by the depolarization field due to the imperfect charge compensation at the counter electrode and preexisting a-domain walls, leading to asymmetric switching. It was found that the preexisting a-domains split into fine a- and c-domains constituting a $90^{\circ}$ stripe domain pattern during the $180^{\circ}$ polarization switching process, revealing that these domains also actively participated in the out-of-plane polarization switching. The real-time observations uncovered the origin of the switching asymmetry and further clarified the importance of charged domain walls and the interfaces with electrodes in the ferroelectric switching processes.

• 한국진공학회지
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• 제7권s1호
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• pp.20-24
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• 1998

Photploymer films are widely used in printing and electronic industries, and their usage is expanding to encompass holography, data storage and data processing, optical waveguides and compact disks, etc. One of widely used photoplymerization initiator, 20chloro-hexaarylbiimidazole (o-Cl-HABI), is studied by laser flash photolysis in dichloromethane solution in the absence and presence of the visible light photosensitizing dye, 2, 5-bis[(2, 3, 6, 7 -tetrahydro- 1H, 5H -benzo [i, j,] quinolizin -1-yl) methylene]-cyclopenta-none, (JAW). In the presence of JAW, an increase in triarylimidazolyl radicals L.formation is observed in relative to the absence of JAW. The mechanism of this photosensitizing dissociation is concluded as the dissociation of the o-Cl-HABI radical anion formed by the electron transfer from excited singlet state of JAW to o-Cl-HABI. The observed formation of L.radicals exhibits a linear dependence on o-Cl-HABI concentration. The rate constant of electron transfer obtained from this dependence is equal to (1.0$\pm$0.2) x $10^9 M^{-1}s^{-1}$. No reaction between the excited triplet state of JAW and o-Cl-HABI is found.

• 한국항행학회논문지
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• 제2권2호
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• pp.132-142
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• 1998

스케줄링 문제는 응용 영역에 따라 각기 다른 작업과 차원을 가지며, 이들간에 여러 가지 제약 조건이 존재한다. 이러한 문제에 인공지능 기법을 이용하여 문제를 효율적으로 해결할 수 있다. 그러나, 동적으로 변하는 상황에서 모든 제약조건을 만족하는 최적해를 구할 수 없는 경우, 부분적으로 만족하는 해를 구하기 위해 제약 조건들은 수정 또는 추가되어야 하며, 때로는 고려 대상에서 제외되어야 한다. 부분적으로 만족하는 해를 구하기 위해, 수정되어야 할 적절한 제약 조건을 스케줄링 시스템이 자동으로 선택하기에는 많은 문제가 있다. 이러한 문제를 해결하기 위해, 본 논문에서는 부분적으로 만족하는 해를 구해야 하는 경우, 제약 조건 완화를 위해 사용자와 상호 작용하며 사용자의 의견을 제약 조건에 반영하는 지능형 사용자 인터페이스(Intelligent User Interface)의 모델을 설계하고 구현하였다. 김포공항의 운항 스케줄 데이터를 사용한 실험에서 제약 조건을 모두 적용하여 이를 만족하는 해를 구하지 못한 경우, 사용자의 의견이 반영된 제약 조건완화를 통해 검색 도메인의 증가와 제약 조건을 부분적으로 만족하는 해를 구할 수 있었다.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2002년도 추계학술대회 논문집
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• pp.31-34
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• 2002

Imaging techniques using x-ray beam at high energies (>6KeV) such as contact radiography, projection microscopy, and tomography have been used to nondestructively discern internal structure of objects in material science, biology, and medicine. This paper introduces the x-ray micro-imaging method using 1B2 micro-probe line of PAL (Pohang Accelerator Laboratory). Cross-sectional information on low electron density materials can be obtained by probing a sample with coherent synchrotron x-ray beam in an in-line holography setup. Living organism such as plants, insects are practically transparent to high energy x-rays and create phase shift images of x-ray wave front. X-ray micro-images of micro-bubbles of $20\~120\;{\mu}m$ diameter in an opaque tube were recorded. Clear phase contrast images were obtained at Interfaces between bubbles and surrounding liquid due to different decrements of refractive index.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2008년도 Techno-Fair 및 추계학술대회 논문집 전기물성,응용부문
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• pp.163-164
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• 2008

Chalcogenide glass has been known for many photo induced phenomena and superial electron / optical specific by structure flexibility, unique electronic configuration. It is become known to the greatest specific as photonic material medium that possible to perfect controlling by continuity and photo inducing direction of amorphous chalcogenide. In our experiment, we choose the amorphous As-Ge-Se-S and corning glass as a substrate. And then we have evaporated in the ${\sim}2{\times}10^{-6}$ Torr using a E-beam evaporator, completed thin film sample that have 1um thickness of As-Ge-Se-S in $600{\AA}$, $10{\sim}5{\AA}/s$. At first, we let the change the angle between laser and sample by holography litho method and then, expect that satisfied conclusion which 2-dimension diffraction lattice manufacture and specifics by investing a He-Ne laser for 2000 seconds.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2008년도 추계학술대회 논문집 Vol.21
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• pp.130-131
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• 2008

Chalcogenide glass has been known for many photo induced phenomena and superial electron / optical specific by structure flexibility, unique electronic configuration. It is become known to the greatest specific as photonic material medium that possible to perfect controlling by continuity and photo inducing direction of amorphous chalcogenide. In our experiment, we choose the amorphous As-Ge-Se-S and coming glass as a substrate. And then we have evaporated in the ${\sim}2{\times}10^{-6}$ Torr using a E-beam evaporator, completed thin film sample that have 1um thickness of As-Ge-Se-S 600 $\AA$, 10~5 $\AA$/s. At first, we let the change the angle between laser and sample by holography litho method and then, expect that satisfied conclusion which 2-dimension diffraction lattice manufacture and specifics by investing a He-Ne laser for 2000 seconds.

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