질화갈륨 발광다이오드의 내부 양자 효율에 대한 분석 및 향상

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2014. 8. 박병국.In order to extract the recombination coefficient and the internal quantum efficiency (IQE) of the GaN-based LEDs, a fast and reliable measurement method using transient characteristics is developed. For accurate extraction of the recombination coefficients and the IQE, an improved rate equation model for GaN-based LEDs considering the effective volume of the active region is also proposed. Through TCAD simulations, it is confirmed that the IQE, especially efficiency droop is related with small effective volume. Also, it is confirmed that the effective volume is controlled by polarization charge, the barriers between the quantum wells, and current density. The trap and its impact on the GaN-based LEDs are also analyzed by measurement and TCAD simulation. A reversible increase in the current of GaN-based blue LEDs is observed when constant forward voltage is applied. This characteristic is assumed to be the result of trapping process, and a trap activation energy of 0.30 eV is extracted. Through TCAD simulations, it is confirmed that the multi-quantum well (MQW) barrier height is reduced by the hole trapping process and that the current is increased by lowering this barrier. It is also confirmed that the effect of this trap on the optical characteristics of GaN-based blue LEDs by TCAD simulation and measurement. To improve the IQE of GaN-based LEDs, a novel structure for GaN-based LED featuring p-type trench in the MQW is proposed. Through TCAD simulation, it is confirmed that the proposed structure shows quite uniform hole distribution in the MQW than that of the conventional structure, because holes are injected efficiently into the MQW along the p-type trench. It is also confirmed that the proposed structure also has a significant effect on strain relaxation and reduction in quantum confined stark effect by cathodo-luminescence (CL) measurement. In addition, two simple fabrication methods using e-beam lithography and selective wet etching for manufacturing the proposed structure are also proposed. From the measurement results of the manufactured GaN-based LEDs, it is confirmed that the proposed structure using e-beam lithography or selective wet etching shows improved light output power compared to the conventional structure because of more uniform hole distribution and strain relaxation effect. From this study, methods for analyzing the IQE of the GaN-based LEDs and its limiting factors are proposed and verified. It is also demonstrated that the p-type trench structure in the MQW will be the promising candidate for solving the efficiency droop problem of the GaN-based LEDs.Chapter1 Introduction 1.1 BACKGROUND 1.2 THESIS OUTLINE Chapter 2 Extraction of IQE and recombination coefficients by measuring transient characteristics 2.1 MODEL DESCRIPTION AND EXPERIMENTAL SETUP 2.2 EFFECTIVE VOLUME OF THE ACTIVE REGION 2.3 EXTRACTION OF RECOMBINATION COEFFICIENTS 2.4 CALCULATION OF INTERNAL QUANTUM EFFICIENCY 18 Chapter 3 Analysis of trap and its impact 3.1 EXPERIMENTAL PROCEDURE 3.2 EXTRACTION OF TRAP ACTIVATION ENERGY 3.3 EFFECTS OF TRAPS ON GAN-BASED LEDS Chapter 4 p-type trench structure for improving IQE 4.1 PROPOSED STRUCTURE 4.2 TCAD SIMULATION RESULTS 4.3 TRENCH PATTERNING USING E-BEAM LITHOGRAPHY 4.4 TRENCH PATTERNING USING SELECTIVE WET ETCHING 4.5 MEASUREMENT RESULTS Chapter 5 ConclusionsDocto

Experimental Study of Effects of Transverse Jet on Aerodynamic Characteristics of Supersonic flat plate

학위논문(석사)--서울대학교 대학원 :공과대학 기계항공공학부,2019. 8. 김규홍.초음속 유동 내에서 수직 방향으로 제트가 분사되는 환경은 여러 항공 공학 적용 분야에서 나타난다. 특히 초음속 비행체의 유동 제어를 위해 수직 분사 제트를 활용하는 경우 분사 제트와 초음속 자유류와의 상호 작용에 의한 제트 간섭(jet interaction) 현상이 발생한다. 본 연구에서는 초음속 풍동 실험을 통해 초음속 평판 유동에서 수직 분사 되는 제트가 있을 경우 발생하는 유동 현상과 그에 의한 평판 공력 특성 변화에 대한 연구를 수행하였다. 서울대학교 풍동 시험동에 위치한 초음속 풍동 설비를 이용하여 마하수 3의 자유류 유동 환경을 모사하였고 평판 모델에 압축 가스를 분사하여 실험을 수행하였다. 평판에 압력탭을 설치하여 제트 간섭에 의한 압력 분포 변화를 측정하였으며 쉴리렌 유동 가시화를 통해 barrel shock 및 bow shock 등의 유동 구조를 관측하였다. 분사 제트의 전압 및 작동유체 조건을 변화시켜가며 실험을 수행하였으며 이를 통해 제트 간섭 효과에 영향을 미치는 작동 유체의 특성에 대해 확인하였다. 본 연구를 통해 초음속 환경에서 수직 분사 제트 이용 시 발생하는 제트 간섭 현상에 대한 연구를 수행하였으며 후속 연구로 수행될 스파크 젯 플라즈마 액츄에이터에 의한 유동 제어 연구의 기초 연구로써 활용할 수 있을 것으로 판단된다.Transverse jet in supersonic flow can be used for several aerodynamic applications. When a transverse jet is injected into the supersonic flow, a complex flow field is created which is known as the jet interaction. This paper studies experimentally the aerodynamic characteristics of transverse jet in supersonic flow. The experiments were conducted in Seoul National University supersonic wind tunnel and the Mach number of free stream was 3. A flat plate model was mounted in test section of supersonic wind tunnel. The gaseous injector nozzle was located on the flat plate model. Surface pressure distribution over flat plate is measured by pressure scanner which is connected with pressure tab on the flat plate model. Z-type Schlieren visualization was conducted to measure the structure of flow near the gaseous injector nozzle. Experiments were conducted with changing the injectants and stagnation pressure of jet to investigate the effects of transverse jet on aerodynamic characteristics of supersonic flat plate. Through these experiments, jet interaction which occur at transverse jet in supersonic free stream was investigated. The results can be used for basic research for developing spark jet actuator to control supersonic flow.목 차 국문 초록 i 목 차 iii 그림 목차 v 표 목차 viii 기호 및 약어 ix 1 서 론 1 1.1 수직 분사 제트 1 1.2 제트 간섭(jet interaction) 3 2 준 1차원 유동 및 지배 방정식 5 3 실험 장비 8 3.1 서울대학교 초음속 풍동 8 3.2 평판 모델 10 3.2.1 평판 모델 형상 10 3.2.3 Viscous interaction을 고려한 압력탭 위치 13 3.3 수직 분사 제트 15 3.4 압력 계측 시스템 17 3.4.1 WTR01 Industrial pressure transmitter 17 3.4.2 9116 NetScanner System 17 3.5 쉴리렌 유동 가시화 20 4 초음속 풍동 성능 시험 21 4.1 초음속 풍동 유질 평가 21 4.2 시험부 정상 상태 22 5 초음속 평판 유동에서 수직 제트 분사 실험 24 5.1 Baseline 24 5.2 Compressed air 분사 실험 26 5.2.1 Compressed air 분사 실험 조건 26 5.2.2 Compressed air 분사 실험 결과 27 5.3 Helium 분사 실험 33 5.3.1 Helium 분사 실험 조건 33 5.3.2 Helium 분사 실험 결과 34 5.4 Argon 분사 실험 39 5.4.1 Argon 분사 실험 조건 39 5.4.2 Argon 분사 실험 결과 40 5.5 작동 유체 종류의 영향 43 6 결 론 47 7 참고문헌 48 Abstract 50Maste

유방암에서 자기공명영상 근거 영상표현형과 유전자 발현 프로파일 근거 위험도의 관계

Purpose To investigate the correlation between magnetic resonance (MR) image-based radiomics features and the genomic features of breast cancer by focusing on biomolecular intrinsic subtypes and gene expression profiles based on risk scores. Materials and Methods We used the publicly available datasets from the Cancer Genome Atlas and the Cancer Imaging Archive to extract the radiomics features of 122 breast cancers on MR images. Furthermore, PAM50 intrinsic subtypes were classified and their risk scores were determined from gene expression profiles. The relationship between radiomics features and biomolecular characteristics was analyzed. A penalized generalized regression analysis was performed to build prediction models. Results The PAM50 subtype demonstrated a statistically significant association with the maximum 2D diameter (p = 0.0189), degree of correlation (p = 0.0386), and inverse difference moment normalized (p = 0.0337). Among risk score systems, GGI and GENE70 shared 8 correlated radiomic features (p = 0.0008–0.0492) that were statistically significant. Although the maximum 2D diameter was most significantly correlated to both score systems (p = 0.0139, and p = 0.0008), the overall degree of correlation of the prediction models was weak with the highest correlation coefficient of GENE70 being 0.2171. Conclusion Maximum 2D diameter, degree of correlation, and inverse difference moment normalized demonstrated significant relationships with the PAM50 intrinsic subtypes along with gene expression profile-based risk scores such as GENE70, despite weak correlations.ope

Feasibility study using multifocal Doppler twinkling artifacts to detect suspicious microcalcifications in ex vivo specimens of breast cancer on US

Multifocal Doppler twinkling artifact (MDTA) imaging has shown high detection rates of microcalcifications in phantom studies. We aimed to evaluate its performance in detecting suspicious microcalcifications in comparison with mammography by using ex vivo breast cancer specimens. We prospectively included ten women with breast cancer that presented with calcifications on mammography. Both digital specimen mammography and MDTA imaging were performed for ex vivo breast cancer specimens on the day of surgery. Five breast radiologists marked cells that included suspicious microcalcifications (referred to as 'positive cell') on specimen mammographic images using a grid of 5-mm cells. Cells that were marked by at least three readers were considered as 'consensus-positive'. Matched color Doppler twinkling artifact (CDTA) signals were compared between reconstructed US-MDTA projection images and mammographic images. The median detection rate for each case was 74.7% for positive cells and 96.7% for consensus-positive cells. Of the 10 cases, 90% showed a detection rate of ≥ 80%, with 50% of cases showing a 100% detection rate for consensus-positive cells. The proposed MDTA imaging method showed high performance for detecting suspicious microcalcifications in ex vivo breast cancer specimens, and may be a feasible approach for detecting suspicious breast microcalcifications with US.ope

Sonographic features and ultrasonography-guided fine-needle aspiration of metastases to the thyroid gland

PURPOSE: To evaluate the characteristic ultrasonographic (US) features of metastatic carcinoma to the thyroid, and how accurate US features and ultrasonography-guided fine-needle aspiration (US-FNA) are for the diagnosis of thyroid metastases. METHODS: Twenty-three thyroid lesions in 23 patients (mean age, 66.7 years; range, 46 to 85 years) that had been diagnosed as thyroid metastases were included. The composition, echogenicity, margin, shape, presence of calcifications, underlying parenchymal echotexture, and vascularity were analyzed in US images of the thyroid metastases. Final US assessments were categorized into probably benign and suspicious malignancy. The presence of suspicious metastatic cervical lymph nodes was noted. The medical records, US-FNA cytology, and pathology reports of these patients were retrospectively reviewed. RESULTS: Of the 23 thyroid lesions, the general US appearance was mass-forming in 21 (91.3%) and non-mass-forming in 2 (8.7%). All 23 lesions showed a solid tumor composition. Common US features among the 21 mass-forming thyroid metastases were hypoechogenicity (81.0%), non-circumscribed margins (90.5%), no calcifications (76.2%), and parallel shape (81.0%). Suspicious cervical lymph nodes were present in 18 patients (78.3%). Of the 23 lesions, 21 (91.3%) were classified as suspicious malignancy, and 2 (8.7%) as probably benign. US-FNA showed diagnostic results specific for metastases in 21 of the 22 patients (95.5%) who had undergone US-FNA. CONCLUSION: Common US features in thyroid metastasis were hypoechogenicity, non-circumscribed margins, no calcifications, parallel shape, and the presence of suspicious cervical lymph nodes. US-FNA can be effectively used in the diagnosis of thyroid metastasis, preventing unnecessary surgery.ope

Benign Aspirates on Follow-Up FNA May Be Enough in Patients with Initial Atypia of Undetermined Significance/Follicular Lesion of Undetermined Significance

Background. Management of thyroid nodules with benign aspirates following atypia of undetermined significance/follicular lesion of undetermined significance (AUS/FLUS) is not well established. We reviewed the risk of malignancy and the role of ultrasound (US) features among thyroid nodules with benign results following initial AUS/FLUS diagnoses. Methods. From December 2009 to February 2011, a total of 114 nodules in 114 patients diagnosed as benign on follow-up fine-needle aspiration (FNA) after AUS/FLUS results were included in our study. Eight among 114 nodules were confirmed pathologically and 106 were clinically observed by a follow-up FNA or US. Suspicious US features were defined as markedly hypoechogenicity, irregular or microlobulated margin, presence of microcalcifications, and taller than wide shape. Results. There were 110 (96.5%) benign nodules and 4 (3.5%) malignant nodules. Two (4.8%) among 42 nodules without suspicious US features and 2 (2.8%) out of 72 nodules with suspicious US features were confirmed as malignancy, but there were no significant associations between the malignancy rate and US features (P = 0.625). Conclusion. Clinical follow-up instead of surgical excision or continuous repeat FNA may be enough for benign thyroid nodules after AUS/FLUS. The role of US features might be insignificant in the management of these nodules.ope

Photoacoustic Imaging of Breast Microcalcifications: A Preliminary Study with 8-Gauge Core-Biopsied Breast Specimens

BACKGROUND: We presented the photoacoustic imaging (PAI) tool and to evaluate whether microcalcifications in breast tissue can be detected on photoacoustic (PA) images. METHODS: We collected 21 cores containing microcalcifications (n = 11, microcalcification group) and none (n = 10, control group) in stereotactic or ultrasound (US) guided 8-gauge vacuum-assisted biopsies. Photoacoustic (PA) images were acquired through ex vivo experiments by transmitting laser pulses with two different wavelengths (700 nm and 800 nm). The presence of microcalcifications in PA images were blindly assessed by two radiologists and compared with specimen mammography. A ratio of the signal amplitude occurring at 700 nm to that occurring at 800 nm was calculated for each PA focus and was called the PAI ratio. RESULTS: Based on the change of PA signal amplitude between 700 nm and 800 nm, 10 out of 11 specimens containing microcalcifications and 8 out of 10 specimens without calcifications were correctly identified on blind review; the sensitivity, specificity, accuracy, positive predictive and negative predictive values of our blind review were 90.91%, 80.0%, 85.71%, 83.33% and 88.89%. The PAI ratio in the microcalcification group was significantly higher than that in the control group (the median PAI ratio, 2.46 versus 1.11, respectively, P =  .001). On subgroup analysis in the microcalcification group, neither malignant diagnosis nor the number or size of calcification-foci was proven to contribute to PAI ratios. CONCLUSION: Breast microcalcifications generated distinguishable PA signals unlike breast tissue without calcifications. So, PAI, a non-ionizing and non-invasive hybrid imaging technique, can be an alternative in overcoming the limitations of conventional US imaging.ope

Analysis of tumor markers in cytological fluid obtained from computed tomography–guided needle aspiration biopsies for the diagnosis of ground-glass opacity pulmonary lesions

BACKGROUND: The purpose of this study was to assess whether analyses of tumor markers in cytological fluid can improve the performance of computed tomography (CT)-guided needle aspiration biopsy (NAB) for the diagnosis of ground-glass opacity (GGO) pulmonary lesions. METHODS: Forty-two patients were prospectively enrolled for CT-guided NAB. Levels of cytokeratin 19 fragments (CYFRA 21-1) and carcinoembryonic antigen (CEA) from serum and cytological fluid were measured. The cutoff values of 3.3 ng/mL for CYFRA 21-1 and 5.0 ng/mL for CEA (threshold A) or thresholds by adding 2 standard deviations to the mean levels of markers found in patients without malignancy (threshold B) were used to identify malignancy. The sensitivity and area under the curve (AUC) of NAB alone were compared with those of NAB combined with serum or cytological tumor markers. RESULTS: Among the 42 patients, 30 (71.4%) had malignant and 12 (28.6%) had benign lesions. For NAB alone, the sensitivity, specificity, and AUC for diagnosing GGO were 70.0%, 100%, and 0.850, respectively. The sensitivity and AUC increased significantly for NAB with cytological CYFRA 21-1 compared with NAB alone, using both thresholds (threshold A: 86.7%, P=.026 and .933, P=.016; threshold B: 93.3%, P=.008 and .925, P=.046). CONCLUSIONS: Cytological fluid measurements of CYFRA 21-1 can improve the diagnostic performance of CT-guided NAB for GGO pulmonary lesions.ope

비원어민 화자의 억양이 한국인 고등학생들의 영어듣기에 미치는 영향

학위논문(석사) --서울대학교 대학원 :외국어교육과(영어전공),2008.2Maste

심근염 및 교모세포종 모델에서 PET과 형광영상을 이용한 전이체 단백질 18 kDa 발현의 평가

학위논문 (석사)-- 서울대학교 대학원 : 융합과학기술대학원 융합과학부, 2018. 2. 김상은.Translocator protein 18 kDa (TSPO), which is located mainly in the outer membrane of the mitochondria, is considered as a potential biomarker of inflammation or specific cancers because of its overexpression in such lesions. This protein is closely associated with inflammatory responses in various diseases, including myocarditis or glioblastoma multiforme (GBM). Here, we evaluated TSPO overexpression using TSPO radiotracers, [18F]fluoromethyl-PBR28 [18F]1) and [18F]CB251 ([18F]2), and TSPO-targeted iron oxide nanoparticles in different rodent disease models to precisely diagnose each lesion. Both the radiotracers, [18F]1 and [18F]2, were successfully prepared in an automated module and compared between in an experimental autoimmune myocarditis (EAM) model and a healthy control by positron emission tomography (PET) imaging to determine which is more suitable for in vivo TSPO assessment. [18F]2 showed a more specific TSPO uptake in the heart of EAM rats (1.32-fold higher heart-to-lung uptake ratio) than in that of healthy rats, whereas [18F]1 showed similar heart uptake patterns between the two groups. Histopathological analysis of the heart tissues from each group demonstrated abnormal TSPO expression in inflammatory myocardial tissues compared with control tissues. Nanoparticles were prepared with TSPO ligands (CB235) and Cy5.5 dyes which were introduced on the surface of the nanoparticles, and were evaluated in a subcutaneously implanted glioblastoma xenograft mouse model by fluorescence imaging. The fluorescence signal from tumors was strongest at 8 h, and 71.3% of the uptake at 8 h was maintained after 24 h despite rapid clearance from other organs (highest tumor-to-background ratio at 24 h). These results demonstrated that the imidazole[1,2-a]pyridine-based radiotracer [18F]2 is a sensitive tool for the noninvasive diagnosis of myocarditisthus, it may be applied to clinical settings for the early diagnosis of human myocarditis. Moreover, the tumors were visualized using TSPO-targeted fluorescent nanoparticles. The findings suggest that the specificity and selectivity of TSPO-targeted nanoparticles may facilitate tumor diagnosis, serving as a guide for surgical treatment.I. INTRODUCTION 1 1. Translocator protein 18 kDa (TSPO) 1 2. PET and radiotracers for diagnosing myocarditis 2 3. Fluorescent visualization of glioblastoma 4 4. Research objectives 6 II. MATERIALS AND METHODS 7 1. Assessment of TSPO in myocarditis by PET imaging 7 1-1. Synthesis of radiotracers 7 1-2. Metabolite testing 9 1-3. Animal models 10 1-4. microPET imaging 11 1-5. PET image analysis 11 1-6. Western blot analysis 11 1-7. Immunohistochemistry 12 1-8. Statistical analysis 13 2. Fluorescence imaging analysis of tumor models 14 2-1. Fluorescence-labeled and TSPO-targeted nanoparticles 14 2-2. Cell lines 14 2-3. Cytotoxicity testing 15 2-4. In vitro cellular uptake analysis 16 2-5. Animal models 16 2-6. In vivo fluorescence imaging 17 2-7. Fluorescent image analysis 18 2-8. Statistical analysis 18 III. RESULTS AND DISCUSSION 19 1. Diagnosis of myocarditis by TSPO PET imaging 19 1-1. Radiotracers 19 1-2. Metabolite analysis of the heart and plasma 20 1-3. PET imaging of the EAM model and control 20 1-4. Comparison of TSPO expression in inflamed and healthy heart tissues 21 2. Specific visualization of tumors with TSPO-targeted nanoparticles 23 2-1. In vitro toxicity testing 23 2-2. In vitro cellular uptake analysis 23 2-3. Fluorescence imaging 24 IV. CONCLUSION 36 V. REFERENCES 38 국문초록 42Maste