GaN/AlN Multiple Quantum Wells and Nitride-Based Waveguide Structures for Ultrafast All-Optical Switch Utilizing Intersubband Transition

Intersubband transition （ISBT） in multiple quantum wells （MQW） has drawn much attention for ultrafast optoelectronic devices owing to its wide wavelength-tunability and extremely fast energy relaxation process. Recently, the extension of ISBT wavelength to near-infrared wavelength, especially 1.55 μm, is of particular interest because such wavelength is vital for the development of ultrafast photonic devices for silica-fiber-based optical-communication networks. Among various materials proposed for intersubband transition at 1.55 μm, GaN/AlN MQW structures are promising due to their large conduction band offset of approximately 2 eV. Furthermore, the large electron effective mass and the large LO phonon energy in nitrides make their intersubband relaxation extremely fast in the order of sub-picoseconds. This makes intersubband transition in nitrides immensely interesting for the development of ultrafast photonic devices operating at a bit rate higher than 1 Tb/s.//The intersubband transition at 1.55 μm and shorter wavelengths have been achieved by molecular beam epitaxy （MBE） with the shortest wavelength of 1.08 μm. On the other hand, growth by metalorganic vapor phase epitaxy （MOVPE） has not yielded satisfactory results as the shortest ISBT wavelength reported is merely 2.4 μm. The demonstration of 1.55-μm ISBT by MOVPE, however, is still attractive since much better crystalline quality for device fabrication can be achieved. Moreover, MOVPE also has another advantage over MBE in industrial point of view. Indeed, the ultrafast optical switching utilizing intersubband transition has been demonstrated by MBE-grown GaN ridge waveguide structure with a bit rate higher than 1 Tb/s. However, such device requires optical-pulse switching energy higher than 10pJ/μm2 to utilize the saturable intersubband absorption, which is still too large for the applications in conventional optical communication networks. Reduction of the switching energy is therefore another important issue for the intersubband transition devices. In order to reduce the switching energy, not only the waveguide fabrication process, but also the epitaxial growth technique and the device structure have to be improved.//In this dissertation, the GaN/AlN multiple quantum wells and nitride-based waveguide structures are studied and fabricated for the applications of ultrafast all-optical switch utilizing intersubband transition. The ultrafast intersubband transition device is realized by using AlN waveguide structure with GaN/AlN quantum wells. This AlN-waveguide-based intersubband transition device can operate in the optical communication wavelength range, covering 1.3 μm, the shortest wavelength ever demonstrated for the intersubband transition devices.//In order to perform epitaxial growth of such structure with high quality, MOVPE is more preferable than MBE because the AlN layer can be grown with much better quality by the MOVPE. However, since the MOVPE growth of GaN/AlN MQW for the 1.55-μm ISBT is very difficult, the AlN waveguide structure was fabricated with a combination of both MOVPE and MBE growth techniques: MOVPE growth for AlN buffer layer and MBE re-growth for GaN/AlN multiple quantum wells. With this combination, the high quality waveguide with intersubband absorption in a wavelength range of 1.3-1.55 μm is achieved.//In addition to the improvement in the epitaxial growth technique, this dissertation also discusses on the problems in growing the waveguide structure of both MOVPE and MBE. Moreover, the design and fabrication of nitride-based waveguide structures are studied in details to improve the waveguide quality. The high-optical-confinement waveguide structures are proposed and successfully fabricated for the first time thanks to the successful demonstration of epitaxial growth and the improvement of fabrication process. Additionally, a new waveguide characterization method using the supercontinuum light source is also proposed and demonstrated. With this new characterization method, not only are the direct measurements of intersubband absorption in waveguides realized, but the problems in waveguide quality of the MBE-grown waveguide are also revealed. This provides very useful information for the improvement of fabrication process, especially the epitaxial growth process. The achievements in each area of epitaxial growth, waveguide fabrication process, and characterization, have made contributions to the improvement of waveguide characteristic, leading to the successful demonstration of the first AlN-waveguide-based intersubband transition devices with high performance.報告番号: 甲21171 ; 学位授与年月日: 2006-03-23 ; 学位の種別: 課程博士 ; 学位の種類: 博士(工学) ; 学位記番号: 博工第6261号 ; 研究科・専攻: 工学系研究科電子工学専

Financial fragility under implicit insurance scheme: Evidence from the collapse of Thai financial institutions

Using the Thai experience as a clinical study of a financial crisis, we investigate financial failures of Thai financial institutions. This study augments the CAMEL perspective by considering corporate governance and the moral hazard problems under the state of implicit government guarantee. The overall results suggest that high-replicated CAMEL ratings and downgrades of the ratings based on accounting-based information are likely to be important indicators of bank fragility. The ownership-based incentives of the largest shareholders and the level of risk associated with moral hazard problems are also factors that help discriminate sound and unsound financial institutions.CAMEL rating, incentives, deposit insurance, financial failure, Thai financial institutions

Ownership-based Incentives, Internal Corporate Risk and Firm Performance

This study focuses on the incentives and risk-taking behavior of large shareholders in Thailand before and after the 1997 financial crisis. The results show that there is a negative association between risk and firm performance. However, the effect ore cash flow re members of top management. Furthermore, there is weak evidence that a move to more transparent direct control structure benefits the firms. Overall, the results indicate that ownership-based incentives are an effective means of aligning the interests between controlling shareholders and minority shareholders particularly in the post-crisis period.Cash flow rights, Incentives, Financial and business risk, Thailand

Deposit Insurance, Corporate Governance and Discretionary Behavior: Evidence from Thai Financial Institutions

Claiming that the implicit cost of deposit insurance is an alternative proxy for risk-taking behavior, we examine the effects of incentive-inducing ownership and entrenchment of the largest shareholders and discretionary behavior of the management on the risk of Thai financial institutions. Our empirical results suggest that, during 1994-1996, the largest shareholders engage in low risk-taking activities when they hold large cash flow rights and have low deviation of cash flow from control rights. However, the risk is higher when the largest family shareholder enters the board and when Chairman-CEO can manipulate loan loss provisions. After the financial crisis, earnings management through discretion on loan loss provisions reduces risk. Overall, this study suggests that the problems underlying the implicit guarantee scheme are different between banks and finance companies, and between types of governance structure.Implicit guarantee, Risk, Ownership Structure, Managerial Discretion, Thai Financial Institutions

การสอนแบบระดมความคิดในวิชาคอมพิวเตอร์กราฟิก เพื่อส่งเสริมความคิดเชิงสร้างสรรค์

The purposes of this study were to 1) study the learning achievement of students in a Computer Graphics course learned by Brainstorming, 2) study the creative thinking of students in a Computer Graphics course learned by Brainstorming, and 3) survey the students’ satisfaction towards the Brainstorming-Based Approach. The sample group was 17 students from IT program, in Faculty of Sciences and Technology, Muban Chombueng Rajabhat University, who enrolled in Computer Graphics course. The tools consisted of (1) a lesson plan of Brainstorming-Based Approach in the Computer Graphics course, (2) creative thinking test, (3) an evaluation form on creative thinking ability, and (4) a satisfaction survey towards the Brainstorming-Based Approach. The data was analysis by mean, standard deviation, and t-test. The findings revealed that 1) the comparison of the learning achievement scores showed the difference at a significance level of .01, 2) the result of creative thinking assessment of students in a Computer Graphics course learned by Brainstorming founds that an average result of pretest is 4.32 score which signifies as 43.24 percent and an average result of posttest is 7.24 score which signifies as 72.35 percent, and 3) the overall satisfaction of the students towards the Brainstorm- Based Approach was at a high level (mean= 4.28)

Serum noncholesterol sterols in Alzheimer's disease : the Helsinki Businessmen Study

Cerebral cholesterol metabolism is perturbed in late-onset Alzheimer's disease (AD), but whether also the extracerebral cholesterol metabolism is perturbed is not known. Thus, we studied whole-body cholesterol synthesis and absorption with serum noncholesterol sterols in men without AD (n = 114) or with (n = 18) "pure" AD (no concomitant atherosclerotic cardiovascular disease) in a long-term cohort (the Helsinki Businessmen Study) of home-dwelling older men without lipid-lowering drugs and on their habitual home diet. Serum lipids did not differ between AD and controls, but age was higher (78 +/- 1 vs 74 +/- 0.3 years, mean f standard error, P <0.001), age-adjusted plasma glucose concentration was lower (4.8 +/- 0.3 vs 5.7 +/- 0.1 mmol/L, P= 0.011), and APOE epsilon 4 allele and frailty were more frequent in AD than in controls. Of the age and frailty-adjusted serum noncholesterol sterols desmosterol and lathosterol ratios to cholesterol reflecting cholesterol synthesis were lower in AD than in controls (eg, lathosterol 114 +/- 12 vs 137 +/- 5 10(2) mu mol/mmol cholesterol, P= 0.004). Cholestanol ratio to cholesterol was higher in AD than in controls suggesting increased cholesterol absorption. lathosterol/sitosteroll ratio reflecting cholesterol metabolism was lower in AD than in controls (0.95 +/- 0.28 vs 1.52 +/- 0.11 10(2) mu mol/mmol cholesterol, P = 0.027). In AD, plasma glucose correlated negatively with cholesterol synthesis, whereas in controls the correlation was positive. In conclusion, extracerebral cholesterol metabolism was altered in AD. This finding along with the low plasma glucose concentration and its paradoxical interaction with cholesterol synthesis opens new perspectives in the regulation of cholesterol metabolism and glucose homeostasis in AD.Peer reviewe

Lipidomic analysis of patients with microbial invasion of the amniotic cavity reveals upâ regulation of leukotriene B4

Bioactive lipids derived from the metabolism of polyunsaturated fatty acids are important mediators of the inflammatory response. Labor per se is considered a sterile inflammatory process. Intraâ amniotic inflammation (IAI) due to microorganisms (i.e., intraâ amniotic infection) or danger signals (i.e., sterile IAI) has been implicated in the pathogenesis of preterm labor and clinical chorioamnionitis at term. Early and accurate diagnosis of microbial invasion of the amniotic cavity (MIAC) requires analysis of amniotic fluid (AF). It is possible that IAI caused by microorganisms is associated with a stereotypic lipidomic profile, and that analysis of AF may help in the identification of patients with this condition. To test this hypothesis, we analyzed the fatty acyl lipidome of AF by liquid chromatographyâ mass spectrometry from patients in spontaneous labor at term and preterm gestations. We report that the AF concentrations of proinflammatory lipid mediators of the 5â lipoxygenase pathway are significantly higher in MIAC than in cases of sterile IAI. These results suggest that the concentrations of 5â lipoxygenase metabolites of arachidonic acid, 5â hydroxyeicosatetraenoic acid, and leukotriene B4 in particular could serve as potential biomarkers of MIAC. This finding could have important implications for the rapid identification of patients who may benefit from antimicrobial treatment.â Maddipati, K. R., Romero, R., Chaiworapongsa, T., Chaemsaithong, P., Zhou, S.â L., Xu, Z., Tarca, A. L., Kusanovic, J. P., Gomez, R., Chaiyasit, N., Honn, K. V. Lipidomic analysis of patients with microbial invasion of the amniotic cavity reveals upâ regulation of leukotriene B4. FASEBJ. 30, 3296â 3307 (2016). www.fasebj.orgPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154488/1/fsb2fasebj30100583.pd