Si-Based Germanium Tin Photodetectors for Infrared Imaging and High-Speed Detection

Infrared (IR) radiation spans the wavelengths of the windows: (1) near-IR region ranging from 0.8 to 1.0 μm, (2) shortwave IR (SWIR) ranging from 1.0 to 3.0 μm, (3) mid-wave IR (MWIR) region covering from 3.0 to 5.0 μm, (4) longwave IR (LWIR) spanning from 8.0 to 12.0 μm, and (5) very longwave IR extending beyond 12.0 μm. The MWIR and LWIR regions are important for night vision in the military, and since the atmosphere does not absorb at these wavelengths, they are also used for free-space communications and astronomy. Automotive and defect detection in the food industry and electronic circuits also use IR detection as non-contact inspection methods. IR detection is also applied in the medical field. The market of SWIR and MWIR detectors is primarily dominated by mature technology from III-V systems such as indium gallium arsenide (InGaAs and extended InGaAs), indium antimonide (InSb), from II-VI such as mercury cadmium telluride (MCT), lead sulfide (PbS), and from group IV such as silicon (Si) and germanium (Ge) for shorter wavelength. However, the mature IR photodetector technology is expensive, demands to operate at low temperatures, and has complicated fabrication processes. In order to lower cost by mass production, many approaches have been developed towards the hybrid integration of III-Vs or II-VIs on a Si substrate. At the same time, it is desirable to develop an alternative material to reduce the cost and improve the performance for high-temperature operations. The discovery of group IV (Si)GeSn alloys has opened a route for a new generation of IR detectors. The work in this dissertation set out to develop Si-based Ge1-xSnx photodetectors for low-cost infrared imaging and high-speed detection. A study of effective carrier lifetime and optical properties of Ge1-xSnx materials is presented. The carrier lifetime is then applied to model the Ge1-xSnx photodetectors. For optical properties of Ge1-xSnx materials, two empirical formulae with extracted constants and coefficients were developed: (1) Absorption coefficient. The absorption regarding Urbach tail, indirect and direct bandgap transitions were comprehensively considered; (2) refractive index. The developed formulae could simplify the optoelectronic device design process due to their parameter-based expressions. A comprehensive study of Si-based GeSn mid-infrared photodetectors is carried out. A set of photoconductors with Sn compositions ranging from 10.5% to 22.3% show the cutoff wavelength to be extended to 3.65 μm. The devices’ peak D* is comparable to that of commercial extended-InGaAs detectors. The GeSn photodiodes are also explored with an in-depth analysis of a dark current. The dark current is suppressed as the photodiode was passivated. Moreover, mid-infrared images were captured using GeSn photodetectors, showing the comparable image quality with that acquired by using commercial PbSe detectors. The performance of GeSn photodiodes with 6.44 % and 9.24 % Sn is evaluated under high-speed measurements and simulations. The cutoff wavelength is extended up to 2.2 μm and 2.5 μm for 6.44 % and 9.24 % Sn devices, respectively. The photodiodes’ bandwidth is 1.78 GHz, and the simulation shows excellent agreement with measurement results

Resonant Spectra of Metal Nanotoroids of Various Sizes

Nowadays, the manipulation of light by using metallic nanostructures has wide applications in photonics, optoelectronics and energy conversion. Along with other universities all over the world, the University of Arkansas is researching on nano-antennas’ design, fabrication and applications. Current research in Dr. El-Shenawee’s Terahertz Imaging and Spectroscopy Computational Electromagnetics Group, has computationally investigated the behaviors of plasmonic nanostructures by using the commercial finite element electromagnetic solver Ansys® HFSS. This work reproduced the previous work of spectral absorption enhancement of infinite and finite arrays of silver and gold nanotoroids with sizes of the inner radii: 13nm – 21nm, while outer radius of 42nm and more on an amorphous silicon absorbing layer. There are three significant factors in modeling this configuration such as surface resolution, optical properties of materials, and boundary conditions. A convergence study was performed on a gold sphere dimer with 40 nm radii and 1 nm gap between spheres. This illustrated that an at least surface resolution of 0.02 nm was needed to provide converging results in an acceptable computational time for conducted simulations. Furthermore, the Lorentz- Drude models for silver and gold were studied to obtain the optical properties. In addition, in order to reduce computation time and memory consumption by reduction of computational domain, the appropriate symmetry boundary conditions were applied. In this work, three samples of infinite arrays of gold nanotoroids with the sizes of inner radii: 50nm, 60nm and 100nm, respectively, while outer radius of 150nm were simulated as well. These gold nanotoroids were fabricated on glass substrate and then optically characterized by ellipsometry’s transmission measurement. The optical characterization was performed in Dr. Shui-Qing Yu’s Applied Nano and Bio Photonics Group. The observed differences between compute simulations and experimental results in shifting resonance frequencies were analyzed. This thesis is organized as: Part I is discussing about the finite element method, boundary conditions and Lorentz-Drude Model. Part II involves Lorentz-Drude model for gold and silver, convergence study using HFSS, simulations of infinite silver and gold nanotoroid arrays and ellipsometry transmission measurement on gold nanotoroid arrays. Part III is about conclusions and future research. The Appendix A is providing Matlab codes of Lorentz-Drude model for gold and silver

Fostering EFL students’ communicative language competence : Facebook as a platform for a triad of types of talk

Communicative Language Teaching (CLT) is believed to provide students with skills to communicate outside the classroom. It is tailored to engage students in meaningful language use through authentic tasks in real-world contexts focused on information exchange, critical thinking and problem solving. Meanwhile, Facebook also has the potential to enhance students’ communication and collaboration by engaging them in real-world contexts. Apart from creating a sense of community and engagement, Facebook enables more interactive communicative language learning activities. This study used the theoretical framework of CLT to examine how a triad of types of talk enhanced upper secondary students’ communicative competence on Facebook in Vietnam. This study utilised a three-phase convergent mixed method approach, including the collection of both quantitative and qualitative data. Three main ways in which a triad of types of talk fostered students’ communicative competence on Facebook were identified: (a) students became more active and were capable of applying their own previous knowledge; (b) students developed their communicative competence embedded in simulated real-world situations on Facebook; and (c) students cultivated their self-directed learning strategies to communicate with others in the real-world contexts enabled by Facebook. The findings also revealed three main ways that Facebook hindered the development of communicative competence: (a) students’ inexperience in Facebook learning; (b) Vietnamese cultural issues related to English teaching and learning; (c) unavailability of adequate time. The study also raised some implications for language learning in Vietnam and highlighted the important roles of Facebook as a complementary learning platform and a triad of types of talk in strengthening students’ communicative competence. Finally, this study encouraged ongoing research on the impact of a triad of types of talk on communicative competence on Facebook in accordance with the principles of CLT with different populations and subject areas

Pope Saint John Paul II on social justice in human work

In contemporary societies, the issues of industrialization and successive globalization have been affecting deeply the reality and true meaning of human work. The objective dimension of work became more important than the subjective. In contrast, Pope Saint John Paul II affirms that man is “the subject of work” and not merely an object. The human person “performs various actions belonging to the work process; independently of their objective content, these actions must all serve to realize his humanity” (LE. 6), which refers to the eminent subjective dimension of work rooted in the dignity of the worker as person. A lot of research and many interpretations of Pope Saint John Paul II’s innovative teaching in this regard have been proposed and discussed. This study presents how he responded to the signs of his time, by deepening and further developing theology of work that is fundamentally grounded in Scripture, Tradition, and the Social Teaching of the Catholic Church. He demonstrates in depth and extent in his teaching the relevance of social justice as a moral principle by fully respecting human dignity in work, for human beings as subject of work are fundamentally understood as being created in the image and likeness of God

Identification of Medical Interpreter Services in Atlanta Metropolitan Area

Background: Over 73,000 refugees entered the U.S. in 2010. The majority of the refugee population has limited English proficiency (LEP), which is a challenge for those seeking healthcare. People with LEP are often deterred by language barriers from seeking healthcare. Interpreter services are available in some healthcare settings to help improve the communication between physicians and these patients. The availability of interpreters at primary care offices could help encourage the utilization of health services among patients with LEP. Objectives: To identify primary care offices that provide medical interpreter services in areas around the city of Clarkston, GA. To produce a reference sheet that identifies primary care doctors and the languages available for interpretation at those offices. Methods: Data on the availability of medical interpreter services was collected through the phone. Google Map was used to identify public transit routes (bus/train) and time to travel from the center of Clarkston to the medical offices. Results: Less than 50% of family practice, internal medicine, and pediatric offices provide medical interpreter services. It was found that the most common languages available for interpretation are Vietnamese, Spanish, and Mandarin. Interestingly, the current refugee populations arrive from various regions in Africa and Asia. The common languages among these groups are Bhutanese, Burmese dialects, and Farsi. The offices that provide interpreter services with languages relevant to the current refugee population are located more than 30 minutes of travel time by public transit from the center of Clarkston