Growth of Zn-polar BeMgZnO/ZnO heterostructure with two dimensional electron gas (2DEG) and fabrication of silver Schottky diode on BeMgZnO/ZnO heterostructure.

Title of dissertation: GROWTH OF Zn POLAR BeMgZnO/ZnO HETEROSTRUCTURE WITH TWO DIMENSIONAL ELECTRON GAS (2DEG) AND FABRICATION OF SILVER SCHOTTKY DIODE ON BeMgZnO/ZnO HETEROSTRUCTURE By Md Barkat Ullah, Ph.D A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical and Computer Engineering at Virginia Commonwealth University. Virginia Commonwealth University,2017 Major Director: Dr. Hadis Morkoç, Professor, Electrical and Computer Engineering This thesis focuses on growth of Zn polar BeMgZnO/ZnO heterostructure on GaN/sapphire template with two dimensional electron gas (2DEG) for the application of UV photodetector/emitter and high speed electronics. The motivation of using BeMgZnO as a barrier layer originates from the need to reach plasmon-LO phonon resonance in order to obtain minimum longitudinal optical (LO) phonon lifetime. Presence of 2DEG was realized in BeMgZnO/ZnO heterostructure only when the Zn polarity was achieved during the nucleation growth of ZnO on GaN/sapphire template. It was found that, polarity of ZnO on (0001) GaN/sapphire template can be controlled by the oxygen to Zn ratio used during the nucleation growth. To obtain high structural and optical quality of BeMgZnO quaternary alloy, growth kinetics of BeMgZnO layer has been studied at the temperature range from 450°C-500°C. We have achieved the growth of single crystal Be.03Mg00.15ZnO alloy at 500 °C, more than 100°C higher compared to what reported in literature, on the (0001) GaN/sapphire template through the control of Zn/(Be+Mg) flux ratio. We have also observed a thermodynamic limitation of Mg incorporation into the wurtzite BeMgZnO alloy where the excess Mg adatom accumulated in the growing surface as a MgO rich cluster. Two dimensional electron gas with high (1.2×1013cm-2) sheet carrier density was achieved at the Be0.03Mg0.41ZnO/ZnO interface through strain engineering by incorporating Be into MgZnO ternary alloy. To obtain the similar sheet carrier density it would require above 60% of Mg in MgZnO/ZnO heterostructure with reduced structural quality. A systematic comparison of sheet carrier density has been made with the already reported results from Zn polar MgZnO/ZnO heterostructure as well as with the theoretical calculation. Silver Schottky diode on Be0.02Mg0.26ZnO/ZnO heterostructure with barrier height 1.07 eV and ideality factor 1.22 was obtained with 8 order of rectification ratio. The temperature-dependent electrical characteristics were studied by using temperature dependent current-voltage (I-V) measurements. Richardson constant value of 34.8 Acm-2K-2 was found experimentally which was close to the theoretical value of 36 Acm-2K-2 known for Be0.02Mg0.26ZnO alloy

Firm Growth around the World: Financial Constraints, Corruption, and Privatization

This Dissertation consists of three separate essays that are broadly related to firm-level financial constraints, corruption, and privatization and their relation to firm performance and growth across the globe. I employ World Bank Enterprise Survey data collected from 2006-2011 for over one hundred countries. In my first Dissertation essay, I study the effects of accounting information disclosure through auditing on firm level financial constraints and corruption obstacles. I find that audited firms exhibit significantly lower level of financial constraints than unaudited firms. Audited firms also encounter higher level of corruption obstacles than unaudited firms. The second essay examines three firm-level constraints to firm growth (financing, corruption and legal) of privatized firms in comparison to those of the originally private (de novo) firms. I find that the de novo firms experience significantly higher financial, corruption, and legal obstacles than the privatized firms. I further find that the privatized firms significantly underperform the de novo firms. In addition, I find ownership concentration and foreign ownership have an overall moderating effect on firm-level growth constraints. My final essay examines the effect of International Organization for Standardization (ISO) certification on firm performance measured by sales growth and labor productivity. I find that ISO-certified firms exhibit higher sales growth and higher labor productivity than non-certified firms. I also find that the positive link between ISO certification and labor productivity remains highly significant regardless the corruption levels. However, when firms operate in a highly corrupt business environment, the positive effect of ISO certification on sales growth becomes insignificant

Numerical investigation of the Hypersonic Test Facility Delft (HTFD)

The high speed flows with mach number above 5 are generally categorised as hypersonic flows. In order to generate hypersonic flows in wind tunnel for aerothermodynamic testing, a shock tube type setting is generally known as one of the most commonly used methods. Similarly Hypersonic test facility at TUDelft (HTFD) is a Ludwieg tube type wind tunnel which is made of a storage tube connected to the test section through a converging diverging nozzle which expands the high pressure and temperature gas in storage tube to achieve high speed flows in test section. The tunnel is started by a fast opening piston-valve which creates a strong shock progressing in to the nozzle while an expansion wave progresses in to the storage tube. The appeal of this kind of facility to the hypersonic flow regime lies in the relatively long run times (0.1 to 0.2 seconds), large test section (30 cm) and high Reynolds numbers (order of 5 – 50e+06 m-1).But there are also some technological limitations to such facilities like supply of high temperatures and pressures for times long enough to perform a measurement, fast instrumentation and power requirements to run the tunnel, just to name a few. Through various experiments it has been experienced that during HTFD operations, the total temperature does not remain constant and the drop in total temperature has been experienced during expansion of flow in converging-diverging nozzle. Apparently the external walls and piston valve assembly of HTFD do not have adiabatic walls and there is always finite heat transfer taking place through these walls due to temperature difference between the working gas and the tunnel walls. So, in this thesis the heat transfer through HTFD walls is numerically computed during unsteady starting of tunnel. Secondly the unsteady flow topology through the nozzle with strong discontinuities immediately after the opening of piston-valve and before the exit of shock from the nozzle has been numerically studied in detail. In addition the influence of various boundary conditions on the time needed for the flow to be steady at nozzle exit has been computed and steady state flow properties at the exit, obtained with different boundary conditions have been compared. For this task FineHEXA(Development version 2.5-4) is used for computations and Hexpress /Gambit for modelling and mesh generation. The 2D axissymmetric mesh is used for simulations considering the circular cross section of wind tunnel. Jameson’s second order central scheme is used for spatial discretization and second order backward differencing is used for time integration. The dual time stepping is used for iteratively converging the solution at each time level. The starting of wind tunnel by opening of piston-valve is modelled using deforming mesh generated using ALE(Arbitraty Lgrangian Eulerian) approach and RBF(Radial basis functions). The brief outline of the thesis is can be given as follows. First of all the Riemann’s initial value problem(shock tube problem) is simulated to compare the numerical solutions as obtained by all available numerical schemes, mutually and with analytical solution to analyse the performance characteristics and to choose an appropriate numerical scheme for the purpose. In the next step the flow generated by removal of diaphragm in shock tube problem, is simulated through a converging-diverging nozzle instead of a constant area tube in order to investigate the difference in flow topology. Then the flow is simulated in a computational domain resembling the original HTFD geometry with completely open piston-valve using different boundary conditionsAerospace Engineerin

Faking smart industry: A honeypot-driven approach for exploring cyber security threat landscape

The digital evolution of Industry 4.0 enabled Operational Technology (OT) infrastructures to operate and remotely maintain cyber-physical systems bridging over IT infrastructures. It has also expanded new attack surfaces and steadily increased the number of malicious cyber incidents for the interconnected smart critical systems. Within Industrial Control System (ICS), Programmable Logic Controller (PLC) plays a crucial function to bridge between cyber and physical environments which made them the victim of sophisticated cyber-attacks that are designed to interrupt and damage their operations. Honeypots have been used as a key tool for aggregating real threat data e.g., malicious activities and payloads, to observe and determine different attack methods and strategies that can easily affect poorly secured cyber-physical systems. In this research, we deployed T-pot honeypot in Amazon Elastic Compute Cloud (AWS EC2) instance across six different regions to determine the current threat landscape as well as how knowledgeable and ingenious threat actors could be in compromising internet-facing Industrial Control System (ICS)

Polarity Control within One Monolayer at ZnO/GaN Heterointerface: (0001) Plane Inversion Domain Boundary

International audienceIn semiconductor heterojunction, polarity critically governs the physical properties, with an impact on electronic or optoelectronic devices through the presence of pyroelectric and piezoelectric fields at the active heteropolar interface. In the present work, the abrupt O-polar ZnO/Ga-polar GaN heterointerface was successfully achieved by using high O/Zn ratio flux during the ZnO nucleation growth. Atomic-resolution high-angle annular dark-field and bright-field transmission electron microscopy observation revealed that this polarity inversion confines within one monolayer by forming the (0001) plane inversion domain boundary (IDB) at the ZnO/GaN heterointerface. Through theoretical calculation and topology analysis, the geometry of this IDB was determined to possess an octahedral Ga atomic layer in the interface, with one O/N layer symmetrically bonded at the tetrahedral site. The computed electronic structure of all considered IDBs revealed a metallic character at the heterointerface. More interestingly, the presence of two-dimensional (2D) hole gas (2DHG) or 2D electron gas (2DEG) is uncovered by investigating the chemical bonding and charge transfer at the heterointerface. This work not only clarifies the polarity control and interfacial configuration of the O-polar ZnO/Ga-polar GaN heterojunction but, more importantly, also gives insight into their further application on heterojunction field-effect transistors as well as hybrid ZnO/GaN optoelectronic devices. Moreover, such polarity control at the monolayer scale might have practical implications for heterojunction devices based on other polar semiconductors

Factors associated with diarrhea and acute respiratory infection in children under two years of age in rural Bangladesh.

BackgroundDiarrhea and acute respiratory infection (ARI) are major causes of child mortality. We aimed to identify risk factors associated with diarrhea and ARI among children under 2 years of age in rural northern Bangladesh.MethodWe collected information on diarrhea and ARI in the previous 14 days and the previous 6 months at 6, 12, 18 and 24 months of age as part of a longitudinal, cluster randomized effectiveness trial, the Rang-Din Nutrition Study which enrolled 4011 pregnant women at ≤20 gestational weeks. Women and their children were followed up until 2 years postpartum. Information on household socioeconomic status, type of toilet, garbage disposal system, food insecurity, number of under-five children in the household, type of family, maternal characteristics and child characteristics was collected at baseline and/or at 6, 12, 18 and 24 months postpartum. Data on newborn health and feeding behaviors were collected within 72 h of delivery. Associations between potential risk factors and morbidity prevalence outcomes were assessed using logistic regression controlling for potential confounders.ResultsOut of 3664 live born children, we collected information from ~ 3350 children at 6, 12, 18 and 24 months of age. Diarrhea in the previous 14 days, and in the previous 6 months, was associated with maternal depression score and food insecurity; diarrhea in the previous 6 months was also associated with family type (nuclear vs. joint). ARI in the previous 14 days was associated with maternal depression score, type of toilet and garbage disposal, household food insecurity and sex. Cough or nasal discharge in the past 6 months was associated with maternal depression score, type of toilet and garbage disposal, household food insecurity, sex and perceived overall physical condition of the infant after birth.ConclusionMaternal depression and food insecurity appear to be important risk factors for diarrhea and respiratory infection among children under 2 years of age in this setting. These findings suggest that policies and programs that include strategies to address maternal mental health and household food insecurity may contribute to improved child health.Trial registrationThe trial was registered with the US National Institutes of Health at ClinicalTrials.gov, # NCT01715038 , with registration completed October 26, 2012