Reduction of Thermal Resistance and Optical Power Loss Using Thin-Film Light-Emitting Diode (LED) Structure

In this paper, a GaN-LED with sapphire structure and a thin-film LED without sapphire structure are characterized in the photo-electro-thermal (PET) modeling framework for comparison. Starting from the analysis and modeling of internal quantum efficiency as a function of current and temperature of blue LED, this work develops the thin-film LED device model and derives its optical power and the heat dissipation coefficient. The device parameters of the two LED devices with different structural designs are then compared. Practical optical power measurements are compared with theoretical predictions based on the two types of fabricated devices. It is shown that the thin-film LED device has much lower thermal resistance and optical power loss.published_or_final_versio

Theoretical and experimental investigation of quantum well intermediate band solar cells

In order for photovoltaic energy conversion to compete with conventional energy sources and become a realistic alternative source of low-carbon renewable energy, significant cost-per-watt reduction is required. One obvious way to achieve this is to increase the conversion efficiency of solar cells, which is currently limited to around 30% even with modern technology. The Intermediate Band Solar Cell (IBSC), the focus of this project, is a concept that promises photovoltaic power conversion efficiencies of up to 63.2% through the introduction of an extra energy band in the bandgap of a semiconductor. However, many attempts to achieve IBSCs using quantum dots superlattice show poor conversion efficiency due to their small absorption cross-section and short-lived intermediate state. In this project, we attempt to overcome this issue by proposing an innovative Photon Ratchet (PR) quantum well cascade structure designed to improve the efficiency, through increasing the absorption cross-section and the lifetime of electrons in the intermediate state. The goal of this project is to prove the benefits of this concept, both theoretically and experimentally. In this thesis, theoretical and experimental work on quantum well solar cells is presented. The basic concept of solar cells, IBSC and PR-IBSC as well as their advantages and disadvantages are discussed in chapter 1, along with theory of quantum mechanics and optical transitions in quantum wells. Chapter 2 focuses on theoretical work, which includes limiting efficiency calculation and fundamental loss calculation in solar cells, in order to determine the fundamental benefit of the PR-IBSC when compared with conventional IBSCs. The result of this work was published in Applied Physics Letter in 2012 to propose the concept of the PR-IBSC for the first time. Experimental work on existing quantum well solar cells is presented in chapter 3, along with basic characterisation techniques. The InGaAs quantum well with GaAs barrier in a p-i-n diode is optically and electrically characterised and we describe how we have observed an increase in photocurrent due to sequential absorption of photons via the intermediate band (IB), which arises from the one-dimensional confinement in the quantum well, for the first time. This is an important result and has been published in the Journal of Photovoltaics in 2014. In order to study the interband and inter-subband transitions in quantum wells individually, we also designed a new set of samples, along with their reference samples, which consist of n-i-n and p-i-n diodes with identical single quantum wells in the i-region. The details of the samples, along with a model which simulates the transitions in quantum wells and achieves basic characterisation of the samples, are presented in chapter 4. Finally, in chapter 5, we draw up our conclusions and future work on the new samples is discussed.Open Acces

DNA hybridisation sensors for product authentication and tracing : state of the art and challenges

Abstract: The wide use of biotechnology applications in bioprocesses such as the food and beverages industry, pharmaceuticals, and medical diagnostics has led to not only the invention of innovative products but also resulted in consumer and environmental concerns over the safety of biotechnology-derived products. Controlling and monitoring the quality and reliability of biotechnology-derived products is a challenge. Current tracking and tracing systems such as barcode labels and radio frequency identification systems track the location of products from primary manufactures and/or producers throughout globalised distribution channels. However, when it comes to product authentication and tracing, simply knowing the location of the product in the supply chain is not sufficient. DNA hybridisation sensors allows for a holistic approach into product authentication and tracing in that they enable the attribution of active ingredients in biotechnology-derived products to their source. In this article, the state-of-the-art of DNA hybridisation sensors, with a focus on the application of graphene as the backbone, for product authentication and tracing is reviewed. Candidate DNA biocompatible materials, properties and transduction schemes that enable detection of DNA are covered in the discussion. Limitations and challenges of the use of DNA biosensing technologies in real-life environmental, biomedical and industrial fields as opposed to clean-cut laboratory conditions are also enumerated. By considering experimental research versus reality, this article outlines and highlights research needed to overcome commercialisation barriers faced by DNA biosensing technologies. In addition, the content is thought-provoking to facilitate development of cutting edge research activities in the field

Volume 67- Issue 7- April, 1956

The Rose Thorn, Rose-Hulman\u27s independent student newspaper.https://scholar.rose-hulman.edu/rosethorn/2066/thumbnail.jp

Silicon- and Graphene-based FETs for THz technology

[EN] This Thesis focuses on the study of the response to Terahertz (THz) electromagnetic radiation of different silicon substrate-compatible FETs. Strained-Si MODFETs, state-of- the-art FinFETs and graphene-FETs were studied. The first part of this thesis is devoted to present the results of an experimental and theoretical study of strained-Si MODFETs. These transistors are built by epitaxy of relaxed-SiGe on a conventional Si wafer to permit the fabrication of a strained-Si electron channel to obtain a high-mobility electron gas. Room temperature detection under excitation of 0.15 and 0.3 THz as well as sensitivity to the polarization of incoming radiations were demonstrated. A two-dimensional hydrodynamic-model was developed to conduct TCAD simulations to understand and predict the response of the transistors. Both experimental data and TCAD results were in good agreement demonstrating both the potential of TCAD as a tool for the design of future new THz devices and the excellent performance of strained-Si MODFETs as THz detectors (75 V/W and 0.06 nW/Hz0.5). The second part of the Thesis reports on an experimental study on the THz behavior of modern silicon FinFETs at room temperature. Silicon FinFETs were characterized in the frequency range 0.14-0.44 THz. The results obtained in this study show the potential of these devices as THz detectors in terms of their excellent Responsivity and NEP figures (0.66 kV/W and 0.05 nW/Hz0.5). Finally, a large part of the Thesis is devoted to the fabrication and characterization of Graphene-based FETs. A novel transfer technique and an in-house-developed setup were implemented in the Nanotechnology Clean Room of the USAL and described in detail in this Thesis. The newly developed transfer technique enables to encapsulate a graphene layer between two flakes of h-BN. Raman measurements confirmed the quality of the fabricated graphene heterostructures and, thus, the excellent properties of encapsulated graphene. The asymmetric dual grating gate graphene FET (ADGG-GFET) concept was introduced as an efficient way to improve the graphene response to THz radiation. High quality ADGG-GFETs were fabricated and characterized under THz radiation. DC measurements confirmed the high quality of graphene heterostructures as it was shown on Raman measurements. A clear THz detection was found for both 0.15 THz and 0.3 THz at 4K when the device was voltage biased either using the back or the top gate of the G-FET. Room temperature THz detection was demonstrated at 0.3 THz using the ADGG-GFET. The device shows a Responsivity and NEP around 2.2 mA/W and 0.04 nW/Hz0.5 respectively at respectively at 4K. It was demonstrated the practical use of the studied devices for inspection of hidden objects by using the in-house developed THz imaging system

Development of a Hybrid-Electric Aircraft Propulsion System Based on Silicon Carbide Triple Active Bridge Multiport Power Converter

Constrained by the low energy density of Lithium-ion batteries with all-electric aircraft propulsion, hybrid-electric aircraft propulsion drive becomes one of the most promising technologies in aviation electrification, especially for wide-body airplanes. In this thesis, a three-port triple active bridge (TAB) DC-DC converter is developed to manage the power flow between the turbo generator, battery, and the propulsion motor. The TAB converter is modeled based on the emerging Silicon Carbide (SiC) Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) modules operating at high switching frequency, so the size of the magnetic transformer can be significantly reduced. Different operation modes of this hybrid-electric propulsion drive based on the SiC TAB converter are modeled and simulated to replicate the takeoff mode, cruising mode, and regenerative charging mode of a typical flight profile. Additionally, soft switching is investigated for the TAB converter to further improve the efficiency and power density of the converter, and zero voltage switching is achieved at heavy load operating conditions. The results show that the proposed TAB converter is capable of achieving high efficiency during all stages of the flight profile

Atomic-Scale Insights into Light Emitting Diode

In solid-state lightning, GaN-based vertical LED technology has attracted tremendous attention because its luminous efficacy has surpassed the traditional lightning technologies, even the 2014 Nobel Prize in Physics was awarded for the invention of efficient blue LEDs, which enabled eco-friendly and energy-saving white lighting sources. Despite today’s GaN-based blue VLEDs can produce IQE of 90% and EQE of 70-80%, still there exist a major challenge of efficiency droop. Nonetheless, state-of-the-art material characterization and failure analysis tools are inevitable to address that issue. In this context, although LEDs have been characterized by different microscopy techniques, they are still limited to either its semiconductor or active layer, which mainly contributes towards the IQE. This is also one of the reason that today’s LEDs IQE exceeded above 80% but EQE of 70-80% remains. Therefore, to scrutinize the efficiency droop issue, this work focused on developing a novel strategy to investigate key layers of the LED structure, which play the critical role in enhancing the EQE = IQE x LEE factors. Based on that strategy, wafer bonding, reflection, GaN-Ag interface, MQWs and top-textured layers have been systematically investigated under the powerful advanced microscopy techniques of SEM-based TKD/EDX/EBSD, AC-STEM, AFM, Raman spectroscopy, XRD, and PL. Further, based on these correlative microscopy results, optimization suggestions are given for performance enhancement in the LEDs. The objective of this doctoral research is to perform atomic-scale characterization on the VLED layers/interfaces to scrutinize their surface topography, grain morphology, chemical composition, interfacial diffusion, atomic structure and carrier localization mechanism in quest of efficiency droop and reliability issues. The outcome of this research advances in understanding LED device physics, which will facilitate standardization in its design for better smart optoelectronics products

Novel III-V alloys for unassisted water splitting.

Direct conversion of solar energy to fuels by photoelectrochemical (PEC) water splitting has been identified as a promising route for sustainable power generation and storage. However, semiconductor materials with suitable opto-electronic properties have yet to be identified. In this dissertation, band gap engineering of III-V alloys, tuning the light sensitivity of GaP and GaN nitride through alloying Sb, is investigated for developing new semiconductor alloys with appropriate opto-electronic properties and their performance with overall water photoelectrolysis. In addition to III-V materials, copper oxide (Cu2O) electrode was investigated for p-type semiconductor material. The new III-V alloy anodes were studied in single-semiconductor PEC cells, as well as in z-scheme PEC cells involving p-Si substrates. Metal organic chemical vapor deposition, halide vapor phase epitaxy and hot filament chemical vapor deposition methods were employed for the synthesis of tunable III-V alloys and wide band gap semiconductors as photoanodes and photocathodes. Synthesis of p-Cu2O/WO3 core-shell nanowire arrays resulted in a five-fold improvement of the photocurrent density compared to TiO2-coated copper oxide nanowire arrays. The deposition of WO3 and CuWO4 reduced the CuO phase impurities in Cu2O leading to considerable enhancement of the photocathode activity in terms of charge separation and stability. Nearly epitaxial films of GaSbxN(1-x) were grown on lattice matching and miss-matching substrates, studying the effect of substrate temperature and metal precursor ratios in the incorporation of antimony. The photovoltages and the very high carrier concentration of nearly epitaxial GaSbxN(1-x) films suggests that un-intentional doping causes a high degree of degeneracy and a metallic character of the electrodes. This helps explain the lack of a depletion region in the epitaxial films of GaSbxN(1-x) due to continuous density of states between the valence and conduction bands. Un-biased water splitting with 2% solar to hydrogen efficiency under 1.5 AM illumination is reported using GaSbxP1-x. The optoelectronic properties of GaP are modified to study the indirect-to-direct band gap cross-over point in this ternary III-V system. The photoanodes show a photovoltage of 750 mV, photocurrents of 7 mA cm-2 at 10 sun illumination, and corrosion resistance at pH 0. The growth of compact, free-standing films of GaSbxP1-x was acchieved with inexpensive metallic precursors. Experimental results indicate that only photon energies greater than 2.68 eV generate mobile and extractable charges. Here it is demonstrated that a low penetration depth and efficient carrier extraction to the indirect conduction band of the material can be acchieved above the energy level of the hydrogen reduction potential. This observation could potentially challenge the longstanding belief that direct band gap materials outperform indirect ones in regards to absorption. Single semiconductor water splitting can be accomplished by tuning the band structure of GaSbxP1-x through incorporation of antimony into the lattice of GaP. GaSbxP(1-x) was tested with p-type silicon as a wired dual PEC cell. Improved photocurrent with a z-scheme configuration results from enhanced absorption of low energy photons at the p-Si photocathode, and demonstrates the feasibility of using GaSbxP(1-x) in a tandem photoelectrochemical cell