3,598 research outputs found

    Ultrathin compound semiconductor on insulator layers for high performance nanoscale transistors

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    Over the past several years, the inherent scaling limitations of electron devices have fueled the exploration of high carrier mobility semiconductors as a Si replacement to further enhance the device performance. In particular, compound semiconductors heterogeneously integrated on Si substrates have been actively studied, combining the high mobility of III-V semiconductors and the well-established, low cost processing of Si technology. This integration, however, presents significant challenges. Conventionally, heteroepitaxial growth of complex multilayers on Si has been explored. Besides complexity, high defect densities and junction leakage currents present limitations in the approach. Motivated by this challenge, here we utilize an epitaxial transfer method for the integration of ultrathin layers of single-crystalline InAs on Si/SiO2 substrates. As a parallel to silicon-on-insulator (SOI) technology14,we use the abbreviation "XOI" to represent our compound semiconductor-on-insulator platform. Through experiments and simulation, the electrical properties of InAs XOI transistors are explored, elucidating the critical role of quantum confinement in the transport properties of ultrathin XOI layers. Importantly, a high quality InAs/dielectric interface is obtained by the use of a novel thermally grown interfacial InAsOx layer (~1 nm thick). The fabricated FETs exhibit an impressive peak transconductance of ~1.6 mS/{\mu}m at VDS=0.5V with ON/OFF current ratio of greater than 10,000 and a subthreshold swing of 107-150 mV/decade for a channel length of ~0.5 {\mu}m

    Advanced Focused Beam-Induced Processing for Nanoscale Synthesis and 2D Materials Device Architectures

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    Nanofabrication has come to prominence over recent years due to miniaturization of electronic devices as well as interesting physical phenomena that arise in material systems at the nanoscale. Particle beam induced processing enables additive as well as subtractive nanoprocessing techniques. Focused beam induced processing facilitates direct-write processing, thus making it a common technique for fabrication and synthesis on the nanoscale and is typically carried out with charged particles such as electrons or ion species, each of which offer distinct capabilities. This dissertation addresses several challenges which currently plague the focused beam-induced processing community and explores novel applications.Chapter I explores laser based purification strategies for electron beam induced deposition. This addresses the challenge of material purity, which currently limits broader application of the nanofabrication technique. Chapter II covers advanced helium ion beam induced processing using a Gas Field Ionization source. This chapter explores novel applications for the helium ion beam as well as the mitigation of helium-induced subsurface damage, which currently prevents ubiquitous adoption of the helium ion microscope as a nanofabrication tool. Chapter III studies defect introduction in 2D materials under helium ion irradiation, which proves to be an ideal nanoprocessing application for the helium ion beam

    Three transducers for one photodetector: essays for optical communications

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    Dissertation presented to obtain the PhD degree in Electrical and Computer Engineering - ElectronicsOptical processing devices based on a- SiC:H multilayer architectures are expected to become reconfigurable to perform WDM optoelectronic logic functions and provide as well complex photonic functions such as signal amplification and switching. This thesis, entitled ”Three Transducers for One Photodetector: essays for optical communications”, reports the main work areas to design, control, validate and evaluate the research of a voltage-controllable wavelength selective optical switching based on shifting between positive and negative electrically bias and a photodetector, which enables the filtering function with the detector itself and has the potential to be rapidly optically biasing tuned: System Architecture – In this work area it is defined the basic requirements of the device: light-to-dark sensitivity, colour recognition, selective optical and electrical output response, amplification and opto-electronic conversion to transmit, receive, and/or process intelligence(data).The output multiplexed signals should have a strong nonlinear dependence on the light absorption profile, i.e., on the incident light wavelength, bit rate and intensity under unbalanced light generation of carriers. Experimental Design – This test activities work area allows the evaluation of the results. Multiple monochromatic pulsed communication channels were transmitted together, each one with a specific bit sequence. The combined optical signal was analyzed by reading out, under different applied voltages and optical bias, the generated photocurrent across the device. Depending on the wavelength of the external background and irradiation side, it acts either as a short- or a long- pass band filter or as a band-stop filter Optoelectronic Algorithm Interface – To help improve our understanding of the output multiplexed signal, computer models of monolithic photodetectors are developed. Following control theoretic methods we derive state-space representation and an equivalent circuit optoelectronic simulator. We validate each model and calibrate the spectral gain model by background–probe experiments and truth tables lookup that perform 8-to-1 multiplexer (MUX) and 1-to-8 demultiplexer (DEMUX) functions. Applications – The purpose of this work area is to present a new optical logic architecture that offers considerable improvements in reconfigurability. Tunable WDM converters based on amorphous SiC multilayer photonic active filters are used to build blocks to perform standard digital system operations. The transducers combine the simultaneous demultiplexing operation with the photodetection and self amplification. They are optimized for provide the high-sensitivity needed for low-light applications, such as medicine, lighting, sensing and measurement, and manufacturing. The migration to next generation packet based networks can be much easier and smoother than previously thought, using the emerging a-Si solutions and its integration with plastic optical fiber. It will push the limits of functionality, cost/performance and integration level

    Modeling and Simulation of Metal AM

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    Additive manufacturing with metal components is a complex, and currently cyclic, process due to the physical phenomena that are occurring. These phenomena can be mathematically modeled in order to predict the outcome of a specific aspect of the build. Coupling the mathematical models can then be used to develop a complete simulation, which can produce estimates for a range of characteristics for a part built using additive manufacturing techniques. This chapter will investigate the main models used in the simulation of metal AM. These models will include the modeling of thermal behavior, fluid dynamics, stress, and a selection of other auxiliary models, which are necessary to complete the simulations. For each of the models investigated, the various modeling techniques that have been developed will be presented along with their limitations, validation techniques, and parameters necessary to model the process correctly

    Use of a thermal imager for snow pit temperatures

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    Design and Testing of an Automated System using Thermochromatic Liquid Crystals to Determine Local Heat Transfer Coefficients for an Impinging Jet

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    Using thermochromatic liquid crystal to measure surface temperature, an automated transient method with time-varying free-stream temperature is developed to determine local heat transfer coefficients. By allowing the free-stream temperature to vary with time, the need for complicated mechanical components to achieve a step temperature change is eliminated, and by using the thermochromatic liquid crystals as temperature indicators, the labor intensive task of installing many thermocouples is omitted. Bias associated with human perception of the transition of the thermochromatic liquid crystal is eliminated by using a high speed digital camera and a computer. The method is validated by comparisons with results obtained by the steady-state method for a circular Jet impinging on a flat plate. Several factors affecting the accuracy of the method are evaluated

    Dynamic Thermography Derived Perfusion as a Potential Tool for Evaluating Cutaneous Perfusion Changes in Response to Low-level-laser Irradiation

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    The objective of this study is to develop method for extracting cutaneous perfusion information from dynamic thermography, using an improved bio-heat transfer model for the initial application to study the responses of cutaneous perfusion to low level laser irradiation. The bio-heat transfer model developed in this study for dynamic thermography-derived perfusion employs the contributions of heat transfer due to blood circulation, which has been implemented in previous models, and a spatial Laplacian term accounting for heat changes due to conduction, which has been neglected by previous models. The model is applied to dynamic thermography imagery obtained at 23Hz of frame rate from cutaneous tissues of turtle subjects and human volunteers subjected to the same laser irradiation protocol of 20 seconds of irradiation between 20 seconds of idle time prior to and after the laser irradiation. The proposed method yields stable results over all 6 sets of human data with a perfusion range similar to that estimated from other cited works whereas the other models falter for 3 or more sets of data. For turtle data the perfusion pattern is similar to that by the other models attributable to the low changes in heat conduction pertaining to their thermoregulation mechanism. The algorithm is being implemented in a Graphical-User-Interface (GUI) for the clinical testing by physicians.Electrical Engineerin

    Coating Thickness Measurements and Defect Characterization in Non-Metallic Composite Materials by Using Thermography

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    Thermography is a non-destructive testing method (NDT), which is widely used to guarantee the quality of non-metallic materials, such as carbon fiber composite, anti-reflection (AR) film, and coatings. As other NDT methods do, thermography determines a defective area based on the signal difference between suspected defective areas and defective-free areas. Two unavoidable effects are decreasing the credibility of thermography detection: one is uneven heating, and the other is lateral diffusion of heat. To solve this problem, researchers have developed various reconstruction methods. Restoring methods are known to have the capacity to reduce the effect of heat-flux lateral diffusion by de-convoluting a point spread function either along a temporal profile or a spatial profile to process captured thermal images. These methods either require pre-knowledge with depth or are not effective in detecting deep defects. Here we propose a spatial-temporal profile-based reconstruction method to reduce the effect of uneven heating and lateral diffusion. The method evaluates the heat flux deposited onto tested samples based on surface temperature gathered under ideal conditions. Then the proposed method is tested in three real applications – in defect detection on semi-transparent materials, on semi-infinite defects (coatings) and anisotropic materials. The method is evaluated against existing methods. Results suggest that the proposed method is effective and computationally efficiently over all the reconstruction methods reviewed. It reduces the effect of uneven heating by providing a good approximation to the input heat flux at the ending image of the sequence
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