Silicon carbide junction field effect transistor integrated circuits for hostile environments

PhD ThesisSilicon carbide (SiC), in particular its 4H polytype, has long been recognised as an appropriate semiconductor for producing hostile environment electronics due to its wide energy band gap, large chemical bond strength and high mechanical hardness. A strong research foundation has facilitated the development of numerous sensor structures capable of operating at high temperatures and in corrosive atmospheres. Front-end electronics suitable for in situ signal conditioning are however lacking. Junction field effect transistors (JFETs) circumvent the pitfalls of contemporary alternative SiC transistor variants and have been found to operate predictably and consistently under such extreme conditions. This thesis demonstrates for the first time the capability of producing the necessary stable and high-performance interface circuits from n-channel lateral depletion-mode (NLDM) JFETs. The temperature dependence of pertinent bulk 4H–SiC material parameters relevant for describing the operation of macroscopic JFETs were initially studied. An accurate phenomenological model was developed to account for the variation of the thermal equilibrium free carrier concentrations. The position of the electrochemical potential and the distribution of free electron energies were found to change markedly when conduction band nonparabolicity, higher energy intrinsic bands and extrinsic effects were accounted for. These in turn were found to influence the determination of p-n junction contact potentials. The worst case error introduced through use of the Boltzmann approximation when applied to the channel and gate regions of the JFETs under study, having nominal doping concentrations of 1 1017 cm3 and 2 1019 cm3, respectively, were approximately 0:1% and 2%, respectively. A set of efficient and well behaved closed form expressions were subsequently developed for the free carrier concentrations in the framework of the Joyce- Dixon approximation (JDA) which are ideally suited for use in circuit simulations. Expressions for the electron conductively effective mass and an appropriate weighting function for the momentum relaxation time were subsequently identified. While the conductivity effective mass along the basal plane remained almost independent of temperature the non-parabolic band dispersion in the direction of principle axis introduced a temperature variation of 19% and 21% between 25 C and 400 C in the first and second conduction bands, respectively. Monolithically integrated 4H–SiC signal-level homo-epitaxial NLDM JFETs, p-n junction diodes and resistors were electrically characterised between room temperature and 400 C and their static and dynamic properties studied. Their behaviours were found to be well represented by macroscopic drift-diffusion models and were in agreement with predictions based on the bulk material properties. The intrinsic voltage gain of the fabricated JFET structures with nominal 9 μm gate length, 300nm channel depth and 250 μm gate width, under typical bias conditions, was roughly 100. As a consequence of the finite doping concentration in the buffer layer beneath the active device channel, with an experimentally determined value of approximately 3 1015 cm3, the devices under study were found to exhibit a strong body-effect. The thermal performance of the utilised tungsten capped annealed nickel-titanium and aluminium-titanium contacts, on highly doped n- and p-type regions, respectively, were investigated and appropriate methods for their characterisation described. The lowest recorded value of specific contact resistance was 1:90(50) 105 cm2 with a corresponding sheet resistance of 7:89(9) 102 = . Lateral current flow through the contact side wall and the difference in sheet resistance under the contact were found to increase the value of the specific contact resistance determined from transfer length method (TLM) test structures by as much as 10% for n-type contacts. While exhibiting much larger contact resistance, the p-type contacts were found to have negligible impact on device performance due to the high impedance of the gate-channel and body-channel p-n junctions under typical operation. Physics based, Simulation Program with Integrated Circuit Emphasis (SPICE) compatible, integrated circuit (IC) consistent compact models were developed that are congruent with experimental measurements over the aforementioned range of temperature and across all essential bias levels. Most notably, a self-contained, asymmetric double-gated, non-selfaligned JFET model was developed that accurately accounts for the body-effect, voltage dependent mobility and temperature. An accurate yet efficient solver of the charge neutrality equation within each region of the device is utilised to account for incomplete ionisation of dopants and the temperature dependence of the p-n junction contact potentials. Meticulous agreement with experimental measurements was attained from a minimal number of input parameters. The modelled devices were used to simulate pertinent IC building blocks, including single stage and differential amplifiers, level-shifters and voltage buffers. The finite bodytransconductance of active load transistors were identified as a major degrading factor for the voltage gain. Practical methods to circumvent this are discussed with the aid of appropriate small-signal equivalent models. Finally, a design was presented for a two-stage 4H–SiC operational amplifier (op-amp) with direct current (DC) stability over the entire temperature range of study. Low-frequency small-signal voltage gains of 80 dB and 70 dB were achieved at 25 C and 400 C, respectively when utilising a 30V supply. A closed-loop non-inverting op-amp configuration with an ideal gain of 11 was then simulated and found to vary by just 1% between 25 C and 400 C. Such amplifiers are of great utility and form the cornerstone of numerous useful and important electronic systems.Engineering and Physical Sciences Research Council (EPSRC) and BAE Systems Maritime for financially supporting this research project

Fast Compressive 3D Single-pixel Imaging

In this work, we demonstrate a modified photometric stereo system with perfect pixel registration, capable of reconstructing continuous real-time 3D video at ~8 Hz for 64 x 64 image resolution by employing evolutionary compressed sensing

A One-Hole Cu\u3csub\u3e4\u3c/sub\u3eS Cluster with N\u3csub\u3e2\u3c/sub\u3eO Reductase Activity: A Structural and Functional Model for Cu\u3csub\u3eZ\u3c/sub\u3e

During bacterial denitrification, two-electron reduction of N2O occurs at a [Cu4(μ4-S)] catalytic site (CuZ*) embedded within the nitrous oxide reductase (N2OR) enzyme. In this Communication, an amidinate-supported [Cu4(μ4-S)] model cluster in its one-hole (S = 1/2) redox state is thoroughly characterized. Along with its two-hole redox partner and fully reduced clusters reported previously, the new species completes the two-electron redox series of [Cu4(μ4-S)] model complexes with catalytically relevant oxidation states for the first time. More importantly, N2O is reduced by the one-hole cluster to produce N2 and the two-hole cluster, thereby completing a closed cycle for N2O reduction. Not only is the title complex thus the best structural model for CuZ* to date, but it also serves as a functional CuZ* mimic

Socioeconomic and Climate Impacts on the Future of Water: An Integrated Assessment Approach to Demand, Scarcity, and Trade

Changes to socioeconomics and an evolving climate system are likely to play a vital role in how regions around the world use water into the future. Water projections for the future, while prolific, remain highly variable and dependent upon underlying scenario and model assumptions. In this study, the Global Change Assessment Model (GCAM) is used, where interactions between population, economic growth, energy, land, water, and climate systems interact dynamically within a market equilibrium economic modeling framework, to address how changing socioeconomic and climate conditions alter global water futures, and in turn, how water constrains the future of other systems. First, the impacts of efficiency changes are investigated with the addition of socioeconomically consistent water technologies across several sectors. Quantitative assumptions for the Shared Socioeconomic Pathways are extended to the water sector for the first time in a water constrained – Integrated Assessment Modeling framework. It is found that significant water use reductions are possible under certain socioeconomic conditions, provided the ability to adopt appropriate technological advances in lower income regions. Secondly, the relative contributions of climate and human systems on water scarcity are analyzed at global and basin scales under the Shared Socioeconomic Pathway-Representative Concentration Pathway (SSP-RCP) framework. Ninety scenarios are explored to determine how the coevolution of energy-water-land systems affects not only the driver behind water scarcity changes in different water basins, but how human and climate systems interact in tandem to alter water scarcity. Human systems are found to dominate water scarcity changes into the future, regardless of socioeconomic or climate future. However, the sign of these changes has a significant scenario dependence, with an increased number of basins experiencing improving water scarcity conditions due to human interventions in the sustainability focused scenario. Finally, the reliance on international agricultural trade is analyzed to understand how future socioeconomic growth and climatic change will impact the dependency on international water sources. The differentiation between renewable and nonrenewable water sources allow for the quantification of the various water sources needed to produce enough agricultural goods to meet global demands. The first Integrated Assessment Model projection of the evolution of external water sources to meet domestic agricultural demands show that there will be an increasing international dependencies. China, the United States, and portions of South America are pivotal in providing the necessary exports to meet demands in water scarce or high demand areas of the Middle East and Africa

Simultaneous real-time visible and infrared video with single-pixel detectors

Conventional cameras rely upon a pixelated sensor to provide spatial resolution. An alternative approach replaces the sensor with a pixelated transmission mask encoded with a series of binary patterns. Combining knowledge of the series of patterns and the associated filtered intensities, measured by single-pixel detectors, allows an image to be deduced through data inversion. In this work we extend the concept of a ‘single-pixel camera’ to provide continuous real-time video at 10 Hz , simultaneously in the visible and short-wave infrared, using an efficient computer algorithm. We demonstrate our camera for imaging through smoke, through a tinted screen, whilst performing compressive sampling and recovering high-resolution detail by arbitrarily controlling the pixel-binning of the masks. We anticipate real-time single-pixel video cameras to have considerable importance where pixelated sensors are limited, allowing for low-cost, non-visible imaging systems in applications such as night-vision, gas sensing and medical diagnostics

Electron spin resonance studies of transient radical-ions in liquid ammonia

This work describes the use of electron spin resonance spectroscopy in investigations of the reduction of a number of organic compounds by solutions of sodium in liquid ammonia. Solutions of alkali metals in ammonia contain the ammoniated electron, in all its possible forms, and essentially this species is the effective reducing agent. The simple reduction step is the one-electron addition to a suitable substrate molecule, which may then undergo a number of successive reactions before a stable product is formed. The nature of the metal-ammonia solutions, and the paths the reductions may take are discussed in Chapter 2. The simple one-electron reduction product, a radical-anion, is often too short-lived to allow observation by normal static methods, and therefore a relatively high concentration of radicals must be artificially maintained to permit their detection. Basically two different approaches have been utilised: i. Stabilising the radicals either by direct production or trapping in solid matrix, therefore preventing the radicals from reacting, and ii. Production of a high steady state concentration of radicals in solution by either continuous electrolysis or buy use of a flow system, A brief discussion of the methods used in the study of transient radicals is given in Chapter 1. Generally the e.s.r spectra are less well-resolved in solid matrices than in solutions, and for species showing a large amount of hyperfine structure, production of the radicals in solution is preferred. A rapid mixing device has been developed to allow observation of transient intermediates by e.s.r. spectroscopy. The mixer was designed particularly for the type of system and experimental technique under consideration here, and is based on a design used in biological kinetic studies. Observations 2-5 msec. after mixing are possible, and this represents a considerable advantage over the widely used aqueous solution mixing device, where observations are made on a 10-2 sec. time scale. The mixing chamber and experimental technique are presented in Chapter 3. Analysis of the e.s.r. spectrum of a compound allows the calculation of its unpaired electron distribution, the coupling constants being related to the unpaired electron spin density. Spin densities have been calculated theoretically, and it has been found that simple Hückel calculations of pi-electron spin densities show good agreement with “experimentally” determined values. Accordingly, both Hückel and McLachlan spin densities have been calculated for most of the substrates used, and in general it is found that the McLachlan treatment gives better agreement with experiment than the simple Hückel model. A brief discussion of the relevant molecular orbital theory is presented in Chapter 1, and the computer programme used to perform the theoretical calculations is given in Appendix 3. Analysis of the e.s.r. spectra I sometimes very difficult if a large number of lines are present, and two computer programmes used for the simulation of single and mixed e.s.r. spectra are given in Appendices 2 and 3 respectively. In Chapter 4 is described the reduction of a number of aryl halides. With the exception of fluoro-substituted compounds, the halo-pyridines, pyrimidines, benzenes, biphenyls, and naphthalene, all give the e.s.r. spectrum of the radical-anions of the parent compounds on reduction. On the other hand, fluorine is retained as is evident from the spectra, for a much longer period, as shown by observation 0.1 and 1.0 sec. after mixing, while static experiments show some products to be stable for longer periods of time in this system. Mechanisms have been proposed to account for these reductions. Halobenzonitriles have also been studies and the results are in agreement with the proposed reduction mechanisms. Reduction of pyridine, pyrimidine and some simple ring-substituted compounds has been investigated, and their e.s.r. spectra characterised in most cases. Previous attempts to observe the pyridine radical-anion had failed, the spectrum of the 4,4’-bipyridyl being obtained instead. Observations 0.1, 1.0 sec. and 1 min. after mixing allow the reduction path of these nitrogen heterocyclics to be followed, and it is shown that pyridine, pyrimidine and simple alkyl- and alkoxy-substituted pyridines undergo dimerization to give exclusively the respective 4,4’-dimers, unless the 4-position is blocked. Pyridine-N-oxide undergoes a more complex reduction, giving pyridine-N-oxide, pyridine and finally 2,2’-bipridyl radical-anions. Pyridine-2-carboxylic acid gives a spectrum suggesting some form of nitrogen-hydrogen bonded species. Pyridine dicarboxylic acids all have on feature in common, a splitting from an extra proton which arises through a protonated nitrogen atom. Reduction of pyridine in the presence of excess ethanol also shows this feature, and its spectrum is due to C5H5NH. Calculation of the nitrogen sigma-pi interaction parameters gives values of QNN=+28.5 oe. And QCNN=-O.5 oe., in good agreement with results obtained previously. Theoretical calculations have been performed, and comparison with experiment has enabled the assignment of coupling constants to particular positions, and also gives the best set of parameter values required for the calculations, providing a check with values used for comparison with other measurements. The results are presented in Chapter 5. In Chapter 6 is described the reduction of benzoic acid and some of its simple ring-substituted derivate. Ionisation occurs initially, followed by reduction to give the corresponding radical-anions. These species are short-lived, as no spectra are obtained when observations are performed 0.1 sec. after mixing. No further paramagnetic products are found. Molecular orbital calculations have been performed for each compound, and excellent agreement with experiment obtained using the parameter values kC’-C=1, 2, kC-O-1, 6, hO-2.0 for the ionised carboxyl group. Reduction of nitro-substituted isophthalic and terephthalic acids shows the presence of two distinct species, one being unstable while the other is stable for more than one hour, in both cases. The unstable species show a large splitting from an extra proton which is thought to be attached to the nitrogen atom. This proton is lost to give the stable radicals, experimental evidence and comparison with computer calculations suggesting them to be the nitro-substituted radical-anions

Inelastic Dark Matter in Light of DAMA/LIBRA

Inelastic dark matter, in which WIMP-nucleus scatterings occur through a transition to an excited WIMP state ~ 100 keV above the ground state, provides a compelling explanation of the DAMA annual modulation signal. We demonstrate that the relative sensitivities of various dark matter direct detection experiments are modified such that the DAMA annual modulation signal can be reconciled with the absence of a reported signal at CDMS-Soudan, XENON10, ZEPLIN, CRESST, and KIMS for inelastic WIMPs with masses O(100 GeV). We review the status of these experiments, and make predictions for upcoming ones. In particular, we note that inelastic dark matter leads to highly suppressed signals at low energy, with most events typically occurring between 20 to 45 keV (unquenched) at xenon and iodine experiments, and generally no events at low (~ 10 keV) energies. Suppressing the background in this high energy region is essential to testing this scenario. The recent CRESST data suggest seven observed tungsten events, which is consistent with expectations from this model. If the tungsten signal persists at future CRESST runs, it would provide compelling evidence for inelastic dark matter, while its absence should exclude it.Comment: 27 pages, 17 figures, minor revision

Faulting and Folding of the Transgressive Surface Offshore Ventura Records Deformational Events in the Holocene

Identifying the offshore thrust faults of the Western Transverse Ranges that could produce large earthquakes and seafloor uplift is essential to assess potential geohazards for the region. The Western Transverse Ranges in southern California are an E-W trending fold-and-thrust system that extends offshore west of Ventura. Using a high-resolution seismic CHIRP dataset, we have identified the Last Glacial Transgressive Surface (LGTS) and two Holocene seismostratigraphic units. Deformation of the LGTS, together with onlapping packages that exhibit divergence and rotation across the active structures, provide evidence for three to four deformational events with vertical uplifts ranging from 1 to 10 m. Based on the depth of the LGTS and the Holocene sediment thickness, age estimates for the deformational events reveal a good correlation with the onshore paleoseismological results for the Ventura-Pitas Point fault and the Ventura-Avenue anticline. The observed deformation along the offshore segments of the Ventura-Pitas Point fault and Ventura-Avenue anticline trend diminishes toward the west. Farther north, the deformation along the offshore Red Mountain anticline also diminishes to the west with the shortening stepping north onto the Mesa-Rincon Creek fault system. These observations suggest that offshore deformation along the fault-fold structures moving westward is systematically stepping to the north toward the hinterland. The decrease in the amount of deformation along the frontal structures towards the west corresponds to an increase in deformation along the hinterland fold systems, which could result from a connection of the fault strands at depth. A connection at depth of the northward dipping thrusts to a regional master detachment may explain the apparent jump of the deformation moving west, from the Ventura-Pitas Point fault and the Ventura-Avenue anticline to the Red Mountain anticline, and then, from the Red Mountain anticline to the Mesa-Rincon Creek fold system. Finally, considering the maximum vertical uplift estimated for events on these structures (max ∼10 m), along with the potential of a common master detachment that may rupture in concert, this system could generate a large magnitude earthquake (>Mw 7.0) and a consequent tsunami.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEUnión Europea. Horizonte 2020Comunidad de MadridSCECpu

Assessment and restoration of a neighborhood wetland invaded by exotic plant species

The University of Arkansas Crop, Soil, and Environmental Sciences (CSES) Club adopted a local wetland in the spring of 2002 through the Fayetteville Parks and Recreation Department. This project has allowed students to interact with local community and governmental organizations as well as other academic departments within the university. Students have gained valuable laboratory and field experience through characterizing hydric soils, identifying bird and plant species, and analyzing water quality, soil nutrients, and microbial biomass. Under the main goal of restoring the wetland, the club has outlined both short and long-term objectives including soil and water assessments; removal of two invasive species—Lonicera japonica and Festuca arundinacea; revegetation of native species to provide wildlife habitat and forage; establishment of trails and educational signs; and community outreach. To facilitate removal of the invasive species, the club is experimenting with manual removal, implementing physical barriers to prevent plant photosynthesis, and working with city officials to obtain permission for selective use of herbicides. The adoption of the wetland has provided a catalyst for the CSES Club to organize, rebuild itself, and achieve its goals