The variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1% and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy is studied at 4 K. The excitonic linewidth increases as a function of pressure until about 100 kbar after which it tends to saturate. This pressure dependent excitonic linewidth is used to derive the pressure variation of the exciton reduced mass using a theoretical formalism based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The linewidth derived ambient pressure masses are compared and found to be in agreement with other mass measurements. The variation of this derived mass is compared with the results from a nearly first-principles approach in which calculations based on the local density approximation to the Kohn-Sham density functional theory are corrected using a small amount of experimental input.
In this paper, we overview several of the critical materials growth, design and performance issues for nitride-based UV (<400 nm) LEDs. The critical issue of optical efficiency is presented through temperature-dependent photoluminescence studies of various UV active regions. These studies demonstrate enhanced optical efficiencies for active regions with In-containing alloys (InGaN, AlInGaN). We discuss the trade-off between the challenging growth of high Al containing alloys (AlGaN, AlGaInN), and the need for sufficient carrier confinement in UV heterostructures. Carrier leakage for various composition AlGaN barriers is examined through a calculation of the total unconfined carrier density in the quantum well system. We compare the performance of two distinct UV LED structures: GaN/AlGaN quantum well LEDs for λ<360 nm emission, and InGaN/AlGaInN quantum well LEDs for 370 nm<λ<390 nm emission.
The following topics related to radionuclide and colloid transport in the Culebra Dolomite in the 1996 performance assessment for the Waste Isolation Pilot Plant (WIPP) are presented: (i) mathematical description of models: (ii) uncertainty and sensitivity analysis results arising from subjective (i,e. epistemic) uncertainty for individual releases; and (iii) construction of complementary cumulative distribution functions (CCDFs) arising from stochastic (i.e. aleatory) uncertainty. The presented results indicate that radionuclide and colloid transport in the Culebra Dolomite does not constitute a serious threat to the effectiveness of the WIPP as a disposal facility for transuranic waste. Even when the effects of uncertain analysis inputs are taken into account, no radionuclide transport to the boundary with the accessible environment was observed; thus, the associated CCDFs for comparison with the boundary line specified in the US Environmental Protection Agency's standard for the geologic disposal of radioactive waste (40 CFR 191, 40 CFR 194) are degenerate in the sense of having a probability of zero of exceeding a release of zero.
We have designed and assembled two generations of integrated micro-optical systems that deliver pump light and detect broadband laser-induced fluorescence in micro-fluidic chemical separation systems employing electrochromatography. The goal is to maintain the sensitivity attainable with larger, tabletop machines while decreasing package size and increasing throughput (by decreasing the required chemical volume). One type of micro-optical system uses vertical-cavity surface-emitting lasers (VCSELs) as the excitation source. Light from the VCSELs is relayed with four-level surface relief diffractive optical elements (DOEs) and delivered to the chemical volume through substrate-mode propagation. Indirect fluorescence from dye-quenched chemical species is collected and collimated with a high numerical aperture DOE. A filter blocks the excitation wavelength, and the resulting signal is detected as the chemical separation proceeds. Variations of this original design include changing the combination of reflective and transmissive DOEs and optimizing the high numerical aperture DOE with a rotationally symmetric iterative discrete on-axis algorithm. We will discuss the results of these implemented optimizations.
The requirement to efficiently form images over a large range of geometries has a profound impact on the design of a Synthetic Aperture Radar (SAR) system. This article shows how a data set conducive to efficient processing might increase the total RF bandwidth. It also presents examples of how a fixed RF bandwidth might then limit SAR geometries.
Because of limited direct observation, understanding of the interior conditions of the massive storage caverns constructed in Gulf Coast salt domes is realizable only through predictions of salt response. Determination of the potential for formation of salt spalls, leading to eventual salt falls, is based on salt creep and fracture using the Multimechanism-Deformation Coupled Fracture (MCDF) model. This is a continuum model for creep, coupled to continuum damage evolution. The model has been successfully tested against underground results of damage around several test rooms at the Waste Isolation Pilot Plant (WIPP). Model simulations, here, evaluate observations made in the Strategic Petroleum Reserve (SPR), storage caverns, namely, the accumulation of material on cavern floors and evidence of salt falls. A simulation of a smooth cavern wall indicates damage is maximum at the surface but diminishes monotonically into the salt, which suggests the source of salt accumulation is surface sluffing. If a protuberance occurs on the wall, fracture damage can form beneath the protuberance, which will eventually cause fracture, and lead to a salt fall.
This paper will describe an algorithm for detecting and classifying seismic and acoustic signals for unattended ground sensors. The algorithm must be computationally efficient and continuously process a data stream in order to establish whether or not a desired signal has changed state (turned-on or off). The paper will focus on describing a Fourier-based technique that compares the running power spectral density estimate of the data to a predetermined signature in order to determine if the desired signal has changed state. How to establish the signature and the detection thresholds will be discussed as well as the theoretical statistics of the algorithm for the Gaussian noise case with results from simulated data. Actual seismic data results will also be discussed along with techniques used to reduce false alarms due to the inherent nonstationary noise environments found with actual data.
We study the low-temperature in-plane magnetoconductance of vertically coupled double quantum wires. Using a novel flip-chip technique, the wires are defined by two pairs of mutually aligned split gates on opposite sides of a≤1 micron thick AlGaAs/GaAs double quantum well heterostructure. We observe quantized conductance steps due to each quantum well and demonstrate independent control of each 1D wire. A broad dip in the magnetoconductance at approximately 6 T is observed when a magnetic field is applied perpendicular to both the current and growth directions. This conductance dip is observed only when 1D subbands are populated in both the top and bottom constrictions. This data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.
4th North American Rock Mechanics Symposium, NARMS 2000
Rath, J.S.; Pfeifle, T.W.; Hunsche, H.
A simple numerical procedure for predicting damage and permeability in the disturbed rock zone (DRZ) is given. The empirical procedure predicts damage based on a function of stress tensor invariants. For a wttte class of problems hydro logic/mechanical coupling is necessary for proper analysis. The RATDAMPER procedure incorporates dilatant volumetric strain and permeability. It has been implemented in a weakly coupled code, which combines a finite element structural code and a finite difference multi-phase fluid flow code. Using the development of inelastic volumetric strain, a value of permeability can be assigned. This flexibility allows empirical permeability functional relationships to be evaluated.
The design of general-purpose dynamic load-balancing tools for parallel applications is more challenging than the design of static partitioning tools. Both algorithmic and software engineering issues arise. We have addressed many of these issues in the design of the Zoltan dynamic load-balancing library. Zoltan has an object-oriented interface that makes it easy to use and provides separation between the application and the load-balancing algorithms. It contains a suite of dynamic load-balancing algorithms, including both geometric and graph-based algorithms. Its design makes it valuable both as a partitioning tool for a variety of applications and as a research test-bed for new algorithmic development. In this paper, we describe Zoltan's design and demonstrate its use in an unstructured-mesh finite element application.
Intelligent, integrated microsystems combine some or all of the functions of sensing, processing information, actuation, and communication within a single integrated package, and preferably upon a single silicon chip. As the elements of these highly integrated solutions interact strongly with each other, the microsystems can be neither designed nor fabricated piecemeal, in contrast to the more familiar assembled products. Driven by technological imperatives, microsystems will best be developed by multi-disciplinary teams, most likely within flatter, less hierarchical organizations. Standardization of design and process tools around a single, dominant technology will expedite economically viable operation under a common production infrastructure. The production base for intelligent, integrated microsystems has elements in common with the mathematical theory of chaos. Similar to chaos theory, the development of microsystems technology will be strongly dependent on, and optimized to, the initial product requirements that will drive standardization-thereby further rewarding early entrants to integrated microsystems technology.
Monte Carlo (MC) simulations of phosphate tetrahedron connectivity distributions in alkali and alkaline earth phosphate glasses are reported. By utilizing a discrete bond model, the distribution of next-nearest neighbor connectivities between phosphate polyhedron for random, alternating and clustering bonding scenarios was evaluated as a function of the relative bond energy difference. The simulated distributions are compared to experimentally observed connectivities reported for solid-state two-dimensional (2D) exchange and double-quantum (2Q) nuclear magnetic resonance (NMR) experiments of phosphate glasses. These MC simulations demonstrate that the polyhedron connectivity is best described by a random distribution in lithium phosphate and calcium phosphate glasses.
Junction field effect transistors (JFET) were fabricated on a GaN epitaxial structure grown by metal organic chemical vapor deposition. The DC and microwave characteristics, as well as the high temperature performance of the devices were studied. These devices exhibited excellent pinch-off and a breakdown voltage that agreed with theoretical predictions. An extrinsic transconductance (gm) of 48 mS/mm was obtained with a maximum drain current (ID) of 270 mA/mm. The microwave measurement showed an fT of 6 GHz and an fmax of 12 GHz. Both the ID and the gm were found to decrease with increasing temperature, possibly due to lower electron mobility at elevated temperatures. These JFETs exhibited a significant current reduction after a high drain bias was applied, which was attributed to a partially depleted channel caused by trapped electrons in the semi-insulating GaN buffer layer.
An overview of the use of classical mechanical molecular simulations of porphyrins, hydroporphyrins and heme proteins is given. The topics cover molecular mechanics calculations of structures and conformer energies of porphyrins, energies of barriers for interconversion between stable conformers, molecular dynamics of porphyrins and heme proteins, and normal-coordinate structural analysis of experimental and calculated porphyrin structures. Molecular mechanics and dynamics are currently a fertile area of research on porphyrins. In the future, other computational methods such as Monte Carlo simulations, which have yet to be applied to porphyrins, will come into use and open new avenues of research into molecular simulations of porphyrins. Copyright (C) 2000 John Wiley and Sons, Ltd.
Using an interfacial force microscope, the measured elastic response of 100-nm-thick Au films was found to be strongly correlated with the films' stress state and thermal history. Large, reversible variations (2×) of indentation modulus were recorded as a function of applied stress. Low-temperature annealing caused permanent changes in the films' measured elastic properties. The measured elastic response was also found to vary in close proximity to grain boundaries in thin films and near surface steps on single-crystal surfaces. These results demonstrate a complex interdependence of stress state, defect structure, and elastic properties in thin metallic films.
This paper is an overview of the wafer and reticle positioning system of the Extreme Ultraviolet Lithography (EUVL) Engineering Test Stand (ETS). EUVL represents one of the most promising technologies for supporting the integrated circuit (IC) industry's lithography needs for critical features below 100 nm. EUVL research and development includes development of capabilities for demonstrating key EUV technologies. The ETS is under development at the EUV Virtual National Laboratory, to demonstrate EUV full-field imaging and provide data that supports production-tool development. The stages and their associated metrology operate in a vacuum environment and must meet stringent outgassing specifications. A tight tolerance is placed on the stage tracking performance to minimize image distortion and provide high position repeatability. The wafer must track the reticle with less than ±3 nm of position error and jitter must not exceed 10 nm rms. To meet these performance requirements, magnetically levitated positioning stages utilizing a system of sophisticated control electronics will be used. System modeling and experimentation have contributed to the development of the positioning system and results indicate that desired ETS performance is achievable.
Two-phase characteristic curves are necessary for the simulation of water and vapor flow in porous media. Existing functions such as van Genuchten [1980], Brooks and Corey [1966], and Luckner et al. [1989] have significant limitations in the dry region as the liquid saturation goes to zero. This region, which is important in a number of applications, including liquid and vapor flow and vapor-solid sorption, has been the subject of a number of previous investigations. Most previous studies extended standard capillary pressure curves into the adsorption region to zero water content and required a refitting of the revised curves to the data. In contrast, the present method provides for a simple extension of existing capillary pressure curves without the need to refit the experimental data. Therefore previous curve fits can be used, and the transition between the existing fit and the relationship in the adsorption region is easily calculated. The data-model comparison shows good agreement. This extension is a simple and convenient way to extend existing curves to the dry region.
A significant improvement to the classical least-squares (CLS) multivariate analysis method has been developed. The new method, called prediction-augmented classical least-squares (PACLS), removes the restriction for CLS that all interfering spectral species must be known and their concentrations included during the calibration. We demonstrate that PACLS can correct inadequate CLS models if spectral components left out of the calibration can be identified and if their 'spectral shapes' can be derived and added during a PACLS prediction step. The new PACLS method is demonstrated for a system of dilute aqueous solutions containing urea, creatinine, and NaCl analytes with and without temperature variations. We demonstrate that if CLS calibrations are performed with only a single analyte's concentrations, then there is little, if any, prediction ability. However, if pure-component spectra of analytes left out of the calibration are independently obtained and added during PACLS prediction, then the CLS prediction ability is corrected and predictions become comparable to that of a CLS calibration that contains all analyte concentrations. It is also demonstrated that constant-temperature CLS models can be used to predict variable-temperature data by employing the PACLS method augmented by the spectral shape of a temperature change of the water solvent. In this case, PACLS can also be used to predict sample temperature with a standard error of prediction of 0.07°C even though the calibration data did not contain temperature variations. The PACLS method is also shown to be capable of modeling system drift to maintain a calibration in the presence of spectrometer drift.
MicroElectroMechanical Systems (MEMS) were subjected to a vibration environment that had a peak acceleration of 120 g and spanned frequencies from 20 to 2000 Hz. The device chosen for this test was a surface-micromachined microengine because it possesses many elements (springs, gears, rubbing surfaces) that may be susceptible to vibration. The microengines were unpowered during the test. We observed 2 vibration-related failures and 3 electrical failures out of 22 microengines tested. Surprisingly, the electrical failures also arose in four microengines in our control group indicating that they were not vibration related. Failure analysis revealed that the electrical failures were due to shorting of stationary comb fingers to the ground plane.
Scattering to extinction cross-section ratios, ρse, were measured using the NIST Large Agglomerate Optics Facility for soot produced from ethene and acetylene laminar diffusion flames. Measurements were performed using light sources at 543.5 nm, 632.8 nm, and 856 nm. The average scattering to extinction cross-section ratios for these wavelengths are equal to 0.245, 0.195, and 0.195 for ethene and 0.311, 0.228, and 0.237 for acetylene. The 856 nm measurements represent the longest wavelength for which accurate scattering measurements have been performed for soot. The size distribution and fractal properties of the two soots were determined to assess the effects of limited acceptance angle range, finite size of the sensor, and departure from cosine response on the uncertainty in the measurement of ρse. The expanded relative uncertainty (95% confidence level) was found to be ±6% at the two visible wavelengths and ±8% at 856 nm. Both the magnitude and wavelength dependence of ρse for the present experiments are significantly different from those reported by Krishnan et al. [7] for overfire soot produced using a turbulent flame. The results are compared with the predictions of fractal optics.
In order to determine the susceptibility of our MEMS (MicroElectroMechanical Systems) devices to shock, tests were performed using haversine shock pulses with widths of 1 to 0.2 ms in the range from 500 g to 40,000 g. We chose a surface-micromachined microengine because it has all the components needed for evaluation: springs that flex, gears that are anchored, and clamps and spring stops to maintain alignment. The microengines, which were unpowered for the tests, performed quite well at most shock levels with a majority functioning after the impact. Debris from the die edges moved at levels greater than 4,000 g causing shorts in the actuators and posing reliability concerns. The coupling agent used to prevent stiction in the MEMS release weakened the die-attach bond, which produced failures at 10,000 g and above. At 20,000 g we began to observe structural damage in some of the thin flexures and 2.5-micron diameter pin joints. We observed electrical failures caused by the movement of debris. Additionally, we observed a new failure mode where stationary comb fingers contact the ground plane resulting in electrical shorts. These new failures were observed in our control group indicating that they were not shock related.
Dispersion of solutes in a variable aperture fracture results from a combination of molecular diffusion and velocity variations in both the plane of the fracture (macrodispersion) and across the fracture aperture (Taylor dispersion). We use a combination of physical experiments and computational simulations to test a theoretical model in which the effective longitudinal dispersion coefficient D(L) is expressed as a sum of the contributions of these three dispersive mechanisms. The combined influence of Taylor dispersion and macrodispersion results in a nonlinear dependence of D(L) on the Peclet number (Pe = V/D(m), where V is the mean solute velocity,is the mean aperture, and D(m) is the molecular diffusion coefficient). Three distinct dispersion regimes become evident: For small Pe (Pe << 1), molecular diffusion dominates resulting in D(L) proportional to Pe0; for intermediate Pe, macrodispersion dominates (D(L) proportional to Pe); and for large Pe, Taylor dispersion dominates (D(L) proportional to Pe2). The Pe range corresponding to these different regimes is controlled by the statistics of the aperture field. In particular, the upper limit of Pe corresponding to the macrodispersion regime increases as the macrodispersivity increases. Physical experiments in an analog, rough-walled fracture confirm the nonlinear Pe dependence of D(L) predicted by the theoretical model. However, the theoretical model underestimates the magnitude of D(L). Computational simulations, using a particle-tracking algorithm that incorporates all three dispersive mechanisms, agree very closely with the theoretical model predictions. The close agreement between the theoretical model and computational simulations is largely because, in both cases, the Reynolds equation describes the flow field in the fracture. The discrepancy between theoretical model predictions and D(L) estimated from the physical experiments appears to be largely, due to deviations from the local cubic law assumed by the Reynolds equation.
Structural dynamic systems are often attached to a support structure to simulate proper boundary conditions during testing. In some cases, the support structure is fairly simple and can be modeled by discrete springs and dampers. In other cases, the desired test conditions necessitate the use of a support structure which introduces dynamics of its own. For such cases, a more complex structural dynamic model is required to simulate the response of the full combined system. In this paper, experimental frequency response functions, admittance function modeling concepts, and least squares reductions are used to develop a support structure model including both translational and rotational degrees of freedom at an attachment location. Subsequently, the modes of the support structure are estimated, and a NASTRAN model is created for attachment to the tested system.