Showing papers by "Sandia National Laboratories published in 1994"

Robust two-dimensional weighted and unweighted phase unwrapping that uses fast transforms and iterative methods

Abstract: Two-dimensional (2D) phase unwrapping continues to find applications in a wide variety of scientific and engineering areas including optical and microwave interferometry, adaptive optics, compensated imaging, and synthetic-aperture-radar phase correction, and image processing. We have developed a robust method (not based on any path-following scheme) for unwrapping 2D phase principal values (in a least-squares sense) by using fast cosine transforms. If the 2D phase values are associated with a 2D weighting, the fast transforms can still be used in iterative methods for solving the weighted unwrapping problem. Weighted unwrapping can be used to isolate inconsistent regions (i.e., phase shear) in an elegant fashion.

Phase gradient autofocus-a robust tool for high resolution SAR phase correction

Abstract: The phase gradient autofocus (PGA) technique for phase error correction of spotlight mode synthetic aperture radar (SAR) imagery is examined carefully in the context of four fundamental signal processing steps that constitute the algorithm. We demonstrate that excellent results over a wide variety of scene content, and phase error function structure are obtained if and only if all of these steps are included in the processing. Finally, we show that the computational demands of the fun PGA algorithm do not represent a large fraction of the total image formation problem, when mid to large size images are involved. >

Frequency-resolved optical gating with the use of second-harmonic generation

Abstract: We discuss the use of second-harmonic generation (SHG) as the nonlinearity in the technique of frequency-resolved optical gating (FROG) for measuring the full intensity and phase evolution of an arbitrary ultrashort pulse. FROG that uses a third-order nonlinearity in the polarization-gate geometry has proved extremely successful, and the algorithm required for extraction of the intensity and the phase from the experimental data is quite robust. However, for pulse intensities less than ~1 MW, third-order nonlinearities generate insufficient signal strength, and therefore SHG FROG appears necessary. We discuss the theoretical, algorithmic, and experimental considerations of SHG FROG in detail. SHG FROG has an ambiguity in the direction of time, and its traces are somewhat unintuitive. Also, previously published algorithms are generally ineffective at extracting the intensity and the phase of an arbitrary laser pulse from the SHG FROG trace. We present an improved pulse-retrieval algorithm, based on the method of generalized projections, that is far superior to the previously published algorithms, although it is still not so robust as the polarization-gate algorithm. We discuss experimental sources of error such as pump depletion and group-velocity mismatch. We also present several experimental examples of pulses measured with SHG FROG and show that the derived intensities and phases are in agreement with more conventional diagnostic techniques, and we demonstrate the high-dynamic-range capability of SHG FROG. We conclude that, despite the above drawbacks, SHG FROG should be useful in measuring low-energy pulses.

Low threshold voltage vertical-cavity lasers fabricated by selective oxidation

Abstract: Novel vertical-cavity surface emitting lasers fabricated using selective oxidation to form a current aperture under a top monolithic distributed Bragg reflector mirror are reported. Large cross-sectional area lasers (259 µm2) exhibit threshold current densities of 150 A/cm2 per quantum well and record low threshold voltage of 1.33 V. Smaller lasers (36 µm2) possess threshold currents of 900 µA with maximum output powers greater than 1 mW. The record performance of these oxidised vertical-cavity lasers arises from the low mirror series resistance and very efficient current injection into the active region.

Computer-aided mammographic screening for spiculated lesions.

Abstract: PURPOSE: To study the use of a computer vision method as a second reader for the detection of spiculated lesions on screening mammograms. MATERIALS AND METHODS: An algorithmic computer process for the detection of spiculated lesions on digitized screen-film mammograms was applied to 85 four-view clinical cases: 36 cases with cancer proved by means of biopsy and 49 cases with negative findings at examination and follow-up. The computer detections were printed as film with added outlines that indicated the suspected cancers. Four radiologists screened the 85 cases twice, once without and once with the computer reports as ancillary films. RESULTS: The algorithm alone achieved 100% sensitivity, with a specificity of 82%. The computer reports increased the average radiologist sensitivity by 9.7% (P = .005), moving from 80.6% to 90.3%, with no decrease in average specificity. CONCLUSION: The study demonstrated that computer analysis of mammograms can provide a substantial and statistically significant increase ...

Dynamics of irreversible island growth during submonolayer epitaxy.

Abstract: The nucleation and growth of two-dimensional islands during the submonolayer stage of epitaxial growth is studied with kinetic Monte Carlo simulations and mean-field rate equations. Previous work on irreversible growth is extended to include relaxation of island shapes by edge diffusion. Island morphologies range from ramified structures at low temperatures to compact, polygonal shapes at higher temperatures. Using a self-consistent calculation of the rate coefficients, [ital quantitative] agreement is obtained between the solution to coupled, mean-field rate equations and the simulation results for average quantities. The island size distribution function is described by a single universal scaling function.'' The average island size is the only important scale for determining island densities. It is shown that the general form of the scaling [ital ansatz] applies to wider range of coverages than anticipated previously. This scaling form is combined with the solution to the rate equations to explore explicitly the [ital D]/[ital F] dependence of the number density of islands ([ital D] is the surface diffusion coefficient and [ital F] is the deposition flux).

Treatment of Uncertainty in Performance Assessments for Complex Systems

Abstract: When viewed at a high level, performance assessments (PAs) for complex systems involve two types of uncertainty: stochastic uncertainty, which arises because the system under study can behave in many different ways, and subjective uncertainty, which arises from a lack of knowledge about quantities required within the computational implementation of the PA. Stochastic uncertainty is typically incorporated into a PA with an experimental design based on importance sampling and leads to the final results of the PA being expressed as a complementary cumulative distribution function (CCDF). Subjective uncertainty is usually treated with Monte Carlo techniques and leads to a distribution of CCDFs. This presentation discusses the use of the Kaplan/Garrick ordered triple representation for risk in maintaining a distinction between stochastic and subjective uncertainty in PAs for complex systems. The topics discussed include (1) the definition of scenarios and the calculation of scenario probabilities and consequences, (2) the separation of subjective and stochastic uncertainties, (3) the construction of CCDFs required in comparisons with regulatory standards (e.g., 40 CFR Part 191, Subpart B for the disposal of radioactive waste), and (4) the performance of uncertainty and sensitivity studies. Results obtained in a preliminary PA for the Waste Isolation Pilot Plant, an uncertainty and sensitivity analysis of the MACCS reactor accident consequence analysis model, and the NUREG-1150 probabilistic risk assessments are used for illustration.

A determination of the effective viscosity for the Brinkman–Forchheimer flow model

Abstract: The effective viscosity mu{sub e} for the Brinkman-Forchheimer flow (BFF) model has been determined experimentally for steady flow through a wall-bounded porous medium. Nuclear magnetic resonance (NMR) techniques were used to measure non-invasively the ensemble-average velocity profile of water flowing through a tube filled with an open-cell rigid foam of high porosity (phi = 0.972). By comparing these data with the BFF model, for which all remaining parameters were measured independently, it was determined that mu{sub e} = (7.5{sub -2.4}{sup +3.4}) mu{sub f}, where mu{sub f} was the viscosity of the fluid. The Reynolds number, based upon the square root of the permeability, was 17. 14 refs.

Intermetallic growth and mechanical behavior of low and high melting temperature solder alloys

Abstract: The presence of an intermetallic is often an indication of good wetting in a solder joint. However, excessive intermetallic growth and the brittleness of the intermetallic layer may be detrimental to joint reliability. This study examined the growth and mechanical behavior of interfacial intermetallics between copper and six solder alloys commonly used in electronics assembly. The solder alloys tested were 60Sn-40Pb, 63Sn-37Pb, 95Sn-5Sb, 96.5Sn-3.5Ag, 50Pb-50In, 50Sn-50In, and 40In-40Sn-20Pb. The 50Sn-50In and 40In-40Sn-20Pb exhibited faster solid state growth of the intermetallic layer at 100 °C as compared to the near-eutectic Sn-Pb control solder. The 50In-50Pb had a slower growth rate, relative to 63Sn-37Pb, at the aging temperature of 170 °C due to slower reaction rate kinetics of indium with copper. The 96.5Sn-3.5Ag and 95Sn-5Sb had similar intermetallic growth rates at 170 °C and 205 °C, and the aging was comparable to that of the 63Sn-37Pb alloy. The 95Sn-5Sb solder/copper intermetallic had a faster growth rate of the Cu3Sn layer than was observed in the Sn-Ag or Sn-Pb alloys. Modified fracture toughness and low load indentation tests were used to characterize the mechanical behavior of the intermetallics. The intermetallics were harder than both the base metal and the solder alloy. The fracture behavior of the joints in tension was dependent upon the strength of the solder alloy. Solders with low strengths failed in the solder by plastic deformation. The failure of solders with higher strengths was dependent upon intermetallic thickness. When the intermetallic was thin, fracture occurred in the solder or at the solder/ intermetallic interface. As the interfacial intermetallic thickened, the fracture path moved into the intermetallic layer.

Physical mechanisms contributing to enhanced bipolar gain degradation at low dose rates

Abstract: We have performed capacitance-voltage (C-V) and thermally-stimulated-current (TSC) measurements on non-radiation-hard MOS capacitors simulating screen oxides of modern bipolar technologies. For O-V irradiation at /spl sim/25/spl deg/C, the net trapped-positive-charge density (N/sub ox/) inferred from midgap C-V shifts is /spl sim/25-40% greater for low-dose-rate ( 100 rad(SiO/sub 2/)/s) exposure. Device modeling shows that such a difference in screen-oxide N/sub ox/ is enough to account for the enhanced low-rate gain degradation often observed in bipolar devices, due to the /spl sim/exp(N/sub ox//sup 2/) dependence of the excess base current. At the higher rates, TSC measurements reveal a /spl sim/10% decrease in trapped-hole density over low rates. Also, at high rates, up to /spl sim/2.5-times as many trapped holes are compensated by electrons in border traps than at low rates for these devices and irradiation conditions. Both the reduction in trapped-hole density and increased charge compensation reduce the high-rate midgap shift. A physical model is developed which suggests that both effects are caused by time-dependent space charge in the bulk of these soft oxides associated with slowly transporting and/or metastably trapped holes (e.g. in E/sub /spl delta//' centers). On the basis of this model, bipolar transistors and screen-oxide capacitors were irradiated at 60/spl deg/C at 200 rad(SiO/sub 2/)/s in a successful effort to match low-rate damage. These surprising results provide insight into enhanced low-rate bipolar gain degradation and suggest potentially promising new approaches to bipolar and BiCMOS hardness assurance for space applications. >

Structural studies of complex systems using small-angle scattering: a unified Guinier/power-law approach☆

Abstract: A unified analysis method for small-angle scattering data is demonstrated by surveying complex systems that display multiple size-scale structures. Using this approach the relationship between micro- and nano-structures can be ascertained. The method uses a function that is general enough to adequately describe systems ranging from particulates with fractally rough interfaces to mass fractals such as polymer coils. Additionally multiple Guinier and power-law regimes can be treated. The unified method can distinguish Guinier regimes buried between two power-law regimes. Data from particulate filled systems, low crystallinity polymers and low density polymer foams are analyzed.

Photoinduced hysteresis changes and optical storage in (Pb,La)(Zr,Ti)O3 thin films and ceramics

Abstract: Pb(Zr,Ti)O3 and (Pb,La)(Zr,Ti)O3 thin films and bulk ceramics are shown to exhibit two distinct, but related types of photoinduced changes in their hysteresis behavior: (1) a photoinduced suppression of the switchable polarization and (2) a photoinduced voltage shift. Both effects give rise to stable and reproducible hysteresis changes and, thus, either could be the basis of an optical memory. Both phenomena can be explained by trapping of photogenerated charge at domain boundaries to minimize internal depolarizing fields. The space‐charge field that causes the voltage‐shift effect is primarily due to the migration and subsequent trapping of electrons. However, the thickness dependence of the voltage shift implies that the trapped charge is not confined to the interface. The voltage‐shift kinetics exhibit a stretched‐exponential dependence, whereas the polarization‐suppression effect follows an exponential time dependence. However, both effects exhibit similar relaxation times. In addition, the relaxation...

Efficient semiconductor light-emitting device and method

Abstract: A semiconductor light-emitting device and method. The semiconductor light-emitting device is provided with at least one control layer or control region which includes an annular oxidized portion thereof to channel an injection current into the active region, and to provide a lateral refractive index profile for index guiding the light generated within the device. A periodic composition grading of at least one of the mirror stacks in the device provides a reduced operating voltage of the device. The semiconductor light-emitting device has a high efficiency for light generation, and may be formed either as a resonant-cavity light-emitting diode (RCLED) or as a vertical-cavity surface-emitting laser (VCSEL).

PRISM theory of the structure, thermodynamics, and phase transitions of polymer liquids and alloys

Abstract: The recent development of a microscopic theory of the equilibrium properties of polymer solutions, melts and alloys based on off-lattice Polymer Reference Interaction Site Model (PRISM) integral equation methods is reviewed. Analytical and numerical predictions for the intermolecular structure and collective density scattering patterns of both coarse-grained and atomistic models of polymer melts are presented and found to be in good agreement with large scale computer simulations and diffraction measurements. The general issues and difficulties involved in the use of the structural information to compute thermodynamic properties are reviewed. Detailed application of a hybrid PRISM approach to calculate the equation-of-state of hydrocarbon fluids is presented and found to reproduce accurately experimental PVT data on polyethylene. The development of a first principles off-lattice theory of polymer crystallization based on a novel generalization of modern thermodynamic density functional methods is discussed. Numerical calculations for polyethylene and polytetrafluoroethylene are in good agreement with the experimental melting temperatures and liquid freezing densities. Generalization of the PRISM approach to treat phase separating polymer blends is also discussed in depth. The general role of compressibility effects in determining small angle scattering patterns, the effective chi-parameter, and spinodal instability curves are presented. New theoretical concepts and closure approximations have been developed in order to describe correctly long wavelength concentration fluctuations in macromolecular alloys. Detailed numerical and analytical applications of the PRISM theory to model athermal and symmetric blends are presented, and the role of nonmean field fluctuation processes are established. Good agreement between the theory and computer simulations of simple symmetric polymer blends has been demonstrated. Strong, nonadditive compressibility effects are found for structurally and/or interaction asymmetric blends which have significant implications for controlling miscibility in polymer alloys. Recent generalizations of PRISM theory to treat block copolymer melts, and nonideal conformational perturbations, are briefly described. The paper concludes with a brief summary of ongoing work and fertile directions for future research.

Control of vertical-cavity laser polarization with anisotropic transverse cavity geometries

Abstract: We show the two-fold polarization degeneracy of etched air-post vertical-cavity surface emitting laser diodes can be lifted and a dominant polarization state selected through use of anisotropic transverse laser cavity geometries. For lasers with rhombus-shaped cavities, fundamental mode lasing emission linearly polarized along one specified crystal axis is obtained up to twice the threshold current. For dumbbell-shaped lasers, fundamental mode lasing emission linearly polarized along one specified crystal axis is maintained over the entire operating range of the device producing a maximum orthogonal polarization suppression ratio of 14 dB. >

Sensing liquid properties with thickness-shear mode resonators

Abstract: The responses of smooth- and textured-surface thickness-shear mode (TSM) resonators in liquid are examined. Smooth devices, which viscously entrain a layer of contacting liquid, exhibit a response that depends on the product of liquid density and viscosity. Textured-surface devices, which also trap liquid in surface crevices, pores, etc., exhibit an additional response that depends on liquid density alone. Combining smooth- and textured-surface resonators in a monolithic sensor enables the liquid density and viscosity to be extracted simultaneously.

Temperature dependence of gain‐guided vertical‐cavity surface emitting laser polarization

Abstract: We show the polarization characteristics of gain‐guided vertical‐cavity surface emitting lasers are related to the temperature‐dependent cavity optical resonance and laser gain spectral alignment. Simultaneous nearly degenerate orthogonal eigen polarization states are observed at and above lasing threshold. The partitioning of power between the linear polarization states is shown to depend on the relative spectral overlap of the cavity resonance of each state with the gain. Near the condition of cavity resonance/gain alignment, an abrupt switch in the dominant eigen polarization with a region of polarized output fluctuations is evident. Rotation of the eigen polarization directions relative to the crystal axes is also observed at temperatures where the gain is blue shifted from the cavity resonances.

Defect classes-an overdue paradigm for CMOS IC testing

Abstract: The IC test industry has struggled for move than 30 years to establish a test approach that would guarantee a low defect level to the customer. We propose a comprehensive strategy for testing CMOS ICs that uses defect classes based on measured defect electrical properties. Defect classes differ from traditional fault models. Our defect class approach requires that the rest strategy match the defect electrical properties, while fault models require that IC defects match the fault definition. We use data from Sandia Labs failure analysis and test facilities and from public literature. We describe test pattern requirements for each defect class and propose a test paradigm.

Ion-assisted pulsed laser deposition of cubic boron nitride films

Abstract: Ion‐assisted pulsed laser deposition has been used to produce films containing ≳85% sp3‐bonded cubic boron nitride (c‐BN) By ablating from a target of hexagonal boron nitride (h‐BN), BN films have been deposited on heated (50–800 °C) Si(100) surfaces The growing films are irradiated with ions from a broad beam ion source operated with Ar and N2 source gasses Successful c‐BN synthesis has been confirmed by Fourier transform infrared (FTIR) spectroscopy, high‐resolution transmission electron microscopy (TEM), selected‐area electron diffraction, electron energy‐loss spectroscopy, and x‐ray diffraction The films are polycrystalline and show grain sizes up to 300 A In addition, Rutherford backscattering, elastic recoil detection, and Auger electron spectroscopies have been used to further characterize the samples The effects of varying ion current density, substrate growth temperature, growth time, and ion energy have been investigated It is found that stoichiometric films with a high c‐BN percentage ca

Micromirror optical-fiber hydrogen sensor

Abstract: In a recent letter (M.A. Butler, J. Electrochem. Soc., 138 (1991) L46) a new hydrogen sensor was reported that operates by measuring changes in the reflectivity of thin palladium films deposited on the end of an optical fiber when exposed to hydrogen gas. In this paper we explore in more detail the mechanisms of operation of this sensor. Two effects are observed: a reversible micro-blistering of the films on exposure to hydrogen and changes in optical constants with hydriding. Use of a thin nickel layer under the palladium to improve adhesion results in large changes in optical response. These changes are attributed to clamping effects on the film.

Pulse retrieval in frequency-resolved optical gating based on the method of generalized projections.

Abstract: We use the algorithmic method of generalized projections (GP's) to retrieve the intensity and phase of an ultrashort laser pulse from the experimental trace in frequency-resolved optical gating (FROG). Using simulations, we show that the use of GP's improves significantly the convergence properties of the algorithm over the basic FROG algorithm. In experimental measurements, the GP-based algorithm achieves significantly lower errors than previous algorithms. The use of GP's also permits the inclusion of an arbitrary material response function in the FROG problem.