Publications

Probabilistic forecasting techniques can be used in the treatment of uncertainties in the cost and duration of programmatic alternatives on risk and performance assessment projects. Where significant uncertainties exist and where programmatic decisions must be made despite existing uncertainties, probabilistic techniques may yield important insights into decision options, especially when used in a decision analysis framework and when properly balanced with deterministic analyses. An example application of probabilistic forecasting is presented and described.

We have realized a new class of high-Q resonant cavity using two-dimensional photonic bandgap (PBG) structures and showed that its Q-value can be as high as approximately 23,000 in the mm-wave regime. We further show that its modal properties, such as the resonant frequency, modal linewidth and number of modes, can be tuned by varying the cavity size. In addition, we present a new nano-fabrication technique for constructing PBG resonant cavities in the near infrared and visible spectral regime.

This paper presents a set of motion planners for an exploration vehicle on a simulated rugged terrain. The vehicle has four wheels for its movement and a robotic arm mounted on the vehicle for object manipulation. Given a target point to reach with the hand of the arm, our planners first compute a path for the vehicle to the vicinity of the target, then compute an optimal vehicle position from which the arm can reach the target point, and then plans a path for the arm to reach the target. The vehicle path is planned in two stages. A rough path is planned considering only global features of the terrain, and the path is modified by a local planner to avoid more detailed features of the terrain. The planners are expected to increase the autonomy of robots and improve the efficiencies of exploration missions.

Metallorganic chemical vapor deposition (MOCVD) technology is increasingly recognised as a superior platform for growth of vertical-cavity surface-emitting lasers (VCELs) because of its high throughput, low surface defect density, continuous compositional grading control, and the flexibility for materials and dopant choices. In this paper, it is shown that it is also capable of extremely high wafer uniformity and run-to-run reproducibility.

Frequency modulation is demonstrated in a ring-cavity KTP OPO seeded by frequency-modulated Ti:SAP light. The singly resonant OPO is pumped by a single-longitudinal-mode 532-nm Nd:YAG light, and the 800-nm signal seed is modulated at 3.7 GHz to match the OPO cavity's free spectral range. A comparison is presented of OPO operation with FM and AM seeds that demonstrates the dramatic difference in spectral properties and pulse profiles for the two modulation types. FM modulated absorption measurements made using FM OPO is also demonstrated.

The ability to predict the mechanical response of rock in three dimensions over the spatial and time scales of geologic interest would give the oil and gas industry the ability to reduce risk on prospects, improve pre-project initial reserve estimates, and lower operating costs. A program has recently been initiated, under the auspices the Advanced Computational Technology Initiative (ACTI), to achieve such a computational technology breakthrough by adapting the unique advanced quasistatic finite element technology developed by Sandia to the mechanics applications important to exploration and production activities within the oil and gas industry. As a pre-cursor to that program, in an effort to evaluate the feasibility of the approach, several complex geologic structures of interest were analyzed with the existing two-dimensional quasistatic finite element code, SANTOS, developed at Sandia. Some examples will be presented and discussed in this paper.

The surface stoichiometry, surface morphology, and electrical conductivity of AlN, GaN, InN, InGaN, and InAlN were examined at rapid thermal annealing temperatures up to 1150 °C. The sheet resistance of the AlN dropped steadily with annealing, but the surface showed signs of roughening only above 1000 °C. Auger electron spectroscopy (AES) analysis showed little change in the surface stoichiometry even at 1150 °C. GaN root mean square (rms) surface roughness showed an overall improvement with annealing, but the surface became pitted at 1000 °C, at which point the sheet resistance also dropped by several orders of magnitude, and AES confirmed a loss of N from the surface. The InN surface had roughened considerably even at 650 °C, and scanning electron microscopy showed significant degradation. In contrast to the binary nitrides, the sheet resistance of InAlN was found to increase by ∼102 from the as grown value (3.2×10-3 Ω cm) after annealing at 800 °C and then remain constant up to 1000 °C, while that of InGaN increased by two orders of magnitude between 700 and 900 °C. The rms roughness increased above 800 and 700 °C, respectively, for InAlN and InGaN samples. In droplets began to form on the surface at 900 °C for InAlN and at 800 °C for InGaN, and then evaporate at 1000 °C, leaving pits. AES analysis showed a decrease in the N concentration in the top 500 Å of the sample for annealing ≥800 °C in both materials. © 1996 American Vacuum Society.

The performance of 21 PV-powered low pressure sodium lighting systems on a multi-use pathway has been documented in this paper. Specific areas for evaluation include the constant voltage and on/off PV charge controllers, flooded deep-cycle lead-antimony and valve regulated lead-acid (VRLA) gel batteries, low pressure sodium ballasts and lights, and vandal resistant PV modules. The PV lighting system lessons learned and maintenance intervals have been documented over the past 2.5-years. The above performance data has shown that with careful hardware selection, installation, and maintenance intervals the PV lighting systems will operate reliably.

We have synthesized periodic mesoporous silica thin films (PMSTF) from homogeneous solutions. To synthesize the films a thin layer of a pH = 7 micellar coating solution that contains TMOS is dip- or spin-coated onto silicon wafers, borosilicate glass, or quartz substrates. Ammonia gas is diffused into the solution and causes rapid hydrolysis and condensation of the TMOS and the formation of periodic mesoporous thin films within approximately 10 seconds. The combination of homogeneous solutions and rapid product formation maximizes the concentration of desired product and provides a controlled, predictable microstructure. The films have been made continuous and crack-free by optimizing initial silica concentration and film thickness.

Understanding high-pressure material behavior is crucial to address the physical processes associated with a variety of hypervelocity impact events related to space sciences such as orbital-debris impact on a debris shield. At very high impact velocities material properties will be dominated by phase-changes, such as melting or vaporization, which cannot be achieved at lower impact velocities. Development of well-controlled and repeatable hypervelocity launch capabilities is the first step necessary to improve our understanding of material behavior at extreme pressures and temperatures not currently available using conventional two-stage light-gas gun techniques. In this paper, techniques used to extend the launch capabilities of a two-stage light gas gun to 16 km/s are described. It is anticipated that this technology will be useful in testing, evaluating, and design of various debris shields proposed for use with many different spacecrafts before deployment.

We have used a novel technique, measurement of stress isotherms in microporous thin films, as a means of characterizing porosity. The stress measurement was carried out by applying sol-gel thin films on a thin silicon substrate and monitoring the curvature of the substrate under a controlled atmosphere of various vapors. The magnitude of macroscopic bending stress developed in microporous films depends on the relative pressure and molar volume of the adsorbate and reaches a value of 180 MPa for a relative vapor pressure, P/Po = 0.001, of methanol. By using a series of molecules, and observing both the magnitude and the kinetics of stress development while changing the relative pressure, we have determined the pore size of microporous thin films. FTIR measurements were used to acquire adsorption isotherms and to compare pore emptying to stress development, about 80% of the change in stress takes place with no measurable change in the amount adsorbed. We show that for sol-gel films, pore diameters can be controlled in the range of 5-8 angstroms by `solvent templating'.

Electron cyclotron resonance (ECR) etching of GaP, GaAs, InP, and InGaAs are reported as a function of percent chlorine-containing gas for Cl2/Ar, Cl2/N2, BCl3/Ar, and BCl3/N2 plasma chemistries. GaAs and GaP etch rates were faster than InP and InGaAs, independent of plasma chemistry due to the low volatility of the InClx etch products. GaAs and GaP etch rates increased as %Cl2 was increased for Cl2/Ar and Cl2/N2 plasmas. The GaAs and GaP etch rates were much slower in BCl3-based plasmas due to lower concentrations of reactive Cl, however enhanced etch rates were observed in BCl3/N2 at 75% BCl3. Smooth etched surfaces were obtained over a wide range of plasma chemistries.

Development of a complementary heterostructure field effect transistor (CHFET) technology for low-power, mixed-mode digital-microwave applications is presented. An earlier digital CHFET technology with independently optimizable transistors which operated with 319 ps loaded gate delays at 8.9 fJ is reviewed. Then work demonstrating the applicability of the digital nJFET device as a low-power microwave transistor in a hybrid microwave amplifier without any modification to the digital process is presented. A narrow band amplifier with a 0.7 × 100 μm nJFET as the active element was designed, constructed, and tested. At 1 mW operating power, the amplifier showed 9.7 dB of gain at 2.15 GHz and a minimum noise figure of 2.5 dB. In addition, next generation CHFET transistors with sub 0.5 μm gate lengths were developed. Cutoff frequencies, ft of 49 GHz and 11.5 GHz were achieved for n- and p-channel FETs with 0.3 and 0.4 μm gates, respectively. These FETs will enable both digital and microwave circuits with enhanced performance.

A new semi-Analytical model describing the entry and deformation of meteoroids entering planetary atmospheres has been developed and calibrated against numerical simulations performed using the CTH shock-physics computational hydrocode. The model starts with the classical treatment of meteoroid ablation which is modified to include an explicit treatment of energy conservation during the ablative process. This is reconciled with terrestrial observations by modeling the formation of a vapor/debris layer (the visible bolide) surrounding the central meteoroid. A mechanical deformation model based on long-wavelength hydrodynamic instability growth is added and calibrated against numerical simulations performed with CTH. The analytical model provides initial conditions for numerical fireball simulations which are compared with observations of the Comet Shoemaker-Levy 9 impact on Jupiter and can be used to assess the terrestrial impact hazard. © 1996 American Society of Civil Engineers.

This paper describes a study of the underlying physical mechanisms governing the threshold properties of a VCSEL. In particular, it theoretically and experimentally evaluates the mechanisms that effect the threshold properties as a function of emission wavelength. Other important issues, such as the dependence of the threshold properties on microcavity dimensions, we discussed.

Tunneling absorption is calculated in weakly-coupled n-type asymmetric double quantum wells in an in-plane magnetic field using a linear response theory. Tunneling absorption of photons occurs between the ground sublevels of the quantum wells. We show that the absorption threshold, the resonance energy of absorption, and the linewidth depend sensitively on the magnetic field and the temperature. © 1996 Academic Press Limited.

Thin-film decoupling capacitors based on ferroelectric (Pb,La)(Zr,Ti)O3 films are being developed for use in advanced packaging applications. The increased integration that can be achieved by replacing surface-mount capacitors should lead to decreased package volume and improved high-speed performance. For this application, chemical solution deposition is an appropriate fabrication technique since it is a low-cost, high-throughput process. The use of relatively thick Pt electrodes (approximately 1 μm) to minimize series resistance and inductance is a unique aspect to fabricating these devices. In addition, the important electrical properties are discussed, with particular emphasis on lifetime measurements, which suggest that resistance degradation will not be a severe limitation on device performance. Finally, some of the work being done to develop methods of integrating these thin-film capacitors with ICs and MCMs is presented.

La0.5Sr0.5CoO3 (LSCO) thin films have been deposited, using RF magnetron sputter-deposition for use as an electrode material for Pb(Zr,Ti)O3 (PZT) thin film capacitors. The effect of the O2:Ar sputter gas ratio during deposition, on the LSCO film properties was investigated. It was found that the resistivity of the LSCO films deposited at ambient temperature decreases as the O2:Ar ratio was increased for both the as-deposited and annealed films. In addition, it was found that thin overlayers of LSCO tend to stabilize the underlying Pt//Ti electrode structure during subsequent thermal processing. The LSCO//Pt//Ti composite electrode stack has a low resistivity and provides excellent fatigue performance for PZT capacitors. Furthermore, the LSCO//Pt//Ti electrode sheet resistance does not degrade with annealing temperature and the electrode does not display hillock formation. Possible mechanisms for the stabilization of the Pt//Ti electrode with LSCO overlayers will be discussed.

The relative viscosity of concentrated suspensions of mixtures of rodlike and spherical particles are measured by falling-ball rheometry. The suspensions are well mixed and homogeneous in the sense that the particles are well dispersed and the rods are randomly oriented. For a constant total volume fraction of solids, the addition of spheres to suspensions of rods results in large decrease in the relative viscosity of the suspension. In these experiments the length of the suspended rods is approximately 10 times the diameter of the suspended spheres. Due to this difference in the characteristic sizes of the two types of particles, the spheres may be considered as part of the suspending homogeneous continuum. A simple model based on this physical picture, after Farris [1968], is very successful in predicting the relative viscosity of the mixed suspensions.

In recent studies, we used the Interfacial Force Microscope in a nanoindenter mode to survey the nanomechanical properties of Au films grown on various substrates. Quantitative tabulations of the indentation modulus and the maximum shear stress at the plastic threshold showed consistent values over individual samples but a wide variation from substrate to substrate. These values were compared with film properties such as the surface roughness, average grain size and interfacial adhesion and no correlation was found. However, in a subsequent analysis of the results, we found consistencies which support the integrity of the data and point to the fact that the results are sensitive to some property of the various film/substrate combinations. In the present paper, we discuss these consistencies and show recent measurements which strongly suggest that the property that is being probed is the residual stress in the films caused by their interaction with the substrate surfaces.

The MITS multigroup/continuous-energy electron-photon Monte Carlo transport code system has matured to the point that it is capable of addressing more realistic three-dimensional adjoint applications. It is first employed to efficiently predict point doses as a function of source energy for simple threedimensional experimental geometries exposed to planar sources of monoenergetic electrons up to 4. 0 MeV due to simulated uniform isotropic fluences. Results are in very good agreement with experimental data. It is then used to efficiently simulate dose to a detector in a subsystem of a GPS satellite from the natural electron environment, employing a relatively complex model of the satellite. The capability for survivability analysis of space systems is demonstrated, and results are obtained with and without variance reduction. © 1996 IEEE.

The study of gain properties in group-III nitride quantum wells is complicated by several factors. In view of this, an approach is presented that involves a first-principles bandstructure calculation, the results of which are incorporated into a microscopic laser theory. The band structure calculation applies a density-functional method. This method provides a single analytical model for computing the group-II nitride material properties, thus ensuring consistency in the values for the different bandstructure parameters, and circumventing the discrepancies present in the literature due to different experimental conditions, or different computational methods. With a complete set of the relevant material parameters, it is possible to study the effects of strain and quantum confinement.

Monochromatic imaging was used to investigate the excited-state density distributions of Fe and Fe+ in the inter-electrode gap region of a 3,100 A dc metal vapor arc burning between molten iron surfaces in a vacuum arc furnace. Multiple images were acquired at four wavelengths. The images were corrected and Abel inverted to yield the absolute radial intensity distributions for Fe and Fe+ in the inter-electrode gap region. The results show a structured, axisymmetric plasma consisting of a high density 'core' of Fe+ emitters centered between the electrode surfaces situated against a relatively broad, flat excited-state Fe distribution.

This paper examines a novel optimization technique called genetic algorithms and its application to the optimization of reliability allocation strategies. Reliability allocation should occur in the initial stages of design, when the objective is to determine an optimal breakdown or allocation of reliability to certain components or subassemblies in order to meet system objectives. The reliability allocation optimization is applied to the design of a cluster tool, a highly complex piece of equipment used in semiconductor manufacturing. The problem formulation is presented, including decision variables, performance measures and constraints, and genetic algorithm parameters. Piecewise “effort curves” specifying the amount of effort required to achieve a certain level of reliability for each component or subassembly are defined. The genetic algorithm evolves or picks those combinations of “effort” or reliability levels for each component which optimize the objective of maximizing Mean Time Between Failures while staying within a budget. The results show that the genetic algorithm is very efficient at finding a set of robust solutions. A time history of the optimization is presented, along with histograms of the solution space fitness, MTBF, and cost for comparative purposes.

We present the results of recent failure analysis of an advanced, 0.5 um, fully planarized, triple metallization CMOS technology. A variety of failure analysis (FA) tools and techniques were used to localize and identify defects generated by wafer processing. These include light (photon) emission microscopy (LE), fluorescent microthermal imaging (FMI), focused ion beam cross sectioning, SEM/voltage contrast imaging, resistive contrast imaging (RCI), and e-beam testing using an IDS-5000 with an HP 82000. The defects identified included inter- and intra-metal shorts, gate oxide shorts due to plasma processing damage, and high contact resistance due to the contact etch and deposition process. Root causes of these defects were determined and corrective action was taken to improve yield and reliability.