Publications

We discuss the selective conversion of buried layers of AlGaAs to a stable oxide and the implementation of this oxide into high performance vertical-cavity surface emitting lasers (VCSELs). The rate of lateral oxidation is shown to be linear with an Arrhenius temperature dependence. The measured activation energies vary with Al composition, providing a high degree of oxidation selectivity between AlGaAs alloys. Thus buried oxide layers can be selectively fabricated within the VCSEL through small compositional variations in the AlGaAs layers. The oxidation of AlGaAs alloys, as opposed to AlAs, is found to provide robust processing of reliable lasers. The insulating and low refractive index oxide provides enhanced electrical and optical confinement for ultralow threshold currents in oxide-apertured VCSELs.

The ability of high gain GaAs Photoconductive Semiconductor switches (PCSS) to deliver high peak power, fast risetime pulses when triggered with small laser diode arrays makes them suitable for their use in radars that rely on fast impulses. This type of direct time domain radar is uniquely suited for observation of large structures under ground because it can operate at low frequencies and at high average power. This paper will summarize the state-of-the-art in high gain GaAs switches and discuss their use in a radar transmitter. We will also present a summary of an analysis of the effectiveness of different pulser geometries that result in transmitted pulses with varying frequency content. To this end we developed a simple model that includes transmit and receive antenna response, attenuation and dispersion of the electromagnetic impulses by the soil, and target cross sections.

Gallium nitride (GaN) and related III-Nitride materials (AlN, InN) have recently been the focus of extensive research for photonic and electronic device applications. As this material system matures, ion implantation doping and isolation is expected to play an important role in advance device demonstrations. To this end, we report the demonstration of implanted p-type doping with 24Mg+31P and 40Ca as well as n-type doping with Si in GaN. These implanted dopants require annealing approximately 1100 °C to achieve electrical activity, but demonstrate limited redistribution at this temperature. The redistribution of other potential dopants in GaN (such as Be, Zn, and Cd) will also be reported. Results for a GaN junction field effect transistor (JFET), the first GaN device to use implantation doping, will also be presented.

A survey is made of acoustic devices that are suitable as gas and vapor sensors. This survey focuses on attributes such as operating frequency, mass sensitivity, quality factor (Q), and their ability to be fabricated on a semiconductor substrate to allow integration with electronic circuitry. The treatment of the device surface with chemically-sensitive films to detect species of interest is discussed. Strategies for improving discrimination are described, including sensor arrays and species concentration and separation schemes. The advantages and disadvantages of integrating sensors with microelectronics are considered, along with the effect on sensitivity of scaling acoustic gas sensors to smaller size.

An accurate first-principles analysis to probe the threshold properties of selectively oxidized vertical-cavity surface-emitting lasers (VCSELs) were developed. The analysis indicates that in order to achieve ultralow threshold, oxide aperture scattering loss and leakage currents must be addressed. The agreement between calculations and experiment solidify the understanding and enable the identification of fundamental limitations of low threshold VCSEL operation. The performance and analysis of modified VCSEL designs are presented.

A monochrome 2f optical performance measurement system was developed at Sandia. To meet the goals of an optical test method that can be done in a relatively short period of time, requires little space, uses `off the shelf' test equipment, and provides a quantitative measure adequate to address quality control requirements, necessitates conversion of the system to use color. This test method is based on common ray trace calculations for targets and images at the radius of curvature for spherical and parabolic (f/D>3) concentrators. The implementation of a color system involved changing hardware and software. Target design - the layout, materials, and color selection - is a primary consideration. As the system development neared completion, it was used in several applications to measure solar concentrator facet performance and evaluate system performance. Included in this testing was a side-by-side test with the SHOT system at the National Renewable Energy Laboratory. This paper discusses the development of the color system hardware, reviews the results of testing, and presents requirements for the color system software.

This paper describes our initial investigations to determine the suitability of Ion Mobility Spectrometry (IMS) as an on-line, real-time monitor for ultrapure water systems. We have found that acetone, ethylene glycol, isopropanol, chloroform, and n-methyl pyrrolidone (NMP) can be detected at parts-per-million levels or lower using this technique. As expected, surfactants, such as dodecylsulfate sodium salt, dodecyl-trimethylammonium bromide, and hexadecyl trimethylammonium bromide were not detected. Several sample introduction schemes are proposed for continual monitoring of ultrapure water (UPW) streams.

In this work, high-energy electron beam brazing of a ceramic part is modeled numerically. The part considered consists of a ceramic cylinder and disk between which is sandwiched an annular washer of braze material. An electron beam impinges on the disk, melting the braze metal. The resulting coupled electron-photon and thermal transport equations are solved using Monte Carlo and finite element techniques respectively. Results indicate that increased electron beam current decreases the time required to melt the braze while increasing temperature gradients in the ceramic near the braze. Furnace brazing was also simulated and predicted results indicate increased processing times relative to electron beam brazing.

An oscillator circuit has been developed at Sandia National Laboratories that uses Inverted-Mesa resonators, in a high-precision VCO application, at frequencies historically dominated by SAW designs. This design incorporates a frequency tripler that provides a 600 MHz output capability using a 200 MHz 3rd overtone resonator. This design has some advantages over equivalent SAW alternatives: lower power-consumption, superior aging characteristics, linear frequency pulling and low frequency versus temperature sensitivity. The VCO presented demonstrates <+/-60 ppm pullability (0 to 7 V control), tuning linearity better than +/-5% with phase noise at 1 kHz >-110 dBc/Hz. This oscillator-tripler exploits the nonlinear characteristics of an emitter-coupled-pair differential amplifier to obtain a high-performance oscillator design.

The planarization technology of chemical-mechanical- polishing (CMP), used for the manufacturing of multilevel metal interconnects for high-density integrated circuits, is also readily adaptable as an enabling technology in micro- electro-mechanical systems (MEMS) fabrication, particularly polysilicon surface micromachining. CMP not only eases the design and manufacturability of MEMS devices by eliminating several photolithographic and film issues generated by severe topography, but also enables far greater flexibility with process complexity and associated designs. Thus, the CMP planarization technique alleviates processing problems associated with fabrication of multilevel polysilicon structures, eliminates design constraints linked with non- planar topography, and provides an avenue for integrating different process technologies. Examples of these enhancements include: a simpler extension of surface micromachining fabrication to multiple mechanical layers, a novel method of monolithic integration of electronics and MEMS, and a novel combination of bulk and surface micromachining.

A release etch model for etching sacrificial oxides in aqueous HF solutions is presented. This model is an extension of work done by Monk et. al. and Liu et. al The model is inherently one dimensional, but can be used to model the etching of complex three dimensional parts. Solutions and boundary conditions are presented for a number of geometries.

An electromechanical model of Sandia's microengine is developed and applied to quantify critical performance tradeoffs. This is done by determining how forces impact the mechanical response of the engine to different electrical drive signals. To validate the theoretical results, model- based drive signals are used to operate actual engines, where controlled operation is achieved for the following cases: 1) spring forces are dominant, 2) frictional forces are dominant, 3) linear inertial forces are dominant, 4) viscous damping forces are dominant, and 5) inertial load forces are dominant. Significant improvements in engine performance are experimentally demonstrated in the following areas: positional control, start/stop endurance, constant speed endurance, friction load reduction,and rapid actuation of inertial loads.

For high-speed integrated circuit applications, it is important to interconnect decoupling capacitors and integrated circuits (ICs) as intimately as possible, to minimize parasitic impedances. This can be achieved by mounting freestanding, thin film capacitors directly onto ICs as part of a chip-scale packaging approach. These `applique' capacitors utilize a chemically-prepared PLZT dielectric, which is nominally 1 μm thick. The small size and weight of applique capacitors can be used to improve packaging efficiency. Applique capacitors, which are initially fabricated on silicon wafers, have high permittivity (ε≅1000), low loss (tanδ≅0.01) and high breakdown strength (EB≅1 MV/cm) and leakage resistance (ρ>1014 Ω-cm 125 °C). Various processes being developed to remove the capacitors from the silicon substrate and reattach them to ICs is described. In addition, a concept for interconnecting the capacitors using a repatterning process is discussed.

To demonstrate a system integration process for Micro-Electro-Mechanical Systems (MEMS), we are building an active cooling MEMS unit for microelectronics applications. This integrated unit will incorporate a micropump, temperature sensors, microchannels, and heat exchange devices into a single unit. The first phase of this research project is to develop and test a micropump capable of moving the working fluid within the integrated device. This paper will discuss the design, development, testing, and evaluation of a micropump concept. The micropump which was developed is an electrohydrodynamic (EHD) injection pump. Fabrication of the pump was accomplished using laser micromachining technology, and two initial designs were examined for full fabrication. The first design has two silicon parts stacked vertically on top of each other. Gold is deposited on one side of each stacked plate to serve as electrodes for the electrohydrodynamic pump. A Nd:YAG laser is used to drill an array of circular holes in the "well" region of both silicon parts, leaving an open pathway for fluid movement. Next the silicon parts are aligned and bonded together, thus becoming a EHD pump. Fluid flow has been observed when an electric voltage is applied across the electrodes. The second design has the silicon parts which contain the flow grid oriented "back-to-back" and bonded together. This "back-to-back" design has a shorter grid distance between the anode and cathode plates so that a smaller voltage is required for pumping. Preliminary results from laboratory experiments have demonstrated that this EHD micropump design can achieve a pressure head of about 287 Pa with an applied voltage of 120 V.

In applications where multiple magnetic modulators are used to drive a single Linear Induction Voltage Adder (LIVA) or Linear Accelerator (LINAC), it is essential that the outputs of the modulators be synchronized. Output rise times are typically in the 10 ns to 20 ns range, often making it necessary to synchronize to within less than 1 ns. Microprocessor and electronic feedback schemes have been developed and demonstrated that achieve the required level of synchronization, however, they are sophisticated and potentially complex. In a quest for simplicity, this work seeks to determine the achievable level of modulator to modulator timing jitter that can be obtained with simple design practices and passive techniques. Sources of output pulse time jitter in magnetic modulators are reviewed and some basic modulator design principles that can be used to minimize the intrinsic time jitter between modulators are discussed. A novel technique for passive synchronization is presented.

High network performance is essential to satisfying a wide class of High Performance Computing and Communication (HPCC) user needs. The requirements of these applications for high throughput and low interactive response time have focused this research on the scaling of technology far past the performance of the newest data telecommunication industry standards in order to facilitate the design of communication systems of interest to the HPCC community. These applications require Local Area Network (LAN) and Wide Area Network (WAN) performance in the 10 to 100 Gigabit per second (Gb/s) range, far greater than the 0.05 Gb/s performance typical of today`s LANs or WANs. This research investigated various approaches to achieving 10 to 100 Gigabit per second performance, developed a communication architecture to achieve this performance, and tested the viability of selected techniques through simulation and prototyping.

We review the processes and mechanisms by which voltage offsets occur in the hysteresis loop of ferroelectric materials. Simply stated, voltage shifts arise from near-interfacial charge trapping in the ferroelectric. We show that the impetus behind voltage shifts in ferroelectric capacitors is the net polarization, with the net polarization being determined by the perovskite and the aligned defect-dipole components. Some common defect-dipoles in the PZT system are lead vacancy-oxygen vacancy complexes. One way to change the net polarization in the ferroelectric is to subject the PZT capacitor to a dc bias at elevated temperature; this process is spectroscopically shown to align defect-dipoles along the direction of the applied electric field. The alignment of defect-dipoles can strongly impact several material properties. One such impact is that it can lead to enhanced voltage shifts (imprint). It is proposed that the net polarization determines the spatial location of the asymmetrically trapped charge that are the cause for the voltage shifts. An enhanced polarization at one electrode interface can lead to larger voltage shifts since it lowers the electrostatic potential well for electron trapping, i.e., more electron trapping can occur. Defect-dipole alignment is also shown to increase the UV sensitivity of the ferroelectric.

A prototype sensor fusion framework called the {open_quotes}Knowledge Assistant{close_quotes} has been developed and tested on a gantry robot at Sandia National Laboratories. This Knowledge Assistant guides the robot operator during the planning, execution, and post analysis stages of the characterization process. During the planning stage, the Knowledge Assistant suggests robot paths and speeds based on knowledge of sensors available and their physical characteristics. During execution, the Knowledge Assistant coordinates the collection of data through a data acquisition {open_quotes}specialist.{close_quotes} During execution and post analysis, the Knowledge Assistant sends raw data to other {open_quotes}specialists,{close_quotes} which include statistical pattern recognition software, a neural network, and model-based search software. After the specialists return their results, the Knowledge Assistant consolidates the information and returns a report to the robot control system where the sensed objects and their attributes (e.g. estimated dimensions, weight, material composition, etc.) are displayed in the world model. This paper highlights the major components of this system.

State and Federal regulations have been implemented that are intended to encourage more widespread use of low-emission vehicles. These regulations include requirements of the California Air Resources Board (CARB) and regulations pursuant to the Clean Air Act Amendments of 1990 and the Energy Policy Act. If the market share of electric vehicles increases in response to these initiatives, corresponding growth will occur in quantities of spent electric vehicle batteries for disposal. Electric vehicle battery recycling infrastructure must be adequate to support collection, transportation, recovery, and disposal stages of waste battery handling. For some battery types, such as lead-acid, a recycling infrastructure is well established; for others, little exists. This paper examines implications of increasing electric vehicle use for lead recovery infrastructure. Secondary lead recovery facilities can be expected to have adequate capacity to accommodate lead-acid electric vehicle battery recycling. However, they face stringent environmental constraints that may curtail capacity use or new capacity installation. Advanced technologies help address these environmental constraints. For example, this paper describes using backup power to avoid air emissions that could occur if electric utility power outages disable emissions control equipment. This approach has been implemented by GNB Technologies, a major manufacturer and recycler of lead-acid batteries. Secondary lead recovery facilities appear to have adequate capacity to accommodate lead waste from electric vehicles, but growth in that capacity could be constrained by environmental regulations. Advances in lead recovery technologies may alleviate possible environmental constraints on capacity growth.

The disturbed-rock zone surrounding the air-intake shaft at the Waste Isolation Pilot Plant (WIPP) site was investigated to determine the extent and the permeability of the disturbed-rock zone as a function of radial distance from the 6.1 m diameter shaft, at different elevations within the Salado. Gas- and brine-permeability tests were performed in the bedded halite of the Salado formation at two levels within the air-intake shaft. The gas- and brine-permeability test results demonstrated that the radial distance to an undisturbed formation permeability of 1 {times} 10{sup {minus}21} m{sup 2} was less than 3.0 m.

The primary current-collector materials being used in lithium-ion cells are susceptible to environmental degradation: aluminum to pitting corrosion and copper to environmentally assisted cracking. Pitting occurs at the highly oxidizing potentials associated with the positive-electrode charge condition. However, the pitting mechanism is more complex than that typically observed in aqueous systems in that the pits are filled with a mixed metal/oxide product and exist as mounds or nodules on the surface. Electrochemical impedance was shown to be an effective analytical tool for quantification and verification of visual observations and trends. Two fluorocarbon-based coatings were shown to improve the resistance of Al to localized pitting. Finally, environmental cracking of copper can occur at or near the lithium potential and only if specific metallurgical conditions exist (work hardening and large grain size).

Highly crystalline nanoclusters of MoS{sub 2} were synthesized and their optical absorption and photoluminescence spectra were investigated. Key results include: (1) strong quantum confinement effects with decreasing size; (2) preservation of the quasiparticle (or excitonic) nature of the optical response for clusters down to {approximately} 2.5 nm in size which are only two unit cells thick; (3) demonstration that 3-D confinement produces energy shifts which are over an order of magnitude larger than those due to 1-D confinement; (4) observation of large increases in the spin-orbit splittings at the top of the valence band at the K and M points of the Brillouin zone with decreasing cluster size; and (5) observation of photoluminescence due to both direct and surface recombination. Application is to photocatalysts for solar fuel production and detoxification of chemical waste.

Buried oxides formed from the wet oxidation of AlGaAs alloys, rather than AlAs, are found to be superior in terms of oxidation isotropy, mechanical stability, and strain. It is not surprising that vertical-cavity surface-emitting lasers (VCSELs) using AlGaAs oxide layers as current apertures have shown promising reliability as compared to VCSELs using AlAs layers. Comparisons of lifetime data for VCSELs with differing oxide layers is presented. The beneficial properties of oxides converted from AlGaAs alloys are found to provide robust device processing of reliable VCSELs and may play an important role in other advanced optoelectronic devices.

The Treaty on Open Skies is a precedent-setting agreement that allows signatory states to fly aircraft over each other`s territory with sensor systems. The purpose of the Treaty is to improve confidence and security with respect to military activities of the signatories. This paper reviews the sensor technology that is currently allowed by the Treaty on Open Skies and potential future sensor technology. The Treaty on Open Skies does have provisions to allow for the improvement of the technology of the current sensor systems and for the proposal of new sensors after a period of time. This can occur only after the Treaty has been ratified and has entered into force. If this regime was to be used for other than Treaty on Open Skies applications some modifications to the allowed sensor technology should be examined. This paper presents some ideas on potential improvements to existing allowed sensor technology as well as some suggested new advanced sensor systems that would be useful for future potential monitoring of safeguard`s related activities. This paper addresses advanced imaging sensors and non-imaging sensors for potential use in aerial remote sensing roles that involve international data sharing.

An acoustic emission test for aircraft Halon bottles has been developed in response to a need expressed by the US Airline Industry. During this development many choices had to be made about test methods, procedures and analysis techniques. This paper discusses these choices and how successful they were. The test itself was designed to replace the currently required hydrostatic test for these bottles. The necessary load is applied by heating the sealed bottles. Acoustic emission is monitored, during the heating, by six sensors held in position by a special fixture. A prototype of the test apparatus was constructed and used in two commercial Halon bottle repair and test facilities. Results to date indicate that about 97% of the bottles tested show no indications of flaws. The other 3% have had indications of possible flaws in non-critical areas of the bottles. All bottles tested to date have passed the hydrostatic test subsequent to the acoustic emission test.