Publications

A number of different high bandgap tunnel junctions have been examined for use in advanced monolithic multi-junction solar cells. All of the tunnel junctions are grown by metal-organic vapor phase epitaxy. An Al0.9Ga0.1As/In0.5Ga0.3Al0.2P tunnel junction has the necessary optical and electrical properties for use in an advanced four junction device, and we demonstrate a working device. The bandgap of the Al0.9Ga0.1As and the In0.5Ga0.3Al0.2P are both 2.1 eV. The Jp of the device is 1,500 mA/cm2, and the series resistance is 2.5 × 10-2 Clcm2. The Al0.9Ga0.1As is doped with carbon, while the In0.5Ga0.3Al0.2P is doped with tellurium. SIMS analysis indicates minimal tellurium memory effect and outdiffusion, and hence minimal tellurium doping in ensuing layers.

Abstract not provided.

Monolithic, integrated acoustic wave chemical microsensors are being developed on gallium arsenide (GaAs) substrates. With this approach, arrays of microsensors and the high frequency electronic components needed to operate them reside on a single substrate, increasing the range of detectable analytes, reducing overall system size, minimizing systematic errors, and simplifying assembly and packaging. GaAs is employed because it is both piezoelectric, a property required to produce the acoustic wave devices, and a semiconductor with a mature microelectronics fabrication technology. Many aspects of integrated GaAs chemical sensors have been investigated, including: surface acoustic wave (SAW) sensors; monolithic SAW delay line oscillators; GaAs application specific integrated circuits (ASIC) for sensor operation; a hybrid sensor array utilizing these ASICS; and the fully monolithic, integrated SAW array. Details of the design, fabrication, and performance of these devices are discussed. In addition, the ability to produce heteroepitaxial layers of GaAs and aluminum gallium arsenide (AlGaAs) makes possible micromachined membrane sensors with improved sensitivity compared to conventional SAW sensors. Micromachining techniques for fabricating flexural plate wave (FPW) and thickness shear mode (TSM) microsensors on thin GaAs membranes are presented and GaAs FPW delay line and TSM resonator performance is described.

The authors designed and conducted experiments in a heterogeneous sand pack where gravity-destabilized nonwetting phase invasion (CO{sub 2} and TCE) could be recorded using high resolution light transmission methods. The heterogeneity structure was designed to be reminiscent of fluvial channel lag cut-and-fill architecture and contain a series of capillary barriers. As invasion progressed, nonwetting phase structure developed a series of fingers and pools; behind the growing front they found nonwetting phase saturation to pulsate in certain regions when viscous forces were low. Through a scale analysis, they derive a series of length scales that describe finger diameter, pool height and width, and regions where pulsation occurs within a heterogeneous porous medium. In all cases, they find that the intrinsic pore scale nature of the invasion process and resulting structure must be incorporated into the analysis to explain experimental results. The authors propose a simple macro-scale structural growth model that assembles length scales for sub-structures to delineate nonwetting phase migration from a source into a heterogeneous domain. For such a model applied at the field scale for DNAPL migration, they expect capillary and gravity forces within the complex subsurface lithology to play the primary roles with viscous forces forming a perturbation on the inviscid phase structure.

The authors have used low-energy electron microscopy to investigate the dynamics of the Si(111) 7 x 7 {r_arrow} 1 x 1 phase transition. Because the densities of the two phases differ, the phase transformation is analogous to precipitation in bulk systems: additional material must diffuse to the phase boundaries in order for the transformation to occur. By measuring the size evolution of an ensemble of domains, and comparing the results to simulations, they have identified a new mechanism of precipitate growth. The source of material necessary for the transformation is the random creation of atom/vacancy pairs at the surface. This mechanism contrasts sharply with classical theories of precipitation, in which mass transport kinetics determine the rate of transformation.

On August 4, 1994, a Pollution Prevention Opportunity Assessment (PPOA) for the Print Shop was initiated by Print Shop Manager Bruce Fetzer and the Pollution Reduction Group (PRG).

We present a theory for transforming the system-theory-based realization models into the corresponding physical coordinate-based structural models. The theory has been implemented into computational procedure and applied to several example problems. Our results show that the present transformation theory yields an objective model basis possessing a unique set of structural parameters from an infinite set of equivalent system realization models. For proportionally damped systems, the transformation directly and systematicaly yields the normal modes and modal damping. Moreover, when nonproportional damping is present, the relative magnitude and phase of the damped mode shapes are separately characterized, and a corrective transformation is then employed to capture the undamped normal modes and nondiagonal modal damping matrix.

The point of this work is to create a tool for thorough analysis of the eigenspectrum of the linearized finite element equations for buoyancy driven flow.