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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Fast chemistry in ion wakes

A common aspect of the models of such processes as track formation and induced desorption of molecular ions is the requirement that the electronic excitation have a sufficiently long lifetime (of the order of picoseconds) so that energy can be transferred to atomic motion. Attention is called to several phenomena recently observed indicating that atomic motion/chemical rearrangement can occur even when the electronic excitation is shorter lived (of the order of femtoseconds). Two such processes, which are also induced by MeV ion bombardment, are track damage in heavily doped compound semiconductors and greatly enhanced adhesion of metal films to metallic and semiconducting substrates. Since neither of these effects can easily be accommodated within the existing theoretical models, it is thought possible that an even richer variety of ion-induced effects will be discovered.

Tombrello, T. A.↗
NASA NTRS1983

Cleaning chemistry of GaAs(100) and InSb(100) substrates for molecular beam epitaxy

Ploog (1980) and Bachrach and Krusor (1981) have pointed out the importance of substrate preparation and surface cleaning for obtaining high quality films with the aid of molecular beam epitaxial growth techniques. In the present investigation, high resolution X-ray photoemission (XPS) is used to determine the oxide removal mechanism for GaAs(100) substrates which have undergone a standardized cleaning procedure. Other objectives of the investigation are related to a comparison of different cleaning procedures in order to minimize carbon contamination, the extension of these cleaning techniques to other III-V compound semiconductors such as InSb, and the evaluation of the sensitivity of the compositional results to electron-induced damage effects.

Vasquez, R. P.↗
NASA NTRS1986

Measuring Thermoelectric Properties Automatically

Microcomputer-controlled system speeds up measurements of Hall voltage, Seebeck coefficient, and thermal diffusivity in semiconductor compounds for thermoelectric-generator applications. With microcomputer system, large data base of these parameters gathered over wide temperature range. Microcomputer increases measurement accuracy, improves operator productivity, and reduces test time.

Chmielewski, A.↗
NASA NTRS1985

Crystal growth of device quality GaAs in space

The present program has been aimed at solving the fundamental and technological problems associated with Crystal Growth of Device Quality in Space. The initial stage of the program was devoted strictly to ground-based research. The unsolved problems associated with the growth of bulk GaAs in the presence of gravitational forces were explored. Reliable chemical, structural and electronic characterization methods were developed which would permit the direct relation of the salient materials parameters (particularly those affected by zero gravity conditions) to the electronic characteristics of single crystal GaAs, in turn to device performance. These relationships are essential for the development of optimum approaches and techniques. It was concluded that the findings on elemental semiconductors Ge and Si regarding crystal growth, segregation, chemical composition, defect interactions, and materials properties-electronic properties relationships are not necessarily applicable to GaAs (and to other semiconductor compounds). In many instances totally unexpected relationships were found to prevail.

Gatos, H. C.↗
NASA NTRS1985

The surface tension of liquid gallium

The surface tension of liquid gallium has been measured using the sessile drop technique in an Auger spectrometer. The experimental method is described. The surface tension in mJ/sq m is found to decrease linearly with increasing temperature and may be represented as 708-0.66(T-29.8), where T is the temperature in centigrade. This result is of interest because gallium has been suggested as a model fluid for Marangoni flow experiments. In addition, the surface tension is of technological significance in the processing of compound semiconductors involving gallium.

Hardy, S. C.↗
NASA NTRS1985

Partially confined configuration for the growth of semiconductor crystals from the melt in zero-gravity environment

A novel partially confined configuration is proposed for the crystal growth of semiconductors from the melt, including those with volatile constituents. A triangular prism is employed to contain the growth melt. Due to surface tension, the melt will acquire a cylindrical-like shape and thus contact the prism along three parallel lines. The three empty spaces between the cylindrical melt and the edges of the prism will accommodate the expansion of the solidifying semiconductor, and in the case of semiconductor compounds with a volatile constituent, will permit the presence of the desired vapor phase in contact with the melt for controlling the melt stoichiometry. Theoretical and experimental evidence in support of this new type of confinement is presented.

Lagowski, J.↗
NASA NTRS1987

Boron nitride: Composition, optical properties and mechanical behavior

A low energy ion beam deposition technique was used to grow boron nitride films on quartz, germanium, silicon, gallium arsenide, and indium phosphate. The film structure was amorphous with evidence of a hexagonal phase. The peak boron concentration was 82 at %. The carbon and oxygen impurities were in the 5 to 8 at % range. Boron-nitrogen and boron-boron bonds were revealed by X-ray photoelectron spectroscopy. The index of refraction varied from 1.65 to 1.67 for films deposited on III-V compound semiconductors. The coefficient of friction for boron nitride in sliding contact with diamond was less than 0.1. The substrate was silicon.

Pouch, John J.↗
NASA NTRS1987

Investigation of temperature and concentration oscillations in the directional solidification of Pb-Sn-Te

Directional solidification of the pseudobinary compound semiconductor material Pb sub 1-x Sn sub x Te by the Bridgman crystal growth process will be studied. Natural convection in the molten sample will be visualized with a novel electrochemical cell technique that employs the solid electrolyte material yttria-stabilized zirconia. Mass transfer by both diffusion and convection will be measured by detecting the motion of oxygen tracer in the liquid. Additional applications for electrochemical cells in semiconductor crystal growth are suggested. Unsteady convection in the melt will also be detected by the appearance of temperature oscillations. The purpose of this study is to experimentally characterize the overstable conditions for a Pb sub 1-x Sn sub x Te melt in the vertical Bridgman crystal growth technique and use a linear analysis to predict the onset of convection for this system.

Anderson, T. J.↗
NASA NTRS1987

Computer control of a scanning electron microscope for digital image processing of thermal-wave images

Using a recently developed technology called thermal-wave microscopy, NASA Lewis Research Center has developed a computer controlled submicron thermal-wave microscope for the purpose of investigating III-V compound semiconductor devices and materials. This paper describes the system's design and configuration and discusses the hardware and software capabilities. Knowledge of the Concurrent 3200 series computers is needed for a complete understanding of the material presented. However, concepts and procedures are of general interest.

Gilbert, Percy↗
NASA NTRS1987

Boron nitride - Composition, optical properties, and mechanical behavior

A low energy ion beam deposition technique was used to grow boron nitride films on quartz, germanium, silicon, gallium arsenide, and indium phosphate. The film structure was amorphous with evidence of a hexagonal phase. The peak boron concentration was 82 at. percent. The carbon and oxygen impurities were in the 5 to 8 at. percent range. Boron-nitrogen and boron-boron bonds were revealed by X-ray photoelectron spectroscopy. The index of refraction varied from 1.65 to 1.67 for films deposited on III-V compound semiconductors. The coefficient of friction for boron nitride in sliding contact with diamond was less than 0.1. The substrate was silicon.

Pouch, John J.↗
NASA NTRS1989

Thermal-Wave Microscope

Computer-controlled thermal-wave microscope developed to investigate III-V compound semiconductor devices and materials. Is nondestructive technique providing information on subsurface thermal features of solid samples. Furthermore, because this is subsurface technique, three-dimensional imaging also possible. Microscope uses intensity-modulated electron beam of modified scanning electron microscope to generate thermal waves in sample. Acoustic waves generated by thermal waves received by transducer and processed in computer to form images displayed on video display of microscope or recorded on magnetic disk.

Jones, Robert E.↗
NASA NTRS1989

Temperature profiles in high gradient furnaces

Accurate temperature measurement of the furnace environment is very important in both the science and technology of crystal growth as well as many other materials processing operations. A high degree of both accuracy and precision is acutely needed in the directional solidification of compound semiconductors in which the temperature profiles control the freezing isotherm which, in turn, affects the composition of the growth with a concomitant feedback perturbation on the temperature profile. Directional solidification requires a furnace configuration that will transport heat through the sample being grown. A common growth procedure is the Bridgman Stockbarger technique which basically consists of a hot zone and a cold zone separated by an insulator. In a normal growth procedure the material, contained in an ampoule, is melted in the hot zone and is then moved relative to the furnace toward the cold zone and solidification occurs in the insulated region. Since the primary path of heat between the hot and cold zones is through the sample, both axial and radial temperature gradients exist in the region of the growth interface. There is a need to know the temperature profile of the growth furnace with the crystal that is to be grown as the thermal load. However it is usually not feasible to insert thermocouples inside an ampoule and thermocouples attached to the outside wall of the ampoule have both a thermal and a mechanical contact problem as well as a view angle problem. The objective is to present a technique of calibrating a furnace with a thermal load that closely matches the sample to be grown and to describe procedures that circumvent both the thermal and mechanical contact problems.

Fripp, A. L.↗
NASA NTRS1990

Chemical nature of silicon nitride-indium phosphide interface and rapid thermal annealing for InP MISFETs

A rapid thermal annealing (RTA) process in pure N2 or pure H2 was developed for ion-implanted and encapsulated indium phosphide compound semiconductors, and the chemical nature at the silicon nitride-InP interface before and after RTA was examined using XPS. Results obtained from SIMS on the atomic concentration profiles of the implanted silicon in InP before and after RTA are presented, together with electrical characteristics of the annealed implants. Using the RTA process developed, InP metal-insulator semiconductor FETs (MISFETS) were fabricated. The MISFETS prepared had threshold voltages of +1 V, transconductance of 27 mS/mm, peak channel mobility of 1200 sq cm/V per sec, and drain current drift of only 7 percent.

Biedenbender, M. D.↗
NASA NTRS1990

Investigation of the asymmetric misfit dislocation morphology in epitaxial layers with the zinc-blende structure

The source of the asymmetry in the dislocation morphology exhibited in the epitaxial growth of compound semiconductors on (100) was investigated. A thickness wedge of p- and n-type GaAs(0.95)P(0.05) was grown on GaAs by metalorganic chemical vapor deposition, and the effect of misorientation on the resolved shear stress for each slip system was calculated and eliminated as the source of the asymmetry. Another potential source of asymmetry, the thickness gradient, was also eliminated. Results show that the substrate misorientation and the thickness gradient do not significantly contribute to the asymmetry and that the dominant contributor to the asymmetry of misfit dislocations in the (001) epitaxial interface can be attributed to the differences in the Peierls barriers between the two types of dilocations in GaAsP/GaAs.

Fox, Bradley A.↗
NASA NTRS1990

Growth and characterization of CdS crystals

A growth method for the physical vapor transport of compound semiconductors in closed ampoules is described. With the unique techniques applied in the heat treatment of the starting materials and the temperature profiles provided by the three-zone translational furnace, large crystals of CdS have been grown successfully by the method at lower temperatures than previously used. Both unseeded and seeded growth have been investigated. The CdS crystals were examined using optical and scanning electron microscopies (SEM) to study the microstructure and the dislocation etch-pits. The crystals were further characterized by infrared (IR) and ultraviolet (UV) transmission measurements.

Su, Ching-Hua↗
NASA NTRS1991

Optical processing using photorefractive GaAs and InP

The unique features of photorefractive compound semiconductors are presented. The advantages of this class of nonlinear optical materials for optical processing are illustrated with examples using GaAs and InP. The difference between GaAs and InP in the laser power density requirement is discussed.

Liu, Duncan T. H.↗
NASA NTRS1991

New optical approaches to the quantitative characterization of crystal growth, segregation and defect formation

Elemental and compound semiconductors were characterized using new optical approach based on NIR microscopy in conjunction with computational image analysis and contrast enhancement. The approach made it possible to perform a quantitative microsegregation analysis of GaAs and InP. NIR dark file illumination in transmission mode makes it possible to detect submicron precipitates in semiinsulating GaAs.

Carlson, D. J.↗
NASA NTRS1993

Production of electronic grade lunar silicon by disproportionation of silicon difluoride

Waldron has proposed to extract lunar silicon by sodium reduction of sodium fluorosilicate derived from reacting sodium fluoride with lunar silicon tetrafluoride. Silicon tetrafluoride is obtained by the action of hydrofluoric acid on lunar silicates. While these reactions are well understood, the resulting lunar silicon is not likely to meet electronic specifications of 5 nines purity. Dale and Margrave have shown that silicon difluoride can be obtained by the action of silicon tetrafluoride on elemental silicon at elevated temperatures (1100-1200 C) and low pressures (1-2 torr). The resulting silicon difluoride will then spontaneously disproportionate into hyperpure silicon and silicon tetrafluoride in vacuum at approximately 400 C. On its own merits, silicon difluoride polymerizes into a tough waxy solid in the temperature range from liquid nitrogen to about 100 C. It is the silicon analog of teflon. Silicon difluoride ignites in moist air but is stable under lunar surface conditions and may prove to be a valuable industrial material that is largely lunar derived for lunar surface applications. The most effective driver for lunar industrialization may be the prospects for industrial space solar power systems in orbit or on the moon that are built with lunar materials. Such systems would require large quantities of electronic grade silicon or compound semiconductors for photovoltaics and electronic controls. Since silicon is the most abundant semimetal in the silicate portion of any solar system rock (approximately 20 wt percent), lunar silicon production is bound to be an important process in such a solar power project. The lunar silicon extraction process is discussed.

Agosto, William N.↗