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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

On the origin and distribution of C IV and Si IV ions in the neighboring interstellar medium

IUE spectrometer observational data are reported on the C IV, Si IV, and N V absorption lines in 46 distant stars at FWHM resolution of 30 km/sec. A simple analytic theory is given for these contributions, including the effects of ionization by a soft X-ray background. While for some of the most distant field stars a photoionized contribution may be ruled out, C IV absorption lines are detected which are shallow and broad. It is suggested that what are being detected in these cases are contributions from the hot interstellar gas that is responsible for the O VI absorption lines. Most of the Si IV and C IV lines probably arise in the H II regions surrounding the observed star or its neighbors.

Cowie, L. L.↗
NASA NTRS1980

Nuclear gamma rays from 720-MeV alpha-induced reactions on Al-27 and Si-28

Prompt gamma rays from the interaction of 720-MeV alpha particles with Al-27 and Si-28 were detected and analyzed to identify residual nuclei and to determine cross sections for production of specific levels. No gamma-ray transitions were detected from nuclei heavier than the target. From Doppler broadening, the momentum of the residual nuclei was estimated. The results are compared with previous results for 140- and 1600-MeV alphas on Al-27 and approximately 200-MeV positive or negative pions on Al-27 and Si-28 and fitted to a spallation-yield formula.

Lieb, B. J.↗
NASA NTRS1982

Evidence of redshifts in the average solar line profiles of C IV and Si IV from OSO-8 observations

Line profiles of C IV and Si V obtained by the Colorado spectrometer on OSO-8 are presented. It is shown that the mean profiles are redshifted with a magnitude varying from 6-20 km/s, and with a mean of 12 km/s. An apparent average downflow of material in the 50,000-100,000 K temperature range is measured. The redshifts are observed in the line center positions of spatially and temporally averaged profiles and are measured either relative to chromospheric Si I lines or from a comparison of sun center and limb profiles. The observations of 6-20 km/s redshifts place constraints on the mechanisms that dominate EUV line emission since it requires a strong weighting of the emission in regions of downward moving material, and since there is little evidence for corresponding upward moving materials in these lines.

Roussel-Dupre, D.↗
NASA NTRS1982

Nitridation of Si(111) by nitrogen atoms. II

The reaction between Si(111) and nitrogen atoms has been investigated by LEED, Auger, and electron energy loss (ELS) techniques. The early stage of the reaction at 850-1050 C involves the formation of a chemically reacted monolayer which grows in islands. The sticking coefficient for the submonolayer is estimated to be of the order of unity. Two electronically different surface structures can be formed during these stages, yielding the '8 x 8' and the quadruplet LEED patterns; the quadruplet unit cell is the one with the highest nitrogen density. A pure quadruplet structure is obtained from samples with a carbon contamination of the Si monolayer of about 5 pct. During the multilayer stages, the influence of the substrate is not as important, and the reactions at 850 C and above produce quadruplet-like local structures.

Schrott, A. G.↗
NASA NTRS1982

Crystal nucleation in Pd-Si alloys

A study of the crystal phase nucleation in undercooled droplets of Pd-Si alloys with composition near the Pd(84.5)Si(15.5) eutectic composition is reported. Molten droplets are released at the top of a drop tube and solidify (to either a crystalline or glassy state) during descent. This provides a containerless (and nearly gravity free) environment so that nucleation due to container walls or vibrations is eliminated. It is found that crystallization, due to homogeneous nucleation, is bypassed in droplets of 1 mm diameter when cooled at 760 K/sec. From this an upper limit of the homogeneous nucleation rate is estimated. Results are compared with a previously published study of nucleation in 0.06 mm to 0.33 mm diameter droplets, which indicated that nucleation results from heterogeneous surface nucleation and that the number of these nuclei is dependent on the atmosphere in the drop tube.

Drehman, A. J.↗
NASA NTRS1983

Si III line ratios in the sun

New atomic data for Si III have been used to predict level populations and emission-line intensity ratios for electron densities and temperatures appropriate to the solar transition region. The electron impact collision rates used here are substantially larger than those previously published owing to delineation of the complex resonance structures in the low-energy collision strengths. This together with small changes in the spontaneous radiative rates produces significant changes in the calculated intensity ratios. Generally good agreement is found with observations obtained using the Naval Research Laboratory slit spectrograph aboard Skylab, electron densities from three Si III ratios and from other methods normally agreeing to 0.2 dex or better for a wide variety of solar features. For a fourth ratio, incorporating lines with a wide wavelength separation, the agreement is less satisfactory, possibly owing to uncertainties in the observational data.

Dufton, P. L.↗
NASA NTRS1984

Ultraviolet spectral morphology of the O stars - The remarkable luminosity dependence of the Si IV stellar wind effect

An extensive survey of ultraviolet O type spectra, by means of high-resolution data from the International Ultraviolet Explorer archives, shows a strong correlation between both the photospheric and stellar wind features, and the optical spectral types. In particular, the stellar wind effect in the Si IV resonance doublet displays a strong luminosity dependence which has not been completely described before: there is no effect anywhere on the main sequence, but it develops gradually through the intermediate luminosity classes into a full P Cygni profile in the supergiants. This behavior is in sharp contrast to that of the C IV and N V, which have strong P Cygni profiles throughout most of the O type domain, including the main sequence. The Si IV effect simultaneously provides a sensitive classification criterion and poses an outstanding problem for detailed astrophysical interpretation. In addition, a new emission feature at 1574 A, possibly due to N III or to N IV forbidden line, is reported.

Walborn, N. R.↗
NASA NTRS1985

Measurements of the fragmentation of (40)Ar, (28)Si and (12)C in CH2C and H targets between 300 and 1500 MeV/nuc at the Bevalac

Studies of the fragmentation of various nuclei in CH2 and C targets were continued with the objective of obtaining cross sections in hydrogen for use in the cosmic ray propagation problem. New measurements include Fe-56. Measurements were made at 6 energies between 300 and 1700 MeV/nuc. C-12 measurements were made at six energies. Si-28 measurements were made at three energies and measurements were made at two energies. New data on C.-12 Si-28 and AR-44 nuclei are given. The data are compare it with the earlier semi-empirical predictions.

Webber, W. R.↗
NASA NTRS1984

Ionic charge states of N, Ne, Mg, Si and S in solar energetic particle events

The mean ionic charges and source-region temperatures of flare-accelerated N, Ne, Mg, Si and S in three large solar-energetic-particle events during 1978-1979 are determined from ISEE-3 observations, extending the findings of Hovestadt et al. (1981) and Gloeckler et al. (1981) for C, He, O, and Fe. The results are presented in tables and graphs, and the charge states are shown to correspond to different source temperatures even in the same flare, assuming equilibration in the hot plasma. The electron temperatures range from 2 x 10 to the 6th K for C, N, O, Si, and S, 4 x 10 to the 6th K for Ne and Fe, and 7 x 10 to the 6th K for Mg. The possibility that these temperature inconsistencies reflect different stages in the approach to equilibrium is considered.

Luhn, A.↗
NASA NTRS1986

Passivation of Si solar cells by hetero-epitaxial compound semiconductor coatings

A development status evaluation is made for high efficiency Si solar cells, with emphasis on the suppression of the deleterious effects of surface recombination. ZnS(0.9)Se(0.1) and GaP are identified as candidates for the reduction of surface recombination. Attention is given to methods developed for the deposition of heteroepitaxial compounds designed to block minority carrier transport to the Si solar cell surface without interfering with the majority carrier flow.

Vernon, S. M.↗
NASA NTRS1987

Si(111) cleavage and the (2 x 1) reconstruction process

Using a computer simulation technique with a semiempirical potential, a Si crystal was cleaved along the (111) plane. The pi-bonded chain structural features of the Si(111) cleavage surface are observed and found to be a consequence of the dynamics of this cleavage process and seem not to be influenced by the final energetics.

Pearson, E. M.↗
NASA NTRS1987

Investigation of GaAs/Al(x)Ga(1-x)As and In(y)Ga(1-y)As/GaAs superlattices on Si substrates

The optical properties of lattice-matched GaAs/Al(x)Ga(1-x)As and In(y)Ga(1-y)As/GaAs strained-layer superlattices grown on Si substrates have been studied using the photoreflectance technique. These preliminary results show that good quality III-IV epilayers can be grown on Si. The experimental data were compared with calculations based on the envelope-function approximation and fitted to the third-derivative functional form of reflectance modulation theory.

Reddy, U. K.↗
NASA NTRS1987

Growth and characterization of cubic SiC single-crystal films on Si

Morphological and electrical characterization results are presented for cubic SiC films grown by chemical vapor deposition on single-crystal Si substrates. The films, up to 40 microns thick, were characterized by optical microscopy, (SEM), (TEM), electron channeling, surface profilometry, and Hall measurements. A variety of morphological features observed on the SiC films are described. Electrical measurements showed a decrease in the electron mobility with increasing electron carrier concentration, similar to that observed in Si. Room-temperature electron mobilities up to 520 sq cm/V-s (at an electron carrier concentration of 5 x 10 to the 16th/cu cm) were measured. Finally, a number of parameters believed to be important in the growth process were investigated, and some discussion is given of their possible effects on the film characteristics.

Powell, J. Anthony↗
NASA NTRS1987

Absolute oscillator strengths for 108 lines of Si I between 163 and 410 nanometers

Measurements of neutral silicon oscillator strengths (f-values) obtained by absorption and emission techniques have been combined using the numerical procedure of Cardon et al. (1979) to produce 108 f-values for the Si I lines between 163 and 410 nm. Beam-foil-lifetime measurements were employed to determine the absolute scale. The present measurements have uncertainties of about 0.07 dex (+ or - 16 percent) at the 1-sigma level of confidence. Good agreement is obtained between the results and previous data. The data also provide upper limits for the f-values of 22 other lines and information on the lifetimes for 36 levels in Si I.

Smith, Peter L.↗
NASA NTRS1988

Secondary ion mass spectrometry study of ex situ annealing of epitaxial GaAs grown on Si substrates

Samples of epitaxial GaAs grown on (100) Si substrates using molecular beam epitaxy were annealed at four different temperatures, from 800 to 950 C. Following annealing, the samples were analyzed using secondary ion mass spectrometry. Depth profiles of Ga, As, and Si reveal optimum conditions for annealing, and place a lower limit on a damage threshold for GaAs/Si substrates.

Radhakrishnan, G.↗
NASA NTRS1987

Binary synaptic connections based on memory switching in a-Si:H for artificial neural networks

A scheme for nonvolatile associative electronic memory storage with high information storage density is proposed which is based on neural network models and which uses a matrix of two-terminal passive interconnections (synapses). It is noted that the massive parallelism in the architecture would require the ON state of a synaptic connection to be unusually weak (highly resistive). Memory switching using a-Si:H along with ballast resistors patterned from amorphous Ge-metal alloys is investigated for a binary programmable read only memory matrix. The fabrication of a 1600 synapse test array of uniform connection strengths and a-Si:H switching elements is discussed.

Thakoor, A. P.↗
NASA NTRS1989

Molecular-Beam Epitaxy Of CrSi2 on Si(111)

Crystalline layers grown in commercial apparatus. Experiments show CrSi2 grown on (111) face of single-crystal Si substrate by molecular-beam epitaxy. Epitaxial CrSi2 produced thus far not in desired single-crystal form. Because CrSi2 semiconductor with band gap of 0.3 eV, experimental process potential for monolitic integration of microelectronic devices based on CrSi2 (e.g., infrared detectors) with signal-processing circuitry based on Si.

Fathauer, Robert W.↗
NASA NTRS1988

Dopant redistribution during the solid-phase growth of CrSi2 on Si(100)

The distribution pattern of As, B, and P dopants during the solid-phase growth of CrSi2 on Si(100) was investigated using Rutherford backscattering spectroscopy and SIMS. The results demonstrated that all three dopants did redistribute during the formation of CrSi2 layers; however, the nature of redistribution was different for different dopants. While B and P were transported from the Si substrate to the surface of the growing CrS2 layer, As accumulated at the CrSi2/Si interface. The formation of silicide was found to be inhibited by large concentrations of As. A model was developed to explain these results.

Rockett, A.↗