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At least 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Demonstration of a silicon nitride attrition mill for production of fine pure Si and Si3N4 powders

To avoid metallic impurities normally introduced by milling ceramic powders in conventional steel hardware, an attrition mill (high-energy stirred ball mill) was constructed with the wearing parts (mill body, stirring arms, and media) made from silicon nitride. Commercial silicon and Si3N4 powders were milled to fine uniform particles with only minimal contamination - primarily from wear of the sintered Si3N4 media.

Herbell, T. P.↗
NASA NTRS1984

Lifetimes in Si CVD-epitaxial and other layers determined by spectral LBIC

In an effort to evaluate the minority carrier lifetimes in CVD-epitaxially grown layers in their dependence on the CVD process variables, the absolute spectral LBIC method was found to be the only available method capable of yielding both lifetime and surface recombination velocity in layers, independent of the resistivity of this layer or that of the substrate, as long as they are separated by a pn-junction. With this method, it was not only possible to determine the minority carrier lifetimes in as-grown epi-layers, but to observe their changes through the device fabrication processes. In addition, it was possible for the first time to determine the lifetimes in the alloy-regrowth p(+) layers of base high/low junction structures.

Wolf, M.↗
NASA NTRS1984

Computer modeling results on all-Si cascade solar cells

The properties of a cascade solar cell made entirely of silicon are investigated numerically with the goal of developing an optimal silicon solar cell grown by molecular-beam epitaxy. The cascade cell is modeled as two standard back-surface field cells with abrupt junctions connected by a tunnel junction. A cascade cell would have approximately twice the open-circuit voltage of a single cell. If the minority carriers generated in the front cell can be reflected before reaching the tunnel junction, then the cascade cell will show an increase in efficiency over a single cell by a percentage point.

Sparks, P. D.↗
NASA NTRS1985

I-V-T analysis of radiation damage in high efficiency Si solar cells

A detailed analysis of current-voltage characteristics of N(+)-P/P solar cells indicate that there is a combination of different mechanisms which results in an enhancement in the dark current and in turn deteriorates the photovoltaic performance of the solar cells after 1 MeV e(-) irradiation. The increase in the dark current is due to three effects, i.e., bulk recombination, space charge recombination by deep traps and space charge recombination through shallow traps. It is shown that the increase in bulk recombination current is about 2 to 3 orders of magnitude whereas space charge recombination current due to shallow traps increases only by an order or so and no space charge recombination through deep traps was observed after irradiation. Thus, in order to improve the radiation hardness of these devices, bulk properties should be preserved.

Banerjee, S.↗
NASA NTRS1986

Development of InSb and Si:Ga arrays for ISO camera

The Infrared Space Observatory (ISO) satellite will contain a 60 cm telescope. A 4 faces pyramid mirror feeds each experiment with a 3 arc minute part of the telescope field of view. The camera system is described. The arrays for the 3 to 5 and 5 to 17 micron channel are also discussed.

Sibille, F.↗
NASA NTRS1986

Si:As BIB detector arrays

Highlights of the results obtained on arsenic-doped silicon blocked impurity band (BIB) detectors and arrays since the invention of the BIB concept a few years ago are presented. After a brief introduction and a description of the BIB concept, data will be given on single detector performance. Then different arrays that were fabricated will be described and test data presented.

Bharat, R.↗
NASA NTRS1986

Evaluation of a Si:Ga direct readout array

Preliminary results from the Hughes 58 x 62 gallium-doped silicon infrared array are presented. The device functioned predictably and repeatedly at the low temperatures for the Space Infrared Telescope Facility (SIRTF). Power dissipation was reduced to acceptable levels while still maintaining the device speed. The infrared response was excellent and the degree of anomalous effects was compatible to conventional photoconductors.

Lamb, G. M.↗
NASA NTRS1987

Hot corrosion attack and strength degradation of SiC and Si(sub)3N(sub)4

Thin films of Na2SO4 and Na2CO3 molten salt deposits were used to corrode sintered SiC and Si3N4 at 1000 C. The resulting attack produced pitting and grain boundary etching resulting in strength decreases ranging from 15 to 50 percent. Corrosion pits were the predominant sources of fracture. The degree of strength decrease was found to be roughly correlated with the depth of the pit, as predicted from fracture toughness considerations. Gas evolution and bubble formation were key aspects of pit formation. Many of the observations of furnace exposures held true in a more realistic burner rig test.

Smialek, James L.↗
NASA NTRS1987

Effect of 1.0 MeV electron irradiation on shunt resistance in Si-MINP solar cells

Shunt resistance from 100 K to 400 K is compared for diffused and ion implanted cells before and after irradiation. R sub sh decreases from greater than 10 to the 7th power omega-square cm for T less than 250 K to 10 to the 4th power omega-square cm at 400 K for non-irradiated diffused cells. Electron irradiation causes a more rapid decrease in R sub sh for T greater than 250 K. Ion implanted cells exhibit a similar trend except that R sub sh is significantly less for T less than 250 K and is more sensitive to irradiation at these low temperatures. The mechanism of R sub sh appears to be a combination of multistep tunneling and trapping - detrapping in the defect states of the semiconductor. Radiation serves to increase the density of these states to decrease R sub sh.

Anderson, Wayne A.↗
NASA NTRS1987

TEM investigation of beta-SiC grown epitaxially on Si substrate by CVD

A transmission electron microscopy study is being conducted on the microstructure of beta-SiC films grown epitaxially on 001-plane silicon by chemical vapor deposition. Observations have been made in plain view and in cross-section. A high density of stacking faults has been found in the bulk of the epilayer which are bounded by partials of the Shockley type with Burgers vectors 1/6 211-line. Cross-sectional high resolution electron microscopy of the interface has shown it to be semicoherent with misfit dislocations to accommodate the lattice parameter difference between the substrate and the epilayer. In addition to misfit dislocations, a high density of twins and some stacking faults are present in the SiC near the interface. Mechanisms for the nucleation and growth of these defects are discussed.

Chorey, C. M.↗
NASA NTRS1988

Noble gases, C,N, and Si isotopes in interstellar SiC form the Murchison carbonaceous chondrite

Isotopic measurements of presolar interstellar silicon carbide are presented. Silicon carbide was recently identified in a primitive meteorite Murray. There exists strong evidence that the silicon carbide predates the formation of the solar system and originated in the atmospheres of certain stars. Thus, this material provides a link with its stellar sources and gives the opportunity to study processes taking place in distant stars.

Ming, Tang↗
NASA NTRS1988

Characterization of ZrO2 buffer layers for sequentially evaporated Y-Ba-CuO on Si and Al2O3 substrates

Thin film high temperature superconductors have the potential to change the microwave technology for space communications systems. For such applications it is desirable that the films be formed on substrates such as Al2O3 which have good microwave properties. The use of ZrO2 buffer layers between Y-Ba-Cu-O and the substrate has been investigated. These superconducting films have been formed by multilayer sequential electron beam evaporation of Cu, BaF2 and Y with subsequent annealing. The three layer sequence of Y/BaF2/Cu is repeated four times for a total of twelve layers. Such a multilayer film, approximately 1 micron thick, deposited directly on SrTiO3 and annealed at 900 C for 45 min produces a film with a superconducting onset of 93 K and critical temperature of 85 K. Auger electron spectroscopy in conjunction with argon ion sputtering was used to obtain the distribution of each element as a function of depth for an unannealed film, the annealed film on SrTiO3 and annealed films on ZrO2 buffer layers. The individual layers were apparent. After annealing, the bulk of the film on SrTiO3 is observed to be fairly uniform while films on the substrates with buffer layers are less uniform. The Y-Ba-Cu-O/ZrO2 interface is broad with a long Ba tail into the ZrO2, suggesting interaction between the film and the buffer layer. The underlying ZrO2/Si interface is sharper. The detailed Auger results are presented and compared with samples annealed at different temperatures and durations.

Valco, George J.↗
NASA NTRS1990

Multi-Quantum-Well Spatial Light Modulators On Si Substrates

Proposed new class of two-dimensional spatial light modulators (SLM's) based on fabrication of multi-quantum-well (MQW) structures (e.g. of AlGaAs and InGaAs) on silicon substrates. Concept makes two-dimensional SLM arrays by taking advantage of mature technology of silicon charge-coupled devices (CCD's). Features include short response times, high resolution, and low switching energies.

Cutts, James A.↗
NASA NTRS1989

Interstellar SiC in the Murchison and Murray meteorites - Isotopic composition of Ne, Xe, Si, C, and N

Detailed ion-probe data are reported on Murchison and Murray C2 chondrites, along with noble-gas data on Murchison, with the purpose of characterizing the number and isotopic composition of independent SiC components and to link them to their stellar sources. Results show that coarse-grained and fine-grained SiC (totaling about 6-9 ppm of the two meteorites) contains record concentrations of two exotic components: Ne-E(H) (nearly monoisotopic Ne-22) and Se-S (enriched in isotopes 128, 130, and 132), respectively. The Ne-21 content was found to exceed up to 40-fold the content expected from the recent cosmic-ray irradiation, implying a presolar cosmic-ray exposure age of about 39 Ma. This age is much shorter that the predicted 500 Ma lifetime of refractory interstellar grains and implies either a selective interstellar destruction process for SiC, enhanced supernova activity in the protosolar neighborhood, or late outgassing of greater than 90 percent of the SiC.

Zinner, Ernst↗