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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Exchange and relaxation effects in low-energy radiationless transitions

The effect on low-energy atomic inner-shell Coster-Kronig and super Coster-Kronig transitions that is produced by relaxation and by exchange between the continuum electron and bound electrons was examined and illustrated by specific calculations for transitions that deexcite the 3p vacancy state of Zn. Taking exchange and relaxation into account is found to reduce, but not to eliminate, the discrepancies between theoretical rates and measurements.

Chen, M. H.↗
NASA NTRS1978

Morphology of gold and copper ion-plated coatings

Copper and gold films (0.2 to 2 microns thick) were ion plated onto polished 304-stainless-steel, glass, mica surfaces. These coatings were examined by SEM for defects in their morphological growth. Three types of defects were distinguished: nodular growth, abnormal or runaway growth, and spits. The cause for each type of defect was investigated. Nodular growth is due to inherent substrate microdefects, abnormal or runaway growth is due to external surface inclusions, and spits are due to nonuniform evaporation (ejection of droplets). All these defects induce stresses and produce porosity in the coatings and thus weaken their mechanical properties. During surface rubbing, large nodules are pulled out, leaving vacancies in the coatings.

Spalvins, T.↗
NASA NTRS1978

Radiation damage of germanium detectors

Energetic particles can produce interstitial-vacancy pairs in a crystal by knocking the atoms from their normal positions. Detectors are unique among semiconductor devices in depending on very low concentrations of electrically active impurities, and also on efficient transport of holes and electrons over relatively large distances. Because the dense regions of damage produced by energetic particles may result in donors and/or acceptors, and also provide trapping sites for holes and electrons, detectors are very sensitive to radiation damage. In addition to these effects occurring within the detector, radiation may also change the characteristics of the exposed surfaces causing unpredictable effects on the detector leakage current. Radiation-induced surface degradation has rarely, if ever, been observed for germanium detectors. The possibility of minimizing hole trapping in charge collection by the use of a high-purity germanium coaxial detector configured with the p (+) contact on the coaxial periphery is discussed.

Pehl, R. H.↗
NASA NTRS1977

Effect of prior creep at 1365 K on the room temperature tensile properties of several oxide dispersion strengthened alloys

An experimental study was conducted to determine whether oxide dispersion-strengthened (ODS) Ni-base alloys in wrought bar form are subject to creep degradation effects similar to those found in thin-gage sheet. The bar products evaluated included ODS-Ni, ODS-NiCr, and advanced ODS-NiCrAl types; the alloys included microstructures ranging from an essentially perfect single crystal to a structure consisting of very small elongated grains. Tensile test specimens were exposed to creep at various stress levels at 1365 K and then tensile tested at room temperature. Low residual tensile properties, change in fracture mode, appearance of dispersoid free bands, grain boundary cavitation, and/or internal oxidation are interpreted as creep degradation effects. The amount of degradation depends on creep strain, and degradation appears to be due to diffusional creep which produces dispersoid free bands around grain boundaries acting as vacancy sources.

Whittenberger, J. D.↗
NASA NTRS1979

Reverse annealing in radiation-damaged, silicon solar cells

In order to understand the results in terms of properties of the radiation induced defects, a combination of diffusion length measurements and defect data obtained from Deep Level Transient Spectroscopy were used. The results indicate that the defect at E sub v + 0.30 eV is responsible for the observed reversed annealing. The defect was identified as a boron-oxygen vacancy complex. This identification is a guide to processing efforts aimed at increasing the concentration of these radiation induced defects.

Weinberg, I.↗
NASA NTRS1979

Some TEM observations of Al2O3 scales formed on NiCrAl alloys

The microstructural development of Al2O3 scales on NiCrAl alloys has been examined by transmission electron microscopy. Voids were observed within grains in scales formed on a pure NiCrAl alloy. Both voids and oxide grains grew measurably with oxidation time at 1100 C. The size and amount of porosity decreased towards the oxide-metal growth interface. The voids resulted from an excess number of oxygen vacancies near the oxidemetal interface. Short-circuit diffusion paths were discussed in reference to current growth stress models for oxide scales. Transient oxidation of pure, Y-doped, and Zr-doped NiCrAl was also examined. Oriented alpha-(Al, Cr)2O3 and Ni(Al, Cr)2O4 scales often coexisted in layered structures on all three alloys. Close-packed oxygen planes and directions in the corundum and spinel layers were parallel. The close relationship between oxide layers provided a gradual transition from initial transient scales to steady state Al2O3 growth.

Smialek, J.↗
NASA NTRS1980

Radiation damage annealing mechanisms and possible low temperature annealing in silicon solar cells

The defect responsible for reverse annealing in 2 ohm/cm n(+)/p silicon solar cells was identified. This defect, with energy level at e sub v + 0.30 eV was tentatively identified as a boron oxygen-vacancy complex. Results indicate that its removal could result in significant annealing for 2 ohm/cm and lower resistivity cells at temperatures as low as 200 C. These results were obtained by use of an expression derived from the Shockley-Read-Hall recombination theory which relates measured diffusion length ratios to relative defect concentrations and electron capture cross sections. The relative defect concentrations and one of the required capture cross sections are obtained from Deep Level Transient Spectroscopy. Four additional capture cross sections are obtained using diffusion length data and data from temperature dependent lifetime studied. These calculated results are in reasonable agreement with experimental data.

Weinberg, I.↗
NASA NTRS1979

Some TEM observations of Al2O3 scales formed on NiCrAl alloys

The microstructural development of Al2O3 scales on NiCrAl alloys has been examined by transmission electron microscopy. Voids have been observed within grains in scales formed on a pure NiCrAl alloy. Both voids and oxide grains grew measurably with oxidation time at 1100 C. The size and amount of porosity decreased towards the oxide-metal growth interface. It was postulated that the voids resulted from an excess number of oxygen vacancies near the oxide-metal interface. Short-circuit diffusion paths were discussed in reference to current growth stress models for oxide scales. Transient oxidations of pure, Y-doped, and Zr-doped NiCrAl was also examined. Oriented alpha-(Al,Cr)2O3 and Ni(Al,Cr)2O4 scales often coexisted in layered structures on all three alloys. Close-packed oxygen planes and directions in the corundum and spinel layers were parallel. The close relationships between oxide layers provided a gradual transition from initial transient scales to steady state Al2O3 growth.

Smialek, J.↗
NASA NTRS1980

Origin of reverse annealing in radiation-damaged silicon solar cells

The paper employs relative defect concentrations, energy levels, capture cross sections, and minority carrier diffusion lengths in order to identify the defect responsible for the reverse annealing observed in a radiation damaged n(+)/p silicon solar cell. It is reported that the responsible defect, with the energy level at +0.30 eV, has been tentatively identified as boron-oxygen-vacancy complex. In conclusion, it is shown that removal of this defect could result in significant cell recovery when annealing at temperatures well below the currently required 400 C.

Weinberg, I.↗
NASA NTRS1980

Distribution of oxygen in silicon and its effects on electronic characteristics on a microscale

The microdistribution of oxygen in silicon was obtained by scanning IR absorption in as grown Czochralski crystals. The crystals were subsequently submitted to various heat treatments. The profiles of the generated thermal donors were determined by spreading resistance measurements. Contrary to the prevailing views, it was found that the concentration of the activated thermal donors is not strictly a function of the oxygen concentration, but depends strongly on an additional factor, which was shown to be associated with vacancy concentration. These conclusions could only be reached on the basis of microscale characterization. In fact, commonly employed macroscale analysis has led to erroneous conclusions.

Gatos, H. C.↗
NASA NTRS1980

Radiation tolerance of boron doped dendritic web silicon solar cells

The potential of dendritic web silicon for giving radiation hard solar cells is compared with the float zone silicon material. Solar cells with n(+)-p-P(+) structure and approximately 15% (AMl) efficiency were subjected to 1 MeV electron irradiation. Radiation tolerance of web cell efficiency was found to be at least as good as that of the float zone silicon cell. A study of the annealing behavior of radiation-induced defects via deep level transient spectroscopy revealed that E sub v + 0.31 eV defect, attributed to boron-oxygen-vacancy complex, is responsible for the reverse annealing of the irradiated cells in the temperature range of 150 to 350 C.

Rohatgi, A.↗
NASA NTRS1981

A molecular model for ice nucleation and growth, attachment 2

The central field empirical pair potential model is applied to studying the effects of kinks, ledges, and vacancies on the absorption of water molecules from the vapor. Molecular dynamics simulations indicate that cluster and/or surface modes play a primary role in the absorption process, the flexibility of the hydrogen bond serves to decrease the energy required for structural interconversion, and the rapid distribution of added energy in a hydrogen bonded system lead to aggregate stability which greatly exceeds that predicted by static energy calculations.

Hale, B. N.↗
NASA NTRS1980

Photodegradation in silicon

The state of knowledge concerning the following defects in silicon is briefly surveyed: substitutional impurities; vacancy-related defects; interstitial-related defects; defect pairs. Although a great deal is known, it is argued that not enough is yet known to model radiation damage production in the bulk or in the vicinity of the junction of a solar cell. The results on photon degradation (and enhancement) of solar cells are then reviewed, and it is suggested that defect pairs may be the defects responsible. Mechanisms for photon-induced dissociation of pairs are discussed but a more detailed understanding awaits the identification of the actual defects.

Corbett, J. W.↗
NASA NTRS1980

Radiation damage annealing mechanisms and possible low temperature annealing in silicon solar cells

Deep level transient spectroscopy and the Shockley-Read-Hall recombination theory are used to identify the defect responsible for reverse annealing in 2 ohm-cm n+/p silicon solar cells. This defect, with energy level at Ev + 0.30 eV, has been tentatively identified as a boron-oxygen-vacancy complex. It has been also determined by calculation that the removal of this defect could result in significant annealing at temperatures as low as 200 C for 2 ohm-cm and lower resistivity cells.

Weinberg, I.↗
NASA NTRS1981

Annealing and anomalous high-energy electron irradiation effects in low-cost silicon N+P solar cells

Silicon solar cells of N(+)P type were subjected to 1 MeV electron irradiation (up to 10 to the 16th electrons/sq cm) and then annealed at 450 C for 20 min or annealed with no electron irradiation. Electron irradiation resulted in a degradation of longer wavelength cell response, but produced a marked enhancement of response at shorter wavelengths with a peak change of 40% at 0.44 microns. Subsequent thermal anneal at 450 C reduced the long-wavelength degradation, but enhancement at shorter wavelengths persisted. Excitation at the shorter wavelengths was in the N(+)-diffused layer and in the junction region of the cell. Anneal of unirradiated cells produced shorter-wavelength enhancement with a similar peaking at 0.44 microns, but with a relative change of only 20%. More enhancement was produced in the longer wavelength region (up to 0.8 microns). These effects in the different cell regions are explained by a decrease in the interstitial oxygen-impurity complexes (deep recombination levels) and the formation of substantial oxygen-silicon vacancy centers (donors).

Garlick, G. F. J.↗
NASA NTRS1982

On the cause of the flat-spot phenomenon observed in silicon solar cells at low temperatures and low intensities

A model is presented that explains the "flat-spot" (FS) power loss phenomenon observed in silicon solar cells operating deep space (low temperature, low intensity) conditions. Evidence is presented suggesting that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the PN junction is destroyed and replaced with a metal-semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallurization are presented along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Preliminary data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Weizer, V. G.↗
NASA NTRS1981

Effects of low temperature periodic annealing on the deep-level defects in 200 keV proton irradiated AlGaAs-GaAs solar cells

The GaAs solar cell has shown good potential for space applications. However, degradation in performance occurred when the cells were irradiated by high energy electrons and protons in the space environment. The considered investigation is concerned with the effect of periodic thermal annealing on the deep-level defects induced by the 200 keV protons in the AlGaAs-GaAs solar cells. Protons at a fluence of 10 to the 11th P/sq cm were used in the irradiation cycle, while annealing temperatures of 200 C (for 24 hours), 300 C (six hours), and 400 C (six hours) were employed. The most likely candidate for the E(c) -0.71 eV electron trap observed in the 200 keV proton irradiated samples may be due to GaAs antisite, while the observed E(v) +0.18 eV hole trap has been attributed to the gallium vacancy related defect. The obtained results show that periodic annealing in the considered case does not offer any advantages over the one time annealing process.

Li, S. S.↗
NASA NTRS1982

On the cause of the flat-spot phenomenon observed in silicon solar cells at low temperatures and low intensities

A model is presented that explains the 'flat-spot' power-loss phenomenon observed in silicon solar cells operating under deep space (low temperature, low intensity) conditions. Evidence is presented suggesting that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the PN junction is destroyed and replaced with a metal-semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallization are presented along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Preliminary data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Weizer, V. G.↗