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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 91 records · Page 5
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NASA NTRS1973

Temperature, stress, and annealing effects on the luminescence from electron-irradiated silicon

Low-temperature photoluminescence spectra are presented for Si crystals which have been irradiated with high-energy electrons. Studies of isochronal annealing, stress effects, and the temperature dependences of the luminescence are used to discuss the nature of the luminescent transitions and the properties of defects. Two dominant bands present after room-temperature anneal of irradiated material are discussed, and correlations of the properties of these bands are made with known Si defects. A band between 0.8 and 1.0 eV has properties which are related to those of the divacancy, and a band between 0.6 and 0.8 eV has properties related to those of the Si-G15(K) center. Additional peaks appear in the luminescence after high-temperature anneal; the influence of impurities and the effects of annealing of these lines are discussed.

Jones, C. E.↗
NASA NTRS1973

Recombination luminescence from electron-irradiated Li-diffused Si

Lithium doping has a dramatic effect on the low-temperature photoluminescence of electron-irradiated Si. In oxigen-lean Si with Li doping, a new irradiation-dependent luminescence band between 0.75 and 1.05 eV is observed, which is dominated by a zero-phonon peak at 1.045 eV. This band is believed to be due to radiative transitions involving a Li-modified divacancy. This band is present also in oxygen-rich, Li-diffused Si and is accompanied by bands previously related to the Si-G15(K) center and the divacancy. The intensities of the Li-modified divacancy and Si-G15(K) center bands are relatively weak in the oxygen-rich material, apparently due to the formation of lithium-oxygen complexes which reduce the concentration of unassociated interstitial Li and O.

Johnson, E. S.↗
NASA NTRS1974

Luminescence of powdered uranium glasses

Measurement of cathodoluminescence and photoluminescence efficiencies in powdered borosilicate glasses having different particle size and different uranium content. Excitation with 100 to 350 keV electrons and with 253.7 nm light was found to produce identical absolute radiant exitance spectra in powdered samples. The most efficient glass was one containing 29.4 wt% B2O3, 58.8 wt% SiO2, 9.8 wt% Na2O and 2.0 wt% UO2.

Eubanks, A. G.↗
NASA NTRS1980

Optimization of solar cells for air mass zero operation and study of solar cells at high temperatures, phase 4

The Pd contact to GaAs was studied using backscattering, Auger analysis, and sheet resistance measurements. Several metallurgical phases were present at low temperatures, but PdGa was the dominant phase in samples annealed at 500 C. Ti/Pd/Ag contacts appeared to have the lowest contact resistance. Etchback epitaxy (EBE) was compared to saturated melt epitaxy (SME) method of growing liquid phase epitaxial layers. The SME method resulted in a lower density of Ga microdroplets in the grown layer, although the best solar cells were made by the EBE method. Photoluminescence was developed as a tool for contactless analysis of GaAs cells. Efficiencies of over 8 percent were measured at 250 C.

Hovel, H. J.↗
NASA NTRS1982

Compensation mechanism in liquid encapsulated Czochralski GaAs Importance of melt stoichiometry

It is shown that the key to reproducible growth of undoped semi-insulating GaAs by the liquid encapsulated Czochralski (LEC) technique is the control over the melt stoichiometry. Twelve crystals were grown from stoichiometric and nonstoichiometric melts. The material was characterized by secondary ion mass spectrometry, localized vibrational mode far infrared spectroscopy, Hall-effect measurements, optical absorption, and photoluminescence. A quantitative model for the compensation mechanism in the semi-insulating material was developed based on these measurements. The free carrier concentration is controlled by the balance between EL2 deep donors and carbon acceptors; furthermore, the incorporation of EL2 is controlled by the melt stoichiometry, increasing as the As atom fraction in the melt increases. As a result, semi-insulating material can be grown only from melts above a critical As composition. The practical significance of these results is discussed in terms of achieving high yield and reproducibility in the crystal growth process.

Holmes, D. E.↗
NASA NTRS1983

Residual double acceptors in bulk GaAs

By using infrared absorption, photoluminescence, and Hall measurements we have observed an additional level associated with a residual acceptor in liquid encapsulated Czochralski GaAs. These results indicate that the defect is a double acceptor with levels 78 and 200 meV above the valence band.

Elliott, K. R.↗
NASA NTRS1983

GaInAs-AlInAs heterostructures for optical devices grown by MBE

The band gap of Ga(0.47)In(0.53)As corresponds to an emission wavelength of about 1.65 microns. Lasers have been produced with Al(0.48)In(0.52)As as cladding layers operating at room temperature. The peak emission of Ga(0.47)In(0.53)As can be continuously varied from 1.65 to 1.2 microns by the use of the multiquantum well structures. This range of wavelengths covers the minimum loss and dispersion in optical fibers and will be applicable to integrated optics. Double heterostructure broad area lasers have been fabricated using AlInAs as cladding layers to the GaInAs active layer. Room temperature threshold current densities of 4.3 kA/sq cm have been obtained for lasers with a 4500 A active region. The first data on GaInAs/AlInAs quantum well emitters will be presented. Photoluminescence of 4 K from quantum well layers of 100, 150, and 180 A with 150 A AlInAs barrier layers produced emission at 1.27, 1.35, and 1.41 microns, respectively. Ga(0.47)In(0.52)As quantum well LEDs have also been produced which emit at 1.34 microns.

Welch, D. F.↗
NASA NTRS1984

Calculation of the conduction band discontinuity for Ga(0.47)In(0.53)As/Al(0.48)In(0.52)As heterojunction

Photoluminescence studies at 4 K on Ga(0.47)In(0.53)As/Al(0.48)In(0.52)As single quantum wells exhibit emission ranging from 1.318 eV for a 15-A well to 0.82 eV for thick wells. The emission energy of each single quantum well is compared to theoretical curves which are generated from a finite potential square well model. The closest agreement between the experimental curves and the theoretical curves occurs when the conduction band discontinuity is taken to be 70 percent of the bandgap discontinuity or 0.52 eV.

Welch, D. F.↗
NASA NTRS1985

Processing of Complex Compound Semiconductors in Space

The purpose of this first year ground based research effort was to develop growth and characterization facilities for bulk single crystals of compound semiconductor alloys. As a system of 2-6 alloy crystal growth, C sub x Mn sub (1-x)Te was chosen since it has a relatively narrow solidus-liquidus separation serving as a suitable first model for zone melting and solution growth. Also, it allows, in principle, perfect lattice matching to Cd sub x Hg sub(1-x)Te that needs to be studied as a model for the investigation of heteroepitaxial growth from Te-solutions. A vertical zone melting furnace was built and used in the synthesis of Cd sub x Mn sub(1-x)Te from the elements and crystal growth in carbon coated fused silica crucibles of Bridgman growth geometry. Photoluminescence spectra of the crystals reveal band to band recombination at energies that agree with the reported literature data. In addition, deep luminescence is present at energies corresponding to proportional shifts from the 1.4 eV deep emission observed for bulk single crystals of CdTe.

Bachmann, K.↗
NASA NTRS1986

Sputtering and native oxide formation on (110) surfaces of Cd(1-x)Mn(x)Te

Native oxides on the surface of Cd(1-x)Mn(x)Te (X between 0 and 0.7) have been analyzed on the basis of X-ray photoemission spectroscopy measurements. Depth profile analysis revealed a significant increase in the thickness at higher Mn concentrations and a strong Mn segregation to the surface, respectively. Sputter-induced damage on cleaved (110)-oriented surfaces was analyzed by photoreflectance and photoluminescence measurements. The damage was found to be larger on CdTe than on the alloy. Thermal annealing showed nearly complete restoration for the surface of the alloy, while CdTe revealed irreversible modifications in the near-surface regime upon sputtering and post annealing.

Neff, H.↗
NASA NTRS1985

Absorption and fluorescence of alexandrite and of titanium in sapphire and glass

The fluorescence and absorption data for titanium in crystalline sapphire and titanium doped into two silicate and one phosphate glass structures are analyzed. It is observed that the Ti-doped silicate glass sample exhibits no absorption related to the Ti(III) ion, the Ti-doped phosphate glass is deep blue, the absorption line width of the glass samples are a factor of two larger than that of sapphire, and the absorption peak for the Ti in the glass shifted about 100 nm to the red from the Ti:sapphire absorption peak. This shift reveals that the Ti(III) ion is sensitive to the crystalline environment and not to the glass environment. The photoluminescence spectra for Ti-doped sapphire and alexandrite are compared. It is detected that the Ti:sapphire exhibits a broader spectrum than that for alexandrite with a peak at 750 nm. The three zero phonon transitions of Ti:Al2O3 at liquid nitrogen temperatures are studied.

Byvik, C. E.↗
NASA NTRS1987

Material growth and characterization for solid state devices

During this period InGaAs and InGaAsP were grown on (100)InP by liquid phase electroepitaxy (LPEE). Results of the epitaxial growth of InGaAs on sputtered quartz masked substrates are presented. The resulting surface morphology can be related to the current density distribution near the edges of a masked pattern. The quaternary InGaAs was grown with compositions corresponding to 1.3 micron and 1.5 micron emission wavelengths. Growth rates were found to be linearly dependent upon current density, and a strong dependence upon composition was noted. These compositions lie in the miscibility gap region of the alloy phase diagram at the 645 C growth temperature. Growths were performed at 685 C to avoid the miscibility gap. Epilayers were characterized by photoluminescence, X-ray diffraction, secondary ion mass spectrometry, and Hall effect measurements. Aluminum oxide was deposited on silicon and InGaAs substrates for the characterization of this material as an insulator in a field effect transistor structure. It was determined that the results did not warrant further work with the deposition from an aluminum isopropoxide source. A metallographic vapor phase epitaxy system installation is nearing completion for use in hybrid III-V semiconductor epilayer growths.

Collis, Ward J.↗
NASA NTRS1987

Interstellar polycyclic aromatic hydrocarbons and carbon in interplanetary dust particles and meteorites

Raman spectra of interplanetary dust particles (IDPs) and meteorites containing material similar to polycyclic aromatic hydrocarbons (PAHs) show features that are similar in position and relative strength to interstellar IR emission features attributable to vibrational transitions in free molecular-sized PAHs. In addition, these spectra sometimes show red photoluminescence that has elsewhere been attributed to PAHs, and a part of the carbonaceous phase in IDPs and meteorites contain a degree of deuterium enrichment anticipated in small, free PAHs that are exposed to ISM UV radiation. These observations suggest that some of the IDPs' carbonaceous material may have been produced in circumstellar dust shells, and only slightly modified in interstellar space.

Allamandola, L. J.↗
NASA NTRS1987

II-VI and I-III-VI2 alloy crystals for photovoltaic applications

The results of the photoreflectance, photoconductivity, and photoluminescence studies on Mn(x)Cd(1-x)Te single crystals are discussed from the viewpoint of the utility of these compounds as gap components in multijunction solar cells. Attention is also given to recent results on CuInSe2 and on quaternary I-III-VI2 alloys based on CuInSe2. Recent Kelvin probe and X-ray photoemission spectroscopy studies of the surface properties of CuInSe2 and Mn(x)Cd(1-x)Te, respectively, are described. In addition, recent results on the radiation damage and annealing kinetics of Mn(x)Cd(1-x)Te are considered in the context of future work on the sputtering and ion beam deposition of thin films of this material.

Bachmann, K. J.↗
NASA NTRS1987

Liquid phase epitaxy growth of GaAs/GaAlAs multi-quantum well structures

Experiments in liquid phase epitaxial fabrication of thin GaAs/GaAlAs layers over a planar substrates have been carried out. Layer thicknesses smaller than 300 A were routinely obtained, with the best result being 120 A. Interface sharpness between the layers is approximately 10 A, which is comparable to OMCVD results, but the layers' thicknesses are usually not uniform. Of the experimental parameters, the growth time and the cooling rate seem to have the largest effect on the obtained layer thickness, while the growth temperature and the substrate crystallographic orientation produce less noticeable effects. Quantum effects in the grown layers were observed by photoluminescence measurements.

Cser, J.↗
NASA NTRS1987

Heteroepitaxial growth of Cd(1-x)Mn(x)Te on GaAs by metalorganic chemical vapor deposition

In this letter, preliminary results are reported of heteroepitaxial growth of the dilute magnetic semiconductor alloy Cd(1-x)Mn(x)Te on GaAs by metalorganic chemical vapor deposition. Dimethylcadmium (DMCd), diethyltellurium (DETe), and tricarbonyl (methylcyclopentadienyl) manganese (TCPMn) were used as source materials. The TCPMn had to be heated to as high as 140 C to provide the required vapor pressure. Films with Mn atomic fractions up to 30 percent have been grown over the temperature range 410-450 C. Results of optical absorption/transmission, photoluminescence, and X-ray diffraction measurements are presented along with a scanning electron micrograph showing good surface morphology of the grown layers.

Nouhi, Akbar↗
NASA NTRS1988

Influence of substrate misorientation on defect and impurity incorporation in GaAs/AlGaAs heterostructures grown by molecular-beam epitaxy

GaAS/AlGaAs heterostructures have been grown by molecular-beam epitaxy on GaAs substrates intentionally oriented (tilted) a few degrees (0-6.5) off the (001) plane towards either (111)A, (111)B, or (011). It was observed that the 4-K photoluminescence and low-field electron transport properties of these structures may be functions of the substrate tilt angle and tilt direction, depending on the concentration of impurities incorporated during growth. A substrate tilt during molecular-beam epitaxy is observed to have the largest effect on these properties when the background impurity concentration in the molecular-beam epitaxial machine is high. This supports the contention that the observed changes in material characteristics are due to differences in the incorporation of defects and impurities. The incorporation of defects and impurities are reduced by using substrates tilted toward (111)A, in comparison to nominally flat (001) substrates or substrates tilted toward (111)B.

Radulescu, D. C.↗
NASA NTRS1988

Raman observations on individual interplanetary dust particles

A Raman study of 20 representative interplanetary dust particles (IDPs) belonging to different infrared spectral classes is discussed. Six different groups of Raman spectra were discerned among the IDPs studied. Groups 1-5 exhibit the Raman signature of poorly crystallized carbonaceous material, with the degree of disorder of this material increasing from group 1 (most ordered) to group 5 (least ordered). Group 1 contains IDPs that have infrared spectra characteristic of olivines, and are deuterium depleted, while those in groups 2, 3, and 4 contain less ordered carbonaceous material and are deuterium enriched, suggesting different carbonaceous carrier phases for deuterium depletions and enrichments. Groups 5 and 6 contain little or no carbonaceous material, with an abundance of deuterium. No obvious relationship was found between Raman groups and infrared classes based on the 10 micron absorption band due to silicates. Because silicates are known to be present, but are not seen, it is presumed that silicate grains are coated with and/or imbedded in carbonaceous material. Several IDPs show broad visible laser-induced photoluminescence, probably produced by a carbonaceous component.

Wopenka, Brigitte↗