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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Highly efficient pseudomorphic InGaAs/GaAs/AlGaAs single quantum well lasers for monolithic integration

Highly efficient ridge waveguide pseudomorphic single quantum well lasers, emitting at 980 nm, have been fabricated from an In(0.2)Ga(0.8)As/GaAs/AlGaAs graded-index separate confinement heterostructure grown by molecular beam epitaxy. The laterial index guiding provided by the ridge reduces the anomalously large lateral loss of optical power found in gain-guided structures, thereby reducing the internal loss by more than 50 percent. The low threshold current (7.6 mA) and high differential quantum efficiency (79 percent) obtained under continuous operation as well as the transparency of the GaAs substrate to the emitted radiation render these lasers attractive for Ga-As-based optoelectronic integration.

Larsson, A.↗
NASA NTRS1990

The Bholghati (howardite) consortium - An overview

The Bholghati (howardite) has had a complex history. The consortium studies indicate that eucrite clasts show evidence of rapid crystallization followed by prolonged subsolidus annealing. Dark clasts are carbonaceous CM2 type. Bholghati bulk composition can be modeled by 55 percent eucritic, 45 percent diogenitic, and 3 percent dark clast components. The eucritic clasts show a LREE depleted pattern relative to HREEs, which is not typical of a normal eucrite. The LREE depletion requires two-stage melting from a chondritic source. The Bholghati evolution scenario is (1) early multiple magmatic events (4.53 Ga ago), producing eucrites and diogenites; (2) a metamorphic event (2-3 Ga ago) and prolonged subsolidus annealing; (3) fragmentation and low-temperature mixing of eucrites and diogenites; (4) low-velocity impact and admixing of carbonaceous material; (5) disruption of regolith and ejection of Bholghati 10-17 Ma ago; and (6) Bholghati fell on the earth in 1905.

Laul, J.-C.↗
NASA NTRS1990

Intersubband absorption in Si(1-x)Ge(x/Si superlattices for long wavelength infrared detectors

Researchers calculated the absorption strengths for intersubband transitions in n-type Si(1-x)Ge(x)/Si superlattices. These transitions can be used for the detection of long-wavelength infrared radiation. A significant advantage in Si(1-x)Ge(x)/Si supperlattice detectors is the ability to detect normally incident light; in Ga(1-x)Al(x)As/GaAs superlattices, intersubband absorption is possible only if the incident light contains a polarization component in the growth direction of the superlattice. Researchers present detailed calculation of absorption coefficients, and peak absorption wavelengths for (100), (111) and (110) Si(1-x)Ge(x)/Si superlattices. Peak absorption strengths of about 2000 to 6000 cm(exp -1) were obtained for typical sheet doping concentrations (approx. equals 10(exp 12)cm(exp -2)). Absorption comparable to that in Ga(1-x)Al(x)As/GaAs superlattice detectors, compatibility with existing Si technology, and the ability to detect normally incident light make these devices promising for future applications.

Rajakarunanayake, Yasantha↗
NASA NTRS1990

Possibilities for LWIR detectors using MBE-grown Si(/Si(1-x)Ge(x) structures

Traditionally, long wavelength infrared (LWIR) detection in Si-based structures has involved either extrinsic Si or Si/metal Schottky barrier devices. Molecular beam epitaxially (MBE) grown Si and Si/Si(1-x)Ge(x) heterostructures offer new possibilities for LWIR detection, including sensors based on intersubband transitions as well as improved conventional devices. The improvement in doping profile control of MBE in comparison with conventional chemical vapor deposited (CVD) Si films has resulted in the successful growth of extrinsic Si:Ga, blocked impurity-band conduction detectors. These structures exhibit a highly abrupt step change in dopant profile between detecting and blocking layers which is extremely difficult or impossible to achieve through conventional epitaxial growth techniques. Through alloying Si with Ge, Schottky barrier infrared detectors are possible, with barrier height values between those involving pure Si or Ge semiconducting materials alone. For both n-type and p-type structures, strain effects can split the band edges, thereby splitting the Schottky threshold and altering the spectral response. Measurements of photoresponse of n-type Au/Si(1-x)Ge(x) Schottky barriers demonstrate this effect. For intersubband multiquntum well (MQW) LWIR detection, Si(1-x)Ge(x)/Si detectors grown on Si substrates promise comparable absorption coefficients to that of the Ga(Al)As system while in addition offering the fundamental advantage of response to normally incident light as well as the practical advantage of Si-compatibility. Researchers grew Si(1-x)Ge(x)/Si MQW structures aimed at sensitivity to IR in the 8 to 12 micron region and longer, guided by recent theoretical work. Preliminary measurements of n- and p-type Si(1-x)Ge(x)/Si MQW structures are given.

Hauenstein, Robert J.↗
NASA NTRS1991

Improvements in contact resistivity and thermal stability of Au-contacted InP solar cells

Specific contact resistivities for as-fabricated Au contacts on n-p InP solar cells are typically in the 10(exp -3) ohm/sq cm range, but contact resistivities in the 10(exp -6) ohm/sq cm range can be obtained if the cells are heat treated at 400 C for a few minutes. This heat treatment, however, results in a dramatic drop in the open circuit voltage of the cell due to excessive dissolution of the emitter into the metallization. It was found that low values of contact resistivity can be secured without the accompanying drop in the open circuit voltage by adding Ga and In in the Au metallization. It is shown that Au contacts containing as little as 1 percent atomic Ga can suppress the reaction that takes place at the metal-InP interface during heat treatment, while exhibiting contact resistivity values in the low 10(exp -5) ohm/sq cm. Detailed explanations for the observed superior thermal stability of these contacts are presented.

Fatemi, Navid S.↗
NASA NTRS1991

Radiation resistance of Ge, Ge0.93Si0.07, GaAs and Al0.08Ga0.92 as solar cells

Solar cells made of Ge, Ge(0.93)Si(0.07) alloys, GaAs and Al(0.08)Ga(0.92)As were irradiated in two experiments with 1-meV electrons at fluences as great as 1 x 10(exp 16) cm(exp-2). Several general trends have emerged. Low-band-gap Ge and Ge(0.93)Si(0.07) cells show substantial resistance to radiation-induced damage. The two experiments showed that degradation is less for Al(0.08)Ga(0.92)As cells than for similarly irradiated GaAs cells. Compared to homojunctions, cells with graded-band-gap emitters did not show the additional resistance to damage in the second experiment that had been seen in the first. The thickness of the emitter is a key parameter to limit the degradation in GaAs devices.

Timmons, M. L.↗
NASA NTRS1991

The Estherville mesosiderite - U-Pb, Rb-Sr, and Sm-Nd isotopic study of a polymict breccia

The U-Pb, Rb-Sr, and Sm-Nd isotopic systematics of the Estherville mesoderite were studied using analytical procedures and mass spectrometric techniques similar to those reported by Nakamura et al. (1976), Tatsumoto et al. (1987), and Premo et al. (1989) to analyze 21 separates of a 13-g Estherville clast, obtained either by handpicking or by using density and magnetic separation methods. The results on the Pb-Pb and U-Pb ages (about 4555 and about 4560 Ma, respectively) indicate that at least a part of the Estherville silicate fraction was formed early in the history of solar system. Younger Pb-Pb and U-Pb ages (about 4.43 Ga) were also obtained, confirming the heterogeneity of the Estherville mesoderite that is in agreement with the Wasson and Rubin (1985) suggestion of several generations of meteoritic impacts. The Sm-Nd and Rb-Sr ages were found to be close to 4.56 Ga.

Brouxel, M.↗
NASA NTRS1991

Probable age of Autolycus and calibration of lunar stratigraphy

Ar-39 - Ar-40 analyses of three petrographically distinct, shocked Apollo 15 KREEP (i.e., high K, rare earth element, P, and other trace element contents) basalt samples demonstrate that a major impact event affected all three samples at about 2.1 Ga. The Copernican System craters Aristillus and Autolycus are to the north. Autolycus, the older of the two, is in a particularly appropriate terrain and is the most likely source of the 2.1 Ga heating and delivery event. With this calibration point, and if Autolycus really is a Copernican crater, the Copernican System lasted twice as long as has previously been suggested. Furthermore, the moon was not subjected to a constant cratering rate over the past 3 billion years; the average rate in the preceding Eratosthenian must have been twice that in the Copernican.

Ryder, G.↗
NASA NTRS1991

Nitrogen isotopes in the 74001/74002 double-drive tube from Shorty Crater, Apollo 17

Trapped N in the black/orange glass at different levels of the 74001/74002 double-drive tube can be used to adress two questions: the isotopic composition of indigenous N exhaled from the lunar interior 3.7 Ga ago; and the nature of extralunar N incident upon the lunar surface during the same epoch. Indigenous lunar N is found to have the same delta-N-15 value as N in terrestrial mantle samples with high He-3/He-4 ratios. This suggests that the lunar interior and the terrestrial mantle acquired their N, and possibly other volatiles, from a common nebular reservoir. The ancient extralunar N reveals evidence for a N-15-enriched component present on the lunar surface 3.7 Ga ago, together with N-15-depleted N. However, the role of these components, for neither of which a solar origin can be excluded, in explaining the long-term increase in N-15/N-14 ratio of regolith N on the moon is not yet clear.

Kerridge, J. F.↗
NASA NTRS1991

Nd-142/Nd-144 in SNCs and early differentiation of a heterogeneous Martian mantle

Sm/Nd correlated variations in Nd-142/Nd-144 have been observed for mineral phases of achondrites from decay of live Sm-146 in the early solar system. Crystallization ages of shergottites-nakhlites-Chassigny (SNC) meteorites are less than or = 1.3 Ga, so variations of Nd-142/Nd-144 among mineral phases of the SNC's are not expected. However, if SNC's were derived from source reservoirs of differing Sm/Nd ratios, established while Sm-146 was still alive, and which remained isolated except for magma extraction, then variations in Nd-142/Nd-144 would exist among individual SNC meteorites. Rb-Sr and U-Pb isotopic data for the shergottites imply differentiation of their parent planet approximately 4.6 Ga ago. The confirmation of the conclusion that the nakhlites and shergottites were derived from different source regions, and that, consequently, the shergottite parent body (SPB) mantle was heterogeneous is presented.

Nyquist, L. E.↗
NASA NTRS1990

Studies in general aviation aerodynamics

The Department of Aerospace Engineering at the University of Maryland has completed a research study for NASA Langley on the application of drooped leading edges to high aspect wings. The experimental study conducted for this grant was a natural extension of work previously conducted at NASA Ames, the University of Michigan, NASA Langley and the University of Maryland. Previous research had shown that wing planform modifications (commonly referred to as drooped leading edge mods) could have a significant effect on reducing or eliminating the stall/spin characteristics of General Aviation (GA) aircraft. All aircraft studied in the earlier work had relatively low aspect ratio wings (AR = 6). Since future GA aircraft will feature higher aspect ratio wings, the obvious question was: how well will the dropped leading edge work on higher aspect ratio wings? The focus of the current study was to examine the effectiveness of the drooped leading edge modifications to higher aspect ratio wings with AR = 9 to 12.

Winkelmann, Allen E.↗
NASA NTRS1992

Microwave properties of peeled HEMT devices sapphire substrates

The focus of this research is to demonstrate the first full radio frequency characterization of high electron mobility transistor (HEMT) device parameters. The results of this research are used in the design of circuits with peeled HEMT devices, e.g. 10 GHz amplifiers. Devices were fabricated using two HEMT structures grown by molecular beam epitaxy methods. A 500 A AlAs release layer for 'peel off' was included under the active layers of the structure. The structures are a homogeneously doped Al(0.3)GA(0.7)As/GaAs and a delta doped square well Al(.23)Ga(.77)As/GaAs HEMT structure. Devices were fabricated using a mesa isolation process. Contacts were done by sequentially evaporating Au/Ge/Au/Ni/Au followed by rapid thermal anneal at 400 C for 15 seconds. Gates were wet etch recessed and 1 to 1.4 micron Ti/Au gate metal was deposited. Devices were peeled off the GaAs substrate using Apiezon wax to support the active layer and a HF:DI (1:10) solution to remove the AlAs separation layer. Devices were then attached to sapphire substrates using van der Waals bonding.

Young, Paul G.↗
NASA NTRS1992

Evolution of isotopic signatures in lunar-regolith nitrogen - Noble gases and nitrogen in grain-size fractions from regolith breccia 79035

Ilmenite and pyroxene grain-size separates from 79035 were analyzed for trapped N; the ilmenite was also analyzed for trapped Xe. Ilmenite N contains two or more isotopically distinct trapped components differing in release temperature and therefore plausibly in implantation energy. The isotopically light, higher-temperature (higher-energy?) component has a delta N-15 value equal to -180 percent, significantly above the minimum value observed in bulk 79035, suggesting that parts of 79035 were exposed on the lunar surface earlier than the ilmenite. Using trapped Ar-40/Ar-36 and cosmogenic Ne-21 (Benkert, 1989) a compaction age of about 1 Ga was derived for the ilmenite. This implies a considerably more recent exposure than previously thought, and suggests that the long-term change in delta N-15 of regolith N was more rapid than generally believed. Comparison of these results with those for black/orange glass from 74001/74002 (compaction age 3.7 Ga; delta N-15 in the range -36 to +18 percent (Kerridge et al., 1991) indicates that the long-term trend may have followed a complex evolutionary path. Data for 79035 pyroxenes are consistent with the ilmenite compaction age but suggest a more complex exposure history.

Kerridge, J. F.↗
NASA NTRS1992

U-Th-Pb, Rb-Sr, and Sm-Nd isotopic systematics of lunar troctolitic cumulate 76535 - Implications on the age and origin of this early lunar, deep-seated cumulate

The U-Th-Pb, Rb-Sr, and Sm-Nd isotopic systematics of four lightly leached residues of pristine, high-Mg, troctolitic cumulate 76535 were analyzed in order to determine their ages and magma sources. The data indicate that the cumulate was in isotopic equilibrium with a fluid or magma characterized by a high U-238/Pb-204 (mu) value of 600 at 4.236 Ga. Two and three stage Pb evolution calculations define even greater source mu values of about 1000, assuming low lunar initial mu values between 5 and 40 prior to about 4.43 Ga. These results are similar to mu values for KREEP sources and are also consistent with values from 78235, suggesting that at least some high-Mg suite rocks were derived from magma sources with high-mu values similar to KREEP, and support that idea that these rocks postdate primary lunar differentiation and formation of ferroan anorthosites.

Premo, Wayne R.↗
NASA NTRS1992

Rb-Sr and Sm-Nd chronology of an Apollo 17 KREEP basalt

The paper determines Sm-Nd and Rb-Sr mineral isochrons for an Apollo 17 KREEP (pigeonite) basalt clast from breccia 72275 collected from Boulder 1, Station 2 in the Valley of Taurus-Littrow. Sm-Nd analyses of the basalt yield a precise mineral isochron age of 4.08 +/-0.07 Ga for lambda(Sm-147) = 0.00654/Ga. The concordancy of Sm-Nd and Rb-Sr ages for the basalt suggests that it crystallized about 4.08 Ga ago. Distinct ages and initial Sr isotopic ratios for Apollo 17 KREEP basalts and Apollo 14 and 15 KREEP suggest that these two types of KREEP basalts were not derived from the same source. Apollo 17 KREEP basalts are contemporaneous with some Apollo 14 aluminous mare basalts. The ages and Sr and Nd isotopic data suggest that these two different types of basalts were produced from sources having similar Rb/Sr ratios but different Sm/Nd ratios.

Shih, C.-Y.↗
NASA NTRS1992

Geochemistry of Precambrian carbonates. V - Late Paleoproterozoic seawater

A study of mineralogy, chemistry, and isotopic composition of the Coronation Supergroup (about 1.9 Ga, NWT), Canada, and the McArthur Group (about 1.65 NT), Australia, is reported in order to obtain better constrained data for the first- and second-order variations in the isotopic composition of late Paleoproterozoic (1.9 +/- 0.2 Ga) seawater. Petrologically, both carbonate sequences are mostly dolostones. The McArthur population contains more abundant textural features that attest to the former presence of sulfates and halite, and the facies investigated represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes. It is suggested that dolomitization was an early diagenetic event and that the O-18 depletion of the Archean to late Paleoproterozoic carbonates is not an artifact of postdepositional alteration.

Veizer, Jan↗
NASA NTRS1992

Coincidences in time of the Imbrium Basin impact and Apollo 15 KREEP volcanic series: Impact-induced melting?

On the Earth there may be no firm evidence that impacts can induce volcanic activity. However, the Moon does provide a very likely example of volcanism induced by an immense impact: the Imbrium Basin-forming event was immediately succeeded by a crustal partial melting event that released KREEP lava flows over a wide area. These two events are presently indistinguishable in radiometric age. The sample record indicates that such KREEP volcanism had not occurred in the region prior to that time, and never occurred again. Such coincidence in time implies a genetic relationship between the two events, and impact-induced partial melting appears to be the only candidate process. This conclusion rests essentially on the arguments that: (1) the Imbrium Basin event took place 3.86 +/- 0.02 Ga ago; (2) the Apennine Bench Formation postdates Imbrium; (3) the Apollo 15 KREEP basalts are 3.85 +/- 0.03 Ga old; (4) the Apollo 15 KREEP basalts are derived from the Apennine Bench Formation; and (5) the Apollo 15 KREEP basalts are volcanic. Thus, the Apollo 15 KREEP basalts represent a unique volcanic unit that immediately postdates the Imbrium event (within 20 Ma, possibly much less). The evidence for the links in the argument are sketched, and some implications for initial conditions are described. Ramifications of the process for the early history of the Earth are briefly explored.

Ryder, Graham↗
NASA NTRS1992

Melting of cognetic depleted and enriched reservoirs and the production of high Ti Mare basalts

Implicit in current understanding of the location of terrestrial enriched and depleted reservoirs is the notion that they are spatially separated. The depleted reservoir on Earth is situated in the upper mantle, and the complementary enriched reservoir is located in the crust. However, Earth reservoirs are continually being modified by recycling driven by mantle convection. The Moon is demonstrably different from Earth in that its evolution was arrested relatively early - effectively with 1.5 Ga of its formation. It is possible that crystallized trapped liquids (from the late stages of a magma ocean) have been preserved as LILE-enriched portions of the lunar mantle. This would lead to depleted (cumulate) and enriched (magma ocean residual liquid) reservoirs in the lunar upper mantle. There is no evidence for significant recycling from the highland crust back into the mantle. Therefore, reservoirs created at the Moon's inception may have remained intact for over 4.0 Ga. The topics discussed include the following: (1) radiogenic isotopes in high-Ti mare basalts; (2) formation of cogenetic depleted and enriched reservoirs; and (3) melting of the source to achieve high-Ti mare basalts.

Snyder, Gregory A.↗