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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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

High-frequency noise of In(y)Ga(1-y)As/Al(x)Ga(1-x)As MODFETs and comparison to GaAs/Al(x)Ga(1-x)As MODFETs

Noise parameter measurements for recently developed 1 micron gate In(y)Ga(1-y)As/Al(0.15)Ga(0.85)As MODFETs have been performed at 8 GHz art room and cryogenic temperatures. Owing to the relatively small C(gs/sq rt) g(m) ratio in these devices compared to identical conventional GaAs/AlGaAs MODFETs, both room- and cryogenic temperature noise figures have been reduced. In addition, the light sensitivity and drift in noise figure at cryogenic temperatures observed in conventional GaAs/AlGaAs MODFETs have been substantially reduced.

Morkoc, H.↗
NASA NTRS1986

Identification of the Ga interstitial in Al(x)Ga(1-x)As by optically detected magnetic resonance

A new optically detected magnetic resonance spectrum in Al(x)Ga(1-x)As is reported and assigned to native Ga interstitials. Luminescence-quenching signals were observed over the energy region from 0.75 to 1.1 eV. The optically detected magnetic resonance is nearly isotropic, with spin-Hamiltonian parameters g = 2.025 + or - 0.006, central hyperfine splitting A(Ga-69) = 0.050 + or - 0.001/cm, and A(Ga-71) = 0.064 + or - 0.001/cm for H near the 001 line. The strong hyperfine coupling denotes an electronic state of A1 symmetry, which current theories predict for the Ga interstitial but not the Ga antisite. The slight anisotropy probably indicates that the Ga(i) is paired with a second, unknown defect.

Kennedy, T. A.↗
NASA NTRS1977

Theoretical studies of a new double graded band-gap Al sub x Ga sub 1-x As-Al sub y Ga sub 1-y As

A new double graded band-gap (DGBG) Al sub x Ga sub l-x As-Al sub z Ga sub l-z As solar cell has potential for providing high efficiency performance throughout the entire life of a solar cell in a space environment. A preliminary theoretical analysis indicates that short circuit current available from an optimized DGBG cell is slightly larger than that of a previously reported single graded band-gap cell. However, the DGBG cell potentially offers a substantial improvement in radiation resistance of the base region.

Hutchby, J. A.↗
NASA NTRS1986

Low-loss optical waveguides made with molecular beam epitaxial In(0.012)Ga(0.988)As and In(0.2)Ga(0.8)As-GaAs superlattices

Low-loss optical guiding in In-doped GaAs is demonstrated for the first time. Ridge waveguides are made with single In(0.012)Ga(0.988)As ternary layers and In(0.2)Ga(0.8)As-GaAs superlattices. Attenuation constants of about 1.3 dB/cm are measured and the principal loss mechanism is identified to be scattering at the ridge walls. It is expected that improved fabrication techniques will lead to guides with attenuation less than or equal to 0.5 dB/cm.

Das, U.↗
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 NTRS1988

Al(0.3)Ga(0.7)As / Al(0.05)Ga(0.95)As light-emitting diodes on GaAs-coated Si substrates grown by liquid phase epitaxy

Al(0.3)Ga(0.7)As/Al(0.05)Ga(0.95)As double-heterostructure light-emitting diodes (LEDs) were successfully grown for the first time by liquid phase epitaxy on a GaAs-coated Si substrate that was prepared by a sequential process of migration-enhanced epitaxy and molecular beam epitaxy. The edge-emitting LEDs had diode ideality factors of 1.54 at a forward-biased voltage higher than 0.9 V and external quantum efficiencies of 0.0033 W/A per facet. This efficiency is 50 times higher than the previously reported value, and is on the same order as that of AlGaAs homojunction LEDs fabricated on the GaAs substrates by liquid phase epitaxy.

Sakai, Shiro↗
NASA NTRS1992

Reduced mobility and PPC in In(.20)Ga(.80)As / Al(.23)Ga(.77)As HEMT structure

Transport properties of a pseudomorphic In(.20)Ga(.80)As/Al(.23)Ga(.77)As High Electron Mobility Transistor (HEMT) structure were measured by Hall and SdH techniques. Two samples of identical structures but with different doping levels were compared. Low temperature mobility measurements as a function of concentration coincides with the onset of second subband occupancy, indicating that the decrease in mobility is due to intersubband scattering. In spite of the low Al content (23 percent), large persistent photoconductivity (PPC) was observed in the highly doped sample only, showing a direct correlation between the PPC and doping concentration of the barrier layer.

Schacham, S. E.↗
NASA NTRS2003

Synthesis,and structural characterization of [(CH3(C5H4N))Ga(SCH2(CO)O)]-[(4-MepyH)]+, a novel Ga(III) five coordinate complex.

The synthesis and structural characterization of a novel ionic Ga(III) five coordinate complex [{CH3(C5H4N)}Ga(SCH2(CO)O)2]-[(4-MepyH)]+, (4-Mepy = CH3(C5H5N)) from the reaction between Ga2Cl4 with sodium mercapto-acetic acid in 4-methylpyridine is described. Under basic reaction conditions the mercapto ligand is found to behave as a 2e- bidentate ligand. Single crystal X-ray diffraction studies show the complex to have a distorted square pyramidal geometry with the [(-SCH2(CO)CO-)] ligands in a trans conformation. The compound crystallizes in the P2(sub 1)/c (No. 14) space group with a = 7.7413(6) A, b = 16.744(2) A, c = 14.459(2) A, V = 1987.1(6) A(sup 3), R(F) = 0.032 and R(sub w) = 0.038.

Banger, Kulbinder K.↗
NASA NTRS1976

The optimization of Ga (1-x)Al (x)As-GaAs solar cells for air mass zero operation and a study of Ga (1-x)Al (x)As-GaAs solar cells at high temperatures, phase 1

The three types of solar cells investigated were: (1) one consisting of a nGaAs substrate, a Zn doped pGaAs region, and a Zn doped Ga(1-x)Al(x)As layer, (2) one consisting of an nGaAs substrate, a Ge doped pGaAs region, and a pGa(1-x)Al(x)As upper layer, and (3) one consisting of an n+GaAs substrate, an nGa(1-x)Al(X)As region, a pGa(1-x)Bl(X) As region, and a pGa(1-y)Al(y)As upper layer. In all three cases, the upper alloy layer is thin and of high Al composition in order to obtain high spectral response over the widest possible range of photon energies. Spectral response, capacitance-voltage, current-voltage, diffusion length, sunlight (or the equivalent)-efficiency, and efficiency-temperature measurements were made as a function of device parameters in order to analyze and optimize the solar cell behavior.

Hovel, H. J.↗
NASA NTRS1986

A continuous record of tectonic evolution from 3.5 Ga to 2.6 Ga in Swaziland and northern Natal

The approx. 3.5 Ga-old bimodal suite underlying an extensive area in southwestern Swaziland comprises the oldest-dated sialic rocks in the Kaapvaal structural province. The suite consists of leucocratic, layered tonalitic-trondhjemitic gneisses and amphibolites characterized by the effects of repeated high strains. This suite is considered to represent a sialic basement on which metavolcanic and metasedimentary rocks, now preserved as scattered greenstone remnants, accumulated. Direct evidence to confirm this temporal relationship is lacking, but structural data from the Dwalile, Assegaai and Commondale areas indicate that (1) the bimodal gneisses experienced a complex structural history prior to the first recognizable deformation in the supracrustal rocks (i.e., D1 in the supracrustals is equivalent to Dn + 1 in the gneisses) and (2) scattered remnants of the Dwalile rocks infolded with the bimodal suite structurally overlie the gneisses and are preserved in synformal keels. Significant proportions of metaquartzites and metapelites are present in the Assegaai greenstone sequence, the presence of which implies the existence of felsic crust in the source area from which these sediments were derived, a conclusion that is consistent with the structural data.

Hunter, D. R.↗
NASA NTRS1986

Determination of carrier saturation velocity in high-performance In(y)Ga(1-y)As/Al(x)Ga(1-x)As modulation-doped field-effect transistors (with y between 0 and 0.2)

The relation between the intrinsic transconductance per unit gate width and the carrier saturation velocity, v(sat), is used to determine v(sat) for several high-performance pseudomorphic MODFET's with different InAs mole fractions (y). Measurements of In(y)Ga(1-y)As/AlGaAs MODFET's grown by MBE were found to give accurate v(sat) values at 77 K. Devices with y between 0 and 0.20 were shown to have higher v(sat) than conventional GaAs/AlGaAs MODFET's. An optimum y value for peak v(sat), which may optimize overall device performance, is expected.

Henderson, T. S.↗
NASA NTRS1992

Oxygen-induced Al surface segregation in Al(x)Ga(1-x)As and the effect of Y overlayers on the oxidation of the Y/Al(x)Ga(1-x)As interface

The oxidation of Al(x)Ga(1-x)As (x = 0.15, AlGaAs) was studied by AES and XPS at 350 C and different oxygen exposures (up to 5 x 10 exp 4 L). Also studied were the effects of yttrium overlayers (theta = 3 ML) on the oxidation of the AlGaAs surface. Substantial oxygen-induced Al surface segregation has been observed for both yttriated and nonyttriated AlGaAs surfaces which increased with increasing oxygen exposure. Also observed is a significant Y-enhanced oxidation of the AlGaAs surface. Oxidation of the yttriated AlGaAs surface was found to be a factor of 4 greater than that of the nonyttriated surface. Also, while oxidation of the nonyttriated AlGaAs yielded mainly Al2O(x) (x less than 3) and only little Ga2O3, the yttriated AlGaAs surface oxide layer was principally Ga2O3 and stoichiometric Al2O3. However, both the yttriated and nonyttriated surfaces were found to contain metallic As within the oxide layer.

Mesarwi, A.↗
NASA NTRS2002

Synthetic Vision Systems in GA Cockpit-Evaluation of Basic Maneuvers Performed by Low Time GA Pilots During Transition from VMC to IMC

An experimental investigation was conducted to study the effectiveness of modern flight displays in general aviation cockpits for mitigating Low Visibility Loss of Control and the Controlled Flight Into Terrain accidents. A total of 18 General Aviation (GA) pilots with private pilot, single engine land rating, with no additional instrument training beyond private pilot license requirements, were recruited to evaluate three different display concepts in a fixed-based flight simulator at the NASA Langley Research Center's General Aviation Work Station. Evaluation pilots were asked to continue flight from Visual Meteorological Conditions (VMC) into Instrument Meteorological Conditions (IMC) while performing a series of 4 basic precision maneuvers. During the experiment, relevant pilot/vehicle performance variables, pilot control inputs and physiological data were recorded. Human factors questionnaires and interviews were administered after each scenario. Qualitative and quantitative data have been analyzed and the results are presented here. Pilot performance deviations from the established target values (errors) were computed and compared with the FAA Practical Test Standards. Results of the quantitative data indicate that evaluation pilots committed substantially fewer errors when using the Synthetic Vision Systems (SVS) displays than when they were using conventional instruments. Results of the qualitative data indicate that evaluation pilots perceived themselves to have a much higher level of situation awareness while using the SVS display concept.

Takallu, M. A.↗
NASA NTRS1991

Pseudomorphic In(y)Ga(1-y)As/GaAs/Al(x)Ga(1-x)As single quantum well surface-emitting lasers with integrated 45 deg beam deflectors

The paper reports on the first demonstration of pseudomorphic InGaAs single quantum well surface-emitting lasers (SELs), with etched vertical mirrors and integrated 45-deg beam deflectors fabricated by ion beam etching. 100-micron-wide broad-area SELs exhibited a threshold current of 320 mA, a total power of 126 mW, and a total external differential quantum efficiency of 0.09 W/A for a 500-micron-long cavity. The perpendicular far-field pattern of broad-area SELs showed a full width at half maximum of about 20 deg. Lasers with various types of cavities fabricated from the same wafer were compared. Broad-area edge-emitting lasers had a threshold current of 200 mA, a total power of 700 mW, and a total external differential quantum efficiency of 0.52 W/A.

Kim, Jae-Hoon↗
NASA NTRS1991

Theoretical branching ratios for the 5I7 to 5I7 levels of Ho(3+) in the garnets A3B2C3O12 (A = Y,La,Lu,Gd; B = Al,Lu,Sc,Ga; C = Al,Ga)

Results are reported from an experimental study investigating triply ionized holmium in 10 garnets using the point-change model to predict theoretical energy levels and temperature-dependent branching ratios for the 5I7 to 5I8 manifolds for temperatures between 50 and 400 K. Plots were made for the largest lines at 300 K. YScAG was plotted twice, once for each set of X-ray data available. Energy levels are predicted based on theoretical crystal-field parameters, and good agreement to experiment is found. It is suggested that the present set of theoretical crystal-field parameters provides good estimates of the energy levels for the other hosts on which there are no experimental optical data. X-ray and index-of-refraction data are used to evaluate the performance of 10 lasers via a quantum mechanical model to predict the position of the energy levels and the temperature-dependent branching rations of the 5I7 to 5I8 levels of holmium. The fractional population inversion required for threshold is also evaluated.

Filer, Elizabeth D.↗