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At least 307 records · Page 17

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NASA NTRS1982

Radiation damage

The radiation damage workshop considered a variety of topics among which were the need for equivalent electron fluences in gallium arsenide, the possibility of 15 percent end-of-life efficiencies for silicon, increasing radiation resistance in gallium arsenide, annealing of radiation damage and the need for radiation damage studies in cascade cells. The workshop members agreed that a high priority should be assigned to obtaining equivalent electron fluences for gallium arsenide cells. It was suggested that 1 MeV would be a reasonable electron energy for this purpose. Special care should be given to proton irradiations particularly for energies below 1 MeV. In addition, omnidirectional rather than normal incidence protons should be used. It was also agreed that there was a need for obtaining damage coefficients in gallium arsenide. In silicon, there is a requirement for additional flight data, especially in proton dominated orbits. These data are needed to further check the accuracy of the 1 MeV equivalence fluences.

Weinberg, I.↗

NASA NTRS1995

Method for fabricating semiconductor devices

A process for fabricating gold/gallium arsenide structures, in situ, on molecular beam epitaxially grown gallium arsenide. The resulting interface proves to be Ohmic, an unexpected result which is interpreted in terms of increased electrode interdiffusion. More importantly, the present invention surprisingly permits the fabrication of Ohmic contacts in a III-V semiconductor material at room temperature. Although it may be desireable to heat the Ohmic contact to a temperature of, for example, 200 degrees Centigrade if one wishes to further decrease the resistance of the contact, such low temperature annealing is much less likely to have any deleterious affect on the underlying substrate. The use of the term in situ herein, contemplates continuously maintaining an ultra-high vacuum, that is a vacuum which is at least 10.sup.-8 Torr, until after the metallization has been completed. An alternative embodiment of the present invention comprising an additional step, namely the termination of the gallium arsenide by a two monolayer thickness of epitaxial aluminum arsenide as a diffusion barrier, enables the recovery of Schottky barrier behavior, namely a rectified I-V characteristic. The present invention provides a significant breakthrough in the fabrication of III-V semiconductor devices wherein excellent Ohmic contact and Schottky barrier interfaces to such devices can be achieved simply and inexpensively and without requiring the high temperature processing of the prior art and also without requiring the use of exotic high temperature refractory materials as substitutes for those preferred contact metals such as gold, aluminum and the like.

Kaiser, William J.↗

DOE OSTI2026

Towards advanced polarized electron sources

Polarized electrons play an important role in high-energy and nuclear physics, and their properties have also been exploited in ultrafast electron microscopy. Currently, gallium arsenide crystals illuminated by circular polarized infrared laser light are commonly used for generating polarized electrons. However, the achievable accelerating voltage and the gradient of these electrostatic sources limit the beam quality and quantity. A solution could be to combine gallium arsenide photocathodes with radio-frequency electron guns, which are capable of accelerating beams with significantly higher gradients and voltage. Here we report the successful operation of a gallium arsenide photocathode in a superconducting radio-frequency gun. Our findings are relevant for future sources of polarized electrons.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2014

Demonstration of InAsBi photoresponse beyond 3.5 μm

An Indium Arsenide Bismide photodiode has been grown, fabricated, and characterized to evaluate its performance in the Mid Wave Infrared region of the spectrum. Spectral response from the diode has been obtained up to a diode temperature of 225 K. At this temperature, the diode has a cut off wavelength of 3.95 μm, compared to 3.41 μm in a reference Indium Arsenide diode, indicating that Bismuth has been incorporated to reduce the band gap of Indium Arsenide by 75 meV. Similar band gap reduction was deduced from the cut off wavelength comparison at 77 K. From the dark current data, shunt resistance values of 8 and 39 Ω at temperatures of 77 and 290 K, respectively, were obtained in our photodiode.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Heat load of a GaAs photocathode in an SRF electron gun

A great deal of effort has been made over the last decades to develop a better polarized electron source for high energy physics. Several laboratories operate DC guns with a gallium arsenide photocathode, which yield a highly polarized electron beam. However, the beam's emittance might well be improved by using a superconducting radio frequency (SRF) electron gun, which delivers beams of a higher brightness than that from DC guns because the field gradient at the cathode is higher. SRF guns with metal and CsTe cathodes have been tested successfully. To produce polarized electrons, a Gallium-Arsenide photo-cathode must be used: an experiment to do so in a superconducting RF gun is under way at BNL. Since a bulk gallium arsenide (GaAs) photocathode is normal conducting, a problem arises from the heat load stemming from the cathode. In this paper, we present our measurements of the electrical resistance of GaAs at cryogenic temperatures, a prediction of the heat load and verification by measuring the quality factor of the gun with and without the cathode at 2 K. We simulate heat generation and flow from the GaAs cathode using the ANSYS program. Finally, by following the findings with the heat load model, we designed and fabricated a new cathode holder (plug) to decrease the heat load from GaAs.

2 K test↗

DOE OSTI2018

Is it possible to stabilize the 1144-phase pnictides with tri-valence cations?

A lately discovered 1144 phase has generated significant interest for its high superconducting temperatures, disorder-free doping, and various chemical substitutions. However, it has only been found in iron arsenides ( AB Fe 4 As 4 ), and cations are limited to +1 or +2 valence states (e.g., alkali metals, alkaline earth elements, and Eu). Whether more 1144 phases could be stabilized and whether intriguing properties exist are questions of general interest. In this work, we investigate 1144 iron and cobalt arsenides with tri-valence cations (La, Y, In, Tl, Sm, Gd). We study phase stability among other competing phases: 122 solution phase and phase decomposition. With La as the cation, we predict room-temperature stable 1144 structures: La A Fe 4 As 4 ( A =K, Rb, and Cs). Other La-contained 1144 structures tend to form solution phase. The solubility of La is estimated and compared with the experiment. By contrast, we do not find stable 1144 structures with Y as the cation. For In and Tl as cations, two 122-phase compounds are remarkably stable: InCo 2 As 2 and TlCo 2 As 2 , which adds to our knowledge about the In(Tl)-Co-As phase diagram. Stable 1144 phases are found in InKCo4As4 and InRbCo 4 As 4 . With Sm and Gd as cations, 1144- or 122-phase iron arsenides are generally unstable. Among structures investigated, we recognize two critical factors for 1144-phase stability: size effect and charge balance, which yields a merging picture with the rule found in previous 1144 systems. Furthermore, La A Fe 4 As 4 ( A =K, Rb, and Cs), InCo 2 As 2 , and TlCo 2 As 2 are exhibiting semimetal features and a two-dimensional Fermi surface, similar to iron superconductors.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Controlling and visualizing Dirac physics in topological semimetal heterostructures

A bulk crystal of cadmium arsenide is a three-dimensional Dirac semimetal, but, in a thin film, it can behave like a three-dimensional topological insulator. This tunability provides unique opportunities to manipulate and explore a topological insulator phase. However, an obstacle to engineering such tunability is the subtlety of transport-based discriminants for topological phases. In this work, the quantum capacitance of cadmium arsenide–based heterostructures provides two direct experimental signatures of three-dimensional topological insulator physics: an insulating three-dimensional bulk and a Landau level at zero energy that does not disperse in a magnetic field. We proceed to join our ability to see these fingerprints of the topological surface states with flexibility afforded by our epitaxial heterostructures to demonstrate a route toward controlling the energy of the Dirac nodes on each surface. These results point to new avenues for engineering topological insulators based on cadmium arsenide.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Development of III-Sb Quantum Dot Systems for High Efficiency Intermediate Band Solar Cells

This project aimed to develop solar cells that can help reduce cost per watt. This work focused on developing solar cells that utilize quantum dot (QD) nanomaterials to provide multijunction solar cell efficiency at the cost of single junction solar cell. We focused on a novel concept known as intermediate band solar cells (IBSC) where an additional energy band is inserted in a single solar cell to accommodate sub-bandgap photons absorption which otherwise are lost through transmission. The additional energy band can be achieved by growing QDs within a solar cell p-n junction. Though numerous studies have been conducted to develop such QD systems, very small improvements in solar energy conversion efficiency have been reported. This is mainly due to non-optimal material parameters such as band gap, band offset etc. In this work, we identified and developed a novel QD material system that meets the requirements of IBSC more closely than the current state-of-the-art technology. To achieve these goals, we focused on three important areas of solar cell design: band structure calculations of new materials, efficient device design for high efficiency, and development of new semiconductor materials. In this project, we focused on III-Sb materials as they possess a wide range of energy bandgaps from 0.2 eV to 2eV. Despite the difficulty involved in realizing these materials, we were successfully developed these materials through a systematic approach. Materials studied in this work are AlAsSb (Aluminum Arsenide Antimonide), InAlAs (Indium Aluminum Arsenide) and InAs (Indium Arsenide). InAs was used to develop QD layers within AlAsSb and InAlAs p-n junctions. As the QDs have very small volume, up to 30 QD layers been inserted into the p-n junction to enhance light absorption. These QD multi-stack devices helped in understanding the challenges associated with the development of quantum dot solar cells. The results from this work show that the quantum dot solar cells indeed improve the efficiency via sub-bandgap photon absorption but much lower than the expected theoretical efficiencies. Yet, these devices serve as a proof of concept and the results provide vital information for future IBSC work. With further understanding of the operation of intermediate band solar cells and the identification of less expensive materials can substantially improve the solar cell efficiency and drastically cut the cell cost thereby reducing the dollar/watt cost.

14 SOLAR ENERGY↗

NASA NTRS1977

Space-based solar power conversion and delivery systems study. Volume 4: Energy conversion systems studies

Solar cells and optical configurations for the SSPS were examined. In this task, three specific solar cell materials were examined: single crystal silicon, single crystal gallium arsenide, and polycrystalline cadmium sulfide. The comparison of the three different cells on the basis of a subsystem parametric cost per kW of SSPS-generated power at the terrestrial utility interface showed that gallium arsenide was the most promising solar cell material at high concentration ratios. The most promising solar cell material with no concentration, was dependent upon the particular combination of parameters representing cost, mass and performance that were chosen to represent each cell in this deterministic comparative analysis. The potential for mass production, based on the projections of the present state-of-the-art would tend to favor cadmium sulfide in lieu of single crystal silicon or gallium arsenide solar cells.

Source record↗

NASA NTRS1980

Analysis of GaAs and Si solar cell arrays for earth orbital and orbit transfer missions

Silicon and gallium arsenide arrays were studied and compared for low earth orbit (LE), geosynchronous orbit (GEO), and LEO to GEO electric propulsion orbit transfer missions. The sensitivities of total cost to parameters such as mission duration, array cost, cover glass thickness, and concentration ratio were determined along with cost tradeoffs between silicon and gallium arsenide arrays for selected mission classes. Results indicate that development of the technology for low cost, light weight concentrators should be increased and that cost reduction efforts for gallium arsenide cells be pursued.

Jeffries, K. D.↗

NASA NTRS1980

Study of multi-kW solar arrays for Earth orbit application

Planar and concentrator solar array configurations based on silicon and gallium arsenide solar cells were conceptualized and on-orbit maintainability was addressed. Four basic categories emerged: (1) planar (non concentrated) with silicon cells, (2) low-CR (concentration ratio = 3.4) with silicon cells, (3) low-CR with GaAs, and (4) high-CR (concentration ratio = 62.5) with GaAs. A very high-CR (concentration ratio = 200) was investigated but rejected on thermal grounds. Nonrecurring and recurring cost elements for each of the four concepts selected were compared over a 15 year life cycle. Under conditions where the gallium arsenide cells can be produced for less than $25 per 2 x 2 cm, the low CR concentrator emerges as the most cost effective configuration. However, the producibility risk remains higher on the gallium arsenide cell.

Source record↗

NASA NTRS1984

The friction behavior of semiconductors Si and GaAs in contact with pure metals

The friction behavior of the semiconductors silicon and gallium arsenide in contact with pure metals was studied. Five transition and two nontransition metals, titanium, tantalum, nickel, palladium, platinum, copper, and silver, slid on a single crystal silicon (111) surface. Four metals, indium, nickel, copper and silver, slid on a single crystal gallium arsenide (100) surface. Experiments were conducted in room air and in a vacuum of 10 to the minus 7th power N/sq cm (10 to the minus 9th power torr). The results indicate that the sliding of silicon on the transition metals exhibits relatively higher friction than for the nontransition metals in contact with silicon. There is a clear correlation between friction and Schottky barrier height formed at the metal silicon interface for the transition metals. Transition metals with a higher barrier height on silicon had a lower friction. The same effect of barrier height was found for the friction of gallium arsenide in contact with metals.

Mishina, H.↗

NASA NTRS1985

GaAs Schottky barrier photo-responsive device and method of fabrication

A gallium arsenide photo-responsive device is provided with an intermediate, transparent layer of a refractory metal or alkaline earth metal forming a tenacious bond between a non-hydroscopic oxide layer and a noble metal Schottky barrier layer. The device has a gallium arsenide substrate with a predetermined type conductivity and a gallium arsenide epitaxial layer with the same type conductivity but a lower charge carrier concentration grown on the substrate. The oxide layer is formed to cover the epitaxial layer, and the transparent metal layer followed by the noble metal layer are deposited upon the oxide layer. An interdigitated ohmic contact is then formed upon the noble metal layer.

Alcorn, G. E.↗

NASA NTRS1990

Peeled film GaAs solar cell development

Thin-film, single-crystal gallium arsenide (GaAs) solar cells could exhibit a specific power approaching 700 W/kg including coverglass. A simple process has been described whereby epitaxial GaAs layers are peeled from a reusable substrate. This process takes advantage of the extreme selectivity of the etching rate of aluminum arsenide (AlAs) over GaAs in dilute hydrofluoric acid. The feasibility of using the peeled film technique to fabricate high-efficiency, low-mass GaAs solar cells is presently demonstrated. A peeled film GaAs solar cell was successfully produced. The device, although fractured and missing the aluminum gallium arsenide window and antireflective coating, had a Voc of 874 mV and a fill factor of 68 percent under AM0 illumination.

Wilt, D. M.↗

DOE OSTI2011

Transport and strain relaxation in wurtzite InAs-GaAs core-shell heterowires

Indium-arsenide-gallium-arsenide (InAs-GaAs) core-shell, wurtzite nanowires have been grown on GaAs (001) substrates. The core-shell geometries (core radii 11 to 26 nm, shell thickness >2.5 nm) exceeded equilibrium critical values for strain relaxation via dislocations, apparent from transmission electron microscopy. Partial axial relaxation is detected in all nanowires increasing exponentially with size, while radial strain relaxation is >90%, but undetected in nanowires with both smaller core radii <16 nm and shell thicknesses <5 nm. Electrical measurements on individual core-shell nanowires show that the resulting dislocations are correlated with reduced electron field-effect mobility compared to bare InAs nanowires.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Optical Properties of InGaAs/InAlAs Metamorphic Nanoheterostructures for Photovoltaic Converters of Laser and Solar Radiation

Reflectance spectroscopy has been used to determine the refractive indices of nanoscale In{sub x}Al{sub y}Ga{sub 1–x–y}As and In{sub x}Al{sub 1–x}As layers with indium and aluminum concentrations x = 0.21–0.24 and y = 0, 0.1, and 0.2 on specially created Bragg-reflector heterostructures at wavelengths in the range 700–2000 nm. It was demonstrated that the method based on an analysis of the auto- and cross-correlation coefficients of the wavelength derivatives of the dependence of the reflectance of structures of this kind in order to determine the dispersion curves of the materials forming a reflector. It was found that raising the concentration of indium in InGaAs and AlInAs leads to a substantial rise in the refractive index, with a preserved spectral run of the refractive indices, which is characteristic of gallium arsenide and aluminum arsenide.

14 SOLAR ENERGY↗

DOE OSTI2011

E{sub 1} Gap of Wurtzite InAs Single Nanowires Measured by Means of Resonant Raman Spectroscopy

Indium arsenide nanowires were synthesized with an intermixing of wurtzite and zincblende structure by chemical beam epitaxy with the vapor-liquid-solid mechanism. Resonant Raman spectroscopy of the transverse optical phonon mode at 215 cm{sup -1} reveals an E{sub 1} gap of 2.47 eV which is assigned to the electronic band gap at the A point in the indium arsenide wurtzite phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Lattice distortion in single crystal rare-earth arsenide/GaAs nanocomposites

Epitaxial single crystal nanocomposites comprised of rare-earth arsenide nanoparticles embedded in GaAs (001) layers produce a larger change in lattice parameter than expected from the lattice parameters of relaxed films. Despite similar cubic structures and lattice parameters, elongation of the interfacial bond length between the two materials induces additional strain causing an expansion in the nanocomposite lattice. The interface bond length is material dependent with an average atomic layer spacing at the ErAs:GaAs interface of 1.9 Å while the spacing at the ScAs:GaAs interface is only 1.4 Å. Implications for lattice matching various single crystal epitaxial nanostructures in semiconductors are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗