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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

High-intensity polarized electron gun featuring distributed Bragg reflector GaAs photocathode

The polarized electron source is a critical component in accelerator facilities such as the electron–ion collider, which requires a polarized electron gun with higher voltage and higher bunch charge than existing sources. One challenge we faced was the surface charge limit of the distributed Bragg reflector GaAs/GaAsP superlattice (DBR-SL-GaAs) photocathode. We suppressed this effect by optimizing the surface doping and heat cleaning procedures. We achieved up to 11.6 nC bunch charge of polarized electron beam. In this report, we discuss the performance of tests of a DBR-SL-GaAs photocathode in the high voltage direct current gun. Possible reasons for the observed peak quantum efficiency wavelength shift are analyzed, and we addressed it by using a wavelength tunable laser. In addition, the impact of the DBR layer and laser on the lifetime is investigated in this paper. The optimal DBR-SL-GaAs operating zone has been proposed, which gave us a long lifetime and high polarization at 30 μA operation. Finally, the success of this polarized gun will be key to the future of the nuclear sciences.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Influence of Bi alloying on GaAs valence band structure

Bi alloying is predicted to transform GaAs from a semiconductor to a topological insulator or semimetal. To date, studies of the GaAs 1−x Bi x alloy band structure have been limited, and the origins of Bi-induced enhancement of the spin-orbit splitting energy, Δ SO , are unresolved. Here, in this study, we present high-resolution angle-resolved photoemission spectroscopy of droplet-free epitaxial GaAs 1−x Bi x films with x Bi = 0.06. In addition to quantifying the Bi-induced shifts of the light-hole and heavy-hole valence bands, we probe the origins of the Bi-enhanced Δ SO . Using exact-two-component density functional theory calculations, we identify the key role of Bi p-orbitals in the upward shift of the light-hole and heavy-hole bands that results in the Bi-enhanced Δ SO .

Cooper, Joshua J. P. [Univ. of Michigan, Ann Arbor↗
DOE OSTI2024

Planarizing Spalled GaAs(100) Surfaces by MOVPE Growth

III-V photovoltaic devices have demonstrated remarkable performance in many applications, and spalling is a promising technique for reducing device costs by recovering the substrate for reuse. In this study, we investigate the in situ planarization of A-directionally spalled GaAs(100) substrates using metal-organic vapor phase epitaxy (MOVPE) grown C:GaAs with CCl4 as the carbon source. We have characterized the (100)-oriented growth for various CCl4 flow rates and observed that the CCl4 or its by-products promote material diffusion from the facet tops to the underlying valleys. For facets with a height of 5 ..mu..m on a substrate with an A-spall and 6 degreesB -offcut, it took 7 ..mu..m of C:GaAs to planarize the substrate. For a similar sample, with a 6 degreesA -offcut, it required 2 ..mu..m of growth to fill the valleys but there were remnant facets.

III-V↗
DOE OSTI2025

Prediction of Alpha-Particle-Immune Gate-All-Around Field-Effect Transistors (GAA-FET) Based SRAM Design

Alpha particles are known to be a major source of particles creating soft errors in semiconductor devices, such as content flipping in Static Random-Access Memory (SRAM). Recent advancements in transistor nodes have led to the introduction of Gate-All-Around Field Effect Transistors (GAA-FETs), which have better gate control, thus better electrostatics. Moreover, the introduction of bottom dielectric isolation (BDI) eliminates substrate leakage and thus is expected to enhance its radiation hardness. It is thus important to explore if one can design an SRAM that is completely radiation-hard to alpha particles. In this paper, using 3D Technology Computer-Aided-Design (TCAD) simulations, we show that it is possible to design an SRAM using GAA-FET technology so that it is immune to single alpha particle radiation error. In other words, with the design, there will be no single-event upset (SEU) due to alpha particles. We first use ab initio calculations in PHITS to show that there is a maximum linear energy transfer (LET), LET max , for the alpha particle in Si and Si x Ge 1-x . Based on that, by de signing a sub-7nm GAA-FET-based SRAM with BDI, we show that the SRAM does not flip even if the particle strike is in the worst-case scenario for LET > LET max .

42 ENGINEERING↗
NASA NTRS1977

High efficiency thin-film GaAs solar cells

Several oxidation techniques are discussed which have been found to increase the open circuit (V sub oc) of metal-GaAs Schottky barrier solar cells, the oxide chemistry, attempts to measure surface state parameters, the evolving characteristics of the solar cell as background contamination (has been decreased, but not eliminated), results of focused Nd/YAG laser beam recrystallization of Ge films evaporated onto tungsten, and studies of AMOS solar cells fabricated on sliced polycrystalline GaAs wafers. Also discussed are projected materials availability and costs for GaAs thin-film solar cells.

Stirn, R. J.↗
NASA NTRS1978

The recombination velocity at III-V compound heterojunctions with applications to Al (x) Ga(1-x)As-GaAs(1-y)Sb(y)

Interface recombination velocity in AlxGa1-xAs-GaAs and A10.85 Ga0.15As-GaAs1-ySby heterojunction systems was studied as a function of lattice mismatch. The results are applied to the design of highly efficient III-V heterojunction solar cells. A horizontal liquid-phase epitaxial growth system was used to prepare p-p-p and p-p-n double heterojunction test samples with specified values of x and y. Samples were grown at each composition, with different GaAs and GaAsSb layer thicknesses. A method was developed to obtain the lattice mismatch and lattice constants in mixed single crystals grown on (100) and (111)B oriented GaAs substrates.

Kim, J. S.↗
NASA NTRS1977

Zn diffusion in Al/0.7/Ga/0.3/As compared with that in GaAs

Zinc was diffused into 4 times 10 to the 17th per cu cm n-type Al(0.7)Ga(0.3)As grown by liquid-phase epitaxy and also into n-type 2 times 10 to the 17th per cu cm doped GaAs slices at 600, 650, and 750 C. The Zn diffusion coefficient in the Al(0.7)Ga(0.3)As was about one order of magnitude larger than in GaAs. The significance of this fact is that diffusion of Zn through a 0.5 micron Al(0.7)Ga(0.3)As layer appears to be possible with adequate control of the junction depth in the underlying GaAs.

Flat, A.↗
NASA NTRS1978

Effects of ultrathin oxides in conducting MIS structures on GaAs

Schottky barrier-type GaAs baseline devices (semiconductor surface etched and then immediately metalized) and GaAs conducting metal oxide-semiconductor devices are fabricated and characterized. The baseline surfaces (no purposeful oxide) are prepared by a basic or an acidic etch, while the surface for the MIS devices are prepared by oxidizing after the etch step. The metallizations used are thin-film Au, Ag, Pd, and Al. It is shown that the introduction of purposeful oxide into these Schottky barrier-type structures examined on n-type GaAs modifies the barrier formation, and that thin interfacial layers can modify barrier formation through trapping and perhaps chemical reactions. For Au- and Pd-devices, enhanced photovoltaic performance of the MIS configuration is due to increased barrier height.

Childs, R. B.↗
NASA NTRS1979

Crystal growth of device quality GaAs in space

The optimization of space processing of GaAs is described. The detailed compositional, structural, and electronic characterization of GaAs on a macro- and microscale and the relationships between growth parameters and the properties of GaAs are among the factors discussed. The key parameters limiting device performance are assessed.

Gatos, H. C.↗
NASA NTRS1979

Shallow-homojunction GaAs solar cells

Single-crystal GaAs shallow-homojunction solar cells on GaAs or Ge substrates, without Ga sub 1-x Al sub x As window layers, that have conversion efficiencies exceeding 20% at AM1 (17% at AMO). Using a simple theoretical model, good fits were obtained between computer calculations and experimental data for external quantum efficiency and conversion efficiency of cells with different values of n+ layer thickness. The calculations not only yield values for material properties of the GaAs layers composing the cells but will also permit the optimization of cell designs for space and terrestrial applications. Preliminary measurements indicate that the shallow-homojunction cells are resistant to electron irradiation. In the best test so far, bombardment with 1 x 10 to the 16th power/sq cm fluence of 1 MeV electrons reduced the short-circuit current by only about 6%.

Fan, J. C. C.↗
NASA NTRS1979

Summary of GaAs Solar Cell Performance and Radiation Damage Workshop

The workshop considered the GaAs solar cell capability and promise in several steps: (1) maximum efficiency; (2) space application; (3) major technology problems (AR coating optimization, contacts); (4) radiation resistance; (5) cost and availability; and (6) alternatives. The workshop believes that GaAs solar cells are fast approaching the fulfillment of their potential as candidates for space cells. A maximum efficiency of 20 to 31 percent AMO can be reasonably expected from GaAs based cells, and this may go a little higher with concentration. The use of concentration in space needs to be more carefully evaluated.

Kamath, G. S.↗
NASA NTRS1979

Electron Radiation Damage of (alga) As-gaas Solar Cells

Solar cells (2 cm by 2 cm (AlGa) As-GaAs cells) were fabricated and then subjected to irradiation at normal incidence by electrons. The influence of junction depth and n-type buffer layer doping level on the cell's resistance to radiation damage was investigated. The study shows that (1) a 0.3 micrometer deep junction results in lower damage to the cells than does a 0.5 micrometer junction, and (2) lowering the n buffer layer doping density does not improve the radiation resistance of the cell. Rather, lowering the doping density decreases the solar cell's open circuit voltage. Some preliminary thermal annealing experiments in vacuum were performed on the (AlGa)As-GaAs solar cells damaged by 1-MeV electron irradiation. The results show that cell performance can be expected to partially recover at 200 C with more rapid and complete recovery occurring at higher temperature. For a 0.5hr anneal at 400 C, 90% of the initial power is recovered. The characteristics of the (AlGa)As-GaAs cells both before and after irradiation are described.

Loo, R.↗
NASA NTRS1980

The GaAs solar cell research and development programs of the Air Force

The compound GaAs is of interest for space application photovoltaics due to its inherent advantages over silicon. Higher efficiencies, superior radiation hardness, and a greater temperature resistance are the major advantages of GaAs over Si. Air Force programs look for ways of maximizing these advantages while minimizing disadvantages such as higher costs and weights. Four programs in GaAs photovoltaics are described and each program is discussed in terms of its objective, approach and status.

Masloski, K. T.↗
NASA NTRS1980

Periodic annealing of radiation damage in GaAs solar cells

Continuous annealing of GaAs solar cells is compared with periodic annealing to determine their relative effectiveness in minimizing proton radiation damage. It is concluded that continuous annealing of the cells in space at 150 C can effectively reduce the proton radiation damage to the GaAs solar cells. Periodic annealing is most effective if it can be initiated at relatively low fluences (approximating continuous annealing), especially if low temperatures of less than 200 C are to be used. If annealing is started only after the fluence of the damaging protons has accumulated to a high value 10 to the 11th power sq/pcm), effective annealing is still possible at relatively high temperatures. Finally, since electron radiation damage anneals even more easily than proton radiation damage, substantial improvements in GaAs solar cell life can be achieved by incorporating the proper annealing capabilities in solar panels for practical space missions where both electron and proton radiation damage have to be minimized.

Loo, R. Y.↗
NASA NTRS1980

Epitaxial and polycrystalline GaAs solar cells using OM-CVD techniques

GaAs epitaxial films were grown by chemical vapor deposition using organo-metallic sources (OM-CVD) on single crystal and polycrystalline bulk GaAs, as well as on bulk polycrystalline and recrystallized thin-film Ge substrates. Details of Antireflecting Metal-Oxide-Semiconductor (AMOS) solar cells fabricated on GaAs films grown on bulk polycrystalline Ge and recrystallized Ge thin-film substrates will be discussed, as well as preliminary photovoltaic results obtained for n(+)/p homojunction structures.

Yeh, Y. C. M.↗
NASA NTRS1981

High efficiency thin-film GaAs solar cells

The paper demonstrates the feasibility of producing high-efficiency GaAs solar cells with high power-to-weight ratios by organic metallic chemical vapor deposition (OM-CVD) growth of thin epi-layers on suitable substrates. An AM1 conversion efficiency of 18% (14% AM0), or 17% (13% AM0) with a 5% grid coverage is achieved for a single-crystal GaAs n(+)/p cell grown by OM-CVD on a Ge wafer. Thin GaAs epi-layers OM-CVD grown can be fabricated with good crystallographic quality using a Si-substrate on which a thin Ge epi-interlayer is first deposited by CVD from GeH4 and processed for improved surface morphology

Zwerdling, S.↗
NASA NTRS1981

GaAs-oxide interface states - Gigantic photoionization via Auger-like process

Spectral and transient responses of photostimulated current in MOS structures were employed for the study of GaAs-anodic oxide interface states. Discrete deep traps at 0.7 and 0.85 eV below the conduction band were found with concentrations of 5 x 10 to the 12th/sq cm and 7 x 10 to the 11th/sq cm, respectively. These traps coincide with interface states induced on clean GaAs surfaces by oxygen and/or metal adatoms (submonolayer coverage). In contrast to surfaces with low oxygen coverage, the GaAs-thick oxide interfaces exhibited a high density (about 10 to the 14th/sq cm) of shallow donors and acceptors. Photoexcitation of these donor-acceptor pairs led to a gigantic photoionization of deep interface states with rates 1000 times greater than direct transitions into the conduction band. The gigantic photoionization is explained on the basis of energy transfer from excited donor-acceptor pairs to deep states.

Lagowski, J.↗
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.↗