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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 1,747 records · Page 97
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DOE OSTI2016

Estimation of the efficiency of the introduction of a porous layer into a silicon-on-sapphire structure substrate to enhance the reliability of devices under irradiation

We investigate the efficiency of the introduction of a porous layer into the substrate of a silicon-onsapphire structure by the implantation of He ions to enhance the radiation resistance of devices. The properties of the introduced layer and its parameters affecting the concentration of minority charge carriers generated by irradiation are analyzed. The reported results of the analysis and calculations can be used to optimize He-ion implantation conditions during the formation of a porous layer.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of active-region “volume” on the radiative properties of laser heterostructures with radiation output through the substrate

The radiative properties of InGaAs/GaAs/InGaP laser structures with radiation output through the substrate depending on the number of quantum wells in the active region and laser diodes on their basis are investigated. It is established that the presence of six–eight quantum wells in the active region is optimum from the viewpoint of observable values of the threshold current and the output optical power of lasers.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Study of the structural and optical properties of GaP(N) layers synthesized by molecular-beam epitaxy on Si(100) 4° substrates

The structural and optical properties of GaP and GaPN layers synthesized by molecular-beam epitaxy on Si(100) substrates misoriented by 4° are studied. The possibility of producing GaP buffer layers that exhibit a high degree of heterointerface planarity and an outcropping dislocation density of no higher than ~2 × 10{sup 8} cm{sup –2} is shown. Emission from the Si/GaP/GaPN structure in the spectral range of 630–640 nm at room temperature is observed. Annealing during growth of the Si/GaP/GaPN structure makes it possible to enhance the room-temperature photoluminescence intensity by a factor of 2.6, with no shift of the maximum of the emission line.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Terahertz-radiation generation and detection in low-temperature-grown GaAs epitaxial films on GaAs (100) and (111)A substrates

The efficiency of the generation and detection of terahertz radiation in the range up to 3 THz by LT-GaAs films containing equidistant Si doping δ layers and grown by molecular beam epitaxy on GaAs (100) and (111)Ga substrates is studied by terahertz spectroscopy. Microstrip photoconductive antennas are fabricated on the film surface. Terahertz radiation is generated by exposure of the antenna gap to femtosecond optical laser pulses. It is shown that the intensity of terahertz radiation from the photoconductive antenna on LT-GaAs/GaAs (111)Ga is twice as large as the intensity of a similar antenna on LT-GaAs/GaAs(100) and the sensitivity of the antenna on LT-GaAs/GaAs (111)Ga as a terahertz-radiation detector exceeds that of the antenna on LT-GaAs/GaAs(100) by a factor of 1.4.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of intercalated hydrogen on the electron state of quasi-free graphene on a SiC substrate

To judge the role of intercalated hydrogen in the doping of quasi-free epitaxial graphene, two systems, specifically, (i) the graphene–NH-SiC{0001} and (ii) graphene–hydrogen single layer–NH-SiC{0001} structures (N = 4, 6) are considered. In case (i), the shift of the Dirac point of epitaxial graphene is induced by the electrostatic potential of spontaneous polarization of the substrate; in case (ii), the shift is due to the field of a double electrical layer formed because of the absorption of hydrogen atoms. It is shown that, in case (ii) compared to case (i), the concentration of holes in graphene is higher and the concentration of electrons is lower. This result is consistent with the currently available experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Structure of bismuth films obtained using an array of identically oriented single-crystal bismuth islands preliminarily grown on a substrate

An array of identically oriented single-crystal bismuth islands formed on mica plates is used as a substrate to improve the structure of bismuth thin films grown by vacuum thermal evaporation. The array of islands is formed by the chemical etching of a single-crystal bismuth film 1 μm thick grown by floating-zone recrystallization under a coating. The structure of the obtained films is studied by X-ray diffraction, atomicforce microscopy, and electron backscatter diffraction scanning electron microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2017

CeB{sub 6} thin films produced on different substrates by electron-beam deposition

Films of cerium hexaboride, a material promising for use in thermoelectric devices at liquidhelium temperatures, are produced by electron-beam deposition. Deposition is carried out from ceramic targets onto insulator, semiconductor, and metal substrates at different temperatures. The microstructure, the elemental and phase compositions, the temperature dependences of the resistivity and the Seebeck coefficient are thoroughly studied. CaB{sub 6}-structured films, for which the structure is characteristic of cerium hexaboride and the elemental composition is close to the stoichiometric composition, are obtained. At low temperatures, the resistivity of the films is somewhat higher than that of single-crystal samples, and the Seebeck coefficient is close to the corresponding coefficient for single-crystal samples. The main cause of the difference between the resistance values is a high concentration of oxygen impurity detected in the films.

36 MATERIALS SCIENCE↗
DOE OSTI2017

On the high characteristic temperature of an InAs/GaAs/InGaAsP QD laser with an emission wavelength of ~1.5 μm on an InP substrate

We report on a study of lasers with an emission wavelength of about 1.5 μm and high temperature stability, synthesized on an InP (001) substrate. Self-organized InAs quantum dots capped with a thin GaAs layer are used as the active region of the laser. A quaternary InGaAsP solid solution with a band-gap width of 1.15 eV serves as the waveguide/matrix layer. A high characteristic temperature of the threshold current, T{sub 0} = 205 K, is reached in the temperature range 20–50°C in ridge-waveguide laser diodes. A correlation between the values of T{sub 0} and the band-gap width of the waveguide layers is found.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Bimodality in Arrays of In{sub 0.4}Ga{sub 0.6}As Hybrid Quantum-Confined Heterostructures Grown on GaAs Substrates

Hybrid quantum-confined heterostructures grown by metal-organic vapor-phase epitaxy (MOVPE) via the deposition of In{sub 0.4}Ga{sub 0.6}As layers with various nominal thicknesses onto vicinal GaAs substrates are studied by photoluminescence spectroscopy and transmission electron microscopy. The photoluminescence spectra of these structures show the superposition of two spectral lines, which is indicative of the bimodal distribution of the size and/or shape of light-emitting objects in an array. The dominant spectral line is attributed to the luminescence of hybrid “quantum well–dot” nanostructures in the form of a dense array of relatively small quantum dots (QDs) with weak electron and hole localization. The second, lower intensity line is attributed to luminescence from a less dense array of comparatively larger QDs. Analysis of the behavior of the spectral line intensities at various temperatures showed that the density of larger QDs grows with increasing thickness of the InGaAs layer.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Infrared Reflection Spectra of Pb{sub 1–x}Sn{sub x}Se (x = 0.2, 0.34) Topological Insulator Films on a ZnTe/GaAs Substrate and the Vibrational Modes of Multilayer Structures

The reflectance spectra of the topological insulator Pb{sub 1–x}Sn{sub x}Se (x = 0.2, 0.34) films grown by molecular-beam epitaxy on a ZnTe/GaAs substrate are measured in the range of 12–2500 cm{sup –1} at room temperature. Using dispersion analysis, the frequencies of transverse optical phonons, plasma frequencies, highfrequency permittivities, and layer thicknesses are determined. In the quasi-steady-state approximation, the interface mode frequencies of the four-layer structure are calculated as a function of the overlap parameter χ{sub 1} (0 ≤ χ{sub 1} ≤ 1). The parameter describes the degree of overlap of two interface modes localized at planes bounding the layer to the right and left. The existence of interacting interface modes in the structure makes its spectrum different from the sum of component spectra. These differences manifest themselves in the experiment. The conditions of the interaction of interface modes with IR radiation are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Raman Scattering of Cu{sub 2}ZnSnSe{sub 4} Thin Films Deposited onto Flexible Metal Substrates

Cu{sub 2}ZnSnSe{sub 4} thin films are produced by selenizing electrochemically layer-by-layer deposited and preliminarily annealed Cu–Zn–Sn precursors. For flexible metal substrates, Mo and Ta foils are used. The morphology, elemental and phase compositions, and crystal structure of Cu{sub 2}ZnSnSe{sub 4} films are studied by scanning electron microscopy, X-ray spectral microanalysis, X-ray phase analysis, and Raman spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Site-Controlled Growth of GaN Nanorods with Inserted InGaN Quantum Wells on μ-Cone Patterned Sapphire Substrates by Plasma-Assisted MBE

We report on a new approach to fabricate regular arrays of GaN nanorods (NRs) with InGaN QWs by plasma-assisted molecular-beam epitaxy (PA MBE) on micro-cone patterned sapphire substrates (μ-CPSSs). A two-stage PA MBE fabrication process of GaN NRs has been developed, starting with a high temperature nucleation layer growth at metal-rich conditions to aggregate selectively GaN nucleus on c-oriented areas of the μ-CPSSs and followed by growth of 1-μm-thick GaN NRs at strongly nitrogen-rich conditions exactly on the cone tips. These results are explained by energetically favorable GaN growth on the (000-) oriented sapphire surface. Both micro-photoluminescence and micro-cathodoluminescence confirm the formation of regular array of optically and spectrally isolated NRs without usage of any nanolithography.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Growth of GaN Layers on Si(111) Substrates by Plasma-Assisted Molecular Beam Epitaxy

The studies of the growth kinetics of GaN layers grown on nitridated Si(111) substrates by plasmaassisted molecular beam epitaxy are presented. The nucleation and overgrowth of the separate GaN/Si(111) nanocolumns during single growth run is demonstrated. The technique of the in situ control of the GaN/Si(111) nanocolumns lateral size is proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Photoreflectance Spectroscopy Study of LT-GaAs Layers Grown on Si and GaAs Substrates

The mechanical strains and densities of surface charge states in GaAs layers grown by low-temperature (LT) molecular-beam epitaxy on Si(100) and GaAs(100) substrates are investigated by photoreflectance spectroscopy. Lines corresponding to the fundamental transition (E{sub g}) and the transition between the conduction band and spin-orbit-split valence subband (E{sub g} + Δ{sub SO}) in GaAs are observed in the photoreflectance spectra of Si/LT-GaAs structures at 1.37 and 1.82 eV, respectively. They are shifted to lower and higher energies, respectively, relative to the corresponding lines in GaAs/LT-GaAs structures. Comparing the spectra of the Si/LT-GaAs and GaAs/LT-GaAs structures, it is possible to estimate mechanical strains in LT-GaAs layers grown on Si (by analyzing the spectral-line shifts) and the density of charge-carrier states at the GaAs/Si heterointerface (by analyzing the period of Franz–Keldysh oscillations).

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Epitaxial-Structure Design and Growth Parameters on the Characteristics of Metamorphic Lasers of the 1.46-μm Optical Range Based on Quantum Dots Grown on GaAs Substrates

The characteristics of lasers of the 1.44–1.46-μm optical range grown on GaAs substrates using a metamorphic buffer are studied. The active region of the laser contains 10 rows of InAs/In{sub 0.4}Ga{sub 0.6}As/In{sub 0.2}Ga{sub 0.8}As quantum dots. It is shown that the use of special selective high-temperature annealing along with the application of short-period In{sub 0.2}Ga{sub 0.8}As/In{sub 0.2}Al{sub 0.3}Ga{sub 0.5}As short-period superlattices makes it possible to substantially decrease the density of threading dislocations in the active region. A threshold current density of 1300 A cm{sup –2}, external differential quantum efficiency of 38%, and maximal pulsed-mode output power of 13 W are attained for a laser with a broad area 3 mm in length.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Heterostructures with InAs/AlAs Quantum Wells and Quantum Dots Grown on GaAs/Si Hybrid Substrates

Heterostructures with InAs/AlAs quantum dots are grown on GaAs/Si hybrid substrates. The experimentally observed low-temperature (5–80 K) photoluminescence spectra of InAs/AlAs/GaAs/Si heterostructures exhibit bands defined by excitonic recombination in quantum dots and a wetting layer, i.e., a thin quantum well lying at the base of the array of quantum dots. Temperature quenching of the photoluminescence of quantum dots occurs due to the direct trapping of charge carriers at defects localized in the AlAs matrix, in the vicinity of the quantum dots.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Stimulated Emission in the 1.3–1.5 μm Spectral Range from AlGaInAs Quantum Wells in Hybrid Light-Emitting III–V Heterostructures on Silicon Substrates

Hybrid laser structures with AlGaInAs quantum wells are grown by metalorganic vapor phase epitaxy on Ge/Si(100) “virtual” substrates using GaAs and InP buffer layers. Stimulated emission is achieved under optical pumping of the prepared samples in the range of 1.3–1.5 μm at liquid-nitrogen temperature. The stimulated-emission threshold is 30–70 kW/cm{sup 2}.

36 MATERIALS SCIENCE↗