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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 1,459 records · Page 81
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

β-(Al{sub x}Ga{sub 1−x}){sub 2}O{sub 3}/Ga{sub 2}O{sub 3} (010) heterostructures grown on β-Ga{sub 2}O{sub 3} (010) substrates by plasma-assisted molecular beam epitaxy

By systematically changing growth parameters, the growth of β-(Al{sub x}Ga{sub 1−x}){sub 2}O{sub 3}/Ga{sub 2}O{sub 3} (010) heterostructures by plasma-assisted molecular beam epitaxy was optimized. Through variation of the Al flux under O-rich conditions at 600 °C, β-(Al{sub x}Ga{sub 1−x}){sub 2}O{sub 3} (010) layers spanning ∼10% to ∼18% Al{sub 2}O{sub 3} were grown directly on β-Ga{sub 2}O{sub 3} (010) substrates. Nominal β-(Al{sub x}Ga{sub 1−x}){sub 2}O{sub 3} (010) compositions were determined through Al:Ga flux ratios. With x = ∼0.18, the β-(Al{sub x}Ga{sub 1−x}){sub 2}O{sub 3} (020) layer peak in a high-resolution x-ray diffraction (HRXRD) ω-2θ scan was barely discernible, and Pendellösung fringes were not visible. This indicated that the phase stability limit of Al{sub 2}O{sub 3} in β-Ga{sub 2}O{sub 3} (010) at 600 °C was less than ∼18%. The substrate temperature was then varied for a series of β-(Al{sub ∼0.15}Ga{sub ∼0.85}){sub 2}O{sub 3} (010) layers, and the smoothest layer was grown at 650 °C. The phase stability limit of Al{sub 2}O{sub 3} in β-Ga{sub 2}O{sub 3} (010) appeared to increase with growth temperature, as the β-(Al{sub x}Ga{sub 1−x}){sub 2}O{sub 3} (020) layer peak with x = ∼0.18 was easily distinguishable by HRXRD in a sample grown at 650 °C. Cross-sectional transmission electron microscopy (TEM) indicated that β-(Al{sub ∼0.15}Ga{sub ∼0.85}){sub 2}O{sub 3} (010) layers (14.4% Al{sub 2}O{sub 3} by energy dispersive x-ray spectroscopy) grown at 650 °C were homogeneous. β-(Al{sub ∼0.20}Ga{sub ∼0.80}){sub 2}O{sub 3} (010) layers, however, displayed a phase transition. TEM images of a β-(Al{sub ∼0.15}Ga{sub ∼0.85}){sub 2}O{sub 3}/Ga{sub 2}O{sub 3} (010) superlattice grown at 650 °C showed abrupt layer interfaces and high alloy homogeneity.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effect of substrate composition on atomic layer deposition using self-assembled monolayers as blocking layers

The authors have examined the effect of two molecules that form self-assembled monolayers (SAMs) on the subsequent growth of TaN{sub x} by atomic layer deposition (ALD) on two substrate surfaces, SiO{sub 2} and Cu. The SAMs that the authors have investigated include two vapor phase deposited, fluorinated alkyl silanes: Cl{sub 3}Si(CH{sub 2}){sub 2}(CF{sub 2}){sub 5}CF{sub 3} (FOTS) and (C{sub 2}H{sub 5}O){sub 3}Si(CH{sub 2}){sub 2}(CF{sub 2}){sub 7}CF{sub 3} (HDFTEOS). Both the SAMs themselves and the TaN{sub x} thin films, grown using Ta[N(CH{sub 3}){sub 2}]{sub 5} and NH{sub 3}, were analyzed ex situ using contact angle, spectroscopic ellipsometry, x-ray photoelectron spectroscopy (XPS), and low energy ion-scattering spectroscopy (LEISS). First, the authors find that both SAMs on SiO{sub 2} are nominally stable at T{sub s} ∼ 300 °C, the substrate temperature used for ALD, while on Cu, the authors find that HDFTEOS thermally desorbs, while FOTS is retained on the surface. The latter result reflects the difference in the head groups of these two molecules. The authors find that both SAMs strongly attenuate the ALD growth of TaN{sub x} on SiO{sub 2}, by about a factor of 10, while on Cu, the SAMs have no effect on ALD growth. Results from LEISS and XPS are decisive in determining the nature of the mechanism of growth of TaN{sub x} on all surfaces. Growth on SiO{sub 2} is 2D and approximately layer-by-layer, while on the surfaces terminated by the SAMs, it nucleates at defect sites, is islanded, and is 3D. In the latter case, our results support growth of the TaN{sub x} thin film over the SAM, with a considerable delay in formation of a continuous thin film. Growth on Cu, with or without the SAMs, is also 3D and islanded, and there is also a delay in the formation of a continuous thin film as compared to growth on SiO{sub 2}. These results highlight the power of coupling measurements from both LEISS and XPS in examinations of ultrathin films formed by ALD.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Inherent substrate-dependent growth initiation and selective-area atomic layer deposition of TiO{sub 2} using “water-free” metal-halide/metal alkoxide reactants

Titanium dioxide atomic layer deposition (ALD) is shown to proceed selectively on oxidized surfaces with minimal deposition on hydrogen-terminated silicon using titanium tetrachloride (TiCl{sub 4}) and titanium tetra-isopropoxide [Ti(OCH(CH{sub 3}){sub 2}){sub 4}, TTIP] precursors. Ex situ x-ray photoelectron spectroscopy shows a more rapid ALD nucleation rate on both Si–OH and Si–H surfaces when water is the oxygen source. Eliminating water delays the oxidation of the hydrogen-terminated silicon, thereby impeding TiO{sub 2} film growth. For deposition at 170 °C, the authors achieve ∼2 nm of TiO{sub 2} on SiO{sub 2} before substantial growth takes place on Si–H. On both Si–H and Si–OH, the surface reactions proceed during the first few TiCl{sub 4}/TTIP ALD exposure steps where the resulting products act to impede subsequent growth, especially on Si–H surfaces. Insight from this work helps expand understanding of “inherent” substrate selective ALD, where native differences in substrate surface reaction chemistry are used to promote desired selective-area growth.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Transfer of a Holographic Structure from Dihromated Gelatin Layers on a Polymethylmethacrylate Substrate

The possibility of the transfer of a holographic structure that was originally registered on layers of dichromated gelatin (DCG) onto a substrate material of polymethylmethacrylate (PMMA) has been shown. The use of a selective destructive effect of short-wave UV radiation with a wavelength that is less than 270 nm is the basis of the mechanism of formation of secondary relief-phase holographic structure on the surface of PMMA. The optimization of processing modes and selection of developing compositions which are based on isopropanol and methylisobutylketone (MIBK) have been carried out, which made it possible to create reliefphase holographic gratings with high diffraction efficiency (DE) of about 25% and the maximum depth of the surface relief of the order of 1 μm on the substrates of PMMA.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Epitaxy of CdTe on sapphire substrates with titanium buffer layers

The formation of a developed electrical relief on the sapphire substrate surface is investigated. A technique is proposed for introducing Ti{sup 4+} impurity atoms into the sapphire crystal lattice by depositing titanium layers with a thickness of about 5 nm and their annealing in air (oxidizing atmosphere) to a temperature of 1400°C. It is shown that this preliminary treatment of the sapphire substrate surface results in epitaxial growth of (111) СdTe films parallel to the sapphire (0001) plane at a temperature of 350°C.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Gallium vacancy ordering in Ga{sub 2}Se{sub 3} thin layers on Si(100), Si(111), and Si(123) substrates

Thin Ga{sub 2}Se{sub 3} layers deposited on silicon substrates with the (100), (111), and (123) orientations are studied by transmission electron microscopy and X-ray microanalysis. Some features and regularities of the stoichiometric gallium vacancy ordering at different substrate orientations are discussed. The Ga{sub 3}Se{sub 4}(100)с(2 × 2) and Ga{sub 2}Se{sub 3}(111)(√3 × √3)-R30° ordered structures are formed on the Si(100) and Si(111) surfaces, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Epitaxial low-temperature growth of In{sub 0.5}Ga{sub 0.5}As films on GaAs(100) and GaAs(111)A substrates using a metamorphic buffer

A complex investigation of epitaxial In{sub 0.5}Ga{sub 0.5}As films grown on GaAs substrates with crystallographic orientations of (100) and (111)A in the standard high- and low-temperature modes has been performed. The parameters of the GaAs substrate and In{sub 0.5}Ga{sub 0.5}As film were matched using the technology of step-graded metamorphic buffer. The electrical and structural characteristics of the grown samples have been studied by the van der Pauw method, atomic force microscopy, scanning electron microscopy, and transmission/ scanning electron microscopy. The surface morphology is found to correlate with the sample growth temperature and doping with silicon. It is revealed that doping of low-temperature In{sub 0.5}Ga{sub 0.5}As layers with silicon significantly reduces both the surface roughness and highly improves the structural quality. Pores 50–100 nm in size are found in the low-temperature samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Ga{sub 2}O{sub 3}–In{sub 2}O{sub 3} thin films on sapphire substrates: Synthesis and ultraviolet photoconductivity

The structure and electrical and optical properties of β-Ga{sub 2}O{sub 3}–In{sub 2}O{sub 3} thin films on sapphire substrates with different orientations have been investigated. The samples have been prepared by annealing of gallium–indium metallic films on sapphire substrates in air at different gallium-to-indium ratios in the initial mixture. The photoconductivity of these structures in the solar-blind ultraviolet spectral region has been examined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

XANES and IR spectroscopy study of the electronic structure and chemical composition of porous silicon on n- and p-type substrates

The differences in the electronic structure and composition of porous silicon samples obtained under identical conditions of electrochemical etching on the most commonly used n- and p-type substrates with different conductivities are demonstrated by X-ray absorption near-edge spectroscopy (XANES) and Fourier transform IR spectroscopy (FTIR) methods. It is shown that significantly higher oxidation and saturation with hydrogen is observed for the porous layer on n-type substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Photosensitivity of ZnO/CdS/Cu(In,Ga)Se{sub 2}/Mo thin-film solar cells fabricated on various substrates

The results of measuring the first spectra of relative quantum efficiency for photoconversion in thin-film ZnO/CdS/Cu(In,Ga)Se{sub 2}/Mo solar cells fabricated on rigid (glass) and flexible (polyimide) substrates are reported. The character of interband transitions has been studied and the values of the band gap for direct and indirect transitions in thin Cu(In,Ga)Se{sub 2} films are determined. It is found that a shift of the maximal photosensitivity for the obtained solar cells to shorter wavelengths is observed as rigid substrates are replaced by flexible ones. It is concluded that thin-film Cu(In,Ga)Se{sub 2} structures can be used as broad-band photoconverters of solar radiation.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Measurement of Young's modulus of GaAs nanowires growing obliquely on a substrate

A convenient and fast method for measuring Young's modulus of semiconductor nanowires obliquely standing on the growth substrate is presented. In this method, the nanowire is elastically bent under the force exerted by the probe of an atomic-force microscope, and the load-unload dependences for the bending of the probe cantilever are recorded. Next, these curves are used to find the bending stiffness of the tilted nanowires, after which, taking into account the nanowire dimensions, Young's modulus is obtained. The implementation of this method is demonstrated for tilted GaAs nanowires growing on a GaAs (111) substrate. Young's modulus is determined by applying finite-element analysis to the problem of the stationary elastic bending of a nanowire taking into account the actual nanowire shape and faceting. It proves that a fairly accurate estimate of Young's modulus can be obtained even if the nanowire shape is approximated by a circular cylinder with a single cross-sectional area. The values of Young's modulus obtained for GaAs nanowires of cubic lattice symmetry are 2 to 3 times smaller than its value for bulk GaAs. This difference is attributed to the presence of stacking faults in the central part of the nanowires.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Comparative study of 3C-SiC layers sublimation-grown on a 6H-SiC substrate

n-3C-SiC/n-6H-SiC heterostructures grown by vacuum sublimation on CREE commercial 6H-SiC substrates are studied. Transmission electron microscopy (TEM) demonstrated that a transitional layer of varying thickness, composed of a mixture of 3C- and 6H-SiC polytypes, is formed on the substrate. A 3C polytype layer was obtained on the interlayer. Cathodoluminescence study of the surface of the film demonstrated that defects in the form of inclusions of another phase (6H-polytype), stacking faults, and twin boundaries (separating domains of cubic modification, grown in various orientations) are found on the surface and in the surface layer with a thickness on the order of 100 {mu}m. Varying the growth conditions changes the concentration of various types of defects.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Control over the morphology of AlN during molecular beam epitaxy with the plasma activation of nitrogen on Si (111) substrates

The results of studies of the growth kinetics of AlN layers during molecular beam epitaxy with the plasma activation of nitrogen using Si (111) substrates are presented. The possibility of the growth of individual AlN/Si (111) nanocolumns using growth conditions with enrichment of the surface with metal near the formation mode of Al drops, at a substrate temperature close to maximal, during molecular beam epitaxy with the plasma activation of nitrogen (T{sub s} ≈ 850°C) is shown. The possibility of growing smooth AlN layers on a nanocolumnar AlN/Si (111) buffer with the use of T{sub s} ≈ 750°C and growth conditions providing enrichment with metal is shown.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Study of a MHEMT heterostructure with an In{sub 0.4}Ga{sub 0.6}As channel MBE-grown on a GaAs substrate using reciprocal space mapping

The crystallographic characteristics of the design elements of a metamorphic high-electron-mobility (MHEMT) heterostructure with an In{sub 0.4}Ga{sub 0.6}As channel are determined based on reciprocal space mapping. The heterostructure is grown by molecular beam epitaxy on the vicinal surface of a GaAs substrate with a deviation angle from the (001) plane of 2° and consists of a stepped metamorphic buffer containing six layers including an inverse step, a high-temperature buffer layer with constant composition, and active HEMT layers. The InAs content in the layers of the metamorphic buffer is varied from 0.1 to 0.48. Reciprocal space maps are constructed for the (004) symmetric reflection and (224)+ asymmetric reflection. It is found that the heterostructure layers are characterized both by a tilt angle relative to the plane of the (001) substrate and a rotation angle around the [001] axis. The tilt angle of the layer increases as the InAs concentration in the layer increases. It is shown that a high-temperature buffer layer of constant composition has the largest degree of relaxation compared with all other layers of the heterostructure.

36 MATERIALS SCIENCE↗
DOE OSTI2015

On the shift of the electroluminescence spectra of In{sub x}Ga{sub 1−x}N/GaN structures with various indium contents and various substrate materials caused by the stark effect and mechanical stresses

The shift between the maxima of the electroluminescence spectra of In{sub x}Ga{sub 1−x}N/GaN structures is measured at forward and reverse bias depending on the indium content x in the quantum well and on the substrate material (SiC, AuSn/Si, and Al{sub 2}O{sub 3}). It is established that this shift increases as the indium concentration in the In{sub x}Ga{sub 1−x}N layer and mechanical stresses from the substrate increase.

36 MATERIALS SCIENCE↗
DOE OSTI2015

On a semiconductor laser with a p–n tunnel junction with radiation emission through the substrate

A multiwell interband cascade laser with a tunnel junction within a single waveguide and radiation emission through the substrate is implemented for the first time. It is shown that such a laser heterostructure design provides the more efficient population of quantum wells in comparison with a conventional multiwell laser with radiation emission through the substrate, due to which the lasing threshold is significantly lowered.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Possibility of the use of intermediate carbidsiliconoxide nanolayers on polydiamond substrates for gallium nitride layers epitaxy

The results of using carbidsiliconoxide (a-C:SiO1{sub .5}) films with a thickness of 30–60 nm, produced by the pyrolysis annealing of oligomethylsilseskvioksana (CH{sub 3}–SiO{sub 1.5}){sub n} with cyclolinear (staircased) molecular structure, as intermediate films in the hydride vapor phase epitaxy of gallium nitride on polycrystalline CVD-diamond substrates are presented. In the pyrolysis annealing of (CH{sub 3}–SiO{sub 1.5}){sub n} films in an atmosphere of nitrogen at a temperature of 1060°C, methyl radicals are carbonized to yield carbon atoms chemically bound to silicon. In turn, these atoms form a SiC monolayer on the surface of a-C:SiO{sub 1.5} films via covalent bonding with silicon. It is shown that GaN islands grow on such an intermediate layer on CVD-polydiamond substrates in the process of hydride vapor phase epitaxy in a vertical reactor from the GaCl–NH{sub 3}–N{sub 2} gas mixture.

36 MATERIALS SCIENCE↗