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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 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Lattice and grain-boundary diffusions of boron atoms in BaSi{sub 2} epitaxial films on Si(111)

A 180-nm-thick boron (B) layer was deposited on a 300-nm-thick a-axis-oriented BaSi{sub 2} epitaxial film grown by molecular beam epitaxy on Si(111) and was annealed at different temperatures in ultrahigh vacuum. The depth profiles of B were investigated using secondary ion mass spectrometry (SIMS) with O{sup 2+}, and the diffusion coefficients of B were evaluated. The B profiles were reproduced well by taking both the lattice and the grain boundary (GB) diffusions into consideration. The cross-sectional transmission electron microscopy (TEM) image revealed that the GBs of the BaSi{sub 2} film were very sharp and normal to the sample surface. The plan-view TEM image exhibited that the grain size of the BaSi{sub 2} film was approximately 0.6 {mu}m. The temperature dependence of lattice and GB diffusion coefficients was derived from the SIMS profiles, and their activation energies were found to be 4.6 eV and 4.4 eV, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Polarization induced hole doping in graded Al{sub x}Ga{sub 1-x}N (x = 0.7 {approx} 1) layer grown by molecular beam epitaxy

Polarization induced hole doping on the order of {approx}10{sup 18} cm{sup -3} is achieved in linearly graded Al{sub x}Ga{sub 1-x}N (x = 0.7 {approx} 1) layer grown by molecular beam epitaxy. Graded Al{sub x}Ga{sub 1-x}N and conventional Al{sub 0.7}Ga{sub 0.3}N layers grown on AlN are beryllium (Be) doped via epitaxial growth. The hole concentration in graded Al{sub x}Ga{sub 1-x}N:Be (x = 0.7 {approx} 1) layers demonstrates that polarization generates hole charges from Be dopant. The Al{sub 0.7}Ga{sub 0.3}N layer is not conductive owing to the absence of carriers generated from the Be dopant without the inducement of polarization. Polarization doping provides an approach to high efficiency p-type doping in high Al composition AlGaN.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Strain effects in epitaxial Mn{sub 2}O{sub 3} thin film grown on MgO(100)

We report on the epitaxial growth and magnetic properties of Mn{sub 2}O{sub 3} thin films grown on MgO(001) substrate by molecular beam epitaxy. We observed the reduction in binding energy of Mn valence states, the increase in satellite separation up to 12.7 eV, and the smaller band gap of 3.32 eV. In addition, the antiferromagnetic ordering below 90 K in bulk changed to ferrimagnetic up to 175 K. The results were possibly to be explained by a lattice mismatch strain on Mn{sub 2}O{sub 3} film on MgO(001) substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Investigation of ZnO thin films deposited on ferromagnetic metallic buffer layer by molecular beam epitaxy toward realization of ZnO-based magnetic tunneling junctions

Deposition of ZnO thin films on a ferromagnetic metallic buffer layer (Co{sub 3}Pt) by molecular beam epitaxy technique was investigated for realization of ZnO-based magnetic tunneling junctions with good quality hexagonal ZnO films as tunnel barriers. For substrate temperature of 600 Degree-Sign C, ZnO films exhibited low oxygen defects and high electrical resistivity of 130 {Omega} cm. This value exceeded that of hexagonal ZnO films grown by sputtering technique, which are used as tunnel barriers in ZnO-MTJs. Also, the effect of oxygen flow during deposition on epitaxial growth conditions and Co{sub 3}Pt surface oxidation was discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2013

The study of in situ scanning tunnelling microscope characterization on GaN thin film grown by plasma assisted molecular beam epitaxy

The epitaxial growth of GaN by Plasma Assisted Molecular Beam Epitaxy was investigated by Scanning Tunnelling Microscope (STM). The GaN film was grown on initial GaN (0001) and monitored by in situ Reflection High Energy Electron Diffraction and STM during the growth. The STM characterization was carried out on different sub-films with increased thickness. The growth of GaN was achieved in 3D mode, and the hexagonal edge of GaN layers and growth gradient were observed. The final GaN was of Ga polarity and kept as (0001) orientation, without excess Ga adlayers or droplets formed on the surface.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Effect of growth temperature on magnetic and electronic properties of epitaxially grown MnAs thin films on GaAs(100) substrates

We grew epitaxial MnAs thin films on GaAs(100) substrates using molecular-beam epitaxy and investigated the growth temperature dependences of their physical properties. With increasing growth temperature from 200 Degree-Sign C to 350 Degree-Sign C, the crystal orientation of the film changed from type-A to type-B. The Curie temperatures of all type-B samples were {approx}346 K, while the type-A sample showed a lower bulk-like value of {approx}318 K, indicating an improvement in magnetic properties. Samples grown at above 550 Degree-Sign C exhibited semiconducting behavior, whereas the lower temperature grown samples show metallic behavior as expected. Our results indicate that growth temperature plays an important role in determining the crystal structure, magnetic, and electrical-transport properties of MnAs/GaAs(100) thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Perpendicularly magnetized {tau}-MnAl (001) thin films epitaxied on GaAs

Perpendicularly magnetized {tau}-MnAl films have been epitaxied on GaAs (001) by molecular-beam epitaxy. Crystalline quality and magnetic properties of the samples were strongly dependent on growth temperature. The highest coercivity of 10.7 kOe, saturation magnetization of 361.4 emu/cm{sup 3}, perpendicular magnetic anisotropy constant of 13.65 Merg/cm{sup 3}, and magnetic energy product of 4.44 MGOe were achieved. These tunable magnetic properties make MnAl films valuable as excellent and cost-effective alternative for not only high density perpendicular magnetic recording storage and spintronics devices but also permanent magnets.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Structure, band gap, and Mn-related mid-gap states in epitaxial single crystal (Zn{sub 1-x}Mg{sub x}){sub 1-y}Mn{sub y}O thin films

Epitaxial (Zn{sub 1-x}Mg{sub x}){sub 1-y}Mn{sub y}O thin films were grown on c-Al{sub 2}O{sub 3} substrates by radio frequency oxygen plasma assisted molecular beam epitaxy. Single crystal structure of the (Zn{sub 1-x}Mg{sub x}){sub 1-y}Mn{sub y}O films was revealed by reflection high energy electron diffraction and X-ray diffraction. The band gap of the films can be tuned dramatically with increasing the Mg concentration, while the onset energy of Mn-related mid-gap absorption band only shows a small blue shift. Photoconductivity measurements indicate the Mn-related mid-gap states in (Zn{sub 1-x}Mg{sub x}){sub 1-y}Mn{sub y}O films can create free carriers and contribute to charge transfer transitions. The conduction band offset {Delta}E{sub C} = 0.13 eV and valence band offset {Delta}E{sub V} = 0.1 eV were obtained for ZnO/Zn{sub 0.8}Mg{sub 0.2}O heterostructures, which increase to {Delta}E{sub C} = 0.21 eV and {Delta}E{sub V} = 0.14 eV for ZnO/Zn{sub 0.7}Mg{sub 0.3}O heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Growth temperature dependent structural and magnetic properties of epitaxial Co{sub 2}FeAl Heusler alloy films

The structural and magnetic properties of a series of Co{sub 2}FeAl Heusler alloy films grown on GaAs(001) substrate by molecular beam epitaxy have been studied. The epitaxial Co{sub 2}FeAl films with an ordered L{sub 21} structure have been successfully obtained at growth temperature of 433 K, with an in-plane cubic magnetic anisotropy superimposed with an unusual uniaxial magnetic anisotropy. With increasing growth temperature, the ordered L{sub 21} structure degrades. Meanwhile, the uniaxial anisotropy decreases and eventually disappears above 673 K. The interfacial bonding between As and Co or Fe atom is suggested to be responsible for the additional uniaxial anisotropy.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Quasi-zero lattice mismatch and band alignment of BaTiO{sub 3} on epitaxial (110)Ge

Growth, structural, and band alignment properties of pulsed laser deposited amorphous BaTiO{sub 3} on epitaxial molecular beam epitaxy grown (110)Ge layer, as well as their utilization in low power transistor are reported. High-resolution x-ray diffraction demonstrated quasi-zero lattice mismatch of BaTiO{sub 3} on (110)Ge. Cross-sectional transmission electron microscopy micrograph confirms the amorphous nature of BaTiO{sub 3} layer as well as shows a sharp heterointerface between BaTiO{sub 3} and Ge with no traceable interfacial layer. The valence band offset, {Delta}E{sub v}, of 1.99 {+-} 0.05 eV at the BaTiO{sub 3}/(110)Ge heterointerface is measured using x-ray photoelectron spectroscopy. The conduction band offset, {Delta}E{sub c}, of 1.14 {+-} 0.1 eV is calculated using the bandgap energies of BaTiO{sub 3} of 3.8 eV and Ge of 0.67 eV. These band offset parameters for carrier confinement and the interface chemical properties of the BaTiO{sub 3}/(110)Ge system are significant advancement towards designing Ge-based p-and n-channel metal-oxide semiconductor field-effect transistors for low-power application.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Molecular beam epitaxial growth of Bi{sub 2}Te{sub 3} and Sb{sub 2}Te{sub 3} topological insulators on GaAs (111) substrates: a potential route to fabricate topological insulator p-n junction

High quality Bi{sub 2}Te{sub 3} and Sb{sub 2}Te{sub 3} topological insulators films were epitaxially grown on GaAs (111) substrate using solid source molecular beam epitaxy. Their growth and behavior on both vicinal and non-vicinal GaAs (111) substrates were investigated by reflection high-energy electron diffraction, atomic force microscopy, X-ray diffraction, and high resolution transmission electron microscopy. It is found that non-vicinal GaAs (111) substrate is better than a vicinal substrate to provide high quality Bi{sub 2}Te{sub 3} and Sb{sub 2}Te{sub 3} films. Hall and magnetoresistance measurements indicate that p type Sb{sub 2}Te{sub 3} and n type Bi{sub 2}Te{sub 3} topological insulator films can be directly grown on a GaAs (111) substrate, which may pave a way to fabricate topological insulator p-n junction on the same substrate, compatible with the fabrication process of present semiconductor optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Lattice constant and substitutional composition of GeSn alloys grown by molecular beam epitaxy

Single crystal epitaxial Ge{sub 1−x}Sn{sub x} alloys with atomic fractions of tin up to x = 0.145 were grown by solid source molecular beam epitaxy on Ge (001) substrates. The Ge{sub 1−x}Sn{sub x} alloys formed high quality, coherent, strained layers at growth temperatures below 250 °C, as shown by high resolution X-ray diffraction. The amount of Sn that was on lattice sites, as determined by Rutherford backscattering spectrometry channeling, was found to be above 90% substitutional in all alloys. The degree of strain and the dependence of the effective unstrained bulk lattice constant of Ge{sub 1−x}Sn{sub x} alloys versus the composition of Sn have been determined.

36 MATERIALS SCIENCE↗
DOE OSTI2013

A comparative study of electronic and structural properties of polycrystalline and epitaxial magnetron-sputtered ZnO:Al and Zn{sub 1-x}Mg{sub x}O:Al Films—Origin of the grain barrier traps

Homoepitaxial and heteroepitaxial ZnO, ZnO:Al, and Zn{sub 1-x}Mg{sub x}O:Al films have been grown by magnetron sputtering from ceramic targets at substrate temperatures between 200 °C and 500 °C. We studied the relation between the electronic transport and structural properties for the epitaxially grown films and compared it to the properties of polycrystalline films by means of X-ray diffraction, transmission electron microscopy and optical reflectance and transmittance measurements. The results show that the epitaxial growth of ZnO:Al and Zn{sub 1-x}Mg{sub x}O:Al thin films, which has been observed for nearly all films prepared on single crystalline substrates, will not significantly improve the electronic transport properties in comparison to polycrystalline films unless the grain boundaries are eliminated completely. The grain boundary defect densities of about 3 × 10{sup 13} cm{sup −2} are nearly independent on the structural quality of the different polycrystalline, hetero- and homoepitaxial films. This clearly proves that the grain boundary defects are not caused by crystallographic defects, but, most probably, by the dopant aluminium.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Epitaxial growth of the topological insulator Bi{sub 2}Se{sub 3} on Si(111): Growth mode, lattice parameter, and strain state

Using spot profile analysis low energy electron diffraction, we studied the growth mode and strain state of ultra-thin epitaxial Bi{sub 2}Se{sub 3}(111) films grown by molecular beam epitaxy on Si(111). The first layer grows as complete quintuple layer and covers the Si substrate before the next layer nucleates. Its lateral lattice parameter is increased by 1% compared with the value of a{sub ‖} = 4.136 Å for a 6-nm-thick film. With increasing film thickness, a continuous change of the lattice parameter is observed to an asymptotic value, which is explained by a van der Waals-like bonding between the Bi{sub 2}Se{sub 3} film and the Si substrate.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Oxygen vacancy induced photoluminescence and ferromagnetism in SrTiO{sub 3} thin films by molecular beam epitaxy

SrTiO{sub 3} thin films were epitaxially grown on (100) SrTiO{sub 3} substrates using molecular beam epitaxy. The temperature for growth of the films was optimized, which was indicated by x-ray diffraction and further confirmed by microstructural characterization. Photoluminescence spectra show that oxygen-vacancy contributes to red and blue luminescence of oxygen-deficient post-annealed films, and a red shift was observed in blue region. On the other hand, ferromagnetism in film form SrTiO{sub 3} was observed from 5 K to 400 K and could be further enhanced with decreasing oxygen plasma partial pressure in annealing processes, which might be explained by the theory involving d{sup 0} magnetism related to oxygen-vacancy. From the cooperative investigations of optical and magnetic properties, we conclude that intrinsic defects, especially oxygen-vacancy, can induce and enhance luminescence and magnetism in SrTiO{sub 3} films.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Epitaxy and stress of MgO/GaAs(001) heterostructures

We report on the preparation of epitaxial MgO film on GaAs(001) substrates by molecular beam epitaxy at growth temperature of 20–200 °C. Reflection high energy electron diffraction, x-ray diffraction, and high resolution transmission electron microscopy reveal the growth of ordered crystalline cubic MgO(001) film at ∼200 °C with MgO(001)[100]|| GaAs(001)[100] and a 4 : 3 lattice registry. The surface of the MgO films, characterized by atomic force microscopy, exhibits a root mean square roughness of only 0.5 nm. In situ stress measurements reveal tensile stress as low as 1.7 GPa for a growth temperature of 200 °C in good agreement with the calculated residual misfit strain.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Epitaxial films of Heusler compound Co{sub 2}FeAl{sub 0.5}Si{sub 0.5} with high crystalline quality grown by off-axis sputtering

Co{sub 2}FeAl{sub 0.5}Si{sub 0.5} films with a surface roughness of 0.12 nm have been grown epitaxially on lattice-matched MgAl{sub 2}O{sub 4} (001) substrates by off-axis sputtering. X-ray diffraction shows pronounced Laue oscillations, rocking curves as narrow as 0.0043°, and clear Co{sub 2}FeAl{sub 0.5}Si{sub 0.5} (111) peaks indicating L2{sub 1} ordering. Magnetic characterizations show a clear magnetocrystalline anisotropy comprising cubic and epitaxy-induced uniaxial terms. Nuclear magnetic resonance measurements reveal L2{sub 1} order of 81% in the Co{sub 2}FeAl{sub 0.5}Si{sub 0.5} films. Magnetotransport measurements show a distinct separation of anisotropic magnetoresistance and ordinary magnetoresistance. These results demonstrate the state-of-the-art crystalline quality and magnetic uniformity of the Co{sub 2}FeAl{sub 0.5}Si{sub 0.5} films.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Effects of boron dopants of Si (001) substrates on formation of Ge layers by sputter epitaxy method

The formation of Ge layers on boron-doped Si (001) substrates by our sputter epitaxy method has been investigated. The surface morphology of Ge layers grown on Si substrates depends on the substrate resistance, and flat Ge layers are obtained on Si substrates with 0.015 Ω cm resistivity. Highly boron-doped Si substrates cause a transition in the dislocation structure from complex dislocations with 60° dislocation glide planes to 90° pure-edge dislocations, resulting in the formation of flat Ge layers. Furthermore, we have found that the surface morphology of the Ge layers improves with increasing Ge layer thickness. Ge atoms migrating on the deposited Ge layers tend to position themselves at the reactive sites, where the reactivity is related to the number of bonding contacts between the Ge atom and the surface. This modifies the surface morphology, resulting in a flatter surface. Boron dopants together with the sputter epitaxy method effectively suppress the growth of Ge islands and result in the formation of flat Ge layers.

36 MATERIALS SCIENCE↗