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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 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Growth and magnetic property of antiperovskite manganese nitride films doped with Cu by molecular beam epitaxy

Manganese nitrides thin films on MgO (100) substrates with and without Cu-doping have been fabricated by plasma assisted molecular beam epitaxy. Antiperovskite compounds Mn{sub 3.6}Cu{sub 0.4}N have been grown in the case of Cu-doping, and the pure Mn{sub 3}N{sub 2} single crystal has been obtained without Cu-doping. The Mn{sub 3.6}Cu{sub 0.4}N exhibits ferrimagnetism, and the magnetization of Mn{sub 3.6}Cu{sub 0.4}N increases upon the temperature decreasing from 300 K to 5 K, similar to Mn{sub 4}N. The exchange bias (EB) effects emerge in the Mn{sub 3.6}Cu{sub 0.4}N films. The EB behavior is originated from the interfaces between ferrimagnetic Mn{sub 3.6}Cu{sub 0.4}N and antiferromagnetic metal Mn, which is verified to be formed by the data of x-ray photoelectron spectroscopy. The present results not only provide a strategy for producing functional antiperovskite manganese nitrides, but also shed promising light on fabricating the exchange bias part of spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Droplet epitaxial growth of highly symmetric quantum dots emitting at telecommunication wavelengths on InP(111)A

We demonstrate the formation of InAs quantum dots (QDs) on InAlAs/InP(111)A by means of droplet epitaxy. The C{sub 3v} symmetry of the (111)A substrate enabled us to realize highly symmetric QDs that are free from lateral elongations. The QDs exhibit a disk-like truncated shape with an atomically flat top surface. Photoluminescence signals show broad-band spectra at telecommunication wavelengths of 1.3 and 1.5 μm. Strong luminescence signals are retained up to room temperature. Thus, our QDs are potentially useful for realizing an entangled photon-pair source that is compatible with current telecommunication fiber networks.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Dual-polarity GaN micropillars grown by metalorganic vapour phase epitaxy: Cross-correlation between structural and optical properties

Self-assembled catalyst-free GaN micropillars grown on (0001) sapphire substrates by metal organic vapor phase epitaxy are investigated. Transmission electron microscopy, as well as KOH etching, shows the systematic presence of two domains of opposite polarity within each single micropillar. The analysis of the initial growth stages indicates that such double polarity originates at the micropillar/substrate interface, i.e., during the micropillar nucleation, and it propagates along the micropillar. Furthermore, dislocations are also generated at the wire/substrate interface, but bend after several hundreds of nanometers. This leads to micropillars several tens of micrometers in length that are dislocation-free. Spatially resolved cathodoluminescence and microphotoluminescence show large differences in the optical properties of each polarity domain, suggesting unequal impurity/dopant/vacancy incorporation depending on the polarity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Anisotropic orbital occupation and Jahn-Teller distortion of orthorhombic YMnO{sub 3} epitaxial films: A combined experimental and theoretical study on polarization-dependent x-ray absorption spectroscopy

The b-axis oriented orthorhombic YMnO{sub 3} (o-YMnO{sub 3}) epitaxial films on a YAlO{sub 3} (010) substrate were fabricated with pulsed-laser deposition. The anisotropic orbital occupation and Jahn-Teller (JT) distortion of an o-YMnO{sub 3} film were investigated with polarization-dependent x-ray absorption spectra and configuration-interaction multiplet-cluster calculations. A significant energy difference, ∼3.8 eV, for the main white line along E//b and E//a in polarization-dependent Mn K-edge spectra of o-YMnO{sub 3} indicates an extraordinary JT distortion and significant anisotropic Mn–O bonding within the ab plane in the o-YMnO{sub 3} film. Most importantly, although the orbital occupation of 3d electrons in o-YMnO{sub 3} films is almost the same as that in single crystalline o-DyMnO{sub 3}, the JT distortion of o-YMnO{sub 3} films is larger than that of single crystalline o-DyMnO{sub 3}, deduced from the multiplet calculations. We speculate that this JT distortion predominantly contributes to the origin of the cycloidal spin deformation in bulk o-YMnO{sub 3}, because of a suppressed nearest-neighbor superexchange interaction and an enhanced next-nearest-neighbor superexchange interaction. These complementary results provide insight into the origin of the E-type magnetic configuration of o-YMnO{sub 3}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Super-dense array of Ge quantum dots grown on Si(100) by low-temperature molecular beam epitaxy

Ge layer grown on Si(100) at the low temperature of ∼100 °C by molecular beam epitaxy is studied using scanning tunneling microscopy and Raman spectroscopy. It is found that crystalline and pseudomorphic to the Si substrate Ge islands are formed at the initial growth stage. The islands acquire the base size of 1.2–2.6 nm and they form arrays with the super-high density of (5–8) × 10{sup 12} cm{sup −2} at 1–2 nm Ge coverages. Such a density is at least 10 times higher than that of Ge “hut” clusters grown via the Stranski-Krastanov growth mode. It is shown that areas between the crystalline Ge islands are filled with amorphous Ge, which is suggested to create potential barrier for holes localized within the islands. As a result, crystalline Ge quantum dots appear being isolated from each other.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Observing hot carrier distribution in an n-type epitaxial graphene on a SiC substrate

Hot carrier dynamics in the Dirac band of n-type epitaxial graphene on a SiC substrate were traced in real time using femtosecond-time-resolved photoemission spectroscopy. The spectral evolution directly reflects the energetically linear density of states superimposed with a Fermi–Dirac distribution. The relaxation time is governed by the internal energy dissipation of electron–electron scattering, and the observed electronic temperature indicates cascade carrier multiplication.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Hydride vapor phase epitaxy and characterization of high-quality ScN epilayers

The heteroepitaxial growth of ScN films was investigated on various substrates by hydride vapor phase epitaxy (HVPE). Single crystalline mirror-like ScN(100) and ScN(110) layers were successfully deposited on r- and m-plane sapphire substrates, respectively. Homogeneous stoichiometric films (N/Sc ratio 1.01 ± 0.10) up to 40 μm in thickness were deposited. Their mosaicity drastically improved with increasing the film thickness. The band gap was determined by optical methods to be 2.06 eV. Impurity concentrations including H, C, O, Si, and Cl were investigated through energy dispersive X-ray spectrometry and secondary ion mass spectrometry. As a result, it was found that the presence of impurities was efficiently suppressed in comparison with that of HVPE-grown ScN films reported in the past, which was possible thanks to the home-designed corrosion-free HVPE reactor. Room-temperature Hall measurements indicated that the residual free electron concentrations ranged between 10{sup 18}–10{sup 20} cm{sup −3}, which was markedly lower than the reported values. The carrier mobility increased monotonically with the decreasing in carrier concentration, achieving the largest value ever reported, 284 cm{sup 2} V{sup −1} s{sup −1} at n = 3.7 × 10{sup 18} cm{sup −3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Defect study in molecular beam epitaxy-grown HgCdTe films with activated and unactivated arsenic

A defect study was performed on molecular beam epitaxy-grown HgCdTe films in situ doped with arsenic. Doping was performed from either effusion cell or cracker cell, and studied were both as-grown samples and samples subjected to arsenic activation annealing. Electrical properties of the films were investigated with the use of ion milling as a means of “stirring” defects in the material. As a result of the study, it was confirmed that the most efficient incorporation of electrically active arsenic occurs at the cracking zone temperature of 700 °C. Interaction between arsenic and tellurium during the growth was observed and is discussed in the paper.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Dispersion properties and low infrared optical losses in epitaxial AlN on sapphire substrate in the visible and infrared range

Optical waveguiding properties of a thick wurtzite aluminum nitride highly [002]-textured hetero-epitaxial film on (001) basal plane of sapphire substrate are studied. The physical properties of the film are determined by X-ray diffraction, atomic force microscopy, microRaman, and photocurrent spectroscopy. The refractive index and the thermo-optic coefficients are determined by m-lines spectroscopy using the classical prism coupling technique. The optical losses of this planar waveguide are also measured in the spectral range of 450–1553 nm. The lower value of optical losses is equal to 0.7 dB/cm at 1553 nm. The optical losses due to the surface scattering are simulated showing that the contribution is the most significant at near infrared wavelength range, whereas the optical losses are due to volume scattering and material absorption in the visible range. The good physical properties and the low optical losses obtained from this planar waveguide are encouraging to achieve a wide bandgap optical guiding platform from these aluminum nitride thin films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The effect of Bi composition on the properties of InP{sub 1−x}Bi{sub x} grown by liquid phase epitaxy

InP{sub 1−x}Bi{sub x} epilayers (x ≥ 1.2%) on InP (001) are grown reproducibly by liquid phase epitaxy with conventional solution baking in a H{sub 2} environment. The Bi composition and surface morphology of the grown layers are studied by secondary ion mass spectroscopy and atomic force microscopy, respectively. High-resolution x-ray diffraction is used to characterize the lattice parameters and the crystalline quality of the layers. 10 K photoluminescence measurements indicate three clearly resolved peaks in undoped InP layers with band-to-band transition at 1.42 eV which is redshifted with Bi incorporation in the layer with a maximum band gap reduction of 50 meV/% Bi. The effect is attributed to the interaction between the valence band edge and Bi-related defect states as is explained here by valence-band anticrossing model. Room temperature Hall measurements indicate that the mobility of the layer is not significantly affected for Bi concentration up to 1.2%.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

DC current induced metal-insulator transition in epitaxial Sm{sub 0.6}Nd{sub 0.4}NiO{sub 3}/LaAlO{sub 3} thin film

The metal-insulator transition (MIT) in strong correlated electron materials can be induced by external perturbation in forms of thermal, electrical, optical, or magnetic fields. We report on the DC current induced MIT in epitaxial Sm{sub 0.6}Nd{sub 0.4}NiO{sub 3} (SNNO) thin film deposited by pulsed laser deposition on (001)-LaAlO{sub 3} substrate. It was found that the MIT in SNNO film not only can be triggered by thermal, but also can be induced by DC current. The T{sub MI} of SNNO film decreases from 282 K to 200 K with the DC current density increasing from 0.003 × 10{sup 9} A•m{sup −2} to 4.9 × 10{sup 9} A•m{sup −2}. Based on the resistivity curves measured at different temperatures, the MIT phase diagram has been successfully constructed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

High-electron-mobility GaN grown on free-standing GaN templates by ammonia-based molecular beam epitaxy

The dependence of electron mobility on growth conditions and threading dislocation density (TDD) was studied for n{sup −}-GaN layers grown by ammonia-based molecular beam epitaxy. Electron mobility was found to strongly depend on TDD, growth temperature, and Si-doping concentration. Temperature-dependent Hall data were fit to established transport and charge-balance equations. Dislocation scattering was analyzed over a wide range of TDDs (∼2 × 10{sup 6} cm{sup −2} to ∼2 × 10{sup 10} cm{sup −2}) on GaN films grown under similar conditions. A correlation between TDD and fitted acceptor states was observed, corresponding to an acceptor state for almost every c lattice translation along each threading dislocation. Optimized GaN growth on free-standing GaN templates with a low TDD (∼2 × 10{sup 6} cm{sup −2}) resulted in electron mobilities of 1265 cm{sup 2}/Vs at 296 K and 3327 cm{sup 2}/Vs at 113 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Compliant ferroelastic domains in epitaxial Pb(Zr,Ti)O{sub 3} thin films

Ordered patterns of highly compliant ferroelastic domains have been created by use of tensile strained epitaxial Pb(Zr,Ti)O{sub 3} thin films, of very low defect density, grown on DyScO{sub 3} substrates. The effect of 180° switching on well-ordered a/c 90° domain patterns is investigated by a combination of transmission electron microscopy, piezoelectric force microscopy, and X-ray diffraction. It is shown that ferroelastic a-domains, having an in-plane polarization, can be created and completely removed on a local level by an out-of-plane electric field. The modifications of the ferroelastic domain pattern can be controlled by varying the parameters used during switching with a piezoresponse force microscope to produce the desired arrangement.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Quantitative analysis of anisotropic magnetoresistance in Co{sub 2}MnZ and Co{sub 2}FeZ epitaxial thin films: A facile way to investigate spin-polarization in half-metallic Heusler compounds

Anisotropic magnetoresistance (AMR) effect has been systematically investigated in various Heusler compounds Co{sub 2}MnZ and Co{sub 2}FeZ (Z = Al, Si, Ge, and Ga) epitaxial films and quantitatively summarized against the total valence electron number N{sub V}. It was found that the sign of AMR ratio is negative when N{sub V} is between 28.2 and 30.3, and turns positive when N{sub V} becomes below 28.2 and above 30.3, indicating that the Fermi level (E{sub F}) overlaps with the valence or conduction band edges of half-metallic gap at N{sub V} ∼ 28.2 or 30.3, respectively. We also find out that the magnitude of negative AMR ratio gradually increases with shifting of E{sub F} away from the gap edges, and there is a clear positive correlation between the magnitude of negative AMR ratio and magnetoresistive output of the giant magnetoresistive devices using the Heusler compounds. This indicates that AMR can be used as a facile way to optimize a composition of half-metallic Heusler compounds having a high spin-polarization at room temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Dopant-site-dependent scattering by dislocations in epitaxial films of perovskite semiconductor BaSnO{sub 3}

We studied the conduction mechanism in Sb-doped BaSnO{sub 3} epitaxial films, and compared its behavior with that of the mechanism of its counterpart, La-doped BaSnO{sub 3}. We found that the electron mobility in BaSnO{sub 3} films was reduced by almost 7 times when the dopant was changed from La to Sb, despite little change in the effective mass of the carriers. This indicates that the scattering rate of conduction electrons in the BaSnO{sub 3} system is strongly affected by the site at which the dopants are located. More importantly, we found that electron scattering by threading dislocations also depends critically on the dopant site. We propose that the large enhancement of scattering by the threading dislocations in Sb-doped BaSnO{sub 3} films is caused by the combination effect of the change in the distribution of Sb impurities in the films, the formation of the Sb impurity clusters near the threading dislocations, and the conduction electron clustering near the Sb impurities.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Chirped-pulse manipulated carrier dynamics in low-temperature molecular-beam-epitaxy grown GaAs

Chirped pulse controlled carrier dynamics in low-temperature molecular-beam-epitaxy grown GaAs are investigated by degenerate pump-probe technique. Varying the chirped condition of excited pulse from negative to positive increases the carrier relaxation time so as to modify the dispersion and reshape current pulse in time domain. The spectral dependence of carrier dynamics is analytically derived and explained by Shockley-Read Hall model. This observation enables the new feasibility of controlling carrier dynamics in ultrafast optical devices via the chirped pulse excitations.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Core level excitations—A fingerprint of structural and electronic properties of epitaxial silicene

From the analysis of high-resolution Si 2p photoelectron and near-edge x-ray absorption fine structure (NEXAFS) spectra, we show that core level excitations of epitaxial silicene on ZrB{sub 2}(0001) thin films are characteristically different from those of sp{sup 3}-hybridized silicon. In particular, it is revealed that the lower Si 2p binding energies and the low onset in the NEXAFS spectra as well as the occurrence of satellite features in the core level spectra are attributed to the screening by low-energy valence electrons and interband transitions between π bands, respectively. The analysis of observed Si 2p intensities related to chemically distinct Si atoms indicates the presence of at least one previously unidentified component. The presence of this component suggests that the observation of stress-related stripe domains in scanning tunnelling microscopy images is intrinsically linked to the relaxation of Si atoms away from energetically unfavourable positions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Identification of dominant scattering mechanism in epitaxial graphene on SiC

A scheme of identification of scattering mechanisms in epitaxial graphene (EG) on SiC substrate is developed and applied to three EG samples grown on SiC (0001), (112{sup ¯}0), and (101{sup ¯}0) substrates. Hall measurements combined with defect detection technique enable us to evaluate the individual contributions to the carrier scatterings by defects and by substrates. It is found that the dominant scatterings can be due to either substrate or defects, dependent on the substrate orientations. The EG on SiC (112{sup ¯}0) exhibits a better control over the two major scattering mechanisms and achieves the highest mobility even with a high carrier concentration, promising for high performance graphene-based electronic devices. The method developed here will shed light on major aspects in governing carrier transport in EG to harness it effectively.

36 MATERIALS SCIENCE↗