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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 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

GaInN light-emitting diodes using separate epitaxial growth for the p-type region to attain polarization-inverted electron-blocking layer, reduced electron leakage, and improved hole injection

A GaInN light-emitting diode (LED) structure is analyzed that employs a separate epitaxial growth for the p-type region, i.e., the AlGaN electron-blocking layer (EBL) and p-type GaN cladding layer, followed by wafer or chip bonding. Such LED structure has a polarization-inverted EBL and allows for uncompromised epitaxial-growth optimization of the p-type region, i.e., without the need to consider degradation of the quantum-well active region during p-type region growth. Simulations show that such an LED structure reduces electron leakage, reduces the efficiency droop, improves hole injection, and has the potential to extend high efficiencies into the green spectral region.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Ferroelectric Pb(Zr,Ti)O{sub 3} epitaxial layers on GaAs

Ferroelectric epitaxial Pb(Zr,Ti)O{sub 3} (PZT) layers were grown by pulsed laser deposition on SrTiO{sub 3}/GaAs templates fabricated by molecular beam epitaxy. The templates present an excellent structural quality and the SrTiO{sub 3}/GaAs is abrupt at the atomic scale. The PZT layers contain a- and c-domains, as shown by X-Ray diffraction analyses. Piezoforce microscopy experiments and macroscopic electrical characterizations indicate that PZT is ferroelectric. A relative dielectric permittivity of 164 is extracted from these measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2013

k-space spin filtering effect in the epitaxial Fe/Au/Fe/GaAs(001) spin-valve

The hot-electron magnetotransport of epitaxial Fe/Au/Fe/GaAs(001) spin-valves is investigated by ballistic-electron magnetic microscopy. A magnetocurrent amplitude larger than 500% is observed at room temperature close to the Schottky barrier energy. Remarkably, this magnetocurrent is not significantly affected by the thickness reduction of ferromagnetic films, down to 5 atomic layers of the Fe(001) top electrode. This rather suggests a dominant interfacial spin-filtering effect. Finally, the magnetocurrent is strongly reduced when the effective mass of the semiconductor collector is increased. These observations are consistent with recent theoretical prediction of k-space spin-filtering effect in epitaxial spin-valves attached to a semiconducting lead.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Critical thickness and strain relaxation in molecular beam epitaxy-grown SrTiO{sub 3} films

We report on the study of the critical thickness and the strain relaxation in epitaxial SrTiO{sub 3} film grown on (La{sub 0.3}Sr{sub 0.7})(Al{sub 0.65}Ta{sub 0.35})O{sub 3} (001) (LSAT) substrate using the hybrid molecular beam epitaxy approach. No change in the film's lattice parameter (both the in-plane and the out-of-plane) was observed up to a film thickness of 180 nm, which is in sharp contrast to the theoretical critical thickness of ∼12 nm calculated using the equilibrium theory of strain relaxation. For film thicknesses greater than 180 nm, the out-of-plane lattice parameter was found to decrease hyperbolically in an excellent agreement with the relaxation via forming misfit dislocations. Possible mechanisms are discussed by which the elastic strain energy can be accommodated prior to forming misfit dislocations leading to such anomalously large critical thickness.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Evidence for graphite-like hexagonal AlN nanosheets epitaxially grown on single crystal Ag(111)

Ultrathin (sub-monolayer to 12 monolayers) AlN nanosheets are grown epitaxially by plasma assisted molecular beam epitaxy on Ag(111) single crystals. Electron diffraction and scanning tunneling microscopy provide evidence that AlN on Ag adopts a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN. This claim is further supported by ultraviolet photoelectron spectroscopy indicating a reduced energy bandgap as expected for hexagonal AlN.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Effect of substrate temperature on the magnetic properties of epitaxial sputter-grown Co/Pt

Epitaxial Co/Pt films have been deposited by dc-magnetron sputtering onto heated C-plane sapphire substrates. X-ray diffraction, the residual resistivity, and transmission electron microscopy indicate that the Co/Pt films are highly ordered on the atomic scale. The coercive field and the perpendicular magnetic anisotropy increase as the substrate temperature is increased from 100–250 °C during deposition of the Co/Pt. Measurement of the domain wall creep velocity as a function of applied magnetic field yields the domain wall pinning energy, which scales with the coercive field. Evidence for an enhanced creep velocity in highly ordered epitaxial Co/Pt is found.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Epitaxial Si encapsulation of highly misfitting SiC quantum dot arrays formed on Si (001)

This work examines Si overgrowth to encapsulate 3C-SiC quantum dot arrays epitaxially grown on Si substrates. Using transmission electron microscopy, we show how the crystalline quality of the Si cap depends on the growth conditions. Overgrowth at 300 °C leads to a planar, epitaxial Si cap, but with small crystallographic rotations in the cap above each quantum dot. At 400 °C growth temperature, Si exhibits reduced sticking to the SiC, leading to a non-planar cap. However, a two-step process, with thin layer grown at 250 °C followed by growth at 500 °C, leads to a planar cap with a much-reduced density of defects.

36 MATERIALS SCIENCE↗
DOE OSTI2014

p-type GaN grown by phase shift epitaxy

Phase shift epitaxy (PSE) is a periodic growth scheme, which desynchronizes host material growth process from dopant incorporation, allowing independent optimization. p-type doping of GaN with Mg by PSE is accomplished with molecular beam epitaxy by periodic shutter action (in order to iterate between Ga- and N-rich surface conditions) and by adjusting time delays between dopant and Ga shutters. Optimum PSE growth was obtained by turning on the Mg flux in the N-rich condition. This suppresses Mg self-compensation at high Mg concentration and produces fairly high hole concentrations (2.4 × 10{sup 18} cm{sup −3})

36 MATERIALS SCIENCE↗
DOE OSTI2014

Anisotropic spin structure along the easy axis of magnetization in epitaxially grown MnAs/GaAs(100) thin films

We grew epitaxial MnAs thin films on GaAs(100) substrates using molecular-beam epitaxy and investigated their magnetic properties. The crystal orientation of the film was type-B. Measurements of the film's magnetization revealed two-fold symmetric magnetic anisotropy on its surface, with the easy and hard directions of magnetization along GaAs[1{sup ¯}10](MnAs[1{sup ¯}1{sup ¯}20]) and GaAs[110](MnAs[11{sup ¯}02]), respectively. We found breakage of the uniaxial anisotropy in magnetization along the easy direction of magnetization for both the magnetization versus magnetic field and magnetization versus temperature measurements. We suggest that the origin of this peculiar pinned ferromagnetism in the MnAs layer is the spin-exchange interaction between the MnAs film and the ultra-thin Mn layer formed at the interface of MnAs film and GaAs(100) substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Nanodomain induced anomalous magnetic and electronic transport properties of LaBaCo{sub 2}O{sub 5.5+δ} highly epitaxial thin films

A giant magnetoresistance effect (∼46% at 20 K under 7 T) and anomalous magnetic properties were found in a highly epitaxial double perovskite LaBaCo{sub 2}O{sub 5.5+δ} (LBCO) thin film on (001) MgO. Aberration-corrected Electron Microscopy and related analytical techniques were employed to understand the nature of these unusual physical properties. The as-grown film is epitaxial with the c-axis of the LBCO structure lying in the film plane and with an interface relationship given by (100){sub LBCO} || (001){sub MgO} and [001]{sub LBCO} || [100]{sub MgO} or [010]{sub MgO}. Orderly oxygen vacancies were observed by line profile electron energy loss spectroscopy and by atomic resolution imaging. Especially, oxygen vacancy and nanodomain structures were found to have a crucial effect on the electronic transport and magnetic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Integration and structural analysis of strain relaxed bi-epitaxial zinc oxide(0001) thin film with silicon(100) using titanium nitride buffer layer

Epitaxial growth of c-plane ZnO(0001) has been demonstrated on the Si(001) by using TiN as an intermediate buffer layer. Because of different out of plane symmetry of the substrate (Si/TiN) and the film (ZnO), two orientations of ZnO domains were obtained and the ZnO film growth is of bi-epitaxial nature. The ZnO thin film was observed to be nearly strain relaxed from X-ray and Raman measurements. The interface between the ZnO and TiN was investigated by transmission electron microscopy, and atomic arrangement has been modeled to understand the crystallographic orientation and structure of the domain/grain boundaries. Reaction at ZnO/TiN interface at higher growth temperature causing zinc titanate formation was observed. The grain boundary structure between the observed domains investigated by scanning transmission electron microscopy, revealed the ZnO(0001) planes to be contiguous across the grain boundary which is significant from the perspective of conduction electron scattering. In this configuration, the TiN (being electrically conductive) can be effectively used as an electrode for novel vertically integrated device applications (like light emitting diodes) directly on Si(100) substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

X-ray magnetic circular dichroism for Co{sub x}Fe{sub 4−x}N (x = 0, 3, 4) films grown by molecular beam epitaxy

We evaluated orbital (m{sub orb}) and spin magnetic moments (m{sub spin}) of Co{sub x}Fe{sub 4−x}N (x = 0, 3, 4) epitaxial thin films grown by molecular beam epitaxy using x-ray magnetic circular dichroism, and discussed the dependence of these values on x. Site-averaged m{sub spin} value of Fe atoms was deduced to be 1.91 μ{sub B} per atom, and that of Co atoms to be 1.47 μ{sub B} per atom in Co{sub 3}FeN at 300 K. These values are close to 1.87 μ{sub B} per Fe atom in Fe{sub 4}N and 1.43 μ{sub B} per Co atom in Co{sub 4}N, respectively. This result implies that the Fe and Co atoms in the Co{sub 3}FeN films were located both at corner and face-centered sites in the anti-perovskite lattice. Spin magnetic moments per unit cell were decreased linearly with increasing x in Co{sub x}Fe{sub 4−x}N. This tendency is in good agreement with theory predicted by the first-principle calculation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Epitaxial growth of γ-Al{sub 2}O{sub 3} on Ti{sub 2}AlC(0001) by reactive high-power impulse magnetron sputtering

Al{sub 2}O{sub 3} was deposited by reactive high-power impulse magnetron sputtering at 600 °C onto pre-deposited Ti{sub 2}AlC(0001) thin films on α-Al{sub 2}O{sub 3}(0001) substrates. The Al{sub 2}O{sub 3} was deposited to a thickness of 65 nm and formed an adherent layer of epitaxial γ-Al{sub 2}O{sub 3}(111) as shown by transmission electron microscopy. The demonstration of epitaxial growth of γ-Al{sub 2}O{sub 3} on Ti{sub 2}AlC(0001) open prospects for growth of crystalline alumina as protective coatings on Ti{sub 2}AlC and related nanolaminated materials. The crystallographic orientation relationships are γ-Al{sub 2}O{sub 3}(111)//Ti{sub 2}AlC(0001) (out-of-plane) and γ- Al {sub 2}O{sub 3}(22{sup ¯}0)// Ti {sub 2} AlC (112{sup ¯}0) (in-plane) as determined by electron diffraction. Annealing in vacuum at 900 °C resulted in partial decomposition of the Ti{sub 2}AlC by depletion of Al and diffusion into and through the γ-Al{sub 2}O{sub 3} layer.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Inhomogeneous distribution of defect-related emission in Si-doped AlGaN epitaxial layers with different Al content and Si concentration

The spatial distribution of luminescence in Si-doped AlGaN epitaxial layers that differ in Al content and Si concentration has been studied by cathodoluminescence (CL) mapping in combination with scanning electron microscopy. The density of surface hillocks increased with decreasing Al content and with increasing Si concentration. The mechanisms giving rise to those hillocks are likely different. The hillocks induced surface roughening, and the compositional fluctuation and local donor-acceptor-pair (DAP) emission at hillock edges in AlGaN epitaxial layers were enhanced irrespective of the origin of the hillocks. The intensity of local DAP emission was related to Si concentration, as well as to hillock density. CL observation revealed that DAP emission areas were present inside the samples and were likely related to dislocations concentrated at hillock edges. Possible candidates for acceptors in the observed DAP emission that are closely related in terms of both Si concentration and hillock edges with large deformations are a V{sub III}-Si{sub III} complex and Si{sub N}, which are unfavorable in ordinary III-nitrides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Perpendicular magnetic anisotropy in epitaxially strained cobalt-ferrite (001) thin films

We investigated the dependencies of both the magnetization characteristics and the perpendicular magnetic anisotropy of Co{sub x}Fe{sub 3–x}O{sub 4}(001) epitaxial films (x = 0.5 and 0.75) on the growth conditions of the reactive magnetron sputtering process. Both saturation magnetization and the magnetic uniaxial anisotropy constant K{sub u} are strongly dependent on the reactive gas (O{sub 2}) flow rate, although there is little difference in the surface structures for all samples observed by reflection high-energy electron diffraction. In addition, certain dead-layer-like regions were observed in the initial stage of the film growth for all films. Our results suggest that the magnetic properties of Co{sub x}Fe{sub 3–x}O{sub 4} epitaxial films are governed by the oxidation state and the film structure at the vicinity of the interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Epitaxial growth of VO{sub 2} by periodic annealing

We report the growth of ultrathin VO{sub 2} films on rutile TiO{sub 2} (001) substrates via reactive molecular-beam epitaxy. The films were formed by the cyclical deposition of amorphous vanadium and its subsequent oxidation and transformation to VO{sub 2} via solid-phase epitaxy. Significant metal-insulator transitions were observed in films as thin as 2.3 nm, where a resistance change ΔR/R of 25 was measured. Low angle annular dark field scanning transmission electron microscopy was used in conjunction with electron energy loss spectroscopy to study the film/substrate interface and revealed the vanadium to be tetravalent and the titanium interdiffusion to be limited to 1.6 nm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

One-step aluminium-assisted crystallization of Ge epitaxy on Si by magnetron sputtering

In this work, one-step aluminium-assisted crystallization of Ge on Si is achieved via magnetron sputtering by applying an in-situ low temperature (50 °C to 150 °C) heat treatment in between Al and Ge depositions. The effect of heat treatment on film properties and the growth mechanism of Ge epitaxy on Si are studied via X-ray diffraction, Raman and transmission electron microscopy analyses. Compared with the conventional two-step process, the one-step aluminium-assisted crystallization requires much lower thermal budget and results in pure Ge epitaxial layer, which may be suitable for use as a virtual substrate for the fabrication of III-V solar cells.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Step edge influence on barrier height and contact area in vertical heterojunctions between epitaxial graphene and n-type 4H-SiC

Vertical rectifying contacts of epitaxial graphene grown by Si sublimation on the Si-face of 4H-SiC epilayers were investigated. Forward bias preferential conduction through the step edges was correlated by linear current density normalization. This phenomenon was observed on samples with 2.7–5.8 monolayers of epitaxial graphene as determined by X-ray photoelectron spectroscopy. A modified Richardson plot was implemented to extract the barrier height (0.81 eV at 290 K, 0.99 eV at 30 K) and the electrically dominant SiC step length of a Ti/Al contact overlapping a known region of approximately 0.52 μm wide SiC terraces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗