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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 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Lutetium-doped EuO films grown by molecular-beam epitaxy

The effect of lutetium doping on the structural, electronic, and magnetic properties of epitaxial EuO thin films grown by reactive molecular-beam epitaxy is experimentally investigated. The behavior of Lu-doped EuO is contrasted with doping by lanthanum and gadolinium. All three dopants are found to behave similarly despite differences in electronic configuration and ionic size. Andreev reflection measurements on Lu-doped EuO reveal a spin-polarization of 96% in the conduction band, despite non-magnetic carriers introduced by 5% lutetium doping.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Growth of AlN/SiC/AlN quantum wells on Si(111) by molecular beam epitaxy

AlN/SiC/AlN quantum well structures have been grown on Si(111) by molecular beam epitaxy at 700 deg. C. The microstructure is single-crystal wurtzite AlN and cubic SiC with stacking sequence disorder. Depth profiles taken by Auger electron spectroscopy indicate that the ratio of Si to C is about 5:4. Layers with lower carbon content exhibit regions with five-fold superstructures. Fourier transform infrared spectroscopy confirms the presence of Al-N and Si-C bonds. Our work demonstrates the feasibility of a low-temperature synthesis route for epitaxial SiC and AlN/SiC heterostructures on Si.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Epitaxial GaN films by hyperthermal ion-beam nitridation of Ga droplets

Epitaxial GaN film formation on bare 6H-SiC(0001) substrates via the process of transformation of Ga droplets into a thin GaN film by applying hyperthermal nitrogen ions is investigated. Pre-deposited Ga atoms in well defined amounts form large droplets on the substrate surface which are subsequently nitridated at a substrate temperature of 630 Degree-Sign C by a low-energy nitrogen ion beam from a constricted glow-discharge ion source. The Ga deposition and ion-beam nitridation process steps are monitored in situ by reflection high-energy electron diffraction. Ex situ characterization by x-ray diffraction and reflectivity techniques, Rutherford backscattering spectrometry, and electron microscopy shows that the thickness of the resulting GaN films depends on the various amounts of pre-deposited gallium. The films are epitaxial to the substrate, exhibit a mosaic like, smooth surface topography and consist of coalesced large domains of low defect density. Possible transport mechanisms of reactive nitrogen species during hyperthermal nitridation are discussed and the formation of GaN films by an ion-beam assisted process is explained.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Epitaxial relationship of semipolar s-plane (1101) InN grown on r-plane sapphire

The heteroepitaxy of semipolar s-plane (1101) InN grown directly on r-plane sapphire by plasma-assisted molecular beam epitaxy is studied using transmission electron microscopy techniques. The epitaxial relationship is determined to be (1101){sub InN} Parallel-To (1102){sub Al{sub 2O{sub 3}}}, [1120]{sub InN} Parallel-To [2021]{sub Al{sub 2O{sub 3}}}, [1102]{sub InN}{approx} Parallel-To [0221]{sub Al{sub 2O{sub 3}}}, which ensures a 0.7% misfit along [1120]{sub InN}. Two orientation variants are identified. Proposed geometrical factors contributing to the high density of basal stacking faults, partial dislocations, and sphalerite cubic pockets include the misfit accommodation and reduction, as well as the accommodation of lattice twist.

36 MATERIALS SCIENCE↗
DOE OSTI2012

In-plane dielectric properties of epitaxial Ba{sub 0.7}Sr{sub 0.3}TiO{sub 3} thin films grown on GaAs for tunable device application

We have epitaxially deposited ferroelectric Ba{sub 0.7}Sr{sub 0.3}TiO{sub 3} (BST) thin films grown on GaAs substrate via SrTiO{sub 3} buffer layer by laser molecular beam epitaxy. Structural characteristics of the heterostructure were measured by various techniques. The in-plane dielectric properties of the heteroepitaxial structure under different applying frequency were investigated from -190 to 90 Degree-Sign C, indicating Curie temperature of the BST film to be around 52 Degree-Sign C. At room temperature, the dielectric constant of the heterostructure under moderate dc bias field can be tuned by more than 30% and K factor used for frequency agile materials is found to be close to 8. Our results offer the possibility to combine frequency agile electronics of ferroelectric titanate with the high-performance microwave capabilities of GaAs for room temperature tunable device application.

36 MATERIALS SCIENCE↗
DOE OSTI2012

High electron mobility through the edge states in random networks of c-axis oriented wedge-shaped GaN nanowalls grown by molecular beam epitaxy

Transport and optical properties of random networks of c-axis oriented wedge-shaped GaN nanowalls grown spontaneously on c-plane sapphire substrates through molecular beam epitaxy are investigated. Our study suggests a one dimensional confinement of carriers at the top edges of these connected nanowalls, which results in a blue shift of the band edge luminescence, a reduction of the exciton-phonon coupling, and an enhancement of the exciton binding energy. Not only that, the yellow luminescence in these samples is found to be completely suppressed even at room temperature. All these changes are highly desirable for the enhancement of the luminescence efficiency of the material. More interestingly, the electron mobility through the network is found to be significantly higher than that is typically observed for GaN epitaxial films. This dramatic improvement is attributed to the transport of electrons through the edge states formed at the top edges of the nanowalls.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2012

Strain controlled systematic variation of metal-insulator transition in epitaxial NdNiO{sub 3} thin films

We report here the strain dependent structural and electrical transport properties of epitaxial NdNiO{sub 3} thin films. Pulsed laser deposition technique was used to grow the NdNiO{sub 3} thin films on c-axis oriented SrTiO{sub 3} single crystals. Deposited films were irradiated using 200 MeV Ag{sup 15+} ion beam at the varying fluence (1 Multiplication-Sign 10{sup 11}, 5 Multiplication-Sign 10{sup 11}, and 1 Multiplication-Sign 10{sup 12} ions/cm{sup 2}). X-ray diffraction studies confirm the epitaxial growth of the deposited films, which is maintained even up to the highest fluence. Rise in the in-plane compressive strain has been observed after the irradiation. All the films exhibit metal-insulator transition, however, a systematic decrease in the transition temperature (T{sub MI}) has been observed after irradiation, which may be attributed to the increase in the in-plane compression. Raman spectroscopy data reveal that this reduction in T{sub MI}, with the irradiation, is related to the decrease in band gap due to the stress generated by the in-plane compressive strain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

High quality ultrathin Bi{sub 2}Se{sub 3} films on CaF{sub 2} and CaF{sub 2}/Si by molecular beam epitaxy with a radio frequency cracker cell

We report a method to fabricate high quality Bi{sub 2}Se{sub 3} thin films using molecular beam epitaxy with a radio frequency cracker cell as an atomic selenium source. With Se-to-Bi ratios close to exact stoichiometry, optimal layer-by-layer growth of high quality Bi{sub 2}Se{sub 3} thin films with smooth surfaces has been achieved on CaF{sub 2}(111) substrates and Si(111) substrates with a thin epitaxial CaF{sub 2} buffer layer (CaF{sub 2}/Si). Transport measurements show a characteristic weak-antilocalization magnetoresistance in all the films, with the emergence of a weak-localization contribution in the ultrathin film limit. Quantum oscillations, attributed to the topological surface states have been observed, including in films grown on CaF{sub 2}/Si.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Band alignment of epitaxial ZnS/Zn{sub 3}P{sub 2} heterojunctions

The energy-band alignment of epitaxial zb-ZnS(001)/{alpha}-Zn{sub 3}P{sub 2}(001) heterojunctions has been determined by measurement of shifts in the phosphorus 2p and sulfur 2p core-level binding energies for various thicknesses (0.6-2.2 nm) of ZnS grown by molecular beam epitaxy on Zn{sub 3}P{sub 2}. In addition, the position of the valence-band maximum for bulk ZnS and Zn{sub 3}P{sub 2} films was estimated using density functional theory calculations of the valence-band density-of-states. The heterojunction was observed to be type I, with a valence-band offset, {Delta}E{sub V,} of -1.19 {+-} 0.07 eV, which is significantly different from the type II alignment based on electron affinities that is predicted by Anderson theory. n{sup +}-ZnS/p-Zn{sub 3}P{sub 2} heterojunctions demonstrated open-circuit voltages of >750 mV, indicating passivation of the Zn{sub 3}P{sub 2} surface due to the introduction of the ZnS overlayer. Carrier transport across the heterojunction devices was inhibited by the large conduction-band offset, which resulted in short-circuit current densities of <0.1 mA cm{sup -2} under 1 Sun simulated illumination. Hence, constraints on the current density will likely limit the direct application of the ZnS/Zn{sub 3}P{sub 2} heterojunction to photovoltaics, whereas metal-insulator-semiconductor structures that utilize an intrinsic ZnS insulating layer appear promising.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Fabrication and characterization of Bismuth-Cerium composite iron garnet epitaxial films for magneto optical applications

The Bi{sub x}Ce{sub 3-x}Fe{sub 5}O{sub 12} (x = 0.8) epitaxial films of high quality were grown by means of pulsed laser deposition on paramagnetic substrates of Gadolinium Gallium Garnet. We study the modifications of substitutions in the parent garnet Y{sub 3}Fe{sub 5}O{sub 12} that produces a higher magneto-optical response at communication wavelengths. These films displayed a strong in plane textures which are treated in argon as well as reduced atmosphere conditions. The elemental constituents of these films were confirmed by energy dispersive-X ray analysis, elastic recoil detection system, Rutherford backscattering spectroscopy, and X-ray photoelectron spectroscopy measurements. The transmittance spectra were measured and found these films exhibit good transmittance values. The transmittance-spectra were fitted with the theoretical model and the optical constants such as refractive index and absorption edge were evaluated. The highest (negative) Faraday rotation was found for these films treated in the environment of Ar + H{sub 2}. A density matrix theory has been developed for the Faraday rotation and a good agreement between the theory and experiment is found. These epitaxial garnet films can be used in a wide range of frequencies from visible to infrared spectra making them ideal for many magneto optical applications. Therefore, these films may overcome many issues in fabricating all optical isolators which is the viable solution for integrated photonics.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Superconducting epitaxial thin films of CeNi{sub x}Bi{sub 2} with a bismuth square net structure

We have grown highly epitaxial and phase pure thin films of the arsenic-free pnictide compound CeNi{sub x}Bi{sub 2} on (100) MgO substrates by reactive molecular beam epitaxy (RMBE). X-ray diffraction and reflection high-energy electron diffraction of the films confirm the ZrCuSiAs structure with a Bi square net layer. Superconductivity was observed in magnetization and resistivity measurements for x= 0.75 to 0.93 in these CeNi{sub x}Bi{sub 2} thin films with the highest critical temperature of 4.05 K and a resistive transition width of 0.1 K for x= 0.86. Our results indicate that thin film deposition by RMBE provides a tool to synthesize high-quality pnictide superconductors of the novel 112 type.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Solid-phase epitaxy of silicon amorphized by implantation of the alkali elements rubidium and cesium

The redistribution of implanted Rb and Cs profiles in amorphous silicon during solid-phase epitaxial recrystallization has been investigated by Rutherford backscattering spectroscopy and secondary ion mass spectroscopy. For the implantation dose used in these experiments, the alkali atoms segregate at the a-Si/c-Si interface during annealing resulting in concentration peaks near the interface. In this way, the alkali atoms are moved towards the surface. Rutherford backscattering spectroscopy in ion channeling configuration was performed to measure average recrystallization rates of the amorphous silicon layers. Preliminary studies on the influence of the alkali atoms on the solid-phase epitaxial regrowth rate reveal a strong retardation compared to the intrinsic recrystallization rate.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Surface composition of BaTiO{sub 3}/SrTiO{sub 3}(001) films grown by atomic oxygen plasma assisted molecular beam epitaxy

We have investigated the growth of BaTiO{sub 3} thin films deposited on pure and 1% Nb-doped SrTiO{sub 3}(001) single crystals using atomic oxygen assisted molecular beam epitaxy and dedicated Ba and Ti Knudsen cells. Thicknesses up to 30 nm were investigated for various layer compositions. We demonstrate 2D growth and epitaxial single crystalline BaTiO{sub 3} layers up to 10 nm before additional 3D features appear; lattice parameter relaxation occurs during the first few nanometers and is completed at {approx}10 nm. The presence of a Ba oxide rich top layer that probably favors 2D growth is evidenced for well crystallized layers. We show that the Ba oxide rich top layer can be removed by chemical etching. The present work stresses the importance of stoichiometry and surface composition of BaTiO{sub 3} layers, especially in view of their integration in devices.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Site-controlled Ag nanocrystals grown by molecular beam epitaxy-Towards plasmonic integration technology

We demonstrate site-controlled growth of epitaxial Ag nanocrystals on patterned GaAs substrates by molecular beam epitaxy with high degree of long-range uniformity. The alignment is based on lithographically defined holes in which position controlled InAs quantum dots are grown. The Ag nanocrystals self-align preferentially on top of the InAs quantum dots. No such ordering is observed in the absence of InAs quantum dots, proving that the ordering is strain-driven. The presented technique facilitates the placement of active plasmonic nanostructures at arbitrarily defined positions enabling their integration into complex devices and plasmonic circuits.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Electric field-tunable Ba{sub x}Sr{sub 1-x}TiO{sub 3} films with high figures of merit grown by molecular beam epitaxy

We report on the dielectric properties of Ba{sub x}Sr{sub 1-x}TiO{sub 3} (BST) films grown by molecular beam epitaxy on epitaxial Pt bottom electrodes. Paraelectric films (x Less-Than-Or-Equivalent-To 0.5) exhibit dielectric losses that are similar to those of BST single crystals and ceramics. Films with device quality factors greater than 1000 and electric field tunabilities exceeding 1:5 are demonstrated. The results provide evidence for the importance of stoichiometry control and the use of a non-energetic deposition technique for achieving high figures of merit of tunable devices with BST thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Properties of epitaxial BaTiO{sub 3} deposited on GaAs

Single crystal BaTiO{sub 3} (BTO) has been grown epitaxially on GaAs using molecular beam epitaxy with a 2 unit cell SrTiO{sub 3} nucleation layer. The oxide film is lattice-matched to GaAs through an in-plane rotation of 45 Degree-Sign relative to the (100) surface leading to c-axis orientation of the BaTiO{sub 3}. X-ray diffraction confirmed the crystallinity and orientation of the oxide film with a full width half maximum of 0.58 Degree-Sign for a 7.5 nm thick layer. Piezoresponse force microscopy was used to characterize the ferroelectric domains in the BaTiO{sub 3} layer, and a coercive voltage of 1-2 V and piezoresponse amplitude {approx}5 pm/V was measured.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Molecular beam epitaxy of high structural quality Bi{sub 2}Se{sub 3} on lattice matched InP(111) substrates

Epitaxial layers of the topological insulator Bi{sub 2}Se{sub 3} have been grown by molecular beam epitaxy on laterally lattice-matched InP(111)B substrates. High resolution X-ray diffraction shows a significant improvement of Bi{sub 2}Se{sub 3} crystal quality compared to layers deposited on other substrates. The measured full width at half maximum of the rocking curve is {Delta}{omega}=13 arc sec, and the ({omega}-2{theta}) scans exhibit clear layer thickness fringes. Atomic force microscope images show triangular twin domains with sizes increasing with layer thickness. The structural quality of the domains is confirmed on the microscopic level by transmission electron microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Ab initio atomistic thermodynamics calculations of the initial deposition of epitaxial MgO film on GaAs(001)-{beta}2(2 Multiplication-Sign 4)

Ab initio calculations were performed to investigate the initial deposition of epitaxial MgO on GaAs(001)-{beta}2(2 Multiplication-Sign 4). The differences between the chemical bonding of Mg-As and O-As were characterized by the adsorption energies of atomic O and Mg at several symmetrically distinct sites, and O bonding was substantially stronger than that of Mg. Thermodynamics were analyzed through the introduction of environmental chemical potentials simulating in situ growth conditions by the sputtering of a stoichiometric MgO target. A surface phase diagram was generated under Mg and O environments with constrained equilibrium, and the results explained the initial formation of an epitaxial MgO phase on GaAs(001)-{beta}2(2 Multiplication-Sign 4).

36 MATERIALS SCIENCE↗