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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 721 records · Page 40
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DOE OSTI2011

Epitaxy, strain, and composition effects on metal-insulator transition characteristics of SmNiO{sub 3} thin films

SmNiO{sub 3} (SNO) thin films were deposited on LaAlO{sub 3} (LAO), SrTiO{sub 3}, SrLaAlO{sub 4}, Si, and Al{sub 2}O{sub 3} (sapphire) substrates by RF magnetron sputtering and studies were conducted to understand how film structure and composition influence the insulator-metal transition properties. It is observed that the compressive strain induces the insulator to metal transition (MIT), while tensile strain suppresses it. In the case of non-epitaxial films, semiconducting behavior is obtained on sapphire over a broad temperature range, while on heavily-doped Si substrate; an MIT is seen in out-of-plane resistance measurement. In addition, thickness dependence on the resistance behavior and nickel oxidation state has been examined for epitaxial SNO films on LAO substrates. Fine control of the MIT by modifications to the mismatch strain and thickness provides insights to enhance the performance and the functionality of these films for emerging electron devices.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Enhanced performance of room-temperature-grown epitaxial thin films of vanadium dioxide

Stoichiometric vanadium dioxide in bulk, thin film and nanostructured forms exhibits an insulator-to-metal transition (IMT) accompanied by a structural phase transformation, induced by temperature, light, electric fields, doping or strain. We have grown epitaxial films of vanadium dioxide on c-plane (0001) of sapphire using two different procedures involving (1) room temperature growth followed by annealing and (2) direct high temperature growth. Strain at the film-substrate interface due to growth at different temperatures leads to interesting differences in morphologies and phase transition characteristics. Comparison of the morphologies and switching characteristics of the two films shows that contrary to conventional wisdom, the room-temperature grown films have smoother, more continuous morphologies and better switching performance, consistent with the behavior of epitaxially grown semiconductors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Defect Formation in GaN Epitaxial Layers due to SHI Irradiation

GaN epitaxial layers were irradiated with 200 MeV swift heavy Ag ions at various fluences. These samples were then characterized by XRD and TEM. Increase in peak width (FWHM) with incident ion dose showed reduction in crystallinity of epitaxial layers. Cross sectional TEM images confirmed that at highest fluence (5x10{sup 12} ions/cm{sup 2}) electronic energy loss process caused structural defect formation in GaN layer.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Molecular beam epitaxy and characterization of thin Bi{sub 2}Se{sub 3} films on Al{sub 2}O{sub 3} (110)

The structural and electronic properties of thin Bi{sub 2}Se{sub 3} films grown on Al{sub 2}O{sub 3} (110) by molecular beam epitaxy are investigated. The epitaxial films grow in the Frank-van der Merwe mode and are c-axis oriented. They exhibit the highest crystallinity, the lowest carrier concentration, and optimal stoichiometry at a substrate temperature of 200 deg. C determined by the balance between surface kinetics and desorption of Se. The crystallinity of the films improves with increasing Se/Bi flux ratio. Our results enable studies of thin topological insulator films on inert, non-conducting substrates that allow optical access to both film surfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2011

In situ investigation of the early stage of TiO{sub 2} epitaxy on (001) SrTiO{sub 3}

We report on a systematic study of the growth of epitaxial TiO{sub 2} films deposited by pulsed laser deposition on Ti-terminated SrTiO{sub 3} (001) single crystals. By using in situ reflection high energy electron diffraction, low energy electron diffraction, x-ray photoemission spectroscopy, and scanning probe microscopy, we show that the stabilization of the anatase (001) phase is preceded by the growth of a few nanometers thick pseudomorphic Sr{sub x}TiO{sub 2+y} (x, y < 1) intermediate layer. The data demonstrate that the formation of this intermediate phase is related to the activation of a long range Sr migration from the SrTiO{sub 3} substrate into the film. Our results enrich the phase diagram of the Sr-Ti-O system under epitaxial strain opening a route for the study of the electronic and dielectric properties of the reported Sr-deficient SrTiO{sub 3} phase.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Magnetic signature of symmetry reduction in epitaxial La{sub 0.67}Sr{sub 0.33}MnO{sub 3} films

The magnetic properties of epitaxially grown La{sub 0.67}Sr{sub 0.33}MnO{sub 3} perovskite thin films were investigated to elucidate an unexpected broken symmetry between orthogonal [100] and [010] inplane directions, resulting from the magnetostructural coupling between the film and the cubic SrTiO{sub 3} (001) substrate. The films were synthesized by molecular beam epitaxy with either complete or zero A-site cation disorder. Magnetization measured in [100] and [010] directions shows differences that signal a reduction of the in-plane cubic symmetry only for T < 290 K. The magnetization asymmetry is more robust in the film with complete A-site disorder. These results are attributed to a dominant Mn{sup 3+} character at the film-substrate interface and an associated out-of-plane bonding character with hypothesized origins in both charge imbalance and strain effects.

36 MATERIALS SCIENCE↗
DOE OSTI2011

High temperature step-flow growth of gallium phosphide by molecular beam epitaxy and metalorganic chemical vapor deposition

Post-growth surface morphologies of high-temperature homoepitaxial GaP films grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have been studied. Smooth, stepped surface morphologies of MBE-grown layers, measured by atomic force microscopy, were found for a wide range of substrate temperatures and P{sub 2}:Ga flux ratios. A MOCVD-based growth study performed under similar conditions to MBE-grown samples shows a nearly identical smooth, step-flow surface morphology, presenting a convergence of growth conditions for the two different methods. The additional understanding of GaP epitaxy gained from this study will impact its use in applications that include GaP-based device technologies, III-V metamorphic buffers, and III-V materials integration with silicon.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Optimal Cu buffer layer thickness for growing epitaxial Co overlayers on Si(111)7 x 7

Using scanning tunneling microscopy, reflection high energy diffraction and magnetic optical Kerr effect measurements, growth mode and the magnetic properties of epitaxial Co films on Si(111) with epitaxial Cu(111) buffer layers of various thicknesses have been studied. The strained 3.5-monolayer-thick Cu/Si(111) film has been found to be an optimal buffer, in which case an almost ideal layer-by-layer like growth of Co is observed up to six Co monolayers, due to a negligible lattice mismatch. The coercivity of Co films grown in this layer-by-layer like fashion has been determined to be about 10 Oe, testifying to the high quality of the formed Co film and Co/Cu interface. Changeover of the Co film growth mode from layer-by-layer like to multilayer has been found to result in the transition of the film magnetic properties from isotropic to markedly uniaxially anisotropic.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Growth mechanism and electronic properties of epitaxial In{sub 2}O{sub 3} films on sapphire

In this work, we report on the epitaxial growth of high-quality cubic indium oxide thick films on c-plane sapphire substrates using a two-step growth process. The epitaxial relationship of In{sub 2}O{sub 3} on (0001) Al{sub 2}O{sub 3} has been investigated. The (222) plane spacing and lattice parameter of a most strain-relaxed high-quality In{sub 2}O{sub 3} film have been determined to be 292.58 pm and 1013.53 pm, respectively. The electronic properties in dependence of the film thickness are interpreted using a three-region model. The density at the surface and interface totals (3.3{+-}1.5)x10{sup 13}cm{sup -2}, while the background electron density in the bulk was determined to be (2.4{+-}0.5)x10{sup 18}cm{sup -3}. Furthermore, post treatments such as irradiation via ultraviolet light and ozone oxidation have been found to influence only the surface layer, while the bulk electronic properties remain unchanged.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Surfactant enhanced solid phase epitaxy of Ge/CaF{sub 2}/Si(111): Synchrotron x-ray characterization of structure and morphology

The structure and morphology of CaF{sub 2}/Si(111) and Ge/CaF{sub 2}/Si(111) layered structures with film thicknesses in the range of very few nanometers has been studied with synchrotron-based radiation. While the CaF{sub 2} film is grown via molecular beam epitaxy, the Ge film is fabricated by surfactant enhanced solid phase epitaxy with Sb as surfactant. The CaF{sub 2} film forms two laterally separated phases of relaxed CaF{sub 2} and pseudomorphic CaF{sub 2}, respectively, although the film thickness is very homogeneous. The Ge film is completely relaxed and forms A-oriented parts as well as B-oriented parts, due to twinning. In spite of the large surface roughness of the Ge film, it completely wets CaF{sub 2}/Si(111) also after annealing at 600 deg. C, due to the application of Sb during the annealing process.

36 MATERIALS SCIENCE↗
DOE OSTI2011

High-T{sub c} and high-J{sub c} SmFeAs(O,F) films on fluoride substrates grown by molecular beam epitaxy

Superconducting thin films of SmFeAs(O,F) were prepared by molecular beam epitaxy on fluoride substrates. In our process, F-free SmFeAsO films were grown first, and F was subsequently introduced to the films by diffusion from an overlayer of SmF{sub 3}. By this simple process, record high T{sub c}, namely, T{sub c}{sup on} (T{sub c}{sup end}) = 57.8 K (56.4 K) was obtained in a film on CaF{sub 2}. Furthermore, the films on CaF{sub 2} showed high critical current density over 1 MA/cm{sup 2} in the self-field at 5 K. The correlation between superconductivity and epitaxial strain in SmFeAs(O,F) films is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Size dependent bandgap of molecular beam epitaxy grown InN quantum dots measured by scanning tunneling spectroscopy

InN quantum dots (QDs) were grown on Si (111) by epitaxial Stranski-Krastanow growth mode using plasma-assisted molecular beam epitaxy. Single-crystalline wurtzite structure of InN QDs was verified by the x-ray diffraction and transmission electron microscopy. Scanning tunneling microscopy has been used to probe the structural aspects of QDs. A surface bandgap of InN QDs was estimated from scanning tunneling spectroscopy (STS) I-V curves and found that it is strongly dependent on the size of QDs. The observed size-dependent STS bandgap energy shifts with diameter and height were theoretical explained based on an effective mass approximation with finite-depth square-well potential model.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Temperature-controlled epitaxy of In{sub x}Ga{sub 1-x}N alloys and their band gap bowing

In{sub x}Ga{sub 1-x}N alloys (0 {<=} x {<=} 1) have been grown on GaN/sapphire templates by molecular beam epitaxy. Growth temperature controlled epitaxy was proposed to modulate the In composition so that each In{sub x}Ga{sub 1-x}N layer was grown at a temperature as high as possible and thus their crystalline quality was improved. The bandgap energies of the In{sub x}Ga{sub 1-x}N alloys have been precisely evaluated by optical transmission spectroscopy, where the effect of residual strain and electron concentration (the Burstein-Moss effect) on the bandgap energy shift has been considered. Finally, a bowing parameter of {approx}1.9 {+-} 0.1 eV has been obtained by the well fitting In-composition dependent bandgap energy.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Two-domain formation during the epitaxial growth of GaN (0001) on c-plane Al{sub 2}O{sub 3} (0001) by high power impulse magnetron sputtering

We study the effect of high power pulses in reactive magnetron sputter epitaxy on the structural properties of GaN (0001) thin films grown directly on Al{sub 2}O{sub 3} (0001) substrates. The epilayers are grown by sputtering from a liquid Ga target, using a high power impulse magnetron sputtering power supply in a mixed N{sub 2}/Ar discharge. X-ray diffraction, micro-Raman, micro-photoluminescence, and transmission electron microscopy investigations show the formation of two distinct types of domains. One almost fully relaxed domain exhibits superior structural and optical properties as evidenced by rocking curves with a full width at half maximum of 885 arc sec and a low temperature band edge luminescence at 3.47 eV with the full width at half maximum of 10 meV. The other domain exhibits a 14 times higher isotropic strain component, which is due to the higher densities of the point and extended defects, resulting from the ion bombardment during growth. Voids form at the domain boundaries. Mechanisms for the formation of differently strained domains, along with voids during the epitaxial growth of GaN are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Magnetism and transport properties of epitaxial Fe-Ga thin films on GaAs(001)

Epitaxial Fe-Ga thin films in disordered bcc {alpha}-Fe crystal structure (A2) have been grown on GaAs(001) by molecular beam epitaxy. The saturated magnetization (M{sub S}) decreased from 1371 to 1105 kA/m with increasing Ga concentration from 10.5 to 24.3 % at room temperature. The lattice parameter increased with the increase in Ga content because of the larger atomic radius of Ga atom than that of Fe. The increase in carrier density with Ga content caused in lower resistivity.

36 MATERIALS SCIENCE↗
DOE OSTI2012

NO-assisted molecular-beam epitaxial growth of nitrogen substituted EuO

We have investigated a method for substituting oxygen with nitrogen in EuO thin films, which is based on molecular beam epitaxy distillation with NO gas as the oxidizer. By varying the NO gas pressure, we produce crystalline, epitaxial EuO{sub 1-x}N{sub x} films with good control over the films' nitrogen concentration. In situ x-ray photoemission spectroscopy reveals that nitrogen substitution is connected to the formation Eu{sup 3+}4f{sup 6} and a corresponding decrease in the number of Eu{sup 2+}4f{sup 7}, indicating that nitrogen is being incorporated in its 3{sup -} oxidation state. While small amounts of Eu{sup 3+} in over-oxidized Eu{sub 1-{delta}}O thin films lead to a drastic suppression of the ferromagnetism, the formation of Eu{sup 3+} in EuO{sub 1-x}N{sub x} still allows the ferromagnetic phase to exist with an unaffected T{sub c}, thus providing an ideal model system to study the interplay between the magnetic f{sup 7} (J = 7/2) and the non-magnetic f{sup 6} (J = 0) states close to the Fermi level.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Improved photoresponsivity of semiconducting BaSi{sub 2} epitaxial films grown on a tunnel junction for thin-film solar cells

The highest photoresponsivity and an internal quantum efficiency exceeding 70% at 1.55 eV were achieved for 400 nm thick undoped n-type BaSi{sub 2} epitaxial layers formed on a n{sup +}-BaSi{sub 2}/p{sup +}-Si tunnel junction (TJ) on Si(111). The diffusion of Sb atoms was effectively suppressed by an intermediate polycrystalline Si layer grown by solid phase epitaxy, located between the TJ and undoped BaSi{sub 2} layers.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Effect of buffer layer growth temperature on epitaxial GaN films deposited by magnetron sputtering

Epitaxial GaN films were deposited by reactive sputtering of a GaAs target in 100 % nitrogen at 700 deg. C on ZnO buffer layers grown at different substrate temperatures over sapphire substrates. High resolution X-ray diffraction measurements and the corresponding analysis show that the growth temperature of buffer layers significantly affects the micro-structural parameters of GaN epilayer, such as lateral coherence length, tilt and twist, while the vertical coherence length remains unaffected. The optimum substrate temperature for buffer layer growth has been found to be 300 deg. C. High epitaxial quality GaN film grown on such a buffer layer exhibited micro strain of 1.8x10{sup -4} along with screw and edge type dislocation densities of 7.87x10{sup 9} and 1.16x10{sup 11}, respectively.

36 MATERIALS SCIENCE↗