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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 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Influence of Ga/N ratio on morphology, vacancies, and electrical transport in GaN grown by molecular beam epitaxy at high temperature

The effect of Ga/N flux ratio on surface morphology, incorporation of point defects and electrical transport properties of GaN films grown by plasma-assisted molecular beam epitaxy in a recently developed high-temperature growth regime was investigated. The homoepitaxial (0001) GaN films grown at {approx}780-790 deg. C showed smoothest morphologies near the cross-over between N-rich and Ga-rich growth (0.75<Ga/N<1.1) contrasting previous observations for low-temperature growth. The higher-quality growth near Ga/N{approx}1 resulted from lower thermal decomposition rates and was corroborated by slightly lower Ga vacancy concentrations [V{sub Ga}], lower unintentional oxygen incorporation, and improved electron mobilities. The consistently low [V{sub Ga}], i.e., {approx}10{sup 16} cm{sup -3} for all films attribute further to the significant benefits of the high-temperature growth regime.

36 MATERIALS SCIENCE↗
DOE OSTI2010

InAlN/GaN Bragg reflectors grown by plasma-assisted molecular beam epitaxy

We report on molecular beam epitaxy growth and characterization of ten-period lattice-matched InAlN/GaN distributed Bragg reflectors (DBRs), with peak reflectivity centered around 400 nm. Thanks to the well tuned ternary alloy composition, crack-free surfaces have been obtained, as confirmed by both optical and transmission electron microscopy (TEM). Their good periodicity and well-defined interfaces have been confirmed by both x-ray diffraction and TEM measurements. Peak reflectivity values as high as 60% with stop bands of 30 nm have been demonstrated. Optical measurements revealed that discrepancy between the obtained (60%) and the theoretically expected ({approx}75%) reflectivity is a consequence of significant residual absorption ({approx}35%). TEM measurements revealed the coexistence of zinc-blende and wurtzite phases, as well as planar defects, mainly in GaN. These defects are suggested as the potential source of the undesired absorption and/or scattering effects that lowered the DBRs' peak reflectivity.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Improved epitaxy of ultrathin praseodymia films on chlorine passivated Si(111) reducing silicate interface formation

Ultrathin praseodymia films have been deposited on both Cl-passivated and nonpassivated Si(111) substrates by molecular beam epitaxy. Comparative studies on the crystallinity and stoichiometry are performed by x-ray photoelectron spectroscopy, x-ray standing waves, and x-ray reflectometry. On nonpassivated Si(111) an amorphous silicate film is formed. In contrast, praseodymia deposited on Cl-passivated Si(111) form a well-ordered crystalline film with cubic-Pr{sub 2}O{sub 3} (bixbyite) structure. The vertical lattice constant of the praseodymia film is increased by 1.4% compared to the bulk value. Furthermore, the formation of an extended amorphous silicate interface layers is suppressed and confined to only one monolayer.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Magnetic properties of full-Heusler alloy Co{sub 2}Fe{sub 1-x}Mn{sub x}Al films grown by molecular-beam epitaxy

We have investigated magnetic properties of single-crystalline full-Heusler alloy Co{sub 2}Fe{sub 1-x}Mn{sub x}Al films (0<x<1) grown on GaAs (001) by molecular-beam epitaxy at different temperatures. It is found that as x increases, the films grown at 160 deg. C have complex magnetic phases, but show pure ferromagnetic properties when grown at 280 deg. C. We attribute this magnetic behavior to the competition between ferromagnetic and antiferromagnetic Mn-Mn interactions, which are proved by x-ray magnetic circular dichroism measurements. Moreover, we have investigated in-plane magnetic anisotropies of Co{sub 2}Fe{sub 1-x}Mn{sub x}Al films with pure ferromagnetic phases and deduced the corresponding anisotropy constants which also remarkably depend on x.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Growth kinetics in position-controlled and catalyst-free InAs nanowire arrays on Si(111) grown by selective area molecular beam epitaxy

We investigated the interwire distance dependence on the growth kinetics of vertical, high-yield InAs nanowire arrays on Si(111) grown by catalyst-free selective area molecular beam epitaxy (MBE). Utilizing lithographically defined SiO{sub 2} nanomasks on Si(111) with regular hole patterns, catalyst-free and site-selective growth of vertically (111)-oriented InAs nanowires was achieved with very high yields of {approx}90 percent. Interestingly, the yield of vertically ordered nanowires was independent of the interwire distance and the initial growth stages. Significant size variation in the nanowires was found to depend critically on the interwire distance and growth time. Two growth regimes were identified--(i) a competitive growth regime with shorter and thinner nanowires for narrow interwire distances and (ii) a diffusion-limited growth regime for wider distances, providing good estimates for the surface diffusion lengths. Surprisingly, despite these size-dependent effects the nanowire geometries remained unaltered with uniform, almost nontapered morphologies even over large variation in nanowire density ({approx}mid-10{sup 6}-10{sup 9} cm{sup -2} range). X-ray diffraction further confirmed the vertical (111) directionality with low crystal tilt by rocking curve widths ({omega} scans) as low as {approx}0.6 deg. These findings demonstrate the capability to precisely tailor the position and size of well-oriented III-V semiconductor nanowires through noncatalytic MBE selective area growth and provide an important step toward fully integrated, uniform vertical III-V nanowire array-on-Si devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2010

Graphene growth by molecular beam epitaxy on the carbon-face of SiC

Graphene layers have been grown by molecular beam epitaxy (MBE) on the (0001) C-face of SiC and have been characterized by atomic force microscopy, low energy electron diffraction (LEED), and UV photoelectron spectroscopy. Contrary to the graphitization process, the step-terrace structure of SiC is fully preserved during the MBE growth. LEED patterns show multiple orientation domains which are characteristic of graphene on SiC (0001), indicating non-Bernal rotated graphene planes. Well-defined Dirac cones, typical of single-layer graphene, have been observed in the valence band for few graphene layers by synchrotron spectroscopy, confirming the electronic decoupling of graphene layers.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Measurement of valence-band offsets of InAlN/GaN heterostructures grown by metal-organic vapor phase epitaxy

The valence band offsets, {Delta}E{sub V}, of In{sub 0.17}Al{sub 0.83}N/GaN, In{sub 0.25}Al{sub 0.75}N/GaN, and In{sub 0.30}Al{sub 0.70}N/GaN heterostructures grown by metal-organic vapor phase epitaxy were evaluated by using x-ray photoelectron spectroscopy (XPS). The dependence of the energy position and the full width at half maximum of the Al 2p spectrum on the exit angle indicated that there was sharp band bending caused by the polarization-induced electric field combined with surface Fermi-level pinning in each ultrathin InAlN layer. The {Delta}E{sub V} values evaluated without taking into account band bending indicated large discrepancies from the theoretical estimates for all samples. Erroneous results due to band bending were corrected by applying numerical calculations, which led to acceptable results. The evaluated {Delta}E{sub V} values were 0.2{+-}0.2 eV for In{sub 0.17}Al{sub 0.83}N/GaN, 0.1{+-}0.2 eV for In{sub 0.25}Al{sub 0.75}N/GaN, and 0.0{+-}0.2 eV for In{sub 0.30}Al{sub 0.70}N/GaN. Despite the large decrease of around 1.0 eV in the band gap of InAlN layers according to the increase in the In molar fraction, the decrease in {Delta}E{sub V} was as small as 0.2 eV. Therefore, the change in band-gap discontinuity was mainly distributed to that in conduction band offset.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Charge Transport Characteristics Of Cobalt Phthalocyanine Thin Films Grown By Molecular Beam Epitaxy

In the recent times organic semiconductors (OSC) have received attention because of their application in low-cost, flexible, and large area electronics devices. The application of OSC thin films has been limited due to their low charge carrier mobility ({approx}0.1 cm{sup 2}/V-s). We have investigated the effect of substrate on structure and charge transport characteristics of cobalt phthalocyanine (CoPc) films. Thin films have been grown on both single crystal (sapphire and LaAlO{sub 3}) and amorphous (quartz) substrates using molecular beam epitaxy system. The films grown on LaAlO{sub 3} substrates exhibited a higher value of mobility ({approx}4 cm{sup 2}/V-s) while those grown on Al{sub 2}O{sub 3} and quartz showed mobility value of {approx}1 cm{sup 2}/V-s. High mobility for LaAlO{sub 3} substrates has been attributed to the enhanced ordering of the molecules due to natural twin boundaries of substrates. In order to further confirm role of grain boundaries in aligning the CoPc molecules, we measured the charge transport on films deposited at bi-crystal SrTiO{sub 3} substrates. The results showed that current along bi-crystal grain boundary is three orders of magnitude higher than for films on SrTiO{sub 3} substrate without grain boundary, which confirms our hypothesis of ordering of molecules along grain boundaries.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Enhanced critical temperature in epitaxial ferroelectric Pb(Zr{sub 0.2}Ti{sub 0.8})O{sub 3} thin films on silicon

The structural and electrical properties of epitaxial Pb(Zr{sub 0.2}Ti{sub 0.8})O{sub 3} thin films grown on 2 in. (001) silicon wafers were investigated. Using x-ray diffraction, the lattice behavior of the heterostructure has been studied as a function of temperature, suggesting a 250 deg. C increase of the Pb(Zr{sub 0.2}Ti{sub 0.8})O{sub 3} ferroelectric-paraelectric transition temperature with respect to the bulk value. This significant enhancement of the critical temperature is understood in terms of a two-dimensional clamping effect.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Improving stability of photoluminescence of ZnSe thin films grown by molecular beam epitaxy by incorporating Cl dopant

This investigation studies the effect of chlorine (Cl) dopant in ZnSe thin films that were grown by molecular beam epitaxy on their photoluminescence (PL) and the stability thereof. Free excitonic emission was observed at room-temperature in the Cl-doped sample. Photon irradiation with a wavelength of 404 nm and a power density of 9.1 W/cm{sup 2} has a much stronger effect on PL degradation than does thermal heating to a temperature of 150 deg. C. Additionally, this study shows that the generation of nonradiative centers by both photon irradiation and thermal heating can be greatly inhibited by incorporating Cl dopant.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Molecular beam epitaxial growth of metamorphic AlInSb/GaInSb high-electron-mobility-transistor structures on GaAs substrates for low power and high frequency applications

We report on molecular beam epitaxial growth of AlInSb/GaInSb metamorphic high-electron-mobility-transistor structures for low power, high frequency applications on 4 in. GaAs substrates. The structures consist of a Ga{sub 0.4}In{sub 0.6}Sb channel embedded in Al{sub 0.4}In{sub 0.6}Sb barrier layers which are grown on top of an insulating metamorphic buffer, which is based on the linear exchange of Ga versus In and a subsequent exchange of As versus Sb. Precise control of group V fluxes and substrate temperature in the Al{sub 0.4}In{sub 0.6}As{sub 1-x}Sb{sub x} buffer is essential to achieve high quality device structures. Good morphological properties were achieved demonstrated by the appearance of crosshatching and root mean square roughness values of 2.0 nm. Buffer isolation is found to be >100 k{Omega}/{open_square} for optimized growth conditions. Hall measurements at room temperature reveal electron densities of 2.8x10{sup 12} cm{sup -2} in the channel at mobility values of 21.000 cm{sup 2}/V s for single-sided Te volume doping and 5.4x10{sup 12} cm{sup -2} and 17.000 cm{sup 2}/V s for double-sided Te {delta}-doping, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Extreme low-temperature molecular beam epitaxy of ZnO-based quantum structures

We report on extreme low-temperature growth of ZnO by plasma-assisted molecular beam epitaxy. Epilayers and quantum well (QW) structures with very good structural and optical properties are prepared at substrate temperatures as low as 50 deg. C. The growth proceeds in a single crystalline layer-by-layer mode. ZnO QWs prepared on a-plane sapphire show bright excitonic luminescence with a very narrow linewidth of only 6 meV at 5 K. High-resolution transmission electron micrographs confirm that low-temperature single crystalline growth is not restricted to a particular surface termination of ZnO but works also for crystal growth along a nonpolar direction.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Structural and magnetic characterizations of Mn{sub 2}CrO{sub 4} and MnCr{sub 2}O{sub 4} films on MgO(001) and SrTiO{sub 3}(001) substrates by molecular beam epitaxy

In this report, we grow MnCr{sub 2}O{sub 4} (124) films, and Mn{sub 2}CrO{sub 4} (214) films, for comparison, on MgO and SrTiO{sub 3} (STO) substrates by oxygen-plasma-assisted molecular beam epitaxy and also carry out the measurements of the magnetization as a function of temperature (5-200 K) and external field (0-5 T) of these films. The results of magnetization versus temperature further show that ferrimagnetic transition (T{sub c}) occurs at 44 K for 214 films grown on both MgO and STO. The T{sub c} of 124 films raises to 45 K but is only observed for the film grown on STO. No or very weak magnetization is observed for the 124 films grown on MgO(001). The vanishing of the magnetization of the 124 films grown on MgO is attributed to the instability of spinel structure for Cr-rich films. Magnetic hysteresis curves of the films further show the magnetization saturation (M{sub s}) of 124 is <1/2 bohr magneton, which is noticeably smaller than the bulk M{sub s} {approx} 1 bohr magneton. The reduction of M{sub s} in these chromites films may result from the cation distribution in that Cr cations distribute in both A and B sites, which is different from the bulk materials. The distribution of Cr cations in both A and B is attributed to the low temperature growth and similar results have been found in previous growth of various ferrite films.

36 MATERIALS SCIENCE↗
DOE OSTI2011

InN/InGaN multiple quantum wells emitting at 1.5 {mu}m grown by molecular beam epitaxy

This work reports on the growth by molecular beam epitaxy and characterization of InN/InGaN multiple quantum wells (MQWs) emitting at 1.5 {mu}m. X-ray diffraction (XRD) spectra show satellite peaks up to the second order. Estimated values of well (3 nm) and barrier (9 nm) thicknesses were derived from transmission electron microscopy and the fit between experimental data and simulated XRD spectra. Transmission electron microscopy and XRD simulations also confirmed that the InGaN barriers are relaxed with respect to the GaN template, while the InN MQWs grew under biaxial compression on the InGaN barriers. Low temperature (14 K) photoluminescence measurements reveal an emission from the InN MQWs at 1.5 {mu}m. Measurements as a function of temperature indicate the existence of localized states, probably due to InN quantum wells' thickness fluctuations as observed by transmission electron microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Germanium-tin p-i-n photodetectors integrated on silicon grown by molecular beam epitaxy

GeSn heterojunction p-i-n diodes with a Sn content of 0.5% are grown with a special low temperature molecular beam epitaxy. The Sn incorporation in Ge is facilitated by a very low temperature growth step in order to suppress Sn surface segregation. Diodes with sharp doping transitions are realized as double mesa structures with a diameter from 1.5 up to 80 {mu}m. An optical responsivity of these GeSn diodes of 0.1 A/W at a wavelength of {lambda}=1.55 {mu}m is measured. In comparison with a pure Ge detector the optical responsivity is increased by factor of 3 as a result of Sn caused band gap reduction.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Influence of Au capping layer on the magnetic properties of ultrathin epitaxial Fe{sub 3}O{sub 4}/GaAs(001) film

The effect of the Au capping layer on the magnetic anisotropies and molecular magnetic moment of ultrathin epitaxial Fe{sub 3}O{sub 4} films on GaAs(100) substrate was studied by MO Kerr effect, superconducting quantum interference device, and Ferromagnetic resonance, respectively. The films with and without capping layers show a predominant in-plane uniaxial magnetic anisotropy and a minor fourfold anisotropy. The Au capping layer was found to increase significantly the uniaxial magnetic anisotropy related to the magnetoelastic interactions at the Au/Fe{sub 3}O{sub 4} interface, and unexpectedly reduce saturation molecular magnetization due to the interfacial diffusion between Au layer and Fe{sub 3}O{sub 4} layer.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Properties of molecular beam epitaxy grown Eu{sub x}(transition metal){sub y} films (transition metals: Mn, Cr)

The electronic and crystallographic structures, as well as the magnetic properties, of Eu{sub x}(transition metal){sub y} (transition metals: Mn, Cr) thin films grown by molecular beam epitaxy were studied. Relative changes of the Eu/Mn and Eu/Cr ratios derived from the XPS lines, as well as x-ray reflectivity, indicate mixing of the Eu/Mn and Eu/Cr layers. Valency transitions from Eu{sup 2+} to Eu{sup 3+} were observed in both systems for most studied stoichiometries. A transition to a magnetically ordered phase was observed at 15 K, 40 K, and 62 K for selected films in the Eu-Mn system, and at 50 K for the film with a Eu/Cr ratio of 0.5.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Magnetic properties of MnSb inclusions formed in GaSb matrix directly during molecular beam epitaxial growth

Despite of intensive search for the proper semiconductor base materials for spintronic devices working at room temperature no appropriate material based on ferromagnetic semiconductors has been found so far. We demonstrate that the phase segregated system with MnSb hexagonal inclusions inside the GaSb matrix, formed directly during the molecular beam epitaxial growth reveals the ferromagnetic properties at room temperature and is a good candidate for exploitation in spintronics. Furthermore, the MnSb inclusions with only one crystalline structure were identified in this GaMn:MnSb granular material. The SQUID magnetometry confirmed that this material exhibits ferromagnetic like behavior starting from helium up to room temperature. Moreover, the magnetic anisotropy was found which was present also at room temperature, and it was proved that by choosing a proper substrate it is possible to control the direction of easy axis of inclusions' magnetization moment between in-plane and out-of-plane; the latter is important in view of potential applications in spintronic devices.

36 MATERIALS SCIENCE↗