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  • 1985-1989  (26)
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  • 1
    Digitale Medien
    Digitale Medien
    Filamentary structure and transport in corrugated InGaAs/InP superlattices grown by vapor levitation epitaxy (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 66 (1989), S. 1222-1226 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Periodic stimulation of a growth instability on (001) vicinal surfaces of InP and InGaAs induces a corrugated growth interface. When this interface is incorporated into a superlattice the two-dimensional quantum well coalesces into a periodic structure comprised of InGaAs filaments buried in an InP matrix. We have measured the transport in this system by contactless submillimeter wave spectroscopy and show that the electron motion is confined to these filaments, and we have determined the electron density and mobility in these submicrometer filaments.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.343467
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  • 2
    Digitale Medien
    Digitale Medien
    Ohmic, superconducting, shallow AuGe/Nb contacts to GaAs (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 3204-3210 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: An emerging new class of superconductor-semiconductor devices requires Ohmic superconducting contacts which do not diffuse deeply into a semiconductor. We developed a contact to GaAs based on ∼150 A(ring) of AuGe covered with ∼2000 A(ring) of Nb annealed in reducing atmosphere at 390–420 °C for 1–5 s. The resulting contact has linear I-V characteristics at all temperatures down to 4.2 K, resistivity of ∼2×10−6 Ω cm2 (∼0.1 Ω mm), and superconducting transition temperature Tc (approximately-greater-than)8 K. Secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM) studies revealed very shallow penetration of the active dopant Ge into GaAs; the upper limit for the thickness of the doped layer with Ge concentration over 1017 cm−3 is estimated as 200–300 A(ring). Gold, a nonactive dopant (deep level) drops to below 1018 cm−3 within the same distance, with the possible tail extending further. Morphology and uniformity of a contact, as revealed in TEM and optical microscopy, is good, owing to the Nb overlayer which prevents AuGe from "balling up'' upon melting. These contacts can be used in low-temperature devices and conventional devices based on GaAs.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.337738
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  • 3
    Digitale Medien
    Digitale Medien
    Substrate effects on the properties of Y-Ba-Cu-O superconducting films prepared by laser deposition (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 4591-4598 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Superconducting thin films of Y1Ba2Cu3Ox with superconducting transition temperature (Tc) near 90 K have been prepared by a laser deposition technique. We show that films prepared on sapphire, lithium niobate, and strontium titanate under identical processing conditions exhibit different electrical characteristics. The film surfaces, interfaces, and crystallinity have been studied by a number of analytical techniques. We conclude that the substrate influences the film properties primarily in three ways: the thermal expansion mismatch introduces cracks in the film, the interface reaction changes the film composition, and the substrate lattice influences the crystallographic orientation of the film.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.340136
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  • 4
    Digitale Medien
    Digitale Medien
    Auger electron spectroscopy, transmission electron microscopy, and scanning electron microscopy studies of Nb/Al/Nb Josephson junction structures (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 5089-5097 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Al films deposited on Nb can be oxidized and used to make Josephson junction devices. We studied the structure of Al films deposited under "warm'' (estimated to be near 200 °C) and "cold'' (near room temperature) conditions because the cold films produced better Josephson junction devices. For the warm case, the Al film was composed of islands with open channels between them, which we attribute to a high mobility of the Al atoms that lowers the island nucleation density. The Nb surface was extremely flat, which we ascribe to the high surface atom mobility at the higher deposition temperature. The cold Al film was of uniform thickness which can be explained by a high island nucleation density. The cold Nb films had an undulating surface, caused by the lack of surface atom mobility during deposition. There was no evidence of Al-Nb interdiffusion, even after postdeposition heating to 300 °C. Auger spectroscopy, transmission electron microscopy, and scanning electron microscopy were needed to obtain these definitive conclusions.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.338334
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  • 5
    Digitale Medien
    Digitale Medien
    Microstructure of YBa2Cu3O7−x thin films grown on single-crystal SrTiO3 (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 65 (1989), S. 4719-4722 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: YBa2Cu3O7−x thin films on (001) and (014) SrTiO3 were prepared by coevaporation of Y, BaF2, and Cu, and annealing in oxygen at 850 °C. Cross-sectional samples prepared by cutting films and substrates along the SrTiO3 (100) plane were studied with transmission electron microscopy. The growth of YBa2Cu3O7−x in as-deposited films is extremely anisotropic with the [001] being the slow direction and the (001) plane being the fast growth plane. When the (001) planes of YBa2Cu3O7−x grains are parallel to the film plane, as on (001) SrTiO3, there is a dominance of this orientation over the whole film. Three types of YBa2Cu3O7−x grains, 90° misoriented from each other, are observed to grow on both (001) and (014) surfaces of single-crystal SrTiO3 substrates with their c directions aligned with one of the three 〈100〉 directions of cubic SrTiO3. This ambiguity creates a threefold degenerate epitaxy of YBa2Cu3O7−x on SrTiO3, which results in 90° grain boundaries in the final YBa2Cu3O7−x thin films. These boundaries are free of any second phase and decorations. While some extraneous phases were observed in these films, they were located away from the crystallographically special, 90° boundaries. Therefore, the presence of extraneous phases elsewhere in the films was not damaging to the superconducting transport.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.343222
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  • 6
    Digitale Medien
    Digitale Medien
    Investigation of finite size effects in a first order phase transition: High pressure Raman study of CdS microcrystallites (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 2848-2850 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We have used Raman scattering to explore finite size effects in the phase transition of 300 A(ring) colloidal microcrystallites of CdS. By measuring the shifts in LO phonon frequencies as a function of pressure we conclude that these small microcrystallites undergo a well-defined first order phase transition similar to the one occurring in bulk CdS. However, the phase transformation in the colloids exhibits a smaller hysteresis, which we attribute to efficient annealing of defects in the high surface area colloidal material.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.453969
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  • 7
    Digitale Medien
    Digitale Medien
    Clusters in solution: Growth and optical properties of layered semiconductors with hexagonal and honeycombed structures (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 5337-5340 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Transmission electron microscopy and optical absorption were used to examine small clusters of the layered semiconductors, PbI2 and BiI3. In both systems, a layer of metal is sandwiched between two hexagonally closed-packed layers of iodine. We describe a simple solution preparation which gives rise to clusters corresponding to single layer sandwiches, roughly 7 A(ring) thick, whose lateral dimensions vary from 12 to 90 A(ring) depending on the solvent and the nature of the growing cluster interface. The cluster size distributions are markedly different for PbI2 and BiI3 reflecting the different structure in the metal planes of these systems. PbI2 cluster sizes are determined by hexagonal symmetry, with cluster growth achieved by placement of lead atoms symmetrically about a smaller cluster. In BiI3, whose metal plane has a honeycombed structure like graphite, clusters grow to be much larger with their sizes determined by the closure of six-membered rings. The optical absorption spectra of PbI2 and BiI3 can be quantitatively understood in terms of the measured cluster size distributions.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.451677
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  • 8
    Digitale Medien
    Digitale Medien
    Parabolic approximation method for fast magnetosonic wave propagation in tokamaks (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 1608-1619 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Fast magnetosonic wave propagation in a cylindrical tokamak model is studied using a parabolic approximation method in which poloidal variations of the wave field are considered weak in comparison to the radial variations. Diffraction and wave interference effects, which are ignored by ray tracing methods, are included self-consistently using the parabolic method since continuous solutions for the wave electromagnetic fields are computed directly from an approximate form of the wave equation. Numerical results are presented which illustrate the cylindrical convergence of the launched waves into a diffraction-limited focal spot on the cyclotron absorption layer near the magnetic axis for a wide range of plasma confinement parameters.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.865677
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  • 9
    Digitale Medien
    Digitale Medien
    Parabolic approximation method for the mode conversion-tunneling equation (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 340-349 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The derivation of the wave equation that governs ion cyclotron range of frequencies (ICRF) wave propagation, absorption, and mode conversion within the kinetic layer in tokamaks has been extended to include diffraction and focusing effects associated with the finite transverse dimensions of the incident wave fronts. The kinetic layer considered consists of a uniform density, uniform temperature slab model in which the equilibrium magnetic field is oriented in the zˆ direction and varies linearly in the xˆ direction. An equivalent dielectric tensor, as well as a two-dimensional energy conservation equation, are derived from the linearized Vlasov–Maxwell system of equations. A generalized, but approximate form of the mode conversion-tunneling equation is then extracted from the Maxwell equations, using the parabolic approximation method in which transverse variations of the wave fields are assumed to be weak in comparison to the variations in the primary direction of propagation. Numerical solutions of this approximate wave equation agree well with solutions to the exact wave equation within the kinetic layer.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.859208
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  • 10
    Digitale Medien
    Digitale Medien
    High critical currents in epitaxial YBa2Cu3O7−x thin films on silicon with buffer layers (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 754-756 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: As-deposited superconducting thin films (∼0.1 μm) of YBa2Cu3O7−x have been prepared by pulsed laser deposition on (100) Si with buffer layers of BaTiO3/MgAl2O4. X-ray diffraction studies reveal that the films grow epitaxially with the c axis preferentially oriented normal to the substrate surface. This is confirmed by ion channeling measurements along the (100) (normal to the surface) and (110) directions of the Si substrate showing a minimum yield of 54% along the (100), and 78% along the (110) axes using 2.8 MeV He++. Preliminary transmission electron microscopy study also supports these results. The as-deposited films have zero resistance temperatures of 86–87 K, and critical current densities of 6×104 A/cm2 at 77 K and 1.2×105 A/cm2 at 73 K. Our results indicate that the superconducting properties of the films are limited primarily by the quality and degree of epitaxal growth of the buffer layers on the silicon substrate.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.101471
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