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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 3176-3181 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Photoemission results demonstrate that atom deposition of Ti, Cr, and Cu at 20 K on the high-temperature superconductors (HTSs) dramatically reduces interfacial reaction relative to 300 K growth but does not completely eliminate it. Thin Ti-O or Cr-O layers are formed during atom deposition of ∼2 A(ring) of Ti or Cr on YBa2Cu3O7 or Bi2Sr2Ca1Cu2O8 because oxygen is withdrawn from the Bi-O and/or Cu-O layers. Interfacial reactions are diffusion limited at 20 K, and metal overlayers nucleate on the reacted layers. These metal layers are more uniform than those grown at 300 K because clustering is suppressed. There is no additional disruption for Cr/HTS interfaces when warmed to 300 K, but increased disruption is evident for Ti/HTS interfaces. The differences reflect the stabilities of Cr and Ti in contact with their own interfacial oxide. Cu atom deposition on Bi2Sr2Ca1Cu208(100) at 20 K also leads to much less disruption than observed for deposition at 300 K.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4336-4341 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: GaAs oxide films were formed by x-ray illumination of condensed O2 on GaAs(110) at 20 K, and their stability was investigated during subsequent Cr overlayer formation with atoms at 20 and 300 K and with preformed clusters. High resolution synchrotron radiation photoemission results for overlayer formation at 300 K show that Cr atoms reduce the GaAs oxides to form Cr oxides, Cr—As, and Cr—Ga bonding configurations. These reactions first involve the As2O5-like species and then progress to the As2O3-like and Ga2O3-like oxides, reflecting the relative stability of the oxides and their spatial distributions. For overlayer formation at 20 K, the Cr atoms initially reduced the As-oxide surface layers but had little effect on the Ga oxides, and a nonuniform metallic overlayer was formed after 1.5 A(ring) Cr deposition. These low-temperature results demonstrate that kinetic constraints imposed at 20-K limit Cr diffusion and, hence, oxide reduction. In contrast, the deposition of Cr clusters onto oxidized GaAs produced a metallic layer with minimal oxide disruption. This different final state can be understood by noting that the bonding of Cr atoms in metallic Cr clusters establishes an activation barrier for reaction that is not present for atom deposition.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 8442-8448 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Irradiation with 1.7 and 1.97 eV photons of O2 physisorbed on GaAs(110) at 25 K induced strong surface oxidation. The oxidation rate was dependent on photon energy as well as substrate doping type. The 1.97 eV photons induced reaction ∼30 times faster than did the 1.7 eV photons. For fixed photon energy, reaction on p-type substrates was ∼6 times faster than on n-type substrates. These results stand in contrast from those of room temperature experiments where reaction rate was independent of doping type and the dependence on photon energy reflected only the substrate photon absorption coefficient. We show that photoexcited hot electrons are responsible for photo-induced reactions at low temperature. Coupling between hot electrons and physisorbed O2 is via resonant tunneling involving the O2 electron affinity level.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 65 (1994), S. 2267-2275 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We have developed an apparatus that produces an ionized beam of mass selected clusters of controllable energy. It uses a pulse of second-harmonic light from a Nd:YAG laser to vaporize atoms from a target disk. The clustering rate of the atoms in this expanding plume is controlled with the pressure of a He pulse injected from a fast pulsed valve. A Wiley–McLaren type time-of-flight mass spectrometer with a shield lens is used to monitor the cluster distribution and to optimize the control parameters of the system. The shield lens doubles the resolution and the signal-to-noise ratio of the spectrometer. A 4000 amu rf quadrupole with 1.2 amu resolution is used to select the mass of charged clusters. An electrostatic quadrupole is used to separate the charged clusters from the neutral clusters and the He gas. The ionized clusters are then focused onto a suitably prepared substrate by using electrostatic lenses. Time averaged deposition current densities on the order of 100 pA cm−2 have been measured when the vaporization laser is fired at 10 Hz. The deposition energy of the clusters is controlled by applying a bias voltage to the substrate. A mobile vacuum chamber is used to transfer samples to various diagnostic systems.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 2249-2251 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Three different ways of forming oxide overlayers on GaAs(110) have been examined with x-ray photoemission. First, Cr atoms were deposited onto cleaved GaAs(110) at 300 K, producing a disrupted region over which Cr metal grew. Subsequent exposure to O2 resulted in an inhomogeneous overlayer with areas of thick Cr2O3-like oxides in addition to As and Ga oxides. GaAs oxidation was enhanced by Cr-induced surface disruption, but there was no evidence of a catalytic process. Second, metallic clusters of Cr containing hundreds of atoms were condensed onto GaAs(110). In this case, no substrate disruption was observed at low temperature. O2 exposure resulted in Cr2O3 formation with small amounts of Ga2O3 and no detectable As2O3. Third, Cr atoms and O2 molecules were condensed onto a Xe buffer layer on GaAs(110) to produce Cr2O3-like species out of contact with the semiconductor. Buffer layer desorption brought these Cr2O3 aggregates into contact with the substrate. The overlayer produced in this manner was abrupt, and there was no evidence of GaAs oxidation.
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 2809-2811 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This letter discusses band bending induced by the deposition of metal clusters coated with onto GaAs(110). The layer of CO between the metallic clusters and the undisrupted semiconductor simulates a metal-insulator-semiconductor junction. The observed barrier height shows Schottky-limit-like dependence on the work function of the metal.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 56 (1990), S. 2510-2512 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: High-resolution synchrotron radiation photoemission results for O2 physisorbed on GaAs (110) show Ga-O and As-O formation that is a direct result of photon-induced reaction at 20 K. Spatially resolved studies show that the thickness and chemical composition of the semiconductor oxides vary in proporition to total beam irradiation. The extent of reaction can be controlled by varying the amount of oxygen present on the surface, and As5+-like bonding configurations can be formed. These results can only be understood when competition between thermodynamic, kinetic, photon- and electron-mediated processes are considered.
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 56 (1990), S. 671-673 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Metal/single-crystal-silicide interface evolution has been examined for Ti/MoSi2 (001) using high-resolution synchrotron radiation and x-ray photoemission. Reaction between Ti and Si was observed for temperatures 300≤T≤873 K. At 300 K, it was limited to Ti interaction with the single Si layer terminating the cleaved MoSi2 (001) surface. Analysis of the Si 2p core level line shape showed two different interfacial reaction products with bonding characteristics of TiSi and a disordered solution of Si in Ti. Interfacial development was dominated by Si outdiffusion at higher temperatures with Si enrichment of the overlayer but not conversion to a silicide.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 56 (1990), S. 2507-2509 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: High resolution transmission electron microscopy and analytical electron microscopy show direct evidence for defect-free interfaces produced by in situ cluster deposition of Au, Ag, and Ti onto ultrahigh vacuum-cleaved n-GaAs. In contrast to interfaces produced by atom-by-atom deposition, no specific interface reconstruction or orientation relationship and no change of stoichiometry of the GaAs near the interface was observed. Schottky barrier heights correspond to unusual Fermi level pinning positions in the upper half of the GaAs band gap, in clear contrast to values obtained for atom deposition and for diodes prepared by standard technology on GaAs (100). These results give clear evidence that Fermi level pinning for metal/GaAs interfaces formed without defects does not follow the predictions of current metal-induced gap-state models.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 53 (1988), S. 2647-2649 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: With synchrotron radiation photoemission, we contrast the morphology and the Schottky barrier obtained when Au atoms are condensed onto InP(110) at 300 and ∼60 K to what is obtained when preformed, metallic Au clusters are deposited. Atom by atom deposition at either temperature leads to substrate disruption and Fermi level pinning 0.75 eV below the conduction-band minimum (CBM). Deposition of preformed Au clusters induces almost no disruption and a pinning position 0.42 eV below the CBM. Differences reflect the dependence upon the process, and therefore the energetics, of bringing dissimilar atoms in contact.
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