TcSUH
Special Seminar
Optical Properties of Semiconductor Surfaces and Interfaces: First Principle Study
by: Prof. Vladimir I. Gavrilenko
Date: Monday December 12, 2005
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Electron energy structure and linear and non-linear optical properties of group III-nitride compounds, as well as group IV semiconductors (bulk, surfaces, and interfaces) are studied by first principle evaluation of eigen-value and eigen-vector problems using full potential linearized augmented plane wave (FLAPW) and ab initio pseudopotentials (PP) approaches. Equilibrium surface/interface geometries are determined through the total energy minimization method. Optical responses from solid surfaces and effects of the adsorption of inorganic elements and organic molecules are studied. Intermolecular interaction is shown to be responsible for substantial modifications of optical spectra of molecular aggregates. Predictions of second harmonic generation (SHG) from semiconductor surfaces and interfaces are discussed. Effects of rehybridization of atomic bonds and electric field induced second harmonic (EFISH) response are considered. Strong contributions to the SHG efficiency of electron excitations from surface atom orbitals are demonstrated for Si(001) and GaN(0001) surfaces and the Ge/Si(001) interface. Adsorption of Ga on the N-terminated GaN(0001) surface results in a substantial evolution of the surface related SHG features. The predicted spectroscopic results from semiconductor surfaces and interfaces are discussed in comparison with experiment.
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Special Seminar
High-Temperature Superconducting Wire and Power Applications Research and Development in the USA
by: Robert Hawsey
Date: Friday November 11, 2005
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
U.S. efforts to develop and deploy “second generation” (2G) high-temperature superconducting (HTS) wires that use the compound Y1Ba2Cu3Ox (YBCO) or other rare-earth (RE) superconducting materials are described. Wires have been demonstrated in 20-m to >200-m lengths with the RE-BCO deposited using vapor deposition or wet chemical processes in thin layers onto textured templates, which force the grains of the RE-BCO into near perfect alignment. Critical currents for these pre-commercial wires are now within striking distance of those achieved for commercial BSSCO wires. One expected advantage of 2G wire is a projected 5-fold decrease in cost of wire compared with first generation wires. Another advantage of 2G wire is the intrinsic behavior of YBCO in the presence of a strong magnetic field at intermediate temperatures (viz., 50 K), where single-stage cryocoolers may be used for certain applications. Enhancements in flux pinning of at least a factor of two have been demonstrated for MOCVD and MOD deposited YBCO films. U.S. progress towards meeting the challenging goals for the year 2006, including current exceeding 300 A/cm width (77 K, self field) in 100-m lengths and engineering current density exceeding 15,000 A/cm2 (65 K, 3-T) is reported. In addition, initial efforts toward engineering the conductor for the mechanical and electrical properties needed for strong magnetic field applications are described. These projects include striation or other means to subdivide the superconducting film into filaments, as well as lamination or other innovative processes. Finally, an overview of U.S. demonstration projects for superconducting cables, synchronous condensers, motors, generators, and other power applications will be presented.
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Special Seminar
X-ray Diffraction from Functional Perovskites
by: Dr. Wolfgang Donner
Date: Friday October 14, 2005
Time: 1:00 pm – 2:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Thin epitaxial oxide films that adopt the perovskite structure display a variety of phenomena such as ferroelectricity, ferromagnetism, metal-insulator transitions or oxygen permeability. Their physical properties are strongly affected by defects such as strain gradients, point and line defects or even the presence of a surface. X-ray diffraction experiments, notably carried out using highly brilliant Synchrotron radiation, are able to shed light on those structure-property relations. The talk will present an overview of diffraction experiments on perovskite films, performed by various groups using Synchrotron radiation.
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Special Seminar
Development of Coated Conductors
by: Dr. Dean Peterson
Date: Friday October 07, 2005
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Commercial manufacture of high temperature superconducting wires with appropriate properties and costs for full implementation in electric power applications ultimately depends on overcoming several technical barriers. Many organizations across the world have demonstrated the great potential for production of coated conductor tapes in long lengths with appropriate superconducting properties for practical power usage. This review will summarize technical progress and remaining barriers with a focus on the Ion Beam Assisted Deposition (IBAD) approach to fabricating coated conductors. The most promising techniques for depositing practical superconducting films at high rates will also be presented. Current plans for using coated conductors in power prototype demonstrations will be summarized. Opportunities for collaborative research and development areas are also to be highlighted.
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Special Seminar
The Shadow of a Carbon Nanotube
Date: Thursday August 18, 2005
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The features of ion beam proximity lithography include sub-nanometer limits for diffraction, penumbra, and resist scattering. Further, by using atoms, which have essentially the same interaction with resist and mask materials as ions, the technology becomes immune to fixed and mobile charge, either in the mask or on the wafer. Thus, atom beam lithography (ABL) seems ideal for prototyping and later manufacturing nanoscale integrated circuits. The fundamental challenge is to fabricate a stencil mask with atomically smooth sidewalls in a membrane that is thick enough (e.g. 0.5 μm) to stop the atoms. We have shown that a scattering mask can bypass this issue. A thin evaporated film was used to shrink the openings of a conventional stencil mask and, while not thick enough to actually stop the particles, enough were scattered into the walls of the mask to generate very good exposure contrast. This approach gives the possibility of using very thin, potentially self-assembled, scattering layer structures to form complex, high density masks. An important open question is how thick these features need to be for faithful atom beam replication. In this talk, we report the ability of a carbon nanotube, 18 nm in diameter, to cast a well-defined shadow in a broad beam of energetic (30 keV) helium atoms. When imaged in resist and engraved into silicon dioxide, the projected replica retains the natural smoothness of the nanotube and shows, for the first time that ultra-thin, self-assembled structures can be used as masks in nanoscale printing.
Experiments were carried out using a 30 keV atom beam proximity lithography system. Briefly, a beam of helium atoms, generated by charge transfer scattering in the extraction region of a duoplasmatron ion source, drifts through a 10 M long tube, and impinges on a stencil mask where the transmitted beamlets transfer the mask pattern to the substrate. The mask was prepared by sprinkling a dry nanotube powder onto a 3 μm thick silicon stencil mask with 1 μm wide openings. The mask was clamped, with 5 μm thick cleaved mica spacers, to a silicon substrate coated with 44 nm thick thermal SiO2 and 50 nm thick PMMA resist. After exposure, the PMMA was developed and the resist pattern transferred into the oxide to a depth of 37 nm by CHF3-RIE. Atoms incident upon the thick regions of the mask are absorbed while scattering generates image contrast for the nanotube. Experimental data will be presented, showing a 20 nm wide nanotube suspended over an opening in the stencil mask, with its image after resist removal, engraved into oxide. Also shown is linewidth versus exposure dose, normalized to the critical dose, of a different tube, 18 nm in diameter. Since printed and nominal linewidth are generally equal at twice the critical dose, the difference between these values, about 4.6 nm, is a measure of pattern bias, perhaps an artifact of metrology, resist development, and/or etching. After subtracting the bias from the measured data, a threshold development model with a blur of 5 nm (FWHM) describes the experiment reasonably well. Thus, the ultimate resolution may be near 4 nm. This result shows that the edges of a nanotube, just a few atomic layers thick, generate enough scattering to be printed. I will report experiments to better understand the pattern bias issue and to determine the resolution limit.
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