TcSUH
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Special Seminar
The approach for nanolayered semiconductor-on-insulator
by: Dr. Lin Shao
Date: Thursday April 06, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The creation of ever higher density chips with faster speeds and lower power consumption is one of the major goals being persistently pursed by the microelectronics industry. Device integration on silicon-on-insulator (SOI) wafers offer a sustainable, long-term pathway for scaling various devices such as sensors, photodiodes, and most importantly, complementary metal oxide semiconductor (CMOS). Fabrication of SOI wafers is typically performed using an ion-implantation-based technique in which implanted hydrogen atoms react with broken silicon bonds and create hydrogen-filled microcracks which can propagate in a direction parallel to the wafer surface. The top Si layer is then split apart and bonded to an oxide layer, to form SO1 wafers. However, there are technical challenges for the fabrication of nanolayered SO1 wafers, with the top Si layer less than 100 nm thick. We have recently developed novel approaches to transfer an ultrathin Si layer by using a buried strained layer or highly doped layer to provide H trapping centers during hydrogenation, with following advantages: 1) The crack location can be controlled by adjusting the position of the H-trapping layer; and 2) the crystalline quality of the transferred layer is greatly improved due to reduced irradiation damage. We have realized the lift-off of a Si layer with a thickness of 15 nm, which is not achievable by previous techniques. This talk will give an overview of the status and perspective of ion cutting techniques.
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Special Seminar
Colloids in External Fields: Crystallization, Melting, and Dynamics
by: Dr. Charles Reichhardt
Date: Monday April 03, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Colloidal assemblies are ideal model systems in which to study basic equilibrium and non-equilibrium phenomena relevant to a wide range of condensed matter systems. Additionally, there are a variety of technological applications for self-organizing colloid structures, including photonic band gap materials and patterned nanostructures. Here we study the statics and dynamics of colloids interacting with external fields. When the fields are used to create a periodic substrate, we find a variety of novel crystalline states that we term "colloidal molecular crystals." These have interesting multi-step melting transitions and can be used to realize a variety of canonical statistical mechanics models physically. When the substrate is dynamic, we show that novel dynamical phases arise and lead to ratchet effects, which can be used to create new types of logic gates and new fractionation techniques. Many of these results have recently been realized experimentally for colloids interacting with periodic optical arrays.
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Special Seminar
Pseudogap, Superconducting Energy Scale, and Fermi Arcs in Cuprate Superconductors
by: Hai-Hu Wen
Date: Thursday March 02, 2006
Time: 4:00 pm – 5:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Through low temperature specific heat and point contact tunneling measurement, we investigated the pairing symmetry and low energy quasiparticle excitation behavior in cuprate superconductors. The following conclusions are drawn:
1. For hole doped samples, the Volovik's relation predicted for the d-wave pairing symmetry has been well demonstrated by low temperature specific heat in wide doping regime in La2-xSrxCuO4. This is supported by the tunneling spectrum with a zero-bias-conductance peak along nodal direction. The nodal slope of the superconducting gap is thus derived and is found to follow the same doping dependence of the pseudogap Δp. Both indicate a close relationship between the pseudogap and superconductivity.
2. Still in the hole-doped side, it is also found that the critical temperature Tc is determined by the multiplication of the nodal gap slope and the residual Fermi arc length.
3. For electron doped samples, both specific heat and tunneling measurements reveal an unavoidable s-wave component which may be explained by the two-band and thus two-gap pictures. The evidence for pseudogap is presented from the tunneling data under a high magnetic field for electron-doped samples. It is found that the pseudogap does not change with temperature instead of being filled up by thermal excitation.
These observations put strong constraints on the theoretical models of high temperature superconductors.
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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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