Sodium chloride (NaCl) films were grown on an Si(100)-(2 × 1) surface at near room temperature by molecular beam epitaxy (MBE). The atomic structure and growth mode of the prototypical ionic materials on the covalent bonded semiconductor surface is examined by synchrotron core-level x-ray photoemission spectrum (XPS), scanning tunneling microscopy (STM), and first-principles calculations. The Si 2p, Na 2p, and Cl 2p core-level spectra together indicate that adsorbed NaCl molecules at submonolayer coverage [i.e., below 0.4 monolayer (ML)] partially dissociate and form Si-Cl species, and that a significant portion of the dangling-bond characteristics of the clean surface remains after NaCl deposition of 1.8 MLs. The deposition of 0.65-ML NaCl forms a partially ordered adlayer, which includes NaCl networks, Si-Cl species, adsorbed Na species, and isolated dangling bonds. The STM results revealed that the first adlayer consists of bright protrusions which form small c(2 × 4) and (2 × 2) patches. Above 0.65 ML, the two-dimensional NaCl double-layer growth proceeds on top of the first adlayer.
In order to obtain 100% bi-epitaxial 45° grain boundary junctions of YBa2Cu3Ox (YBCO), we have systematically examined the in-plane epitaxy of CeO2 films grown on MgO substrates. The inevitable presence of CeO2[110]∥MgO[100] causes mixtures of in-plane rotation of 0° and 45° between YBCO/CeO2/MgO and YBCO/MgO. We have further developed a new structure, namely YBCO/CeO2/Yttria-stabilized ZrO2/MgO and YBCO/MgO boundary, so that 100% in-plane rotation of 45° can be routinely obtained. The model of the in-plane epitaxial relationship between the multilayers using near coincident site lattices was proposed. The critical current density of the junctions made on the boundary is 3×103 A/cm2 at 77 K, while the order of the Jc of YBCO films on both sides of the grain boundary is 106 A/cm2. The current-voltage characteristics of the junctions show resistively shunted junction behavior. The better epitaxy of our new structure can lead to a better control of grain boundary critical current density.
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