The unified defect model has been successful in explaining a wide variety of phenomena as oxygen or a metal is added to the III–V surface. These phenomena cover a range from a small fraction of a monolayer of adatoms to practical III–V structures with very thick overlayers. The tenets of the unified defect model are outlined, and the experimental results leading to its formulation are briefly reviewed. InP levels 0.4 and 0.1 eV and GaAs levels 0.7 and 0.9 eV below the conduction-band minimum (CBM) are associated with either missing column III or V elements. In InP, it has been found possible by a number of workers to ’’switch’’ between the two defect levels by variations in surface processing, temperature, and/or selection of the deposited atom. The need to apply the proper concepts for surface and interface chemistry and metallurgy is recognized, and the danger of using solely bulk concepts is emphasized. The reason for this is examined for certain cases on an atomic level. The need for new fundamental attacks on interface interaction is shown. The importance of semiconductor–oxide chemical stability is also recognized and, drawing on a large body of work from several laboratories, it is suggested that there will be more difficulties with ’’native’’ oxides on GaAs than on InP. It is concluded that ’’scientific engineering’’ of interfaces to give optimum performance should be a goal and test of the fundamental work described here. Specific possibilities are discussed for Schottky barriers on III–V’s.
GaAs metal insulator semiconductor capacitors and high transconductance metal insulator semiconductor field effect transistorsSimple technique for separating the effects of interface traps and trappedoxide charge in metaloxide semiconductor transistorsWe have used high resolution photoemisslon spectroscopy to probe the electronic structure of a wide variety of gold/high temperature superconductor interfaces, the majority of which were formed by low-temperature (20 K) gold evaporations on cleaved high quality single crystals. For c-axis interfaces formed on the 123 family of superconductors, we find that the gold deposition essentially destroys the metaUicity of the superconducting substrate in the near surface region ( ~ 5 A), while the near surface region of Bi2 Sr 2 CaCu 2 0 8 remains metallic. We have also used photoemission spectroscopy to search for a proximity-effect induced superconducting gap in gold overlayers on c-axis single crystals and a-axis thin films, though no such effect was found.
Photoemission studies have been performed on all classes of high temperature superconductors except the Tl-related compounds. Particular attention was paid to the surface cleanliness. Comparison with band calculation shows that the one-electron picture cannot adequately explain the electronic structure of this type of materials. Most important, Cu satellites were observed both in the valence band and the Cu 2p core level for all the samples studied, signaling the importance of the d-d correlation effects. The Cu 3d character of these satellites in the valence band was verified using resonance photoemission. The results have been interpreted in terms of a cluster model derived from the two band Anderson Hamiltonian, which in the past has been used successfully to describe the electronic structure of highly correlated systems. No clear satellite structure was observed in the O 1 s core spectrum, which is consistent with the bandlike nature of the oxygen states. Examples of changes in the electronic structure, which could be related to Tc, (such as substituting Y by Pr in the Y-Ba-Cu-O system and altering the number of Cu-O layers in the Bi-Ca-Sr-Cu-O system), are also discussed
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