Differences between junctions of metals on ionic or covalent semiconductors persist for junctions, prepared by wet solution methods with a molecular layer at the junctions’ interface. A series of molecules that controls the junction of Au with does so even stronger with ZnO (300 instead of barrier height change). With ZnO the interface behavior index is found to be 0.55, five times that with GaAs. This agrees remarkably well with results for junctions of these materials with different metals, prepared in ultrahigh vacuum. Thus, the free semiconductor surface, e.g., surface state density, rather than direct metal–semiconductor interactions, appears to dominate junction behavior.
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This is discussed in detail in A. Vilan and D. Cahen (unpublished), based on results from several experiments, including controls. In short, for junctions made by LOFO with slow drying, the relevant dipole moment is the one corresponding to that of molecules, chemisorbed on the free surface. This is so because then over most of the junction area the metal and molecules are separated by an air gap. If, however, the molecules’ functional groups and the metal are directly contacting, those groups will influence the metal rather than the remainder of the molecule, thus reversing the dipole moments.
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A direct link to this document may be found in the online article’s HTML reference section. The document may also be reached via the EPAPS homepage (http://www.aip.org/pubservs/epaps.html) or from ftp.aip.org in the directory /epaps/. See the EPAPS homepage for more information.
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