The stability and reproducibility of current–voltage curves of Schottky barrier structures on particular porous silicon surfaces used for obtaining electroluminescence is greatly improved by first coating the surface with a conducting polymer, poly-4-dicyanomethylene-4H-cyclopenta [2,1-b:3,4-b′] dithiophene. With such coated structures it is possible to fit the usual diode formula at room temperatures with a quality factor of 3.0, obviating the need for more complex theories. The stability of electroluminescence, which shows a redshift from 500 to 580 nm after coating, is also greatly improved. The coating appears to chemically react with the hydride surface and also mechanically strengthens the structure.
We have found significant electroluminescence (EL), clearly visible in room light, to be produced from lightly oxidized Si wafers, which have been mechanically scratched, or indented with a diamond tip. The EL occurs in the visible and infrared ranges. Undamaged oxidized samples, where the oxide is chemically thinned to allow current from a top contact to pass, also show similar EL. However, damaged Si wafers that have only normal thin or no oxide, show negligible EL. A search for quantum-confined particles in indentation pits by scanning tunneling microscopy does not show any present. The results point strongly to the EL mechanism being related to the damage creating areas of optimum oxide thickness. The results are compared with those from spark-processed and laser-grooved silicon.
Fibers of cotton and wool, and samples of paper, have been ruptured in tension in vacuum and in air, and give detectable luminescence in the visible range. All have a common emission peak at around 2.0 eV, which is ascribed to the deexcitation of states excited by the rupture of organic chain molecule bonds. Rubber bands give stronger emission in air, but no emission in vacuum, suggesting the material breaks only at weak interchain bonds. Mohair, cat, and horse hair also give emission in air. The phenomena reveal effects that would occur widely in nature.
The work demonstrates that the surface atom layers of variously-prepared samples of porous silicon can, after high-temperature heat treatment in high vacuum, yield low energy electron diffraction patterns and also atomic resolution in scanning tunneling microscopy. The (100) surfaces show some patches of 2 Â 1 reconstruction but no clear reconstructions are visible on (111) surfaces. We also show that although evaporated metal contacts give poor reproducibility of current±voltage characteristics, the latter can be made stable and reproducible if the surfaces are first coated with a conducting polymer.Introduction The properties and structure of porous silicon (PSi) have been well studied and are thoroughly discussed in recent reviews, e.g. [1,2]. The structure of the porous layer, formed by electrochemical anodising, has been shown by transmission electron microscopy (TEM) to consist of columns (ªwiresº) of crystalline Si, plus amorphous Si and crystallites embedded in the latter. The relative content of these structures depends on the Si conductivity type and the preparation conditions.Although most TEM studies, including the associated electron diffraction measurements, reveal the crystallinity of much of the skeleton structure of PSi, they have not been able to show that the surface of the PSi is also crystalline. Microscopic studies reveal pores and solid regions with a range of dimensions down to a few nm. Structures generally have some damage due to the drying process, unless very special drying techniques are used. This can cause tilted and broken columns.There remains uncertainty about the relative importance of columns connected to the bulk, and small independent nanostructures. In the latter case, especially if the crystallites are embedded in any amorphous matrices, the crystalline continuity from the bulk would be impaired, and diffraction data suggest that such misorientations occur. There have also been suggestions based on TEM studies that the top layers are amorphous, even if no ion milling is used to thin the sections.
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