Abstract
We have previously derived seven requirements for the formulation of effective stain etchants and have demonstrated that Fe3+, Ce4+, and VO2+ + HF solutions are highly effective at producing nanocrystalline porous silicon. Here, we show that Cl2, Br2, I2, ClO3–, BrO3–, IO3–, I–, and I3– induce etching of silicon when added to HF. However, using the strict definition that a pore is deeper than it is wide, we observe little evidence for porous layers of significant thickness but facile formation of pits. Iodate solutions are extremely reactive, and by the combined effects of IO3–, I3–, I2, and I–, these etchants roughen and restructure the substrate to form a variety of structures including (circular, rectangular, or triangular) pits, pyramids, or combinations of pits and pyramids without leaving a porous silicon layer of significant thickness.
Highlights
Stain etching is potentially a fast, easy, and inexpensive method of producing porous silicon on arbitrarily shaped surfaces
In electroless etching to form porous silicon (por-Si), the quantum confinementrelated shift of the Si valence band causes fluoride etching of Si to be self-limiting, which is responsible for nanostructure formation [13]
Diffuse reflectance infrared spectroscopy, cross-sectional scanning electron microscopy, photoluminescence, and white light reflectometry have been used to measure the thickness, surface area, and porosity of por-Si thin films and restructured Si substrates made by oxidant + HF etching in various solutions as a function of solution composition and etch time
Summary
Stain etching is potentially a fast, easy, and inexpensive method of producing porous silicon (por-Si) on arbitrarily shaped surfaces. Perhaps more importantly for applications, the ability to etch arbitrarily shaped substrates means that stain etchants can efficiently etch powdered silicon. Loni et al [6] have demonstrated that inexpensive metallurgical-grade silicon powder can be converted to high-surface-area por-Si powder. Oxidation by water dissociation or step flow etching initiated by OH– both favor the formation of uniform surfaces, whereas direct Si atom dissolution via the Gerischer mechanism [11,12] favors por-Si formation. Surface chemistry alone does not lead to por-Si formation. In electroless etching to form por-Si (stain etching), the quantum confinementrelated shift of the Si valence band causes fluoride etching of Si to be self-limiting, which is responsible for nanostructure formation [13]
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