From the vast literature available now on porous silicon, few trends are noticed as follows.
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The morphology, optical and structural properties of porous silicon depend upon (1) Substrate type (n or p), (2) Doping level (low or high), (3) Concentration of HF in the electrolyte, (4) Current density used to etch silicon, (5) Effect of light, and (6) Duration of etching (Box 11.4). Box 11.4: Porous Material Materials become porous when some solid portion is removed from them leaving voids in them.
The volume of the original material does not alter. Therefore density of a porous material is lower than the original material. Materials can be made even 98 % porous. There is a large variety of porous materials, natural (e.g. wood) as well as synthetic (e.g. thermocole). The pores can be of regular, irregular and different sizes and shapes. Pores can be closed, open, penetrating or ink bottle type.
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Internal area increases with pores (Fig. 11.15). Fig. 11.15 Various types of pores (continued) 292 11 Some Special Nanomaterials Box 11.4 (continued) In general, properties of porous materials are quite different than those of corresponding bulk materials. Porous materials are often divided into microscopic (less than 2 nm), mesoscopic (2–50 nm) and macroscopic (larger than 50 nm) depending upon the diameters or size of the pores.
Pore structure is normally formed due to holes and silicon atoms interacting together with fluorine. As was discussed in the mechanism of pore formation, it is necessary to consider the substrate type viz.
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p or n type as they already have excess holes or electrons due to doping. For example it has been found in case of moderately doped n or p type silicon that columnar or sponge-like structure is respectively formed.
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