Porous silicon

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Porous silicon (pSi) is a form of the chemical element silicon which has an extremely large surface to volume ratio.

It is usually manufactured by etching away most of a layer of silicon using wet chemical etches. Many etches are based on hydrofluoric acid. Electrochemical etching is also used. What is left is a fine connected network of sub micrometre silicon threads. These thin structures often have properties that are different to bulk crystalline silicon. Fine control of these etching processes enables the properties of the network to be repeatably controlled. For example, controlling the mean diameter of the threads or the pores.

One property changed is luminescence. Ordinary amorphous, solid Silicon does not exhibit electroluminescence. Porous silicon exhibits electroluminesce at a range of wavelengths from the near infrared to the visible (blue). The wavelength depends on the thickness of the threads and on the chemical state of their surfaces. Porous silicon also emits light under illumination (photoluminescence), first reported by Leigh Canham 1990^[1].

Like bulk silicon, porous silicon is a semiconductor.

Porous silicon exhibits a high degree of biocompatability. The large surface area enables bio-organic molecules to adhere well. It degrades silicic acid, which causes no harm to the body. This has opened potential applications in medicine such as a framework of the growth of bone.

Other materials can be made porous in a similar way, such as the semiconductor gallium arsenide.