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Black silicon
Black silicon is a semiconductor material, a surface modification of silicon with very low reflectivity and correspondingly high absorption of visible (and infrared) light. The modification was discovered in the 1980s as an unwanted side effect of reactive ion etching (RIE).〔(Black Silicon ) as a functional layer of the micro-system technology〕 Other methods for forming a similar structure include electrochemical etching, stain etching, metal-assisted chemical etching, and laser treatment (which is developed in Eric Mazur's laboratory at Harvard University ), and FFC Cambridge process (an electrochemical reduction process).
== Properties ==
Black silicon is a needle-shaped surface structure where needles are made of single-crystal silicon and have a height above 10 µm and diameter less than 1 µm.〔 Its main feature is an increased absorption of incident light—the high reflectivity of the silicon, which is usually 20–30% for quasi-normal incidence, is reduced to about 5%. This is due to the formation of a so-called effective medium by the needles. Within this medium, there is no sharp interface, but a continuous change of the refractive index that reduces Fresnel reflection. When the depth of the graded layer is roughly equal to the wavelength of light in silicon (about one-quarter the wavelength in vacuum) the reflection is reduced to 5%; deeper grades produce even blacker silicon.〔H.M. Branz, V.E. Yost, S. Ward, B. To, K. Jones, and P. Stradins, "Nanostructured black silicon and the optical reflectance of graded-density surfaces," Appl. Phys. Lett., vol 94, pp. 231121-3, 2009〕 For low reflectivity, the nanoscale features producing the index graded layer must be smaller than the wavelength of the incident light to avoid scattering.〔
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