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). Another method for forming a similar structure was developed in Eric Mazur's laboratory at Harvard University (1998).
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.  For low reflectivity, the nanoscale features producing the index graded layer must be smaller than the wavelength of the incident light to avoid scattering. 
The unusual optical characteristics, combined with the semiconducting properties of silicon make this material interesting for sensor applications. Potential applications include:
- Image sensors with increased sensitivity
- Thermal imaging cameras
- Photodetector with high-efficiency through increased absorption.
- Mechanical contacts and interfaces 
- Terahertz applications.
- Solar cells
- Antibacterial surfaces that work by physically rupturing bacteria's cellular membranes.
Production by reactive-ion etching
In semiconductor technology, reactive-ion etching (RIE) is a standard procedure for producing trenches and holes with a depth of up to several hundred micrometres and very high aspect ratios. In Bosch process RIE, this is achieved by repeatedly switching between an etching and passivation. With cryogenic RIE, the low temperature and oxygen gas achieve this sidewall passivation by forming SiO2, easily removed from the bottom by directional ions. Both RIE methods can produce black silicon, but the morphology of the resulting structure differs substantially. The switching between etching and passivation of the Bosch process creates undulated sidewalls, which are visible also on the black silicon formed this way.
During etching, however, small debris remain on the substrate; they mask the ion beam and produce structures that are not removed and in the following etching steps and result in tall silicon pillars. The process can be set so that a million needles are formed on an area of one square millimeter.
Production by Mazur's method
In 1999, a group led by Eric Mazur and James Carey at the Harvard University developed a process in which black silicon was produced by irradiating silicon with femtosecond laser pulses. After irradiation in the presence of a gas containing sulfur hexafluoride and other dopants, the surface of silicon develops a self-organized microscopic structure of micrometer-sized cones. The resulting material has many remarkable properties, such as an enhanced absorption that extends to the infrared below the band gap of silicon, including the wavelengths for which unmodified silicon is transparent. This property is caused by sulfur atoms being forced to the silicon surface, creating a structure with a lower band gap and therefore the ability to absorb longer wavelengths.
Similar surface modification can be achieved in vacuum using the same type of laser and laser processing conditions. In this case, the individual silicon micro-cones lack sharp tips (see image). The reflectivity of such a micro-structured surface is very low, 3-14% in the spectral range 350–1150 nm. Such reduction in reflectivity is considered to be contributed by the geometry of these micro-cones, which increases the light internal reflections between them. Hence, the possibility of light absorption by the silicon is increased. The gain in absorption achieved by fs laser texturization is found to be superior to that achieved by using an alkaline chemical etch method, which is a standard industrial approach for surface texturing of mono-crystalline silicon wafers in solar cell manufacturing. It is also found that such surface modification is independent of local crystalline orientation. A uniform texturing effect can be achieved across the whole surface of a multi-crystalline silicon wafer. The very steep angles lower the reflection to near zero and also increase the probability of recombination, which is why it thus far has not been used in solar cell manufacturing.
When the material is biased by a small electric voltage, absorbed photons are able to excite dozens of electrons. The sensitivity of black silicon detectors is 100–500 times higher than that of untreated silicon (conventional silicon), in both the visible and infrared spectra.
Uses and commercialization
The material has not yet found commercial applications but potentially could find in a number of photodetectors for various imaging and night vision applications. Black silicon is currently being commercialized by SiOnyx, a Massachusetts-based venture-funded startup company which acquired licensing for the process from Harvard in 2006.
Black silicon also has potential application for high-efficiency solar cells, which is being explored by Solasys, an EU Seventh Framework Programme (FP7) funded demonstration project aiming at lowering manufacturing costs while increasing cell efficiency at the same time.
A group at the National Renewable Energy Laboratory has reported black silicon solar cells with 18.2% confirmed efficiency. This black silicon anti-reflective surface was formed by a metal-assisted etch process using nano particles of silver.
A team led by Elena Ivanova at Swinburne University of Technology in Melbourne discovered in 2012 that cicada wings were potent killers of Pseudomonas aeruginosa, an opportunist germ that also infects humans and is becoming resistant to antibiotics. Looking closely, they found that the answer lay not in any biochemical on the wing, but in regularly-spaced "nanopillars" on which bacteria were sliced to shreds as they settled on the surface. The cicada wings and black silicon were put through their paces in a lab, and both were ruthlessly bactericidal. Smooth to the human touch, the surfaces destroyed Gram-negative and Gram-positive bacteria, as well as bacterial spores. The three targeted bacterial species comprised P. aeruginosa, the notorious Staphylococcus aureus and the ultra-tough spore of Bacillus subtilis, a wide-ranging soil germ that is a cousin of anthrax. The killing rate was 450,000 bacterial cells per square centimetre per minute over the first three hours of exposure. This is 810 times the minimum dose needed to infect a person with S. aureus, and 77,400 times that of P. aeruginosa.
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