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).[1][2] Another method for forming a similar structure was developed in Eric Mazur's laboratory at Harvard University (1998).
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Black silicon is a needle-shaped surface structure where needles are made of single-crystal silicon and have a height above 10 microns and diameter <1 micron.[2] 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[3] by the needles. Within this medium, there is no sharp interface, but a continuous change of the refractive index that reduces Fresnel reflection.
The unusual optical characteristics, combined with the semiconducting properties of silicon make this material interesting for sensor applications. The potential applications include:[4]
In semiconductor technology, RIE is a standard procedure for producing trenches and holes with a depth of up to several hundred microns 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.[12] The process can be set so that a million needles are formed on an area of one square millimeter.[10]
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.[13] 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 are lack of sharp tip but more in a penguin-like form. The reflectivity of such a micro-structured surface is very low, 3~14% in the spectral range 350–1150 nm.[14] Such reduction in reflectivity is considered to be contributed by the geometry of these micro-cones, which increases the light internal reflections between themselves and 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 alkaline chemical etch method,[15] which is a standard industrial approach for surface texturization of mono-crystalline silicon wafers in solar cell manufacturing. It is also found that such surface modification is independent to local crystalline orientation. Uniform texturisation 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, the latter is the reason that 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.[16][17]
The material has found commercial applications 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.