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). 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).
Black silicon has become a major asset to the solar photovoltaic industry as it enables greater light to electricity conversion efficiency of standard crystalline silicon solar cells, which significantly reduces their costs.
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. For low reflectivity, the nanoscale features producing the index graded layer must be smaller than the wavelength of the incident light to avoid scattering.
Applications
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. Surface enhanced Raman spectroscopy Ammonia Gas Sensors
Production
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 , 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 and passivation steps result in tall silicon pillars. The process can be set so that a million needles are formed on an area of one square millimeter.
Mazur's method
In 1999, a Harvard University group led by Eric Mazur 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 absorption that extends to the infrared range, below the band gap of silicon, including wavelengths for which ordinary silicon is transparent. sulfur atoms are forced to the silicon surface, creating a structure with a lower band gap and therefore the ability to absorb longer wavelengths.