Black silicon is a

semiconductor material A semiconductor is a material with electrical conductivity between that of a Electrical conductor, conductor and an Insulator (electricity), insulator. Its conductivity can be modified by adding impurities ("doping (semiconductor), doping") to ...

, a surface modification of

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

with very low

reflectivity The reflectance of the surface of a material is its effectiveness in Reflection (physics), reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the respon ...

and correspondingly high absorption of visible (and

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

) 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. Black silicon has become a major asset to the

solar photovoltaic A photovoltaic system, also called a PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to abs ...

industry as it enables greater

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

electricity Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

conversion

efficiency Efficiency is the often measurable ability to avoid making mistakes or wasting materials, energy, efforts, money, and time while performing a task. In a more general sense, it is the ability to do things well, successfully, and without waste. ...

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 In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

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. Slantedblacksilicon

Applications

The unusual optical characteristics, combined with the semiconducting properties of silicon make this material interesting for sensor applications. Potential applications include: *

Image sensor An image sensor or imager is a sensor that detects and conveys information used to form an image. It does so by converting the variable attenuation of light waves (as they refraction, pass through or reflection (physics), reflect off objects) into s ...

s with increased sensitivity * Thermal imaging cameras *

Photodetector Photodetectors, also called photosensors, are devices that detect light or other forms of electromagnetic radiation and convert it into an electrical signal. They are essential in a wide range of applications, from digital imaging and optical ...

with high-efficiency through increased absorption. * Mechanical contacts and interfaces * Terahertz applications.Martin Schaefer:
Velcro in miniature – "silicon grass holds together micro-components"
' In: wissenschaft.de. 21 June 2006. *

Solar cells A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.

* Antibacterial surfaces that work by physically rupturing bacteria's cellular membranes. * Surface enhanced Raman spectroscopy * Semiconductor chemical or gas sensors * Gettering of metallic impurities

Production

Reactive-ion etching

In semiconductor technology,

reactive-ion etching Reactive-ion etching (RIE) is an etching (microfabrication), etching technology used in microfabrication. RIE is a type of dry etching which has different characteristics than Isotropic etching, wet etching. RIE uses chemical reaction, chemically ...

(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 Harvard University is a Private university, private Ivy League research university in Cambridge, Massachusetts, United States. Founded in 1636 and named for its first benefactor, the History of the Puritans in North America, Puritan clergyma ...

group led by Eric Mazur developed a process in which black silicon was produced by irradiating silicon with

femtosecond A femtosecond is a unit of time in the International System of Units (SI) equal to 10 or of a second; that is, one quadrillionth, or one millionth of one billionth, of a second. A femtosecond is to a second, as a second is to approximately 31.6 ...

laser pulses. After irradiation in the presence of a gas containing

sulfur hexafluoride Sulfur hexafluoride or sulphur hexafluoride ( British spelling) is an inorganic compound with the formula SF6. It is a colorless, odorless, non-flammable, and non-toxic gas. has an octahedral geometry, consisting of six fluorine atoms attache ...

and other

dopant A dopant (also called a doping agent) is a small amount of a substance added to a material to alter its physical properties, such as electrical or optics, optical properties. The amount of dopant is typically very low compared to the material b ...

s, 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

range, below the

band gap In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to t ...

of silicon, including wavelengths for which ordinary silicon is transparent. sulfur atoms are forced to the silicon surface, creating a structure with a lower

and therefore the ability to absorb longer

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

Similar surface modification can be achieved in vacuum using the same type of laser and laser processing conditions. In this case, the individual silicon 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 contributed by the cone geometry, which increases the light internal reflections between them. Hence, the possibility of light absorption is increased. The gain in absorption achieved by fs laser texturization was 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 A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.

manufacturing. Such surface modification is independent of local crystalline orientation. A uniform texturing effect can be achieved across the surface of a multi-crystalline silicon wafer. The very steep angles lower the reflection to near zero and also increase the probability of recombination, keeping it from use in solar cells.

Nanopores

When a mix of

copper nitrate Copper(II) nitrate describes any member of the family of inorganic compounds with the formula Cu( NO3)2(H2O)x. The hydrates are hygroscopic blue solids. Anhydrous copper nitrate forms blue-green crystals and sublimes in a vacuum at 150-200 °C ...

phosphorous acid Phosphorous acid (or phosphonic acid) is the Compound (chemistry), compound described by the chemical formula, formula . It is diprotic (readily ionizes two protons), not triprotic as might be suggested by its formula. Phosphorous acid is an in ...

hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...

and water are applied to a silicon wafer, the phosphorous acid reduction reduces the copper ions to copper nanoparticles. The

nanoparticle A nanoparticle or ultrafine particle is a particle of matter 1 to 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At ...

s attract electrons from the wafer's surface, oxidizing it and allowing the hydrogen fluoride to burn inverted pyramid-shaped nanopores into the silicon. The process produced pores as small as 590 nm that let through more than 99% of light.

Chemical etching

Black silicon can also be produced by chemical etching using a process called Metal-Assisted Chemical Etching (MACE).

Function

When the material is biased by a small electric voltage, absorbed

photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that ...

are able to excite dozens of

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s. 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. A group at the National Renewable Energy Laboratory reported black silicon solar cells with 18.2% efficiency. This black silicon anti-reflective surface was formed by a metal-assisted etch process using nano particles of silver. In May 2015, researchers from

Finland Finland, officially the Republic of Finland, is a Nordic country in Northern Europe. It borders Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bothnia to the west and the Gulf of Finland to the south, ...

Aalto University Aalto University (; ) is a public university, public research university located in Espoo, Finland. It was established in 2010 as a merger of three major Finnish universities: the Helsinki University of Technology, the Helsinki School of Economic ...

, working with researchers from Universitat Politècnica de Catalunya announced they had created black silicon solar cells with 22.1% efficiency by applying a thin passivating film on the nanostructures by

Atomic Layer Deposition Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called wiktionary:precu ...

, and by integrating all metal contacts on the back side of the cell. A team led by Elena Ivanova at

Swinburne University of Technology The Swinburne University of Technology (or simply Swinburne) is a public university, public research university in Melbourne, Australia. It is the modern descendant of the Eastern Suburbs Technical College established in 1908, renamed Swinburne ...

Melbourne Melbourne ( , ; Boonwurrung language, Boonwurrung/ or ) is the List of Australian capital cities, capital and List of cities in Australia by population, most populous city of the States and territories of Australia, Australian state of Victori ...

discovered in 2012 that

cicada The cicadas () are a superfamily, the Cicadoidea, of insects in the order Hemiptera (true bugs). They are in the suborder Auchenorrhyncha, along with smaller jumping bugs such as leafhoppers and froghoppers. The superfamily is divided into two ...

wings were potent killers of ''

Pseudomonas aeruginosa ''Pseudomonas aeruginosa'' is a common Bacterial capsule, encapsulated, Gram-negative bacteria, Gram-negative, Aerobic organism, aerobic–facultative anaerobe, facultatively anaerobic, Bacillus (shape), rod-shaped bacteria, bacterium that can c ...

'', an opportunist germ that also infects humans and is becoming resistant to antibiotics. The effect came from regularly-spaced "nanopillars" on which bacteria were sliced to shreds as they settled on the surface. Both cicada wings and black silicon were put through their paces in a lab, and both were bactericidal. Smooth to human touch, the surfaces destroyed

Gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

and

Gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain ...

, as well as

bacterial spore An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria in the phylum Bacillota. The name "endospore" is suggestive of a spore or seed-like form (''endo'' means 'within'), but it is not a true spore (i.e., not ...

s. The three targeted bacterial species were ''P. aeruginosa'', ''

Staphylococcus aureus ''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often posi ...

'' and ''

Bacillus subtilis ''Bacillus subtilis'' (), known also as the hay bacillus or grass bacillus, is a gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants, humans and marine sponges. As a member of the genus ''Bacill ...

'', a wide-ranging soil germ that is a cousin of

anthrax Anthrax is an infection caused by the bacterium '' Bacillus anthracis'' or ''Bacillus cereus'' biovar ''anthracis''. Infection typically occurs by contact with the skin, inhalation, or intestinal absorption. Symptom onset occurs between one ...

. The killing rate was 450,000 bacteria per square centimetre per minute over the first three hours of exposure or 810 times the minimum dose needed to infect a person with ''S. aureus'', and 77,400 times that of ''P. aeruginosa''. However, it was later proven that the quantification protocol of Ivanova's team was not suitable for these kind of antibacterial surfaces. A group led by Gagik Ayvazyan at National Polytechnic University of Armenia in

Yerevan Yerevan ( , , ; ; sometimes spelled Erevan) is the capital and largest city of Armenia, as well as one of the world's List of oldest continuously inhabited cities, oldest continuously inhabited cities. Situated along the Hrazdan River, Yerev ...

demonstrated in 2023 that needle-like nanotextures provide a feasible light-management solution for perovskite/silicon tandem solar cells. The black silicon interlayer enhances light absorption within the bottom silicon solar sub-cell.

References

External links

SiOnyx brings "Black Silicon" into the light

* ttp://sionyx.com/ SiOnyx homepage* ttps://web.archive.org/web/20110720172245/http://www.solasys.eu/file_144922.dat/ Lasers for Photovoltaics – Knowledge Base* {{cite journal , author1=E. Mazur , author2=J. Carey , author3=C. Crouch , author4=R. Younkins , title=Fabrication of micrometer-sized conical field emitters using femtosecond laser-assisted etching of silicon , journal=MRS Spring Meeting , year=2001 , url=http://mazur.harvard.edu/publications/Pub_287.pdf , access-date=6 March 2015 , archive-url=https://web.archive.org/web/20150402111329/http://mazur.harvard.edu/publications/Pub_287.pdf , archive-date=2 April 2015 , url-status=dead
Lasers Improve PV Efficiency

Lasers, Plasmas et Procédés Photoniques – Recherche – Structuration du silicium : Application au Photovoltaïque (in French)
Allotropes of silicon Silicon, black Silicon solar cells Infrared solar cells Thin-film cells