Black silicon
Encyclopedia
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).
that reduces Fresnel reflection.
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.
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 dopant
s, 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 wavelength
s.
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. 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, 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.
, 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.
s 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.
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
with very low reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
and correspondingly high absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
of visible (and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
) light. The modification was discovered in the 1980s as an unwanted side effect of reactive ion etching
Reactive ion etching
Reactive-ion etching is an etching technology used in microfabrication. It uses chemically reactive plasma to remove material deposited on wafers. The plasma is generated under low pressure by an electromagnetic field...
(RIE). Another method for forming a similar structure was developed in Eric Mazur
Eric Mazur
Eric Mazur is a prominent physicist and educator at Harvard University. Mazur is known for his work in experimental ultrafast optics and condensed matter physics and a national leader in science education...
's laboratory at Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
(1998).
Properties
Black silicon is a needle-shaped surface structure where needles are made of single-crystalline silicon and have a height above 10 microns and diameter <1 micron. 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 indexRefractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
that reduces Fresnel reflection.
Applications
The unusual optical characteristics, combined with the semiconducting properties of silicon make this material interesting for sensor applications. The potential applications include:- Image sensorImage sensorAn image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...
s with increased sensitivity - Thermal imaging cameraThermal imaging cameraA Thermal Imaging Camera is a type of thermographic camera used in firefighting. By rendering infrared radiation as visible light, such cameras allow firefighters to see areas of heat through smoke, darkness, or heat-permeable barriers. Thermal imaging cameras are typically handheld, but may be...
s - Photodetector with high-efficiency through increased absorption.
- Mechanical contacts and interfaces
- Terahertz applications.
- Solar cells
Production by RIE
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. The process can be set so that a million needles are formed on an area of one square millimeter.
Material
In 1999, a group led by Eric MazurEric Mazur
Eric Mazur is a prominent physicist and educator at Harvard University. Mazur is known for his work in experimental ultrafast optics and condensed matter physics and a national leader in science education...
and James Carey at the Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
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
Sulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...
and other dopant
Dopant
A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...
s, 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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
below the band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
of silicon, including the wavelengths for which unmodified silicon is transparent. This property is caused by sulfur atoms
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
being forced to the silicon surface, creating a structure with a lower band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
and therefore the ability to absorb longer wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s.
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. 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, which is a standard industrial approach for surface texturization of mono-crystalline silicon wafers in solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
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.
Function
When the material is biased by a small electric voltageVoltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
, 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 found commercial applications in a number of photodetectorPhotodetector
Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:*Active pixel sensors are image sensors consisting of an integrated circuit that contains an array of pixel sensors, each pixel containing a both a light sensor and an active amplifier...
s for various imaging
Imaging
Imaging is the representation or reproduction of an object's outward form; especially a visual representation .- Imaging methodologies and technologies :...
and night vision
Night vision
Night vision is the ability to see in low light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range...
applications. Black silicon is currently being commercialized by SiOnyx, a Massachusetts-based
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
venture-funded
Venture capital
Venture capital is financial capital provided to early-stage, high-potential, high risk, growth startup companies. The venture capital fund makes money by owning equity in the companies it invests in, which usually have a novel technology or business model in high technology industries, such as...
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.
External links
- SiOnyx brings "Black Silicon" into the light
- New New York Times article (needs NYT subscription)
- SiOnyx homepage
- Lasers for Photovoltaics - Knowledge Base
- Lasers Improve PV Efficiency
- Lasers, Plasmas et Procédés Photoniques - Recherche - Structuration du silicium : Application au Photovoltaïque (in French)