Monocrystalline silicon

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A silicon ingot Monokristalines Silizium fur die Waferherstellung.jpg
A silicon ingot

Monocrystalline silicon (also called single-crystal silicon (or Si), mono c-Si or simply mono-Si) is the base material for silicon-based discrete components and integrated circuits used in virtually all modern electronic equipment. Mono-Si also serves as a photovoltaic, light-absorbing material in the manufacture of solar cells.

Integrated circuit

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny transistors into a small chip results in circuits that are orders of magnitude smaller, faster, and less expensive than those constructed of discrete electronic components. The IC's mass production capability, reliability, and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.

Solar cell electrical device that converts the energy of light directly into electricity by the photovoltaic effect

A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Individual solar cell devices can be combined to form modules, otherwise known as solar panels. In basic terms a single junction silicon solar cell can produce a maximum open-circuit voltage of approximately 0.5 to 0.6 volts.

Contents

It consists of silicon in which the crystal lattice of the entire solid is continuous, unbroken to its edges, and free of any grain boundaries. Mono-Si can be prepared as an intrinsic semiconductor that consists only of exceedingly pure silicon, or it can be doped by the addition of other elements such as boron or phosphorus to make p-type or n-type silicon. [1] Due to its semiconducting properties, single-crystal silicon is perhaps the most important technological material of the last few decades—the "silicon era", [2] because its availability at an affordable cost has been essential for the development of the electronic devices on which the present-day electronics and IT revolution is based.

Silicon Chemical element with atomic number 14

Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard and brittle crystalline solid with a blue-grey metallic lustre; and it is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic table: carbon is above it; and germanium, tin, and lead are below it. It is relatively unreactive. Because of its high chemical affinity for oxygen, it was not until 1823 that Jöns Jakob Berzelius was first able to prepare it and characterize it in pure form. Its melting and boiling points of 1414 °C and 3265 °C respectively are the second-highest among all the metalloids and nonmetals, being only surpassed by boron. Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure element in the Earth's crust. It is most widely distributed in dusts, sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates. More than 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust after oxygen.


An intrinsic(pure) semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities. In intrinsic semiconductors the number of excited electrons and the number of holes are equal: n = p. This may even be the case after doping the semiconductor, though only if it is doped with both donors and acceptors equally. In this case, n = p still holds, and the semiconductor remains intrinsic, though doped.

In semiconductor production, doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical, optical and structural properties. The doped material is referred to as an extrinsic semiconductor. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is referred to as a degenerate semiconductor.

Monocrystalline silicon differs from other allotropic forms, such as non-crystalline amorphous silicon—used in thin-film solar cells—and polycrystalline silicon, which consists of small crystals known as crystallites.

Amorphous silicon non-crystalline silicon

Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs.

Thin-film solar cell type of second-generation solar cell

A thin-film solar cell is a second generation solar cell that is made by depositing one or more thin layers, or thin film (TF) of photovoltaic material on a substrate, such as glass, plastic or metal. Thin-film solar cells are commercially used in several technologies, including cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and amorphous thin-film silicon.

Polycrystalline silicon high purity, polycrystalline form of silicon

Polycrystalline silicon, also called polysilicon or poly-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry.

Production

Monocrystalline silicon is generally created by one of several methods that involve melting high-purity, semiconductor-grade silicon (only a few parts per million of impurities) and the use of a seed to initiate the formation of a continuous single crystal. This process is normally performed in an inert atmosphere, such as argon, and in an inert crucible, such as quartz, to avoid impurities that would affect the crystal uniformity.

Seed crystal small piece of a single crystal

A seed crystal is a small piece of single crystal or polycrystal material from which a large crystal of typically the same material is to be grown in a laboratory. Used to replicate material, the use of seed crystal to promote growth avoids the otherwise slow randomness of natural crystal growth and allows manufacture on a scale suitable for industry.

Argon Chemical element with atomic number 18

Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in the Earth's atmosphere, at 0.934%. It is more than twice as abundant as water vapor, 23 times as abundant as carbon dioxide, and more than 500 times as abundant as neon. Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust.

Quartz mineral composed of silicon and oxygen atoms in a continuous framework of SiO₄ silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO₂

Quartz is a mineral composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.

The most common production method is the Czochralski process, which dips a precisely oriented rod-mounted seed crystal into the molten silicon. The rod is then slowly pulled upwards and rotated simultaneously, allowing the pulled material to solidify into a monocrystalline cylindrical ingot up to 2 meters in length and weighing several hundred kilograms. Magnetic fields may also be applied to control and suppress turbulent flow, further improving the uniformity of the crystallization. [3] Other methods are float-zone growth, which passes a polycrystalline silicon rod through a radiofrequency heating coil that creates a localized molten zone, from which a seed crystal ingot grows, and Bridgman techniques, which move the crucible through a temperature gradient to cool it from the end of the container containing the seed. [4] The solidified ingots are then sliced into thin wafers during a process called wafering. After post-wafering processing, the wafers are ready for use in fabrication.

Czochralski process Method of crystal growth

The Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors, metals, salts and synthetic gemstones. The process is named after Polish scientist Jan Czochralski, who invented the method in 1915 while investigating the crystallization rates of metals. He made this discovery by accident: instead of dipping his pen into his inkwell, he dipped it in molten tin, and drew a tin filament, which later proved to be a single crystal.

Float-zone silicon

Float-zone silicon is very pure silicon obtained by vertical zone melting. The process was developed at Bell Labs by Henry Theuerer in 1955 as a modification of a method developed by William Gardner Pfann for germanium. In the vertical configuration molten silicon has sufficient surface tension to keep the charge from separating. Avoidance of the necessity of a containment vessel prevents contamination of the silicon.

Bridgman–Stockbarger technique

The Bridgman–Stockbarger technique is named after Harvard physicist Percy Williams Bridgman (1882-1961) and MIT physicist Donald C. Stockbarger (1895–1952). The technique includes two similar but distinct methods primarily used for growing boules, but which can be used for solidifying polycrystalline ingots as well.

Compared to the casting of polycrystalline ingots, the production of monocrystalline silicon is very slow and expensive. However, the demand for mono-Si continues to rise due to the superior electronic properties—the lack of grain boundaries allows better charge carrier flow and prevents electron recombination [5] —allowing improved performance of integrated circuits and photovoltaics.

In the solid-state physics of semiconductors, carrier generation and carrier recombination are processes by which mobile charge carriers are created and eliminated. Carrier generation and recombination processes are fundamental to the operation of many optoelectronic semiconductor devices, such as photodiodes, light-emitting diodes and laser diodes. They are also critical to a full analysis of p-n junction devices such as bipolar junction transistors and p-n junction diodes.

In electronics

The primary application of monocrystalline silicon is in the production of discrete components and integrated circuits. Ingots made from the Czochralski process are sliced into wafers about 0.75 mm thick and polished to obtain a regular, flat substrate, onto which microelectronic devices are built through various microfabrication processes, such as doping or ion implantation, etching, deposition of various materials, and photolithographic patterning.

A single continuous crystal is critical for electronics, since grain boundaries, impurities, and crystallographic defects can significantly impact the local electronic properties of the material, which in turn affects the functionality, performance, and reliability of semiconductor devices by interfering with their proper operation. For example, without crystalline perfection, it would be virtually impossible to build very large-scale integration (VLSI) devices, in which billions [6] of transistor-based circuits, all of which must function reliably, are combined into a single chip to form a microprocessor. As such, the electronics industry has invested heavily in facilities to produce large single crystals of silicon.

In solar cells

Global market-share in terms of annual production by PV technology since 1990 Global Market Share by PV Technology from 1990 to 2013.svg
Global market-share in terms of annual production by PV technology since 1990

Monocrystalline silicon is also used for high-performance photovoltaic (PV) devices. Since there are less stringent demands on structural imperfections compared to microelectronics applications, lower-quality solar-grade silicon (Sog-Si) is often used for solar cells. Despite this, the monocrystalline-silicon photovoltaic industry has benefitted greatly from the development of faster mono-Si production methods for the electronics industry.

Market share

Being the second most common form of PV technology, monocrystalline silicon is ranked behind only its sister, polycrystalline silicon. Due to the significantly higher production rate and steadily decreasing costs of poly-silicon, the market share of mono-Si has been decreasing: in 2013, monocrystalline solar cells had a market share of 36%, which translated into the production of 12.6 GW of photovoltaic capacity, [7] but the market share had dropped below 25% by 2016. Despite the lowered market share, the equivalent mono-Si PV capacity produced in 2016 was 20.2 GW, indicating a significant increase in the overall production of photovoltaic technologies. [8]

Efficiency

With a recorded single-junction cell lab efficiency of 26.7%, monocrystalline silicon has the highest confirmed conversion efficiency out of all commercial PV technologies, ahead of poly-Si (22.3%) and established thin-film technologies, such as CIGS cells (21.7%), CdTe cells (21.0%), and a-Si cells (10.2%). Solar module efficiencies for mono-Si—which are always lower than those of their corresponding cells—finally crossed the 20% mark for in 2012 and hit 24.4% in 2016. [9] The high efficiency is largely attributable to the lack of recombination sites in the single crystal and better absorption of photons due to its black color, as compared to the characteristic blue hue of poly-silicon. Since they are more expensive than their polycrystalline counterparts, mono-Si cells are useful for applications where the main considerations are limitations on weight or available area, such as in spacecraft or satellites powered by solar energy, where efficiency can be further improved through combination with other technologies, such as multi-layer solar cells.

Manufacturing

Besides the low production rate, there are also concerns over wasted material in the manufacturing process. Creating space-efficient solar panels requires cutting the circular wafers (a product of the cylindrical ingots formed through the Czochralski process) into octagonal cells that can be packed closely together. The leftover material is not used to create PV cells and is either discarded or recycled by going back to ingot production for melting. Furthermore, even though mono-Si cells can absorb the majority of photons within 20 μm of the incident surface, limitations on the ingot sawing process mean commercial wafer thickness are generally around 200 μm. However, advances in technology are expected to reduce wafer thicknesses to 140 μm by 2026. [10]

Other manufacturing methods are being researched, such as direct wafer epitaxial growth, which involves growing gaseous layers on reusable silicon substrates. Newer processes may allow growth of square crystals that can then be processed into thinner wafers without compromising quality or efficiency, thereby eliminating the waste from traditional ingot sawing and cutting methods. [11]

Appearance

See also

Related Research Articles

Wafer (electronics) thin slice of semiconductor material used in the fabrication of integrated circuits

In electronics, a wafer is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serves as the substrate for microelectronic devices built in and upon the wafer. It undergoes many microfabrication processes, such as doping, ion implantation, etching, thin-film deposition of various materials, and photolithographic patterning. Finally, the individual microcircuits are separated by wafer dicing and packaged as an integrated circuit.

Photovoltaics Method of generating electrical power by converting solar radiation into direct current electricity

Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry.

Boule (crystal) Synthetic ingot of crystal

A boule is a single crystal ingot produced by synthetic means.

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PV Crystalox Solar plc is a supplier to solar cell manufacturers, producing multicrystalline silicon wafers for use in solar electricity generation systems. It has operations in Germany, United Kingdom and Japan and its headquarters are in the United Kingdom. It is listed on the London Stock Exchange and was a former constituent of the FTSE 250 Index.

SunEdison, Inc. is a renewable energy company headquartered in the U.S. In addition to developing, building, owning, and operating solar power plants and wind energy plants, it also manufactures high purity polysilicon, monocrystalline silicon ingots, silicon wafers, solar modules, solar energy systems, and solar module racking systems. Originally a silicon-wafer manufacturer established in 1959 as the Monsanto Electronic Materials Company, Monsanto sold the company in 1989.

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Crystalline silicon

Crystalline silicon (c-Si) is the crystalline forms of silicon, either multicrystalline silicon (multi-Si) consisting of small crystals, or monocrystalline silicon (mono-Si), a continuous crystal. Crystalline silicon is the dominant semiconducting material used in photovoltaic technology for the production of solar cells. These cells are assembled into solar panels as part of a photovoltaic system to generate solar power from sunlight.

Jinko Solar(NYSE: JKS) is currently the world's largest solar panel manufacturer, shipping 11.4 GW of modules in 2018. Headquartered in Shanghai, China, the company started out as a wafer manufacturer in 2006 and went public on the New York Stock Exchange in 2010. The company shipped 11.4 GW of modules in 2018.

The Silicon Module Super League (SMSL) is a group of big-six crystalline silicon (c-Si) module suppliers in the modern solar PV industry. The 'big six' industry group members were Canadian Solar, Hanwha Q CELLS, JA Solar, Jinko Solar, and Trina Solar. LONGi the world's largest solar monocrystalline silicon manufacturer and GCL, the world's largest solar poly crystalline silicon manufacturer, both joined the SMSL in mid-2016.

LONGi Green Energy Technology

LONGi Green Energy Technology, formerly Xi'an Longi Silicon Materials Corporation, is one of the six major Chinese manufacturers of photovoltaics and a developer of solar projects. Longi is the world largest manufacturer of monocrystalline silicon wafers and is listed on the Shanghai Stock Exchange.

References

  1. Monkowski, J. R.; Bloem, J.; Giling, L. J.; Graef, M. W. M. (1979). "Comparison of dopant incorporation into polycrystalline and monocrystalline silicon". Appl. Phys. Lett. 35 (5): 410–412. doi:10.1063/1.91143.
  2. W.Heywang, K.H.Zaininger, Silicon: the semiconductor material , in Silicon: evolution and future of a technology, P.Siffert, E.F.Krimmel eds., Springer Verlag, 2004.
  3. Wang, C.; Zhang, H.; Wang, T. H.; Ciszek, T. F. (2003). "A continuous Czochralski silicon crystal growth system". Journal of Crystal Growth. 250 (1–2): 209–214. doi:10.1016/s0022-0248(02)02241-8.
  4. Capper, Peter; Rudolph, Peter (2010). Crystal growth technology: semiconductors and dielectrics. Weinheim: Wiley-VCH. ISBN   9783527325931. OCLC   663434790.
  5. Wenham, S. R.; Green, M. A.; Watt, M. E.; Corkish R. (2007). Applied photovoltaics (2nd ed.). London: Earthscan. ISBN   9781844074013. OCLC   122927906.
  6. Peter Clarke, Intel enters billion-transistor processor era, EE Times, 14 October 2005.
  7. Photovoltaics Report, Fraunhofer ISE, July 28, 2014.
  8. Photovoltaics Report, Fraunhofer ISE, February 26, 2018.
  9. Green, Martin A.; Hishikawa, Yoshihiro; Dunlop, Ewan D.; Levi, Dean H.; Hohl-Ebinger, Jochen; Ho-Baillie, Anita W. Y. (2018-01-01). "Solar cell efficiency tables (version 51)". Progress in Photovoltaics: Research and Applications. 26 (1): 3–12. doi:10.1002/pip.2978. ISSN   1099-159X.
  10. Solar Industry Technology Report 2015–2016, Canadian Solar, October 2016.
  11. Scanlon, Bill (August 27, 2014). "Crystal Solar and NREL Team Up to Cut Costs". NREL. Retrieved 2018-03-01.