Monocrystalline silicon, often referred to assingle-crystal silicon or simplymono-Si, is a critical material widely used in modern electronics and photovoltaics. As the foundation for silicon-based discrete components andintegrated circuits, it plays a vital role in virtually all modern electronic equipment, from computers to smartphones. Additionally, mono-Si serves as a highly efficient light-absorbing material for the production ofsolar cells, making it indispensable in the renewable energy sector.
It consists ofsilicon in which thecrystal lattice of the entire solid is continuous, unbroken to its edges, and free of anygrain boundaries (i.e. asingle crystal). Mono-Si can be prepared as anintrinsic semiconductor that consists only of exceedingly pure silicon, or it can bedoped by the addition of other elements such asboron orphosphorus to makep-type orn-type silicon.[1] Due to itssemiconducting properties, single-crystal silicon is perhaps the most important technological material of the last few decades—the "silicon era".[2] Its availability at an affordable cost has been essential for the development of the electronic devices on which thepresent-day electronics andIT revolution is based.
Monocrystalline silicon differs from otherallotropic forms, such as non-crystallineamorphous silicon—used inthin-film solar cells—andpolycrystalline silicon, which consists of small crystals known ascrystallites.
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 aseed 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 asquartz, to avoid impurities that would affect the crystal uniformity.
The most common production technique is theCzochralski method, which dips a precisely oriented rod-mountedseed 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 arezone melting, which passes a polycrystalline silicon rod through a radiofrequency heating coil that creates a localized molten zone, from which a seed crystal ingot grows, andBridgman 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 thinwafers during a process calledwafering. After post-wafering processing, the wafers are ready for use in fabrication.
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 electronrecombination[5]—allowing improved performance of integrated circuits and photovoltaics.
The primary application of monocrystalline silicon is in the production ofdiscrete components andintegrated circuits. Ingots made by the Czochralski method are sliced into wafers about 0.75 mm thick and polished to obtain a regular, flat substrate, onto whichmicroelectronic devices are built through variousmicrofabrication processes, such asdoping orion implantation,etching,deposition of various materials, andphotolithographic patterning.
A single continuous crystal is critical for electronics, since grain boundaries,impurities, andcrystallographic defects can significantly impact the local electronic properties of the material, which in turn affects the functionality, performance, and reliability ofsemiconductor devices by interfering with their proper operation. For example, without crystalline perfection, it would be virtually impossible to buildvery 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.
Monocrystalline silicon is also used for high-performancephotovoltaic (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.
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]
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 establishedthin-film technologies, such asCIGS cells (21.7%),CdTe cells (21.0%), anda-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.
Besides the low production rate, there are also concerns over wasted material in the manufacturing process. Creating space-efficientsolar 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 waferepitaxial 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]
Monocrystalline silicon differs significantly from other forms ofsilicon used in solar technology, particularly polycrystalline silicon and amorphous silicon:
