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22 SOLAR POWER TECHNOLOGIES

(SiO ). Silicon (Si) was first produced in 1823 by Berzelius when he separated the
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naturally occurring ferrous silica (SiF ) by heat exposure with potassium metal.
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Commercial production of silicon commenced in 1902 and resulted in an iron-
silicon alloy with an approximate weight of 25 percent iron which was used in steel
production as an effective deoxidant. At present more than one million tons of
metallurgical-grade 99 percent pure silicon is used by the steel industry.
Approximately 60 percent of the referenced silicon is used in metallurgy, 35 percent
in the production of silicones, and approximately 5 percent for the production of
semiconductor-grade silicon.
In general, common impurities found in silicon are iron (Fe), aluminum (Al),
magnesium (Mg), and calcium (Ca). The purest grade of silicon used in semiconduc-
tor applications contains about one part per billion (ppb) of contamination. Puri-
fication of silicon involves several different types of complex refining technologies
such as chemical vapor deposition, isotopic enrichment, and a crystallization process.
Figure 2.1 depicts silicon crystals prior to ingot manufacturing process.

Chemical vapor deposition One of the earlier silicon refining processes, known
as chemical vapor deposition, produced a higher grade of metallurgical silicon, which
consisted of a chemical reaction of silicon tetrachloride (SiCl ) and zinc (Zn) under
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Figure 2.1 Silicon crystals.
Photo courtesy of SolarWorld.