Page 76 - Alternative Energy Systems in Building Design

52 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

Figure 3.15 Silicon crystals.

steel industry. Approximately 60 percent of the referenced silicon is used in metallurgy,
35 percent in the production of silicones, and approximately 5 percent in the produc-
tion of semiconductor-grade silicon.
In general, common impurities found in silicon are iron (Fe), aluminium (Al), mag-
nesium (Mg), and calcium (Ca). The purest grade of silicon used in semiconductor
applications contains about 1 ppb contamination. The purification of silicon involves
several different types of complex refining technologies, such as chemical vapor dep-
osition, isotopic enrichment, and a crystallization process. Figure 3.15 shows silicon
crystals prior to the ingot manufacturing process.

Chemical vapor deposition One of the early silicon-refining processes, known as
chemical vapor deposition, produced a high grade of metallurgical silicon and consisted of
a chemical reaction of silicon tetrachloride (SiCl ) and zinc (Zn) under high-temperature
4
vaporization conditions, yielding pure silicon through the following chemical reaction:
SiCl + 2Zn → Si + 2ZnCl
4 2
The main problem with this process was that SiCl always contained boron chloride
4
(BCl ) when combined with zinc-produced boron, which is a serious contaminant. In
3
1943, a chemical vapor deposition was developed that involved replacement of the
zinc with hydrogen (H), giving rise to pure silicon, because hydrogen, unlike zinc,
does not reduce the boron chloride to boron. Further refinement involved replacement
of silicon tetrachloride with trichlorosilane (SiHCl ), which is readily reduced to silicon.
3
Figure 3.16 shows a Czochralski silicon crystallization furnace.

Czochralski crystal growth In 1916, a Polish metallurgist, Jan Czochralski,
developed a technique to produce silicon crystals that bears his name. The crystallization