301At one time, DuPont produced ultra-pure silicon by reacting silicon tetrachloride with high-
purity zinc vapors at 950 °C, producing silicon by SiCl 4 + 2 Zn → Si + 2 ZnCl 2. However,
this technique was plagued with practical problems (such as the zinc chloride byproduct
solidifying and clogging lines) and was eventually abandoned in favor of the Siemens
process. In the Siemens process, high-purity silicon rods are exposed to trichlorosilane at
1150 °C. The trichlorosilane gas decomposes and deposits additional silicon onto the
rods, enlarging them because 2 HSiCl 3 → Si + 2 HCl + SiCl 4. Silicon produced from this
and similar processes is called polycrystalline silicon. Polycrystalline silicon typically has
impurity levels of less than one part per billion.
In 2006 REC announced construction of a plant based on fluidized bed (FB) technology
using silane: 3 SiCl 4 + Si + 2 H 2 → 4 HSiCl 3 , 4 HSiCl 3 → 3 SiCl 4 + SiH 4 , SiH 4 → Si + 2 H 2.
The advantage of fluid bed technology is that processes can be run continuously, yielding
higher yields than Siemens Process, which is a batch process.
Today, silicon is purified by converting it to a silicon compound that can be more easily
purified by distillation than in its original state, and then converting that silicon compound
back into pure silicon. Trichlorosilane is the silicon compound most commonly used as the
intermediate, although silicon tetrachloride and silane are also used. When these gases
are blown over silicon at high temperature, they decompose to high-purity silicon.
In addition, there is the Schumacher process, which utilizes tribromosilane in place of
trichlorosilane and fluid bed technology. It requires lower deposition temperatures, lower
capital costs to build facilities and operate, no hazardous polymers nor explosive material,
and produces no amorphous silicon dust waste, all of which are drawbacks of the Siemens
process. However, there are yet to be any major factories built using this process.
Compounds
Silicon forms binary compounds called silicides with many metallic elements
whose properties range from reactive compounds, e.g. magnesium silicide, Mg 2 Si
through high melting refractory compounds such as molybdenum disilicide, MoSi 2.
Silicon carbide, SiC (carborundum) is a hard, high melting solid and a well-known
abrasive. It may also be sintered into a type of high-strength ceramic used in armor.
Silane, SiH 4 , is a pyrophoric gas with a similar tetrahedral structure to methane,
CH 4. When pure, it does not react with pure water or dilute acids; however, even
small amounts of alkali impurities from the laboratory glass can result in a rapid
hydrolysis. There is a range of catenated silicon hydrides that form a homologous
series of compounds, SinH 2 n+2 where n = 2–8 (analogous to the alkanes). These
are all readily hydrolyzed and are thermally unstable, particularly the heavier
members.
Disilenes contain a silicon-silicon double bond (analogous to the alkenes) and are
generally highly reactive requiring large substituent groups to stabilize them A
disilyne with a silicon-silicon triple bond was first isolated in 2004; although as the
compound is non-linear, the bonding is dissimilar to that in alkynes.
Tetrahalides, SiX 4 , are formed with all the halogens. Silicon tetrachloride, for
example, reacts with water, unlike its carbon analogue, carbon tetrachloride.
Silicon dihalides are formed by the high temperature reaction of tetrahalides and
silicon; with a structure analogous to a carbene they are reactive compounds.
Silicon difluoride condenses to form a polymeric compound, (SiF 2 )n.
Silicon dioxide is a high melting solid with a number of crystal forms; the most
familiar of which is the mineral quartz. In quartz each silicon atom is surrounded