Silicon

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Table 1: Some physical properties of silicon

Atomic number	14
Atomic mass	28.086
Lattice constant:	0.5431 nm (edge length of the cubic unit cell)
Interatomic distance in <111> direction	0.2352 nm
Atomic density	5.00 × 1022 atoms/cm3
Density at 300 K	2.329 g/cm3
Volume increase at trans. from liq. to solid	+ 9.1 %
Specific heat (300 K)	0.713 J g-1 K-1
Thermal expansion (300 K)	2.6 × 10-6 K-1
Thermal conductivity (300 K)	1.5 W cm-1 K-1
Melting point	1687 K
Boiling point	3504 K
Latent heat of fusion	50.66 kJ/mol
Heat of evaporation	385 kJ/mol
Combustion heat, ΔH0 (Si/SiO2) (298 K)	-911 kJ/mol
Bulk modulus (300 K)	97.84 Pa
Band gap (300 K)	1.126 eV
Electron mobility	1440 cm2 V-1 s-1
Hole mobility	484 cm2 V-1 s-1

Isotopes

Silicon has numerous known isotopes, with mass numbers ranging from 22 to 44. 28Si (the most abundant isotope, at 92.23%), 29Si (4.67%), and 30Si (3.1%) are stable; 32Si is a radioactive isotope produced by cosmic ray spallation of argon. Its half-life has been determined to be approximately 170 years (0.21 MeV), and it decays by beta - emission to 32P (which has a 14.28 day half-life ) and then to 32S.

Production

Silicon is commercially prepared by the reaction of high-purity silica with wood, charcoal, and coal, in an electric arc furnace using carbon electrodes. At temperatures over 1,900 °C (3,450 °F), the carbon reduces the silica to silicon according to the chemical equations:

SiO2 + C → Si + CO2

SiO2 + 2 C → Si + 2 CO

Liquid silicon collects in the bottom of the furnace, and is then drained and cooled. The silicon produced via this process is called metallurgical grade silicon and is at least 98% pure. Using this method, silicon carbide (SiC) may form. However, provided the concentration of SiO2 is kept high, the silicon carbide can be eliminated:

2 SiC + SiO2 → 3 Si + 2 CO

Pure silicon (>99.9%) can be extracted directly from solid silica or other silicon compounds by molten salt electrolysis. This method, known from 1854 (see also FFC Cambridge Process) has the potential to directly produce solar grade silicon without any CO2 emission and at much lower energy consumption.

Crystallization

Silicon, like carbon and germanium, crystallizes in a diamond cubic
crystal structure. The lattice spacing for silicon is 0.5430710 nm (5.430710 Å). The majority of silicon crystals grown for device production are produced by the Czochralski process, (CZ-Si) since it is the cheapest method available and it is capable of producing large size crystals. However, silicon single-crystals grown by the Czochralski method contain impurities since the crucible which contains the melt dissolves. For certain electronic devices, particularly those required for high power applications, silicon grown by the Czochralski method is not pure enough. For these applications, float-zone silicon (FZ-Si) can be used instead. With the CZ-Si method the seed is dipped into the silicon melt, and the growing crystal is pulled upward, whereas with the FZ-Si method the thin seed crystal sustains the growing crystal as well as the polysilicon rod from the bottom. As a result, it is difficult to grow large size crystals using the float-zone method. Today, all the dislocation-free silicon crystals used in semiconductor industry with diameter 300 mm or larger are grown by the Czochralski method with purity level significantly improved.