Silicide

A silicide is a compound that has silicon with more electropositive elements.

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Silicon is more electropositive than carbon. Silicides are structurally closer to borides than to carbides.

Similar to borides and carbides, the composition of silicides cannot be easily specified as covalent molecules. The chemical bonds in silicides range from conductive metal-like structures to covalent or ionic. Silicides of all non-transition metals, with exception of beryllium, have been described.

Mercury, thallium, bismuth, and lead are nonmiscible with liquid silicon.

Silicon atoms in silicides can have many possible organizations:

• Isolated silicon atoms: electrically conductive Cu₅Si, (V,Cr,Mn)₃Si, Fe₃Si, Mn₃Si, and nonconductive (Mg,Ge,Sn,Pb)₂Si, (Ca,Ru,Ce,Rh,Ir,Ni)₂Si)
• Si₂ pairs: U₃Si₂, Hf and Th silicides
• Si₄ tetrahedra: KSi, RbSi, CsSi
• Si_n chains: USi, (Ti, Zr, Hf, Th, Ce, Pu)Si, CaSi, SrSi, YSi
• Planar hexagonal graphite-like Si layers: β-USi₂, silicides of other lanthanoids and actinoids
• Corrugated hexagonal Si layers: CaSi₂
• Open three-dimensional Si skeletons: SrSi₂, ThSi₂, α-USi₂

Group 1 and 2 silicides e.g. Na₂Si and Ca2Si react with water to yielding hydrogen and/or silanes. The transition metal silicides are, in contrast, usually inert to aqueous solutions of everything with exception of hydrofluoric acid; however, they react with more aggressive agents, eg. melted potassium hydroxide, or fluorine and chlorine when red-hot.

Silicide prepared by a self-aligned process is called salicide. This is a process in which silicide contacts are formed only in those areas in which deposited metal (which after annealing becomes a metal component of the silicide) is in direct contact with silicon, hence, the process is self-aligned. It is commonly implemented in MOS/CMOS processes for ohmic contacts of the source, drain, and poly-Si gate.

Examples

• sodium silicide, Na₂Si
• magnesium silicide, Mg₂Si
• Platinum silicide, PtSi
• Titanium silicide, TiSi₂
• Tungsten silicide, WSi₂

See category for a list.

References

Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.