Silicide

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

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 like with covalent molecules. The chemical bonds in silicides range from conductive metal-like structures to covalent bond to ionic bond. Silicides of all non-transition metals, with exception of beryllium, were 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 Cu₃Si, (V,Cr,Mn)Si, Fe₃Si, Mn₃Si, and nonconductive (Mg,GeSn,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

Transition metal silicides are 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.