Silicide

A silicide is a type of chemical compound that combines silicon and a usually more electropositive element. 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 have been described except beryllium. Silicon atoms in silicides can have many possible organizations: Isolated silicon atoms: electrically conductive (or semiconductive) CrSi, MnSi, FeSi, CoSi, , , , , and nonconductive , Si2 pairs: , hafnium and thorium silicides Si4 tetrahedra: KSi, RbSi, CsSi Sin chains: USi, , 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 A silicide prepared by a self-aligned process is called a 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. Group 1 and 2 silicides e.g. Na2Si and Ca2Si react with water, yielding hydrogen and/or silanes. When magnesium silicide is placed into hydrochloric acid, HCl(aq), the gas silane, SiH4, is produced. This gas is the silicon analogue of methane, CH4, but is more reactive. Silane is pyrophoric, that is, due to the presence of oxygen, it spontaneously combusts in air: Mg2Si(s) + 4HCl(aq) → SiH4(g) + 2MgCl2(s) SiH4 + 2O2 → SiO2 + 2H2O These reactions are typical of a Group 2 silicide. Mg2Si reacts similarly with sulfuric acid. Group 1 silicides are even more reactive. For example, sodium silicide, Na2Si, reacts rapidly with water to yield sodium silicate, Na2SiO3, and hydrogen gas.