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, with exception of beryllium, have been described. Overview. Silicon atoms in silicides can have many possible organizations: 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. Alkali and alkaline earth metals. Group 1 and 2 silicides e.g. NaSi and CaSi react with water, yielding hydrogen and/or silanes. When magnesium silicide is placed into hydrochloric acid, HCl(aq), the gas silane, SiH, is produced. This gas is the silicon  analogue of methane, CH, but is more reactive. Silane is pyrophoric, that is, due to the presence of oxygen, it spontaneously combusts in air: These reactions are typical of a Group 2 silicide. MgSi reacts similarly with sulfuric acid. Group 1 silicides are even more reactive. For example, sodium silicide, NaSi, reacts rapidly with water to yield sodium silicate, NaSiO, and hydrogen gas. Rubidium silicide is pyrophoric, igniting in contact with air. Transition metals. 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 , e.g. melted potassium hydroxide, or fluorine and chlorine when red-hot. Other elements. Mercury, thallium, bismuth, and lead are immiscible with liquid silicon.