Silicide
Encyclopedia
A silicide is a compound that has silicon
with (usually) more electropositive elements.
Silicon is more electropositive
than carbon. Silicides are structurally closer to boride
s than to carbide
s.
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:
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 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, e.g. melted potassium hydroxide
, or fluorine
and chlorine
when red-hot.
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:
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.
See category for a list.
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
with (usually) more electropositive elements.
Silicon is more electropositive
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
than carbon. Silicides are structurally closer to boride
Boride
In chemistry a boride is a chemical compound between boron and a less electronegative element, for example silicon boride . The borides are a very large group of compounds that are generally high melting and are not ionic in nature. Some borides exhibit very useful physical properties. The term...
s than to carbide
Carbide
In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and "intermediate" transition metal carbides...
s.
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
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
or ionic
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
. Silicides of all non-transition metals, with exception of beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
, have been described.
Mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
, thallium
Thallium
Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...
, bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
, and lead are nonmiscible
Miscibility
Miscibility is the property of liquids to mix in all proportions, forming a homogeneous solution. In principle, the term applies also to other phases , but the main focus is usually on the solubility of one liquid in another...
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: 3Si2, and silicides
- Si4 tetrahedraTetrahedronIn geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
: Si, Si, Si - Sin chains: USi, (Ti, Zr, Hf, Th, Ce, Pu)Si, CaSi, SrSi, YSi
- Planar hexagonal graphiteGraphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
-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
Salicide
The term salicide refers to a technology used in the microelectronics industry used to form electrical contacts between the semiconductor device and the supporting interconnect structure...
. 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
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
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 to yielding hydrogen and/or silanes. The transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
silicides are, in contrast, usually inert to aqueous solutions of everything with exception of hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
; however, they react with more aggressive agents, e.g. melted potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
, or fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
and chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
when red-hot.
When magnesium silicide is placed into hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
, HCl(aq), the gas silane
Silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...
, SiH4, is produced. This gas is the silicon analogue of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, CH4, but is more reactive. Silane is pyrophoric
Pyrophoricity
A pyrophoric substance is a substance that will ignite spontaneously in air. Examples are iron sulfide and many reactive metals including uranium, when powdered or sliced thin. Pyrophoric materials are often water-reactive as well and will ignite when they contact water or humid air...
, that is, due to the presence of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, 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
Alkaline earth metal
The alkaline earth metals are a group in the periodic table. In the modern IUPAC nomenclature, the alkaline earth metals are called the group 2 elements. Previously, they were called the Group IIA elements . The alkaline earth metals contain beryllium , magnesium , calcium , strontium , barium and...
silicide. Mg2Si reacts similarly with sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
. Group 1
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
silicides are even more reactive. For example, sodium silicide, Na2Si, reacts rapidly with water to yield sodium silicate
Sodium silicate
Sodium silicate is the common name for a compound sodium metasilicate, Na2SiO3, also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and automobiles...
, Na2SiO3, and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas.
Examples
- Nickel silicide, NiSi
- Sodium silicide, Na2Si
- Magnesium silicideMagnesium silicideMagnesium silicide, Mg2Si, is an inorganic compound consisting of magnesium and silicon. As a powder magnesium silicide is dark blue or slightly purple in color. Silicon dioxide, SiO2, found in sand and glass, when heated with magnesium forms magnesium oxide, and, if an excess of magnesium is...
, Mg2Si - Platinum silicidePlatinum silicidePlatinum silicide is a semiconductor material used in infrared detectors. It is used in detectors for infrared astronomy.Platinum silicide is capable of operating at 1–5 µm wavelength range. It has a good sensitivity and high stability...
, PtSi (platinum is actually more electronegative than silicon) - Titanium silicide, TiSi2
- Tungsten silicide, WSi2
See category for a list.