Arsenide; Gallium Arsenide

Preparation and chemistry

In the compound, gallium has a +3 oxidation state. Gallium arsenide can be prepared by direct reaction from the elements which is used in a number of industrial processes:[3]
Crystal growth using a horizontal zone furnace in the Bridgman-Stockbarger technique, in which gallium and arsenic vapors react and free molecules deposit on a seed crystal at the cooler end of the furnace.
Liquid encapsulated Czochralski (LEC) growth is used for producing high purity single crystals that exhibit semi-insulating characteristics.
Alternative methods for producing films of GaAs include:[3][4]
VPE reaction of gaseous gallium metal and arsenic trichloride
2 Ga + 2 AsCl3 → 2 GaAs + 3 Cl2
MOCVD reaction of trimethylgallium and arsine:
Ga(CH3)3 + AsH3 → GaAs + 3 CH4
Wet etching of GaAs industrially uses an oxidizing agent, e.g., hydrogen peroxide or bromine water,[5] and the same strategy has been described in a patent relating to processing scrap components containing GaAs where the Ga3+ is complexed with a hydroxamic acid ("HA"), for example:[6]:
GaAs + H2O2 + "HA" → "GaA" complex + H3AsO4 + 4 H2O
Oxidation of GaAs occurs in air and degrades performance of the semiconductor, the surface can be passivated by depositing a cubic gallium(II) sulfide layer using a tert-butyl gallium sulfide compound such as (tBuGaS)7.[7]

Notable characteristics

Elemental gallium is not found in nature, but it is easily obtained by smelting. Very pure gallium metal has a brilliant silvery color and its solid metal fractures conchoidally like glass. Gallium metal expands by 3.1 percent when it solidifies, and therefore storage in either glass or metal containers is avoided, due to the possibility of container rupture with freezing. Gallium shares the higher-density liquid state with only a few materials like silicon, germanium, bismuth, antimony and water.

Gallium attacks most other metals by diffusing into their metal lattice. Gallium for example diffuses into the grain boundaries of Al/Zn alloys or steel, making them very brittle. Also, gallium metal easily alloys with many metals, and was used in small quantities as a plutonium-gallium alloyin the plutonium cores of the first and third nuclear bombs, to help stabilize the plutonium crystal structure.

The melting point of 302.9146 K (29.7646°C, 85.5763°F) is near room temperature. Gallium's melting point (mp) is one of the formal temperature reference points in the International Temperature Scale of 1990 (ITS-90) established by BIPM. The triple point of gallium of 302.9166 K (29.7666°C, 85.5799°F), is being used by NIST in preference to gallium's melting point.

Gallium is a metal that will melt in one's hand. This metal has a strong tendency to supercool below its melting point/freezing point. Seeding with a crystal helps to initiate freezing. Gallium is one of the metals (with caesium, rubidium, francium and mercury) which are liquid at or near normal room temperature, and can therefore be used in metal-in-glass high-temperature thermometers. It is also notable for having one of the largest liquid ranges for a metal, and (unlike mercury) for having a low vapor pressure at high temperatures. Unlike mercury, liquid gallium metal wets glass and skin, making it mechanically more difficult to handle (even though it is substantially less toxic and requires far fewer precautions). For this reason as well as the metal contamination problem and freezing-expansion problems noted above, samples of gallium metal are usually supplied in polyethylene packets within other containers.

Crystallization of gallium from the melt

Gallium does not crystallize in any of the simple crystal structures. The stable phase under normal conditions is orthorhombic with 8 atoms in the conventional unit cell. Each atom has only one nearest neighbor (at a distance of 244 pm) and six other neighbors within additional 39 pm. Many stable andmetastable phases are found as function of temperature and pressure.

The bonding between the nearest neighbors is found to be of covalent character, hence Ga2 dimers are seen as the fundamental building blocks of the crystal. This explains the drop of the melting point compared to its neighbour elements aluminium and indium. The compound with arsenic, gallium arsenide is a semiconductor commonly used in light-emitting diodes.

High-purity gallium is dissolved slowly by mineral acids.

Gallium has no known biological role, although it has been observed to stimulate metabolism.[