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Concept
Gallium arsenide
Summary
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated circuits, infrared light-emitting diodes, laser diodes, solar cells and optical windows.
GaAs is often used as a substrate material for the epitaxial growth of other III-V semiconductors, including indium gallium arsenide, aluminum gallium arsenide and others.
In the compound, gallium has a +3 oxidation state. Gallium arsenide single crystals can be prepared by three industrial processes:
The vertical gradient freeze (VGF) process.
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 can exhibit semi-insulating characteristics (see below). Most GaAs wafers are produced using this process.
Alternative methods for producing films of GaAs include:
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
Molecular beam epitaxy (MBE) of gallium and arsenic: 4 Ga + As4 → 4 GaAs or 2 Ga + As2 → 2 GaAs
Oxidation of GaAs occurs in air, degrading 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.
In the presence of excess arsenic, GaAs boules grow with crystallographic defects; specifically, arsenic antisite defects (an arsenic atom at a gallium atom site within the crystal lattice). The electronic properties of these defects (interacting with others) cause the Fermi level to be pinned to near the center of the band gap, so that this GaAs crystal has very low concentration of electrons and holes.
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