Gallium phosphide


Gallium phosphide, a phosphide of gallium, is a compound semiconductor material with an indirect band gap of 2.24eV at room temperature. Impure polycrystalline material has the appearance of pale orange or grayish pieces. Undoped single crystals are orange, but strongly doped wafers appear darker due to free-carrier absorption. It is odorless and insoluble in water.
GaP has a microhardness of 9450 N/mm2, a Debye temperature of, and a thermal expansion coefficient of 5.3 K−1 at room temperature. Sulfur, silicon or tellurium are used as dopants to produce n-type semiconductors. Zinc is used as a dopant for the p-type semiconductor.
Gallium phosphide has applications in optical systems. Its static dielectric constant is 11.1 at room temperature. Its refractive index varies between ~3.2 and 5.0 across the visible range, which is higher than in most other semiconducting materials.

Light-emitting diodes

Gallium phosphide has been used in the manufacture of low-cost red, orange, and green light-emitting diodes with low to medium brightness since the 1960s. It is used standalone or together with gallium arsenide phosphide.
Pure GaP LEDs emit green light at a wavelength of 555 nm. Nitrogen-doped GaP emits yellow-green light, zinc oxide doped GaP emits red.
Gallium phosphide is transparent for yellow and red light, therefore GaAsP-on-GaP LEDs are more efficient than GaAsP-on-GaAs.

Crystal growth

At temperatures above ~900 °C, gallium phosphide dissociates and the phosphorus escapes as a gas. In crystal growth from a 1500 °C melt, this must be prevented by holding the phosphorus in with a blanket of molten boric oxide in inert gas pressure of 10–100 atmospheres. The process is called liquid encapsulated Czochralski growth, an elaboration of the Czochralski process used for silicon wafers.

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