Ammonothermal

Ammonothermal synthesis is a crystal growth technique that uses dense liquid ammonia as the solvent in a high-pressure reactor to dissolve and transport material to a seed crystal, where it crystallizes. The method is especially associated with group III-nitrides, notably GaN, and has been explored as an alternative to hydrothermal growth and vapor-phase methods for obtaining high-quality bulk crystals and substrates.

In typical ammonothermal growth, a mixture of ammonia and mineralizers is loaded into a sealed autoclave. Under elevated pressure and temperature, ammonia remains liquid and solubilizes selected precursors, often using additives such as ammonium salts (for example ammonium chloride or ammonium fluoride) or alkali metal salts to enhance solubility. A temperature gradient within the vessel promotes diffusion of dissolved species from the source zone to seed crystals, where they deposit as solid crystals. Growth can produce relatively large, low-defect GaN crystals, with ongoing research into quality, reproducibility, and doping.

Advantages include the ability to grow high-quality nitrides at comparatively moderate temperatures and the potential for large crystal sizes; limitations include the need for specialized high-pressure equipment and stringent safety controls due to the toxic and corrosive nature of liquid ammonia and mineralizers, as well as slower growth rates and the challenge of controlling impurities and reproducibility. The ammonothermal approach remains an area of active research for scalable production of high-purity nitride crystals.