It's a wonder that gallium nitride technology exists at all. Light emitting diodes got their start at the red end of the visible spectrum by an arduous process of material improvement. It took decades to perfect the chemistry of growing pure and perfect crystals of a class of light emitting diode materials with the same symmetry as silicon. Getting to gallium arsenide was hard because arsenic is volatile. It tends to boil off at the temperatures needed to grow crystals directly from a melt, so it took decades to perfect it and a class of chemically related crystals , the so called III-V compounds , in ways that allowed the direct conversion of electrons into photons-- the quantized stuff of light.
Can Exobiology Recapituate Exomineralogy ?
Funded by an effort rooted in science fiction, the scientific search for extraterrestrial intelligence ( SETI) began three decades ago. But searching for intelligence is very different from searching for life. SETI asumes we can eavesdrop on distant civilizations talking to themselves , whether as broadcast journalism or the extraterestrial equivalent of the Internet. This human effort has coincided with the dawn of synthetic biology, and some hope that discovering earthly life's origins will provide insight into how distant intelligences may be embodied. Yet strangely, very little has been done to discover what can happen on alien planets in the eons before life evolves.One of their stranger--and more colorful--sights may be metal saturated drops of ammonia rain.
On planets where that happens, crystals of hard to synthesize semiconductors may be as commonplace as those of fools gold are on Earth .
Conditions on Earth scarcely resemble those elsewhere in our own solar system. We live on a wet and tepid exception to the chemical and physical norms of the planets that contain most of the solar sysytems mass. Being made largely of water like the rest of the life on Earth, we think nothing of life's inorganic substrate being the product of wet chemistry- most of the material in the Earths crust has been transformed by interaction with superheated water, often several times over, as the tectonic conveyor belt recycles rocks from the mid-ocean ridges and subducts those at the edges of the continents.
Earthly quartz and feldspars, micas and clays , all contain water and have been re-arranged by it. Likewise, compounds that are decomposed by water and elements that react vigorously with it are largely alien to the surface of the Earth . Not only have we never seen them in the state of nature , but they scarcely figure in our imagined view of the chemistry that gave rise to life. 19th century mineralogists cataloged species representing the entire Periodic Chart long before modern chemistry isolated most of the elements it contains. Yet no earthly mineralogist is likely to encounter crystals of many of the semiconductor compounds on which modern technology depends - the superheated water useful for growing quartz crystals would simply decompose transistors or light emitting diodes made of the nitrides , phosphides, and arsenides of metals like aluminum , gallium and indium.
Since they do not exist as minerals , nobody thinks of them as the basis of rocks. Nor have those who speculate on extraterrestrial life given much thought to the sorts of un-speculative but utterly alien geology that must arise when solvents other than water dominate the geochemical cyles of worlds hot and cold where water is the exception and compounds like ammonia, hydrocyanic acid ,and carbon dioxide are the basic feedstocks of natural history.
We cannot begin to think about the origins of truly alien life until we acquire some understanding of truly alien geology, we will be locked into the Earthly paradigm-- we need to study the phase diagrams that may relate how alien rain and ground fluids re-arrange chemicals that are incompatible with water. It is a rich area for research,for not just nitrides , but alkali metals and their sulfides are easily as amenable to recrystallization and reaction in , for example , ammonia. as silicates and carbonates are in water . We have to experimentally master , and think through rock-forming and altering processes alien to Earth, or remain literally clueless as to what our unmanned probes will encounter in the geology of both the outer solar system, and ultimately, un-Earthly planets orbiting nearby stars.
Early electronics made extensive use of earth materials- 'crystal radios ' used bits of lead ore ( galena is a natural 'II-Vi semiconductor) as rectifying diodes and natural quartz was the basis of piezoelectric frequency standards until the growth of synthetic quartz from superheated water was perfected in the 1960's . Now it's alien mineralogy's turn to come down to earth- and reveal new regimes of surface phenomena and catalysis relevant to the origins of universal biology . That may be speculation, but there is no doubt it will pay some electronic dividends in the process.
Gallium and aluminum both have three bonding electrons, whic balance with the five available in such elements as nitrogen, phosphorus and arsenic to yield stable crystals , but while they share the same sort of covalent bonding , not all of these compounds exhibit the high symmetry of diamond or silicon- elemets of group IV.
Some , instead of forming cubic crystals, , with axes of symmetry at right angles, are based instead on atoms arranged in hexagonal arrays. Instead of forming platonic solids like cubes or tetrahedrons, they grow as six sided pyramids, and their crystals are prone to plague of growth defects arising from their lower symmetry. This tendency is worsened by the strain induced by atoms lost in growing them at the high temperatures necessitated by their high melting points. These high defect density crystals suffer from severely degraded performance when used as the scaffolding for the multilayer structures of laser and light emitting diodes, and it is a testimony to perseverance that blue emitting diodes have been developed at all. As it is , they have taken decades longer than red and yellow solid state lights in everyday use.
But why grow high-voltage , short wavelength semiconductors at high temperatures at all ? Large and perfect crystals of any materials , like quartz , are grown instead at temperatures far below their melting points by crystallizing them instead like rock candy, from solutions in very hot water, inside steamy autoclaves . But expose gallium nitride to superheated water and it falls apart- the nitrogen becomes ammonia. Yet ammonia is an excellent solvent too, and though a toxic and corrosive gas at room temperature, it is less hairy to handle than many chemicals common in Silicon Valley foundries and the crystal growth labs of Gallium Gulch. Hence the very welcome and promising news that U Cal researchers have indeed begun to grow gallium nitride from ammonia.
Hashimoto , Speck et al. write that:
A bulk GaN crystal with improved structural quality was grown via ammonthermal growth with polycrystalline GaN nutrient and a sodium amide mineralizer. The threading dislocation density estimated by plan-view transmission electron microscopy observations was less than 1×106 cm-2 for the Ga-face and 1×107 cm-2 for the N-face. There was no dislocation generation observed at the interface on the Ga-face although a few defects were generated at the interface on the N-face. The chemical etching revealed macroscopic grains on the N-face.