MICROCHIPS made of diamond have come a step closer after engineers succeeded in making the material into a semiconductor. Diamond chips could be invaluable in radar and satellite transmitters, as they would need less energy than silicon chips to cool them, and they wouldn鈥檛 burn out as quickly.
Engineers have been interested in diamond as an alternative to silicon for more than a decade. Silicon circuits become unreliable at 150 掳C and stop working altogether at 200 掳C. Diamond-based circuits, on the other hand, could work at 400 掳C and above. A radar trasmitter with diamond electronics could transmit 100 times as much power.
But until now no one has been able to make diamond pure enough to do the job. Natural diamond contains too many impurities, while synthetic diamonds are made up of a number of small crystals, and their borders present too high a resistance to current.
Advertisement
Now scientists in Britain and Sweden have managed to grow a synthetic diamond film as a single crystal (Science, vol 297, p 1670). This has few impurities and many characteristics that will make it useful for high-power electronics.
To make their diamond chip, the researchers used a technique called microwave plasma chemical vapour deposition, in which carbon atoms ripped from molecules of methane gas are deposited on an inert surface to make a film of diamond. Others have tried to do the same thing, but always ended up with many current-resisting crystals.
To get a large, single crystal, the trick is to use a synthetic diamond as a substrate. The researchers made some by subjecting graphite to high pressures and temperatures, then choosing the purest and polishing its surface. Like silicon, the lattice of diamond atoms can be 鈥渄oped鈥� with traces of substances that alter the way current flows through it. The researchers introduced a small amount of boron into the diamond film to provide the extra 鈥渉oles鈥� or spaces in the crystal that allow current to flow. This makes the diamond a semiconductor.
When they tested the film, researchers from De Beers Industrial Diamonds in Ascot, near London, and the Innovative Materials Group in Vasteras, Sweden, found the material allowed electrons and holes to move around freely. The mobility was much higher than in silicon carbide or gallium nitride, two materials being considered for use as high-power semiconductor applications.