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17 Jun 2002
American Xtal Technology, Inc., has grown its first siliconcarbide crystals with the company's vertical gradient freezetechnology.
The silicon carbide (SiC) crystals will be used as asubstrate for gallium nitride semiconductors and can also be usedto produce lasers and light emitting diodes in the 520 nm greenrange to the 390 nm UV range.
Currently about 75 percent of American Xtal's revenue isfrom the production of gallium arsenide crystals. The substratemaker also produces indium phosphide and germanium substrates.
The development of gallium nitride outer layers hasincreased the efficiency of LEDs, but gallium nitride has beenusing a sapphire substrate. "Silicon carbide actually has abetter lattice match between it and the gallium nitride," saidMorris Young, the president and CEO of American Xtal Technology.
Silicon carbide has both electronic and optoelectronicfunctions. It can power cell phones as well as integratedcircuits and transistors.
"It is also very strong and very hard," noted Young. Silicon carbide can operate at 400 degrees Centigrade, allowingit to be used in the monitoring of jet or automobile engineoperations.
Silicon carbide also has a breakdown field more than 20times higher than silicon. While the tolerance is dependent upondistance, silicon carbide has the potential to handle up to 1million volts over long distances, making it ideal for high-voltagedevices.
The ability to be a substrate for gallium nitride will alsoallow proliferation of blue LEDs in the 430 to 475 nm range. Silicon carbide's range also creates the possibility of its usein UV detection.
Although those applications provide potential for siliconcarbide crystal substrates, specific areas of market have not yetbeen determined. "Silicon carbide is still in its infancy inresearch and development," said Young. "This is still fairlyearly in the development process."
Unlike silicon, which melts at 1460 degrees Centigrade,silicon carbide doesn't melt until temperatures of 3000 degreesCentigrade are reached. While silicon carbide is more durablethan silicon, it's also more complex.
"Silicon carbide is a totally different animal than siliconitself," said Young. "Silicon is a single element so you don'thave to worry about any other element."
The compound forces silicon carbide crystals to be createdby a different process. "It's virtually impossible to growsilicon carbide crystal from the traditional method."
AXT's vertical gradient freeze technology provides a lowertemperature gradient than the two traditional methods. "Thehigher the temperature gradient the more the stress will build upin the crystal," Young explained. "Because we don't fight withnature we don't have to pay for it."
AXT achieved a typical growth rate of millimeters per hour. "I think the difficulty is not the growth rate but how well youcan control it," noted Young. "Everything has defects."
AXT has demonstrated 2 inch crystals and is working ongrowing 3 inch silicon carbide crystals.
Young expects commercialization in nine to twelve months. American Xtal Technology is working with leaders in the field tocooperate on the development of better methods.
"We think this is a very exciting market and we want to getinto it," Young said.
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