Researchers study ultralow wear of GaN
GaN has emerged as one of the most important and widely used semiconducting materials. Now, four Lehigh University engineers have reported a lesser known property for GaN: Its wear resistance approaches that of diamonds and promises to open up applications in touch screens, space vehicles and RF MEMS, all of which require high-speed, high-vibration technology.
The researchers reported their findings in August in Applied Physics Letters (APL) in an article titled "˜Ultralow wear of gallium nitride' by Guosong Zeng et al.
GaN's electronic and optical properties have been studied extensively for several decades, said Zeng, the lead author of the APL article, but virtually no studies have been done of resistance to the mechanical wear imposed by reciprocated sliding. "Our group is the first to investigate the wear performance of GaN," said Zeng. "We have found that its wear rate approaches that of diamonds, the hardest material known."
Wear rate is expressed in negative cubic millimeters of Newton meters (Nm). The rate for chalk, which has virtually no wear resistance, is on the order of 10 2 mm3/Nm, while that of diamonds is between 10-9 and 10-10, making diamonds eight orders of magnitude more wear resistant than chalk. The rate for GaN ranges from 10-7 to 10-9, approaching the wear resistance of diamonds and three to five orders of magnitude more wear resistant than silicon (10-4).
Nelson Tansu, one of the paper's authors, said the group's discovery of GaN's hardness and wear performance could have a dramatic effect on the electronic and digital device industries. In a device such as a smartphone, he said, the electronic components are housed underneath a protective coating of glass or sapphire. This poses potential compatibility problems which could be avoided by using GaN.
"The wear resistance of GaN," said Tansu, "gives us the opportunity to replace the multiple layers in a typical semiconductor device with one layer made of a material that has excellent optical and electrical properties and is wear-resistant as well.
"Using GaN, you can build an entire device in a platform without multiple layers of technologies. You can integrate electronics, light sensors and light emitters and still have a mechanically robust device. This will open up a new paradigm for designing devices. And because GaN can be made very thin and still strong, it will accelerate the move to flexible electronics."
In addition to its unexpectedly good wear performance, said Zeng, GaN also has a favorable radiation hardness, which is an important property for the solar cells that power space vehicles.
The Lehigh group has begun collaborating with Bruce E. Koel, a surface chemistry expert and professor of chemical and biological engineering at Princeton University, to gain a better understanding of the interaction of GaN and water under contact. Koel was formerly a chemistry professor and vice president for research and graduate studies at Lehigh.
