Gallium Nitride Tribology
December 1, 2016
The first light-emitting diodes (LEDs) produced light in the infrared. While they were not useful for display applications, they enabled optical free-space communication devices, such as television remote controls, and fiberoptic communication over short links. The first visible light LED, a red light emitter, was invented by Nick Holonyak, Jr., in 1962. This led to a plethora of LED numerical displays and indicator lamps on electronic equipment.
In just a few years thereafter, new materials enabled fabrication of LEDs in colors from green through red, with yellow and orange in between. However, blue light LED emission was elusive for the principal reason that blue light is energetic, so a high energy electron transition is required for its emission as well as a material that can host such a transition.
A decade after the invention of the red LED, researchers at Stanford University made a blue LED from magnesium-doped gallium nitride (GaN) combined with an n-doped layer of gallium nitride, prepared on a sapphire substrate. Undoped gallium nitride is n-type, as grown, with a carrier concentration greater than 1018 cm-3. The addition of magnesium produces deep acceptors that compensates the native donors in GaN to make it intrinsic. This allowed the formation of an intrinsic-n junction diode.
Blue LEDs finally arrived when Shuji Nakamura discovered an inexpensive thermal annealing technique to fabricate p-type GaN. Nakamura shared the 2014 Nobel Prize in Physics with two other Japanese scientists for their work on blue LEDs.
The award of this Nobel Prize was not without controversy, since no Nobel Prize was awarded to anyone involved with the initial discovery of LEDs. My analysis of the matter is that there were so many of these early LED researchers that it's hard to select certain prominent individuals from that group. Interestingly, the LED award follows also in the "tradition" of the Nobel Prize for the laser, in which Theodore Maiman, who made the first working laser, was excluded.
The Nobel Prizes are awarded "to those who, during the preceding year, shall have conferred the greatest benefit on mankind." While it's nice to have an LED alarm clock, blue LEDs have enabled replacement of less efficient lighting technologies, and such an energy savings has a definite benefit to mankind.
While the electronic and optical properties of gallium nitride have been thoroughly investigated, there's still much to learn about its mechanical properties. Nitrides are diverse materials, with boron nitride (BN) having graphite-like properties, and titanium nitride (TiN) being an extremely hard ceramic. Titanium nitride is the golden coating seen on drill bits that increases their useful life.
Mechanical engineers and electrical engineers at Lehigh University (Bethlehem, Pennsylvania) have looked at the tribological (friction/wear) properties of GaN, and they found that it is extremely wear resistant, approaching the wear rates reported for diamond.[3-4] The research was conducted by Guosong Zeng, a graduate student in mechanical engineering, Nelson Tansu, a professor of Electrical and Computer Engineering at Lehigh and director of the Lehigh Center for Photonics and Nanoelectronics, Brandon A. Krick, an assistant professors of Mechanical Engineering and Mechanics, and Chee-Keong Tan, a recent Lehigh Ph.D. who is now an assistant professor of Electrical and Computer Engineering at Clarkson University.
This research group was the first to examine the wear performance of gallium nitride. Says paper co-author, Brandon Krick,
"Nelson [Tansu] asked me if anyone had ever investigated the friction and wear properties of gallium nitride... and I said I didn't know. We checked later and found a wide-open field."
The research team used a custom microtribometer to measure the wear rate and friction coefficients of GaN by dry sliding wear experiments. Wear is typically seen for most materials after just a thousand sliding cycles, but the GaN wear wasn't evident for 30,000 cycles. They found that the wear rate ranged from 10-7 to 10-9 mm3/Nm. For comparison, the wear rate for chalk is the high value of 102, silicon is 10-4, and diamond ranges from 10-9 to 10-10.
The wear resistance depended on the crystallographic direction, with sliding in the <⟨1210⟩> direction having significantly lower wear than for <⟨1100⟩>. One surprise was the affect of humidity. Wear increases by two orders of magnitude with a relative humidity change from 0% to 50%. Says Zeng,
"The first time we observed the ultralow wear rate of GaN was in winter... These results could not be replicated in summer, when the material's wear rate increased by two orders of magnitude."
Subsequent experiments were undertaken under controlled humidity. It's suspected that humidity strains the surface lattice, leading to the enhanced wear.
Since GaN has excellent radiation resistance, it's a candidate material for solar cells on spacecraft. The wear resistance will inhibit erosion by cosmic dust. Tansu points out that the wear properties of GaN, combined with its high strength, would allow thin layers to be used in flexible electronics.
- D. Stevenson, W. Rhines, and H. Maruska, "Gallium nitride metal-semiconductor junction light emitting diode, US Patent No. 3,819,974, June 25, 1974.
- Full Text of Alfred Nobel's Will, Nobelprize.org
- Guosong Zeng, Chee-Keong Tan, Nelson Tansu, and Brandon A. Krick, "Ultralow wear of gallium nitride," Appl. Phys. Lett., vol. 109, no. 5 (August 3, 2016), Article No. 051602, http://dx.doi.org/10.1063/1.4960375.
- Researchers surprised at the unexpected hardness of gallium nitride, Lehigh University Press Release, October 28, 2016.
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