Scientists make 2D GaN
Above: An illustration of the Migration Enhance Encapsulated Growth (MEEG) process
Penn State University materials scientists have developed the first-ever growth of 2D GaN using graphene encapsulation. They believe the work could lead to applications in deep ultraviolet lasers, next-generation electronics and sensors.
"These experimental results open up new avenues of research in 2D materials," says Joshua Robinson, associate professor of materials science and engineering.
GaN in its 3D form is known to be a wide-bandgap semiconductor. Wide-bandgap semiconductors are important for high frequency, high power applications. When grown in its 2D form, GaN transforms from a wide-bandgap material to an ultrawide-bandgap material, effectively tripling the energy spectrum it can operate in, including the whole ultraviolet, visible and infrared spectrum. This work will have a particular impact on electro-optic devices that manipulate and transmit light.
"This is a new way of thinking about synthesising 2D materials," said Zak Al Balushi, a Ph.D. candidate coadvised by Robinson and Joan Redwing, professor of materials science and engineering and electrical engineering. Al Balushi is lead author on a paper appearing online (Aug.29) in the journal Nature Materials titled 'Two-Dimensional GaN Realized via Graphene Encapsulation'.
"We have this palette of naturally occurring 2D materials," he continued. "But to expand beyond this, we have to synthesize materials that do not exist in nature. Typically, new material systems are highly unstable. But our growth method, called Migration Enhanced Encapsulated Growth (MEEG), uses a layer of graphene to assist the growth and stabilise a robust structure of 2D GaN."
The graphene is grown on a substrate of SiC. When heated, the silicon on the surface decomposes and leaves a carbon-rich surface that can reconstruct into graphene. The advantage of producing the graphene in this way is that the interface where the two materials meet is perfectly smooth.
Robinson believes that in the case of 2D GaN, the addition of a layer of graphene makes all the difference. Graphene, a one-atom-thick layer of carbon atoms, is known for its remarkable electronic properties and strength.
"It's the key," Robinson says. "If you try to grow these materials the traditional way, on SiC, you normally just form islands. It doesn't grow in nice layers on the SiC."
When gallium atoms are added to the mix, they migrate through the graphene and form the middle layer of a sandwich, with graphene floating on top. When nitrogen atoms are added, a chemical reaction takes place that turns the gallium and nitrogen into GaN.
"The MEEG process not only produces ultra-thin sheets of GaN but also changes the crystal structure of the material, which may lead to entirely new applications in electronics and optoelectronics," said Redwing.
Additional coauthors include Ke Wang, Rafael Vila, Sarah Eichfield, Yu-Chuan Lin and Shruti Subramanian of Penn State, Ram Krishna Ghosh and Suman Datta of Notre Dame, Joshua Caldwell, US Naval Research Laboratory, Xiaoye Qin and Robert Wallace The University of Texas at Dallas and Dennis Paul, Physical Electronics USA.
The Asahi Glass, Japan, and the US National Science Foundation provided funding for this project. Other funding was provided by the Alfred P. Sloan Foundation, the Penn State Materials Characterisation Laboratory and the Center for Low Energy Systems Technology (LEAST), funded by the Semiconductor Research Corporation and DARPA.
