A room-temperature bonding technique for integrating wide bandgap materials such as GaN with thermally conducting materials such as diamond is being developed, to improve GaN device cooling to achieve higher output power, improved reliability and reduced manufacturing costs.

The technique, called surface-activated bonding, uses an ion source in a high-vacuum environment to clean the surfaces of the GaN and diamond, which activates them by creating “dangling” bonds. Introducing small amounts of silicon into the ion beams helps form strong atomic bonds at room temperature, enabling direct bonding of the GaN with single-crystal diamond to fabricate HEMTs. Performed at room temperature, the new process reduces the thermal stress applied to the device.

“This technique allows us to place high thermal conductivity materials much closer to the active device regions in GaN,” said Samuel Graham, a professor in Georgia Tech’s George W. Woodruff school of mechanical engineering. “The performance allows us to maximize the performance for GaN on diamond systems.”

The resulting interface layer from the GaN to single-crystal diamond is just 4 nm thick, enabling up to 2x better thermal transfer than achieved with previous GaN-on-diamond HEMTs.

GaN-diamond interface
Cross-section of GaN-diamond interfaces bonded by the surface-activated bonding technique.

The new process eliminates lower quality diamond left over from nanocrystalline diamond growth. With current approches, diamond is integrated with GaN using crystalline growth techniques that produce a thicker interface layer and lower quality nanocrystalline diamond near the interface. When diamond films are grown on GaN, they must be seeded with nanocrystalline particles around 30 nm in diameter, and this layer of nanocrystalline diamond has low thermal conductivity, which adds resistance to the flow of heat into the bulk diamond film. The growth takes place at high temperatures, which can create stress-producing cracks in the resulting transistors.

“In the currently used growth technique, you don’t really reach the high thermal conductivity properties of the microcrystalline diamond layer until you are a few microns away from the interface. The materials near the interface just don’t have good thermal properties,” Graham said.

By creating a thinner interface, the surface-activated bonding technique moves the thermal dissipation closer to the GaN heat source.

“Our bonding technique brings high thermal conductivity single crystal diamond closer to the hotspots in the GaN devices, which has the potential to reshape the way these devices are cooled,” said Zhe Cheng, a recent Georgia Tech Ph.D. graduate who is the paper’s first author. “And because the bonding takes place near room temperature, we can avoid thermal stresses that can damage the devices.”

That reduction in thermal stress can be significant, going from as much as 900 to less than 100 MPa with the room temperature technique.

“This low stress bonding allows for thick layers of diamond to be integrated with the GaN and provides a method for diamond integration with other semiconductor materials,” Graham said.

Beyond the GaN and diamond, the technique can be used with other semiconductors, such as Ga2O3, and other thermal conductors, such as SiC. Graham said the technique has broad applications to bond electronic materials where thin interfacial layers are advantageous.

“This new pathway gives us the ability to mix and match materials,” he said. “This can provide us with great electrical properties, but the clear advantage is a vastly superior thermal interface. We believe this will prove to be the best technology available so far for integrating wide bandgap materials with thermally conducting substrates.”

In future work, the researchers plan to study other ion sources and evaluate other materials that could be integrated using the technique.

“We have the ability to choose processing conditions, as well as the substrate and semiconductor material to engineer heterogenous substrates for wide bandgap devices,” Graham said. “That allows us to choose the materials and integrate them to maximize electrical, thermal and mechanical properties.”

The research, which was conducted in collaboration with scientists from Meisei University and Waseda University in Japan, was reported February 19 in the journal ACS Applied Materials and Interfaces.

Georgia Tech researchers
Researchers Cheng Zhe (left) and Samuel Graham (right).

The work was supported by a multidisciplinary university research initiative (MURI) project from the U.S. Office of Naval Research (ONR), grant N00014-18-1-2429.