  To achieve high device performance and high reliability for the gallium
nitride (GaN)-based high electron mobility transistors (HEMTs), efficient heat
dissipation is important but remains challenging. Enormous efforts have been
made to transfer a GaN device layer onto a diamond substrate with a high
thermal conductivity by bonding. In this work, two GaN-diamond bonded
composites are prepared via modified surface activated bonding (SAB) at room
temperature with silicon interlayers of different thicknesses (15 nm and 22
nm). Before and after post-annealing process at 800 oC, thermal boundary
conductance (TBC) across the bonded interface including the interlayer and the
stress of GaN layer are investigated by time-domain thermoreflectance and Raman
spectroscopy, respectively. After bonding, the 15 nm Si interlayer achieved a
higher TBC. The post-annealing significantly increased the TBC of both
interfaces, while the TBC of 22 nm silicon interlayer increased greater and
became higher than that of 15 nm. Detailed investigation of the microstructure
and composition of the interfaces were carried out to understand the difference
in interfacial thermal conduction. The obtained stress was no more than 230 MPa
for both before and after the annealing, and this high thermal stability of the
bonded composites indicates that the room temperature bonding can realize a
GaN-on-diamond template suitable for further epitaxial growth or device
process. This work brings a novel strategy of SAB followed by high-temperature
annealing to fabricate a GaN-on-diamond device with a high TBC.
