Investigation of Bond Quality and Interfacial Strain at a Room Temperature Bonded GaN-Diamond Interface via Raman Spectroscopy

Vertical gallium nitride (GaN) power devices have the potential to improve power conversion efficiency in electrical circuits due to enhanced mobility and high critical electric field. Since device performance and reliability are directly related to thermal performance, there is significant interest in improving options for passive thermal management. To this end, the heterogeneous integration of diamond with vertical GaN devices is explored. In this current work, the bond quality of a bulk GaN substrate with single-crystal diamond via an intermetallic surface activated Au bond is investigated. The intermetallic layer in the GaN-diamond stack potentially creates a phonon bridge for enhanced heat dissipation out of GaN and into the diamond. However, it is unclear whether/how the bonding will introduce strain in the GaN crystal, which can negatively impact electrical performance and device reliability. Here, we use high-resolution micro-Raman spectroscopy to characterize the strain in the GaN near the interface. The quality of the interfacial bonding between GaN and diamond is evaluated via Scanning Acoustic Microscopy (C-SAM). The C-SAM images are referenced to bonded and non-bonded regions for further investigation via micro-Raman. The micro-Raman maps revealed localized regions of strain when moving from a non-bonded to bonded area. This result highlights two important aspects of the interface when optimizing for thermal boundary conductance: 1) Interfacial bonding is not necessarily uniform and requires spatial mapping for full characterization. 2) Although bonded and non-bonded regions of an interface may not show strain changes post bonding, the transition between these regions may be strained and have an impact on the device performance. Moving forward, Raman strain maps and C-SAM images of bonded and non-bonded regions will be combined with thermal mapping, via frequency-domain thermoreflectance (FDTR), to fully characterize the thermal and mechanical properties across an entire interfacial region. This work will provide additional insight into the inhomogeneity of a room temperature bonded GaN-diamond interface and the subsequent non-uniform strain profiles that exist near bonding sites. (SAND Number: SAND2022-14970 A)<br/><br/><b>Acknowledgements</b>: Sandia National Laboratories is a multi-mission laboratory managed and operated by the National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract No. DE-NA0003525.