A nanoindentation induced blistering method has been used to extract the GaN/diamond interfacial toughness (adhesion energy) from four types of GaN-on-diamond samples with varying SiN_x interlayer thicknesses. The mode I energy release rate (G_IC) was quantified and is presented. Additionally, transient thermoreflectance has been used to measure the thermal boundary resistance (TBR) between the GaN and the diamond substrate. It was found that a thin SiN_x interlayer resulted in a lower TBR (15 m² K GW^-1) whilst maintaining a reasonable interfacial toughness (1.4±0.5 J m^-2). For interlayers of a similar thickness, samples with a high interfacial toughness and high residual stresses in the GaN had a smaller TBR. This indicates that the intrinsic interfacial characteristics that enhanced the interfacial toughness could be beneficial in improving the TBR.

4B.4 Final.2025

Abstract
Herein, we demonstrate top, bottom, and double-side thermal management strategies for gallium nitride (GaN) high electron mobility transistors (HEMTs). The cooling technologies investigated include GaN/SiC (reference), GaN/diamond (bottom-side), diamond/GaN/SiC (top-side), and diamond/GaN/diamond (double-side). We review processing methods to realize these device structures as well as the intricacies of the fabrication process. From DC output characteristics, the diamond/GaN/diamond HEMTs demonstrate over 0.6 A/mm at VGS = 2 V. From a thermal perspective, the double-side diamond cooling approach enabled operation at DC power densities of ~30 W/mm with a peak temperature rise of ~50 K at the drain-side edge of the gate electrode. Finally, we demonstrate our initial efforts towards diamond encasement of AlGaN/GaN epilayers to further reduce device-level thermal resistance.