## 3 Effects of TBR on Thermal Management in GaN-on-SiC and GaN-on-Diamond Devices
### Effects of TBR on Thermal Management in GaN-on-Diamond Devices
However, the successful growth of a thick diamond layer was achieved on AlN surfaces pretreated with 10% N\({}_{2}\)/H\({}_{2}\) plasma for a minimum of 10 min. The effective TBR of the diamond/AlN interface was measured at approximately 16 m\({}^{2}\) K/GW. Characterization employing XPS revealed that pretreatment increased the oxygen content on the AlN surface. After pretreatment, O-terminated seeds led to reduced stress at the AlN/diamond interface, resulting in a low TBR. [PERSON] et al. [103] fabricated various GaN/diamond interfaces by growing a polycrystalline diamond layer on a GaN device, with SiN and AlN barrier layers, as well as without any barrier layer, using the MPCVD method. These authors measured and systematically compared the effective TBRs of the GaN/diamond interfaces. The results show that an extremely low TBR of \(\sim\)6.5 m\({}^{2}\) K/GW was obtained by using a 5 nm thick SiN barrier layer, whereas the TBR was \(\sim\)15.9 and \(\sim\)61.1 m\({}^{2}\) K/GW for the GaN/diamond interfaces formed by using an AlN barrier layer and without any barrier layer, respectively. In comparison, the DMM-predicted TBR between GaN and diamond was \(\sim\)3 m\({}^{2}\) K/GW. Furthermore, no clear correlation was observed between the TBR and diamond growth conditions, such as growth temperature and growth recipes, as shown in Figure 7a,b. The cross-sectional TEM images of the GaN/diamond interfacial region in Figure 7c shows that the low effective TBR of the GaN/SiN/diamond structures could be attributed to the smooth diamond/SiN and SiN/GaN interfaces, leading to low phonon scattering rates. For the GaN/AlN/diamond structures, both the rougher interface and the thicker diamond nucleation layer are responsible for the higher TBR compared with those grown with the SiN barrier layer. When no barrier layer was used, an even larger roughness was observed at the interface between GaN and diamond, resulting in significantly higher TBR. [PERSON] et al. [104] prepared various GaN/diamond interfaces by growing diamonds on three separate structures (namely, a 5 nm thick SiN interfacial layer, a 5 nm thick AlN interfacial layer, and no interfacial layer). The measured effective TBRs of these GaN/diamond interfaces were \(\sim\)9.5, \(\sim\)18.2, and \(\sim\)41.4 m\({}^{2}\) K/GW for the SiN and AlN interfacial layers, and no interlayer, respectively. The results showed a trend in TBR difference similar to that measured by [PERSON] et al. Cross-sectional TEM imaging demonstrated etching of the GaN device layer by the harsh diamond growth environment when an AlN interfacial layer was used, resulting in a rough interface and increased TBR. When no interlayer was used, the diamond was delaminated completely from the GaN layer for most samples, leading to a significantly high TBR. In comparison, high-resolution TEM imaging and EELS analysis revealed that SiN acts as an etch barrier between the diamond and GaN, and a relatively smooth and ordered elemental transition appears throughout the interlayer, thereby reducing disorder and enhancing phonon transport across the GaN/diamond interface. [PERSON] et al. [105] investigated the effects of interlayer materials on the TBR of a diamond/GaN interface using ab initio calculations incorporating the phonon Boltzmann transport equations. The TBRs of three diamond/GaN interfaces with 5 nm thick Si\({}_{3}\)N\({}_{4}\), 5 nm thick AlN, and 5 nm thick Si interlayers were calculated at \(\sim\)4.58, \(\sim\)5.04, and \(\sim\)8.48 m\({}^{2}\) K/GW, respectively. This trend in the TBR difference is in good agreement with the experimental results reported by [PERSON] et al. and [PERSON] et al. In addition, the effect of  
Figure 7: (**a**) Effective TBRs of GaN/diamond interfaces with different interlayers prepared under various diamond growth temperatures. (**b**) Effective TBRs of GaN/SiN/diamond samples as a function of growth temperature for different growth recipes.