## 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  
Diamond is considered the superior candidate to replace SiC as a substrate for fully exploiting the potential of GaN. Several methods have been employed to integrate GaN with diamond, with the most widely used being the transfer of pregrown GaN from Si to diamond. In this method, the GaN device layer is first grown on a Si substrate, and then polycrystalline diamond is grown by MWCVD on the back of GaN using a transition layer [95]. The second method involves the direct epitaxial growth of GaN on diamond substrates. However, this method is currently uneconomical owing to the unavailability of large diamond substrates [96]. The last method involves direct bonding of diamond wafers to GaN, such as SAB. In this method, the surfaces of diamond and GaN are irradiated simultaneously by an Ar fast atom beam and, after completion of the irradiation process, the diamond and GaN are brought into contact by applying a load for a period [97; 98]. In this section, we review the TBRs of various GaN/diamond interfaces integrated using different methods. Figure 6: TBR of GaN/SiC interface as a function of AlN interlayer thickness [60; 84; 85; 86; 87; 88]. TBR predicted by the DMM is also shown for comparison [91]. Only the data with clear interlayer information are included. The TBRs measured by different research groups differ significantly and show no thickness dependence, ranging from 5 to 100 m\({}^{2}\) K/GW, although the same growth methods are used for growing the AlN nucleation layer. [PERSON] et al. [99] prepared a GaN/diamond interface by directly growing diamonds on GaN (van der Waals bonding). The measured TBR of the GaN/diamond interface was approximately 220 m\({}^{2}\) K/GW, closely matching the value calculated using the weakly bonded AMM model (200 m\({}^{2}\) K/GW). This value is much higher than that of GaN/diamond with an interlayer. These results indicate that a strong bond is crucial for the successful heterogeneous integration of GaN and diamond, despite the interlayer itself having a lower thermal conductivity and additional extrinsic thermal resistance. A suitable interlayer not only protects the GaN surface during growth but also enables carbide bond formation, which greatly increases the interface adhesion energy and, consequently, facilitates phonon transmission. To reduce the high TBR of the weakly bonded GaN/diamond interface, an interlayer is required to enhance bonding between GaN and diamond. Numerous research groups have investigated the effects of the interlayer type, including Si, SiC, SiN, and AlN. [PERSON] et al. [100] prepared two GaN-on-diamond samples (namely, one with diamond grown directly on the AlGaN interlayer and the other incorporating a thin crystalline SiC interlayer between AlGaNN and diamond). The measured effective TBRs were \(\sim\)30 and \(\sim\)107 m\({}^{2}\) K/GW for the sample with a SiC interlayer and without an interlayer, respectively. The reduced TBR was attributed to the improved adhesion between the SiC and diamond compared with diamond directly on AlGaN because of the increased propensity for carbide bond formation between the SiC and diamond. Stronger carbide bonds aid in the transmission of phonons across the interface, improving heat transport. [PERSON] et al. [101] deposited a hot filament (HF) CVD diamond on an AlGaN/GaN HEMT with a 46 nm thick SiN\({}_{\mathrm{x}}\) interlayer. Extremely smooth surface morphology of SiN\({}_{\mathrm{x}}\) was obtained, with an RMS roughness of 0.43 nm. Even with some selective degradation of the in situ SiN\({}_{\mathrm{x}}\) layer, a \(>\)20 nm intact SiN\({}_{\mathrm{x}}\) remained that protected the underlying GaN layers. The effective TBR of the GaN/diamond interface measured by TDTR was \(\sim\)52.8 m\({}^{2}\) K/GW. [PERSON] et al. [102] deposited a thick (\(>\)100 \(\upmu\)m) diamond layer on 250 nm thick AlN layers. These authors found that a thick diamond layer could not be grown on the untreated as-grown AlN surfaces.