## 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
These results indicate that the nanocrystalline diamond layer near the nucleation surface contributed to the TBR  
[PERSON] et al. [114] bonded GaN and a single-crystal diamond using two modified SAB techniques and measured the effective TBR of the GaN/diamond interfaces. The TBR of the first sample with a sputtering-deposited 10 nm thick Si interlayer (Samp1) was \(\sim\)19.2 m\({}^{2}\) K/GW. In comparison, a relatively lower TBR of \(\sim\)10.9 m\({}^{2}\) K/GW was achieved by mixing Si atoms into the Ar ion beam during SAB processing in the second sample (Samp2), which formed a \(\sim\)4 nm ultrathin interlayer. Figure 8a shows the temperature dependence of the measured thermal conductivity of the two diamond substrates, the measured thermal conductivity of the GaN layer, the measured TBC of the bonded GaN/diamond interfaces, and the phonon density of states (DOS) of GaN, Si, and diamond. A comparison of the phonon DOS indicates that Si is not an ideal interlayer material from the point of view of phonon DOS mismatch, but it does facilitate strong bonding of GaN with diamond. Therefore, the TBR of bonded GaN/diamond interfaces retains the potential for further reduction using other interfacial layers, such as SiC, AlN, or SiN\({}_{\rm x}\). The cross-sectional TEM images of the two samples in Figure 8b show that the interlayer thickness in Samp2 was much smaller than that in Samp1, resulting in a relatively lower TBR. Furthermore, device modeling showed that the measured TBR could enable high-power GaN devices by fully exploiting the ultrahigh thermal conductivity of single-crystal diamonds. For the modeled devices, the power density of GaN-on-diamond could reach values \(\sim\)2.5 times higher than those of GaN-on-SiC and \(\sim\)5.4 times higher than those of GaN-on-Si with a maximum device temperature of 250 \({}^{\circ}\)C.  
The TBR has been shown to contribute significantly to the total thermal resistance of GaN-on-diamond devices. Accordingly, the effect of the TBR on the channel temperature rise in the thermal design of GaN-on-diamond HEMTs was investigated. [PERSON] et al. [115] showed that the effective TBR could be decreased to 18 m\({}^{2}\) K/GW using a 50 nm thick proprietary transition layer, leading to a more than 25% lower channel temperature rise for GaN-on-diamond HEMTs compared with that of standard GaN-on-SiC HEMTs under a fixed power dissipation condition. [PERSON] et al. [116] investigated the effects of the TBR of a GaN/diamond interface on the overall thermal management of GaN HEMTs. Employing the finite element method, three-dimensional thermal simulation was performed to analyze the heat dissipation capabilities of different thermal designs. The effect of the TBR on the thermal design of GaN-on-diamond HEMTs, including the GaN buffer, diamond substrates, gate-gate pitch spacing, and chip size, was investigated as the TBR increased from 3 to 140 m\({}^{2}\) K/GW. The measured TBR of GaN/diamond interfaces with different interlayers and different growth or integration methods, and the TBRs predicted using simulation methods are summarized in Table 2.  
[PERSON] et al. [117] performed extensive reverse nonequilibrium molecular dynamics (MD) simulations on a GaN/diamond interface, finding that changing the conventional planar interface to nanoengineered, interlaced architecture with optimal geometry resulted in >80% reduction in TBR. Moreover, introducing a conformal graphene buffer layer further reduced the TBR by ~33%. Figure 8: (**a**) Temperature dependence of the measured thermal conductivity of two diamond substrates (**upper left**). Measured thermal conductivity of the GaN layer (**upper right**). Measured TBC of bonded GaN/diamond interfaces (**lower left**), and phonon density of states of GaN, Si, and diamond (**lower right**). (**b**) Cross-sectional HR-STEM images of GaN/diamond interfaces of Samp1 (**upper**) and Samp2 (**lower**). (Reprinted from Ref. [114]. Reproduced with permission from ACS Publications. All rights reserved).