## 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-SiC Devices
Compared with that of the CMP substrate, the RMS roughness of the MP substrate, which is six times greater, is considered responsible for its order-of-magnitude-greater TBR. Atomic-resolution TEM imaging revealed that the near-interface planar defects in the AlN films grown on rough SiC were the source of the increased TBR, as shown in Figure 5b. Figure 5: (**a**) Effective thermal conductivity (**left**) and total thermal resistance (**right**) as a function of AlN film thickness on different substrates. These plots indicate that the total thermal resistance mainly derives from the TBR rather than the film itself. (**b**) Comparison of the atomic-resolution TEM images of the interfaces between AlN/CMP SiC and AlN/MP SiC. AlN grown on CMP 4H-SiC substrate has a clearly defined interface with no apparent additional strain in the film (**left**). In contrast, microstructural roughness of the surface of the MP 6H-SiC substrate generates stresses and causes planar defects within the first few atomic layers of the AlN film (**right**). (Reprinted from Ref. [60]. Reproduced with permission from AIP Publishing. All rights reserved). [PERSON] et al. [88] reported that a lower TBR of \(\sim\)20 m\({}^{2}\) K/GW could be achieved by employing an ultrathin AlGaN buffer layer with a low Al content between the GaN films and SiC substrate in comparison with the TBR of \(\sim\)25 m\({}^{2}\) K/GW for the sample with a 90 nm thick conventional AlN buffer layer. An ultrathin AlGaN buffer layer was introduced through trimethylaluminum pretreatment of the SiC substrates. High-resolution XRD and TEM characterization showed that the dislocation density in the GaN layer could be significantly reduced by using an ultrathin AlN buffer layer. The ultrathin buffer layer not only reduces the TBR at the GaN/SiC interface but also improves the crystal quality of the subsequent GaN layer, which is beneficial for improving the device performance. AlN nucleation layers are conventionally used to improve the wettability of GaN and SiC. Furthermore, AlN layers are expected to serve as phonon bridges to enhance thermal transport between GaN and SiC. However, using an AlN nucleation layer introduces extrinsic thermal resistance owing to the intrinsic thermal resistance of the AlN layer and the TBRs of the GaN/AlN and AlN/SiC interfaces. Therefore, the TBRs of the GaN/SiC interfaces without an AlN nucleation layer were also investigated. Using an RF-plasma-assisted MBE method, [PERSON] et al. [89] deposited an epitaxial GaN layer directly on a SiC substrate without a transition layer. The TBR of the GaN/SiC interface was measured at \(\sim\)4.3 m\({}^{2}\) K/GW using the FDTR method, which was significantly lower than the TBR for a GaN/SiC interface with an AlN transition layer. Room-temperature surface-activated bonding (SAB) is a promising technique for heterogeneous integration of semiconductor materials and microelectronic packaging. The SAB technique is insensitive to lattice mismatch and can be performed at room temperature and wafer scale, resulting in low thermal stress. [PERSON] et al. [90] directly bonded high-quality GaN to SiC using the room-temperature SAB method. The TBR of the as-bonded GaN/SiC was measured at \(\sim\)5.9 m\({}^{2}\) K/GW. The TBR decreased to \(\sim\)4.3 m\({}^{2}\) K/GW after annealing at 1273 K for 10 min in flowing N\({}_{2}\) gas, which is almost the same value as the TBR of directly grown GaN on SiC by MBE. High-resolution scanning TEM and electron energy loss spectroscopy (EELS) were used to study the interface structure and local chemical distribution. The results showed that both the interfacial amorphous layer and interfacial mixing caused by diffusion could have contributed to the low TBR of the as-bonded interface. The decrease in the TBR after annealing is attributed to the disappearance of the amorphous layer and redistribution of Ar atoms.