## 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
The TBR measurements were performed at an interface temperature of GaN/SiC of approximately 160 \({}^{\circ}\)C. Surface pretreatment using H\({}_{2}\) on the SiC substrate was performed at different temperatures, varying from 1200 to 1280 \({}^{\circ}\)C, prior to the growth of GaN/ AlN structures. Characterization using X-ray photoelectron spectroscopy (XPS) revealed that the oxygen- and carbon-related contaminants on the SiC substrates were efficiently removed after H\({}_{2}\) treatment at 1320 \({}^{\circ}\)C. The effective TBR was observed to first decrease from \(\sim\)20 to \(\sim\)14.5 m\({}^{2}\) K/GW as the pretreatment temperature increased from 1200 to 1220 \({}^{\circ}\)C and, subsequently, it increased to \(\sim\)33 m\({}^{2}\) K/GW as the pretreatment temperature reached 1240 \({}^{\circ}\)C. The highest TBR value was measured from the sample containing the superior crystalline quality of the AlN nucleation layer in the series, which could be attributed to the degraded surface morphology of the SiC substrates because of the aggressive H\({}_{2}\) etching process above 1240 \({}^{\circ}\)C. This finding revealed the significance of the interfacial structures in the AlN/SiC and AlN/GaN regions in the TBR. Furthermore, by reducing the thickness of the high-quality AlN nucleation layer from 105 to 35 nm, the TBR was reduced from \(\sim\)33 to \(\sim\)13 m\({}^{2}\) K/GW. [PERSON] et al. [86] compared the TBR values of various GaN-on-SiC device structures from US, Japanese, and European suppliers, which were all fabricated using standard MOCVD methods. The thickness of the AlN nucleation layers ranged from 40 to 200 nm. Measured using Raman thermography, TBR was found to differ by a factor of four between the different device suppliers, ranging from 15 to 50 m\({}^{2}\) K/GW at 150 \({}^{\circ}\)C. Microstructure/defects within the nucleation layer or near the interfaces hinder heat transport by enhancing the phonon scattering rates, resulting in an increase in the effective TBR. The large variation in the TBR values between different suppliers indicates the potential for nucleation-layer growth optimization using standard MOCVD methods. [PERSON] et al. [87] demonstrated that changing over from a MOCVD-grown standard 40 nm thick AlN nucleation layer to a hot-wall MOCVD-grown 80 nm thick AlN nucleation layer could decrease the TBR from \(\sim\)43 to \(\sim\)25 m\({}^{2}\) K/GW at an interface temperature of 100 \({}^{\circ}\)C. Such a decrease in TBR leads to a \(\sim\)10% reduction in the operating temperature of AlGaN/GaN HEMTs. The TEM characterization showed that the AlN nucleation layer, grown using hot-wall MOCVD, was more monocrystalline, thereby resulting in fewer defects and grains and less domain boundary phonon scattering and, consequently, a low TBR. [PERSON] et al. [60] grew AlN thin films on mechanically polished (MP) and chemomechanically polished (CMP) SiC substrates. The TBRs of the AlN/SiC interfaces in the MP and CMP samples were approximately 94 and \(\sim\)5.1 m\({}^{2}\) K/GW. The intrinsic thermal conductivities of the AlN thin films for both types of samples showed no remarkable differences at \(\sim\)47 W/mK. The TBR of \(\sim\)5.1 m\({}^{2}\) K/GW is equivalent to \(\sim\)240 nm of highly dislocated AlN thin films. Figure 5a shows the effective thermal conductivity and total thermal resistance as a function of the AlN film thickness on different substrates. These results indicate that the AlN layer made only a small contribution to the overall thermal resistance. Furthermore,the differences in thermal conductivity between films grown on MP and CMP SiC substrates of differing roughness were found to be more significant than the differences owing to the growth method or substrate material. Atomic force microscopy (AFM) characterization showed that the RMS roughness of MP and CMP substrates are \(\sim\)1.2 and \(\sim\)0.2 nm, respectively.