## 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
(**c**) Cross-sectional TEM images of GaN-on-diamond interfaces grown with different interlayers of SiN (**left**), AlN (**middle**), and no interlayer (**right**). (Reprinted from Ref. [103]. Reproduced with permission from ACS Publications. All rights reserved). the interlayer thickness on the TBR was investigated. The results showed that the optimal interlayer thicknesses were approximately 50-60 nm for the AlN interlayer and 70-80 nm for the Si\({}_{3}\)N\({}_{4}\) interlayer, matching the spectral phonon transport features. The difference between the experimental and calculated results could be ascribed to the calculations assuming perfectly crystalline interlayer materials without impurities or structural defects. The interlayer materials used in the experiments were either amorphous or contained numerous defects, impurities, and dislocations. In addition, at the nanoscale level, the TBRs measured by different research groups showed substantial measurement uncertainties. [PERSON] et al. [106] measured the effective TBRs of two types of GaN/diamond interfaces. One had a 100 nm thick SiN interlayer and the other had a 100 nm thick AlN interlayer. The SiN and AlN interlayers were deposited using radio frequency (RF) magnetron sputtering. The TBRs of samples with the SiN and an AlN interlayers were measured at \(\sim\)38.5 and \(\sim\)56.4 m\({}^{2}\) K/GW, respectively, i.e., much higher than the TBRs measured by [PERSON] et al. and [PERSON] et al. Typically, crystalline SiN and AlN exhibit high thermal conductivities. However, amorphous SiN and AlN thin films deposited by sputtering exhibit much lower thermal conductivities. Therefore, the measured highly effective TBR could be attributed to a thicker interlayer with low thermal conductivity. In addition to the effects of interfacial roughness on the TBR characterized by TEM imaging, the peak shift by XPS analysis demonstrated that compared with AlN, the enhancement in nanodiamond seeding attachment facilitated diamond nucleation and diamond growth on the SiN interlayer, resulting in a lower TBR. Furthermore, interfacial adhesion evaluation using a microscratch test showed that stronger Si-C bonding during diamond nucleation was beneficial for strong film adhesion and lower TBR when SiN was used as an interlayer. In addition to the effects of the interlayer type, the effects of the SiN interlayer thickness on the TBR have been investigated by several research groups. [PERSON] et al. [107] measured the effective TBRs of a series of GaN/diamond interfaces with SiN\({}_{x}\) interlayers. Two diamond-growth methods were used (namely, HFCVD and MPCVD). Seventeen wafers were prepared, with a SiN\({}_{x}\) layer thickness ranging from 28 to 100 nm. These authors found the effective TBR scales with the thickness of the SiN\({}_{x}\) interlayers. However, no significant differences were observed among the different diamond growth methods. The effective TBR could be reduced from \(\sim\)50 to \(\sim\)12 m\({}^{2}\) K/GW by decreasing the thickness of SiN\({}_{x}\) from 100 to 28 nm. From the dependence of TBR on the interlayer thickness, the thermal conductivity of the amorphous SiN\({}_{x}\) layer is estimated at \(\sim\)1.9 W/mK, in agreement with the values for amorphous silicon nitride thin films. [PERSON] et al. [108] observed reduction by a factor of two in TBR for samples with SiN of different thicknesses. The measured effective TBRs of GaN/diamond interfaces with 22-nm-thick and 31-nm-thick SiN interlayers were \(\sim\)19.8 and \(\sim\)31.8 m\({}^{2}\) K/GW, respectively. Furthermore, these authors calculated the GaN/SiN TBR at \(\sim\)1 m\({}^{2}\) K/GW and the SiN/diamond at \(\sim\)2 m\({}^{2}\) K/GW using the DMM. Using the calculated DMM value, the intrinsic thermal conductivity of the SiN interlayer was calculated at 1.1-1.5 W/mK. These results show that the low thermal conductivity of the amorphous SiN interlayer is a bottleneck for heat dissipation that needs to be minimized to fully exploit the ultrahigh thermal conductivity of diamond.