## 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
Using a combination of Raman thermography and thermal modeling, [PERSON] et al. [109] investigated the role of the diamond substrate thermal conductivity and GaN/diamond TBR in determining the thermal resistance of GaN-on-diamond devices. Two samples were prepared, one with a 95 \(\upmu\)m thick HFCVD diamond layer and a 25 nm thick dielectric interlayer and the other with a 120 \(\upmu\)m thick MPCVD diamond layer and a 50 nm thick dielectric interlayer. Based on analyses of the lateral and depth temperature profiles, effective substrate thermal conductivities of \(\sim\)710 and \(\sim\)1200 W/mK were obtained for the HFCVD and MPCVD polycrystalline diamond wafers, respectively. Effective TBRs of \(\sim\)27 and \(\sim\)36 m\({}^{2}\) K/GW were measured for GaN/diamond interfaces with 25 nm and 50 nm thick proprietary dielectric interlayers, respectively. [PERSON] et al. [110] deposited polycrystalline diamond on GaN-based HEMTs using MPCVD. By reducing the thickness of the Si\({}_{3}\)N\({}_{4}\) interlayer to \(\sim\)1 nm, a record low TBRof \(\sim\)3.1 m\({}^{2}\) K/GW was achieved without damaging the electrical performance of the GaN channel, which is close to the values predicted by DMM. However, if the SiN\({}_{\rm x}\) interlayer is too thin, the harsh H\({}_{2}\)-plasma diamond growth conditions could etch the GaN device layer underneath and degrade its electrical performance. This challenge requires solving. [PERSON] et al. [111] investigated the effects of a SiN\({}_{\rm x}\) interlayer structure on the effective TBR of a GaN/diamond interface. Three types of SiN\({}_{\rm x}\) interlayers were used: 100 nm thick SiN\({}_{\rm x}\), 80 nm thick SiN\({}_{\rm x}\), and 100 nm thick SiN\({}_{\rm x}\) with a 20 nm \(\times\) 20 nm periodic structure. The effective TBR of the GaN/diamond interfaces was measured with TDTR. The results show that the effective TBR was reduced from \(\sim\)40.5 to \(\sim\)38.8 m\({}^{2}\) K/GW when the thickness of the SiN\({}_{\rm x}\) interlayer was decreased from 100 to 80 nm. The effective TBR was reduced from \(\sim\)40.5 to \(\sim\)32.2 m\({}^{2}\) K/GW when a 100 nm thick, 20 nm \(\times\) 20 nm periodically patterned SiN\({}_{\rm x}\) interlayer was used to replace the 100 nm thick SiN\({}_{\rm x}\) with no pattern. Imaging employing TEM showed that the periodically patterned structure formed a wavelike SiN\({}_{\rm x}\)/diamond interface, which increased the interfacial contact area and phonon transmission efficiency. In addition, the periodic structure improved the interface bonding strength and seeding density, further enhancing the interfacial heat transfer. [PERSON] et al. [112] deposited polycrystalline diamonds on GaN using a 30-40 nm thick amorphous SiN\({}_{\rm x}\) interlayer. At the beginning of diamond nucleation, all samples were pretreated in CH\({}_{4}\)/H\({}_{2}\) mixtures at positive bias voltage of 400 to 700 V for 60 min at approximately 700 \({}^{\circ}\)C. The lowest and highest effective TBRs were measured at \(\sim\)26 and \(\sim\)83 m\({}^{2}\) K/GW for the samples prepared under 700 V and 600 V bias nucleation conditions, respectively. Characterization employing TEM showed that the thicknesses of the SiN\({}_{\rm x}\) interlayers for the two samples were \(\sim\)35 and \(\sim\)70 nm, respectively. Furthermore, Raman spectroscopy showed that different bias voltages led to changes in the interface roughness and microstructure of the transition layer, resulting in differing TBRs. [PERSON] et al. [113]. fabricated GaN/diamond interfaces using a 40 nm thick amorphous SiN\({}_{\rm x}\) interlayer. Two samples were prepared: one with a thin diamond nucleation/transition region (<10 nm) at the interface and the other with a thicker nucleation/transition region (estimated 50-100 nm). This variation was controlled using different seeding methods for diamond growth. The former and latter have an effective TBR of \(\sim\)26 and \(\sim\)33 m\({}^{2}\) K/GW, respectively.