Crystalline Interlayers for Reducing the Effective Thermal Boundary Resistance in GaN-on-Diamond

Integrating diamond with GaN high electron mobility transistors (HEMTs) improves thermal management, ultimately increasing the reliability and performance of high-power high-frequency radio frequency amplifiers. Conventionally, an amorphous interlayer is used before growing polycrystalline diamond onto GaN in these devices. This layer contributes significantly to the effective thermal boundary resistance (TBR_eff) between the GaN HEMT and the diamond, reducing the benefit of the diamond heat spreader. Replacing the amorphous interlayer with a higher thermal conductivity crystalline material would reduce TBR_eff and help to enable the full potential of GaN-on-diamond devices. In this work, a crystalline Al_0.32Ga_0.68N interlayer has been integrated into a GaN/AlGaN HEMT device epitaxy. Two samples were studied, one with diamond grown directly on the AlGaN interlayer and another incorporating a thin crystalline SiC layer between AlGaN and diamond. The TBR_eff, measured using transient thermoreflectance, was improved for the sample with SiC (30 ± 5 m² K GW^-1) compared to the sample without (107 ± 44 m² K GW^-1). The reduced TBR_eff is thought to arise from improved adhesion between SiC and the diamond compared to the diamond directly on AlGaN because of an increased propensity for carbide bond formation between SiC and the diamond. The stronger carbide bonds aid transmission of phonons across the interface, improving heat transport.