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

Abstract

Integrating diamond with GaN high electron mobility transistors (HEMTs) improves thermal management, ultimately increasing the reliability and performance of high-power high-frequency 2 RF amplifiers. Conventionally, an amorphous interlayer is used before growing polycrystalline diamond onto the GaN in these devices. This layer contributes significantly to the effective thermal boundary resistance (TBReff) 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 TBReff and help to enable the full potential of GaN-on-diamond devices. In this work, a crystalline Al0.32Ga0.68N 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 the AlGaN and diamond.The TBReff, measured using transient thermoreflectance, was improved for the sample with SiC (30 ± 5 m 2 K GW -1 ) compared to the sample without (107 ± 44 m 2 K GW -1 ). The reduced TBReff is thought to arise from improved adhesion between the SiC and the diamond compared to the diamond directly on the AlGaN due to an increased propensity for carbide bond formation between SiC and diamond. The stronger carbide bonds aid transmission of phonons across the interface, improving heat transport.

Diamond/GaN HEMTs: Where from and Where to?

“…Due to the sample layout, a 10 nm-thick SiC layer was formed between the Al 0.32 Ga 0.68 N/diamond interface, which improved the heat transport across the two materials. A TBR of 30 m 2 •K/GW was measured, a value still much higher than the theoretical minimum of 4 m 2 •K/GW obtained for this interface [106].…”

“…This layer would act as an etch stop during the device epitaxy, as well as a seed layer for the diamond growth [168]. This is the approach proposed by Field and co-workers [106]; since integrating a thin AlN or high Al content AlGaN layers at this point in the epitaxy is challenging because of alloying with surrounding layers, they used a relatively low Al content crystalline Al 0.32 Ga 0.68 N layer as the etch stop and interlayer and grew diamond following the same procedure as in [84]. Due to the sample layout, a 10 nm-thick SiC layer was formed between the Al 0.32 Ga 0.68 N/diamond interface, which improved the heat transport across the two materials.…”

DFT-Based Studies on Carbon Adsorption on the wz-GaN Surfaces and the Influence of Point Defects on the Stability of the Diamond–GaN Interfaces

“…The present paper aims to demonstrate that certain reconstruction patterns involving substitutional atoms can improve the energetic stability of the diamond-GaN interfaces, as was observed in the case of diamond-AlN interfaces [26]. Since the proposed reconstruction patterns take place in the GaN substrate, our findings are closely related to GaN-on-diamond experimental approaches where the formation of carbide bonds [27] or a silicon-carbon-nitrogen layer between GaN and diamond [16] was reported.…”

Thermal stress modelling of diamond on GaN/III-Nitride membranes

“…From a stress perspective, the adhesion strength needs investigation as high stresses in this layer will result in delamination. This layer may be incredibly important for creating an interface with N-polar faces and CVD diamond, subject of another study [60].…”