(Invited) Challenges and Opportunities for GaN on Diamond Devices

The integration of CVD diamond is seen as an important technology for the passive thermal management of high power GaN devices. Diamond films are often attached to GaN devices either through direct growth on the GaN or through a plasma activated attachment process. Regardless of the method used, a thermal resistance between the GaN and diamond will exist at the interface either through the nucleation and growth of low thermal conductivity diamond at the interface or through a large phonon mismatch at the GaN diamond interace for the bonded approach. While recent demonstrations have shown the benefits of using CVD diamond in lowering the temperature of GaN HEMTs, the theoretical benefits of CVD diamond have not been achieved due to the need to control thermal boundary resistance and the elimination of low conductivity diamond near the interace with GaN. In this work, we will present data on the thermal performance of GaN on Diamond HEMTs fabricated using bonding as well as CVD growth on the GaN. To better understand the thermal response of these devices, the thermal conductivity of the diamond and quality of the diamond measured using Raman spectroscopy and microscopy approaches will be presented. In addition, the thermal boundary resistance between the GaN and diamond will be presented. A link between the thermal conductivity, thermal boundary resistance, and the bonding or growth approach will be used to elucidate their impact on the total thermal resistance of the GaN on Diamond system. Furthermore, the mechanical integrity of the GaN on diamond interface which can lead to a degradation in TBR measured using a transient thermoreflectance method will be shown. Finally, the temperature distribution in GaN on Diamond HEMTs will be measured and correlated to models of the devices employind properties measured in our experiments. Finally, the propspects for GaN on Diamond based on CVD growth and direct bonding approaches will be discussed.