Effect of AlN interlayer thickness on thermal conductances of GaN epilayer and GaN/SiC interface in GaN-on-SiC heterostructures

The temperature rise in GaN-on-SiC based high electron mobility transistors (HEMTs) is firmly dependent on the thermal conductivity (k) of GaN epilayer and the interfacial thermal conductance (G) between GaN and SiC. The AlN buffer is usually utilized during the heteroepitaxial growth of GaN on SiC substrate, while the effects of its thickness on k and G are still not clear. In this study, the GaN/AlN/SiC multilayer structure is prepared by metal-organic chemical vapor deposition, and aiding by time-domain thermoreflectance, we detect how the thickness of AlN interlayer influences k and G. The results reveal that the AlN interlayer evolves from serrated island shape to smooth planar form with increasing its thickness from 13 to 104 nm, which induces that the tensile stress of the subsequently grown GaN firstly decreases and then increases, giving a minimum value of 339 MPa at 52 nm-thick AlN. Consequently, a maximal k of 150 W m-1 K- 1 for the GaN epilayer is achieved. Moreover, the AlN interlayer is beneficial to the enhancement of G due to the improved overlap of phonon density of states, and an increase of G by up to 64% can be realized via an insertion of 104 nm-thick AlN, which could be the consequence of both atomically smooth interfaces and the improved crystal quality of thicker AlN. The findings clearly manifest the effect of AlN interlayer thickness on the k and G of GaN/AlN/SiC structures, which provides guidelines for preparation of multilayer structures helping to minimize the thermal resistance of HEMTs.