Interfacial fracture toughness of GaN film on diamond substrate for application in ultra-high power RF devices

Achieving a mechanically stable interface with low thermal boundary resistance in GaN-on-diamond is crucial for reliable operation of high-power devices . As one cannot grow diamond directly to GaN due to crystallographic restrictions, an interlayer of SiN_x is usually introduced between GaN and diamond. This layer not only affects the thermal resistance of the heterogeneously integrated GaN-on-diamond material, but also impact its mechanical strength. In this work, a series of GaN-on-diamond materials with varying SiN_x thicknesses were studied. First of all, the GaN/diamond interfacial toughness was measured by nanoindentation. Secondly, the effective thermal boundary resistance was evaluated by transient thermoreflectance method. In addition, to understand the complex stress-strain field at the interface, a 2D axisymmetric finite element model has been established and validated by experimental results. It was found that the reduction in SiNx interlayer thickness benefits the thermal properties of GaN-on-diamond without compromising its mechanical stability.