Probing local thermal conductivity variations in CVD diamond with large grains by time-domain thermoreflectance

Chemical vapor deposited (CVD) diamond, due to its high thermal conductivity, is an attractive candidate for thermal management of GaN-based high-electron mobility transistors (HEMTs). However, because of its heterogeneous grain structure, CVD diamond has a spatially inhomogeneous thermal conductivity at the microscale. To understand this inhomogeneity and the effect of structural imperfections on thermal conduction, time-domain thermoreflectance (TDTR) is used to study the local thermal conductivity of two samples: a heavily boron-doped ~534 µm-thick diamond sample with an average surface grain size of ~23 µm, and an undoped diamond sample that was cut from a bulk piece of CVD diamond. For the doped diamond, large thermal conductivity variations (of nearly 50 %) are observed across the surface of the sample. For the undoped sample, the large average grain size (several hundred µm) results in a high local thermal conductivity (>2000 W/m-K, close to the conductivity of bulk diamond). The thermal conductivity is not seen to change significantly with grain size (127 - 260 µm), and we measure up to ~8 % variation in the local thermal conductivity. We speculate that grain boundary scattering affects phonon transport differently in the two samples, possibly due to varying amounts of near-boundary disorder. This work provides insights to understand the local thermal conductivity inhomogeneity and phonon transport across grain boundaries in CVD diamond with large grains, which is important for thermal management applications in high-power electronics.