Large-area single-crystal “mosaic” diamond wafers, produced by homoepitaxial diamond chemical vapor deposition (CVD) on seed crystals aligned in close proximity, are of increasing interest for high-power electronic and optical applications, not least due to their extraordinary thermal properties. However, thermal conductivity might be reduced if a significant thermal-barrier resistance (TBR) forms on junctions between the single-crystal blocks of the mosaic. Here, using a steady-state longitudinal heat-flow method, we measure with a high accuracy the in-plane thermal conductivity $\kappa (T)$ in the broad temperature range of 6–410 K for a diamond mosaic crystal grown by microwave plasma CVD. At room temperature, the conductivity as high as $24.0\pm 0.5\mathrm{W}{\mathrm{cm}}^{-1}{\mathrm{K}}^{-1}$ is determined within a single block, reducing by less than 2% only for $\kappa (T)$ measured across the junction due to low enough TBR of approximately ${10}^{-4}{\mathrm{cm}}^2\mathrm{K}{\mathrm{W}}^{-1}$. However, below 100 K the TBR strongly increases with the temperature decrease, resulting in a dramatic reduction of the mosaic thermal conductivity compared to that for the single-crystal block. We associate the appearance of TBR with a layer (approximately equal to $20\mu \mathrm{m}$ thick) of defected and stress material near the junction, as revealed with a confocal Raman mapping and transmission electron microscopy. The phonon scattering from defects in the zone around the junction strongly reduces the local thermal conductivity, as confirmed by modeling of heat transport. The observed temperature dependence $\kappa (T)\sim T^2$ of the local conductivity at $T<83$ K suggests the dominance of the phonon-dislocation scattering among other scattering processes. Our results show that the diamond mosaics preserve excellent thermal properties of the constituent single-crystal blocks at room and higher temperatures, and can be effectively used in the applications where the thermal conductivity is of primary relevance.

• Received 22 April 2021
• Revised 17 June 2021
• Accepted 30 June 2021

DOI:https://doi.org/10.1103/PhysRevApplied.16.014049