Phonon scattering in chemical-vapor-deposited diamond

The in-plane thermal conductivity κ_∥ has been measured over the temperature range 5--400 K for samples of chemical-vapor-deposited (CVD) diamond made by both the microwave and hot-filament processes. The samples span a range of defect level, grain size, and degree of thinning. Comparison with a model of heat transport suggests that κ_∥ is limited by scattering of phonons from point defects, extended defects of {~}1.5 nm diameter, dislocations, grain boundaries, and microcracks, as well as by phonon-phonon scattering at high temperatures. The Callaway model of thermal conductivity is used to include the effects of normal three-phonon scattering processes. In the higher-conductivity samples, scattering of long-wavelength phonons is very weak even at grain boundaries, indicating relatively smooth boundaries. The value of κ_∥=20 W cm^-1 K^-1 at room temperature for some of the microwave CVD samples is the highest reported to date for CVD diamond. Measurements of the anisotropy in conductivity obtained from the measured perpendicular conductivity κ_⊥ consistently show a higher conductivity along the (columnar) grains. The hot-filament-CVD sample measured exhibits a room-temperature conductivity approaching that of the best microwave-plasma samples, indicating that the thermal conductivity is determined more by the specific conditions of growth than by the type of CVD growth (microwave or hot filament).