PHONON-SCATTERING IN CHEMICAL-VAPOR-DEPOSITED DIAMOND

The in-plane thermal conductivity kappa(parallel-to) has been measured over the temperature range 5-400 K for samples of chemical-vapor-depo sited (CVD) diamond made by both the microwave and hot-filament proces ses. The samples span a range of defect level, grain size, and degree of thinning. Comparison with a model of heat transport suggests that k appa(parallel-to) is limited by scattering of phonons from point defec ts, extended defects of approximately 1.5 nm diameter, dislocations, g rain boundaries, and microcracks, as well as by phonon-phonon scatteri ng at high temperatures. The Callaway model of thermal conductivity is used to include the effects of normal three-phonon scattering process es. In the higher-conductivity samples, scattering of long-wavelength phonons is very weak even at grain boundaries, indicating relatively s mooth boundaries. The value of kappa(parallel-to) = 20 W cm-1 K-1 at r oom temperature for some of the microwave CVD samples is the highest r eported to date for CVD diamond. Measurements of the anisotropy in con ductivity obtained from the measured perpendicular conductivity kappa( perpendicular-to) consistently show a higher conductivity along the (c olumnar) 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 (m icrowave or hot filament).