Laser-induced transient gratings application for measurement of thermal conductivity of CVD diamond

In-plane D-II, (k(II)) and normal D-perpendicular to (k(perpendicular to)) thermal diffusivity (thermal conductivity) of CVD diamond plates have been measured by method of transient gratings and laser flash technique. Samples of insulating and semiconducting boron-doped polycrystalline diamond were synthesized in microwave plasma maintained in methane-hydrogen mixtures. Averaged over the film thickness k(II) values at room temperature are determined to be in the range of 15-20 W/cmK approaching to those known for single crystal diamonds. From comparison of k(II) and k(perpendicular to) values a thermal anisotropy of 10-20% for in-plane and normal directions (k(perpendicular to) > k(II)) is found, this being ascribed to specific columnar growth of diamond grains. A strong variation (up to 2.7 times) in thermal conductivity across film thickness is revealed for selected specimens using a quasi-surface thermal grating excitation in VUV spectral range. In this case an additional fast-decay diffraction is observed due to nonthermal, owing to generated free charge carriers, recording of the grating.