Measurements of Thermal Conductivity of Undoped and Boron-Doped CVD Diamond by Transient Grating and Laser Flash Techniques

In plane (k||) and perpendicular (k⊥) thermal conductivity of high quality polycrystalline diamond films have been measured using transient thermal grating (TG) and laser flash (LFT) techniques, respectively. Undoped and boron-doped films have been grown by microwave plasma CVD. The TG tests the films were excited in bulk by Nd:YAG laser pulses at 1064 and 266 nm, while in LFT 1064 nm radiation was used. The in-plane values k||, averaged over the film thickness (0.3-0.5 mm), fall in the range of 15-20 W/cmK at room temperature. The anisotropy of 10-20% in thermal conductivity (k||⊥) caused by specific columnar growth of diamond grains is revealed. In the temperature range T = 25 - 200°C the thermal conductivity follows the relationship k ~ T-1 typical for dielectrics at high T. Using a quasi-surface TG excitation in VUV spectral range (213 nm wavelength) k|| values of thin sub-surface layers were measured, and the depth nonhomogeneity of thermal conductivity was estimated. A strong, up to 2.7 times, variation in k|| across the film thickness is revealed for some specimens. Thermal conductivity as high as 22.4 W/cmK is measured at the growth (coarse-grained) surface of a B-doped film.