Photothermal examination of the heat diffusion inhomogeneity in diamond films of sub-micron thickness

We present measurements of the in-plane thermal diffusivity of free-standing diamond films of thicknesses from 0.3 to 6 μm. The photothermal method used for the measurements is non-destructive and contact-free, and does not require any sample preparation. The measured values lie in the range of 0.1 to 1.0 cm² s^-1, which is an order of magnitude below typical results obtained on bulk material. The results show a strong dependence of the thermal diffusivity on the film thickness and morphology. Phonon scattering at grain boundaries was identified as the dominating process limiting the heat diffusion. The availability of sets of films with different characteristics allowed the determination of the spatial dependence of the heat diffusivity even in very thin membranes. The observed effects are in good accordance with the extrapolation of measurements previously performed on much thicker samples. The data are correlated to results gained by Raman spectroscopy and spectroscopic ellipsometry.