Thermal transport across boundaries in diamond structure materials

Previous measurements of the thermal conductivity of chemical vapour deposition diamond films show values of near, or even exceeding, that of natural diamond in spite of the polycrystalline nature of the films. These data have led us to consider whether there can be a `resonant' transfer of energy between identical crystallites separated by a material with different vibrational properties. We consider here a model of energy transfer between diamond-structure crystallites (Stillinger - Weber silicon) separated by a barrier region in which the mass is altered. We find that, for a pulse of energy deposited in one crystallite, there can be an efficient transfer of energy through the barrier region and subsequent build-up in a neighbouring crystallite if the vibrational spectrum of the barrier region is harder than that of the crystallites. If the vibrational spectrum of the barrier material is softer, the energy accumulation in the barrier region is at least as rapid as in the neighbouring crystallite and energy is retained longer in the region where it was deposited. The microscopic reasons behind this behaviour are discussed, and we conclude that the transmission probability between neighbouring crystallites leads to a more physical interpretation than a resonant transfer between next-neighbour crystallites.