Thermal conductivity computed for vitreous silica and methyl-doped silica above the plateau

The thermal conductivity of carbon-doped silicon dioxide films, fabricated to potentially serve as interlayer materials with low dielectric permittivity in small circuits, has been recently measured [B. C. Daly , Physica B 316-317, 254 (2002)]. Trends in the results were found to be inconsistent with the minimum thermal conductivity model. We report computational results for the thermal conductivity of vitreous silica and two methyl-doped silica systems of different composition, which serve as models for vitreous silica with nanoscale defects, in terms of their vibrational modes. Addition of methyl groups to silica influences the nature of the vibrational modes, particularly in the THz region. These modes, while apparently not localized, exhibit enhanced amplitude around methyl groups and are better characterized as resonant modes. We find the thermal conductivity to decrease with increased methyl doping, corresponding to a decrease in the participation ratio of the heat-carrying modes.