We report on molecular dynamics studies of heat flow in superlattices. The computer simulations are performed using classical mechanics with periodic boundary conditions. The heat flow is in the direction normal to the layers. We have studied the variation of the conductivity with the repeat distance and the effect of interfacial roughness. We discuss the relation of these results to experimental data in the literature.

We report ultrafast optical measurements of the attenuation of 50 and 100 GHz longitudinal acoustic-phonon pulses in Si. Picosecond acoustic measurements were made at temperatures 50Ͻ T Ͻ 300 K on thinned ͑50-m-thick͒ wafers. The measured phonon lifetimes at 300 K, Ϸ5 -7 ns, are an order of magnitude less than expected based on three-phonon scattering rates derived from thermal conductivity data. We find instead that relaxational damping is the dominant mechanism in this frequency and temperature range. This attenuation sets an intrinsic limit on the quality factor of nanomechanical resonators that operate near room temperature.

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We report on measurements of the thermal conductivity of epitaxially grown nitride thin films. These semiconductor materials are of considerable technological importance for applications such as blue-light emitters and high-power, high frequency electronic devices. Measurements were made in the temperature range of 150–400 K using an optical technique in which the sample is heated with an ultrashort (∼150 fs) light pulse, and a time-delayed probe light pulse is used to measure the temperature of the sample as a function of time. The conductivity of the polycrystalline sample and the alloys are found to be significantly reduced compared to published values for bulk GaN.