Interface thermal conductance and the thermal conductivity of multilayer thin films

Accurate and simple measurements of the thermal conductivity of thin films deposited on high thermal conductivity substrates have been recently enabled by the development of an Ac hot-wire method, the 3 omega method. Recent progress in the measurement and understanding of heat transport in ultra-thin films (much less than 1 mu m thick) and multilayers is reviewed, and the possibility of using solid-solid interfaces on nanometer length scales to control heat transport in thin film materials is explored. The finite thermal conductance of solid-solid interfaces becomes important when considering heat transport in single layer films <100 nm thick. Through the use of multilayer films-for example, epitaxial superlattices of crystalline semiconductors or nanometer-thick layers of amorphous and microcrystalline oxides-we can study materials with an extremely high and controllable density of internal interfaces, and evaluate the effect of these interfaces on heat transport. For the case of Si-Ge superlattices, the relatively large mismatch of the vibrational properties of silicon and germanium creates a larger reduction in thermal conductivity than for GaAs-A1As superlattices. Surprisingly, heat conduction in multilayers of disordered oxides is essentially unchanged by a high interface density.