Due to the high surface–to–volume ratio in nanostructured components and devices, thermal transport across the solid–solid interface strongly affects the overall thermal behavior. Materials such as Si, Ge, SiO2 and GaAs are widely used in advanced semiconductor devices. These materials may have differences in both crystal structure and Debye temperature. We have shown that the thermal transport across such interfaces can be improved by inserting an interlayer between the two confining solids.
If the two confining solids are similar in crystal structure and lattice constant but different in Debye temperature, it is predicted from the molecular dynamics modeling that an over 50% reduction of the thermal boundary resistance can be achieved by inserting a 1– to 2–nm–thick interlayer which has similar crystal structure and lattice constant as the two solids. In this case, the Debye temperature of the optimized interlayer is approximately the square root of the product of the Debye temperatures of the two solids. However, if the interlayer has large lattice mismatches with the two confining solids, a thin disordered layer is formed in the solid and in the interlayer adjacent to their interface. Such a disordered layer can distort the phonon density of states at the interface and strongly affects the interfacial phonon transport. In this case, it is found that a 70% reduction of the thermal boundary resistance can be achieved if the lattice constant of the interlayer is smaller than that of the two solids and the Debye temperature of the interlayer is approximately the average of the Debye temperatures of the two solids.
On the other hand, if the two solids have a large difference in both lattice constant and Debye temperature, the optimized interlayer should have a lattice constant near the average of the lattice constants of the two solids. For this case, an over 60% reduction of the thermal boundary resistance can be achieved if the Debye temperature of the interlayer is equal to or slightly higher than the square root of the product of the Debye temperatures of the two solids. The calculated phonon density of states shows that the distorted phonon spectra induced by large lattice mismatches are generally broader than the phonon spectra of the corresponding undistorted case. The broader interfacial phonon spectra increase the overlap between the phonon spectra of the two solids at the interface which leads to improved thermal boundary transport.