Thermal conductivity of amorphous Al₂O₃/TiO₂ nanolaminates deposited by atomic layer deposition

The thermophysical properties of Al₂O₃/TiO₂ nanolaminates deposited by atomic layer deposition (ALD) are studied as a function of bilayer thickness and relative TiO₂ content (0%-100%) while the total nominal thickness of the nanolaminates was kept at 100 nm. Cross-plane thermal conductivity of the nanolaminates is measured at room temperature using the nanosecond transient thermoreflectance method. Based on the measurements, the nanolaminates have reduced thermal conductivity as compared to the pure amorphous thin films, suggesting that interfaces have a non-negligible effect on thermal transport in amorphous nanolaminates. For a fixed number of interfaces, we find that approximately equal material content of Al₂O₃ and TiO₂ produces the lowest value of thermal conductivity. The thermal conductivity reduces with increasing interface density up to 0.4 nm^-1, above which the thermal conductivity is found to be constant. The value of thermal interface resistance approximated by the use of diffuse mismatch model was found to be 0.45 m² K GW^-1, and a comparative study employing this value supports the interpretation of non-negligible interface resistance affecting the overall thermal conductivity also in the amorphous limit. Finally, no clear trend in thermal conductivity values was found for nanolaminates grown at different deposition temperatures, suggesting that the temperature in the ALD process has a non-trivial while modest effect on the overall thermal conductivity in amorphous nanolaminates.