Thermal Relaxation in NbN Films on Crystalline Substrates

Here we focus on thermal transport in NbN films, deposited on crystalline substrates. Using the noise thermometry, we found that thermal relaxation in thin NbN films at temperatures above the critical superconducting temperature is mainly restricted by the thermal boundary resistance, and it can be modified by a structural disorder on a substrate surface. In addition, for thick NbN films deposited on a sapphire substrate, we observe that thermal relaxation occurs in the diffusive phonon regime when the phonon mean free path is shorter than the film thickness. The findings can be exploited for low-temperature devices based on NbN films.