Noise measurements on NbN thin films with a negative temperature resistance coefficient deposited on sapphire and on SiO2

We characterize granular NbNx thin cermet films deposited on either sapphire substrate or on SiO2 and compare the 1/f noise at 300 K and 80 K. The films were characterized with an impedance analyzer from 20 Hz to 1 MHz and analyzed as a resistor R in parallel with a capacitor C. The calculated noise voltage spectral density SvTh of the sample is in agreement with the experimentally observed noise for unbiased samples. The noise measurements on biased samples show 1/f noise. We checked that contact noise does not contribute to our measurements. The 1/f noise was compared for the films deposited on the silicon substrate and on the sapphire. Finally a model is proposed to explain the observed trends in the temperature resistance coefficient (TRC <0 for all samples) and the relative 1/f noise at 300 K and 80 K with the composition x = N/Nb of the layers.