Grain boundary scattering in the normal state resistivity of superconducting NbN thin films

NbN films prepared by reactive dc sputtering at high ambient pressure exhibit anomalous normal-state resistivity, as pervasively reported in literature. Most of the prepared films exhibit negative temperature coefficient of resistivity (TCR), while a few films are metallic (TCR≳0) near room temperature and are nonmetallic (TCR<0) at temperatures between 200 and 300 K. Theoretical fittings by the model of grain-boundary scattering yield a better congruence with the experimental data, than those by exploiting the model of electron localization. This suggests that the transport mechanism of sputtered film prevails manipulated by grain boundary scattering rather than by defect scattering or by impurity scattering.