Point-Contact Spectroscopy of Thin Superconducting NbN Films

Superconducting niobium nitride films are ideal candidates for electronics applications among low-Tc materials due to comparatively high critical temperature, large critical magnetic fields and relative ease of fabrication. The superconducting properties of NbN are known to be strongly dependent on the formation of a correct crystallographic phase. The tunneling spectroscopy data for NbN films are in good agreement with existing theoretical concepts, while contact methods provide significantly lower values of the superconducting order parameter. In this paper, we present corresponding results for a representative NbN film with Tc about 14 K, obtained using a point contact made by bringing a sharp metallic tip of silver in touch with the sample surface. We have observed fundamental difference between the gap values extracted from the differential conductance measurements and those following from the standard BCS theory. Despite the almost perfect contact of the normal tip with the film surface, the energy gap was nearly two times smaller than theoretically expected. At the same time, the gap values did not vanish at appreciably lower temperatures compared to the bulk of the NbN film, but rather continued to decrease slowly up to the bulk critical temperature. This finding is explained by significant deformation of the near-surface layer leading to the local suppression of superconducting parameters. Such behavior is a typical example of the superconducting proximity effect when the temperature dependence of the smaller gap strongly deviates from the BCS prediction but remains however finite up to the critical temperature of the larger gap, which is the same for two contacting spaces with superconducting parameters that differ significantly from each other. This conclusion is important for practical applications, in particular, for creating integration circuits based on superconducting NbN thin layers.