Superconducting Properties of $NbN-SiO_2$ Sol-Gel Derived Thin Films

This work presents results of superconducting properties studies of $xNbN-(100-x)SiO_2$ (where x = 80 mol%) films of the thickness from 450 to 1950 nm. The films were prepared by thermal nitridation of sol-gel derived $xNb_2O_5-(100-x)SiO_2$ coatings. The nitridation process of $Nb_2O_5-SiO_2$ coatings leads to the formation of weakly disordered structures, with NbN metallic grains dispersed in the matrix of insulating $SiO_2$. All the samples in the normal state exhibit negative temperature coefficient of resistivity. Superconducting transition was not observed for the sample 450 nm thick. To examine the influence of magnetic field on superconducting properties of the films, the resistance versus temperature of 1350 and 1650 nm thick samples was measured in high magnetic fields. Resistive superconducting transitions are broadened due to the magnetic flux creep. The perpendicular upper critical magnetic fields for 1350 and 1650 thick samples are about 4.4 T. The critical temperatures of the superconducting transitions are about 4.5 K. Specific features of the magnetic field induced superconductor-insulator transition are found.