Effect of nitrogen content on the structure and superconductivity of reactive sputtered NbTiN thin films

In this research, we have studied the structural and electrical properties of NbTiN films deposited on MgO and SiO2/Si substrates by reactive dc sputtering. The formation of stoichiometric NbTiN is very sensitive to N concentration and can be easily adjusted by changing the discharge current and Ar: N2 ratio along the current–voltage curves (IVCs) of the NbTi target. Excessive or insufficient N concentration in NbTiN leads to sublattice expansion or distortion, resulting in a decrease in critical temperature Tc. At Ar: N2 ratio of 30:4 and discharge current of 2.2 A, Tc as high as 15.8 K and 15.3 K has been obtained for 200 nm thick NbTiN/MgO and NbTiN/SiO2/Si samples, respectively. In addition, the critical density Jc of the 4 μm-wide and 7 nm-thick NbTiN film grown on MgO substrate at 2 K reaches 19.2 MA cm−2, which is approximately twice as high as the 10.9 MA cm−2 of the same-sized NbTiN film grown on SiO2/Si substrate. Therefore, by further fine-tuning the N concentration in combination with the IVCs of the target, high-quality stoichiometric NbTiN can be obtained.