Ultrathin NbTiN Films With High Ti Composition for Superconducting Nanowire Single Photon Detectors

We have fabricated and characterized ultrathin NbTiN films with high Titanium (Ti) composition for applications of superconducting nanowire single photon detectors (SNSPD). An alloy with 47 wt. % Ti and 53 wt. % Nb as the target material is employed. NbTiN thin films with thicknesses ranging from 5 nm to 20 nm are grown onto MgO (100), Si (100) and SiO_X-Si substrates by DC magnetron sputtering. The zero resistance superconducting transition temperature (T_C0) of about 8.8 K has been obtained for the NbTiN film of 8 nm thick on MgO substrate. With twice Ti composition than usual NbTiN film, the lattice mismatch and low resistivity are further improved, which will be more favorable for the performance of SNSPD. The results of the films´ structural characterizations by AES, AFM, TEM and XRD are also presented.