Microstructure of NbN epitaxial ultrathin films grown on A-, M-, and R-plane sapphire

We compared ultrathin NbN films (2.5–10nm) simultaneously grown by dc reactive sputtering at 600°C on A, M, and R orientations of sapphire substrates. Film structures and superconducting properties were characterized. We show that actual NbN device films on R-Al2O3 are (135) oriented and suffer from detrimental disoriented twin domains. On the contrary, NbN on M-Al2O3 is shown to be untwined, leading to a lower resistivity, an increased critical current density Jc (>4MAcm−2 at 4.2K), and a higher critical temperature Tc (11.3K for 4.4nm). These results offer promising alternative for better performances of superconducting detectors and mixers.