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

We compared ultrathin NbN films (2.5-10 nm) simultaneously grown by dc reactive sputtering at 600 degrees 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 J(c) (>4 MA cm(-2) at 4.2 K), and a higher critical temperature T-c (11.3 K for 4.4 nm). These results offer promising alternative for better performances of superconducting detectors and mixers. (c) 2007 American Institute of Physics.