Effect of substrate miscut angle on critical thickness, structural and electronic properties of MBE-grown NbN films on c-plane sapphire

We report the structural and electronic properties of niobium nitride (NbN) thin films grown by molecular beam epitaxy on c-plane sapphire with miscut angles of 0.5°, 2°, 4°, and 10° toward m-axis. X-ray diffraction scans reveal that the full width at half maximum of the rocking curves around the 1 1 1 reflection of these NbN films decreases with increasing miscut. Starting from 76 arcsecs on 0.5° miscut, the FWHM reduces to almost 20 arcsecs on 10° miscut sapphire, indicating improved structural quality. Scanning transmission electron microscopy images indicate that NbN on c-sapphire has around 10 nm critical thickness, irrespective of the substrate miscut, above which it turns columnar. The improved structural property is correlated with a marginal increment in superconducting transition temperature Tc from 12.1 K for NbN on 0.5° miscut sapphire to 12.5 K for NbN on 10° miscut sapphire.