Ambient temperature growth of mono- and polycrystalline NbN nanofilms and their surface and composition analysis

Journal article, 2016

This paper presents the studies of high-quality 5 nm thin NbN films deposited by means of reactive DC magnetron sputtering at room temperature. The deposition without substrate heating offers major advantages from a processing point of view and motivates the extensive composition- and surface characterization and comparison of the present films with high quality films grown at elevated temperatures. Monocrystalline NbN films have been epitaxially grown onto hexagonal GaN buffer-layers (0002) and show a distinct, low defect interface as confirmed by High-Resolution TEM. The critical temperature Tc of films on the GaN buffer-layer reached 10.4 K. Furthermore, a poly-crystalline structure was observed on films grown onto Si (100) substrates, exhibiting a Tc of 8.1 K albeit a narrow transition from the normal to the superconducting state. X-ray photoelectron spectroscopy and reflected electron energy loss spectroscopy verified that the composition of NbN was identical irrespectively of applied substrate heating. Moreover, the native oxide layer at the surface of NbN has been identified as NbO2 and thus, is in contrast to the Nb2O5, usually claimed to be formed at the surface of Nb when exposed to air. These findings are of significance since it was proven the possibility of growing epitaxial NbN onto GaN buffer layer in the absence of high temperatures hence paving the way to employ NbN in more advanced fabrication processes involving a higher degree of complexity. The eased integration and employment of lift-off techniques could, in particular, lead to improved performance of cryogenic ultra-sensitive terahertz electronics.