Ambient Temperature Growth of Mono- and Polycrystalline NbN Nanofilms and Their Surface and Composition Analysis

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 T
_c
of the films on the GaN buffer layer reached 10.4 K. Furthermore, a polycrystalline structure was observed on films grown onto Si (100) substrates, exhibiting a T
_c
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 irrespective of applied substrate heating. Moreover, the native oxide layer at the surface of NbN has been identified as NbO
₂
and, thus, is in contrast to the Nb
₂
O
₅
, 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 liftoff techniques could, in particular, lead to improved performance of cryogenic ultrasensitive terahertz electronics.