Epitaxial Growth of Sputtered Ultra-Thin NbN Layers and Junctions on Sapphire

High crystalline quality of ultra-thin NbN layers and of NbN-MgO-NbN tri-layers, epitaxially grown by DC-magnetron sputtering in the superconducting B1-cubic phase has been achieved in a reproducible way on three different orientations of sapphire substrates i.e. R-, A- and M-planes. Significant improvements such as higher Tc, higher Jc and lower resistivity have been obtained by growing un-twined (110) oriented NbN layers on M-plane orientation of sapphire. Uniform, low roughness, 3-5 nm thick films with Tc above 12 K and Jc above 5 MA/cm² at 4.2 K were obtained. Characterizations by TEM, AFM and X-Ray diffraction evidence that growth of un-twined NbN on M-plane lead to a better epitaxy in comparison with twinned films observed on other sapphire orientations. We observe that the reduction of the substrate temperature from 600degC to 300degC during the deposition of NbN or NbN-MgO-NbN layers thicker than 20 nm prevents the nucleation of the competing HCP NbN phase. Moreover, 1.5 nm thick AlN or MgO over-layers sputtered in-situ prevent ultra-thin NbN films degradation through aging. The formation of Nb₂N_yO_5-x ( ~ 2.2 nm) at the unprotected NbN surface and of interfacial NbO ( ~ 0.7 nm) native oxides has been observed by XPS. It is forecasted that such improvements in ultra-thin NbN films deposited uniformly on 3 and 4 inch sapphire wafers is a key in the future development of superconducting single photon detectors, THz HEB mixers and also in low noise quantum analogical and digital Josephson devices.