Effect of oxygen plasma treatment on the structural and superconducting properties of atomic-layer-deposited NbN thin films

Abstract

Surface oxidation is a common concern for ultrathin NbN superconducting devices, yet which superconducting properties it affects has not been established for films grown by plasma-enhanced atomic layer deposition. We expose NbN films about 11.4 nm thick to an O$_2$ plasma for 0 to 60 s, which oxidizes the surface without etching the film, and characterize the same sample series by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and transport measurements on 500 nm wide nanowires. XPS of the surface and the underlying layer shows that the plasma converts the surface to Nb$_2$O$_5$, whereas the bulk of the treated film is chemically identical to that of the untreated film. Crystalline $δ$-NbN remains intact. During exposure, the total film thickness rises and saturates between 40 and 60 s, indicating self-limiting surface oxide growth. The normal-state resistivity rises by 21% (dc) and 25% (optical) with the plasma dose, while the critical temperature and transition width are unchanged. The switching current densities of 4.9-6.2 MA/cm$^2$ at 4.2 K show no systematic dependence on the plasma dose. Their temperature dependence follows the dirty-limit depairing current of Kupriyanov and Lukichev, and the extrapolated switching currents reach 39-46% of the depairing current calculated from the optical resistivity and diffusion coefficient, in agreement with the width dependence reported for nanowires from the same process. The oxygen signature is confined to temperatures near $T_c$, where the supercurrent of the treated wires vanishes at least 0.5 K earlier than that of the untreated wire, consistent with a partially oxidized surface layer that acts on the film by the proximity effect. At these doses, oxygen narrows the temperature range of the supercurrent from above but does not systematically reduce the low-temperature current-carrying capacity of the wires.