Superconducting properties and Hall effect of epitaxial NbN thin films

Abstract

We have measured the magnetotransport properties and Hall effect of a series of epitaxial NbN films grown on (100) oriented single crystalline MgO substrate under different conditions using reactive magnetron sputtering. Hall effect measurements reveal that the carrier density in NbN thin films is sensitive to the growth condition. The carrier density increases by a factor of 3 between the film with highest normal state resistivity $({\ensuremath{\rho}}_{n}\ensuremath{\sim}3.83\text{ }\ensuremath{\mu}\ensuremath{\Omega}\text{ }\text{m})$ and lowest transition temperature $({T}_{c}\ensuremath{\sim}9.99\text{ }\text{K})$ and the film with lowest normal state resistivity $({\ensuremath{\rho}}_{n}\ensuremath{\sim}0.94\text{ }\ensuremath{\mu}\ensuremath{\Omega}\text{ }\text{m})$ and highest transition temperature $({T}_{c}\ensuremath{\sim}16.11\text{ }\text{K})$, while the mobility of carriers does not change significantly. Our results show that ${T}_{c}$ of NbN is governed primarily by the carrier density rather than disorder scattering. By varying the carrier concentration during growth, we can vary the effective disorder $({k}_{F}l)$ from the moderately clean limit to the dirty limit, which makes this system ideal to study the interplay of carrier density and disorder on the superconducting properties of an $s$-wave superconductor.