Superconductivity Dependence on Epitaxial NbN Film Thickness

A series of NbN films (2.2-200 nm thick) were epitaxially grown on (100) oriented single crystal MgO substrates using reactive de magnetron sputtering. As the film thickness decreased, the superconducting critical temperature (T-C) and the residual resistivity ratio both decreased, the normal state resistivity increased, while the zero temperature coherence length remained basically unchanged. In addition, a negative linear relationship existed between the T-C and the normal state resistivity at 20 K. Hall Effect measurements showed that the carrier density of the 200-nm-thick NbN film that exhibited the highest T-C (16.63 K) decreased from 1.12 x 10(23) to 5.56 x 10(22) e/cm(-3) in going to the 2.2-nm-thick NbN film that exhibited the lowest T-C (9.28 K). By fitting the data using McMillan theory for strong coupling superconductors, the T-C for the NbN films was determined directly from the thickness-dependent carrier density.