Abstract
A series of NbN films (2.2–200 nm thick) were epitaxially grown on (100) oriented single crystal MgO substrates using reactive dc 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 × 1023 to 5.56 × 1022 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.
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