  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 (rho_n~3.83micro-Ohm-m) and
lowest transition temperature (Tc~9.99K) and the film with lowest normal state
resistivity (rho_n~0.94micro-Ohm-m) and highest transition temperature
(Tc~16.11K) while the mobility of carriers does not change significantly except
for the most resistive films. Our results show that the Tc 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
(kF_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.
