  We report on the structural and superconducting properties of nanocrystalline
tungsten thin films growth by sputtering at room temperature with an N2:Ar
mixture (N2 from 3% to 50%). The crystalline phases were identified by
comparing pristine and thermal annealed thin films. For N2/(Ar+N2) mixtures
between 3 and 10 %, the films display nanocrystalline beta-W phase. Coexistence
of beta-W and W2N phases are observed for gas mixtures with N2 between 20 % and
40%. A detailed study of the superconducting properties as function of the
thickness was performed for W films growth with 8 % N2 mixtures. For this
concentration, the nitrogen atoms increase the disorder at the nanoscale
reducing the grain size and avoiding the crystallization of alpha-W. The
superconducting critical temperature (Tc = 4.7 K) is thickness independent for
films thicker than ~ 17 nm. Below this thickness, the Tc value decreases
systematically being 3.1 K for 4 nm thick films. Our study provides a simple
method for the fabrication of nanocrystalline beta-W thin films with potential
applications in superconducting devices.
