Elucidating the role of disorder introduced by nitrogen in the superconducting properties of tungsten thin films

Abstract

Tungsten thin films are successfully prepared by reactive sputtering at room temperature using different Ar/N₂ gas mixtures. The role of disorder and chemical composition in the presence of superconductivity is analyzed. The results show that the crystalline structure of the samples changes drastically due to nitrogen in the mixture increases. A small addition of nitrogen stabilizes the metastable β-W phase, which successively increases its disorder as more nitrogen is added. These changes in the microstructure manifest as an increment in the lattice parameter followed by an amorphization. The electrical transport shows that the β-W stabilized at low N₂ concentrations does not display superconductivity above 2.5 K. Superconductivity with T_c between 4 K and 4.7 K emerges for disordered β-W and remains in amorphous films. A detailed study of the chemical state and composition indicates that superconductivity may be related more to amorphous structures (nitrides + W) and strained β-W than to β-W as it is stabilized with low nitrogen impurities.