Growth of nano-composites and nano-alloys of Cu-Nb thin films

We report on the growth of immiscible Nb-Cu nano-composite thin films by DC co-sputtering. The ratio of Nb:Cu was varied in the films by changing the relative DC power ratio during sputtering. One set of films was grown at 600 °C while the other set was grown at room temperature. For the former set, the films formed phase-separated nano-composites of FCC-Cu and BCC-Nb irrespective of the Cu content. The microstructure revealed the presence of small crystallites of Nb and Cu, with their sizes changing only slightly with decreasing Cu content. These films were superconducting with the superconducting transition temperature (Tc) decreasing marginally with increased Cu content. For the second set of films, nano-alloys of Nb-Cu were formed as the Cu concentration was about 30% thereby showing that solid solutions can be formed in a metastable state for systems which is otherwise thermodynamically unfavourable. Interestingly, these nano-alloys were also found to be superconducting with a Tc around 3.2 ± 0.5 K.