On the superconducting transition temperature of A-15 "Nb3Si" synthesized by ion implantation

A new technique has been applied towards the synthesis of metastable A-15 Nb3Si, namely ion implantation of Si into an A-15 substrate. The substrate material was A-15 Nb3Al0.9Si0.1. Its surface was depleted of Al by a diffusion anneal. The Al deficiency was then replaced with Si by sequential Si implantations at different doses and energies. This produced a surface with a disordered bcc Nb structure. During subsequent heat treatments, the surface layer recrystallized epitaxially on the A-15 substrate into A-15 Nb3Al0.2Si0.8. In an effort to optimize the superconducting transition temperatures of the implanted layer, the stoichiometries, recrystallization anneals and ordering anneals were varied. The (Al + Si) concentration was varied in steps by varying the length of the diffusion anneal and keeping the Si dopant profile constant. Recrystallization anneals were varied from 800 to 980°C and were followed by ordering anneals of 500 to 700°C. The highest Tcobtained was 5.6K. The low Tccould be intrinsic to Nb3Si, but may also be due to structural or stoichiometric defects in the implanted layer.