Control ofTcfor Niobium by N Ion Implantation

Nitrogen ion implantation in niobium films and single crystals has been investigated as a novel method for controlling superconducting transition temperature ( T c ) over a wide temperature range. T c measurements and He ion backscattering measurements were done as a function of dose to characterize the implanted layer. Reflection electron diffraction (RHEED) was utilized to investigate the lattice structure in the implanted layer. T c of films was found to decrease with increasing a dose up to ∼6 × 10 16 N 2 /cm 2 where T c decreased below 4.2 K from the initial value of 9 K, but increased rapidly to 10∼11 K at a dose of 1 × 10 17 N 2 /cm 2 . He ion channeling spectra of Nb single crystals also showed a sudden change at a dose of 6 × 10 16 N 2 /cm 2 , a value above which a rapid increase in T c was observed, which suggests that some change in structure occurs. The highest T c of 12.9 K (4 K higher than the initial value) was observed for samples implanted at a dose of 3 × 10 17 N 2 /cm 2 and annealed at 900°C for 10 min. RHEED pattern suggests that δ-NbN were formed in these implanted layers.