Superconducting and normal properties of nbn produced by ion implantation of niobium thin films

Abstract

Niobium films (120 nm thick) deposited by electron-beam evaporation were implanted near 5 K with 100 keV N₂ ions to maximum fluences of 4 to 7 × 10¹⁷ N/cm². STEM diffraction patterns showed a structural phase transformation from the initial Nb lattice with [110] fiber texture (bcc, a ₀ = 0.330 nm) to randomly oriented δ-NbN (fcc, a ₀ = 0.435–0.439 nm), via an intermediate, highly disordered Nb-(interstitial N) structure (bcc, maximum a ₀ = 0.342 nm). Decreases in transition temperature T _c from 9 K to a minium of 3 K and rapid linear increases in residual resistivity (ρ ₁₀) were observed to fluence of 1.0–1.5 × 10¹⁷ N/cm² and were attributed to the accumulation of radiation-induced defects. Continued implantation resulted in δ-NbN formation and T _c increases to maximum values of 9–10 K which were achieved at fluences from 3.5 to 5.0 × 10¹⁷ N/cm², corresponding to average substoichiometric N/Nb ratios of 0.52–0.75, at which point the phase transformation was complete. The δ-NbN formation was accompanied by significant lattice reordering and grain growth. At still higher fluences T _c gradually declined to 8–9 K and sharp increases in ρ ₁₀ were observed, possibly due to film sputtering.

Key Words:

• ion implantation
• niobium nitride
• superconductivity
• thin films