Structural modifications induced by ion implantation in Nb–N thin-film superconductors

First-order structural phase transitions have been induced in thin-film samples of Nb–N by means of ion implantation. Samples originally deposited in the hexagonal β phase (Nb₂N) were implanted with each of the following ions: N⁺, C⁺, O⁺, Ne⁺ and Si⁺. Both the N⁺ and C⁺ implants resulted in a transition from the β to the cubic (δ) and the hexagonal (ε) phases with a concomitant improvement in the superconducting transition temperature, T_c, from 3.8 to 10.7 K, in the best case. The O⁺, Si⁺ and Ne⁺ implants had the effect of enlarging the β lattice and increasing the amount of crystalline disorder. The relative degree of crystallinity was significantly reduced after the Si⁺ implant; the Ne⁺ implant is the singular example in which T_c was depressed as a result of implantation. Films originally deposited in the δ phase could be partially transformed to the ε phase by suitable annealing treatment; the δ-to-ε transformation, however, could not be induced by implantation techniques alone. Possible mechanisms for the transition are discussed.