Effects of low energy ion bombardment on the formation of cubic iron mononitride thin films

• We have grown iron nitride FeNx(0.6 ≤ x ≤ 1) thin films by dual ion beam sputtering.
• Effects of N2+ ion assistance in the formation of Fe mononitride phases are studied.
• Nanocrystalline Fe mononitride with a composition FeNx ≈ 1 is obtained.
• A phase evolution ε → γ‴ + γ″ → ε is observed as EFe increases.
• γ‴-FeN phase coexists with γ″-FeN at room temperature deposition conditions.

The formation of cubic nitrides with stoichiometry close to FeN obtained by ion assisted sputter deposition has been studied as a function of deposition parameters. In particular, we have explored the influence of the energy deposited by the assistant beam per deposited Fe atom to understand changes in composition, phase formation and nanocrystallinity of the films. An optimum N2+ ion energy and a JN/JFe ratio (JN and JFe represent the current density of N2+ ions and Fe atoms respectively) have been determined in order to obtain only iron mononitride phases. X-ray diffraction and Mössbauer spectroscopy revealed a phase evolution from ε-Fex(x ≈ 2)N to γ″ and γ‴-FeN as the N2+ ion energy and the JN/JFe flux ratio increase. Pure nanocrystalline iron mononitride, with nitrogen content close to 50%, is obtained when JN/JFe ratio reaches 5.9 and the N2+ ion energy is 63.4 eV. Further increments of N2+ energies and JN/JFe values reverse this behavior and a phase evolution from γ″ and γ‴-FeN to ε-Fex(x ≈ 2)N is found. This behavior is attributed to energy damage and resputtering phenomena. It has also been found that γ‴-FeN phase coexists with γ″-FeN phase when the deposition is performed at room temperature.