Self-diffusion processes in stoichiometric iron mononitride

In this work, we studied atomic self-diffusion and structural phase transformation in a single phase iron mononitride (FeN) thin film deposited at an optimized substrate temperature (Ts) of 423 K. At this Ts, the FeN film exhibits a tetrahedral coordination between Fe and N atoms (ZnS-type structure with a lattice parameter of 4.28 Å). The structure of the FeN film was studied by combining x-ray diffraction with Fe and N K-edge x-ray absorption spectroscopy and conversion electron Mössbauer spectroscopy measurements. Self-diffusion of Fe and N was measured using secondary ion mass spectroscopy depth profiling in trilayer structures: [FeN(50 nm)/ 57FeN(2 nm)/FeN(50 nm)] and [FeN(50 nm)/Fe 15N(2 nm)/FeN(50 nm)] deposited on an amorphous quartz substrate using reactive magnetron sputtering. It was found that atomic self-diffusion is strongly associated with thermal stability. Before reaching the phase decomposition temperature, the self-diffusion of N was found to be slower than Fe. Upon phase decomposition, both Fe and N diffuse rapidly, and at this stage, the self-diffusion of N takes over Fe. Within the thermally stable state, slower N diffusion indicates that Fe–N bonds are stronger than Fe–Fe bonds in FeN. This behavior was predicted theoretically and has been evidenced in this work.