Finding pathways for stoichiometric Co4N thin films

In this work, we studied the pathways for formation of stoichiometric tcn~thin films. Polycrystalline and epitaxial tcn~films were prepared using reactive direct current magnetron (dcMS) sputtering technique. A systematic variation in the substrate temperature (Ts) during the dcMS process reveals that the lattice parameter (LP) decreases as Ts~increases. We found that nearly stoichiometric tcn~films can be obtained when Ts~= 300,K. However, they emerge from the transient state of Co target ($phi$3,inch). By reducing the target size to $phi$1,inch, now the tcn~phase formation takes place from the metallic state of Co target. In this case, LP of tcn~film comes out to be $sim$99p~of the value expected for tcn. This is the largest value of LP found so far for tcn. The pathways achieved for formation of polycrystalline tcn~were adopted to grow an epitaxial tcn~film, which shows four fold magnetic anisotropy in magneto-optic Kerr effect measurements. Detailed characterization using secondary ion mass spectroscopy indicates that N diffuses out when Ts~is raised even to 400,K. Measurement of electronic structure using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy further confirms it. Magnetization measurements using bulk magnetization and polarized neutron reflectivity show that the saturation magnetization of stoichiometric tcn~film is even larger than pure Co. Since all our measurements indicated that N could be diffusing out, when tcn~films are grown at high Ts, we did actual N self-diffusion measurements in a CoN sample and found that N self-diffusion was indeed substantially higher. The outcome of this work clearly shows that the tcn~films grown prior to this work were always N deficient and the pathways for formation of a stoichiometric tcn~have been achieved.