Annealing study in oxygen environment of Iron (Fe) and Nickel (Ni) thin films

Metals, ceramics, and polymers are just a few of the materials that can be used with thin film technology. The majority of early material science research was primarily concerned with well-defined metals but, as a result of the various important industrial uses of metal oxide systems, there has been a phenomenal increase in this area of study. To tailor the physical properties of metal-metal oxide films placed onto ceramic substrates for specific purposes, it is essential to understand how growth and/or annealing conditions affect the crystallography and microstructure of the films. In order to study the differences in the growth of single and polycrystalline structures, as well as, to understand the impact of annealing on the surface morphology and structure of the films, Fe and Ni films were grown epitaxially on a MgO(001) substrate using the molecular beam epitaxy (MBE) technique and were annealed in-situ in the MBE chamber under a controlled temperature of 400⁰C and oxygen partial pressure of 5×10-6 mbar. Reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), scanning electron microscopy (SEM) / energy-dispersive x-ray spectroscopy (EDS), and atomic force microscopy (AFM) techniques were used to characterise the structure of the films. According to the structural analyses, Fe films grew on the MgO(001) substrate with more uniformity than Ni films. Comparing Ni films to Fe films, higher surface roughness was visible on Ni film, which is possibly due to the larger mismatch (∼16%) between Ni film and MgO substrate. The analysis of annealed film lattice shows that the films get oxidised and RMS surface roughness increased with annealing. The lattice d-spacing analysis, from high-resolution TEM (HRTEM) images and selected area electron diffraction (SAED) pattern on the annealed samples, indicated that the Fe films were half oxidised towards Fe3O4 (magnetite) along with non-oxidised iron and Ni films were fully oxidised to NiO.