Two stage oxidation in epitaxial Ni (111)/GaN (0001) thin films

We present the oxidation process of epitaxial Ni (111)/GaN (0001) thin films studied by in situ synchrotron x-ray scattering, scanning electron microscopy, and transmission electron microscopy. By monitoring the evolution of the Ni (111) Bragg reflection, we reveal that two distinct oxidation processes occur. Initially, a continuous NiO layer of about 50 Å thickness is formed on the surface of Ni. The planar oxide layer saturates immediately and passivates the film from further surface oxidation. From this stage, the oxidation proceeds by means of the growth of surface oxide islands. The Ni atoms diffuse out through the defect sites running vertically through the initial oxide layer to form the oxide islands. Voids are generated underneath the oxide layer in this process. The oxide in the second stage grows logarithmically in time with the activation energy of about 0.15 eV.