CoO epitaxial growth on Cu(111) by reactive PVD and plasma oxidation

Heteroepitaxial layers of ordered cobalt monoxide were prepared on single crystal Cu(111) substrate by two physical deposition methods – reactive physical vapor deposition (PVD) of Co in pure oxygen and by step-wise oxygen plasma oxidation of small Co deposits. Volume and surface-specific diffraction methods were utilized to characterize the structure and orientation of the CoO thin films, the surface morphology of the PVD grown structures was probed by scanning tunneling microscopy (STM). Photoelectron spectroscopy (XPS) was used to monitor the chemical state of the samples during preparation stage, as well as during the following thermal treatments up to 700 K. At low oxygen pressures (below 1 × 10−4 Pa), unlike for some other single crystal surfaces, on Cu(111) only cobalt monoxide phase with rocksalt structure can be established. The resulting CoO thin film exhibits a 3-dimensional mesa-like character with large flat (111) terraces aligned with the (111) face of Cu substrate and shaped by (111) and (100) side facets. The natural surface polarity of the (111) facets of CoO is suggested to be compensated primarily by a presence of aliovalent surface Co3+ cations, forming an ultrathin surface spinel-like phase, and surface-bound hydroxyls.