Highly oriented (111) CoO and Co3O4 thin films grown by ion beam sputtering

We demonstrate that ion beam sputtering is a suitable deposition technique to obtain single-phase and highly (111)-oriented CoO and Co₃O₄ thin films. The control of substrate temperature and composition of the reactive atmosphere during deposition allows tailoring the phase structure and preferential growth of the films. Optimum substrate temperatures of 773 K and 603 K for CoO and Co₃O₄, respectively, have been determined and the presence of the single CoO and Co₃O₄ phases on the films has been confirmed by X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. Transmittance differences in the visible region higher than 50% have been observed between low transmittance in Co₃O₄ films and high transmittance in CoO films. The semiconducting behavior of Co₃O₄ thin films was confirmed by temperature dependent resistance measurements. The Co₃O₄ thin films also show a dual optical band gap at energies of 1.47 and 2.14 eV associated to d-d and p-d transition, respectively. Additionally, at intermediate temperatures between the optimum ones to growth (111)-oriented single phase CoO and Co₃O₄ films, only one single phase, either CoO or Co₃O₄, is present in the films, in which the electrical and structural properties can be controlled by the oxygen partial pressure used during deposition.