Controllable growth, microstructure and electronic structure of copper oxide thin films

Abstract

Copper oxide (Cu2O, Cu4O3 and CuO) thin films have been deposited on unmatched substrates by sputtering at room temperature. The influence of oxygen flow rate and total pressure on the film structure and preferred orientation has been studied. The total pressure is a relevant parameter to control the texture of Cu2O and Cu4O3 films, while the oxygen flow rate is effective to tune the preferred orientation of CuO films. Local epitaxial growth, where epitaxial relationship exists in columns of sputtered films, has been observed in Cu2O and Cu4O3 films by using a seed layer. The seed layer will govern the growth orientation of top layer via the local epitaxy, independently of the deposition conditions of top layer. Unusual microstructure that both phases have the vertically aligned columnar growth has been evidenced in biphase Cu2O and Cu4O3, which may relate to the local epitaxial growth of Cu2O. The lower resistivity than that in single phase films has been observed in this biphase film. Annealing in air can increase the transmittance of Cu2O films in visible region by the reduction of the impurity scattering, while the optical band gap is enlarged due to the partial removal of defect band tail. The optical properties and electronic structure of copper oxides calculated by GW approach with an empirical on-site potential for Cu d orbital, are in good accordance with experimental results from optical absorption, photoemission and electron energy loss spectroscopies