Crystalline Structure and Optical Properties of Cobalt Nickel Oxide Thin Films Deposited with a Pulsed Hollow-Cathode Discharge in an Ar+O₂ Gas Mixture

Cobalt nickel oxide films are deposited on Si(111) or fluorine-doped tin-oxide-coated (FTO) glass substrates employing a pulsed hollow-cathode discharge. The hollow cathode is operated with argon gas flowing through the nozzle and with O-2 gas admitted to the vacuum chamber. Three different cathode compositions (Co20Ni80, Co50Ni50, and Co80Ni20) are investigated. Deposited and annealed thin films are characterized by X-ray diffraction, infrared (Raman) spectroscopy, and ellipsometry. As-deposited films consist of a single mixed cobalt nickel oxide phase. Upon annealing at 600 degrees C, the mixed cobalt nickel oxide phase separates into two cystalline sub-phases which consist of cubic NiO and cubic Co3O4. Annealed films are investigated by spectroscopic ellipsometry and the optical bandgaps are determined.