Electrical, optical and surface characterization of reactive RF magnetron sputtered molybdenum oxide films for solar cell applications

Thin films of molybdenum oxide (MoOx) were fabricated by radio frequency (RF) reactive magnetron sputtering technique with various oxygen flow rates and the deposition pressure fixed at 4 mTorr. The stoichiometry, composition, chemical binding energy, electrical, and optical properties of the sputtered MoOx films were examined. The deposited films were characterised by x-ray photoelectron spectroscopy (XPS), capacitance-voltage measurement (CV), spectroscopic ellipsometer (SE), and ultraviolet-visible spectroscopy (UV-VIS). XPS studies of the film indicated the presence of Mo5+ and Mo6+ oxidation states within the film. The deposited sputtered films were semiconducting in nature with the highest work function of 5.92 eV observed for MoOx. The stoichiometry for RF sputtered MoOx (x < 3) was found to be 2.73. CV analysis indicated that density of defect states in the MoOx films decreased with the corresponding increase in the oxygen flow rate. SE studies have shown that the refractive indices of the as-deposited films ranged from -1.8-2.05 at nominal wavelength of 632.8 nm. Similarly, high transmittance and large band gap values of sputtered MoOx were observed. It was proved that deposited MoOx is of n-type character with the Fermi level variation observed within the band gap of MoOx upon changing the oxygen flow rate.