Tuning of structural and optical properties of reactively sputtered MoO_x films

MoO_x films were deposited using reactive sputtering of Mo under varying oxygen flow in the plasma ambient. The films showed a broad X-ray diffraction peak corresponding to MoO_x structure. The surface morphology has been influenced significantly with the oxygen content in the plasma during the process of film deposition. The as-deposited films were porous at lower oxygen flow rate (< 10 sccm), while it showed coarse structure at the higher flow rates (> 15 sccm). The compositional analysis revealed that the films are sub-stoichiometric and the weight ratio of O/Mo varied from 1.5 to 2 in the O₂ flow range of 5–25 sccm. The sub-stoichiometric phases of MoO₃ were detected from FTIR analysis. The optical characteristics showed that the films were highly transparent (> 85%) in the visible region, and exhibited direct band gap energy of 3.66 eV. The optical constants, i.e., refractive index (n) and extinction coefficient (k) evaluated from spectroscopic ellipsometry were found to be 2.03 and 0.0035, respectively at the wavelength of 600 nm, for the films deposited at the optimum conditions. The films showed a broad luminescence with a strong emission at 400 nm, which would help improve the blue response of solar cells. The Al/MoO_x/Si device showed a maximum barrier height, ϕ_b ~0.86 eV and a minimum leakage current, I_o ~ 5.81 × 10^–11 A at the higher O₂ flow rates (> 15 sccm), which changed significantly with the oxygen flow rate. The study demonstrates the tunability of structural and optical characteristics, which have been significantly influenced by tuning the plasma ambient during the film deposition.