Effect of post-annealing on the properties of thermally evaporated molybdenum oxide films: Interdependence of work function and oxygen to molybdenum ratio

Molybdenum oxide (MoOx, x < 3) thin films were deposited on glass and (100) silicon substrates by thermal evaporation technique. Post-annealing of the samples was performed in O-2 by rapid thermal process at 373-673 K. The surface chemical, structural, morphological and optical properties of the samples were investigated. X-ray photoelectron spectra characterization verified the growth of sub-stoichiometric MoOx films as the coexistence of oxidation states of Mo6+ and Mo5+. The O/Mo ratio of MoOx films increases from 2.58 to 2.75 when the annealing temperature increases to 473 K, but this ratio rapidly decreases at higher temperatures. The work function of MoOx films is proportional to O/Mo ratio and within the range of 5.40-5.95 eV. This correlation can be ascribed to the variation of Fermi level and surface dipole with oxygen vacancy. The MoOx films are polycrystalline with nanoparticles, and both alpha-and beta-phase MoO3 are co-existed with the annealing temperature from 373 K to 673 K. The optical band gap of MoOx films is in the scope of 2.72-2.95 eV. The three critical properties of MoOx films, such as work function, optical band gap and transmittance, are well-correlated to the variation of chemical configuration with the annealing temperature.