Optical and electrical properties of MoO ₂ and MoO ₃ thin films prepared from the chemically driven isothermal close space vapor transport technique

Chemically-driven isothermal close space vapour transport was used to prepare pure MoO₂ thin films which were eventually converted to MoO₃ by annealing in air. According to temperature-dependent Raman measurements, the MoO₂/MoO₃ phase transformation was found to occur in the 225 °C-350 °C range while no other phases were detected during the transition. A clear change in composition as well as noticeable modifications of the band gap and the absorption coefficient confirmed the conversion from MoO₂ to MoO₃. An extensive characterization of these two pure phases was carried out. In particular, a procedure was developed to determine the dispersion relation of the refractive index of MoO₂ from the shift of the interference fringes of the used SiO₂/Si substrate. The obtained data of the refractive index was corrected taking into account the porosity of the samples calculated from elastic backscattering spectrometry. The Debye temperature and the residual resistivity were extracted from the electrical resistivity temperature dependence using the Bloch-Grüneisen equation. MoO₃ converted samples presented a very high resistivity and a typical semiconducting behavior. They also showed intense and broad luminescence spectra composed by several contributions whose temperature behavior was examined. Furthermore, surface photovoltage spectra were taken and their relation with the photoluminescence is discussed.