In situ spectroscopic ellipsometry as a pathway toward achieving VO2 stoichiometry for amorphous vanadium oxide with magnetron sputtering

As a special class of materials, transition metal oxides exhibit in their crystalline phase a variety of interesting properties, such as metal–insulator transition, ferroelectricity, magnetism, superconductivity, and so forth. However, for industrially widely applied methods such as room temperature magnetron sputtering, during initial fabrication steps of these materials, they are mostly amorphous, and control of stoichiometry during fabrication is challenging. It is, therefore, of pivotal importance to control the stoichiometry of transition metal oxides during growth in the amorphous state. One particularly important example for the necessity of stoichiometry control is vanadium dioxide (VO2), where small deviations in stoichiometry during fabrication result in unfavorable changes in the electronic and structural properties, for example, the metal–insulator transition temperature and optical permittivity. In this work, the stoichiometry of amorphous vanadium oxides is adjusted to VO2 using in situ spectroscopic ellipsometry (in situ SE) and verified by x-ray photoelectron spectroscopy. After an annealing process, a monoclinic VO2 crystalline structure is observed through x-ray diffraction at 30 °C. At an elevated temperature of 150 °C, which is higher than the typical metal–insulator transition temperature in VO2 of around 67 °C, a rutile crystalline structure is observed, which verifies the correctness of the stoichiometry of VO2. A Mott metal–insulator transition is revealed by the change in the imaginary part of optical permittivity through SE as well.