Optical properties, band structures, and phase transition of UO_2+x epitaxial films deposited by polymer-assisted deposition

Optical properties of the UO2+x film deposited by a polymer-assisted deposition method have been investigated by spectroscopic ellipsometry (SE). This epitaxial film contains at least two kinds of uranium oxides of U3O8 and UO3, and the O/U ratio is 2.74, which is confirmed by x-ray diffraction (XRD) and scanning Auger microscopy methods. By investigating the optical constants, the bandgaps of U3O8 and UO3 are determined as 2.3 and 1.0 eV, respectively, and 80% of the epitaxial film is U3O8 and 20% is UO3. The speciation signatures from the XRD and band structures show that the UO2+x epitaxial film reduced to U3O8 with the heating treatment at 480 K in a vacuum while oxidized to UO3 at 650 K. This work demonstrates a useful tool for studying the optical properties, band structures, and phase transition of uranium oxide film by SE. (c) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)